Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last Five Years
---
paper_title: Channel Estimation for OFDM

paper_content:

Orthogonal frequency division multiplexing (OFDM) has been widely adopted in modern wireless communication systems due to its robustness against the frequency selectivity of wireless channels. For coherent detection, channel estimation is essential for receiver design. Channel estimation is also necessary for diversity combining or interference suppression where there are multiple receive antennas. In this paper, we will present a survey on channel estimation for OFDM. This survey will first review traditional channel estimation approaches based on channel frequency response (CFR). Parametric model (PM)-based channel estimation, which is particularly suitable for sparse channels, will be also investigated in this survey. Following the success of turbo codes and low-density parity check (LDPC) codes, iterative processing has been widely adopted in the design of receivers, and iterative channel estimation has received a lot of attention since that time. Iterative channel estimation will be emphasized in this survey as the emerging iterative receiver improves system performance significantly. The combination of multiple-input multiple-output (MIMO) and OFDM has been widely accepted in modern communication systems, and channel estimation in MIMO-OFDM systems will also be addressed in this survey. Open issues and future work are discussed at the end of this paper.
---
paper_title: Dirty RF: a new paradigm

paper_content:

The implementation challenge for new low-cost low-power wireless modem transceivers has continuously been growing with increased modem performance, bandwidth, and carrier frequency. Up to now we have been designing transceivers in a way that we are able to keep the analog (RF) problem domain widely separated from the digital signal processing design. However, with today’s deep sub-micron technology, analog impairments – “dirt effects” – are reaching a new problem level which requires a paradigm shift in the design of transceivers. Examples of these impairments are phase noise, non-linearities, I/Q imbalance, ADC impairments, etc. In the world of “Dirty RF” we assume to design digital signal processing such that we can cope with a new level of impairments, allowing lee-way in the requirements set on future RF sub-systems. This paper gives an overview of the topic and presents analytical evaluations of the performance losses due to RF impairments as well as algorithms that allow to live with imperfect RF by compensating the resulting error effects using digital baseband processing.
---
paper_title: Synchronization Techniques for Orthogonal Frequency Division Multiple Access (OFDMA): A Tutorial Review

paper_content:

Orthogonal frequency division multiple access (OFDMA) has recently attracted vast research attention from both academia and industry and has become part of new emerging standards for broadband wireless access. Even though the OFDMA concept is simple in its basic principle, the design of a practical OFDMA system is far from being a trivial task. Synchronization represents one of the most challenging issues and plays a major role in the physical layer design. The goal of this paper is to provide a comprehensive survey of the latest results in the field of synchronization for OFDMA systems, with tutorial objectives foremost. After quantifying the effects of synchronization errors on the system performance, we review some common methods to achieve timing and frequency alignment in a downlink transmission. We then consider the uplink case, where synchronization is made particularly difficult by the fact that each user's signal is characterized by different timing and frequency errors, and the base station has thus to estimate a relatively large number of unknown parameters. A second difficulty is related to how the estimated parameters must be employed to correct the uplink timing and frequency errors. The paper concludes with a comparison of the reviewed synchronization schemes in an OFDMA scenario inspired by the IEEE 802.16 standard for wireless metropolitan area networks.
---
paper_title: Fundamentals of Large Sensor Networks: Connectivity, Capacity, Clocks and Computation

paper_content:

Sensor networks potentially feature large numb- ers of nodes. The nodes can monitor and sense their environ- ment over time, communicate with each other over a wireless network, and process information that they exchange with each other. They differ from data networks in that the network as a whole may be designed for a specific application. We study the theoreticalfoundationsofsuchlarge-scalesensornetworks.We address four fundamental organizational and operational issues related to large sensor networks: connectivity, capacity, clocks, and function computation. To begin with, a sensor network must be connected so that information can indeed be exchanged between nodes. The connectivity graph of an ad hoc network is modeledas arandomgraphandthe criticalrangefor asymptotic connectivity is determined, as well as the critical number of neighbors that a node needs to connect to. Next, given connectivity, we address the issue of how much data can be transported over the sensor network. We present funda- mental bounds on capacity under several models, as well as architectural implications for how wireless communication should be organized. Temporal information is important both for the applications of sensor networks as well as their operation. We present fundamental bounds on the synchroniz- ability of clocks in networks, and also present and analyze algorithms for clock synchronization. Finally, we turn to the issue of gathering relevant information, which sensor networks are designed to do. One needs to study optimal strategies for in- network aggregation of data, in order to reliably compute a composite function of sensor measurements, as well as the complexity of doing so. We address the issue of how such computation can be performed efficiently in a sensor network and the algorithms for doing so, for some classes of functions.
---
paper_title: What Will 5G Be?

paper_content:

What will 5G be? What it will not be is an incremental advance on 4G. The previous four generations of cellular technology have each been a major paradigm shift that has broken backward compatibility. Indeed, 5G will need to be a paradigm shift that includes very high carrier frequencies with massive bandwidths, extreme base station and device densities, and unprecedented numbers of antennas. However, unlike the previous four generations, it will also be highly integrative: tying any new 5G air interface and spectrum together with LTE and WiFi to provide universal high-rate coverage and a seamless user experience. To support this, the core network will also have to reach unprecedented levels of flexibility and intelligence, spectrum regulation will need to be rethought and improved, and energy and cost efficiencies will become even more critical considerations. This paper discusses all of these topics, identifying key challenges for future research and preliminary 5G standardization activities, while providing a comprehensive overview of the current literature, and in particular of the papers appearing in this special issue.
---
paper_title: Multi-Cell MIMO Cooperative Networks: A New Look at Interference

paper_content:

This paper presents an overview of the theory and currently known techniques for multi-cell MIMO (multiple input multiple output) cooperation in wireless networks. In dense networks where interference emerges as the key capacity-limiting factor, multi-cell cooperation can dramatically improve the system performance. Remarkably, such techniques literally exploit inter-cell interference by allowing the user data to be jointly processed by several interfering base stations, thus mimicking the benefits of a large virtual MIMO array. Multi-cell MIMO cooperation concepts are examined from different perspectives, including an examination of the fundamental information-theoretic limits, a review of the coding and signal processing algorithmic developments, and, going beyond that, consideration of very practical issues related to scalability and system-level integration. A few promising and quite fundamental research avenues are also suggested.
---
paper_title: Digital Communication Receivers: Synchronization, Channel Estimation, and Signal Processing: Digital E-BK

paper_content:

From the Publisher: ::: Digital Communication Receivers offers a complete treatment on the theoretical and practical aspects of synchronization and channel estimation from the standpoint of digital signal processing. The focus on these increasingly important topics, the systematic approach to algorithm development, and the linked algorithm-architecture methodology in digital receiver design are unique features of this book. The material is structured according to different classes of transmission channels. In Part C, baseband transmission over wire or optical fiber is addressed. Part D covers passband transmission over satellite or terrestrial wireless channels. Part E deals with transmission over fading channels. Designed for the practicing communication engineer and the graduate student, the book places considerable emphasis on helpful examples, summaries, illustrations, and bibliographies. Contents include basic material, baseband communications, passband transmission, receiver structure for PAM signals, synthesis of synchronization algorithms, performance analysis of synchronizers, bit error degradation caused by random tracking errors, frequency estimation, timing adjustment by interpolation, DSP system implementation, characterization, modeling, and simulation of linear fading channels, detection and parameter synchronization on fading channels, receiver structures for fading channels, parameter synchronization for flat fading channels, and parameter synchronization for selective fading channels.
---
paper_title: OFDM and Its Wireless Applications: A Survey

paper_content:

Orthogonal frequency-division multiplexing (OFDM) effectively mitigates intersymbol interference (ISI) caused by the delay spread of wireless channels. Therefore, it has been used in many wireless systems and adopted by various standards. In this paper, we present a comprehensive survey on OFDM for wireless communications. We address basic OFDM and related modulations, as well as techniques to improve the performance of OFDM for wireless communications, including channel estimation and signal detection, time- and frequency-offset estimation and correction, peak-to-average power ratio reduction, and multiple-input-multiple-output (MIMO) techniques. We also describe the applications of OFDM in current systems and standards.
---
paper_title: Wireless Visions: A Look to the Future by the Fellows of the WWRF

paper_content:

In less than two decades, mobile communication has developed from a niche application to a mass-market high-tech product, having experienced an unprecedented growth, never achieved by any other technology, whether radio, television, or even the Internet. Thirteen well-known experts, all of them honored as WWRF Fellows, have been interviewed and shared their expertise and opinions on ten questions about the wireless future, as presented here. The answers span a wide field from air interfaces, networks, devices, applications, to new ways of interaction, to name a few. Although the ideas and views presented here are not one common vision, they should provide stimulating ideas and questions for future research, and it will be exciting to see how things are really going to develop. The Fellows' ideas also clearly show the fascination, impact, and opportunities wireless communications has and will have in the future.
---
paper_title: Emergent Slot Synchronization in Wireless Networks

paper_content:

This paper presents a biologically inspired approach for distributed slot synchronization in wireless networks. This is facilitated by modifying and extending a synchronization model based on the theory of pulse-coupled oscillators. The proposed Meshed Emergent Firefly Synchronization (MEMFIS) multiplexes synchronization words with data packets and adapts local clocks upon the reception of synchronization words from neighboring nodes. In this way, a dedicated synchronization phase is mitigated, as a network-wide slot structure emerges seamlessly over time as nodes exchange data packets. Simulation results demonstrate that synchronization is accomplished regardless of the arbitrary initial situation. There is no need for the selection of master nodes, as all nodes cooperate in a completely self-organized manner to achieve slot synchrony. Moreover, the algorithm is shown to scale with the number of nodes, works in meshed networks, and is robust against interference and collisions in dense networks.
---
paper_title: Distributed synchronization in wireless networks

paper_content:

This article has explored history, recent advances, and challenges in distributed synchronization for distributed wireless systems. It is focused on synchronization schemes based on exchange of signals at the physical layer and corresponding baseband processing, wherein analysis and design can be performed using known tools from signal processing. Emphasis has also been given on the synergy between distributed synchronization and distributed estimation/detection problems. Finally, we have touched upon synchronization of nonperiodic (chaotic) signals. Overall, we hope to have conveyed the relevance of the subject and to have provided insight on the open issues and available analytical tools that could inspire further research within the signal processing community.
---
paper_title: Timing and Carrier Synchronization With Channel Estimation in Multi-Relay Cooperative Networks

paper_content:

Multiple distributed nodes in cooperative networks generally are subject to multiple carrier frequency offsets (MCFOs) and multiple timing offsets (MTOs), which result in time varying channels and erroneous decoding. This paper seeks to develop estimation and detection algorithms that enable cooperative communications for both decode-and-forward (DF) and amplify-and-forward (AF) relaying networks in the presence of MCFOs, MTOs, and unknown channel gains. A novel transceiver structure at the relays for achieving synchronization in AF-relaying networks is proposed. New exact closed-form expressions for the Cramer-Rao lower bounds (CRLBs) for the multi-parameter estimation problem are derived. Next, two iterative algorithms based on the expectation conditional maximization (ECM) and space-alternating generalized expectation-maximization (SAGE) algorithms are proposed for jointly estimating MCFOs, MTOs, and channel gains at the destination. Though the global convergence of the proposed ECM and SAGE estimators cannot be shown analytically, numerical simulations indicate that through appropriate initialization the proposed algorithms can estimate channel and synchronization impairments in a few iterations. Finally, a maximum likelihood (ML) decoder is devised for decoding the received signal at the destination in the presence of MCFOs and MTOs. Simulation results show that through the application of the proposed estimation and decoding methods, cooperative systems result in significant performance gains even in presence of impairments.
---
paper_title: Bounds and Algorithms for Multiple Frequency Offset Estimation in Cooperative Networks

paper_content:

The distributed nature of cooperative networks may result in multiple carrier frequency offsets (CFOs), which make the channels time varying and overshadow the diversity gains promised by collaborative communications. This paper seeks to address multiple CFO estimation using training sequences in space-division multiple access (SDMA) cooperative networks. The system model and CFO estimation problem for cases of both decode-and-forward (DF) and amplify-and-forward (AF) relaying are formulated and new closed-form expressions for the Cramer-Rao lower bound (CRLB) for both protocols are derived. The CRLBs are then applied in a novel way to formulate training sequence design guidelines and determine the effect of network protocol and topology on CFO estimation. Next, two computationally efficient iterative estimators are proposed that determine the CFOs from multiple simultaneously relaying nodes. The proposed algorithms reduce multiple CFO estimation complexity without sacrificing bandwidth and training performance. Unlike existing multiple CFO estimators, the proposed estimators are also accurate for both large and small CFO values. Numerical results show that the new methods outperform existing algorithms and reach or approach the CRLB at mid-to-high signal-to-noise ratio (SNR). When applied to system compensation, simulation results show that the proposed estimators significantly reduce average-bit-error-rate (ABER).
---
paper_title: The Evolution of Cellular Backhaul Technologies: Current Issues and Future Trends

paper_content:

The rapid increase of the number of mobile subscribers as well as the deployment of 3G technologies are putting strain on mobile backhaul operational expenditures (OPEX) which amount to 20-40% of total mobile operator's OPEX due to their reliance on T1/E1 copper lines. For these reasons, the current backhaul systems, a term commonly used to describe connectivity between base stations and radio controllers, are increasingly integrating more cost-effective, packet switched technologies, especially Ethernet/Internet technologies. In addition, Wi-Fi and WiMAX are emerging as promising backhaul solutions and initial findings have demonstrated their feasibility. However, the notion of network migration unavoidably raises new technical challenges relevant to aspects of TDM and packet network timing synchronization, QoS, and packet efficiency. This survey aims to provide a comprehensive study of state-of-the-art circuit switched and emerging packet switched backhaul technologies based on research articles and standard documents. For packet switched backhaul, we focus on the practically important Pseudowire approaches which are used to transport TDM services over packet switched networks. We also discuss the features and research findings on the use of Wi-Fi and WiMAX technologies which illustrate their potential for rapid and cost-efficient backhaul deployment. Finally, we highlight some open issues relevant to timing synchronization in wireless mesh backhaul and femtocells deployments, which offer a rich ground for further research.
---
paper_title: Relay-based deployment concepts for wireless and mobile broadband radio

paper_content:

In recent years, there has been an upsurge of interest in multihop-augmented infrastructure-based networks in both the industry and academia, such as the seed concept in 3GPP, mesh networks in IEEE 802.16, and converge extension of HiperLAN/2 through relays or user-cooperative diversity mesh networks. This article, a synopsis of numerous contributions to the working group 4 of the wireless world research forum and other research work, presents an overview of important topics and applications in the context of relaying. It covers different approaches to exploiting the benefits of multihop communications via relays, such as solutions for radio range extension in mobile and wireless broadband cellular networks (trading range for capacity), and solutions to combat shadowing at high radio frequencies. Furthermore, relaying is presented as a means to reduce infrastructure deployment costs. It is also shown that through the exploitation of spatial diversity, multihop relaying can enhance capacity in cellular networks. We wish to emphasize that while this article focuses on fixed relays, many of the concepts presented can also be applied to systems with moving relays.
---
paper_title: Cooperative diversity in wireless networks: Efficient protocols and outage behavior

paper_content:

We develop and analyze low-complexity cooperative diversity protocols that combat fading induced by multipath propagation in wireless networks. The underlying techniques exploit space diversity available through cooperating terminals' relaying signals for one another. We outline several strategies employed by the cooperating radios, including fixed relaying schemes such as amplify-and-forward and decode-and-forward, selection relaying schemes that adapt based upon channel measurements between the cooperating terminals, and incremental relaying schemes that adapt based upon limited feedback from the destination terminal. We develop performance characterizations in terms of outage events and associated outage probabilities, which measure robustness of the transmissions to fading, focusing on the high signal-to-noise ratio (SNR) regime. Except for fixed decode-and-forward, all of our cooperative diversity protocols are efficient in the sense that they achieve full diversity (i.e., second-order diversity in the case of two terminals), and, moreover, are close to optimum (within 1.5 dB) in certain regimes. Thus, using distributed antennas, we can provide the powerful benefits of space diversity without need for physical arrays, though at a loss of spectral efficiency due to half-duplex operation and possibly at the cost of additional receive hardware. Applicable to any wireless setting, including cellular or ad hoc networks-wherever space constraints preclude the use of physical arrays-the performance characterizations reveal that large power or energy savings result from the use of these protocols.
---
paper_title: Synchronization Protocols and Implementation Issues in Wireless Sensor Networks: A Review

paper_content:

Time synchronization in wireless sensor networks (WSNs) is a topic that has been attracting the research community in the last decade. Most performance evaluations of the proposed solutions have been limited to theoretical analysis and simulation. They consequently ignored several practical aspects, e.g., packet handling jitters, clock drifting, packet loss, and mote limitations, which affect real implementation on sensor motes. Authors of some pragmatic solutions followed empirical approaches for the evaluation, where the proposed solutions have been implemented on real motes and evaluated in testbed experiments. This paper gives an insight on issues related to the implementation of synchronization protocols in WSN. The challenges related to WSN environment are presented; the importance of real implementation and testbed evaluation are motivated by some experiments we conducted. The most relevant implementations of the literature are then reviewed, discussed, and qualitatively compared. While there are several survey papers that present and compare the protocols from the conception perspectives, as well as others that deal with mathematical and signal processing issues of the estimators, a survey on practical aspects related to the implementation is missing. To our knowledge, this paper is the first one that takes into account the practical aspect of existing solutions.
---
paper_title: Fractionally Spaced Frequency-Domain MMSE Receiver for OFDM Systems

paper_content:

Based on frequency-domain oversampling and the Bayesian Gauss-Markov theorem, we propose a fractionally spaced frequency-domain minimum mean-square error (FSFDMMSE) receiver for orthogonal frequency-division multiplexing systems. It is shown that frequency diversity inherent in a frequency-selective fading channel can be extracted and exploited by the proposed FSFD-MMSE receiver. This diversity advantage outweighs the effect of intercarrier interference generated by frequency-domain oversampling, due largely to the MMSE receiver's interference suppression capability. Numerical results show that the FSFD-MMSE receiver outperforms the conventional MMSE receiver under both ideal and practical situations (i.e., with frequency offset, channel estimation errors, and even doubly selective channel fading). In addition, the FSFD-MMSE receiver only needs a fast Fourier transform size that is no larger than N + Q -1 (TV = number of data subcarriers, and Q = number of resolvable multipath components).
---
paper_title: Reconfigurable architecture of a hybrid synchronisation algorithm for LTE

paper_content:

Time and frequency synchronisation are significant parts of the cell-search procedure and one of the first processing blocks within a mobile communication system. Particularly for an OFDM transmission within an initial synchronisation process, the algorithm has to deal with carrier frequency offsets up to several subcarrier spacings. To be able to still operate under those conditions, the cell-search procedure consists of different processing steps, combined in a hybrid algorithm. Beside good performance properties, hybrid algorithms lead to a high computational demand and implementation effort. To overcome these challenges, flexible architectures, which are able to select the most suitable algorithm during runtime, are the base for an efficient hardware realization. In this paper, as an example, we are introducing an hybrid initial synchronisation algorithm for an LTE-system, which still operates under the effect of a carrier frequency offset greater than the subcarrier spacing. Consecutively, we are showing a reconfigurable architecture as well as the results of an FPGA implementation of the time synchronisation part of that algorithm. The architecture is able to switch between different correlators, namely a reverse-auto, a cross- and a CP-based auto-correlation during runtime, which enables a flexible and low complexity realization of the computational complex synchronisation process.
---
paper_title: Joint Channel and Frequency Offset Estimation for Oversampled Perfect Reconstruction Filter Bank Transceivers

paper_content:

Recently, DFT-based oversampled perfect reconstruction filter banks (OPRFB), as a special form of filtered multitone, have shown great promises for applications to multicarrier modulation. Still, accurate frequency synchronization and channel equalization are needed for their reliable operation in practical scenarios. In this paper, we first derive a data-aided joint maximum likelihood (ML) estimator of the carrier frequency offset (CFO) and the channel impulse response (CIR) for OPRFB transceiver systems operating over frequency selective fading channels. Then, by exploiting the structural and spectral properties of these systems, we are able to considerably reduce the complexity of the proposed estimator through simplifications of the underlying likelihood function. The Cramer Rao bound on the variance of unbiased CFO and CIR estimators is also derived. The performance of the proposed ML estimator is investigated by means of numerical simulations under realistic conditions with CFO and frequency selective fading channels. The effects of different pilot schemes on the estimation performance for applications over time-invariant and mobile time-varying channels are also examined. The results show that the proposed joint ML estimator exhibits an excellent performance, where it can accurately estimate the unknown CFO and CIR parameters for the various experimental setups under consideration.
---
paper_title: Localized or Interleaved? A Tradeoff between Diversity and CFO Interference in Multipath Channels

paper_content:

Carrier frequency offset (CFO) damages the orthogonality between sub-carriers and thus causes multiuser interference in uplink OFDMA/SC-FDMA systems. For a given CFO, such multiuser interference is mainly dictated by channel (sub-carrier) allocation, which also specifies the diversity gain of one user over multi-path channels. In particular, the positions of one user's sub-channels will determine its diversity gain, while the distances between sub-channels of the concerned user and those of others will govern the CFO interference. Two popular channel allocation methods are the localized and interleaved (distributed) schemes where the former has less CFO interference but the latter achieves more diversity gain. In this paper, we will consider the channel allocation scheme for uplink LTE systems by investigating the effects of channel allocation on both of the diversity gain and CFO interference. By combining these two effects, we will propose a semi-interleaved scheme, which achieves full diversity gain with minimum CFO interference.
---
paper_title: Efficient OFDM Symbol Timing Estimator Using Power Difference Measurements

paper_content:

This paper presents an efficient blind symbol timing estimation scheme for orthogonal frequency-division multiplexing (OFDM) systems with constant modulus constellation. The proposed technique is designed to estimate symbol timing offsets by minimizing the power difference between subcarriers with similar indices over two consecutive OFDM symbols based on the assumption that the channel slowly changes over time. The proposed power difference estimator (PDE) is totally blind because it requires no prior information about the channel or the transmitted data. Monte Carlo simulation is used to assess the PDE performance in terms of the probability of correct timing estimate Plock-in. Moreover, we propose a new performance metric denoted as the deviation from safe region (DSR). Simulation results have demonstrated that the PDE performs well in severe frequency-selective fading channels and outperforms the other considered estimators. The complexity of the PDE can be significantly reduced by incorporating a low-cost estimator to provide initial coarse timing information. The proposed PDE is realized using feedforward and early-late gate (ELG) configurations. The new PDE-ELG does not suffer from the self-noise problem inherent in other ELG estimators reported in the literature.
---
paper_title: Pilot Subset Partitioning Based Integer Frequency Offset Estimation for OFDM Systems With Cyclic Delay Diversity

paper_content:

Cyclic delay diversity (CDD) is a simple transmit diversity technique for coded OFDM systems with multiple transmit antennas. However, high frequency selectivity caused by CDD degrades the performance of post- FFT estimation, i.e., integer frequency offset (IFO). This paper suggests a simple way of improving the performance of the IFO estimator based on the pilot subset partitioning which is designed to reduce the effect of frequency selective fading by adopting the CDD. By partitioning uncorrelated pilot subcarriers into subsets to satisfy high correlation, and performing frequency estimation for each pilot subset, a robust IFO estimation scheme is derived. The simulation results show that the proposed method can provide benefit to the overall system performance .
---
paper_title: MCMOE-Based CFO Estimator Aided With the Correlation Matrix Approach for Alamouti's STBC MC-CDMA Downlink Systems

paper_content:

This paper addresses the estimation problem of carrier frequency offset (CFO) in the downlink transmission of space-time block-coded multicarrier code-division multiple-access (STBC MC-CDMA) systems over multipath fading. This study proposes a multiply constrained minimum output energy (MCMOE)-based blind CFO estimator, which is simply assisted by the presented correlation matrix approach to efficiently achieve the CFO estimate. We formulate a two-level CFO estimator by optimizing the receiver output power, as well as the data correlation matrix. At the first level, all possible CFO candidates are found by evaluating the well-defined estimated merit figure. Then, exploiting multiple constraints in the design of the MCMOE receiver, a criterion is used in level two to determine the exact CFO estimate. Numerical results are presented to verify that both precise CFO estimation and reliable error performance can be achieved, even when the channel is dominated by noise because the impact of CFO on the output signal-to-interference-plus-noise ratio (SINR) and bit error rate (BER) are effectively removed by the proposed CFO estimator.
---
paper_title: A practical equalizer for cooperative delay diversity with multiple carrier frequency Offsets

paper_content:

Cooperative transmission in wireless networks provides diversity gain in multipath fading environments. Among all the proposed cooperative transmission schemes, delay diversity has the advantage of needing less coordination and higher spectrum efficiency. However, in a distributed network, the asynchrony comes from both carrier frequency and symbol timing between the cooperating relays. In this paper, a minimum mean square error fractionally spaced decision feedback equalizer (MMSE-FS-DFE) is developed to extract the diversity with large multiple carrier frequency offsets, its performance approaches the case without multiple carrier frequency offsets. The front end design for the receiver in this scenario is discussed, and a practical frame structure is designed for carrier frequency offsets and channel estimation. A subblock-wise decision-directed adaptive least squares (LS) estimation method is developed to solve the problem caused by error in frequency offset estimation. The purpose of this paper is to provide a practical design for cooperative transmission (CT) with the delay diversity scheme.
---
paper_title: Estimation scheme of the receiver IQ imbalance under carrier frequency offset in communication system

paper_content:

IQ signal processing is widely utilised in today's communication systems. However, it usually faces a common problem of front-end distortion such as IQ imbalance and carrier frequency offset (CFO). Effective algorithms exist for estimating and compensating for IQ imbalance as well as CFO, when the two problems are treated separately. With both effects present, most of those algorithms suffer from degraded quality parameter estimates. In this study, the authors proposed a scheme to estimate and compensate for IQ imbalance in the presence of CFO by using a known preamble of a repeated training sequence. In addition, the authors present a modified algorithm to suit the particular case when CFO is small. The performance of our proposed scheme has been examined with computer simulations on IEEE 802.11a signals. It is shown that the proposed method is more robust and renders better performance than the existing method.
---
paper_title: Performance study of fast frequency-hopped/M-ary frequency-shift keying systems with timing and frequency offsets over Rician-fading channels with both multitone jamming and partial-band noise jamming

paper_content:

In this study, the effects of timing and frequency offsets on bit-error rate (BER) performance of fast frequency-hopped M-ary frequency-shift keying communication systems over Rician-fading channels with both multitone jamming (MTJ) and partial-band noise jamming (PBNJ) are investigated. Analytical BER expressions are derived for both linear-combining and product-combining receivers. Numerical results show that under both MTJ and PBNJ conditions, the BER performance falls between the two extreme cases, in which either MTJ or PBNJ presents. It is found that the BER performance is severely degraded as the timing or frequency offset increases. The product-combining receiver is found to be more sensitive to the timing and frequency offsets than the linear-combining receiver. It is also observed that for the linear-combining receiver, the optimum diversity order increases as the timing and frequency offset increases over both Rician-fading and Rayleigh-fading channels; however, the reverse is true for the product-combining receiver.
---
paper_title: A Low-Power Low-Cost Design of Primary Synchronization Signal Detection

paper_content:

Synchronization is an important component of a practical communication system. Furthermore, network entry including synchronization is important. Since the detection of primary synchronization signal (PSS) is the first step of network entry in long term evolution (LTE) systems, thus it may be a critical path for practical systems. Therefore, tradeoff between performance and low power consumption and low cost of PSS detection needs to be made carefully. This paper presents a new synchronization method for low power and low cost design. The approach of a 1-bit analog-to-digital converter (ADC) with down-sampling is compared with that of a 10-bit ADC without down-sampling under multi-path fading conditions defined in LTE standard for user equipment (UE) performance test . The simulation results of PSS are obtained on several kinds of channels. The simulation results explicitly show that the performance of the method with down-sampling for 1-bit ADC does not degrade even if frequency offset exists. Based on the simulation results, different implementation architectures and their synthesis report and analysis are present. A low-power low-cost design with high performance to detect PSS is derived in this paper.
---
paper_title: Improved Detection of Uplink OFDM-IDMA Signals with Carrier Frequency Offsets

paper_content:

This letter proposes an improved detection scheme to mitigate the influence of carrier frequency offsets (CFOs) in uplink orthogonal frequency division multiplexing-interleave division multiple access (OFDM-IDMA) systems. The basic principle is to iteratively estimate and cancel the combined interference from multiple users and CFOs at the receiver so that the additional interference due to the residual CFOs from other users can be suppressed. Simulation results show that the proposed scheme can effectively eliminate the interference and significantly improve the system performance in the presence of CFOs.
---
paper_title: A computationally efficient sampling frequency offset estimation for OFDM-based digital terrestrial television systems

paper_content:

Sampling frequency offset (SFO) that occurs due to a sampling frequency mismatch between the transmitter and receiver oscillators is one of the main problem in the orthogonal frequency division multiplexing (OFDM) based digital terrestrial television (DTTV) system. The SFO can cause an intersymbol interference and an intercarrier interference that may degrade the performance of an OFDM system and result in high bit error rate. Since the value of SFO is usually very low (part per million scale), the SFO estimation is very susceptible to a noise. In this paper, we propose the estimation method that can minimize the influence of the noise in SFO estimation and also minimize the computational complexity. The performance of the proposed method has been verified with computer simulation. The computer simulation results show that the proposed sampling frequency offset estimation is more efficient in computational complexity compared to the conventional method to achieve the similar performance.
---
paper_title: A SAGE Approach to Frequency Recovery in OFDM Direct-Conversion Receivers

paper_content:

In-phase/quadrature (I/Q) imbalances are front-end impairments which may greatly complicate the synchronization task in a low-cost direct-conversion receiver (DCR). In this paper we investigate the possibility of using the space-alternating generalized expectation-maximization (SAGE) algorithm to recover the carrier frequency offset in OFDM terminals with a DCR architecture. Our study leads to a novel scheme that operates in a recursive fashion and exploits a conventional OFDM training preamble composed by several repeated parts. At each iteration, interference arising from I/Q impairments is subtracted from the received samples before updating the frequency estimate. The performance of the new scheme is assessed in terms of estimation accuracy and processing load by considering a wireless local area network (WLAN) compliant with the IEEE 802.11a standard. The main goal is to check whether the SAGE approach exhibits some advantages compared to existing alternatives.
---
paper_title: Joint Estimation of Channel Impulse Response and Carrier Frequency Offset for OFDM Systems

paper_content:

In this paper, an order recursive method is proposed to solve the joint estimation of channel impulse response (CIR) and carrier frequency offset (CFO) for orthogonal frequency-division multiplexing (OFDM) transmission. As long as one can obtain the solution for Qth-order Taylor expansion, the solution for (Q + 1)th order can also be obtained via a simple recursive relation. The proposed recursive algorithm actually provides a method to handle any Qth-order Taylor expansion, instead of just the second order adopted in the technical literature. Significant improvement can be observed by adopting higher order approximation. Analytical mean-square-error (MSE) performance results are given, demonstrating the efficiency of the proposed algorithm.
---
paper_title: FADAC-OFDM: Frequency-Asynchronous Distributed Alamouti-Coded OFDM

paper_content:

We propose frequency-asynchronous distributed Alamouti-coded orthogonal frequency-division multiplexing (FADAC-OFDM). The proposed scheme effectively mitigates the intercarrier interference (ICI) due to frequency offset (FO) between two distributed antennas. The transmitter side of the proposed scheme transmits each of the two subslots in Alamouti code through two remote subcarriers symmetric to the center frequency and is referred to as space–frequency reversal schemes by Wang et al. or Choi. The receiver side of the proposed scheme is significantly different from the conventional scheme or the scheme proposed by Wang et al. in that it performs two discrete Fourier transform (DFT), each of which is synchronized to each transmit antenna's carrier frequency. The decision variables are generated by performing a simple linear combining with the same complexity as that of the conventional Alamouti code. The derivation shows that in flat-fading channels, the dominant ICI components due to FO cancel each other during the combining process. The proposed scheme achieves almost the same performance as the ideal Alamouti scheme, even with large FO. To use this ICI self-cancellation property for selective fading channels or in cases with timing offset (TO) between two transmit antennas, the total subcarriers are divided into several subblocks, and the proposed scheme is applied to each subblock. For mildly selective channels or cases with practically small TO, the proposed scheme achieves significantly improved performance compared with the conventional space–frequency Alamouti-coded OFDM.
---
paper_title: Modeling and Estimation of Transient Carrier Frequency Offset in Wireless Transceivers

paper_content:

Future wireless devices have to support many applications (e.g., remote robotics, wireless automation, and mobile gaming) with extremely low latency and reliability requirements over wireless connections. Optimizing wireless transceivers while switching between wireless connections with different circuit characteristics requires addressing many hardware impairments that have been overlooked previously. For instance, switching between transmission and reception radio functions to facilitate time-division duplexing can change the load on the power supply. As the supply voltage changes in response to the sudden change in load, the carrier frequency drifts. Such a drift results in transient carrier frequency offset (CFO) that cannot be estimated by conventional CFO estimators and is typically addressed by inserting or extending guard intervals. In this paper, we explore the modeling and estimation of the transient CFO, which is modeled as the response of an underdamped second order system. To compensate for the transient CFO, we propose a low complexity parametric estimation algorithm, which uses the null space of the Hankel-like matrix constructed from phase difference of the two halves of the repetitive training sequence. Furthermore, to minimize the mean squared error of the estimated parameters in noise, a weighted subspace fitting algorithm is derived with a slight increase in complexity. The Cramer-Rao bound for any unbiased estimator of the transient CFO parameters is derived. The performance of the proposed algorithms is also confirmed by the experimental results obtained from the real wireless transceivers.
---
paper_title: Carrier frequency synchronization in the downlink of 3GPP LTE

paper_content:

In this paper, we investigate carrier frequency synchronization in the downlink of 3GPP Long Term Evolution (LTE). A complete carrier frequency offset estimation and compensation scheme based on standardized synchronization signals and reference symbols is presented. The estimation performance in terms of mean square error is derived analytically and compared to simulation results. The impact of estimation error on the system performance is shown in terms of uncoded bit error ratio and physical layer coded throughput. Compared to perfect synchronization, the presented maximum likelihood estimator shows hardly any performance loss, even when the most sophisticated MIMO schemes of LTE are employed.
---
paper_title: MMSE Solution for OFDMA Systems with Carrier Frequency Offset Correction

paper_content:

The multi-access orthogonal frequency division multiplexing (OFDMA) technology has drawn a lot of attention in next generation wireless mobile communications. It is well-known that carrier frequency offsets in OFDMA systems can destroy the orthogonality among subcarriers and produce intercarrier interference (ICI) and multiuser interference (MUI). In our previous works, we proposed a common carrier frequency offset (CCFO) correction scheme at the OFDMA receiver, which can reduce the ICI/MUI effect and the bit error rate. In the scheme, the CFO correction can be performed by adaptively converging the MSE between the demodulated output and the decision feedback data. This paper studies the minimum MSE solution for the CCFO value, and the result is exploited to verify the adaptive CCFO estimation algorithm by means of the decision feedback. Simulation results show that the adaptive decision feedback scheme for CCFO estimation is effective and the minimum MSE performance is well achieved.
---
paper_title: Carrier Frequency Offset Estimation for OFDM Systems Over Mobile Radio Channels

paper_content:

In this paper, a new technique is proposed for blind estimation of carrier frequency offset (CFO) in wireless orthogonal frequency-division multiplexing (OFDM) systems with constant-modulus constellations. The proposed scheme is based on the assumption that the channel slowly changes in the time domain with respect to the OFDM symbol duration. As a consequence, the channel effect on a given subcarrier in two consecutive OFDM symbols is approximately the same. Based on this assumption, a cost function is derived such that the power difference between all subcarriers in two consecutive OFDM symbols is minimized. Using Monte Carlo simulation, we demonstrate that the proposed scheme has superior performance in both static and time-varying frequency-selective fading channels. The proposed system can rapidly and accurately estimate the CFO using only three trial values, given that the CFO is less than half of the subcarriers' frequency spacing.
---
paper_title: Sensitivity analysis of interleaved OFDMA system uplink to carrier frequency offset

paper_content:

This paper investigates the sensitivity analysis of orthogonal frequency division multiple access (OFDMA) systems to carrier frequency offset (CFO) in the uplink. This analysis uses simple superposition principle approach, where the effects of different users are studied separately. We calculate a closed-form expression for signal-to-interference ratio (SIR) and derive very simple expressions for inter-carrier interference (ICI) and multiple access interference (MAI) in interleaved subcarrier allocation scheme. Finally theoretical results are verified using Monte Carlo simulation.
---
paper_title: Maximum likelihood algorithms for joint estimation of synchronisation impairments and channel in multiple input multiple output–orthogonal frequency division multiplexing system

paper_content:

Maximum likelihood (ML) algorithms, for the joint estimation of synchronisation impairments and channel in multiple input multiple output–orthogonal frequency division multiplexing (MIMO–OFDM) system, are investigated in this work. A system model that takes into account the effects of carrier frequency offset, sampling frequency offset, symbol timing error and channel impulse response is formulated. Cramer–Rao lower bounds for the estimation of continuous parameters are derived, which show the coupling effect among different impairments and the significance of the joint estimation. The authors propose an ML algorithm for the estimation of synchronisation impairments and channel together, using the grid search method. To reduce the complexity of the joint grid search in the ML algorithm, a modified ML (MML) algorithm with multiple one-dimensional searches is also proposed. Further, a stage-wise ML (SML) algorithm using existing algorithms, which estimate less number of parameters, is also proposed. Performance of the estimation algorithms is studied through numerical simulations and it is found that the proposed ML and MML algorithms exhibit better performance than SML algorithm.
---
paper_title: A Semidefinite Relaxation Approach to Blind Despreading of Long-Code DS-SS Signal With Carrier Frequency Offset

paper_content:

Blind despreading of long-code direct sequence spread spectrum (DS-SS) signal with unknown carrier frequency offset (CFO) is considered. The maximum likelihood estimate (MLE) of spreading waveform is first derived, and to cope with the unknown CFO, we then use the semidefinite relaxation (SDR) technique to approximate our MLE problem as a convex semidefinite programming (SDP) problem, which can be solved efficiently using modern convex optimization methods. Simulation results demonstrate that the proposed approach significantly outperforms the dominant mode despreading estimator whether or not CFO exists at low signal to noise ratio (SNR).
---
paper_title: Joint AOD and CFO estimation in wireless sensor networks localization system

paper_content:

Self-localization system of wireless sensor networks based on angle of departure (AOD) is studied in this paper. In AOD model, anchor node with multi-antenna transmits orthogonal pilot signals to sensor node with single antenna, which entitles the sensor node to function as equipping multi-antenna. Given that the limited number of antenna at anchor nodes will result in degree of freedom (DOF) deficiency at sensor node when dealing with more multipath components (MPC), we adopt a novel method without requiring extra antennas. The proposed algorithm simultaneously includes the oscillator mismatch between anchor node and sensor node, i.e., carrier frequency offset (CFO). With the aid of anchor's movement and CFO, the equivalent antenna array at sensor node is expended by synthetic aperture procedure to a much larger one, which subsequently improve the estimation ability to MPCs. In addition, the close-form solutions of CFO and AOD are also derived. The effectiveness and performance of proposed algorithm are demonstrated by numerical simulations.
---
paper_title: Classification of Space-Time Block Codes Based on Second-Order Cyclostationarity with Transmission Impairments

paper_content:

Signal classification is important in various commercial and military applications. Multiple antenna systems complicate the signal classification problem since there is now the issue of estimating the number and configuration of transmit antennas. The novel blind classification algorithm proposed in this paper exploits the cyclostationarity property of space-time block codes (STBCs) for the classification of multiple antenna systems in the presence of possible transmission impairments. Analytical expressions for the second-order cyclic statistics used as the basis of the algorithm are derived, and the computational cost of the proposed algorithm is considered. This algorithm avoids the need for a priori knowledge of the channel coefficients, modulation, carrier phase, and timing offsets. Moreover, it does not need accurate information about the transmission data rate and carrier frequency offset. Monte Carlo simulation results demonstrate a good classification performance with low sensitivity to phase noise and channel effects, including frequency-selective fading and Doppler shift.
---
paper_title: Joint Maximum Likelihood Estimation of CFO, Noise Power, and SNR in OFDM Systems

paper_content:

Estimation of noise power and signal-to-noise ratio (SNR) are fundamental tasks in wireless communications. Existing methods to recover these parameters in orthogonal frequency-division multiplexing (OFDM) are derived by following heuristic arguments and assuming perfect carrier frequency offset (CFO) synchronization. Hence, it is currently unknown how they compare with an optimum scheme performing joint maximum likelihood (ML) estimation of CFO, noise power and SNR. In the present work, the joint ML estimator of all these parameters is found by exploiting the repetitive structure of a training preamble composed of several identical parts. It turns out that CFO recovery is the first task that needs to be performed. After CFO compensation, the ML estimation of noise power and SNR reduces to a scheme that is available in the literature, but with a computational saving greater than 60% with respect to the original formulation. To assess the ultimate accuracy achievable by the ML scheme, novel expressions of the Cramer-Rao bound for the joint estimation of all unknown parameters are provided.
---
paper_title: Efficient Phase-Error Suppression for Multiband OFDM-Based UWB Systems

paper_content:

This paper proposes an efficient phase-error suppression scheme for multiband (MB) orthogonal frequency-division multiplexing (OFDM)-based ultrawideband (UWB) communication systems. The proposed scheme consists of a clock-recovery loop and a common phase-error (CPE) tracking loop. The clock-recovery loop performs estimation of the sampling frequency offset (SFO) and its 2-D (time and frequency) compensation, while the CPE tracking loop estimates and corrects the phase errors caused by residual carrier frequency offset (CFO), residual SFO, and phase noise (PHN). The SFO and CPE estimators employ pilot-tone-based and channel-frequency-response (CFR)-weighted low-complexity approaches, each of which uses a robust error-reduction scheme without using angle calculations or divisions. Analytical results and numerical examples show the effectiveness of the proposed scheme in different multipath fading scenarios and signal-to-noise ratio (SNR) regimes.
---
paper_title: A Subspace-Based Two-Way Ranging System Using a Chirp Spread Spectrum Modem, Robust to Frequency Offset

paper_content:

Herein, we propose and implement a subspace-based two-way ranging system for high resolution indoor ranging that is robust to frequency offset. Due to the frequency offset between wireless nodes, issues about sampling frequency offset (SFO) and carrier frequency offset (CFO) arise in range estimation. Although the problem of SFO is resolved by adopting the symmetric double-sided two-way ranging (SDS-TWR) protocol, the CFO of the received signals impacts the time-of-arrival (TOA) estimation, obtained by conventional subspace-based algorithms such as ESPRIT and MUSIC. Nevertheless, the CFO issue has not been considered with subspace-based TOA estimation algorithms.Our proposed subspace-based algorithm, developed for the robust TOA estimation to CFO, is based on the chirp spread spectrum (CSS) signals. Our subspace-based ranging system is implemented in FPGA with CSS modem using a hardware/software co-design methodology. Simulations and experimental results show that the proposed method can achieve robust ranging between the CSS nodes in an indoor environment with frequency offset.
---
paper_title: Interference mitigation techniques for asynchronous multiple access communications in SIMO FBMC systems

paper_content:

In this paper we derive linear equalizers for FBMC systems. We focus on the multiple access channel where signals transmitted by different users may have different carrier frequency offsets and time delays. Aiming at reducing the bandwidth requirements of the periodic ranging messages we formulate two SIMO solutions that are tolerant to time and frequency misalignments. Simulation-based results show that the same performance can be achieved in the BER range [10−2, 10−4] in comparison to an OFDM multi-user minimum mean square error receiver. Considering a guard interval between users, the BER range in which FBMC and OFDM perform equally can be broadened. However, the OFDM solution requires a complexity 8.6 times higher and its spectral efficiency is reduced by 0.72 b/s/Hz due to the cyclic prefix.
---
paper_title: OFDM Transmission scheme for asynchronous two-way multi-relay cooperative networks with analog network coding

paper_content:

For two-way relaying assisted by analog network coding, most investigation so far is based on perfect synchronization assumption. In contrast, in this paper we consider the more practical asynchronism assumption, and develop a new OFDM transmission scheme that is robust to the lack of synchronization in both timing and carrier frequency. In our scheme, the relays' signals are constructed by fusing several OFDM symbols received from the source nodes transmissions. The source node receivers can successfully demodulate the received OFDM signals after mitigating effectively multiple carrier frequency offsets and multiple timing phase offsets. Simulations are conducted to demonstrate its superior performance. This scheme has the same bandwidth efficiency as the conventional OFDM transmission, and can achieve the same relaying gain as the existing multiple relay transmissions. By relieving the stringent synchronization requirement, this scheme leads to simplified relay design, which makes it more practical to exploit multiple relays in two-way relaying networks.
---
paper_title: Differential Carrier Frequency Offset and Sampling Frequency Offset Estimation for 3GPP LTE

paper_content:

The Long Term Evolution (LTE) system, like other OFDM based systems, is very sensitive to synchronization errors which must be estimated and compensated. In this paper, maximum-likelihood (ML) estimates of the Carrier Frequency Offset (CFO) and Sampling Frequency Offset (SFO) are investigated in the framework of LTE downlink, aiming at precisely tracking time-varying fluctuations of the impairments. In order to make the tracking independent of the time index of the OFDM symbol, the differential CFO and SFO tracking algorithm are proposed. Combined with the phase ambiguity cancellation scheme, the differential tracking algorithm can work with high accuracy. The performances are analyzed and evaluated through simulations.
---
paper_title: Blind CFO estimation algorithm for OFDM systems by using generalized precoding and trilinear model

paper_content:

This paper discusses the blind carrier frequency offset (CFO) estimation for orthogonal frequency division multiplexing (OFDM) systems by utilizing trilinear decomposition and generalized precoding. Firstly, the generalized precoding is employed to obtain multiple covariance matrices which are requisite for the trilinear model, and then a novel CFO estimation algorithm is proposed for the OFDM system. Compared with both the joint diagonalizer and estimation of signal parameters via rotational invariant technique (ESPRIT), the proposed algorithm enjoys a better CFO estimation performance. Furthermore, the proposed algorithm can work well without virtual carriers. Simulation results illustrate the performance of this algorithm.
---
paper_title: A Unified Framework for Interference Analysis of Noncoherent MFSK Wireless Communications

paper_content:

This paper presents a new unified analytical technique for accurate interference analysis of noncoherent M-ary frequency shift keying (MFSK) wireless communication systems in the presence of additive white Gaussian noise (AWGN) and multiple arbitrary interfering signals. New exact bit error rate expressions are derived for nonorthogonal noncoherent binary FSK in the presence of an arbitrary number of interfering signals having arbitrary spectral shapes at arbitrary frequency offsets. Furthermore, tight upper bounds for the symbol and bit error rates are given for noncoherent MFSK with arbitrary tone spacings. These results are extended to include MFSK signals experiencing Rayleigh, Nakagami-m or Rician fading. Some specific interference scenarios are analyzed in details, including jammers with a given Doppler-spread, direct sequence spread spectrum and orthogonal frequency division multiplexing signals, and a Poisson field of arbitrary interferers.
---
paper_title: A non-coherent neighbor cell search scheme for LTE/LTE-A systems

paper_content:

A new neighbor cell search algorithm for LTE/LTE-A systems is presented in this paper. To improve the interference problem in channel estimation for coherent SSS detection in the conventional neighbor cell search approaches, we propose a non-coherent scheme that takes advantage of the similarity of channel responses at adjacent subcarriers. The proposed neighbor cell search procedure not only includes both PSS and SSS detection, but also can combat different carrier frequency offsets that the home cell signal and the neighbor cell signal may suffer. The removal of the home cell synchronization signals in our algorithm converts the neighbor cell PSS and SSS into new sequences for recognition, respectively. By examining the cross-correlation properties of the new sequences, we show that partial correlation can well detect the neighbor cell sector ID and group ID through the new sequences. From simulation results, it is also clear that the proposed algorithm has good detection results and outperforms the conventional coherent approaches.
---
paper_title: Carrier Frequency Offset Estimation for OFDM Direct-Conversion Receivers

paper_content:

We investigate the problem of carrier frequency offset (CFO) recovery in an OFDM direct-conversion receiver plagued by both dc-offset and frequency-selective I/Q imbalance. In order to enlarge the frequency acquisition range, the CFO is divided into an integer part, which is multiple of the subcarrier spacing, plus a remaining fractional part. The fractional CFO is firstly estimated by resorting to the least-squares (LS) principle using a suitably designed training sequence. Since the exact LS solution requires a complete search over the frequency uncertainty range, we propose a simpler scheme that dispenses from any peak-search procedure. We also derive an approximated closed-form expression of the estimation accuracy that reveals useful for assessing the impact of various design parameters on the system performance. After computing the fractional CFO, the integer frequency error is eventually retrieved by following a weighted LS approach. Numerical simulations and theoretical analysis indicate that the proposed scheme can be used to obtain accurate CFO estimates with affordable complexity.
---
paper_title: Alamouti Coding Scheme for AF Relaying With Doppler Shifts

paper_content:

In this paper, we propose an Alamouti-code-based relaying scheme for frequency asynchronous amplify-and-forward (AF) relay networks. Both the oscillator frequency offsets and the Doppler shifts among the distributed nodes are considered in our design. We employ orthogonal frequency-division multiplexing (OFDM) modulation at the source node and let the two relay nodes implement only simple operations, such as time reversal, conjugation, and amplification. We show that without Doppler shifts, the multiple carrier frequency offsets (CFOs) can be directly compensated at the destination, and the received signals exhibit an Alamouti-like structure. We further prove that full spatial diversity can be achieved by the fast symbol-wise detection when the oscillator frequency offset between the relay nodes is smaller than a certain threshold, which yields lower decoding complexity compared with the existing schemes. In the case with Doppler shifts, where the direct CFO compensation becomes impossible, we develop a repetition-aided Alamouti coding approach, by which full diversity can be nearly achieved from the fast symbol-wise detection. Numerical results are provided to corroborate the proposed studies.
---
paper_title: Sequence Designs for Interference Mitigation in Multi-Cell Networks

paper_content:

We propose a training sequence that can be used at the handshaking stage for multi-cell networks. The proposed sequence is theoretically proved to enjoy several nice properties including constant amplitude, zero autocorrelation, and orthogonality in multipath channels. Moreover, the analytical results show that the proposed sequence can greatly reduce the multi-cell interference (MCI) induced by carrier frequency offset (CFO) to a negligible level. Therefore, the CFO estimation algorithms designed for single-user or single-cell environments can be slightly modified, and applied in multi-cell environments; an example is given for showing how to modify the estimation algorithms. Consequently, the computational complexity can be dramatically reduced. Simulation results show that the proposed sequences and the CFO estimation algorithms outperform conventional schemes in multi-cell environments.
---
paper_title: Optimized Joint Timing Synchronization and Channel Estimation for OFDM Systems

paper_content:

This paper addresses training-sequence-based joint timing synchronization and channel estimation for orthogonal frequency division multiplexing (OFDM) systems. The proposed approach consists of three stages. First, a coarse timing offset estimate is obtained. Then an advanced timing, relative timing indices, and channel impulse response estimates are obtained by maximum-likelihood estimation based on a sliding observation vector. Finally, the fine time adjustment based on the minimum mean squared error criterion is performed. The simulation results show that the proposed approach has excellent performance of timing synchronization in several channel models at low signal-to-noise ratio (SNR) which is smaller than 1dB. Moreover, for a low-density parity-check coded 1x2 single-input multiple-output OFDM system with maximum ratio combining, zero bit-error-rate is achievable using our proposed approach when SNR exceeds 1dB.
---
paper_title: A Front End for Discriminative Learning in Automatic Modulation Classification

paper_content:

This work presents a novel method for automatic modulation classification based on discriminative learning. The features are the ordered magnitude and phase of the received symbols at the output of the matched filter. The results using the proposed front end and support vector machines are compared to other techniques. Frequency offset is also considered and the results show that in this condition the new method significantly outperforms two cumulant-based classifiers.
---
paper_title: Physical-layer transceiving techniques on data-aided orthogonal frequency-division multiplexing towards seamless service on vehicular communications

paper_content:

Frequency error, non-ideal channel estimation (CE) and inefficient seamless road side unit (RSU) service are critical issues occurring in conventional systems that are completely specified by current IEEE 802.11p standards. This study investigates novel techniques for achieving accurate frequency offset compensation and effective CE and RSU selection for handover in signal-overlapping areas. Recently, communications using data-aided orthogonal frequency-division multiplexing (DA-OFDM), such as pseudo-random-postfix OFDM and time-domain-synchronous OFDM (TDS-OFDM), have been actively studied because of their higher effectiveness, efficiency and better transmission quality. This study consists of three parts: (i) derivation of the maximum likelihood estimation for DA-OFDM, (ii) design of an accurate RSU selection scheme and (iii) implementation of a superior TD CE technique. Performance comparisons between the conventional and the proposed techniques are conducted through comprehensive computer simulations.
---
paper_title: Implementation-friendly synchronisation algorithm for DVB-T2

paper_content:

To support high-definition television, the revision of DVB-T, the terrestrial digital video broadcasting standard, has recently been developed under the abbreviation DVB-T2. The provision of the anticipated high data rate services requires highly accurate synchronisation and frequency acquisition and tracking. Setting out from the procedure defined by the DVB-T2 implementation guidelines provided by the DVB-T2 standardisation community, a modified, yet implementation-friendly synchronisation algorithm has been developed. Simulation results show a similar performance for synchronisation and frequency offset estimation, as well as the detection of the signalling information contained in the preamble P1 with a significant reduction of the implementation complexity.
---
paper_title: Initial Synchronization Assisted by Inherent Diversity over Time-Varying Frequency-Selective Fading Channels

paper_content:

An initial synchronization technique based on novel estimations of the time error and carrier frequency offset (CFO) is investigated in this paper to operate in frequency-selective fading environments. Based on motivation from statistical derivations, a novel estimator is proposed by embedding matched filters (MFs) into the RAKE fingers to approach the modified Cramer-Rao lower bounds (MCRLBs). Meanwhile, a dual chirp signal is proven to have the ability to decorrelate the performances between the time-error and the CFO estimators. By taking advantage of pseudo-noise (PN) MFs, the individual channel tap-weighting coefficients can be extracted from the interpath interference on a path-by-path basis. The proposed technique is then built to approach the MCRLBs by taking advantage of the maximum ratio combining (MRC) criterion. In practice, the proposed technique can significantly outperform a conventional initial synchronization technique that is not assisted by the diversity in terms of higher probabilities of burst acquisition and lower mean-square errors (MSEs) on the time-error and CFO estimations over multipath fading channels. Comprehensive computer simulations were conducted to verify the improvements achieved using the technique that is statistically derived in this paper.
---
paper_title: Maximum-likelihood based lock detectors for M-PSK carrier phase tracking loops

paper_content:

Carrier phase synchronisation is essential for coherent communications. Receivers typically use digital phase-locked loops (DPLLs) to acquire the carrier phase. The lock range of DPLLs, i.e. the range of frequency offsets that they can acquire, is usually significantly less than the initial frequency uncertainty in typical systems. Hence, acquisition is achieved by sweeping through the frequency uncertainty range, and stopping the sweep when the DPLL acquires the signal. Since the transmitted data symbols are in general unknown, successful acquisition is determined by a non-data aided carrier lock detector (CLD). In this reported work, a maximum-likelihood based CLD is derived which has low implementation complexity, and is better than existing CLDs while being impervious to errors in the receive Automatic Gain Control (AGC).
---
paper_title: A Novel Algebraic Carrier Frequency Offset Estimator for ASTC-MIMO-OFDM Systems Over a Correlated Frequency-Selective Channel

paper_content:

This paper presents a new algebraic carrier frequency offset (CFO) estimation technique for multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) system, to overcome the sensitivity of algebraic space-time codes (ASTCs) to frequency synchronization in quasi-static correlated Rayleigh frequency-selective fading channels. The technique uses a preamble and is thus particularly suitable for burst-mode communications. The preamble consists of orthogonal training sequences that are simultaneously transmitted from the various transmit antennas. The proposed system exploits all subcarriers in the frequency domain, which provides a remarkable performance improvement, and reaches the Cramer-Rao lower bound (CRLB) at a high signal-to-noise ratio. The proposed method is compared with three known CFO estimators in the literature: Cyclic-Prefix-based (CP), Moose, and Classen techniques that show clear advantages.
---
paper_title: Simultaneous Multiple Carrier Frequency Offsets Estimation for Coordinated Multi-Point Transmission in OFDM Systems

paper_content:

Orthogonal frequency division multiplexing (OFDM) combined with the coordinated multi-point (CoMP) transmission technique has been proposed to improve performance of the receivers located at the cell border. However, the inevitable carrier frequency offset (CFO) will destroy the orthogonality between subcarriers and induce strong inter-carrier interference (ICI) in OFDM systems. In a CoMP-OFDM system, the impact of CFO is more severe because of the mismatch in carrier frequencies among multiple transmitters. To reduce performance degradation, CFO estimation and compensation is essential. For simultaneous estimation of multiple CFOs, the performance of conventional CFO estimation schemes is significantly degraded by the mutual interference among the signals from different transmitters. In this work, our goal is to propose an effective approach that can simultaneously estimate multiple CFOs in the downlink by using the composite signal coming from multiple base stations corresponding to CoMP transmission. Based on the Zadoff-Chu sequences, we design an optimal set of training sequences, which minimizes the mutual interference and is robust to the variations in multiple CFOs. Then, we propose a maximum likelihood (ML)-based estimator, the robust multi-CFO estimation (RMCE) scheme, for simultaneous estimation of multiple CFOs. In addition, by incorporating iterative interference cancellation into the RMCE scheme, we propose an iterative scheme to further improve the estimation performance. According to the simulations, our scheme can eliminate the mutual interference effectively, approaching the Cramer-Rao bound performance.
---
paper_title: Frequency-immune and low-complexity symbol timing synchronization scheme in OFDM systems

paper_content:

In this paper, we propose a simple time-domain replica correlation (TDRC) based symbol timing synchronization scheme in orthogonal frequency division multiplexing systems. By allocating a locally concatenated sequence of a base sequence and its modified sequence to total feasible frequency resources, we can achieve both hardware complexity and frequency offset reductions. The hardware complexity of a searcher is less than halved over existing TDRC-based schemes, such as long-term evolution (LTE) scheme [3] and a performance-efficient scheme [8]. Also the proposed scheme provides significant frequency offset immunity over the LTE scheme. The performance analysis and comparisons are made in terms of complexity and detection error probability.
---
paper_title: Closed Form BER Expressions for BPSK OFDM Systems with Frequency Offset

paper_content:

This letter addresses the performance degradation caused by the presence of carrier frequency offset (CFO) in orthogonal frequency division multiplexing (OFDM) systems. Accurate closed form bit error rate (BER) expressions for BPSK-OFDM systems impaired by frequency offset are derived. The analysis is carried out for flat and frequency selective Rayleigh fading channels. Simulation results have been used to cross-check the accuracy of the theoretical analysis.
---
paper_title: Blind carrier frequency offset estimator for multi-input multi-output-orthogonal frequency division multiplexing systems over frequency-selective fading channels

paper_content:

This study presents a new blind carrier frequency offset (CFO) estimation technique for multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems employing space-time coding (STC). CFO estimation is crucial for OFDM systems to avoid the performance degradation because of the inter-carrier interference that results when the CFO is not estimated and compensated accurately. Based on the assumptions that the data symbols are selected from a constant modulus constellation and the channel is varying slowly over time, a new blind CFO estimator is proposed by minimising the power difference between all subcarriers in two consecutive STC blocks. Therefore the proposed system exploits all subcarriers in time and frequency domain, which provides a remarkable performance improvement over other techniques reported in the literature. The complexity of the proposed estimator is substantially reduced by approximating the cost function by a sinusoid that can be minimised using direct closed-form computations within one OFDM symbol period. Monte Carlo simulations are used to assess the performance of the proposed system by means of mean squared error (MSE) in both static and time-varying frequency-selective fading channels. The simulation results demonstrate that the proposed estimator can eliminate the MSE error floors that usually appear at moderate and high signal-to-noise ratios for the estimators that work only in frequency domain.
---
paper_title: Multi-User Interference Cancellation Schemes for Carrier Frequency Offset Compensation in Uplink OFDMA

paper_content:

Each user in the uplink of an Division Multiple Access (OFDMA) system may experience a different carrier frequency offset (CFO). These uncorrected CFOs destroy the orthogonality among subcarriers, causing inter-carrier interference and multi-user interference, which degrade the system performance severely. In this paper, novel time-domain multi-user interference cancellation schemes for OFDMA uplink are proposed. They employ an architecture with multiple OFDMA-demodulators to compensate for the impacts of multi-user CFOs at the base station's side. Analytical and numerical evaluations show that the proposed schemes achieve a significant performance gain compared to the conventional receiver and a reference frequency-domain multi-user interference cancellation scheme. In a particular scenario, a maximum CFO of up to 40% of the subcarrier spacing can be tolerated, and the CFO-free performance is maintained in the OFDMA uplink.
---
paper_title: Resource Block Basis MMSE Beamforming for Interference Suppression in LTE Uplink

paper_content:

This paper proposes a new method to suppress interference using antenna array for LTE uplink. The proposed method does not require knowledge on resource allocation of either interfering or communicating mobile stations, and thus it has a significant advantage of ease of implementation. An additional advantage of this method is parallelization and scalability for multi-core processing. This paper also proposes a novel iterative timing offset compensation to enable effective interference suppression. The proposed method has been successfully implemented on System-on-Chip consisting of multi-core DSP and ARM microprocessors and verified that it successfully suppresses interference in real time.
---
paper_title: Frequency Synchronization for Multiuser MIMO-OFDM System Using Bayesian Approach

paper_content:

This paper addresses the problem of frequency synchronization in multiuser multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) systems. Different from existing work, a Bayesian approach is used in the parameter estimation problem. In this paper, the Bayes estimator for carrier frequency offset (CFO) estimation is proposed and the Bayesian Cram'er-Rao bound (BCRB) is also derived in closed form. Direct implementation of the resultant estimation scheme with conventional methods is challenging since a high degree of mathematical sophistication is always required. To solve this problem, the Gibbs sampler is exploited with an efficient sample generation method. Simulation results illustrate the effectiveness of the proposed estimation scheme.
---
paper_title: A Low-Power Low-Cost GFSK Demodulator With a Robust Frequency Offset Tolerance

paper_content:

A low-power low-cost Gaussian frequency shift keying demodulator with a robust frequency offset tolerance is presented. A novel automatic pulse duration calibration is employed to keep the pulse durations of the zero-crossing detection output constant against process variation. Additionally, a discrete-time differentiator is adopted to eliminate the negative effect of frequency offset and drift. The demodulator is implemented in a 0.18-μm CMOS process and the area is approximately 0.08 mm ::: 2 ::: . It can recover data even as the intermediate frequency drifts from 1.5 up to 3.7 MHz during demodulation and requires a signal-to-noise ratio of less than 16.5 dB for a 0.1% bit error rate. The circuit consumes only a 0.51-mA current from a 1.8-V supply.
---
paper_title: A New Synchronization Scheme for OFDM-Based Cooperative Relay Systems

paper_content:

In cooperative relay systems, relayed signals propagate through different channels and are received at the destination node with distinct timing and carrier frequency offsets (CFOs). This feature makes synchronization a rather challenging task as compared with centralized multiple-input multiple-output systems. Unlike conventional preamble designs with good correlation properties only in time domain or frequency domain, a new preamble that has good correlation properties in both time domain and frequency domain is proposed for synchronization in orthogonal frequency-division multiplexing (OFDM) based cooperative relay systems. According to the proposed preamble, a synchronization scheme is developed and a practical threshold is derived that jointly considers the statistic distribution of the correlation function output and the inevitable interference originated from residual CFOs to assist fine timing synchronization. Simulation results verify that the proposed scheme provides notable performance improvement as compared with a previous related work.
---
paper_title: An Unscented Kalman Filter for ICI Cancellation in High-Mobility OFDM System

paper_content:

OFDM system suffers from inter-carrier interference due to frequency offset produced by the movement of terminals. Several schemes have been proposed to mitigate this type of interference. In this paper, an unscented Kalman filter (UKF) based methodology is addressed to estimate the carrier frequency offset (CFO).We have compared the BER performance of UKF with other schemes and also analyzed the convergence as well as accuracy behavior between UKF and EKF. The simulation result shows that comparing to conventional non-iterative methods, UKF and EKF have higher accuracy and efficiency. Furthermore, UKF surpasses EKF in convergence rate and consistency.
---
paper_title: A Compact Preamble Design for Synchronization in Distributed MIMO OFDM Systems

paper_content:

In distributed multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems, signals arrive at the receiver on different timing and are characterized by distinct carrier frequency offsets (CFOs), which makes synchronization rather challenging than that associated with centralized MIMO systems. Current solutions to this problem are mainly based on special preamble designs, where different training sequences are cascaded and then separately used to assist timing synchronization and CFO estimation. Such preamble designs not only increase system overhead but also burden the receivers with independent algorithms for timing synchronization and CFO estimation. In this paper, we propose a low-overhead (compact) preamble having the same length as one OFDM symbol, along with a unified algorithm for both timing synchronization and CFO estimation. Furthermore, the CFO estimation range can be flexibly extended to cope with larger CFOs in the proposed approach. Under the same training overhead and power consumption, simulation results indicate that the proposed approach outperforms a timing synchronization scheme that based on unequal period synchronization patterns.
---
paper_title: Improved schemes for tracking residual frequency offset in DVB-T systems

paper_content:

Carrier frequency offset (CFO) synchronization is a crucial issue in the implementation of orthogonal frequency division multiplexing (OFDM) systems. Synchronization is performed in two stages: acquisition and tracking. After a first estimation and correction of the CFO performed in the acquisition stage, there still remains a residual frequency offset (RFO) due to real system conditions. This paper presents two new schemes for RFO tracking affecting a digital video broadcasting (DVB-T) system. Due to the nature of DVB-T, where data transmission is performed continuously, RFO tracking needs to be performed accurately during all the data length in order to correct the ICI and the phase rotation impairment produced by this RFO. Comparisons of the novel algorithms with two previously reported estimators in terms of mean square error (MSE), complexity and speed of convergence are performed. The proposed schemes improve performance and/or speed of convergence compared to the conventional estimators with a reduction of the computational complexity. Furthermore, a combined technique for RFO tracking involving the two new schemes is proposed. This method obtains the best results in terms of accuracy, convergence and complexity.
---
paper_title: Semi-blind MIMO OFDM systems with precoding aided CFO estimation and ICA based equalization

paper_content:

We propose a semi-blind multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system, with a precoding aided carrier frequency offset (CFO) estimation approach, and an independent component analysis (ICA) based equalization structure. A number of reference data sequences are carefully designed offline and are superimposed to source data via a non-redundant linear precoding process, which can kill two birds with one stone, without introducing any extra total transmit power and spectral overhead. First, the reference data sequences are selected from a pool of carefully designed orthogonal sequences. The CFO estimation is to minimize the sum cross-correlation between the CFO compensated signals and the rest orthogonal sequences in the pool. Second, the same reference data enable elimination of the permutation and quadrant ambiguity in the ICA equalized signals by maximizing the cross-correlation between the ICA equalized signals and the reference data. Simulation results show that, without extra bandwidth and power needed, the proposed semi-blind system achieves a bit error rate (BER) performance close to the ideal case with perfect channel state information (CSI) and no CFO. Also, the precoding aided CFO estimation outperforms the constant amplitude zero autocorrelation (CAZAC) sequences based CFO estimation approach, with no spectral overhead.
---
paper_title: A Novel Timing Synchronization Method for OFDM Systems

paper_content:

In this letter, a novel timing offset estimation method is presented for orthogonal frequency division multiplexing (OFDM) systems. The proposed method is developed on the basis of the cyclic structure of OFDM symbol and uses a new noise subspace based metric to estimate the timing offset. Simulation results show that the proposed method has a significantly higher probability of correct estimation of the timing offset than the other methods in multipath channels.
---
paper_title: Blind Estimation of OFDM Parameters in Cognitive Radio Networks

paper_content:

This paper presents a blind parameter estimation algorithm for orthogonal frequency division multiplexing (OFDM) signal affected by a time-dispersive channel, timing offset, carrier frequency offset and additive Gaussian noise. Unlike the previous studies, this paper presents the second-order cyclostationarity of OFDM signal considering the effect of time-dispersive channel. The cyclostationarity properties of received OFDM signal in time-dispersive channel is exploited to estimate the OFDM parameters. These parameters includes OFDM symbol period, useful symbol period, cyclic prefix factor, number of subcarriers and carrier frequency offset. Simulations are performed to investigate the performance of OFDM parameter estimation algorithm in diverse channel conditions.
---
paper_title: Highly Accurate Blind Carrier Frequency Offset Estimator for Mobile OFDM Systems

paper_content:

For orthogonal frequency division multiplexing (OFDM) communication systems, the orthogonality among subcarriers is lost in mobile applications due to frequency offset resulting from either transmitter-receiver local oscillator differences or Doppler shift caused by mobility. As a direct result, inter-carrier interference (ICI) is observed on each and every subcarrier, leading to significant performance degradation. There are a lot of OFDM carrier frequency offset (CFO) estimation schemes classified as data aided estimation and blind estimation. Due to the system power and high bandwidth efficiencies, blind estimators have received a lot of attention recently. Many blind CFO schemes were proposed for OFDM systems, some of which are based on power spectrum smoothing, kurtosis-type cost functions and minimum output variance. In this paper, we propose a novel blind CFO estimator based on Minimum Reconstruction Error (MRE). In contrast to other blind CFO estimators, the proposed technique can be used for any constellation schemes, does not require a large number of blocks to reach acceptable estimation error and provides reliable estimation performance with very low mean square error (MSE). Simulation results in AWGN and multi-path fading channels confirm that performance of the proposed highly accurate blind CFO estimator is superior when frequency offset or time variation occurs in the channel - the proposed technique outperforms most existing blind CFO estimation methods.
---
paper_title: Frequency synchronization and phase offset tracking in a real-time 60-GHz CS-OFDM MIMO system

paper_content:

The performance of an Orthogonal Frequency-Division-Multiplexing (OFDM)-based 60-GHz system can be strongly degraded due to carrier frequency impairment and Phase Noise (PN). In this paper we present a practical approach to the design of a frequency synchronization and phase offset tracking scheme for a 60-GHz, Non-Line-of-Sight (NLOS) capable wireless communication system. We first analyse the architecture of the 60-GHz system and propose a simple algorithm for Carrier Frequency Offset (CFO) estimation on the basis of numerical investigations. Then, we explore pilot based and blind tracking methods for mitigation of Residual Frequency Offset (RFO) and Common Phase Error (CPE). Provided are also analysis and implementation results on an Altera Startix III FPGA.
---
paper_title: Pilot-Aided Carrier Frequency Estimation for Filter-Bank Multicarrier Wireless Communications on Doubly-Selective Channels

paper_content:

Multicarrier modulation techniques are currently the key technology in the area of high-data-rate transmission over wireless fading channels. Their considerable vulnerability to carrier frequency offsets, however, hinders their appealing features and demands adequate countermeasures. This paper contributes with a class of frequency estimation algorithms intended for filter bank burst-mode multicarrier transmission over time-frequency selective fading channels. All algorithms are derived from the maximum likelihood principle, exhibit a feedforward structure and are based on the use of pilot symbols scattered throughout the burst. The accuracy of the proposed schemes is investigated in typical mobile wireless scenarios, showing that they outperform maximum likelihood non-data-aided frequency recovery in spite of a substantially lower computational requirement.
---
paper_title: Linear Least Squares CFO Estimation and Kalman Filtering Based I/Q Imbalance Compensation in MIMO SC-FDE Systems

paper_content:

This paper investigates carrier frequency offset (CFO) estimation and inphase/quadrature (I/Q) imbalance compensation in time-varying frequency-selective channels. We first propose a linear least squares (LLS) CFO estimation approach which has a lower complexity and a higher accuracy than the previous nonlinear CFO estimation methods. We then propose a Kalman filtering based I/Q imbalance compensation approach in the presence of CFO, which demonstrates a good ability to track the channel time variations with a fast convergence speed, by nulling the cyclic prefix (CP) and including the CFO in the state vector of the equivalent channel model. The proposed Kalman filtering based I/Q imbalance compensation approach with associated equalization tracks the time variation with a fast convergence speed. Simulation results show that the proposed compensation approach for CFO and I/Q imbalance provides a bit error rate (BER) performance close to the ideal case with perfect channel state information (CSI), no CFO and no I/Q imbalance.
---
paper_title: Reduced-complexity baseband compensation of joint Tx/Rx I/Q imbalance in mobile MIMO-OFDM

paper_content:

Direct-conversion multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) transceivers enjoy high data rates and reliability at practical implementation complexity. However, analog front-end impairments such as I/Q imbalance and high mobility requirements of next-generation broadband wireless standards result in performance-limiting inter-carrier interference (ICI). In this paper, we study the effects of ICI due to these impairments for OFDM with space frequency block codes and spatial multiplexing, derive a generalized linear model and propose a non-iterative reduced-complexity digital baseband joint compensation scheme. Furthermore, we present a pilot scheme for joint estimation of the channel and the I/Q imbalance parameters and evaluate its performance through simulations. Our proposed scheme is effective in estimating and compensating for frequency-independent and frequency-dependent transmit and receive I/Q imbalances even in the presence of a residual frequency offset.
---
paper_title: Training-Based Synchronization and Demodulation With Low Complexity for UWB Signals

paper_content:

In this paper, we propose a low-complexity data-aided (DA) synchronization and efficient demodulation technique for an ultrawideband (UWB) impulse radio system. Depending on the autocorrelation property of a judiciously chosen training sequence, a redundance-included demodulation template (RDT) can be extracted from the received signal by separating, shifting, and realigning two connected portions in the observation window. After constructing the RDT, two receiver designs are available. One approach is to demodulate transmitted symbols by correlating the RDT in a straightforward manner, which does not require the explicit timing acquisition and, thus, considerably reduces the complexity of the receiver. An alternative approach is accomplished with the assistance of a non-RDT (NRDT). The NRDT-based receiver is able to remove the redundant noisy component of the RDT by acquiring timing offset via a simple synchronization scheme, therefore achieving a better bit error rate (BER) performance. Both the schemes can counteract the effects of interframe interference (IFI) and unknown multipath channels. Furthermore, analytical performance evaluations of the RDT- and NRDT-based receivers are provided. Simulations verify the realistic performance of the proposed receivers in the presence of multiuser interference (MUI) and timing errors.
---
paper_title: BER analysis of direct conversion OFDM systems with MRC under channel estimation errors

paper_content:

In this letter, we calculate an exact closed-form expression for the bit error rate (BER) of orthogonal frequency division multiplexing (OFDM) systems with direct conversion that employ maximal ratio combining (MRC) reception in multipath Rayleigh fading channels, using binary phase-shitf keying (BPSK) modulation. We assume a realistic system model where direct current (DC) offset, carrier frequency offset (CFO) and imperfect channel state information (ICSI) are simultaneously considered. Results show the appearance of an irreducible BER floor due to DC offset and ICSI. As a rule of thumb, we provide a simple expression for the maximum DC offset allowable in a direct conversion receiver.
---
paper_title: Cluster-Based Differential Energy Detection for Spectrum Sensing in Multi-Carrier Systems

paper_content:

This paper presents a novel differential energy detection scheme for multi-carrier systems, which can form fast and reliable decision of spectrum availability even in very low signal-to-noise ratio (SNR) environment. For example, the proposed scheme can reach 90% in probability of detection (PD) and 10% in probability of false alarm (PFA) for the SNRs as low as -21 dB, while the observation length is equivalent to 2 multi-carrier symbol duration. The underlying initiative of the proposed scheme is applying order statistics on the clustered differential energy-spectral-density (ESD) in order to exploit the channel frequency diversity inherent in high data-rate communications. Specifically, to enjoy a good frequency diversity, the clustering operation is utilized to group uncorrelated subcarriers, while, the differential operation applied onto each cluster can effectively remove the noise floor and consequently overcome the impact of noise uncertainty while exploiting the frequency diversity. More importantly, the proposed scheme is designed to allow robustness in terms of both, time and frequency offsets. In order to analytically evaluate the proposed scheme, PFA and PD for Rayleigh fading channel are derived. The closed-form expressions show a clear relationship between the sensing performance and the cluster size, which is an indicator of the diversity gain. Moreover, we are able to observe up to 10 dB gain in the performance compared to the state-of-the-art spectrum sensing schemes.
---
paper_title: Iterative Receiver Design With Joint Doubly Selective Channel and CFO Estimation for Coded MIMO-OFDM Transmissions

paper_content:

This paper is concerned with the problem of turbo (iterative) processing for joint channel and carrier frequency offset (CFO) estimation and soft decoding in coded multiple-input-multiple-output (MIMO) orthogonal frequency-division-multiplexing (OFDM) systems over time- and frequency-selective (doubly selective) channels. In doubly selective channel modeling, a basis expansion model (BEM) is deployed as a fitting parametric model to reduce the number of channel parameters to be estimated. Under pilot-aided Bayesian estimation, CFO and BEM coefficients are treated as random variables to be estimated by the maximum a posteriori technique. To attain better estimation performance without sacrificing spectral efficiency, soft bit information from a soft-input-soft-output (SISO) decoder is exploited in computing soft estimates of data symbols to function as pilots. These additional pilot signals, together with the original signals, can help to enhance the accuracy of channel and CFO estimates for the next iteration of SISO decoding. The resulting turbo estimation and decoding performance is enhanced in a progressive manner by benefiting from the iterative extrinsic information exchange in the receiver. Both extrinsic information transfer chart analysis and numerical results show that the iterative receiver performance is able to converge fast and close to the ideal performance using perfect CFO and channel estimates.
---
paper_title: Carrier frequency offset tracking for constant modulus signalling-based orthogonal frequency division multiplexing systems

paper_content:

This study presents a blind carrier frequency offset (CFO) tracking algorithm for orthogonal frequency division multiplexing systems with constant modulus signalling. Both single-input single-output and multiple-input multiple-output (MIMO) systems are considered. Based on the assumption that the channel frequency response (CFR) has been estimated by training symbols and keeps constant over one frame duration, we discover that the CFO can be estimated via minimising the power difference between the received signals and the CFR. The polynomial rooting method is exploited to derive a low complexity solution. The expectation and mean-square error of the proposed method are derived mathematically. Besides, the effect of channel estimation error on the performance of CFO tracking is addressed and it is shown that the proposed algorithm is robust to this error. At last, this blind scheme can be applied to a MIMO system with the aid of space time block codes.
---
paper_title: Frame detection and timing acquisition for OFDM transmissions with unknown interference

paper_content:

Frame detection and timing acquisition are challenging tasks in orthogonal frequency-division multiplexing systems plagued by narrowband interference (NBI). Most existing solutions operate in the time domain by exploiting the repetitive structure of a training symbol and suffer from considerable performance loss in the presence of NBI. In this work, a novel solution in which frame detection is accomplished in the frequency domain on the basis of a suitable likelihood ratio test is presented. In order to increase the resilience to NBI, the interference power is treated as a nuisance parameter that is averaged out from the corresponding likelihood functions. The resulting test statistic depends on the fractional carrier frequency offset (CFO), which is easily estimated. An alternative scheme that dispenses from CFO estimation is also proposed. After frame detection, the test statistic is employed as a timing metric to accurately locate the position of the training symbol within the received data stream. Computer simulations indicate that the proposed solutions are remarkably robust to NBI and outperform existing alternatives in a severe interference scenario. The price for this advantage is a substantial increase in the computational burden.
---
paper_title: Autocorrelation Based Coarse Timing with Differential Normalization

paper_content:

Two novel differential normalization factors, depending on the severity of carrier frequency offset, are proposed for autocorrelation based coarse timing scheme. Compared with the conventional normalization factor based on signal energy, they improve the robustness of the timing metric to signal-to-noise ratio (SNR), improve the mainlobe sharpness of the timing metric and reduce both missed detection and false alarm probabilities.
---
paper_title: A Cooperative Scheme for ZP-OFDM with Multiple Carrier Frequency Offsets over Multipath Channel

paper_content:

In the cooperative communication system, due to the distributed nature of the relay system, multiple carrier frequency offsets (CFOs) at different relays and multipath channels between relays cause the whole cooperative communication channel to be a doubly time-frequency selective channel. Zero-Padding Orthogonal Frequency Division Multiplexing (ZP-OFDM), which has been adopted for some wideband communication standards and applications, possesses a unique linear structure. In this paper, we use a cooperative tall Toeplitz scheme to achieve the full cooperative and multipath diversity, and combat the CFOs simultaneously. Importantly, this full diversity scheme only requires linear equalizers (LEs), such as zero-forcing (ZF) and minimum mean square error (MMSE) receivers, an issue which reduces the system complexity. Theoretical analysis of the proposed cooperative tall Toeplitz scheme is provided based on the analytical upper bound of the channel orthogonality deficiency derived in this paper. The above mentioned theoretical issues are verified by the simulation results as well.
---
paper_title: Cyclostationarity-Based Robust Algorithms for QAM Signal Identification

paper_content:

This letter proposes two novel algorithms for the identification of quadrature amplitude modulation (QAM) signals. The cyclostationarity-based features used by these algorithms are robust with respect to timing, phase, and frequency offsets, and phase noise. Based on theoretical analysis and simulations, the identification performance of the proposed algorithms compares favorably with that of alternative approaches.
---
paper_title: An Efficient Time Synchronization Scheme for Broadband Two-Way Relaying Networks Based on Physical-Layer Network Coding

paper_content:

We present an efficient time synchronization scheme for broadband two-way relaying networks based on two-phase physical layer network coding. Especially, a preamble structure is proposed in this letter for the synchronization. The synchronization approach exploits the preamble in frequency domain and time domain to effectively separate the mixed signals, and jointly estimate timing-offsets and channel parameters, respectively. Numerical results confirm that the suggested method is superior to the conventional scheme, and is very suitable for the synchronization in broadband two-way relaying networks based on two-phase physical layer network coding.
---
paper_title: Jamming Rejection Using FFH/MFSK ML Receiver Over Fading Channels With the Presence of Timing and Frequency Offsets

paper_content:

The composite effect of hostile multitone jamming and partial-band noise jamming on bit-error rate (BER) performance of a fast frequency-hopped M-ary frequency-shift-keying system is studied over Rayleigh fading channels in the presence of timing and frequency offsets. The maximum-likelihood (ML) diversity-combining method is employed to improve BER performance of the system. Analytical BER expression of the proposed ML receiver is derived. The analytical results, validated by computer simulation, show that the proposed ML receiver can suppress the composite hostile jamming more effectively than some existing conventional diversity-combining receivers. The ML receiver is also found to be robust against inaccurate estimation of the required side information.
---
paper_title: Timing-delay and frequency-offset estimations for initial synchronisation on time-varying Rayleigh fading channels

paper_content:

A technique for timing-delay and frequency-offset estimations that takes advantage of a single dual-chirp preamble burst operating in frequency-non-selective fading environments is proposed. As the shape of the autocorrelation function of the dual-chirp signal does not essentially change by different values of frequency offset, the proposed technique provides a maximum-likelihood (ML) timing-delay estimation by exploiting only one pair of pseudo-noise matched filters (PN MFs), instead of conventionally exploiting a continuum of MFs, therefore approaching the Miller-Chang bound (MCB) in the timing-delay estimation with a channel gain and an initial phase error as nuisance parameters. Using the transform invariance property of the dual-chirp signal, the proposed technique estimates a frequency offset by exploiting frequency-domain PN MFs, therefore approaching the MCB in the frequency-offset estimation. The proposed technique accomplishes ML timing-delay and frequency-offset estimations to attain the performance bounds. The advantages of the proposed technique are verified by rigorous statistical analysis in conjunction with comprehensive computer simulations.
---
paper_title: One-Shot Blind CFO and Channel Estimation for OFDM With Multi-Antenna Receiver

paper_content:

In this paper, we design a new blind joint carrier frequency offset (CFO) and channel estimation method for orthogonal frequency-division multiplexing (OFDM) with multiantenna receiver. The proposed algorithm requires only one received OFDM block and thus belongs to the category of one-shot estimation methods. Other advantages of the proposed algorithm include 1) it supports fully loaded data carriers and is thus spectral efficient; 2) the channel from the transmitter to each receive antenna can be blindly estimated with only a scaling ambiguity; and 3) the algorithms outperforms the existing methods. Moreover, we derive the Cramer-Rao Bounds (CRB) of joint CFO and channel estimation in closed forms. Numeral results not only show the effectiveness of the proposed algorithm but also demonstrate its closed performance to CRB.
---
paper_title: Robust Timing and Frequency Synchronization for OFDM Systems

paper_content:

This paper deals with timing and frequency synchronization in orthogonal frequency-division multiplexing (OFDM) systems. A robust multistage scheme that works in the time domain, independent of the preamble structure, is proposed. After coarse-timing estimation, joint timing and integer frequency synchronization is performed. Then, fractional frequency correction is carried out, and finally, fine-timing estimation completes the synchronization process. The new timing estimation method is flexible and can be adjusted according to the degree of channel distortion. Furthermore, frequency synchronization is efficiently accomplished with an estimation range that is as large as the bandwidth of the OFDM signal. The performance of the proposed method is evaluated in terms of the mean square error. The results indicate that the new method significantly improves performance compared with the previous methods.
---
paper_title: Blind Carrier Frequency Offset Estimation for OFDM Systems with Constant Modulus Constellations

paper_content:

This paper proposes a blind carrier frequency offset (CFO) estimation scheme for OFDM systems. In the proposed scheme, the covariance matrix of the received signal is obtained through the circular shifts of OFDM blocks in the time-domain. From the fact that the covariance matrix has a banded structure in the absence of CFO, we estimate CFO through minimizing the powers of the elements that are outside the band. We show that the proposed scheme is better than the conventional schemes through simulations.
---
paper_title: Fast Kalman Equalization for Time-Frequency Asynchronous Cooperative Relay Networks With Distributed Space-Time Codes

paper_content:

Cooperative relay networks are inherently time and frequency asynchronous due to their distributed nature. In this correspondence, we propose a transceiver scheme to combat both time and frequency offsets for cooperative relay networks with multiple relay nodes. At the relay nodes, a distributed linear convolutive space-time coding is adopted, which has the following advantages: 1) Full cooperative diversity can be achieved using a minimum mean square error (MMSE) or MMSE decision feedback equalizer (MMSE-DFE) detector, instead of a maximum-likelihood receiver when only time asynchronism exists. 2) The resultant equivalent channel possesses some special structure, which can be exploited to reduce the equalization complexity at the destination node. By taking full advantage of such a special structure, fast Kalman equalizations based on linear MMSE and MMSE-DFE are proposed for the receiver, where the estimation of the state vector (information symbols) can be recursively taken and become very computationally efficient, compared with direct equalization. The proposed scheme can achieve considerable diversity gain with both time and frequency offsets and applies to frequency-selective fading channels.
---
paper_title: Space–Frequency Convolutional Coding for Frequency-Asynchronous AF Relay Networks

paper_content:

In this paper, we design a space-frequency (SF) convolutional coding scheme for amplify-and-forward (AF) relay networks that contain multiple distributed relay nodes. The frequency-asynchronous nature of the distributed system is considered in our design. Orthogonal frequency-division multiplexing (OFDM) modulation is adopted, which is robust to certain timing errors. We exploit the signal space diversity technique and employ an extended cyclic prefix (CP) at the source node. The relay nodes need to perform only simple operations, e.g., convolution and amplification, and they need no information about the channels and the frequency offsets. Attributed to the extended CP, the multiple frequency offsets can directly be compensated at the destination. We further prove that both spatial and multipath diversity can be achieved by the proposed scheme. Numerical results are provided to corroborate the proposed studies.
---
paper_title: Improved CIR-Based Receiver Design for DVB-T2 System in Large Delay Spread Channels: Synchronization and Equalization

paper_content:

This paper proposes to implement an improved orthogonal frequency division multiplexing (OFDM) receiver by utilizing a channel impulse response (CIR)-based synchronization and sparse equalization for DVB-T2 system operating in both the single-input single-output (SISO) and multi-input single-output (MISO) transmission modes. First, the proposed OFDM receiver performs a pilot-aided CIR estimation after a coarse symbol timing recovery (STR). Then, the proposed CIR-based fine STR compensates for a false symbol timing offset (STO). In particular, the fine STR resolves an ambiguity effect of CIR, which is the main problem caused by a false coarse STO in exploiting the CIR. Upon the completion of the fine synchronization, the proposed CIR-based sparse equalization is performed in order to minimize the noise and interference effects by shifting or selecting a basic frequency interpolation (FI) filter according to an echo delay (phase) or maximum delay spread, respectively. Performance evaluations are accomplished in large delay spread channels in which the maximum delay spread is less or longer than a guard interval (GI). It is shown that the proposed receiver is not only capable of estimating the fine STO but also minimizing effectively the noise effects. In particular, the performance gain in a single pre-echo channel being longer than GI is remarkable as compared with a conventional receiver.
---
paper_title: MCRB for Timing and Phase Offset for Low-Rate Optical Communication with Self-Phase Modulation

paper_content:

We derive the modified Cramer-Rao bound (MCRB) for symbol timing and phase offset estimation in the presence of nonlinear self-phase modulation (SPM) in a dispersion compensated long-haul optical fiber link with coherent detection at data rates below 10 Gigabaud. In the presence of a low-pass filter at the receiver front-end, we find that SPM degrades the MCRB. Moreover, depending on the pulse shape, SPM induces underdamped oscillation on the bounds.
---
paper_title: Design and Analysis of Data-Aided Coarse Carrier Frequency Recovery in DVB-S2

paper_content:

An improved data-aided (DA) frequency error detector (FED) and a frequency lock detector are proposed under large frequency offset for Digital Video Broadcasting Satellite Second Generation (DVB-S2) system. Computer simulations results show that the proposed error detector can increase the frequency acquisition range and decrease the acquisition time without complexity increase. Its closed-loop normalized frequency root mean square error (RMSE) improves at least 1.5 dB compared with that of conventional error detector. The proposed lock detector shows good lock indication. Its modified version can save more symbols to indicate the locking status.
---
paper_title: Correction of the CFO in OFDM Relay-Based Space-Time Codes

paper_content:

In this paper, we analyze the impact of carrier frequency offset (CFO) on the performance of orthogonal frequency division multiplexing (OFDM) transmission employing space-frequency coding over relay channels. The challenge in such systems lies in the difficulty of canceling the interference resulting from the different CFOs that correspond to the relays involved in the transmission. We first analyze the CFO correction schemes and examine their impact on the achievable information rates. Further, we analyze the interference cancellation (IC) technique based on the so-called turbo-principle, that is, which jointly detects and decodes the received data. The increase of the rates achievable thanks to IC is assessed via parametric description of the iterative process. We provide examples that demonstrate the efficacy of the proposed scheme and numerical results are contrasted with theoretical performance limits.
---
paper_title: ICI Mitigation for OFDM Using PEKF

paper_content:

Orthogonal frequency division multiplexing (OFDM) is widely known as the promising broadband wireless communication technology due to the high spectral efficiency and robustness to the multi-path interference. However, the inter-carrier interference (ICI) caused by Doppler frequency offset is an important issue for OFDM in high mobility. In this paper, the method to mitigate ICI for OFDM system is investigated, and a novel ICI mitigation method using planar extended Kalman filter (PEKF) is proposed to reduce the ICI effects. Simulation results demonstrate that the proposed ICI mitigation method in this letter outperforms traditional ICI mitigation methods.
---
paper_title: Multiple CFO Mitigation in Amplify-and-Forward Cooperative OFDM Transmission

paper_content:

In cooperative orthogonal frequency division multiplexing (OFDM) systems, accurate frequency synchronization is critical to achieving any potential gains brought by the cooperative operation. The carrier frequency offsets (CFOs) present among multiple nodes (source, relays and destination) are more difficult to tackle than the single CFO problem in point-to-point systems. Multiple CFOs cause phase drift, inter-carrier interference (ICI) and inter-block interference (IBI) in the received signal. This paper deals with the CFO induced interference mitigation problem in distributed space time block coded (STBC) amplify-and-forward (AF) cooperative OFDM systems. We propose a two step approach to recover the phase distortion and suppress the ICI and IBI using low complexity methods to achieve high performance. The first step is time domain (TD) compensation and the second step is frequency domain (FD) decoding. Two TD compensation schemes are proposed, i.e., IBI-removal and ICI-removal. The IBI-removal scheme decouples the two blocks of one STBC codeword completely and then decodes the ICI degraded blocks individually. The ICI-removal scheme removes ICI first and the subsequent decoding requires joint decoding of the two blocks. Simulation results show that the IBI-removal scheme which is of lower complexity performs well with small CFO. For large CFO, the ICI-removal with modified iterative joint maximum likelihood decoding (MIJMLD) outperforms other schemes.
---
paper_title: New joint algorithm of blind doppler parameters estimation for high-order QAM signals

paper_content:

Aiming at the difficulties of blind doppler parameters estimation for QAM signals in the satellite communication on-the-move (SCOTM) system, a new joint algorithm of blind doppler frequency and doppler-rate estimation based on cyclic-statistical tests and phase difference is proposed. By detecting one of the cyclic frequencies of QAM signals, located at the quartic frequency offset, doppler frequency is effectively estimated, meanwhile, the data division method is exploited to calculate the doppler-rate. Finally, phase lock loop (PLL) is proposed to make the estimation more accurately. All-sided Monte Carlo simulations are employed to confirm the theoretical analysis, simulation results indicate that this algorithm can track time-varying frequency of the QAM signals accurately.
---
paper_title: Frequency Offset Invariant Multiple Symbol Differential Detection of MPSK

paper_content:

Multiple-symbol differential detection (MSDD) of differentially encoded phase-shift keying (PSK) signals in the presence of random frequency variation and additive white Gaussian noise is studied. It is shown that frequency variation distorts the transmitted signal through attenuating its amplitude and introducing a time-varying phase shift to the information symbols. A double differential PSK (DDPSK) modulation scheme is then introduced and a MSDD for detecting DDPSK signals in the presence of frequency offset is proposed. It is shown that the proposed receiver is robust to the distortions caused by the random frequency variation. A lower bound on the error probability of the proposed MSDD receiver is also derived and compared to that of an autocorrelation demodulator for the case where the observation interval approaches infinity.
---
paper_title: SNR Estimation in a Non-Coherent BFSK Receiver With a Carrier Frequency Offset

paper_content:

This correspondence deals with the problem of estimating average signal-to-noise ratio (SNR) for a communication link employing binary frequency shift keying (BFSK) in the presence of a carrier frequency offset (CFO). The transmitted symbols are corrupted by Rayleigh fading and additive white Gaussian noise (AWGN). We treat the CFO as a nuisance parameter and estimate it using a data statistics based estimator. This estimate is then used to design a maximum likelihood (ML) estimator to get the estimates of SNR. We also derive the Cramer-Rao bound (CRB) for the estimators and have shown the performance of both the data-aided and non-data-aided estimators.
---
paper_title: Semiblind Iterative Receiver for Coded MIMO-OFDM Systems

paper_content:

In this paper, a semiblind iterative receiver is proposed for coded multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) systems. A novel iterative extended soft-recursive least square (IES-RLS) estimator for joint channel and frequency offset estimation is introduced. Extrinsic bit information obtained from the channel decoder and expected symbol decisions obtained from the demodulator are simultaneously exploited by the IES-RLS. The proposed receiver combines the MIMO data demodulator, the proposed channel estimator, and the channel decoder in an iterative manner to yield an improved bit error rate (BER) performance. To arrive at a feasible algorithm, the first-order linearization of the received vector signal with respect to the frequency offset is used in the IES-RLS channel estimator. The BER performance, a constellation-constrained mutual information analysis, and an EXIT chart analysis are used to verify the effectiveness of the proposed receiver. Simulation results show the superiority of the proposed semiblind receiver, compared with conventional semiblind receivers.
---
paper_title: Detection of cooperative OFDM signals in time-varying channels with partial whitening of intercarrier interference

paper_content:

Cooperative communication can yield spatial diversity gains, but its performance can also suffer severely from the multiple carrier frequency offsets (MCFOs) induced by high-speed relative motion, in addition to Doppler spread. In orthogonal frequency-division multiplexing (OFDM) systems, such performance degradation arises as a result of intercarrier interference (ICI). We have shown previously that, in conventional non-cooperative communication, certain ICI components (called residual ICI) display high normalized autocorrelation and a partial whitening of the residual ICI can significantly benefit OFDM signal detection. In this work, we show that similar whitening can also be employed in cooperative communication to address the MCFO issue mentioned above. As an illustration, we simulate a distributed spatial-frequency block coding (SFBC) OFDM system subject to highly disparate MCFOs, where in the receiver we perform partial whitening of the residual ICI followed by maximum-likelihood sequence estimation (MLSE). The results show that the proposed method can comfortably accommodate a maximum MCFO span well exceeding one subcarrier spacing.
---
paper_title: Fractional timing offset and channel estimation for MIMO OFDM systems over flat fading channels

paper_content:

This paper addresses the problem of fractional timing offset and channel estimation in Multiple input Multiple output orthogonal frequency division multiplexing (MIMO OFDM) systems. The estimators have been derived assuming a flat fading channel and using the maximum likelihood criterion. Closed form Cramer Rao bound (CRB) expressions for fractional timing offset and channel response are also derived. Simulation results have been used to cross-check the accuracy of the proposed estimation algorithm.
---
paper_title: Maximal power path detection for OFDM timing-advanced synchronization schemes

paper_content:

Fine timing estimation in timing synchronization scheme of orthogonal frequency division multiplexing systems gives an estimate of symbol starting time index corresponding to the path with maximal power within an interval suggested by coarse timing stage. The actual starting index fed to the following stages is brought forward by an amount that should be adaptive to the estimated index to optimize system performance. In this paper, a method of detecting the estimated starting time index of path with maximal power in channel impulse response is proposed based on conventional preamble with repetitive structure. To deal with the adverse effect of fractional timing offset on the detection metric, we propose a preamble composed of cyclic-shifted parts and the accompanying fine timing and detection scheme. Simulation with time-varying wireless channel shows the detection methods makes use of the time diversity provided by time-varying paths and has good error performance. The scheme with proposed preamble further reduces probability of error detection with the diversity in fractional timing offset provided inherently in the parts of preamble.
---
paper_title: A Message Passing Approach to Joint Channel Estimation and Decoding with Carrier Frequency Offset in Time Selective Rayleigh Fading Channel

paper_content:

This paper presents a message passing approach to joint channel estimation, data detection and decoding over time-selective Rayleigh fading channel with residual carrier frequency offset (CFO). The proposed algorithm utilizes the sum product algorithm (SPA) implemented on a factor graph (FG) representing the joint a posteriori probability distribution of the unknown CFO, information bits and channel coefficients vector given the channel output. A combination of particle filtering and Gaussian parameterization is employed to approximate the exact probability density function in message passing for CFO and channel estimation. Computer simulations demonstrate the effectiveness of the proposed algorithm in combating the CFO over unknown Rayleigh fading channels.
---
paper_title: ML Estimation of Timing and Frequency Offsets Using Distinctive Correlation Characteristics of OFDM Signals Over Dispersive Fading Channels

paper_content:

Orthogonal frequency-division multiplexing (OFDM) is a promising technology for communication systems. However, the synchronization of OFDM over dispersive fading channels remains an important and challenging issue. In this paper, a synchronization algorithm for determining the symbol timing offset and the carrier frequency offset (CFO) in OFDM systems, based on the maximum-likelihood (ML) criterion, is described. The new ML approach considers time-dispersive fading channels and employs distinctive correlation characteristics of the cyclic prefix at each sampling time. The proposed symbol timing estimation is found to be a 2-D function of the symbol timing offset and channel length. When compared with previous ML approaches, the proposed likelihood function is optimized at each sampling time without requiring additional pilot symbols. To practically realize the proposed method, a suboptimum approach to the ML estimation is adopted, and an approximate but closed-form solution is presented. Nonlinear operations of the approximate solution can be implemented using a conventional lookup table to reduce the computational complexity. The proposed CFO estimation is also found to depend on the channel length. Unlike conventional schemes, the proposed method fully utilizes the delay spread of dispersive fading channels (which usually reduces the accuracy of estimations). Furthermore, the Cramer-Rao lower bound (CRLB) on the CFO estimate is analyzed, and simulations confirm the advantages of the proposed estimator.
---
paper_title: Joint Frequency-Domain Equalization & Spectrum Combining for the Reception of SC Signals in the Presence of Timing Offset

paper_content:

Frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion is a promising equalization technique for the broadband single-carrier (SC) transmission. However, the presence of timing offset produces the inter-symbol interference (ISI) and degrades the bit error rate (BER) performance. As the roll-off factor of the transmit filter increases, the performance degradation gets larger. In this paper, we propose joint MMSE-FDE & spectrum combining which can achieve the frequency diversity gain while suppressing the negative impact of timing offset for the SC transmission.
---
paper_title: SINR Lower Bound Based Multiuser Detector for Uplink MC-CDMA Systems with Residual Frequency Offset

paper_content:

For uplink multi-carrier code-division multiple access (MC-CDMA) systems, we propose a multiuser detector that is robust to a small residual frequency offset existing after frequency offset estimation and compensation. In this paper, when the residual frequency offset is normalized by subcarrier spacing, it is called a normalized residual frequency offset (NRFO). In the proposed scheme, we first derive a lower bound of the signal-to-interference plus noise ratio (SINR) of the desired user when the NRFO of the desired user is bounded by a small value and the value is known to the receiver. We then design a detection filter to maximize the SINR lower bound. Simulation results show that the proposed scheme has better SINR and bit error rate (BER) performances in a high signal-to-noise ratio (SNR) region than a conventional minimum mean square error (MMSE) receiver that ignores the NRFO.
---
paper_title: Semiblind Iterative Data Detection for OFDM Systems with CFO and Doubly Selective Channels

paper_content:

Data detection for OFDM systems over unknown doubly selective channels (DSCs) and carrier frequency offset (CFO) is investigated. A semiblind iterative detection algorithm is developed based on the expectation-maximization (EM) algorithm. It iteratively estimates the CFO, channel and recovers the unknown data using only limited number of pilot subcarriers in one OFDM symbol. In addition, efficient initial CFO and channel estimates are also derived based on approximated maximum likelihood (ML) and minimum mean square error (MMSE) criteria respectively. Simulation results show that the proposed data detection algorithm converges in a few iterations and moreover, its performance is close to the ideal case with perfect CFO and channel state information.
---
paper_title: Parameter Estimation and Tracking in Physical Layer Network Coding

paper_content:

In this paper, we present an algorithm for joint decoding of the modulo-2 sum of the bits transmitted from two unsynchronized transmitters using Physical Layer Network Coding (PLNC). We address the problems that arise when the boundaries of the signals do not align with each other and when the channel parameters are slowly varying and are not known to the receiver at the relay node. Our approach first estimates jointly the timing and fading gains of both the signals, and uses a state-based Viterbi decoding scheme that takes into account the timing offsets between the interfering signals. We also track the amplitude and phase of the channel which may be slowly varying. Simulation results demonstrate the sensitivity of the detection performance at the relay node to the relative offset of the timings of the two user's signals as well as the advantage of our algorithm over previously published algorithms.
---
paper_title: Impact of pilot pattern on carrier frequency recovery for TETRA-like multitone modulations

paper_content:

A study of maximum-likelihood pilot-aided frequency offset recovery for filtered multitone modulations such as that employed in the TETRA Release 2 Enhanced Data Service is presented. An approach is proposed to improve on previously published algorithms. When pilot symbols are arranged on a rectangular time–frequency grid, as envisaged by the cited standard, the acquisition range of a pilot-based frequency synchroniser may seem to be very narrow as it cannot exceed the inverse of pilot spacing in the time-domain. It is shown that the above drawback can be relieved by resorting to a pilot pattern where the pilot symbols are simply shifted in time along the subcarriers with respect to the rectangular arrangement.
---
paper_title: Joint ml estimation of frame timing and carrier frequency offset for OFDM systems employing time-domain repeated preamble

paper_content:

When a preamble signal is repeated multiple times in OFDM systems, we derive joint maximum likelihood (ML) estimation of the frame timing (FT) and carrier frequency offset (CFO). Unlike conventional estimators which use correlation of adjacent repetition patterns only or some specific sets of patterns, the joint ML estimation exploits correlation of any pair of repetition patterns, providing optimized performance. To reduce the implementation complexity involved in the joint ML estimation, we also propose a near-ML estimation method that separates estimation of the FT and the CFO. The performance of the proposed methods is verified by computer simulation.
---
paper_title: Blind timing and carrier synchronisation in distributed multiple input multiple output communication systems

paper_content:

This study addresses the problem of joint blind timing and carrier synchronisation in a (distributed-M ) × N antenna system where the objective is to estimate the M carrier offsets, the M timing offsets and to recover the transmitted symbols for each of the M users given only the measured signal at the N antennas of the receiver. The authors propose a modular receiver structure that exploits blind source separation to reduce the problem into more tractable sub-problems of estimating individual timing and carrier offsets for multiple users. This leads to a robust solution of low complexity. The authors investigate the performance of the estimators analytically using modified Cramer- Rao bounds and computer simulations. The results show that the proposed receiver exhibits robust performance over a wide range of parameter values, even with worst-case Doppler of 200- 300 Hz and frame size as small as 400 symbols. This work is relevant to future wireless networks and is a complete solution to the problem of estimating multiple timing and carrier offsets in distributed multiple input multiple output (MIMO) communication systems.
---
paper_title: Preamble Based Joint CFO, Frequency-Selective I/Q-Imbalance and Channel Estimation and Compensation in MIMO OFDM Systems

paper_content:

A very promising technical approach for future wireless communication systems is to combine MIMO OFDM and Direct (up/down) Conversion Architecture (DCA). However, while OFDM is sensitive to Carrier Frequency Offset (CFO), DCA is sensitive to I/Q-imbalance. Such RF impairments can seriously degrade the system performance. For the compensation of these impairments, a preamble-based scheme is proposed in this paper for the joint estimation of CFO, transmitter (Tx) and receiver (Rx) frequency-selective I/Q-imbalance and the MIMO channel. This preamble is constructed both in time- and frequency domain and requires much less overhead than the existing designs. Moreover, Closed-Form Estimators (CLFE) are allowed, enabling efficient implementation. The advantages and effectiveness of the proposed scheme have been verified by numerical simulations.
---
paper_title: Pilot-Aided Joint CFO and Doubly-Selective Channel Estimation for OFDM Transmissions

paper_content:

This paper studies the problem of pilot-aided joint carrier frequency offset (CFO) and channel estimation using Fisher and Bayesian approaches in orthogonal frequency division multiplexing (OFDM) transmissions over time- and frequency-selective (doubly selective) channels. In particular, the recursive-least-squares (RLS) and maximum-likelihood (ML) techniques are used to facilitate the Fisher estimation implementations. For the Bayesian estimation, the maximum-a-posteriori (MAP) principle is employed in formulating the joint estimation problem. With known channel statistics, the MAP-based estimation is expected to provide better performance than the RLS- and ML-based ones. To avoid a possible identifiability issue in the joint estimation problem, various basis expansion models (BEMs) are deployed as fitting parametric models for capturing the time-variation of the channels. Numerical results and related Bayesian Cramer Rao bounds (BCRB) demonstrate that the deployment of BEMs is able to alleviate performance degradation in the considered estimation techniques using the conventional block-fading assumption over time-varying channels. Among the considered schemes, the MAP-based estimation using the discrete prolate spheroidal (DPS) or Karhuen Loeve (KL) basis functions would be the best choice that can provide mean-squared-error (MSE) performance comparable to BCRB in low signal-to-noise ratio (SNR) conditions (e.g., coded OFDM transmissions).
---
paper_title: Iterative Joint Estimation Procedure for Channel and Frequency Offset in Multi-Antenna OFDM Systems With an Insufficient Cyclic Prefix

paper_content:

This paper addresses a strategy to improve the joint channel and frequency offset (FO) estimation in multi-antenna systems, widely known as multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM), in the presence of intersymbol interference (ISI) and intercarrier interference (ICI) occasioned by an insufficient cyclic prefix (CP). The enhancement is attained by the use of an iterative joint estimation procedure (IJEP) that successively cancels the interferences located in the preamble of the OFDM frame, which is used for the joint estimation and initially contains the interferences due to a CP shorter than the channel length. The IJEP requires at certain steps a proper iterative interference cancellation algorithm, which makes use of an initial FO compensation and channel estimation obtained due to the use of a symmetric sequence in the preamble. After the iterative cancellation of interferences, the procedure performs an additional joint channel and FO estimation whose mean square error converges to the Cramér-Rao bound (CRB). Later on, this subsequent joint estimation permits the removal of the interferences in the data part of the frame, which are also due to an insufficient CP, in the same iterative fashion but saving iterations compared with the use of other estimation strategies. The appraisal of the procedure has been performed by assessing the convergence of the simulated estimators to the CRB as a function of the number of iterations. Additionally, simulations for the evaluation of the bit error rate (BER) have been carried out to probe how the utilization of the proposed IJEP clearly improves the performance of the system. It is concluded that, with a reduced number of iterations in the preamble, the IJEP converges to the theoretical bounds, thus reducing the disturbances caused by a hard wireless channel or a deliberately insufficient CP. Therefore, this eases the interference cancellation in the data part, leading to an improvement in the BER that approximates to the ideal case of a sufficient CP and, consequently, an improvement in the computational cost of the whole procedure that has been analyzed.
---
paper_title: Estimation algorithms of multiple channels and carrier frequency offsets in application to multiuser OFDM systems

paper_content:

This paper introduces new joint estimation methods of multiple carrier frequency offsets (CFOs) and channel impulse responses (CIR) in application to multiuser orthogonal frequency division multiplexing (OFDM) systems. The estimators are derived from the optimal maximum-likelihood (ML) principle. Complexity reductions are achieved by exploiting the correlation properties of the training sequence (TS). The grid search algorithm is converted into a polynomial root finding procedure which leads to low-complexity closed-form estimators for moderate CFOs. Furthermore, iterative estimators for larger CFOs are proposed. Numerical results confirm that the performance degradation due to the approximations compared to the Cramer-Rao bound (CRB) is small and may be negligible in practice.
---
paper_title: A Low-Delay Low-Complexity EKF Design for Joint Channel and CFO Estimation in Multi-User Cognitive Communications

paper_content:

Parameter estimation in cognitive communications can be formulated as a multi-user estimation problem, which is solvable under maximum likelihood solution but involves high computational complexity. This paper presents a time-sharing and interference mitigation based EKF (Extended Kalman Filter) design for joint CFO (carrier frequency offset) and channel estimation at multiple cognitive users. The key objective is to realize low implementation complexity by decomposing high-dimensional parameters into multiple separate low-dimensional estimation problems, which can be solved in a time- shared manner via pipelining operation. We first present a basic EKF design that estimates the parameters from one TX user to one RX antenna. Then such basic design is time-shared and reused to estimate parameters from multiple TX users to multiple RX antennas. Meanwhile, we use interference mitigation module to cancel the co-channel interference at each RX sample. In addition, we further propose adaptive noise variance tracking module to improve the estimation performance. The proposed design enjoys low delay and low buffer size (because of its online real-time processing), as well as low implementation complexity (because of time-sharing and pipeling design). Its estimation performance is verified to be close to Cramer-Rao bound.
---
paper_title: Frequency-domain processing for synchronization and channel estimation in OQAM-OFDM systems

paper_content:

In this work the design of a frequency-domain synchronization and channel estimation scheme for OQAM-OFDM systems is presented and evaluated. The need for signal processing schemes for wireless communication systems, that are processed solely in the frequency domain results from the desire to build frequency-agile radios. Driven by the transition from exclusive spectrum resource allocation towards a dynamic usage of the spectrum a non-contiguous spectrum utilization in fragmented spectrum is needed. Well-known time-domain algorithms severely suffer from encapsulated spectral components not belonging to the non-contiguous OQAM-OFDM waveform. We show that frequency-domain processing can achieve similar system performance as comparable time-domain methods over a wide range of frequency offsets while offering a new degree of freedom in preamble design for FBMC systems.
---
paper_title: Self-encoded multi-carrier spread spectrum with iterative despreading for random residual frequency offset

paper_content:

In this study, we investigate the multi-carrier spread spectrum (MCSS) communication system which adopts the self-encoded spread spectrum in a downlink synchronous channel. It is very difficult to completely eliminate the frequency offset in practical channel scenarios. We demonstrate that the self-encoded MCSS (SE-MCSS) with iterative despreading manifests a remarkable immunity to residual frequency offset. The SE-MCSS can be an excellent candidate for the future generation of wireless services.
---
paper_title: Tight PEP Lower Bound for Constellation-Rotated Vector-OFDM Under Carrier Frequency Offset and Fast Fading

paper_content:

In this paper, we study pairwise error probability (PEP) of a system employing constellation-rotated vector orthogonal frequency-division multiplexing (CRV-OFDM), which is an enhancement of vector OFDM (V-OFDM) with enhanced diversity, over a frequency-selective channel with both carrier frequency offset (CFO) and Doppler frequency spread. First, we extend the work of Rugini and Banelli, which is known to give a good bit-error rate (BER) approximation for the standard OFDM system only with CFO, to PEP evaluation for CRV-OFDM and discuss its limitations. Then, we discuss the conditionally Gaussian characteristi2cs of the Doppler-induced interference and propose a conditionally Gaussian PEP approximation. Confirming a good match of the conditionally Gaussian PEP approximation and with simulation results, we propose a new semianalytical PEP lower bound. The new lower bound is computationally feasible and is shown by simulation to be tight for small to moderate CFO. Finally, a computationally more feasible approximation of the lower bound is also proposed. Our discussion reveals, as a by-product, why the Gaussian assumption for the Doppler-induced interference works for standard OFDM systems although the assumption is actually incorrect.
---
paper_title: Cooperative Relaying Using OFDM in the Presence of Frequency Offsets

paper_content:

We study the performance of cooperative relaying using orthogonal frequency division multiplexing (OFDM) in the presence of frequency offsets due to Doppler shifts and oscillator instabilities. Through this study, several aspects of transmitter and receiver design, including channel coding, subcarrier mapping and channel estimation are brought to light. We develop two linear front-end receiver architectures based on practical single-user OFDM receivers, and demonstrate the performance using simulations under quasi-static multipath fading channel conditions.
---
paper_title: Joint CFO and Channel Estimation for Asynchronous Cooperative Communication Systems

paper_content:

This letter addresses the joint maximum likelihood carrier frequency offset (CFO) and channel estimation for asynchronous cooperative communication systems. We first present a space-alternating generalized expectation-maximization (SAGE) based iterative estimator (SAGE-IE). Then a low-complexity approximate SAGE-IE (A-SAGE-IE) is developed. Our proposed algorithms decouple the multi-dimensional optimization problem into many one-dimensional optimization problems where the CFO and channel coefficients of each relay-destination link can be determined separately. Simulations indicate that, even though timing offsets are present, the proposed estimators can asymptotically achieve the Cramer-Rao bround (CRB) for the perfectly timing synchronized case.
---
paper_title: Traffic-reduced precise ranging protocol for asynchronous UWB positioning networks

paper_content:

This letter proposes a precise two-way ranging (TWR) protocol toward low traffic for asynchronous UWB positioning networks. The proposed TWR protocol pursuing instantaneous ranging update enables the estimation of clock frequency offset to achieve high ranging accuracy. Theoretical analysis and simulation results verify the performance.
---
paper_title: A novel channel estimation technique for OFDM systems with robustness against timing offset

paper_content:

A new interpolation based channel estimation technique has been proposed to eliminate the effect of phase rotation caused by symbol timing offset (STO) in comb-type pilot-aided OFDM systems. STO is one of the most severe impairments that leads to inter-symbol interference (ISI) and phase rotation of FFT outputs and results in performance degradation in OFDM systems. The main advantage of the proposed channel estimation technique is its insensitivity to the impact of phase rotation. Subsequently, the fine timing synchronization process (which tries to cancel the residual STO) in conventional timing synchronization methods can be eliminated. The main idea behind the proposed technique is to separate the phase and the magnitude components of the estimated channel frequency response at the pilot subcarriers. In this way, two steps of interpolation are performed for the both phase and magnitude, separately and independently. Computational complexity of the proposed channel estimation method is approximately equal to that of the conventional one. Analytical and simulation results of applying the proposed technique to the DVB-T system show that the achieved BER performance is close to that of conventional method in the absence of any timing offset, while in the presence of STO, the proposed technique outperforms the conventional method, considerably.
---
paper_title: An Efficient Blind Deterministic Frequency Offset Estimator for OFDM Systems

paper_content:

This paper proposes an efficient blind deterministic carrier frequency offset (CFO) estimation method for orthogonal frequency division multiplexing (OFDM) systems. In the proposed method, two OFDM symbols with time difference are generated by exploiting both the oversampled OFDM signal and the cyclic prefix, and a cost function is introduced for CFO estimation. It is shown that the cost function can be expressed as a cosine function. Using a property of the cosine function, a formula for estimating the CFO is derived. The estimator of the CFO requires three independent cost function values calculated at three different frequency offsets. Using the formula, the CFO can be estimated without searching all the frequency offset range. The proposed method is very suitable for real wireless environments since it requires only one OFDM symbol for blind reliable estimation of CFO. The computer simulations show that the performance of the proposed method is superior to those of the MUSIC method and the oversampling method . Unlike the conventional methods such as MUSIC method and the oversampling method, the accuracy of the proposed method is independent of the searching step.
---
paper_title: DoA Estimation with Compensation of Hardware Impairments

paper_content:

We consider the estimation of the direction of arrival (DoA) in the presence of hardware impairments that include the RF carrier frequency offsets, the phase offsets generated by an uncalibrated array, and the DC offset. The impairments model has been derived from the analysis of a hardware multiple antenna test-bed that uses direct down-conversion RF front-ends. The performance of the proposed algorithm has been investigated both analytically and via simulations. We show that the estimator provides good performance for a wide range of angles and it is robust to the hardware impairments.
---
paper_title: Robust, Low-Complexity, and Energy Efficient Downlink Baseband Receiver Design for MB-OFDM UWB System

paper_content:

This paper presents optimized synchronization algorithms and architecture designs of a downlink baseband receiver for multiband orthogonal frequency division multiplexing ultra wideband (MB-OFDM UWB). The receiver system targets at low complexity and low power under the premise of good performance. At algorithm level, dual-threshold (DT) detection method is proposed for robust detection performance in timing synchronization; multipartite table method (MTM) is employed to implement arctangent and sin/cos functions in coarse frequency synchronization. MTM outperforms other state-of-the-art methods in power and area. A highly simplified phase tracking method is proposed with better performance in fine frequency synchronization. At architecture level, we focus on optimizing matched filter of packet detector, carrier frequency offset (CFO) corrector and FFT output reorder buffer. The proposed downlink baseband receiver is implemented with 0.13 /mi CMOS technology. The core area of layout is 2.66 × 0.94 mm2, which saves 45.1% hardware cost due to the low-complexity synchronization algorithms and architecture optimization. The postlayout power consumption is 170 mW at 132 MHz clock frequency, which is equivalent to 88 pJ/b energy efficiency at 480 Mbps data rate.
---
paper_title: A fine carrier recovery algorithm robustto doppler shift for OFDM systems

paper_content:

In this paper, a fine carrier recovery algorithm is proposed to maintain the synchronization even in a severe Doppler shift for orthogonal frequency division multiplexing (OFDM) systems such as digital video broadcasting for handheld (DVB-H), DVB for terrestrial (DVB-T) and DVB-T2. The proposed algorithm estimates the frequency offset by utilizing correlation values in the received OFDM symbols by adding intentional frequency offsets. Its performance is compared with conventional algorithms for the DVB-H system in fading with Doppler.
---
paper_title: Exact SINR analysis of wireless OFDM in the presence of carrier frequency offset

paper_content:

This paper presents a new mathematical analysis for the evaluation of the average signal-to-interference plus noise ratio (SINR) of orthogonal frequency division multiplexing (OFDM) in the presence of carrier frequency offset (CFO). CFO destroys the orthogonality between different subcarriers giving rise to inter-carrier interference (ICI). The SINR of wireless OFDM in the presence of CFO and frequency-selective fading becomes a ratio of correlated random variables, and the exact evaluation of its average by using direct methods requires a huge computational effort. In this paper, we present an indirect method that leads to a new exact simpler expression for the average SINR over Rayleigh multipath fading channels.
---
paper_title: IQ Imbalance Estimation Scheme with Intercarrier Interference Self-Cancellation Pilot Symbols in OFDM Direct Conversion Receivers

paper_content:

Direct conversion receivers in orthogonal frequency division multiplexing (OFDM) systems suffer from direct current (DC) offset, frequency offset, and IQ imbalance. We have proposed an IQ imbalance estimation scheme in the presence of DC offset and frequency offset, which uses pilot subcarriers in the frequency domain. In this scheme, the DC offset is eliminated by a differential filter. However, the accuracy of IQ imbalance estimation is deteriorated due to the intercarrier interference (ICI) components caused by frequency offset. To overcome this problem, a new IQ imbalance estimation scheme with intercarrier interference self-cancellation pilot symbols in the frequency domain has been proposed in this paper. Numerical results obtained through computer simulation show that estimation accuracy and bit error rate (BER) performance are improved.
---
paper_title: An Efficient Reduced-Complexity Two-Stage Differential Sliding Correlation Approach for OFDM Synchronization in the AWGN Channel

paper_content:

In this paper, we propose a new scheme for data- aided time and frequency synchronization for OFDM systems, based on a single-symbol preamble. The preamble, of useful length $2^{m}-2$, is composed of two consecutive identical m-sequences (with length $2^{m-1}-1$ each). This preamble is extended by a cyclic prefix of convenient length. This stucture is adequate for a two-stage synchronization scheme, namely a reduced complexity coarse synchronization stage, followed by a finer synchronization one. The first stage, based on Cox and Schmidl-like sliding correlation, determines a reduced uncertainty interval over which the fine stage is carried. The second stage is indeed based on a differential correlation, which is more complex compared to the first stage. The combined use of m-sequences and differential correlation offers an almost perfect peak of the computed metric at the preamble start. To assess the performance degradation occasioned by the reduction of complexity characterizing the proposed two-stage approach, we also consider the brute force single-stage approach, where differential correlation is exclusively used. As a byproduct of our two-stage approach, the fractional frequency offset is estimated and its performance is assessed and compared for both two- stage and one-stage approaches. The brute force approach outperfoms all the considered benchmarks. Compared to the reduced complexity scheme, the brute force one provides similar performance, at the expense of a significant complexity overload. Only for SNR lower than $-5$ dB, the brute force scheme presents a slight enhancement with respect to the reduced complexity one. The simulation results show that the proposed method gives better performance than any other considered estimator. Although our technique is expected to be well suited to multipath channels, thanks to the underlying properties of m-sequences, in this paper we focus on the Additive White Gaussian Noise channel.
---
paper_title: An SFBC-OFDM receiver to combat multiple carrier frequency offsets in cooperative communications

paper_content:

In this paper, a new space-frequency combination technique is proposed for Alamouti coded Orthogonal Frequency Division Multiplexing (OFDM) in the context of cooperative communications. Since cooperative antennas are distributed, there may exist multiple carrier frequency offsets (MCFOs) which cause problems for conventional space-frequency decoding. The proposed algorithm, taking cues from existing MCFO-compensating algorithms [13][19], combines two sets of separately synchronized signal to mitigate inter-carrier interference (ICI). Iterative interference cancellation and a maximum-ratio-combining-like technique are also deployed to further improve performance with low computational complexity. It is observed that the proposed method achieves better bits error rate (BER) performance and has a superior tolerance of multiple CFOs, compared to existing methods.
---
paper_title: Comments on “Estimation of Carrier Frequency Offset With I/Q Mismatch Using Pseudo-Offset Injection in OFDM Systems”

paper_content:

In a recently published paper, an estimation algorithm of carrier frequency offset (CFO) with I/Q mismatch was proposed. Errors in the derivation of the algorithm show that its precision can only be asserted in the case of relatively small I/Q mismatch. Averaging the intermediate variables among one short preamble period improves the estimation accuracy under conditions where I/Q mismatch is large.
---
paper_title: Blind frequency-offset tracking scheme for multiband orthogonal frequency division multiplexing using time-domain spreading

paper_content:

A blind scheme for estimating the residual carrier-frequency offset of multiband orthogonal frequency division multiplexing (MB-OFDM)-based ultra-wideband (UWB) systems is proposed. In the MB-OFDM UWB system, time-domain spreading (TDS) is used by transmitting the same information across two adjacent OFDM symbols. By using the TDS structure, the proposed frequency estimation scheme does not require the use of pilot symbols. To demonstrate the usefulness of the proposed estimator, analytical expression of the mean square error is derived and the performance is compared with a conventional pilot-assisted estimator.
---
paper_title: Joint Carrier Frequency Offset and Channel Estimation for Uplink MIMO-OFDMA Systems Using Parallel Schmidt Rao-Blackwellized Particle Filters

paper_content:

Joint carrier frequency offset (CFO) and channel estimation for uplink MIMO-OFDMA systems over time-varying channels is investigated. To cope with the prohibitive computational complexity involved in estimating multiple CFOs and channels, pilot-assisted and semi-blind schemes comprised of parallel Schmidt Extended Kalman filters (SEKFs) and Schmidt-Kalman Approximate Particle Filters (SK-APF) are proposed. In the SK-APF, a Rao-Blackwellized particle filter (RBPF) is developed to first estimate the nonlinear state variable, i.e. the desired user's CFO, through the sampling-importance-resampling (SIRS) technique. The individual user channel responses are then updated via a bank of Kalman filters conditioned on the CFO sample trajectories. Simulation results indicate that the proposed schemes can achieve highly accurate CFO/channel estimates, and that the particle filtering approach in the SK-APF outperforms the more conventional Schmidt Extended Kalman Filter.
---
paper_title: A Joint Channel and Frequency Offset Estimator for the Downlink of Coordinated MIMO-OFDM Systems

paper_content:

The issues of frequency offset and channel estimation are considered for the downlink of coordinated multiple-input multiple-output orthogonal frequency-division multiplexing systems. Multiple carrier frequency offsets exist in this coordinated system. Without implementing an appropriate compensation for these offsets, both inter-carrier and inter-cell interference will degrade the system performance. Here, we adopt a parallel interference cancelation strategy to iteratively mitigate inter-cell interference, and propose a frequency offset estimator, approximated by a Hadamard product and a Taylor series expansion, to eliminate the inter-carrier interference. Our scheme is significantly less complex than existing methods. Furthermore, the proposed channel estimator is robust to frequency offsets and performs comparably well to these conventional approaches.
---
paper_title: Joint Frequency Offset Tracking and PAPR Reduction Algorithm in OFDM Systems

paper_content:

This paper presents an algorithm that aims to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (QFDM) communication systems while maintaining frequency tracking. The algorithm achieves PAPR reduction by applying the complex conjugates of the data symbol obtained from the frequency domain to cancel the phase of the data symbol. A likelihood estimator is used to obtain the sub-carrier phase error due to the residual carrier frequency offset (RCFO) using the same complex conjugates as a pilot signal. Furthermore, a joint time and frequency domain multicarrier phase locked loop (MPLL) is developed to compensate additional frequency offset. Simulation results show that this algorithm is capable of reducing PAPR without impacting the frequency tracking performance.
---
paper_title: General total inter-carrier interference cancellation for OFDM high speed aerial vehicle communication

paper_content:

Orthogonal Frequency Division Multiplexing (OFDM) has been considered as a strong candidate for next generation high speed aerial vehicle communication systems. However, OFDM systems suffers severe performance degradation due to inter-carrier interference (ICI) in high mobility channel, if no ICI cancellation is performed. Traditionally, training symbols have been employed in one packet to help the OFDM receiver to estimate the multi-path channel and the carrier frequency offset (CFO) between the transmitter local oscillator and the receiver local oscillator. However, in aerial vehicle communication, the relative transmitter-receiver speed changes so rapidly that it is unreasonable to assume a constant speed (and CFO) during the entire packet transmission. Hence, to accurately estimate the CFO, training symbols need to be transmitted for every OFDM symbol. Obviously, this significantly reduces OFDM throughput while adding complexity due to repeated CFO estimation. In this paper, we extend our previous work to propose a joint channel/CFO estimation and ICI cancellation algorithm. Specifically, in our previous work, we have proposed a total ICI cancellation algorithm using parallel processing for OFDM system which offers the excellent ICI cancellation and BER performance. However, in this work, perfect channel information was assumed. In this paper, we combine the channel estimation with the ICI cancellation together. The proposed general total ICI cancellation algorithm has the ability to jointly estimate the carrier frequency offset and channel information, and improve the performance significantly. Meanwhile, a serial processing is proposed to reduce the computation complexity. Simulation results in different scenarios confirm the performance of the proposed scheme in multipath fading channels for high speed aerial vehicle communication.
---
paper_title: Low-Complexity Data-Detection Algorithm in Cooperative SFBC-OFDM Systems With Multiple Frequency Offsets

paper_content:

This paper addresses the problem of data detection in cooperative space-frequency block-coding (SFBC) orthogonal frequency-division multiplexing (OFDM) systems in the presence of multiple carrier frequency offsets (CFOs). A new ordered-successive parallel interference cancellation (OSPIC) detector is proposed. This method consists of first carrying out coarse ordered interference cancellation detection, where the interference components are successively eliminated, and then performing fine interference cancellation detection using parallel interference reduction. Simulation results show that the proposed detector significantly outperforms existing detectors and performs close to that of CFO-free systems for practical signal-to-noise ratios (SNRs). To further reduce the computational complexity, a minimum mean-square error (MMSE)-based reduced-complexity OSPIC (RC-OSPIC) detector is proposed. Compared with the MMSE detector, which requires a full-size complex matrix inversion, the proposed RC-OSPIC detector performs better and is computationally much more effective.
---
paper_title: Integer frequency offset estimation by pilot subset selection for OFDM system with CDD

paper_content:

Cyclic delay diversity (CDD) is a simple transmit diversity technique for an OFDM system using multiple transmit antennas. However, the performance of post-FFT estimation, i.e. integer frequency offset (IFO) is deteriorated by high frequency selectivity introduced by CDD. Proposed is an IFO estimation scheme for an OFDM system with CDD. Based on the pilot subset partitioning, the proposed IFO estimation scheme reduces the effect of frequency selective fading by adopting the CDD.
---
paper_title: A Novel Subspace Decomposition-Based Detection Scheme with Soft Interference Cancellation for OFDMA Uplink

paper_content:

Abstract-In this paper we propose a novel subspace decomposition-based detection scheme with the assistance of soft interference cancellation in the uplink of interleaved orthogonal frequency division multiple access (OFDMA) systems. By utilizing the inherent data structure, the interference is first separated with the desired symbol and then further decomposed into the one caused by the residues of decision errors and the other one by the undetected symbols in the successive interference cancellation (SIC) process.With such an ingenious interference decomposition along with the soft processing scheme, the new receiver can render more thorough interference cancellation, which in turn entails enhanced system performance. Moreover, for practical implementations, a low-complexity version, which only deal with the principal components of inter-carrier interference (ICI), is also addressed. Conducted simulations show that the developed receiver and its low-complexity implementation can provide superior performance compared with pervious works and is resilient to the presence of carrier-frequency offsets (CFOs). The low complexity implementation, in particular, requires substantially lower computational overhead with only slight performance loss.
---
paper_title: Repeated correlative coding scheme for mitigation of inter-carrier interference in an orthogonal frequency division multiplexing system

paper_content:

In this study, a repeated correlative coding scheme is proposed to combat the inter-carrier interference (ICI) caused by the frequency offset in orthogonal frequency division multiplexing (OFDM) communication systems. This proposed scheme combine two ideas of the well-known methods, which are the coding of adjacent subcarriers with antipodal of the same data symbol (ICI self-cancellation) and correlative coding. A mathematical expression for the carrier-to-interference ratio (CIR) by using this proposed repeated correlative coding scheme is derived. The simulated result of CIR for the proposed scheme is significantly improved compared to the correlative coding as well as a self-cancellation scheme. The bit-error-rate (BER) of the proposed scheme is also compared with the ICI self-cancellation scheme and correlative coding scheme, which is comparable to that of the ICI self-cancellation scheme and better than the correlative coding scheme.
---
paper_title: Concatenated Precoded OFDM for CFO Effect Mitigation

paper_content:

Orthogonal frequency-division multiplexing (OFDM) is highly sensitive to carrier frequency offset (CFO), which not only causes intercarrier interference (ICI) among subcarriers but introduces complex multiplicative distortion (CMD) to all detected subcarrier symbols as well. Due to unknown CFO, both ICI and CMD are time variant, thus complicating the data demodulation at the receiver. In conjunction with a training-prefixed data frame structure, a concatenated precoder that is constructed by concatenating an outer modified correlative precoder with an inner reduced Hadamard precoder is proposed in this paper to process data symbols prior to OFDM modulation and to enable joint estimation on channel multipath and constant CMD (CCMD), time-variant CMD (TCMD) estimation and compensation, and ICI suppression at the receiver in the presence of CFO. Simulation results show that the proposed system provides much better error performance than conventional signal coding approaches in the presence of CFO and multipath fading.
---
paper_title: A Fast Time-Delay Estimator of PN Signals

paper_content:

This work proposes an effective time-delay estimator for PN satellite signals, exploiting a fast triangular interpolator, running on three estimated ambiguity samples in the neighborhood of the coarse estimate. Performance analysis (theory and simulation) is carried out in comparison with conventional approaches based on the interpolation, usually carried out by (time-consuming) narrow-band over-sampling or (fast) fitting of few samples of a smoothed function of the ambiguity function around its maximum. The theoretical results, substantiated by computer simulations, have evidenced that the devised method outperforms the conventional estimator for all the timing offsets and is well suited for satellite spread-spectrum communications.
---
paper_title: CFO estimation in OFDM systems under timing and channel length uncertainties with model averaging

paper_content:

In this letter, we investigate the problem of CFO estimation in OFDM systems when the timing offset and channel length are not exactly known. Instead of explicitly estimating the timing offset and channel length, we employ a multi-model approach, where the timing offset and channel length can take multiple values with certain probabilities. The effect of multimodel is directly incorporated into the CFO estimator. Results show that the proposed estimator outperforms the estimator selecting only the most probable model and the method taking the maximal model.
---
paper_title: Repeated preamble based carrier frequency offset estimation in the presence of I/Q imbalance

paper_content:

The paper proposes a novel estimation of carrier frequency offset (CFO) and I/Q imbalance, based on the popular repeated preamble. By investigating the nonlinear least squares cost function of CFO estimation, we discover a general relation among three arbitrary pilot symbols and derive several useful linear equations. Then, we propose a corresponding linear least squares estimation of unsigned CFO with explicit closed-form solution. Using the tentative CFO estimate, the I/Q imbalance can be estimated with sign ambiguity. Notice that the taps of the FIR filter representing frequency-dependent imbalance in practical frond-ends are constrained, we also propose a novel computation-free detection method to solve the sign ambiguity.
---
paper_title: ESPRIT-Based Carrier Frequency Offset Estimation for OFDM Direct-Conversion Receivers

paper_content:

Over the last years, there has been growing interest in making user terminals more efficient in terms of cost, size, and power consumption. A step forward in this direction is represented by direct-conversion receiver (DCR) architectures, which directly convert the received waveform from the radio frequency (RF) to baseband, thereby avoiding any intermediate frequency (IF) stage. One major issue of DCR devices is the presence of RF impairments, which greatly complicate fundamental receiver functions, including the synchronization task. In this paper we propose a carrier frequency offset (CFO) recovery scheme for OFDM DCRs plagued by frequency-selective In-phase/Quadrature (I/Q) imbalances. Our approach is based on the ESPRIT algorithm and relies on the transmission of a typical OFDM training preamble having a repetitive structure in the time domain. Numerical simulations indicate that the proposed scheme outperforms existing state-of-the-art alternatives at the price of a certain increase of the processing load.
---
paper_title: An interference self-cancellation technique for SC-FDMA systems

paper_content:

A new interference self-cancellation (ISC) method for Single Carrier-FDMA (SC-FDMA) systems is proposed to mitigate the inter-user interference caused by frequency offset or Doppler effect. By transmitting a compensation symbol at the first symbol location in each resource block, the energy leakage can be significantly suppressed. With little bandwidth and power sacrifice, the proposed method can greatly improve the system robustness against frequency offset. Simulation results show that the signal-to-interference ratio (SIR) can be improved by 7 dB on average for the entire system band, and up to 11.7 dB for an individual user.
---
paper_title: Joint Channel, Carrier-Frequency-Offset and Noise-Variance Estimation for OFDM Systems Based on Expectation Maximization

paper_content:

In this paper, a joint channel, carrier-frequency-offset (CFO) and noise-variance estimation scheme is proposed for OFDM systems which is based on Expectation and Maximization (EM) algorithm. The channel parameters are estimated using training sequences incorporated at the beginning of each transmission frame. Based on the assumption that the amplitude and CFO of different paths are independent, the received multipath components may be decomposed into $L$ independent data sets of the $L$ resolvable propagation paths. Hence the associated multi-dimensional minimization problem may be decomposed into separate single-dimensional minimization processes, the maximum likelihood and yet, remains capable of approaching performance at a signifucantly reduced complexity.
---
paper_title: Exact BER analysis of FRFT-OFDM system over frequency selective Rayleigh fading channel with CFO

paper_content:

The bit error rate expression of the binary phase-shift keying modulation scheme has been derived in a frequency selective fading channel for the fractional Fourier transform (FRFT) based orthogonal frequency-division multiplexing (OFDM) system in the presence of carrier frequency offset (CFO). The performance of the FRFT based OFDM system has been found to be better than FFT-based OFDM at different values of FRFT angle parameter `α'.
---
paper_title: Gabor Division/Spread Spectrum System Is Separable in Time and Frequency Synchronization

paper_content:

Recently proposed new Time-Domain (TD) synchronization using frequency integration and TD Spread Spectrum (SS) codes has been shown to be robust to frequency offset, that has its dual Frequency-Domain (FD) synchronization using time integration and FD SS codes which is robust to timing offset. Separable Property (SP) is defined for time-frequency synchronization under the condition containing time and frequency deviations to be performed separately and cooperatively. The SP compels us to design phase correction on SS codes and transmitted data.
---
paper_title: Sensing orthogonal frequency division multiplexing systems for cognitive radio with cyclic prefix and pilot tones

paper_content:

The detection of orthogonal frequency division multiplexing (OFDM) for cognitive radio is considered in this paper. A frequency-selective fading channel is considered and the receiving process is modeled with timing and frequency offsets. Firstly, the authors propose a new decision statistic based on time-domain cross-correlation of the cyclic prefix (CP) embedded in OFDM signals. The probability distribution functions (PDFs) of the statistics under both hypotheses of primary signal absence and presence are derived. Estimation of the timing and frequency offset is obtained through the maximum likelihood method and the received signals are modified. Then another new decision statistic based on frequency-domain cross-correlation of the pilot tones (PTs) is proposed whose PDF is also analyzed. Then, through the likelihood ratio test, the authors utilize CP and PT jointly and propose a global test statistic. The theoretical probabilities of false alarm (PFA) and detection are derived, and the theoretical threshold for any given PFA is proposed. The simulation results show that the proposed spectrum-sensing scheme has excellent performance, especially under very low signal-to-noise ratio (SNR).
---
paper_title: Widely Linear MVDR Beamformers for Noncircular Signals Based on Time-Averaged Second-Order Noncircularity Coefficient Estimation

paper_content:

The optimal widely linear (WL) minimum variance distortionless response (MVDR) beamformer, which has a powerful performance for the reception of a noncircular signal, was proposed by Chevalier in 2009. Nevertheless, in spectrum monitoring or passive listening, the optimal WL MVDR beamformer is hard to implement due to an unknown second-order (SO) noncircularity coefficient. This paper aims at estimating the time-averaged SO noncircularity coefficient of a desired noncircular signal, whose waveform is unknown but whose steering vector is known, in the context of the optimal WL MVDR beamformer. The proposed noncircularity coefficient estimator can process 2N - 1 rectilinear signals at most using an array of N sensors. Moreover, a frequency-shift WL MVDR beamforming algorithm is proposed for a noncircular signal having a nonnull frequency offset or carrier residue, jointly with the estimation of the frequency offset of the rectilinear signal. Due to the inevitable estimation error of the time-averaged SO noncircularity coefficient, a diagonal loading technique is used to enhance the robustness of the optimal WL beamformers. Simulations are shown to verify the effectiveness of the proposed algorithms.
---
paper_title: Joint estimation of Carrier and Sampling Frequency Offset, phase noise, IQ Offset and MIMO channel for LTE Advanced UL MIMO

paper_content:

In LTE Advanced Uplink MIMO the pilot symbols on a subcarrier and OFDM symbol are not transmitted exclusively by one layer. If the pilots are transmitted exclusively, like in LTE Advanced Downlink or Mobile WiMAX, the estimation of Carrier Frequency Offset (CFO) and Sampling Frequency Offset (SFO) can be based on correlating two pilot symbols at different OFDM symbols. In addition the estimation of CFO / SFO can be performed separately from IQ Offset and channel estimation. In LTE Advanced Uplink (UL) the pilot symbols on a subcarrier and OFDM symbol are transmitted by all layers simultaneously. As the received symbol consists of the sum of all transmitted pilot symbols, the CFO / SFO estimation approaches used for SISO seem not applicable anymore. This paper introduces a joint estimation of carrier and sampling frequency offset, phase noise, IQ offset and MIMO channel for LTE Advanced UL MIMO.
---
paper_title: E2KF based joint multiple CFOs and channel estimate for MIMO-OFDM systems over high mobility scenarios

paper_content:

An enhanced extended Kalman filtering (E2KF) algorithm is proposed in this paper to cope with the joint multiple carrier frequency offsets (CFOs) and time-variant channel estimate for MIMO-OFDM systems over high mobility scenarios. It is unveiled that, the auto-regressive (AR) model not only provides an effective method to capture the dynamics of the channel parameters, which enables the prediction capability in the EKF algorithm, but also suggests an method to incorporate multiple successive pilot symbols for the improved measurement update.
---
paper_title: An Improved Frequency Offset Estimation Based on Companion Matrix in Multi-User Uplink Interleaved OFDMA Systems

paper_content:

In this letter, we consider a multiuser uplink orthogonal frequency-division multiple access (OFDMA) system. To estimate the carrier frequency offset (CFO) efficiently, a modified pilot structure has been proposed to allow sufficient frequency separation between the subcarriers allocated to any two users. The proposed structure can reduce the ambiguity problem caused by the multiple signal classification (MUSIC) based algorithm when a CFO of one user is close to that of a different user. We present a CFO estimation method based on the companion matrix obtained using the received signal from the proposed pilot structure. Simulation results show that the proposed CFO estimator performs better than the conventional estimator and maintains its performance as the CFO range increases.
---
paper_title: Channel Equalization and Symbol Detection for Single-Carrier MIMO Systems in the Presence of Multiple Carrier Frequency Offsets

paper_content:

A new frequency-domain channel equalization and symbol detection scheme is proposed for multiple-input-multiple-output (MIMO) single-carrier broadband wireless systems in the presence of severely frequency-selective channel fading and multiple unknown carrier-frequency offsets (CFOs). Multiple CFOs cause severe phase distortion in the equalized data for large block lengths and/or constellation sizes, thus yielding poor detection performance. Instead of explicitly estimating the CFOs and then compensating them, the proposed scheme estimates the rotated phases (not frequencies) caused by multiple unknown CFOs and then removes the phase rotations from the equalized data before symbol detection. The estimation accuracy of the phase rotation is improved by utilizing a groupwise method rather than symbol-by-symbol methods. This paper differs from other related work in orthogonal frequency division multiplexing (OFDM) studies in that it can combat multiple CFOs that are time varying within each block. Numerical examples for 4 × 2 and 8 × 4 single-carrier systems with quaternary phase-shift keying (QPSK) and eight-phase-shift keying (8PSK) modulation illustrate the effectiveness of the proposed scheme in terms of scatter plots of constellation, mean square error (MSE), and bit error rate (BER).
---
paper_title: Simple Carrier Recovery Approach for RF-Pilot-Assisted PDM-CO-OFDM Systems

paper_content:

For RF-pilot-assisted PDM-CO-OFDM systems, we propose and demonstrate a carrier recovery method applying a simple moving average filter (MAF) to extract the central RF-pilot for phase noise compensation. Because only two additions per output sample would be required to implement such a MAF, its computational complexity is rather simple compared to any other direct-form finite impulse response (FIR) filter. To handle its weak side-lobe attenuation, which would cause the spectrally nearby OFDM signal to interfere with the extracted pilot signal, we further propose using multiple MAFs in cascade to enhance the side-lobe attenuation. We evaluate the performance of a 16-QAM and 40-Gbps PDM-CO-OFDM signal, in terms of the bandwidth tolerance, optical signal to noise ratio (OSNR) tolerance, nonlinear tolerance, linewidth tolerance, and residual carrier frequency offset (RFO) tolerance, with different filters including the simple MAF, cascaded MAFs, and the previously-demonstrated multi-stage decimation and interpolation filter (MDIF). We've found that 1) all the filters exhibit similar performance to the ideal brick-wall filter in terms of noise and nonlinear tolerances, 2) the MAF1 exhibits the worse tolerances against linewidth and RFO, and 3) the MDIF has to be carefully designed to enhance its tolerances against both the linewidth and RFO.
---
paper_title: Novel Coarse Timing Synchronization Methods in OFDM Systems Using Fourth-Order Statistics

paper_content:

In this paper, the problem of coarse timing synchronization in orthogonal frequency-division multiplexing (OFDM) systems is investigated, and two new timing metrics with better performance are presented. The new metrics take advantage of two novel differential normalization functions that are based on the fourth-order statistics and are designed depending on the value of carrier frequency offset (CFO). The proposed timing metrics are theoretically evaluated using two different class separability criteria. It is shown that the new schemes considerably increase the difference between the correct and wrong timing points in comparison with previous methods. The computational complexity of the new methods is obtained, and their superior detection performances are also demonstrated in terms of probabilities of false alarm and missed detection. The results indicate that due to a significant improvement in missed detection probability (MDP), the new methods offer a considerably wider range of acceptable thresholds.
---
paper_title: Joint Clock and Frequency Synchronization for OFDM-Based Cellular Systems

paper_content:

In cellular systems, a basestation and mobile stations need to be synchronized before data exchange. Since the basestation clock reference is more accurate, a mobile station typically derives its clock reference from the basestation. But the carrier frequency offset due to Doppler shift may have harmful effects on the local clock derivation. This letter proposes a joint clock and frequency synchronization technique between a basestation and a mobile station, which is effective even with Doppler shift. We derive the joint estimation algorithm by analyzing the phase and the amplitude distortion caused by the sampling frequency offset and the carrier frequency offset. Simulation results showing the effectiveness of the proposed algorithm will also be presented.
---
paper_title: Maximum Likelihood Estimation of Time and Carrier Frequency Offset for DVB-T2

paper_content:

The new terrestrial digital video broadcasting standard DVB-T2 provides a specific symbol - called P1-symbol - for the initial time and frequency synchronization. In this paper the maximum likelihood (ML) time and carrier frequency offset (CFO) synchronization scheme, which exploits the structure of the P1 symbol in both time and frequency domains is derived. Two lower-complexity solutions are then proposed: (1) a ML estimator that only exploits the time structure of the P1 symbol and (2) a pseudo ML (PML) scheme that resorts to a suboptimal CFO estimator while still performing ML time synchronization. The proposed schemes are compared in terms of both synchronization capabilities and implementation complexity. The Cramer-Rao bounds for the CFO estimators are also evaluated. Simulation results in a typical DVB-T2 scenario show that both ML and PML schemes have a very close performance while significantly outperforming existing schemes.
---
paper_title: An improved ESPRIT-based blind CFO estimation for OFDM in the presence of I/Q imbalance

paper_content:

The estimation of carrier frequency offset (CFO) is an important issue in the study of OFDM systems. In the past, many CFO estimation methods have been proposed. In particular, the ESPRIT-based blind CFO estimation method is attractive because of its bandwidth efficiency. It can yield accurate estimates and it works for non constant modulus modulation symbols. This paper improves the ESPRIT-based method in two ways. Firstly, we show how the information of virtual carrier location can be exploited to further enhance the estimation accuracy. Secondly, we also derive a new algorithm that can estimate CFO in the presence of I/Q imbalance.
---
paper_title: Data-Aided Timing Synchronization for FM-DCSK UWB Communication Systems

paper_content:

Frequency-modulated differential chaos shift keying (FM-DCSK) ultrawideband (UWB) communication systems convey information by transmitting ultrashort chaotic pulses (in the nanosecond scale). Since such pulses are ultrashort, timing offset may severely degrade the bit error rate (BER) performance. In this paper, a fast data-aided timing synchronization algorithm with low complexity is proposed for FM-DCSK UWB systems, which capitalizes on the excellent correlation characteristic of chaotic signals. Simulation results show that the BER performance of such systems is fairly close to that of perfect timing thanks to the proposed new algorithm. Moreover, the new algorithm requires less synchronization searching time and lower computational complexity than the conventional one for transmitted reference UWB systems existing in the current literature.
---
paper_title: Blind symbol rate estimation using autocorrelation and zero crossing detection

paper_content:

In this paper we introduce a new and simple method to estimate the symbol rate for single carrier systems of unknown modulation. The introduced technique detects the symbol rate from a continuous range of symbol rates, i.e. it does not assume a finite set of candidate symbol rates. The method does not require any knowledge about system parameters and is therefore totally blind. The method belongs to the family of autocorrelation-based symbol rate estimators, yet, unlike many such schemes, its performance is insensitive to the value of the roll-off factor. We then propose a simple method for frequency offset compensation also based on the autocorrelation function. The proposed estimator is implemented as part of a complete DVB-C [1] receiver and is verified using simulations and results in robust performance even at low SNR and high frequency offsets.
---
paper_title: Performance Analysis of Code-Aided Symbol Timing Recovery on AWGN Channels

paper_content:

We analyze the performance of a code-aided (CA) decision-directed (DD) timing synchronizer, which can exploit the dependence structure across coded symbols to improve the timing recovery accuracy. Due to the inherent coupling between timing recovery and decoding, most existing studies rely on extensive simulation rather than on analytical methods to evaluate performance of timing recovery for coded systems. We propose analytical methods in this paper towards this end. A first key step is to approximate timing-offset-induced inter-symbol interference (ISI) as an additive Gaussian noise, since in the low signal-to-noise ratio (SNR) regime the background noise is large enough to mask the ISI. Then, we derive semi-analytical expressions for the mean and variance of extrinsic information as functions of timing offset, building on which we characterize both open-loop and closed-loop performance of decision-directed timing synchronizers. Monte Carlo simulation results corroborate that the proposed method accurately characterizes the performance of CA DD timing recovery, for systems with a wide range of channel bandwidth and different channel codes.
---
paper_title: Intercarrier Interference Cancellation Using General Phase Rotated Conjugate Transmission for OFDM Systems

paper_content:

In this paper, we propose a general phase rotated conjugate cancellation (PRCC) scheme for intercarrier interference (ICI) cancellation in orthogonal frequency division multiplexing (OFDM) systems. It is shown that the previous conjugate cancellation (CQ scheme is equivalent to a special case of our proposed scheme. The general PRCC scheme contains advantages of the conventional CC scheme, such as backward compatibility with the existing OFDM systems, low receiver complexity, and two-path diversity, but provides better performance, especially at high frequency offset situations.
---
paper_title: Improved fine CFO synchronization for MB-OFDM UWB

paper_content:

Proposed is an improved blind carrier frequency offset (CFO) estimator suitable for Multi-Band OFDM Ultra Wideband (MB-OFDM UWB) system. By exploiting the conjugate symmetry of the physical layer convergence protocol (PLCP) the need for training symbols can be avoided and estimation performance is improved as well. Computer simulations show that the proposed method achieves better estimation performance than existing method.
---
paper_title: Schmidl-Cox-like Frequency Offset Estimation in Time-Hopping UWB

paper_content:

This paper presents a time hopping ultra wide band (TH-UWB) receiver design targeted to high performance and low complexity. The classical Schmidl and Cox idea of extracting the frequency offset from the phase of a correlation measure between two identical transmitted signals is here extended to a TH format. The algorithm exploits the low duty cycle of the time hopping access, and combines received samples in order to strengthen the signal-to-noise ratio. A clever selection of which samples to use in the correlation measure (first and last third of the received signal) is proved to perform 0.5 dB from the Cramer-Rao lower bound, thus providing an improvement of 0.75 dB with respect to the standard approach of the literature (correlating the first and second halves of the received signal). A modified algorithm that suitably weights received samples is also proposed for achieving robustness in impulsive multiple user access scenarios.
---
paper_title: Joint Carrier Frequency Offset and Channel Estimation for MIMO-OFDM Systems Using Extended H_{∞} Filter

paper_content:

We address the problem of joint carrier frequency offset (CFO) and channel estimation for multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems over time-varying channels. The CFOs between different pairs of transmit and receive antennas are considered to be different. The method is derived based on the extended H∞ filter (EHF). Compared to the conventional extended Kalman filter (EKF)-based method, the EHF-based method does not require any knowledge of the noise distributions, so it is more flexible and robust to the unknown disturbances. Simulation results are provided to illustrate the performance of this method.
---
paper_title: Pilot-aided estimation of carrier frequency offsets and channel impulse responses for OFDM cooperative communications

paper_content:

We consider a cooperative communication system where the destination node receives an orthogonal frequency-division multiplexing (OFDM) signal transmitted from two relay nodes. We propose and compare two pilot-aided algorithms for the estimation of the carrier frequency offsets (CFOs) and the channel impulse responses (CIRs) associated with the two relay-destination links. The first algorithm uses a basis expansion model (BEM) to estimate a time-varying (TV) multiple-input single-output (MISO) channel that incorporates both CFOs and CIRs effects. The CFOs are then extracted from the estimated TV-MISO channel by applying estimation of signal parameters via rotational invariance techniques (ESPRIT). The second algorithm exploits the structure of a specific pilot sequence and estimates the CFOs by directly applying ESPRIT on the observed signal. Simulation results show the effectiveness of the proposed algorithms.
---
paper_title: A Subspace-Based CFO Estimation Algorithm for General ICI Self-Cancellation Precoded OFDM Systems

paper_content:

This paper presents a carrier-frequency-offset (CFO) estimation algorithm for time-dispersive orthogonal frequency division multiplexing (OFDM) system using a general inter-carrier interference (ICI) self-cancellation scheme. This study uses the time shift invariant property in the precoded signal to estimate the CFO. To achieve this, the proposed algorithm first collects the highly correlated receive time samples into snapshot vectors. The snapshot vectors can be expressed in a form having a CFO-directed response structure, which enables the proposed approach to estimate the CFO by using the multiple signal classfication (MUSIC) algorithm in the time domian. This study also develops a time sample selection scheme to mitigate the noise enhancement caused in the equalization process before the MUSIC algorithm. As compared to conventional algorithms, in addition to having an estimation error approaching to the Cramer-Rao lower bound (CRLB), the proposed algorithm has an adjustable CFO estimation range linearly proportional to the order of the ICI self-cancellation scheme.
---
paper_title: A Novel Distributed Translator for an ATSC Terrestrial DTV System

paper_content:

This paper presents a new design and implementation method of distributed translator (DTxR), termed the equalization DTxR (EDTxR), for distributed translator network in ATSC systems. EDTxR has a simple structure and does not require any devices to be added to studio or transmitter facilities already deployed. EDTxRs which retransmit the same transmitter signal can have identical output symbol streams among them without additional synchronization information by utilizing the structure of the equalization digital on-channel repeater (EDOCR). Moreover, they can synchronize output frequency among them without global positioning system (GPS) clock receivers by adopting a crystal oscillator for each translator and compensating the frequency offset of the each crystal oscillator through the carrier and timing recovery. To verify the proposed method, multiple EDTxRs are implemented and tested in the laboratory and in the field. Through the tests, it is confirmed that EDTxR is a simple and economic distributed translator for distributed transmission system which does not need additional synchronization devices. Therefore, EDTxR can be a promising translator for coverage extension of digital terrestrial television broadcasting under spectrum deficient situations.
---
paper_title: The Cramer-Rao Bound for Training Sequence Design for Burst-Mode CPM

paper_content:

In this paper, we study the Cramer-Rao bound (CRB) for continuous phase modulation (CPM) signals where frequency offset, carrier phase, and symbol timing are jointly estimated when transmitted over an additive white Gaussian noise (AWGN) channel. We consider a data-aided (DA) estimation scenario in which the estimator takes advantage of a known training sequence at the start of each burst. Thus, we first derive the joint CRBs as functions of a known training sequence and CPM parameters. By analyzing the CRB expressions, we propose the optimum training sequence for which the CRB is minimized. We show that the same training sequence is optimum for all three estimation parameters. Additionally, we compare the performance of the optimum training sequence with a random one by providing a closed-form expression for the unconditional CRB (UCRB) for symbol timing estimation of CPM signals. Comparing the UCRB and the CRB for the optimum training sequence reveals that a DA estimator with the optimum training sequence leads to significant gains in terms of the mean-square error of the estimation parameter when the underlying CPM scheme is non-binary and/or partial response.
---
paper_title: Integer Frequency Offset Estimation for OFDM Systems With Residual Timing Offset Over Frequency Selective Fading Channels

paper_content:

Accurate integer frequency offset (IFO) estimation is crucial for orthogonal frequency-division multiplexing (OFDM) systems, particularly in the presence of frequency-selective fading and residual timing offset (RTO). For existing algorithms, however, it is still a challenge to obtain a good tradeoff among the estimation performance, complexity, and spectrum overhead. In this paper, we propose a novel cross ambiguity function (CAF)-based IFO estimator using only one training sequence. By designing the training sequence, which has only a single sharp peak on its ambiguity function surface, a highly accurate and full acquisition range estimation of the IFO can be obtained in the presence of frequency-selective fading and RTO. Moreover, the adoption of the CAF expression in terms of time-domain signals ensures that the complexity of the proposed algorithm is relatively low. Simulation results verify its superior accuracy in frequency-selective fading channels and in the presence of RTO.
---
paper_title: Low complexity scheme for carrier frequency offset estimation in orthogonal frequency division multiple access uplink

paper_content:

Maximum likelihood (ML) carrier-frequency offset estimation for orthogonal frequency-division multiple access uplink is a complex multi-parameter estimation problem. The ML approach is a global optima search problem, which is prohibitive for practical applications because of the requirement of multidimensional exhaustive search for a large number of users. There are a few attempts to reduce the complexity of ML search by applying evolutionary optimisation algorithms. In this study, the authors propose a novel canonical particle swarm optimisation (CPSO)-based scheme, to reduce the computational complexity without compromising the performance and premature convergence. The proposed technique is a two-step process, where, in the first step, low resolution alternating projection frequency estimation (APFE) is used to generate a single better positioned particle for CPSO, followed by an actual CPSO procedure in second step. The mean square error performance of the proposed scheme is compared with existing low complexity algorithms namely APFE and linear particle swarm optimisation with mutation. Simulation results presented in this study show that the new scheme completely avoids premature convergence for a large number of users as high as 32.
---
paper_title: A ranging method for OFDMA uplink system

paper_content:

In orthogonal frequency division multiple access (OFDMA) uplink system, estimations of time delay and carrier frequency offset (CFO) are both particularly challenging. In IEEE 802.16-based networks, such synchronization problem can be resolved in initial ranging process. In this paper, exploiting the unique scheme of interleaved OFDMA, a special ranging symbol which has repetitive scheme in time-domain is designed. Furthermore, a novel ranging method for OFDMA system is proposed by utilizing such repetitive characteristic. Compared with the existing methods, this algorithm just need one ranging symbol and can get more correct estimation of time delay, at the same time, it can estimate the integral CFO in OFDMA uplink. Simulations illustrate efficiency of the proposed algorithm over the existing method.
---
paper_title: Adaptive schemes and analysis for blind beamforming with insufficient cyclic prefix in single carrierfrequency division multiple access systems

paper_content:

When the duration of the cyclic prefix (CP) is shorter than that of the channel impulse response in single carrier-frequency division multiple access systems, inter-symbol interference and inter-carrier interference will degrade the system performance. Previously, one solution of this problem while considering the effect of carrier frequency offsets (CFOs) and the co-channel interference is a blind received beamforming scheme based on eigenanalysis in a batch mode. As the capability in suppressing the multipath signal with the delay larger than the CP length has not previously been analysed theoretically for the scheme, the theoretical analysis regarding the capability in suppressing the long-delayed multipath signal is provided in this study. The analysis provided in this study is also utilised to design an adaptive processing scheme. The adaptive algorithm is developed to find the beamforming weight vector updated on a per symbol basis without using reference signals. The proposed adaptive algorithm reduces the computational complexity of and shows competitive performance under the insufficient CP, the CFOs, the co-channel interference and the time-varying scenarios. The simulation results reveal that the proposed adaptive algorithm provides better performance than the previously proposed algorithm.
---
paper_title: Initial-Estimation-Based Adaptive Carrier Recovery Scheme for DVB-S2 System

paper_content:

A data-aided (DA) carrier recovery (CR) method, which can select different coarse frequency estimation (CFE) paths adaptively according to the initial-estimation of frequency offset, is proposed for the 2nd Generation Digital Video Broadcasting Satellite (DVB-S2) system to accomplish the convergence within at most 400 pilot blocks. The well-known two-step CR scheme is used for the consideration of precision and speed, whereas both the CFE and the fine frequency estimation (FFE) adopt L&R algorithm as their basic frequency estimator so that the same correlation operation can be used by both of them to decrease the realization complexity. Optimum threshold values for different CFE paths are determined by analyzing the ill-selection probability and robust Root Mean Square Error (RMSE) performance has been confirmed by our simulations.
---
paper_title: FS-FBMC: A flexible robust scheme for efficient multicarrier broadband wireless access

paper_content:

An alternative implementation of the filter bank multicarrier (FBMC) concept is introduced. It is based on an FFT whose size is the length of the prototype filter. The approach clarifies the connection with OFDM and its main benefit is in the receiver, where high performance sub-channel equalization and timing offset compensation are achieved in a straightforward manner without additional delay. The scheme is particularly appropriate for broadband wireless access, to cope with fragmented frequency bands and to optimize the utilization of the spectrum, for example with the help of water-filling based sub-channel loading algorithms. The context of TV white spaces is taken for illustration. An issue with the proposed scheme is the computational complexity in the receiver and an approach having the potential for substantial savings is mentioned.
---
paper_title: Diversity analysis of distributed linear convolutive space-time codes for time-frequency asynchronous cooperative networks

paper_content:

This study analyses the achievable cooperative diversity order of the distributed linear convolutional space-time coding (DLC-STC) scheme for time–frequency asynchronous cooperative networks. The authors first prove that perfect time or frequency synchronisation is impractical for cooperative networks with multiple relays serving multiple destinations even when the relays know all accurate time delays and frequency offsets. Then the DLC-STC scheme, in which the exact time synchronisation at the relay nodes is unnecessary, is introduced into this type of cooperative networks. This study proves that the achievable time–frequency asynchronous cooperative diversity order of the DLC-STC scheme with maximum-likelihood receivers is equal to the number of relays. Simulation results verify the analysis.
---
paper_title: On the Cyclostationarity of OFDM and Single Carrier Linearly Digitally Modulated Signals in Time Dispersive Channels: Theoretical Developments and Application

paper_content:

Previous studies on the cyclostationarity aspect of orthogonal frequency division multiplexing (OFDM) and single carrier linearly digitally modulated (SCLD) signals assumed simplified signal and channel models or considered only second-order cyclostationarity. This paper presents new results concerning the cyclostationarity of these signals under more general conditions, including time dispersive channels, additive Gaussian noise, and carrier phase, frequency, and timing offsets. Analytical closed-form expressions are derived for time- and frequency-domain parameters of the cyclostationarity of OFDM and SCLD signals. In addition, a condition to eliminate aliasing in the cycle and spectral frequency domains is derived. Based on these results, an algorithm is developed for recognizing OFDM versus SCLD signals. This algorithm obviates the need for commonly required signal preprocessing tasks, such as signal and noise power estimation and the recovery of symbol timing and carrier information.
---
paper_title: Comments on "A Technique for Orthogonal Frequency Division Multiplexing Frequency Offset Correction"

paper_content:

This comment corrects a few errors found in the derivation of the maximum likelihood estimate of differential phase in the paper, "A Technique for Orthogonal Frequency Division Multiplexing Frequency Offset Correction." We show that the problem of differential phase estimation can be considered as an estimation problem in the presence of nuisance parameters, which does not satisfy strong ancillarity. The approach in the above paper to solve this problem can be understood as conditioning for elimination of nuisance parameters but without taking proper steps. After making corrections on the proof, it is demonstrated that the estimator in the above paper is inherently suboptimal and thus prior knowledge on the nuisance parameters, if available, can be utilized to further improve the estimation performance.
---
paper_title: Error probability expressions for frame synchronization using differential correlation

paper_content:

Probabilistic modeling and analysis of correlation metrics have been receiving considerable interest for a long period of time because they can be used to evaluate the performance of communication receivers, including satellite broadcasting receivers. Although differential correlators have a simple structure and practical importance over channels with severe frequency offsets, closed-form expressions for the output distribution of differential correlators do not exist. In this paper, we present detection error probability expressions for frame synchronization using differential correlation, and demonstrate their accuracy over channel parameters of practical interest. The derived formulas are presented in terms of the Marcum Q-function, and do not involve numerical integration, unlike the formulas derived in some previous studies. We first determine the distributions and error probabilities for single-span differential correlation metric, and then extend the result to multi-span differential correlation metric with certain approximations. The results can be used for the performance analysis of various detection strategies that utilize the differential correlation structure.
---
paper_title: An efficient algorithm for space-time block code classification

paper_content:

This paper proposes a novel and efficient algorithm for space-time block code (STBC) classification, when a single antenna is employed at the receiver. The algorithm exploits the discriminating features provided by the discrete Fourier transform (DFT) of the fourth-order lag products (FOLPs) of the received signal. It does not require estimation of the channel, signal-to-noise ratio (SNR), and modulation of the transmitted signal. Computer simulations are conducted to evaluate the performance of the proposed algorithm. The results show the validity of the algorithm, its robustness to carrier frequency offset, and low sensitivity to timing offset.
---
paper_title: Gaussian Particle Filtering Approach for Carrier Frequency Offset Estimation in OFDM Systems

paper_content:

We propose Gaussian particle filtering (PF) approach for estimating carrier frequency offset (CFO) in OFDM systems. PF is more powerful especially for nonlinear problems where classical approaches (e.g., maximum likelihood estimators) may not show optimal performance. Standard PF undergoes the particle impoverishment (PI) problem resulting from resampling process for this static parameter (i.e., CFO) estimation. Gaussian PF (GPF) avoids the PI problem because resampling process is not needed in the algorithm. We show that GPF outperforms current approaches in this nonlinear estimation problem.
---
paper_title: Frame Synchronization of Coded Modulations in Time-Varying Channels via Per-Survivor Processing

paper_content:

In this letter, an optimum frame synchronizer is proposed for coded modulations in channels with uncertainties. Coded modulations include various frame synchronization scenarios, e.g., convolutionally coded transmissions and nonlinear modulations with memory. Frame synchronization is proposed as a maximum a posteriori probability estimation implemented using trellis path search for Markov chain decoding. In addition, time-varying uncertainties such as frequency offset and phase noise are jointly estimated via per-survivor processing as frame synchronization proceeds. The proposed frame synchronizer exploits the coding gain of coded modulations to achieve better performance than conventional frame synchronizers. We show that the resulting frame synchronizer consists of a correlation term and two data correction terms. Numerical results show that the proposed frame synchronizer is robust to uncertainties at the receiver and it exhibits improved performance.
---
paper_title: Timing Estimation and Resynchronization for Amplify-and-Forward Communication Systems

paper_content:

This paper proposes a general framework to effectively estimate the unknown timing and channel parameters, as well as design efficient timing resynchronization algorithms for asynchronous amplify-and-forward (AF) cooperative communication systems. In order to obtain reliable timing and channel parameters, a least squares (LS) estimator is proposed for initial estimation and an iterative maximum-likelihood (ML) estimator is derived to refine the LS estimates. Furthermore, a timing and channel uncertainty analysis based on the Crame?r-Rao bounds (CRB) is presented to provide insights into the system uncertainties resulted from estimation. Using the parameter estimates and uncertainty information in our analysis, timing resynchronization algorithms that are robust to estimation errors are designed jointly at the relays and the destination. The proposed framework is developed for different AF systems with varying degrees of timing misalignment and channel uncertainties and is numerically shown to provide excellent performances that approach the synchronized case with perfect channel information.
---
paper_title: Second-Order Cyclostationarity of Mobile WiMAX and LTE OFDM Signals and Application to Spectrum Awareness in Cognitive Radio Systems

paper_content:

Spectrum sensing and awareness are challenging requirements in cognitive radio (CR). To adequately adapt to the changing radio environment, it is necessary for the CR to detect the presence and classify the on-the-air signals. The wireless industry has shown great interest in orthogonal frequency division multiplexing (OFDM) technology. Hence, classification of OFDM signals has been intensively researched recently. Generic signals have been mainly considered, and there is a need to investigate OFDM standard signals, and their specific discriminating features for classification. In this paper, realistic and comprehensive mathematical models of the OFDM-based mobile Worldwide Interoperability for Microwave Access (WiMAX) and third-Generation Partnership Project Long Term Evolution (3GPP LTE) signals are developed, and their second-order cyclostationarity is studied. Closed-from expressions for the cyclic autocorrelation function (CAF) and cycle frequencies (CFs) of both signal types are derived, based on which an algorithm is proposed for their classification. The proposed algorithm does not require carrier, waveform, and symbol timing recovery, and is immune to phase, frequency, and timing offsets. The classification performance of the algorithm is investigated versus signal-to-noise ratio (SNR), for diverse observation intervals and channel conditions. In addition, the computational complexity is explored versus the signal type. Simulation results show the efficiency of the algorithm is terms of classification performance, and the complexity study proves the real time applicability of the algorithm.
---
paper_title: Self-Cancellation of Sample Frequency Offset in OFDM Systems in the Presence of Carrier Frequency Offset

paper_content:

A self-cancellation scheme is proposed to cope with the sample frequency offset (SFO) problem in OFDM systems in the presence of a carrier frequency offset (CFO). Making use of the symmetry between the phase shifts caused by SFO and the subcarrier index, we put the same constellation symbol on symmetrical subcarriers, and combine the pairs at the receiver coherently. In this way, the SFO effects are approximately cancelled with the price of cutting down the bandwidth efficiency by half. However some array gain and diversity gain are obtained from the symmetrical combining. Our scheme can work well together with the phase tracking for residual CFO, so that both SFO and residual CFO can be removed with low complexity. Simulations show that, our scheme effectively removes the effect of SFO; the proposed system outperforms the ideal normal OFDM systems (with no SFO) under the same energy efficiency at high SNR, so the proposed system will also outperform the normal system that using the same overhead for SFO estimation. Finally, a mixed system is proposed to mitigate the drawbacks of our design and the normal OFDM systems for the SFO compensation. Our scheme may be helpful for the synchronization of multiple SFOs in the cooperative transmission.
---
paper_title: Joint estimation and suppression of phase noise and carrier frequency offset in multiple-input multiple-output single carrier frequency division multiple access with single-carrier space frequency block coding

paper_content:

Carrier frequency offset (CFO) and phase noise are challenging problems in single carrier frequency division multiple access (SC-FDMA) system. In this study, the authors have studied single-carrier space frequency block coding (SC-SFBC) to reduce the peak to average power ratio (PAPR) of the multiple-input multiple-output (MIMO) SC-FDMA signal, in which Alamouti SFBC will change the signal spectrum structure, break the single-carrier property and increase PAPR. Also, the authors propose a joint algorithm to suppress the inter-carrier interference (ICI) caused by phase noise and CFO. Conventional methods are to estimate phase noise and CFO in other algorithms, which are pretty difficult and complicated to obtain accurate estimation results. Unlike the conventional works, the novelty of the authors proposed algorithm is that it directly calculates the interference components and then reconstructs the ICI matrix. Thus, it avoids the degrading interactions between phase noise and CFO estimations. The proposed algorithm exploits block-type pilot, which is a common pilot pattern in SC-FDMA communications and is used in other wireless communication standards. Simulation results show that the suppression performance keeps smooth while phase noise and CFO varies, and BER performance degradation can be significantly reduced in 3 dB.
---
paper_title: Frequency Offset Estimation for Unknown QAM Constellations

paper_content:

We introduce a novel, both gain and signal-to-noise ratio independent, constellation unaware, blind frequency offset estimation procedure for QAM signals. Asymptotic performance analysis and numerical simulations show that the herein presented method outperforms a selected state of the art blind constellation unaware estimator, especially for cross constellations.
---
paper_title: A Robust Estimation of Residual Carrier Frequency Offset With I/Q Imbalance in OFDM Systems

paper_content:

In this paper, we focus on improving the accuracy of frequency offset estimation for an orthogonal frequency-division multiplexing (OFDM) system under the conditions of the joint impairments in both carrier frequency offset (CFO) and in-phase/quadrature (I/Q) imbalance. To propose a robust CFO estimation scheme and to benchmark its performance, the performance of the conventional frequency estimation algorithm in the presence of I/Q imbalance is analyzed, and some modifications to the conventional estimation scheme are highlighted. We show via simulations that such a design achieves a remarkable robustness against I/Q imbalance; thereby, the proposed scheme can efficiently estimate the CFO, irrespective of the I/Q mismatch.
---
paper_title: Mixture Kalman filtering for joint carrier recovery and channel estimation in time-selective Rayleigh fading channels

paper_content:

This paper proposes a new blind algorithm, based on Mixture Kalman Filtering (MKF), for joint carrier recovery and channel estimation in time-selective Rayleigh fading channels. MKF is a powerful tool for estimating unknown parameters in non-linear, non-Gaussian, real-time applications. We use a combination of Kalman filtering and Sequential Monte Carlo Sampling to estimate the channel fading coefficients and joint posterior probability density of the unknown carrier offset and transmitted data respectively. We study the effect of Signal to Noise Ratio (SNR) and doppler shift on Mean Square Error (MSE) and Bit Error Rate (BER) performance of the proposed algorithm through computer simulations. The results show that BER of the proposed algorithm achieves the theoretical performance slope for the full acquisition range of normalized carrier frequency offset.
---
paper_title: Analysis and suppression of effects of CFO and phase noise in WFMT modulation for vehicular communications

paper_content:

We investigate the WFMT (wavelet filtered multitone) modulation scheme for vehicular system and describe the effect of the carrier frequency offset and phase noise on the performance of this scheme. WFMT modulation scheme is based on the wavelet theory and complex filter banks for synthesis and analysis of multi-channel signal. WFMT modulation scheme keeps on the advantage of filter banks system and decreases the system complexity and can be easily possible for the implementation of filter banks. In this paper, we compare ISI (inter symbol interference) and ICI distortions of the WFMT and vehicular OFDM system due to the CFO (carrier frequency offset) and phase noise. And we analyze the PAPR performance of these systems and show the BER performance curve in the HPA and ICI situation owing to the Doppler frequency shift and the frequency offset. Also, the performance of WFMT system versus vehicular OFDM is calculated to find the SNR degradation on DMT (discrete multitone)/OFDM and WFMT system with number of FFT points in the presence of ICI, ISI, phase noise, PAPR and Doppler Effect. In the simulation results, the PAPR performance of WFMT system becomes worse because of characteristic of wavelet component coefficients.
---
paper_title: Joint sector identity and integer part of carrier frequency offset detection by phase-difference in long term evolution cell search process

paper_content:

In the Long Term Evolution (LTE) system, the initial cell search needs not only to achieve the timing synchronisations of symbols, slots and frames, but also to detect the sector identity (ID), the cell ID group and the carrier frequency offset (CFO), which includes an integer part (ICFO) and a fractional part. To accomplish this cell search process, two synchronisation signals, the primary synchronisation signal (PSS) and the secondary synchronisation signal, are periodically broadcasted from base stations in the LTE system. The PSS is mainly used for sector ID detection. In this study, an innovative algorithm for detecting the sector ID and ICFO together via the PSS-matching process is presented. In the proposed scheme, the phases of the differential correlation instead of the absolute values are adopted to determine the ICFO. Furthermore, a new set of PSS is proposed in order to improve the success probability of ICFO detection. Compared with conventional schemes, the proposed scheme can detect both the sector ID and ICFO with higher accuracy and much lower complexity.
---
paper_title: Estimation of residual carrier and sampling frequency offsets in OFDM-SDMA uplink transmissions

paper_content:

This paper investigates the estimation of multiple residual carrier frequency offsets (RCFOs) and sampling frequency offsets (SFOs) in the uplink of a multiuser OFDM network with space division multiple access (SDMA). The proposed solutions are based on the presence of some dedicated pilot tones within the signal spectrum, which are traditionally employed in OFDM systems to track channel variations and residual frequency errors. The main obstacle is the large number of parameters involved in the estimation process, which makes the uplink synchronization a rather challenging task. A practical solution to this problem relies on the separation of the received signals before the estimation procedure is started. This way, the RCFOs and SFOs of different users are estimated independently with affordable complexity. We propose two alternative approaches for users' separation. The former is based on the use of orthogonal pilot sequences, while the latter exploits the distinct spatial signatures of the uplink signals. Computer simulations and theoretical analysis are employed to assess the performance of the proposed schemes and to make comparison with existing alternatives.
---
paper_title: Uplink single-carrier frequency division multiple access system with joint equalisation and carrier frequency offsets compensation

paper_content:

Similar to the orthogonal frequency division multiple access (OFDMA) system, the single-carrier frequency division multiple access (SC-FDMA) system also suffers from frequency mismatches between the transmitter and the receiver. As a result, in this system, the carrier frequency offsets (CFOs) disrupt the orthogonality between subcarriers and give rise to inter-carrier interference (ICI) and multiple access interference (MAI) among users. The authors present a new minimum mean square error (MMSE) equaliser, which jointly performs equalisation and carrier frequency offsets (CFOs) compensation. The mathematical expression of this equaliser has been derived taking into account the MAI and the channel noise. A low complexity implementation of the proposed equalisation scheme using a banded matrix approximation is presented here. From the obtained simulation results, the proposed equalisation scheme is able to enhance the performance of the SC-FDMA system, even in the presence of estimation errors.
---
paper_title: Hybrid time/frequency domain compensator for RF impairments in OFDM systems

paper_content:

I/Q signal processing based communication systems suffer from analog front-end (FE) imperfections such as in-phase and quadrature-phase (I/Q) imbalance and carrier frequency offset (CFO). These impairments are commonly encountered in all practical implementations, and severely degrade the obtainable link performance. Moreover, orthogonal frequency division multiplexing (OFDM)-based systems are particularly sensitive to radio frequency (RF) impairments. In this paper, we analyze the impact of transmitter and receiver I/Q imbalance together with channel distortion and CFO error on an ideal transmit signal, and propose low-complexity DSP algorithms and compensation structure for coping with such imperfections. Based on our proposed estimation/compensation structure, we are able to decouple the impairments and process them individually with rather low-complexity. More specifically, we first apply a blind algorithm for receiver I/Q imbalance compensation, followed by an efficient time domain CFO estimator and compensator. The transmitter I/Q imbalance and channel are then equalized jointly, in the frequency domain, with maximum-likelihood (ML) or zero-forcing (ZF) schemes, respectively. The applied algorithms are either blind working without aid of any training symbol or use only one OFDM symbol for impairments estimation, providing an efficient alternative solution with reduced complexity. The computer simulation results indicate a close to ideal performance of ZF scheme, and suggest that additional performance improvement due to frequency diversity can be obtained when ML estimation technique is employed.
---
paper_title: BER Analysis of Uplink OFDMA in the Presence of Carrier Frequency and Timing Offsets on Rician Fading Channels

paper_content:

In orthogonal frequency-division multiple access (OFDMA) on the uplink, the carrier frequency offsets (CFOs) and/or timing offsets (TOs) of other users with respect to a desired user can cause multiuser interference (MUI). Analytically evaluating the effect of these CFO/TO-induced MUI on the bit error rate (BER) performance is of interest. In this paper, we analyze the BER performance of uplink OFDMA in the presence of CFOs and TOs on Rician fading channels. A multicluster multipath channel model that is typical in indoor/ultrawideband and underwater acoustic channels is considered. Analytical BER expressions that quantify the degradation in BER due to the combined effect of both CFOs and TOs in uplink OFDMA with M-state quadrature amplitude modulation (QAM) are derived. Analytical and simulation BER results are shown to match very well. The derived BER expressions are shown to accurately quantify the performance degradation due to nonzero CFOs and TOs, which can serve as a useful tool in OFDMA system design.
---
paper_title: On Frequency Offset Estimation for OFDM

paper_content:

This paper presents a comparative study of Schmidl-Cox (SC) and Morelli-Mengali (MM) algorithms for frequency offset estimation in OFDM, along with a new least squares (LS) and a new modified SC algorithm. All algorithms have comparable accuracy approaching asymptotically the Cramer-Rao bound. The complexity of the LS algorithm is between O(N) and O(N log N) operations, where N is the length of the training sequence, while the complexity of the SC algorithm is between O(N log N) and O(N2) operations, and the complexity of the MM algorithm is O(N2) operations. The modified version of the SC algorithm requires only one training sequence as opposed to two required by the original SC algorithm, and significantly reduced O(N log N) complexity. The sensitivity of the three algorithms to quantization of the arg function (the argument of a complex number) is analyzed and quantified. The analysis and simulation results demonstrate that while all considered algorithms can be used with coarse quantization of the arg function, the LS algorithm is least affected and the SC algorithm is most affected by this quantization error.
---
paper_title: Iterative frequency-domain fractionally spaced receiver for zero-padded multi-carrier code division multiple access systems

paper_content:

In this study, the authors propose an improved frequency-domain fractionally spaced (FDFS) minimum mean square error (MMSE) receiver for zero-padded multi-carrier code division multiple access (MC-CDMA) systems when the guard interval is not enough to avoid the inter-symbol interference (ISI) caused by the multipath channel. The proposed novel iterative FDFS-based receivers firstly reconstruct the received symbol to reduce the ISI and then followed by the FDFS-based equalisers to minimise the effect of ISI and inter-carrier interference (ICI) caused by carrier frequency offset (CFO) and Doppler shifts. A few iterations are performed to achieve the expected bit error rate (BER) performance. To reduce the receiver complexity, the novel simplified diagonal FDFS-based receivers with a fixed noise variance are developed with slight performance degradation. The proposed iterative receivers have never been studied in the existing literature. Simulation results show that the proposed iterative FDFS-based receivers can significantly improve the BER performance of the conventional FDFS-MMSE receiver in severe multiple interferences environments caused by multipath, CFO and Doppler shift.
---
paper_title: An Adaptive Receiver Design for OFDM-Based Cooperative Relay Systems Using Conjugate Transmission

paper_content:

Cooperative relay systems have attracted much attention in wireless communications in recent years due to its various potentials in the enhancement of diversity, achievable rates and coverage range. By using spatially distributed relays capable of forwarding data through statistically independent links, cooperative relaying can fulfill conventional multiple-input multiple-output (MIMO) transmission in a more feasible way. It is more appealing to wireless high rate multimedia application to employ orthogonal frequency division multiplexing (OFDM) in cooperative relay systems, since OFDM has desirable properties for wireless transmission such as high bandwidth efficiency and resistance to the multipath delay spread. However, the performance of OFDM-based cooperative relay systems is still sensitive to the intercarrier interference (ICI) induced by the frequency offset. The ICI problem becomes more complicated since the signals forwarded by relays would suffer from statistically independent channel fadings and frequency offsets, which has rarely been dealt with in conventional ICI cancellation schemes. Therefore, the thesis first focuses on some existing ICI cancellation schemes based on the two-path conjugate transmission, including conjugate cancellation (CC), phase rotated conjugate cancellation (PRCC), and the adaptive receivers, which provide remarkable performances for OFDM systems not using relays. We point out the performance deficiency of the receiver designs in the above schemes when conjugate transmission is carried out in the two-relay cooperation scenario. Then, we develop an adaptive receiver that is suitable for OFDM-based cooperative relay systems based on conjugate transmission. In the proposed scheme, not only the phase rotations are applied, but also the amplitudes are adjusted on the two receiving paths. We provide theoretical derivation for the optimal values of both the phase rotations and the amplitude scale using criteria of maximizing the carrier-to-interference ratio and minimizing the ICI power, respectively. We also develop an adaptive process for updating the phase rotations and the amplitude scale using the normalized block least mean-squared (BLMS) algorithm to track channel and frequency offset variation under time-varying environments. Simulation results show that the proposed adaptive receiver is better than other related works for OFDM-based cooperative relaying systems. It is also demonstrated that the proposed scheme is robust against limited channel estimation errors
---
paper_title: Wireless Visions: A Look to the Future by the Fellows of the WWRF

paper_content:

In less than two decades, mobile communication has developed from a niche application to a mass-market high-tech product, having experienced an unprecedented growth, never achieved by any other technology, whether radio, television, or even the Internet. Thirteen well-known experts, all of them honored as WWRF Fellows, have been interviewed and shared their expertise and opinions on ten questions about the wireless future, as presented here. The answers span a wide field from air interfaces, networks, devices, applications, to new ways of interaction, to name a few. Although the ideas and views presented here are not one common vision, they should provide stimulating ideas and questions for future research, and it will be exciting to see how things are really going to develop. The Fellows' ideas also clearly show the fascination, impact, and opportunities wireless communications has and will have in the future.
---
paper_title: An average Cramer-Rao bound for frequency offset estimation in frequency-selective fading channels

paper_content:

Several variations of Cramer-Rao bounds for carrier frequency offset (CFO) estimation in frequency-selective fading channels have been used to benchmark practical estimators' performance or to design training signals for CFO estimation. Among them, the extended Miller-Chang bound (EMCB) provides a tighter bound than the CRB for locally unbiased estimators. However, there is no closed-form expression of the EMCB for the CFO estimation in frequency-selective fading channels with an arbitrary training signal. In this letter, we derive a closed-form exact average CRB (the EMCB) valid for any training signal and any signal structure for the CFO estimation over frequency-selective Rayleigh fading channels with uncorrelated or arbitrarily correlated taps. The accuracy and generality of the proposed average CRB expression, and its advantages over the existing expressions are corroborated by numerical and simulation results.
---
paper_title: A Comment on “A Blind OFDM Synchronization Algorithm Based on Cyclic Correlation”

paper_content:

This comment points out several errors in the above letter. The correct frequency offset estimator is then proposed. Monte Carlo simulation results validate our analytical observations.
---
paper_title: Full Diversity Space-Frequency Codes for Frequency Asynchronous Cooperative Relay Networks with Linear Receivers

paper_content:

In a previous work, we presented a technique that allows verifying the conformance between Java implementations and UML class diagrams, using design tests. While it allowed verifying conformance it does so in a sense that does not deal adequately with some design patterns. In such scenarios, there are semantic constraints among the involved elements that UML does not allow our technique to recognize them. Therefore, if one evolves the implementation violating the design pattern, the generated design tests will no longer be capable of detecting the design violation. To address this problem, we propose in this paper an approach based on: (1) UML profiles to explicitly tag UML incorporating design patterns; and (2) a set of design test templates able to recognize the appropriate implementation of these design patterns on Java code. We also present a prototype capable of automatically generating the design tests to verify the design patterns explicated by the UML profile.
---
paper_title: Adaptive Frequency-Domain RLS DFE for Uplink MIMO SC-FDMA

paper_content:

It is well known that, in the case of highly frequency-selective fading channels, the linear equalizer (LE) can suffer significant performance degradation compared with the decision feedback equalizer (DFE). In this paper, we develop a low-complexity adaptive frequency-domain DFE (AFD-DFE) for single-carrier frequency-division multiple-access (SC-FDMA) systems, where both the feedforward and feedback filters operate in the frequency domain and are adapted using the well-known block recursive least squares (RLS) algorithm. Since this DFE operates entirely in the frequency domain, the complexity of the block RLS algorithm can be substantially reduced when compared with its time-domain counterpart by exploiting a matrix structure in the frequency domain. Furthermore, we extend our formulation to multiple-input–multiple-output (MIMO) SC-FDMA systems, where we show that the AFD-DFE enjoys a significant reduction in computational complexity when compared with the frequency-domain nonadaptive DFE. Finally, extensive simulations are carried out to demonstrate the robustness of our proposed AFD-DFE to high Doppler and carrier frequency offset (CFO).
---
paper_title: Single-Carrier Systems With MMSE Linear Equalizers: Performance Degradation due to Channel and CFO Estimation Errors

paper_content:

We assess the impact of the channel and carrier frequency offset (CFO) estimation errors on the performance of single-carrier systems with MMSE linear equalizers. Performance degradation is caused by the fact that a mismatched MMSE linear equalizer is applied to channel output samples with imperfectly canceled CFO. We develop asymptotic expressions for the excess mean square error (EMSE) induced by the channel and CFO estimation errors. Under some realistic assumptions, we derive a simple EMSE approximation which reveals that performance degradation is mainly caused by the imperfectly canceled CFO. Furthermore, the EMSE is approximately proportional to the CFO estimation error variance, with the proportionality factor being independent of the training sequence. Thus, optimal training sequence (TS) design for CFO estimation is also highly relevant for joint channel and CFO estimation.
---
paper_title: Joint Bit and Power Loading Algorithm for OFDM Systems in the Presence of ICI

paper_content:

It is well known that orthogonal frequency division multiplexing (OFDM) is robust to frequency-selective fading in wireless channels. However, it is sensitive to carrier frequency offset (CFO) which results in inter-carrier interference (ICI) and then degrades the transmission performance significantly. In this paper, a resource allocation scheme for OFDM systems in the presence of ICI is proposed. The aim of the proposed scheme is to maximize the system throughput under a total transmission power constraint. By allocating the resource appropriately, the effect of ICI can be reduced without any ICI cancellation scheme at the receiver, resulting in the increase of system throughput. Numerical results show that the proposed scheme has better throughput as compared to the adaptive subcarrier bandwidth method with equal power allocation or power allocation method with conventional water-filling algorithm.
---
paper_title: Blind Maximum Likelihood Carrier Frequency Offset Estimation for OFDM With Multi-Antenna Receiver

paper_content:

In this paper, based on the maximum likelihood (ML) criterion, we propose a blind carrier frequency offset (CFO) estimation method for orthogonal frequency division multiplexing (OFDM) with multi-antenna receiver. We find that the blind ML solution in this situation is quite different from the case of single antenna receiver. As compared to the conventional MUSIC-like CFO searching algorithm, our proposed method not only has the advantage of being applicable to fully loaded systems, but also can achieve much better performance in the presence of null subcarriers. It is demonstrated that the proposed method also outperforms several existing estimators designed for multi-antenna receivers. The theoretical performance analysis and numerical results are provided, both of which demonstrate that the proposed method can achieve the Cramer-Rao bound (CRB) under the high signal-to-noise ratio (SNR) region.
---
paper_title: Joint Carrier Frequency Offset and Direction of Arrival Estimation via Hierarchical ESPRIT for Interleaved OFDMA/SDMA Uplink Systems

paper_content:

In this paper, we propose an efficient algorithm to jointly estimate the directions of arrival (DOAs) and carrier frequency offsets (CFOs) in interleaved orthogonal frequency division multiple access / space division multiple access (OFDMA/SDMA) uplink networks. The algorithm makes use of the signal structure by estimating the CFOs and DOAs in a hierarchical tree structure, in which two CFO estimations and one DOA estimation are employed alternatively. One special feature in the proposed algorithm is that the algorithm proceeds in a coarse-fine manner with temporal filtering or spatial beamforming being invoked between the parameter estimations to decompose the signals progressively into subgroups so as to enhance the estimation accuracy and lower the computational overhead. Simulations show that the proposed algorithm can provide satisfactory performance with increased channel capacity.
---
paper_title: A Novel Code-Aided Carrier Recovery Algorithm for Coded Systems

paper_content:

We present a novel code-aided carrier recovery method for low-density parity-check (LDPC) coded systems, which comprises two supporting algorithms: 1) a coarse synchronization by maximizing a cost function, the mean absolute value of the soft outputs of the LDPC decoder, followed by a simple interpolation operation to improve the estimation accuracy and 2) a fine synchronization based on the soft decisions produced by the LDPC decoder. With moderate computational complexity, this proposed algorithm is designed to synchronize signals with large carrier frequency offset and phase offset at low signal-to-noise ratio (SNR). When applied to the case of an 8-PSK system with (1944,972) LDPC code, the performance loss compared to the case of ideal synchronization is negligible.
---
paper_title: Effective Adaptive Iteration Algorithm for Frequency Tracking and Channel Estimation in OFDM Systems

paper_content:

For joint maximum-likelihood (ML) frequency tracking and channel estimation using orthogonal frequency-division multiplexing (OFDM) training blocks in OFDM communications over mobile wireless channels, a major difficulty is the local extrema or multiple-solution complication arising from the multidimensional log-likelihood function. To overcome this, we first obtain crude ML frequency-offset estimators using single-time-slot samples from the received time-domain OFDM block. These crude frequency estimators are shown to have unique closed-form solutions. We then optimally combine these crude frequency estimators in the linear-minimum-mean-square-error (LMMSE) sense for a more accurate solution. Finally, by alternatively updating the LMMSE frequency estimator and the ML channel estimator through adaptive iterations, we successfully avoid the use of a multidimensional log-likelihood function, hence obviating the complex task of global solution search and, meanwhile, achieve good estimation performance. Our estimators have mean square errors (MSEs) tightly close to Cramer-Rao bounds (CRBs) with a wide tracking range.
---
paper_title: MMSE-Based CFO Compensation for Uplink OFDMA Systems with Conjugate Gradient

paper_content:

In this paper, we present a low-complexity carrier frequency offset (CFO) compensation algorithm based on the minimum mean square error (MMSE) criterion for uplink orthogonal frequency division multiple access systems. CFO compensation with an MMSE filter generally requires an inverse operation on an interference matrix whose size equals the number of subcarriers. Thus, the computational complexity becomes prohibitively high when the number of subcarriers is large. To reduce the complexity, we employ the conjugate gradient (CG) method which iteratively finds the MMSE solution without the inverse operation. To demonstrate the efficacy of the CG method for our problem, we analyze the interference matrix and present several observations which provide insight on the iteration number required for convergence. The analysis indicates that for an interleaved carrier assignment scheme, the maximum iteration number for computing an exact solution is at most the same as the number of users. Moreover, for a general carrier assignment scheme, we show that the CG method can find a solution with far fewer iterations than the number of subcarriers. In addition, we propose a preconditioning technique which speeds up the convergence of the CG method at the expense of slightly increased complexity for each iteration. As a result, we show that the CFO can be compensated with substantially reduced computational complexity by applying the CG method.
---
paper_title: Blind iterative frequency offset estimator for orthogonal frequency division multiplexing systems

paper_content:

This study presents an iterative carrier frequency offset estimator for orthogonal frequency division multiplexing (OFDM) systems. The proposed estimator is based on the efficient Viterbi-and-Viterbi (VAV) algorithm. The proposed estimator is blind and can be used with non-constant modulus subcarrier modulations such as quadrature amplitude modulation (QAM). The performance of the proposed estimator is assessed theoretically and via Monte Carlo simulations over various channel models and compared to the performance of other well established blind techniques in addition to the Cramer–Rao lower bound. The comparison results demonstrate that the proposed estimator outperforms other well-established blind estimators by more than 12 dB at moderate and high signal-to-noise ratios (SNRs).
---
paper_title: Channel Estimation and Equalization for Asynchronous Single Frequency Networks

paper_content:

Single carrier frequency-domain equalization (SC-FDE) modulations are known to be suitable for broadband wireless communications due to their robustness against severe time-dispersion effects and the relatively low envelope fluctuations of the transmitted signals. In this paper, we consider the use of SC-FDE schemes in broadcasting systems. A single frequency network transmission is assumed, and we study the impact of distinct carrier frequency offset (CFO) between the local oscillator at each transmitter and the local oscillator at the receiver. We propose an efficient method for estimating the channel frequency response and CFO associated to each transmitter and propose receiver structures able to compensate the equivalent channel variations due to different CFO for different transmitters. Our performance results show that we can have excellent performance, even when transmitters have substantially different frequency offsets.
---
paper_title: Joint channel, phase noise, and carrier frequency offset estimation in cooperative OFDM systems

paper_content:

Cooperative communication systems employ cooperation among nodes in a wireless network to increase data throughput and robustness to signal fading. However, such advantages are only possible if there exist perfect synchronization among all nodes. Impairments like channel multipath, time varying phase noise (PHN) and carrier frequency offset (CFO) result in the loss of synchronization and diversity performance of cooperative communication systems. Joint estimation of these multiple impairments is necessary in order to correctly decode the received signal in cooperative systems. In this paper, we propose an iterative pilot-aided algorithm based on expectation conditional maximization (ECM) for joint estimation of multipath channels, Wiener PHNs, and CFOs in amplify-and-forward (AF) based cooperative orthogonal frequency division multiplexing (OFDM) system. Numerical results show that the proposed estimator achieves mean square error performance close to the derived hybrid Cramer-Rao lower bound (HCRB) for different PHN variances.
---
paper_title: Progressive Frequency-Offset Compensation in Turbo Receivers

paper_content:

Based on the recently-proposed iterative receiver by Colavolpe et al., we extend it to further incorporate the mechanism of frequency-offset compensation. As a progressive approach, the factor graph of a long frame is first divided into numerous subgraphs of short block, iterative decoding can successfully start its work as the effect of frequency-offset can be well mitigated due to the small size of subgraph. As the iteration goes on, the extrinsic information from decoder can be employed to get finer estimate of the frequency-offset. The system performance can be steadily improved as the block size is progressively expanded to the final frame size.
---
paper_title: Joint Semi-Blind Channel Estimation and Synchronization in Two-Way Relay Networks

paper_content:

In this paper, we propose a synchronization and channel estimation method for amplify-and-forward two-way relay networks (AF-TWRNs) based on a low-complexity maximum-likelihood (LCML) algorithm and a joint synchronization and channel estimation (JSCE) algorithm. For synchronous AF-TWRNs, the LCML algorithm blindly estimates general nonreciprocal flat-fading channels. We formulate the channel estimation as a convex optimization problem and obtain a closed-form channel estimator. Based on the mean square error (MSE) analysis of the LCML algorithm, we propose a generalized LCML (GLCML) algorithm to perform channel estimation in the presence of the timing offset. Based on the approximation of the LCML algorithm, the JSCE algorithm is proposed to estimate jointly the timing offset and channel parameters. The theoretical analysis shows that the closed-form LCML channel estimator is consistent and unbiased. The analytical MSE expression shows that the estimation error approaches zero in scenarios with either a high signal-to-noise ratio (SNR) or a large frame length. Monte Carlo simulations are employed to verify the theoretical MSE analysis of the LCML algorithm. In the absence of perfect timing synchronization, the GLCML algorithm selects an estimation sample, which produces the optimal channel estimation, according to the MSE analysis. Simulation results also demonstrate that the JSCE algorithm is able to achieve accurate timing offset estimation.
---
paper_title: Multi-tone CDMA design for arbitrary frequency offsets using orthogonal code multiplexing at the transmitter and a tunable receiver

paper_content:

The authors propose a new multi-tone (MT) code division multiple access (CDMA) design which has a superior bit error rate (BER) performance than conventional MT CDMA in the presence of frequency offset. The design involves multiplexing of Walsh codes onto the sub-carriers in conjunction with double differential modulation. To exploit the full potential of the design a partial correlation receiver has been proposed. Depending on the signal-to-noise ratio (SNR) and frequency offset it is possible to tune this receiver for the best possible performance. The simulated BER performance of the proposed system has been found to be better than MT CDMA for small as well as large frequency offsets for both single and multi-user systems in additive white Gaussian noise (AWGN) and Rayleigh fading channels.
---
paper_title: Combined MMSE-FDE and Interference Cancellation for Uplink SC-FDMA with Carrier Frequency Offsets

paper_content:

Due to its lower peak-to-average power ratio (PAPR) compared with orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA) has been recently accepted as the uplink multiple access scheme in the Long Term Evolution (LTE) of cellular systems by the Third Generation Partnership Project (3GPP). However, similar to OFDMA, carrier frequency offset (CFO) can destroy the orthogonality among subcarriers and degrade the performance of SC-FDMA. To mitigate the effect of CFOs, we propose a combined minimum mean square error frequencydomain equalization (MMSE-FDE) and interference cancellation scheme. In this scheme, joint FDE with CFO compensation (JFC) is utilized to obtain the initial estimation for each user. In contrast to previous schemes, where the FDE and CFO compensation are done separately, in JFC, the MMSE FDE is designed to suppress the MUI after CFO compensation. To further eliminate the MUI, we combine JFC with parallel interference cancellation (PIC). In particular, we iteratively design the MMSE FDE equalizer to suppress the remaining MUI at each stage and obtain better estimation. Simulation results show that the proposed scheme can significantly improve the system performance.
---
paper_title: Maximum Likelihood Frequency Offset Estimation in Multiple Access Time-Hopping UWB

paper_content:

Frequency offset estimation for time-hopping (TH) ultra-wide-band (UWB) is addressed in the literature by relying on an AWGN assumption and by exploiting a periodic preamble appended to each packet. In this paper we generalize these techniques with two aims. First, we identify a solution which does not rely on any periodic structure, but can be implemented with a generic TH format. Second, we identify a solution which is robust to multiple access interference (MAI) by assuming a Gaussian mixture (GM) model for MAI. In fact, GMs have recently been identified as good descriptors of UWB interference, and they provide closed form and limited complexity results. With these ideas in mind, we build a data aided maximum likelihood (ML) estimator. The proposed ML solution shows quasi optimum performance in the Cramer-Rao bound sense, and proves to be robust in meaningful multiple user scenarios.
---
paper_title: Accurate Two-Stage Frequency Offset Estimation for Coherent Optical Systems

paper_content:

We present a two-stage feedforward frequency offset estimation (FOE) algorithm on the basis of the phase-difference method. The first stage of the proposed FOE generates a rough FOE through averaging the phase difference between two adjacent symbols and compensates the signals. The second stage of the FOE estimates the residual FO through averaging the phase difference of two symbols with distance L , which provides a significantly larger noise-tolerance and thus improves system performance. The proposed FOE can be applied to both quadrature phase-shift keying and 16-quadrature amplitude modulation systems. Simulations show that the mean square error of the proposed FOE can be reduced through an order of two in magnitude under small laser linewidth situation compared with the classic phase-difference-based FOE. The experimental result shows that the proposed FOE gives 0.6-dB optical-signal-to-noise-ratio improvement against the classic FOE, which validates its usefulness.
---
paper_title: Blind Carrier Frequency Offset Estimation for Interleaved OFDMA Uplink

paper_content:

In this paper, we develop two novel blind carrier frequency offset (CFO) estimators for interleaved orthogonal frequency division multiple access (OFDMA) uplink transmission in a multiantenna system. The first estimator is the subspace-based one and could blindly estimate multiple CFOs from a rank reduction approach. The second estimator is based on the maximum likelihood (ML) approach and improves the performance as compared to the first one. The higher computational complexity of the ML estimator is alleviated by the alternating projection (AP) method. Both the proposed estimators support fully loaded data transmissions, i.e., all subcarriers being occupied, which provides higher bandwidth efficiency as compared to the existing schemes. The numerical results are then provided to corroborate the proposed studies.
---
paper_title: Practical Timing and Frequency Synchronization for OFDM-Based Cooperative Systems

paper_content:

In this paper, we investigate the timing and carrier frequency offset (CFO) synchronization problem in decode and forward cooperative systems operating over frequency selective channels. A training sequence which consists of one orthogonal frequency-division multiplexing (OFDM) block having a tile structure in the frequency domain is proposed to perform synchronization. Timing offsets are estimated using correlation-type algorithms. By inserting some null subcarriers in the proposed tile structure, we propose a computationally efficient subspace decomposition-based algorithm for CFO estimation. The issue of optimal tile length is studied both theoretically and through simulations. By judiciously designing the tile size of the pilot, the proposed algorithms are shown to have better performance, in terms of synchronization errors and bit error rate, than the time-division multiplexing-based training method and the computationally demanding space-alternating generalized expectation-maximization algorithm.
---
paper_title: A Low-Complexity ML Estimator for Carrier and Sampling Frequency Offsets in OFDM Systems

paper_content:

This letter considers the joint acquisition of the carrier frequency offset (CFO) and sampling frequency offset (SFO) in OFDM systems using two long training symbols in the preamble. Conventional maximum-likelihood (ML) methods require a two-dimensional exhaustive search. To overcome this problem, a low-complexity closed-form ML estimator is proposed. It is shown that the CFO can be solved in closed-form. Then we develop an approximate ML estimation algorithm for the SFO by taking the second-order Taylor series expansion. Simulation results show that the proposed algorithm achieves almost the same performance as existing ML methods, but no exhaustive search is needed.
---
paper_title: Low complexity LS and MMSE based CFO compensation techniques for the uplink of OFDMA systems

paper_content:

Orthogonal frequency division multiple access (OFDMA), where different subcarriers are allocated to different users, has been adopted for the uplink of several standards and has attracted a great deal of attention as a result. However, OFDMA is highly sensitive to carrier frequency offset (CFO) between the transmitter and receiver. In the uplink, different carrier frequency offsets due to different users can adversely affect subcarrier orthogonality. We propose a low complexity CFO compensation approach that addresses this problem while maintaining optimal performance. This approach is based on the least squares and minimum mean square error criteria applicable to interleaved and block interleaved carrier assignment schemes. The proposed algorithms use the special block circulant property of the interference matrix. In contrast to existing CFO compensation techniques, our algorithms do not rely on iterations or approximations. We present our approach in this paper and describe how a considerable reduction in computational complexity can be achieved by adopting it.
---
paper_title: A Time Domain Inverse Matrix Receiver for CFO Suppression in WIMAX Uplink System

paper_content:

In orthogonal frequency division multiple access (OFDMA) uplink system, orthogonal multiple subcarriers are assigned to different users for parallel high data rate communications. However, carrier frequency offset (CFO), which is mainly due to oscillator mismatches and/or Doppler shift, will destroy the orthogonality among subcarriers and introduce intercarrier interference (ICI) as well as multiple-access interference (MAI) in uplink scenario. Thus, system performance will be seriously degraded. To overcome this problem, it is of great importance to do research on suppression of the interferences caused by CFO. In this paper, we proposed a novel time domain inverse matrix receiver to suppress the interference of multiple CFOs. Compared with the conventional frequency domain direct ZF inverse matrix method, which has high complexity in obtaining the ICI matrix and its inverse matrix, the proposed method has very low complexity in obtaining the interference matrix. Furthermore, the signal after interference suppression is a frequency domain signal. Thus the receiver complexity can be simplified. Simulation results show that this algorithm has almost the same performance to the frequency domain direct ZF inverse matrix method.
---
paper_title: Energy efficient M2M signaling with enhanced gateway: Detection and offset compensation

paper_content:

Machine to machine (M2M) communication networks are designed to enable communications between an ultra low-power, low-cost wireless sensor motes and the unconstrained access network. Given such asymmetric constraints in the M2M communications, effective techniques should aim to minimize the overall energy consumed in the sensor mote. We consider an M2M network with an enhanced sensor gateway (ESG), where the ESG is designed to compensate for limitations at the sensor mote. We consider energy efficient transmission at the mote, and propose spreading codes as well as a low-complexity detector at the ESG. Subsequently, we specify the frequency offset problem at the sensor mote and propose algorithms to compensate for frequency offset at ESG. We observe that the proposed detection and offset compensation scheme can perform 0.6 dB worse than the optimal detector without frequency offset.
---
paper_title: Combined PA/NPA CFO Recovery for FBMC Transmissions over Doubly-Selective Fading Channels

paper_content:

This paper deals with carrier frequency offset recovery for burst-mode filter-bank multicarrier transmission over channels affected by severe time-frequency selective fading. Unlike previously proposed algorithms, whereby frequency is recovered either relying on known pilot symbols multiplexed with the data stream (pilot-aided, or PA, approach), or exploiting specific properties of the multicarrier signal structure in a non-pilot-aided (NPA) fashion, here we present and discuss an algorithm based on the maximum likelihood principle, which can be qualified as combined PA/NPA since it takes advantage both of pilot symbols and also indirectly of data symbols through knowledge and exploitation of their specific modulation format. The algorithm requires the availability of the statistical properties of channel fading up to second-order moments. It is shown that the above approach allows to improve on both estimation accuracy and frequency acquisition range of previously published schemes.
---
paper_title: Hard decision directed frequency tracking for OFDM on frequency selective channel

paper_content:

This paper presents a decision directed frequency tracking scheme to improve the spectrum efficiency of Orthogonal Frequency Division Modulation (OFDM) transmission for frequency selective channels. OFDM divides a broadband channel into parallel narrowband subchannels with different channel characteristics. The subcarriers with lower attenuation are selected to do the decision directed based frequency tracking. A phase error detection method is needed to keep the hard decision working stable during tracking. The algorithm ensures that the carrier frequency offset could be solved from the detected phase error. The Mean Square Error (MSE) estimate and the loop bandwidth are also presented. Simulations illustrate that this scheme has better MSE and bit error rate performance than the traditional scheme. Furthermore, the pilots are saved in this scheme, which lead to improved spectrum efficiency.
---
paper_title: Differential modulation for amplify-and-forward two-way relaying with carrier offsets

paper_content:

In this paper, differential modulation (DM) schemes, including single differential and double differential, are proposed for amplify-and-forward two-way relaying (TWR) networks with unknown channel state information (CSI) and carrier frequency offsets. Most existing work in TWR assumes perfect channel knowledge at all nodes and no carrier offsets. However, accurate CSI can be difficult to obtain for fast varying channels while increases computational complexity in channel estimation, and commonly existing carrier offsets can greatly degrade the system performance. Therefore, we propose two schemes to remove the effect of unknown frequency offsets for TWR networks, when neither the sources nor the relay has any knowledge of CSI. Simulation results show that the proposed differential modulation schemes are both effective in overcoming the impact of carrier offsets with linear computational complexity.
---
paper_title: Channel, Phase Noise, and Frequency Offset in OFDM Systems: Joint Estimation, Data Detection, and Hybrid Cramer-Rao Lower Bound

paper_content:

Oscillator phase noise (PHN) and carrier frequency offset (CFO) can adversely impact the performance of orthogonal frequency division multiplexing (OFDM) systems, since they can result in inter carrier interference and rotation of the signal constellation. In this paper, we propose an expectation conditional maximization (ECM) based algorithm for joint estimation of channel, PHN, and CFO in OFDM systems. We present the signal model for the estimation problem and derive the hybrid Cramer-Rao lower bound (HCRB) for the joint estimation problem. Next, we propose an iterative receiver based on an extended Kalman filter for joint data detection and PHN tracking. Numerical results show that, compared to existing algorithms, the performance of the proposed ECM-based estimator is closer to the derived HCRB and outperforms the existing estimation algorithms at moderate-to-high signal-to-noise ratio (SNR). In addition, the combined estimation algorithm and iterative receiver are more computationally efficient than existing algorithms and result in improved average uncoded and coded bit error rate (BER) performance.
---
paper_title: Eigenvalue-Based Spectrum Sensing of Orthogonal Space-Time Block Coded Signals

paper_content:

We consider spectrum sensing of signals encoded with an orthogonal space-time block code (OSTBC). We propose a CFAR detector based on knowledge of the eigenvalue multiplicities of the covariance matrix which are inherent owing to the OSTBC and derive theoretical performance bounds. In addition, we show that the proposed detector is robust to a carrier frequency offset, and propose a detector that deals with timing synchronization using the detector for the synchronized case as a building block. The proposed detectors are shown numerically to perform well.
---
paper_title: Design of Data-Aided SNR Estimator Robust to Frequency Offset for MPSK Signals

paper_content:

Data-aided (DA) signal-to-noise ratio (SNR) estimation is required especially at low SNR. The conventional maximum likelihood (ML) DA SNR estimator requires perfect carrier phase estimation and frequency recovery. In this paper, we propose a novel carrier frequency robust DA SNR estimator with its improved variant using autocorrelation of received MPSK symbols. Computer simulations are used to examine their performance in terms of mean estimation value (MEV) and normalized mean square error (NMSE). For the example system in simulations, the MEV of proposed estimator is accurate enough with normalized frequency error on the order of symbol rate. However, its NMSE can not reach DA normalized Cramer-Rao bound (NCRB) even with large observatory length, whereas its NMSE may perform a little worse at high SNR for short pilot symbols. On the other hand, fortunately the its improved variant can reach NCRB with enough pilot symbols. What's more, the proposed DA SNR estimators can operate under large frequency errors or before the frequency recovery unit with baud rate. The implementation complexity is also analyzed.
---
paper_title: ICI Analysis for FRFT-OFDM Systems to Frequency Offset in Time-Frequency Selective Fading Channels

paper_content:

This letter presents the effects of frequency offset to orthogonal frequency division multiplexing systems based on fractional Fourier transform (FRFT-OFDM) in time-frequency selective fading channels. FRFT-OFDM systems generalize the OFDM systems based on discrete Fourier transform by the deployment of FRFT. The expressions of signal-to-interference ratio (SIR) due to ICI are derived in different fading channels. In a flat channel, the performances of both systems are the same. In a frequency selective channel, the FRFT-OFDM systems have superior SIR performances by choosing the optimal fractional factor, when Doppler spread is comparable to the inverse of the symbol duration or carrier offset exists in the system.
---
paper_title: Blind carrier frequency offset estimation for interleaved orthogonal frequency division multiple access uplink with multi-antenna receiver: algorithms and performance analysis

paper_content:

In this study, for interleaved orthogonal frequency division multiple access uplink with multi-antenna receiver, we propose two generalised carrier frequency offset estimators which, respectively, exploit the subspace theory and maximum-likelihood (ML) criterion. We find that, as long as the numbers of multipaths from two of the users are smaller than the number of antennas at the receiver, both the proposed estimators support fully loaded transmissions. We also derive the theoretical performance lower bound for the proposed ML estimator. The numerical results are then provided, which corroborate the proposed studies.
---
paper_title: Frequency Offset Estimation with Increased Nyquist Frequency

paper_content:

In wireless communication systems, there is a need to estimate and compensate for frequency offsets. What is common to frequency offset estimators, is that they only can give correct results within a certain range, up to the Nyquist frequency. This makes a difference especially in OFDM systems with pilots only at certain symbols, such as LTE. In this paper we present an algorithm where the Nyquist frequency is increased, up to the same value as if pilots were present in all OFDM symbols. The algorithm is based on using two or more frequency offset estimators, with lower, but different, Nyquist frequencies.
---
paper_title: Effective Symbol Timing Recovery Based on Pilot-Aided Channel Estimation for MISO Transmission Mode of DVB-T2 System

paper_content:

This paper proposes an effective symbol timing recovery (STR) based on a pilot-aided channel impulse response (CIR) estimation for multi-input single-output (MISO) transmission mode of DVB-T2 system. In particular, this paper focuses on fine STR capable of resolving an ambiguity effect of the CIR which is caused by an inaccurate coarse symbol timing offset (STO). In the proposed fine STR, the CIR of the MISO channel is estimated after performing coarse STR. Then, the ambiguity of the CIR is investigated by categorizing it into four regions under the assumption of an inaccurate STO. Finally, accurate STO is estimated by changing the fast Fourier transform (FFT) window with respect to the ambiguity categorization. Performance evaluations are accomplished by comparing the proposed STR with the conventional STR in large delay channels.
---
paper_title: A Time-Domain Joint Estimation Algorithm for CFO and I/Q Imbalance in Wideband Direct-Conversion Receivers

paper_content:

Carrier frequency offset (CFO) and in-phase and quadrature-phase (I/Q) imbalance are two of the common front-end impairments in low-cost communication devices. It is known that CFO can cause significant performance degradation in multi-carrier modulation (MCM) systems. Also, the existence of the I/Q imbalance usually reduces the accuracy of CFO estimation. In this paper, we propose a new data-aided scheme for the joint estimation of CFO and I/Q imbalance using simple matrix formulation. The proposed algorithms utilize only the periodicity of the generalized periodic pilot (GPP). They do not need to know the channel impulse response and the exact values of the training sequence. Moreover, our method has a low complexity and its performance compares favorably with the existing methods.
---
paper_title: Universal-filtered multi-carrier technique for wireless systems beyond LTE

paper_content:

In this paper, we propose a multi-carrier transmission scheme to overcome the problem of intercarrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems. In the proposed scheme, called universal-filtered multi-carrier (UFMC), a filtering operation is applied to a group of consecutive subcarriers (e.g. a given allocation of a single user) in order to reduce out-of-band sidelobe levels and subsequently minimize the potential ICI between adjacent users in case of asynchronous transmissions. We consider a coordinated multi-point (CoMP) reception technique, where a number of base stations (BSs) send the received signals from user equipments (UEs) to a CoMP central unit (CCU) for joint detection and processing. We examine the impact of carrier frequency offset (CFO) on the performance of the proposed scheme and compare the results with the performance of cyclic prefix based orthogonal frequency division multiplexing (CP-OFDM) systems. We use computer experiments to illustrate the efficiency of the proposed multi-carrier scheme. The results indicate that the UFMC scheme outperforms the OFDM for both perfect and non-perfect frequency synchronization between the UEs and BSs.
---
paper_title: Resource Efficient Implementation of Low Power MB-OFDM PHY Baseband Modem With Highly Parallel Architecture

paper_content:

The multi-band orthogonal frequency-division multiplexing modem needs to process large amount of computations in short time for support of high data rates, i.e., up to 480 Mbps. In order to satisfy the performance requirement while reducing power consumption, a multi-way parallel architecture has been proposed. But the use of the high degree parallel architecture would increase chip resource significantly, thus a resource efficient design is essential. In this paper, we introduce several novel optimization techniques for resource efficient implementation of the baseband modem which has highly, i.e., 8-way, parallel architecture, such as new processing structures for a (de)interleaver and a packet synchronizer and algorithm reconstruction for a carrier frequency offset compensator. Also, we describe how to efficiently design several other components. The detailed analysis shows that our optimization technique could reduce the gate count by 27.6% on average, while none of techniques degraded the overall system performance. With 0.18-μm CMOS process, the gate count and power consumption of the entire baseband modem were about 785 kgates and less than 381 mW at 66 MHz clock rate, respectively.
---
paper_title: A Novel Hierarchical Low Complexity Synchronization Method for OFDM Systems

paper_content:

A new hierarchical synchronization method is proposed for initial timing synchronization in orthogonal frequency-division multiplexing(OFDM) systems. Based on the proposal of new training symbol, a threshold based timing metric is designed for accurate estimation of start of OFDM symbol in a frequency selective channel. Threshold is defined in terms of noise distributions and false alarm, which makes it independent of type of channel it is applied. Frequency offset estimation is also done for the proposed training symbol. The performance of the proposed timing metric is evaluated using simulation results. The proposed method achieves low mean squared error(MSE) in timing offset estimation at five(5 x) times lower computational complexity compared to cross-correlation based method in a frequency selective channel. It is also computationally efficient compared to hybrid approaches for OFDM timing synchronization.
---
paper_title: Blind Identification of Spatial Multiplexing and Alamouti Space-Time Block Code via Kolmogorov-Smirnov (K-S) Test

paper_content:

A novel algorithm for blind identification of spatial multiplexing and Alamouti space-time block code is proposed in this paper. It relies on the Kolmogrov-Smirnov test, and employs the maximum distance between the empirical cumulative distribution functions of two statistics derived from the received signal. The proposed algorithm does not require estimation of the channel coefficients, noise statistics and modulation type, and is robust to the carrier frequency offset and impulsive noise. Additionally, it outperforms the algorithms in the literature under a variety of transmission impairments.
---
paper_title: Performance impact of asynchronous off-tone jamming attacks against OFDM

paper_content:

This paper presents power efficient asynchronous off-tone jamming attacks on orthogonal frequency division multiplexing (OFDM). Often wireless communication systems have to operate in environments, which are prone to unknown interference from adversaries such as jamming, which can lead to degradation of performance. This paper begins with presenting existing conventional jamming attacks, such as barrage jamming (BJ), partial band jamming (PBJ), single-tone jamming (STJ), and multi-tone jamming (MTJ) on OFDM and then proposes new power efficient jamming strategies. It is known that signal with frequency offset can affect more spectrum than the occupied bandwidth as the signal energy gets smeared into adjacent spectrum while performing FFT in the OFDM receiver, and thus create inter-channel interference (ICI). This paper builds on this idea, and presents new asynchronous single off-tone and multiple off-tone jamming attacks on OFDM. We first present our channel model, and then undertake an analysis of OFDM systems under off-tone jamming attacks, verifying the idea through simulation. Both analysis and simulation show that off-tone jamming attacks have more adverse effect than jammer aligned with the received signal.
---
paper_title: A Novel Doppler Frequency Offset Estimation Method for DVB-T System in HST Environment

paper_content:

A novel Doppler frequency offset estimation method is proposed for the digital video broadcasting-terrestrial (DVB-T) system in the high-speed-train (HST) environment. Based on the analysis of the effect of the scattered components of the wireless channel on the accuracy of the Doppler frequency offset estimation, we separate the path with the Line-of-Sight(LOS) component from the multipath channel, and only use it to estimate the frequency offset caused by Doppler shift. Simulation results show that the mean square error (MSE) of the proposed scheme is far better than those of the available schemes.
---
paper_title: SIR Analysis for OFDM Transmission in the Presence of CFO, Phase Noise and Doubly Selective Fading

paper_content:

This paper presents an analysis of the detrimental effects of carrier frequency offset (CFO), phase noise (PHN) and doubly-selective fading on orthogonal frequency division multiplexing (OFDM) transmission performance. In particular, we derive a closed-form expression for the signal-to-interference ratio (SIR) at an OFDM receiver in the presence of CFO, PHN and doubly-selective fading. Simulation and analytical results under various OFDM system settings are in good agreement and reveal the contributions of these channel impairments in the SIR degradation.
---
paper_title: Iterative Timing Recovery with Turbo Decoding at Very Low SNRs

paper_content:

Turbo codes are near Shannon limit channel codes widely used in space communication systems and so on at low signal-to-noise rate (SNR). Timing recovery is one of the key technologies for these systems to work effectively. In this paper, an efficiently iterative timing recovery with Turbo decoding is presented. By maximizing the sum of the square of soft decision metrics from Turbo decoding, it can obtain accurate timing acquisition. And a computation-efficient approximate gradient descent method is adopted to obtain rough estimate of timing offset. Another merit of it is that, by the proposed method, a rate-1/6 Turbo coded binary phase shift keying (BPSK) system can even work at very low SNR (Es/N0) about -7.44 dB without any pilot symbol. Finally, the whole timing recovery scheme is accomplished where the proposed method is combined with Mueller-Muller (M&M) timing recovery which performs the timing track. Simulation results indicate that the Turbo coded BPSK system with rather large timing errors by the proposed scheme can achieve performance within 0.1 dB of the ideal code with reasonable computations and storages.
---
paper_title: Simplified Multiaccess Interference Reduction for MC-CDMA With Carrier Frequency Offsets

paper_content:

Multicarrier code-division multiple-access (MC-CDMA) system performance can severely be degraded by multiaccess interference (MAI) due to the carrier frequency offset (CFO). We argue that MAI can more easily be reduced by employing complex carrier interferometry (CI) codes. We consider the scenario with spread gain N, multipath length L, and N users, i.e., a fully loaded system. It is proved that, when CI codes are used, each user only needs to combat 2(L - 1) (rather than N - 1) interferers, even in the presence of CFO. It is shown that this property of MC-CDMA with CI codes in a CFO channel can be exploited to simplify three multiuser detectors, namely, parallel interference cancellation (PIC), maximum-likelihood, and decorrelating multiuser detectors. The bit-error probability (BEP) for MC-CDMA with binary phase-shift keying (BPSK) modulation and single-stage PIC and an upper bound for the minimum error probability are derived. Finally, simulation results are given to corroborate theoretical results.
---
paper_title: Interference Analysis in Time and Frequency Asynchronous Network MIMO OFDM Systems

paper_content:

It is well known that symbol timing offsets larger than the cyclic prefix as well as carrier frequency offsets between transmitter and receiver stations destroy the orthogonality among OFDM subcarriers and induce additional interference. In conjunction with MIMO transmission on frequency selective fading channels where different users interfere with each other, these effects strongly degrades the signal detection performance. In this paper we consider fully asynchronous spatially multiplexed transmission with different symbol timing and carrier frequency offsets on each transmitter-receiver link which appear in distributed MIMO systems with multiple users and base stations. We derive a factorized system model for signal transmission in frequency domain where the different effects of inter-carrier, inter-symbol and inter-block interference are separated and analyzed in terms of signal-to-interference-noise-ratio degradation. Finally, we evaluate the interference levels at a receiver station for different link-level as well as system-level simulation setups.
---
paper_title: Physical-Layer Network Coding Using FSK Modulation under Frequency Offset

paper_content:

Physical-layer network coding is a protocol capable of increasing throughput over conventional relaying in the two- way relay channel, but is sensitive to phase and frequency offsets among transmitted signals. Modulation techniques which require no phase synchronization such as noncoherent FSK can compensate for phase offset, however, the relay receiver must still compensate for frequency offset. In this work, a soft- output noncoherent detector for the relay is derived, under the assumption that the source oscillators generating FSK tones lack frequency synchronization. The derived detector is shown through simulation to improve error rate performance over a conventional detector which does not model offset, for offset values on the order of a few hundredths of a fraction of FSK tone spacing.
---
paper_title: A New ML Detector for SIMO Systems with Imperfect Channel and Carrier Frequency Offset Estimation

paper_content:

The objective of this paper is to develop a new detection algorithm for single input multiple output (SIMO) systems, using maximum likelihood (ML) scheme. The proposed method takes into account both channel and carrier frequency offset (CFO) estimation errors for detection of the transmitted data. Simulation results show that the new algorithm improve performance in the presence of multiple estimation error variances as compared to the conventional method for different modulation schemes.
---
paper_title: A Novel Effective ICI Self-Cancellation Method

paper_content:

Orthogonal Frequency Division Multiplexing (OFDM) systems may suffer from both carrier frequency offset (CFO) and in-phase/quadrature-phase (I/Q) imbalance at the receiver front end. In this paper, a novel type of intercarrier interference (ICI) self-cancellation method using mirror mapping is proposed to combat these impairment. Based on the proposed novel method and by adopting two widely used mapping operations, we altogether derive two different mirror mapping schemes. Analysis and simulation results verify that the proposed mirror mapping schemes not only inherit the efficiency of the existing self-cancellation schemes in suppressing ICI caused by CFO but also have the ability of compressing or even eliminating IQ imbalance.
---
paper_title: Synchronization for TDS-OFDM over multipath fading channels

paper_content:

Time domain synchronous orthogonal frequency division multiplex (TDS-OFDM) is an effective multi-carrier modulation scheme to improve the spectrum efficiency. However, conventional synchronization algorithm based on the correlation property of the time-domain pseudo-noise (PN) sequence may not work well over severe multipath fading channels when large carrier frequency offset (CFO) exits. In this paper, a robust synchronization algorithm is proposed where the PN sequence in the frame head is considered as a cyclic prefix (CP) OFDM training symbol. The time-domain slide auto-correlation based on the cyclic structure of the PN sequence is adopted for timing and fine CFO estimation, while in the frequency domain, a differential correlation is applied for large CFO estimation. Simulations demonstrate that the proposed method could achieve the satisfactory performance over multipath fading channels. In addition, to reduce the implementation complexity, the frame head of the TDS-OFDM system is preferred to be binary PN sequences in the frequency domain with cyclic structure in the time domain.
---
paper_title: Blind carrier frequency offset estimation for tile-based orthogonal frequency division multiple access uplink with multi-antenna receiver

paper_content:

In this study, the authors propose a blind carrier frequency offset (CFO) estimation method for the tile structure orthogonal frequency division multiple access (OFDMA) uplink with multi-antenna receiver. They employ an iterative approach to extract the signal component for each user gradually and propose a carefully designed CFO estimator to update the CFO estimate during the iterative procedure. The key ingredient of the proposed method is using few subcarriers on both sides within each tile as the ‘guard subcarriers’, which can greatly mitigate the effect of multi-user interference. The proposed method supports not only fully loaded transmissions, but also the generalised assignment scheme that provides the flexibility for dynamical resource allocation. The numerical results are provided, which indicate that the proposed method can almost converge to the analytical lower bound within a few iterative cycles. It is seen that the proposed method also outperforms the existing competitor with multi-antenna receiver in terms of estimation performance, especially with few adopted blocks.
---
paper_title: Estimation, Training, and Effect of Timing Offsets in Distributed Cooperative Networks

paper_content:

Successful collaboration in cooperative networks require accurate estimation of multiple timing offsets. When combined with signal processing algorithms the estimated timing offsets can be applied to mitigate the resulting inter-symbol interference (ISI). This paper seeks to address timing synchronization in distributed multi-relay amplify-and-forward (AF) and decode-and-forward (DF) relaying networks, where timing offset estimation using a training sequence is analyzed. First, training sequence design guidelines are presented that are shown to result in improved estimation performance. Next, two iterative estimators are derived that can determine multiple timing offsets at the destination. The proposed estimators have a considerably lower computational complexity while numerical results demonstrate that they are accurate and reach or approach the Cramer-Rao lower bound (CRLB).
---
paper_title: Timing and Carrier Synchronization With Channel Estimation in Multi-Relay Cooperative Networks

paper_content:

Multiple distributed nodes in cooperative networks generally are subject to multiple carrier frequency offsets (MCFOs) and multiple timing offsets (MTOs), which result in time varying channels and erroneous decoding. This paper seeks to develop estimation and detection algorithms that enable cooperative communications for both decode-and-forward (DF) and amplify-and-forward (AF) relaying networks in the presence of MCFOs, MTOs, and unknown channel gains. A novel transceiver structure at the relays for achieving synchronization in AF-relaying networks is proposed. New exact closed-form expressions for the Cramer-Rao lower bounds (CRLBs) for the multi-parameter estimation problem are derived. Next, two iterative algorithms based on the expectation conditional maximization (ECM) and space-alternating generalized expectation-maximization (SAGE) algorithms are proposed for jointly estimating MCFOs, MTOs, and channel gains at the destination. Though the global convergence of the proposed ECM and SAGE estimators cannot be shown analytically, numerical simulations indicate that through appropriate initialization the proposed algorithms can estimate channel and synchronization impairments in a few iterations. Finally, a maximum likelihood (ML) decoder is devised for decoding the received signal at the destination in the presence of MCFOs and MTOs. Simulation results show that through the application of the proposed estimation and decoding methods, cooperative systems result in significant performance gains even in presence of impairments.
---
paper_title: Frequency estimation of single tone signals with bit transition

paper_content:

Frequency estimation of a single tone signal is important in many applications. In this study, a frequency estimation algorithm is proposed for single tone signals modulated with unknown data. The proposed unbiased frequency estimator (UFE) uses discrete Fourier transform (DFT) coefficients to calculate the accurate frequency offset. To achieve better performance in a noisy environment, a biased frequency estimator (BFE) is obtained by applying trigonometric approximation to UFE. The Taylor expansion of BFE is also proposed to meet the computational complexity requirement in some real-time applications. The performance of the proposed algorithm is evaluated through Monte Carlo simulations, compared with conventional interpolated DFT-based algorithm and typical non-data-aided frequency estimation algorithm.
---
paper_title: Quasi-maximum likelihood initial downlink synchronization for IEEE 802.16m

paper_content:

We consider initial downlink synchronization for the emerging IEEE 802.16m standard. Such synchronization involves estimation of the carrier frequency offset, signal timing, and the index of “Primary Advanced Preamble” (PA-Preamble) over a multipath channel. We first give a brief overview of some relevant signal properties in IEEE 802.16m. These properties are rather different from other recent standards to the point of needing a new synchronization algorithm. We develop a synchronization method based on the maximum likelihood (ML) principle. It turns out that the multipath channel response is also estimated along the way. The resulting method is considered quasi-ML because some approximations are used to simplify computation. Simulation results are presented to illustrate its performance.
---
paper_title: Are SC-FDE Systems Robust to CFO?

paper_content:

This paper investigates the impact of carrier frequency offset (CFO) on Single Carrier wireless communication systems with Frequency Domain Equalization (SC-FDE). We show that CFO in SC-FDE systems causes irrecoverable channel estimation error, which leads to inter-symbol-interference (ISI). The impact of CFO on SC-FDE and OFDM is compared in the presence of CFO and channel estimation errors. Closed form expressions of signal to interference and noise ratio (SINR) are derived for both systems, and verified by simulation results. We find that when channel estimation errors are considered, SC-FDE is similarly or even more sensitive to CFO, compared to OFDM. In particular, in SC-FDE systems, CFO mainly deteriorates the system performance via degrading the channel estimation. Both analytical and simulation results highlight the importance of accurate CFO estimation in SC-FDE systems.
---
paper_title: Joint Carrier Frequency Offset and Channel Estimation for OFDM Systems via the EM Algorithm in the Presence of Very High Mobility

paper_content:

In this paper, the problem of joint carrier frequency offset (CFO) and channel estimation for OFDM systems over the fast time-varying frequency-selective channel is explored within the framework of the expectation-maximization (EM) algorithm and parametric channel model. Assuming that the path delays are known, a novel iterative pilot-aided algorithm for joint estimation of the multipath Rayleigh channel complex gains (CG) and the carrier frequency offset (CFO) is introduced. Each CG time-variation, within one OFDM symbol, is approximated by a basis expansion model (BEM) representation. An autoregressive (AR) model is built to statistically characterize the variations of the BEM coefficients across the OFDM blocks. In addition to the algorithm, the derivation of the hybrid Cramer-Rao bound (HCRB) for CFO and CGs estimation in our context of very high mobility is provided. We show that the proposed EM has a lower computational complexity than the optimum maximum a posteriori estimator and yet incurs only an insignificant loss in performance.
---
paper_title: Timing, Carrier, and Frame Synchronization of Burst-Mode CPM

paper_content:

In this paper, we propose a complete synchronization algorithm for continuous phase modulation (CPM) signals in burst-mode transmission over additive white Gaussian noise (AWGN) channels. The timing and carrier recovery are performed through a data-aided (DA) maximum likelihood algorithm, which jointly estimates symbol timing, carrier phase, and frequency offsets based on an optimized synchronization preamble. Our algorithm estimates the frequency offset via a one-dimensional grid search, after which symbol timing and carrier phase are computed via simple closed-form expressions. The mean-square error (MSE) of the algorithm's estimates reveals that it performs very close to the theoretical Cramer-Rao bound (CRB) for various CPMs at signal-to-noise ratios (SNRs) as low as 0 dB. Furthermore, we present a frame synchronization algorithm that detects the arrival of bursts and estimates the start-of-signal. We simulate the performance of the frame synchronization algorithm along with the timing and carrier recovery algorithm. The bit error rate results demonstrate near ideal synchronization performance for low SNRs and short preambles.
---
paper_title: Decentralised ranging method for orthogonal frequency division multiple access systems with amplify-and-forward relays

paper_content:

In this study, a decentralised ranging method for uplink orthogonal frequency division multiple access (OFDMA) systems with half-duplex (HD) amplify-and-forward (AF) relay stations (RSs) is proposed. In the OFDMA systems with HD AF RSs, twice more resources and delays are required as ranging without RS. To reduce the required resources and delays for ranging, the authors propose a two-phase ranging scheme based on the decentralised timing-offset estimation at each ranging mobile station (MS). At the first phase, RS occasionally broadcasts timing reference signal, and at the second phase RS retransmits the collected ranging signals from the MSs. Then, each ranging MSs can individually estimate its own timing offset from the received signals. In the proposed ranging method, the base station does not need to send a timing-adjustment message, and the overhead associated with ranging in the downlink resources, and computational complexity can be significantly reduced without degrading the timing-offset-estimation performance. Moreover, the delay associated with ranging can be maintained as same as ranging without RS.
---
paper_title: Maximum Likelihood Frequency Estimation and Preamble Identification in OFDMA-based WiMAX Systems

paper_content:

In multi-cellular WiMAX systems based on orthogonal frequency-division multiple-access (OFDMA), the training preamble is chosen from a set of known sequences so as to univocally identify the transmitting base station. Therefore, in addition to timing and frequency synchronization, preamble index identification is another fundamental task that a mobile terminal must successfully complete before establishing a communication link with the base station. In this work we investigate the joint maximum likelihood (ML) estimation of the carrier frequency offset (CFO) and preamble index in a multicarrier system compliant with the WiMAX specifications, and derive a novel expression of the relevant Cramer-Rao bound (CRB). Since the exact ML solution is prohibitively complex in its general formulation, suboptimal algorithms are developed which can provide a reasonable trade-off between estimation accuracy and processing load. Specifically, we show that the fractional CFO can be recovered by combining the ML estimator with an existing algorithm that attains the CRB in all practical scenarios. The integral CFO and preamble index are subsequently retrieved by a suitable approximation of their joint ML estimator. Compared to existing alternatives, the resulting scheme exhibits improved accuracy and reduced sensitivity to residual timing errors. The price for these advantages is a certain increase of the system complexity.
---
paper_title: Bounds and Algorithms for Multiple Frequency Offset Estimation in Cooperative Networks

paper_content:

The distributed nature of cooperative networks may result in multiple carrier frequency offsets (CFOs), which make the channels time varying and overshadow the diversity gains promised by collaborative communications. This paper seeks to address multiple CFO estimation using training sequences in space-division multiple access (SDMA) cooperative networks. The system model and CFO estimation problem for cases of both decode-and-forward (DF) and amplify-and-forward (AF) relaying are formulated and new closed-form expressions for the Cramer-Rao lower bound (CRLB) for both protocols are derived. The CRLBs are then applied in a novel way to formulate training sequence design guidelines and determine the effect of network protocol and topology on CFO estimation. Next, two computationally efficient iterative estimators are proposed that determine the CFOs from multiple simultaneously relaying nodes. The proposed algorithms reduce multiple CFO estimation complexity without sacrificing bandwidth and training performance. Unlike existing multiple CFO estimators, the proposed estimators are also accurate for both large and small CFO values. Numerical results show that the new methods outperform existing algorithms and reach or approach the CRLB at mid-to-high signal-to-noise ratio (SNR). When applied to system compensation, simulation results show that the proposed estimators significantly reduce average-bit-error-rate (ABER).
---
paper_title: Space-time coding for time and frequency asynchronous CoMP transmissions

paper_content:

This paper deals with time and frequency offset in coordinated multipoint (CoMP) transmission/reception networks by using distributed linear convolutional space-time coding (DLC-STC). We first prove that perfect time synchronization is impractical for CoMP transmission/reception networks. Then the DLC-STC scheme, in which exact time synchronization at the relay nodes is unnecessary, is proposed for the CoMP joint processing mode (CoMP-JP). Finally, we show the detecting method by minimum mean-squared error decision-feedback equalizer (MMSE-DFE) receivers with any frequency offsets. Simulation results show that with MMSE-DFE receivers, the proposed DLC-STC scheme outperforms the delay diversity scheme and the MMSE-DFE receivers can achieve the same diversity orders as the maximum likelihood sequence detection (MLSD) receivers.
---
paper_title: Low Complexity Pilot Assisted Carrier Frequency Offset Estimation for OFDMA Uplink Systems

paper_content:

In this letter, we propose a low complexity pilot aided carrier frequency offset (CFO) estimation algorithm for orthogonal frequency division multiplexing access (OFDMA) uplink systems based on two consecutive received OFDMA symbols. Assuming that the channels and the CFOs are static over the two consecutive symbols, we express the second received OFDMA symbol in terms of the CFOs and the first OFDMA symbol. Based on this signal model, a new estimation algorithm which obtains the CFOs by minimizing the mean square distance between the received OFDMA symbol and its regenerated signal is provided. Also, we implement the proposed algorithm via fast Fourier transform (FFT) operations by utilizing the block matrix inversion lemma and the conjugate gradient method. Simulation results show that the proposed algorithm approaches the average Cramer Rao bound for moderate and high signal to noise ratio (SNR) regions. Moreover, the algorithm can be applied for any carrier assignment schemes with low complexity.
---
paper_title: Preamble Design Using Embedded Signaling for OFDM Broadcast Systems Based on Reduced-Complexity Distance Detection

paper_content:

The second-generation digital terrestrial television broadcasting standard adopts the so-called P1 symbol as the preamble for initial synchronization. The P1 symbol also carries a number of basic transmission parameters, including the fast Fourier transform size and the single-input/single-output as well as multiple-input/single-output mode, to appropriately configure the receiver for carrying out the subsequent processing. In this paper, an improved preamble design is proposed, where a pair of training sequences is inserted in the frequency domain, and their distance is used for transmission parameter signaling. At the receiver, only a low-complexity correlator is required for the detection of the signaling. Both the coarse carrier frequency offset and the signaling can simultaneously be estimated by detecting the aforementioned correlation. Compared with the standardized P1 symbol, the proposed preamble design significantly reduces the complexity of the receiver while retaining high robustness in frequency-selective fading channels. Furthermore, we demonstrate that the proposed preamble design achieves better signaling performance than the standardized P1 symbol despite reducing the numbers of multiplications and additions by about 40% and 20%, respectively.
---
paper_title: Challenges in Reconfigurable Radio Transceivers and Application of Nonlinear Signal Processing for RF Impairment Mitigation

paper_content:

The design of compact and reconfigurable radio transceivers with low power consumption and low cost is a challenging task in future wireless communications systems. Transceiver architectures that are amenable to high-level integration will inevitably suffer from various radio frequency (RF) impairments, which limits the communications system performance and hence hinders their wide-spread use in commercial products. In this paper, we present the mitigation of RF impairments as a system identification problem. Four major classes of RF impairments are presented: power amplifier (PA) nonlinearity, in-phase/ quadrature (I/Q) impairments, group delay distortion, and carrier frequency offset and phase noise. Their models and up-todate identification techniques are described here. In particular, various nonlinear signal processing techniques that are effective in mitigating these impairments are also presented here. Theoretical and experimental results show that these mitigation techniques can significantly improve the communications system performance.
---
paper_title: H-inf channel estimation for MIMO-OFDM systems in the presence of carrier frequency offset

paper_content:

An H-infinity (H-inf) channel estimation algorithm is proposed for estimating the channels in MIMO-OFDM systems in the presence of carrier frequency offset (CFO). The goal is to contribute to an algorithm with low complexity, good estimate performance and better suppression to CFO. For this purpose, the H-inf with simplified objective function is first developed, and then, its computational load is reduced by using the iterative expectation maximization (EM) process. To resistant the CFO, we derive a precise equivalent signal model (ESM) to identify the channels. It is observed that the H-inf estimator could be regarded as a substitute for optimal maximum a posteriori (MAP) estimator, but with much less complexity. At the same time, by using ESM, a remarkable improvement for the performance degradation caused by CFO will appear.
---
paper_title: Joint estimation of CFO and receiver I/Q imbalance using virtual subcarriers for OFDM systems

paper_content:

In this paper, we study the estimation of carrier frequency offset (CFO) in the presence of the receiver in-phase and quadrature-phase (I/Q) imbalance for orthogonal frequency division multiplexing (OFDM) systems. By minimizing the energy of the samples on the virtual subcarriers, our proposed algorithm can jointly estimate the CFO and I/Q parameters using one OFDM block. When the CFO is small, a closed-form solution can be obtained. Simulation results show that our proposed method can provide a good performance for both of the I/Q and CFO estimation and it compares favorably with a recent method.
---
paper_title: A Low Complexity Equalization Method for Cooperative Communication Systems Based on Distributed Frequency-Domain Linear Convolutive Space-Frequency Codes

paper_content:

In this paper, we consider the cooperative communication system based on distributed frequency-domain linear convolutive space-frequency codes (FLC-SFC) with multiple carrier frequency offsets (CFOs) when the channels from relay nodes to destination node are flat fading. Through the mathematical derivation, the cooperative system model is simplified. Then, the equivalent banded system model is obtained and the related analysis shows the banded property of the equivalent channel matrix. Furthermore, a banded block minimum mean square error (MMSE) equalization method applying LDLH matrix decomposition is proposed for the banded system model. Besides, a special class of mask matrix is utilized to realize the banded operation. Compared with the traditional MMSE equalization method, the proposed equalization method has a relatively low computational complexity with satisfactory system performance.
---
paper_title: Multiplication-Free Estimation of Integer Frequency Offset for OFDM-Based DRM Systems

paper_content:

This letter proposes a low complexity integer frequency offset (IFO) estimation scheme in an orthogonal frequency division multiplexing (OFDM) based digital radio mondiale plus (DRM+) system with nonuniform phased pilot symbols. To reduce the computational complexity, the pilot symbols used for frequency estimation in the DRM+ system are partitioned into a number of pilot subsets, and pilots in each subset are phase-rotated to have a unique phase so that the estimator is made computationally efficient.
---
paper_title: Timing and Frequency Synchronization for Cooperative Relay Networks

paper_content:

This paper deals with timing and frequency synchronization in multi-relay cooperative networks operating with both large and small carrier frequency offset (CFO) over frequency-selective channels. A novel preamble based on constant amplitude zero auto-correlation (CAZAC) sequence is proposed, and a corresponding practical multistage scheme is presented. Joint timing and integral frequency synchronization is involved to resist multi-relay interference (MRI). Then, fractional frequency estimation is carried out, and fine timing estimation completes the synchronization scheme. The performance is evaluated in terms of the mean square error (MSE). Simulation results show that the method is robust under both flat-fading and multipath fading channels, and provides accurate estimation results in the presence of both large and small multiple CFO values.
---
paper_title: Ultra-Wideband TOA Estimation in the Presence of Clock Frequency Offset

paper_content:

The paper is concerned with the impact of clock frequency offsets on the accuracy of ranging systems based on time of arrival (TOA) measurements. It is shown that large TOA errors are incurred if the transmitter and receiver clocks are mistuned by more than just one part per million (ppm). This represents a serious obstacle to the use of commercial low-cost quartz oscillators, as they exhibit frequency drifts in the range of ± 10 ppm and more. A solution is to estimate first the transmitter clock frequency relative to the receiver's and then compensate for the difference by acting on the receiver clock tuning. An algorithm is proposed that estimates the transmitter clock frequency with an accuracy better than 0.1 ppm. Computer simulations indicate that its use in ranging systems makes TOA measurements as good as those obtained with perfectly synchronous clocks.
---
paper_title: Joint Maximum Likelihood Estimation of Carrier and Sampling Frequency Offsets for OFDM Systems

paper_content:

In orthogonal-frequency division multiplexing (OFDM) systems, carrier and sampling frequency offsets (CFO and SFO, respectively) can destroy the orthogonality of the subcarriers and degrade system performance. In the literature, Nguyen-Le, Le-Ngoc, and Ko proposed a simple maximum-likelihood (ML) scheme using two long training symbols for estimating the initial CFO and SFO of a recursive least-squares (RLS) estimation scheme. However, the results of Nguyen-Le's ML estimation show poor performance relative to the Cramer-Rao bound (CRB). In this paper, we extend Moose's CFO estimation algorithm to joint ML estimation of CFO and SFO using two long training symbols. In particular, we derive CRBs for the mean square errors (MSEs) of CFO and SFO estimation. Simulation results show that the proposed ML scheme provides better performance than Nguyen-Le's ML scheme.
---
paper_title: Coordinated Multi-Cell Systems: Carrier Frequency Offset Estimation and Correction

paper_content:

We consider a coordinated multi-cell (CMC) system and the associated problem of independent carrier frequency offsets (CFOs) at the basestations (BSs). These BS CFOs cause accumulated phase errors that compromise downlink beamforming accuracy, and consequently degrade the spectral efficiency of the CMC system. Since the inherent structure of coordinated downlink beamforming techniques makes it impossible to correct for the BS CFOs at the mobile subscriber (MS), our topic is estimation and correction of the BS CFOs at the BSs. Our method begins with the formation of MS-side estimates of the BS CFOs, which are then fed back to the coordinated BSs. We then derive an optimum maximum likelihood (ML) estimator for the BS CFOs that uses independent MS-side CFO estimates and average channel signal-to-noise ratios. However, it is demonstrated that the CFOs of the MSs themselves introduce a bias to the optimal BS CFO estimator. To compensate for this bias, a joint BS and MS CFO estimator is derived, but shown both to require high computation and have rank deficiency. This motivates an improved technique that removes the rank problem and employs successive estimation to reduce computation. It is demonstrated to overcome the MS CFO bias and efficiently solve the joint BS and MS CFO problem in systems that have low to moderate shadowing. We term the full BS CFO estimation and correction procedure "BS CFO tightening".
---
paper_title: Total Inter-Carrier Interference Cancellation for MC-CDMA System in Mobile Environment

paper_content:

Multi-carrier code division multiple access (MC-CDMA)has been considered as a strong candidate for next generation wireless communication system due to its excellent performance in multi-path fading channel and simple receiver structure. However, like all the multi-carrier transmission technologies such as OFDM, the inter-carrier interference (ICI) produced by the frequency offset between the transmitter and receiver local oscillators or by Doppler shift due to high mobility causes significant BER (bit error rate) performance degradation in MC-CDMA system. Many ICI cancellation methods such as windowing and frequency domain coding have been proposed in the literature to cancel ICI and improve the BER performance for multi-carrier transmission technologies. However, existing ICI cancellation methods do not cancel ICI entirely and the BER performance after ICI cancellation is still much worse than the BER performance of original system without ICI. Moreover, popular ICI cancellation methods like ICI self-cancellation reduce ICI at the price of lowering the transmission rate and reducing the bandwidth efficiency. Other frequency-domain coding methods do not reduce the data rate, but produce less reduction in ICI as well. In this paper, we propose a novel ICI cancellation scheme that can eliminate the ICI entirely and offer a MC-CDMA mobile system with the same BER performance of a MC-CDMA system without ICI. More importantly, the proposed ICI cancellation scheme (namely Total ICI Cancellation) does not lower the transmission rate or reduce the bandwidth efficiency. Specifically, by exploiting frequency offset quantization, the proposed scheme takes advantage of the orthogonality of the ICI matrix and offers perfect ICI cancellation and significant BER improvement at linearly growing cost. Simulation results in AWGN channel and multi-path fading channel confirm the excellent performance of the proposed Total ICI Cancellation scheme in the presence of frequency offset or time variations in the channel, outperforming existing ICI cancellation methods.
---
paper_title: Wide-range frequency offset estimation method for a DD-OFDM-PON downstream system

paper_content:

We propose a wide-range frequency offset estimation algorithm used in a direct detection orthogonal frequency division multiplexing passive optical network (DD-OFDM-PON) downstream system. Using this method, frequency offset is estimated according to the spectrum of the received DD-OFDM signal. The estimated frequency offset is used for frequency down-conversion in the digital domain to obtain the baseband OFDM signal. With this method, all data-bearing subcarriers should fall into the OFDM bandwidth after frequency down-conversion. The validity of the method is confirmed by an experiment with 20 Gb/s OFDM-PON downstream transmission.
---
paper_title: Coarse frame synchronization for OFDM systems using SNR estimation

paper_content:

In wireless orthogonal frequency division multiplexing (OFDM) systems, coarse frame synchronization and signal-to-noise ratio (SNR) values are two crucial parameters for OFDM receivers. In this paper, we develop a coarse timing estimator to overcome the plateau phenomenon by using delayed correlation between two estimated SNR values. The training symbol is designed for multiple pieces instead of two parts to mitigate the Doppler effect for SNR estimation. Compared with the Minn's scheme, our proposed scheme has smaller estimation variance both in EVA and ETU channels, which can also provide an estimated SNR value in the time domain. The obtained SNR value can be further reused to carrier frequency offset (CFO) estimation or decoding for system performance improvement.
---
paper_title: High resolution range profile analysis based on multicarrier phase-coded waveforms of OFDM radar

paper_content:

Orthogonal frequency division multiplexing (OFDM) radar with multicarrier phase-coded waveforms has been recently introduced to achieve high range resolution. The conventional method for obtaining the high resolution range profile (HRRP) is based on matched filters. A method of synthesizing HRRP based on the fast Fourier transform (FFT) and decoding is proposed. The mathematical expressions of HRRP are derived by assuming an elementary scenario of point-scattering targets. Based on the characteristic of OFDM multicarrier signals, it mainly analyzes the influence on HRRP exerted by several factors, such as velocity compensation errors, the sampling frequency offset, and so on. The conclusions are significant for the design of the OFDM imaging radar. Finally, the simulation results demonstrate the validity of the conclusions.
---
paper_title: Robust Timing Estimation Method for OFDM Systems With Reduced Complexity

paper_content:

As a modification of Hamed and Mahrokh's (HM) method, a preamble based timing offset estimation method for orthogonal frequency division multiplexing (OFDM) systems is presented. To make the estimator simpler in computation and keep its immunity to carrier frequency offset (CFO), the local autocorrelation sequence of the known preamble is mapped into an elaborately simplified one. The performance of the proposed estimator is evaluated by mean square error (MSE). Computer simulations under three different Rayleigh fading channels show that the proposed method achieves the same performance as HM's method with only half of its computational complexity.
---
paper_title: Time and frequency synchronisation scheme for orthogonal frequency division multiplexing-based cooperative systems

paper_content:

In the cooperative relay system, different frames from different relays are not aligned in the time domain, and have different carrier frequency offsets (CFOs). Estimation of these timing offsets and CFOs is a crucial and challenging task. In this study, a new training structure is proposed, and a novel time and frequency synchronisation scheme is presented for orthogonal frequency division multiplexing-based (OFDM-based) cooperative systems. The coarse time synchronisation is accomplished by using the symmetric conjugate of the training symbol and the fine time synchronisation is accomplished by segment-moving correlation. The fractional frequency offset is estimated by using the phase difference of the received signal and the integral frequency offset is estimated by utilising the good autocorrelation of the training symbol in frequency domain. The analysis and the simulation results show that, compared with the traditional scheme, the proposed scheme has better timing and frequency offset estimation performance and lower complexity for OFDM-based cooperative systems.
---
paper_title: Improved least squares channel estimation for orthogonal frequency division multiplexing

paper_content:

The authors consider the design of pilot sequences for channel estimation in the presence of carrier frequency offset (CFO) in systems that employ orthogonal frequency division multiplexing (OFDM). The CFO introduces intercarrier interference (ICI) which degrades the accuracy of the channel estimation. In order to minimise this effect, the authors design pilot sequence that minimises the mean square error (MSE) of the modified least squares (mLS) channel estimator. Since the identical pilot sequence, which minimises this MSE, has high peak-to-average power ratio of the OFDM signal, an alternative approach is proposed for channel estimation. The authors first introduce a new estimator as an alternative to the mLS estimator and design a low PAPR pilot sequences tailored to this new estimator. They show that the proposed procedure completely eliminates the effect of the ICI on the channel estimate. They then extend their design of pilot sequences for realistic sparse channels. Both analytical and computer simulation results presented in this study demonstrate the superiority of the proposed approach over conventional methods for channel estimation in the presence of ICI.
---
paper_title: Trade off between Frequency Diversity and Robustness to Carrier Frequency Offset in Uplink OFDMA System

paper_content:

In this paper, we investigate the effect of subcarrier allocation on the CFO for uplink OFDMA systems. Carriers are allocated to the users in order to get maximum throughput by making use of the channel frequency diversity. But in systems with CFO, while allocating the carriers to the users, attention must be paid to the ICI resulting due to CFO. In this paper we propose a carrier allocation scheme that provides a good compromise between the throughput maximization and robustness to the CFO induced ICI for systems with and without channel state information (CSI).
---
paper_title: Frequency Offset Estimation in 3G LTE

paper_content:

3G Long Term Evolution (LTE) technology aims at addressing the increasing demand for mobile multimedia services in high user density areas while maintaining good performance in extreme channel conditions such as high mobility of High Speed Trains. This paper focuses on the latter aspect and compares different algorithms for the uplink frequency offset estimation in LTE Base Stations (eNodeB). A frequency-domain maximum-likelihood based solution is proposed, taking profit of the available interference-free OFDM symbols de-mapped (or de-multiplexed) at the output of the FFT of an OFDMA multi-user receiver. It is shown to outperform the state-of-the-art CP correlation approach on both link-level performance and complexity aspects.
---
paper_title: CFO Estimation and Compensation in SC-IFDMA Systems

paper_content:

Single carrier interleaved frequency division multiple access (SC-IFDMA) has been recently receiving much attention for uplink multiuser access in the next generation mobile systems because of its lower peak-to-average transmit power ratio (PAPR). In this paper, we investigate the effect of carrier frequency offset (CFO) on SC-IFDMA and propose a new low-complexity time domain linear CFO compensation (TD-LCC) scheme. The TD-LCC scheme can be combined with successive interference cancellation (SIC) to further improve the system performance. The combined method will be referred to as TD-CC-SIC. We shall study the use of user equipment (UE) ordering algorithms in our TD-CC-SIC scheme and propose both optimal and suboptimal ordering algorithms in the MMSE sense. We also analyze both the output SINR and the BER performance of the proposed TD-LCC and TD-CC-SIC schemes. Simulation results along with theoretical SINR and BER results will show that the proposed TD-LCC and TD-CC-SIC schemes greatly reduce the CFO effect on SC-IFDMA. We also propose a new blind CFO estimation scheme for SC-IFDMA systems when the numbers of subcarrier sets allocated to different UEs are not the same due to their traffic requirements. Compared to the conventional blind CFO estimation schemes, it is shown that by using a virtual UE concept, the proposed scheme does not have the CFO ambiguity problem, and in some cases can improve the throughput efficiency since it does not need to increase the length of cyclic prefix (CP).
---
paper_title: Particle Filters for Joint Timing and Carrier Estimation: Improved Resampling Guidelines and Weighted Bayesian Cramer-Rao Bounds

paper_content:

This paper proposes a framework for joint blind timing and carrier offset estimation and data detection using a Sequential Importance Sampling (SIS) particle filter in Additive White Gaussian Noise (AWGN) channels. We assume baud rate sampling and model the intractable posterior probability distribution functions for sampling timing and carrier offset particles using beta distributions. To enable the SIS approach to estimate static synchronization parameters, we propose new resampling guidelines for dealing with the degeneracy problem and fine tuning the estimated values. We derive the Weighted Bayesian Cramer Rao Bound (WBCRB) for joint timing and carrier offset estimation, which takes into account the prior distribution of the estimation parameters and is an accurate lower bound for all considered Signal to Noise Ratio (SNR) values. Simulation results are presented to corroborate that the Mean Square Error (MSE) performance of the proposed algorithm is close to optimal at higher SNR values (above 20 dB). In addition, the bit error rate performance approaches that of the perfectly synchronized case for small unknown carrier offsets and any unknown timing offset. The advantage of our particle filter algorithm, compared to existing techniques, is that it can work for the full range acquisition of carrier offsets.
---
paper_title: Analysis of the Frequency Offset Effect on Random Access Signals

paper_content:

Zadoff-Chu (ZC) sequences have been used as random access sequences in modern wireless communication systems, replacing the conventional pseudo-random-noise (PN) sequences due to their superior autocorrelation properties. An analytical framework quantifying the ZC sequence's performance and its fundamental limitation as a random access sequencein the presence of frequency offset between the transmitter and the receiver is introduced. We show that a ZC sequence's perfect autocorrelation properties can be severely impaired by the frequency offset thereby limiting the overall performance of the random access signals formed from these sequences. First, we derive the autocorrelation function of these random access sequences as a function of the frequency offset. Next, we introduce the concept of critical frequency offsets and the spectrum associated with a ZC sequence set to characterize the frequency offset properties of the random access signals. Finally, we demonstrate that the frequency offset immunity of a ZC sequence set can be controlled by shaping the spectrum of the ZC sequence set.
---
paper_title: Practical analysis of codebook design and frequency offset estimation for virtual-multiple-input-multipleoutput systems

paper_content:

A virtual-multiple-input-multiple-output (MIMO) wireless system using the receiver-side cooperation with the compress-and-forward (CF) protocol, is an alternative to a point-to-point MIMO system, when a single receiver is not equipped with multiple antennas. It is evident that the practicality of CF cooperation will be greatly enhanced if an efficient source coding technique can be used at the relay. It is even more desirable that CF cooperation should not be unduly sensitive to carrier frequency offsets (CFOs). This study presents a practical study of these two issues. Firstly, codebook designs of the Voronoi vector quantisation (VQ) and the tree-structure VQ (TSVQ) to enable CF cooperation at the relay are described. A comparison in terms of the codebook design and encoding complexity is analysed. It is shown that the TSVQ is much simpler to design and operate, and can achieve a favourable performance-complexity tradeoff. Furthermore, this study demonstrates that CFO can lead to significant performance degradation for the virtual-MIMO system. To overcome this, it is proposed to maintain clock synchronisation and jointly estimate the CFO between the relay and the destination. This approach is shown to provide a significant performance improvement.
---
paper_title: DSTBC based DF cooperative networks in the presence of timing and frequency offsets

paper_content:

In decode-and-forward (DF) relaying networks, the received signal at the destination may be affected by multiple impairments such as multiple channel gains, multiple timing offsets (MTOs), and multiple carrier frequency offsets (MCFOs). This paper proposes novel optimal and sub-optimal minimum mean-square error (MMSE) receiver designs at the destination node to detect the signal in the presence of these impairments. Distributed space-time block codes (DSTBCs) are used at the relays to achieve spatial diversity. The proposed sub-optimal receiver uses the estimated values of multiple channel gains, MTOs, and MCFOs, while the optimal receiver assumes perfect knowledge of these impairments at the destination and serves as a benchmark performance measure. To achieve robustness to estimation errors, the estimates statistical properties are exploited at the destination. Simulation results show that the proposed optimal and sub-optimal MMSE compensation receivers achieve full diversity gain in the presence of channel and synchronization impairments in DSTBC based DF cooperative networks.
---
paper_title: Comments on "Timing Estimation and Resynchronization for Amplify-and-Forward Communication Systems"

paper_content:

This comment first shows that the Cramer-Rao lower bound (CRLB) derivations in the above paper are not exact. In addition, contrary to the claims in the above paper, the assumptions of perfect timing offset estimation and matched-filtering at the relays affect the generality of the analytical results and cannot be justified.
---
paper_title: Three-Stage Treatment of TX/RX IQ Imbalance and Channel with CFO for SC-FDE Systems

paper_content:

Direct-conversion transceiver obtains increasing attentions due to its low power consumption, but it confronts with serious IQ imbalance. In this paper, a three-stage treatment of transmitter/receiver (TX/RX) IQ imbalance and channel, in the presence of carrier frequency offset (CFO), is proposed for single-carrier-frequency-domain-equalization (SC-FDE) systems. First, RX IQ imbalance and CFO are handled with a repetitive preamble. Second, joint responses of TX IQ imbalance and channel are estimated by designing another preamble via complementary Golay codes. Third, joint compensation of TX IQ imbalance and time-varying channel is conducted using the minimal-mean-square-error criterion. Simulations are presented to verify the proposed method, in terms of normalized mean square error for the estimation and bit error rate for the overall system.
---
paper_title: Receiver Design for Single-Frequency Networks with Fast-Varying Channels

paper_content:

SC-FDE (Single Carrier Frequency-Domain Equalization) modulations are known to be suitable for broadband wireless communications due to their robustness against severe time-dispersion effects and the relatively low envelope fluctuations of the transmitted signals. In this paper we consider the use of SC-FDE schemes in broadcast and multicast systems with SFN (Single Frequency Network) operation where we do not have perfect carrier synchronization between different transmitters. We study the impact of different CFO (Carrier Frequency Offset) between the local oscillator at the receiver and the local oscillator at each transmitter. We also propose receiver structures able to reduce the performance degradation caused by different CFO at different transmitters. Our receivers can be regarded as modified turbo equalizer implemented in the frequency-domain, where a frequency offset compensation is performed before the iterative receiver.
---
paper_title: Two-Dimensional ESPRIT-Like Shift-Invariant TOA Estimation Algorithm Using Multi-Band Chirp Signals Robust to Carrier Frequency Offset

paper_content:

In this paper, a two-dimensional (2-D) ESPRIT-like shift-invariant time-of-arrival (TOA) estimation (ELSITE) algorithm for multi-band chirp signals in the presence of a carrier frequency offset (CFO) is presented. For the shift invariant TOA estimation, the received signals must be transformed into a sinusoidal form. When the received signals are perturbed by a CFO, the frequency of the transformed sinusoids is also shifted such that the TOA estimation results are biased. The TOA-induced phase shift of the multi-band chirp signals is determined according to the parameters of the signal, while the CFO-induced phase shift is only proportional to the elapsed time. Based on this property, the proposed ELSITE algorithm achieves robust TOA estimation against CFO from the signal subspace of the stacked matrix. The root mean square error of the proposed algorithm was analyzed and verified in both an AWGN channel and a multipath channel with CFO via Monte-Carlo simulations.
---
paper_title: ML Detection with Successive Group Interference Cancellation for Interleaved OFDMA Uplink

paper_content:

To mitigate the interference caused by multiple carrier frequency offsets (CFO) of distributed users, a maximum likelihood (ML) detection with group-wise successive interference cancellation (GSIC) is proposed for interleaved Orthogonal Frequency Division Multiplexing Access (OFDMA) uplink. By exploiting the interference characteristics and the finite alphabet property of transmitted symbols, the proposed scheme first extracts the block diagonal of the frequency domain channel matrix and then employs ML detection via sphere decoding in each block. The decisions obtained from the detected blocks are used to mitigate the interference to the residual blocks. Numerical results show that the proposed scheme outperforms both the minimum mean square error (MMSE) detection and parallel interference cancellation (PIC) with affordable computational complexity.
---
paper_title: Gain-Control-Free Blind Carrier Frequency Offset Acquisition for QAM Constellations

paper_content:

This paper introduces a novel blind frequency offset estimator for quadrature amplitude modulated (QAM) signals. Specifically, after a preliminary frequency compensation, the estimator is based on the ?/2-folded phase histogram of the received data. Then, the frequency offset estimate is taken as the frequency compensation value that minimizes the mean square error between the phase histogram measured on the received samples and the reference phase probability density function analytically calculated in the case of zero frequency offset. The ?/2 -folded phase histogram of the received data is here called Constellation Phase Signature, since it definitively characterizes the phase distribution of signal samples belonging to a particular QAM constellation, and it has already been employed to develop a gain-control-free phase estimator that well performs both for square and cross constellations. Also the here described frequency offset estimator has the remarkable property to be gain-control-free and, thus, it can be fruitfully employed in frequency acquisition stages. The asymptotic performance of the estimator has been analytically evaluated and assessed by numerical simulations. Theoretical analysis and numerical results show that the novel frequency offset estimator outperforms state-of-the art estimators in a wide range of signal-to-noise ratio (SNR) values.
---
paper_title: An efficient reduced-complexity two-stage differential sliding correlation approach for OFDM synchronization in the multipath channel

paper_content:

In this paper we propose a reduced-complexity two-stage time and frequency synchronization approach for OFDM systems, operating in multipath channels. The proposed approach exploits a single-symbol preamble with a repetitive structure, composed of two identical m-sequences. The first coarse stage, based on a sliding correlation, finds out the reduced uncertainty interval over which the second fine stage, based on a differential correlation, is performed. The combined use of the sliding correlation, characterized by its low complexity, and the differential correlation, which is much more complex, carried for a limited number of times results in an overall reduced complexity approach. For the time synchronization, the performance is evaluated in terms of correct detection rate of the frame start and the estimation variance. For the frequency synchronization, we focus on the fractional part of the frequency offset which is evaluated in terms of mean squared error. The simulation results prove that, compared to the considered benchmarks, the accuracy of the frame start detection and the fractional frequency offset estimation are greatly enhanced, even at very low SNRs. The proposed two-stage reduced-complexity approach is also compared to the single-stage brute-force approach, where differential correlation is exclusively used, to assess the performance degradation occasioned by the complexity reduction.
---
paper_title: Optimal Training Sequences for Joint Timing Synchronization and Channel Estimation in Distributed Communication Networks

paper_content:

For distributed multi-user and multi-relay cooperative networks, the received signal may be affected by multiple timing offsets (MTOs) and multiple channels that need to be jointly estimated for successful decoding at the receiver. This paper addresses the design of optimal training sequences for efficient estimation of MTOs and multiple channel parameters. A new hybrid Cramer-Rao lower bound (HCRB) for joint estimation of MTOs and channels is derived. Subsequently, by minimizing the derived HCRB as a function of training sequences, three training sequence design guidelines are derived and according to these guidelines, two training sequences are proposed. In order to show that the proposed design guidelines also improve estimation accuracy, the conditional Cramer-Rao lower bound (ECRB), which is a tighter lower bound on the estimation accuracy compared to the HCRB, is also derived. Numerical results show that the proposed training sequence design guidelines not only lower the HCRB, but they also lower the ECRB and the mean-square error of the proposed maximum a posteriori estimator. Moreover, extensive simulations demonstrate that application of the proposed training sequences significantly lowers the bit-error rate performance of multi-relay cooperative networks when compared to training sequences that violate these design guidelines.
---
paper_title: Cooperative Space-Time Coded OFDM with Timing Errors and Carrier Frequency Offsets

paper_content:

The use of distributed space-time codes in cooperative communications promises to increase the rate and reliability of data transmission. These gains were mostly demonstrated for ideal scenarios, where all nodes are perfectly synchronized. Considering a cooperative uplink scenario with asynchronous nodes, the system suffers from two effects: timing errors and individual carrier frequency offsets. In effect, timing errors can completely cancel the advantages introduced by space-time codes, while individual carrier frequency offsets provide a great challenge to receivers. Indeed, frequency offsets are perceived as a time-variant channel (even if the individual links are static) in distributed cooperative communications. We show that using OFDM, space-time codes (STCs) become tolerant to timing errors. Channel estimation and tracking takes care of frequency offsets. Our simulations demonstrate that the bit error rate performance improves by an order of magnitude, when using a cooperative system design, which takes these two effects into account.
---
paper_title: Compensation of multiple carrier frequency offsets in amplify-and-forward cooperative networks

paper_content:

In this paper, we propose a method to find an optimal correction value for multiple carrier frequency offsets (CFO) compensation, where two received signals have different path gains, in orthogonal frequency division multiplexing (OFDM) systems. Multiple CFOs are occurred when two spatially-separated transmitters are used to transmit the same signal simultaneously. In this case, the sidelobes of the subcarrier spectrums of the undesired signals have similar magnitude with opposite signs. Based on this fact, we propose a self-cancellation method of intercarrier interference caused by multiple CFOs. In the proposed scheme, we compensate the multiple CFOs for the signal to intercarrier interference power ratio of the received signal to be maximized, instead only the signal power is maximized in the ordinary methods.
---
paper_title: Improved Code-Aided Symbol Timing Recovery with Large Estimation Range for LDPC-Coded Systems

paper_content:

We present an improved code-aided symbol timing recovery method for low-density parity-check (LDPC) coded systems, which comprises two supporting algorithms: 1) a coarse estimation by utilizing a cost function, the mean absolute value of soft output of the LDPC decoder and 2) a fine estimation based on the Expectation-Maximization (EM) algorithm with a simple maximization step. With low computational complexity, this proposed algorithm can achieve symbol timing recovery at low signal-to-noise ratio (SNR) with large estimation range, eliminating the biased results of many algorithms for large timing offset values. Simulation results for the case of 8-PSK system with (1944,972) LDPC code show that the root-mean-square-error (RMSE) performance of the proposed algorithm approaches the modified Cramer-Rao lower bound (MCRB) at Eb/No=4dB, and the bit-error-rate (BER) curve is very close to that of ideal synchronization.
---
paper_title: Exact Signal Model and New Carrier Frequency Offset Compensation Scheme for OFDM

paper_content:

Carrier frequency offset (CFO) causes spectrum misalignment of transmitter and receiver filters. This misalignment leads to energy loss and distortion of received signal, resulting in performance degradation of the whole system. However, these issues are often overlooked by existing works. This paper presents an exact signal model for OFDM that takes into account the CFO-induced spectral misalignment and related aliasing effects. Incorporating these practical issues, we also propose a new preamble structure and a new CFO compensation approach to mitigate their negative effects. Theoretical analysis and simulation results substantiate the importance of the exact signal model and the advantage of the proposed CFO compensation scheme.
---
paper_title: Optimum Pilot Sequences for Data-Aided Synchronization

paper_content:

We compute the pilot sequences that minimize the Cramer-Rao bound(CRB) for carrier phase offset, carrier frequency offset, and symbol timing offset estimation. If we require the pilot sequence to be drawn from a discrete set of constellation points, then the alternating sequence is the CRB-minimizing sequence. If we relax the constellation constraint and apply a total energy constraint, we obtain three different sequences that minimize the CRB for each of the synchronization parameters. We show that the maximum-likelihood (ML) estimator achieves the CRB for any of the sequences presented. We also explore the impact of using the pilot symbols in a preamble with unknown data symbols immediately following. The interference caused by the unknown data symbols reduces the performance of the ML estimator beyond an SNR threshold.
---
paper_title: Low-Complexity Semiblind Multi-CFO Estimation and ICA-Based Equalization for CoMP OFDM Systems

paper_content:

We propose a low-complexity semiblind structure with multiple-carrier-frequency-offset (CFO) estimation and independent component analysis (ICA)-based equalization for multiuser coordinated multipoint (CoMP) orthogonal frequency-division-multiplexing (OFDM) systems. A short pilot is carefully designed for each user and has a twofold advantage. On the one hand, using the pilot structure, a complex multidimensional search for multiple CFOs is divided into a number of low-complexity monodimensional searches. On the other hand, the cross correlations between the transmitted and the received pilots are explored to allow simultaneous elimination of permutation ambiguity and quadrant ambiguity in the ICA equalized signals. Simulation results show that with a low training overhead of 1.6%, the proposed semiblind system not only outperforms the existing multi-CFO estimation schemes in terms of bit error rate (BER) and mean square error (MSE) of multi-CFO estimation but achieves a BER performance close to the ideal case with perfect channel state information (CSI) and no CFO at the receiver as well.
---
paper_title: Low-complexity frequency offset and phase noise estimation for burst-mode digital transmission

paper_content:

The presence of a frequency offset (FO) and phase noise can cause severe performance degradation in digital communication systems. This work combines a simple FO estimation technique with a low-complexity phase noise estimation method, inspired by the space-alternating generalized expectation-maximization algorithm. Using a truncated discrete-cosine transform (DCT) expansion, the phase noise estimate is derived from the estimated DCT coefficients of the phase. A number of implementations of the proposed algorithm are discussed. Numerical results indicate that when estimating the FO from pilot symbols only, comparable performance can be reached as the computationally more complex case where the FO is updated iteratively, with small convergence time. The phase noise estimation step is well capable of compensating for the residual FO. For the considered scenario, performing FO compensation before iterative phase noise estimation yields a bit-error rate performance degradation close to the case where the FO is known.
---
paper_title: Multiple carrier frequency offsets tracking in co-operative space-frequency block-coded orthogonal frequency division multiplexing systems

paper_content:

This study addresses the problem of carrier frequency offset (CFO) tracking in co-operative space-frequency block-coded orthogonal frequency division multiplexing (OFDM) systems with multiple CFOs. Considering that the inserted pilot tones are decayed by data subcarriers in the presence of multiple CFOs, a novel recursive residual CFO tracking (R-RCFOTr) algorithm is proposed. This method first removes CFO-induced inter-carrier interference from data subcarriers, and then updates the residual CFO (RCFO) estimation of each OFDM block recursively. When used in conjunction with a multiple CFOs estimator, the proposed R-RCFOTr can effectively mitigate the impacts from the multiple RCFOs with affordable complexity. Finally, simulation results are provided to validate the effectiveness of our proposed R-RCFOTr algorithm, which has performance close to that of perfect CFO estimation at moderate and high signal-to-noise ratio, and significantly outperforms conventional CFO tracking algorithm for large CFOs.
---
paper_title: Accurate Pilot-Aided Sampling Frequency Offset Estimation Scheme for DRM Broadcasting Systems

paper_content:

In this paper, we propose an improved sampling frequency offset (SFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) based digital radio mondiale (DRM) broadcasting systems. To demonstrate the performance of the proposed frequency estimator, analytical expression of the mean square error (MSE) is derived and performances are compared with conventional pilot-assisted estimators. Based on the simulation and theoretical analysis, our proposed estimator shows an improved estimation performance compared to the conventional estimators.
---
paper_title: Sequential Compensation of RF Impairments in OFDM Systems

paper_content:

Direct-conversion OFDM transceivers are seriously affected by front-end distortions like IQ imbalance and phase noise. Moreover, OFDM systems are sensitive to nonlinear distortions in power amplifiers and A/D converters, and to carrier frequency offset (CFO). We present a novel baseband compensation technique that compensates these main impairments in OFDM systems. Unlike compensation scheme that target a subset of impairments, our method jointly mitigates the effects of IQ imbalance, phase noise, carrier frequency offset and nonlinear distortion. Simulations of the proposed scheme with large levels of CFO, IQ imbalance and phase noise show an excellent performance in terms of bit error rate.
---
paper_title: Suppression of ICI and MAI in SC-FDMA communication system with carrier frequency offsets

paper_content:

Similar to other orthogonal frequency division multiplexing (OFDM)-based systems, carrier frequency offset (CFO) is a challenging problem in uplink communications of single carrier frequency division multiple access (SC-FDMA) system. It must be noticed that CFO, which is mainly due to oscillator instability and/or Doppler shift, would generate inter carrier interference (ICI) as well as multi-access interference (MAI) to disturb the received signals and seriously degrade system performance. Frequency synchronization in uplink communications is difficult because different users always experience different CFOs and one user's CFO correction would misalign other users. In this paper, we proposed a suppression method to overcome the multi CFOs problem. To implement this algorithm, block type pilots would be exploited, which is also utilized in LTE uplink standard. The proposed algorithm is based on the following two assumptions. Firstly the proposed algorithm is applied in this scenario, where different users start to communicate with the base station at different symbol periods, and secondly, during the pilot block and the following data blocks, the CFO of each user is quasi static, which is feasible since CFO is slow varying. Compared with other interference suppression methods, the proposed method could directly estimate the interference components from the inverse pilot matrix, thus it does not need to do the CFO estimation. Further more, since block type pilots is a common pilot pattern in wireless communication system, this algorithm can be easily extended to other communication system. Simulation results show that the proposed suppression algorithm can significantly improved system performance.
---
paper_title: A Novel Initial Cell Search Scheme in TD-LTE

paper_content:

In LTE system, in order to access the network, user equipment (UE) should detect the primary synchronization signal (PSS) and secondary synchronization signal (SSS) in downlink (DL) signal from the surrounding base stations (BS). This paper presents a novel initial cell search (ICS) scheme in TD-LTE system that contains two steps. In the first step, modified normalization based PSS detection is proposed to combat carrier frequency offset (CFO) and uplink (UL) interference. After CFO estimation and compensation, coherent SSS detection is adopted in frequency domain in the second period. Furthermore, in order to combat channel fading and noise, a method of flexible combination of PSS and SSS within several frames is proposed. Simulation results demonstrate that the proposed scheme is more robust and effective than conventional approaches in TDD system.
---
paper_title: Time and frequency offset estimation for distributed multiple-input multiple-output orthogonal frequency division multiplexing systems

paper_content:

This study addresses the problem of time and frequency offset estimation in the case of the delays, and the frequency offsets of all the transmitter/receiver pairs are different for distributed multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. A new training structure is designed and an efficient time and frequency offset estimator is proposed. With the symmetric conjugate property of the preamble, the timing metric proposed has an impulse shape, making the time offset estimation accurate. Using the repetition property of the preamble, iterative frequency offset estimation algorithm proposed can acquire a high precision and a wide estimation range. To distinguish different transmit antennas, the preamble is weighted by pseudo-noise (PN) sequences with low cross-correlations. The analysis and the simulation results show that the proposed estimator offers an accurate estimation on the different time delays and frequency offsets caused by the distributed transmitters of the MIMO-OFDM system. This is a situation that the conventional estimators cannot handle as far.
---
paper_title: Frequency Synchronization for the OFDMA Uplink Based on the Tile Structure of IEEE 802.16e

paper_content:

The multiple carrier frequency offsets (CFOs) of multiple users make frequency synchronization a challenging task in the orthogonal frequency-division multiple-access (OFDMA) uplink. In this paper, a computationally efficient iterative CFO estimation and compensation method for the OFDMA uplink with the generalized subcarrier allocation scheme (CAS) based on the tile structure of IEEE 802.16e is proposed. The proposed method only needs a few iteration cycles for its convergence. It greatly lowers the computational cost for the existing methods and can achieve better CFO estimation and compensation performance.
---
paper_title: Multiple Carrier Frequency Offset and Channel Estimation for Distributed Relay Networks

paper_content:

Cooperative communication using decode and forward (DF)-based distributed relays is one important solution to achieve better connectivity and higher data rates in wireless fading channels. In distributed relay networks, each relay has an independent local oscillator which when unsynchronized results in the presence of multiple carrier frequency offsets (CFOs) at the destination. The maximum likelihood estimator (MLE) for estimating the multiple CFOs requires a two dimensional grid search involving matrix inversion for each search point. Thus, the multiple CFO and channel estimation is computationally prohibitive and challenging in distributed relay networks. In this paper, we propose a simple estimator for multiple CFO and channel estimation at the destination by exploiting the fact that the relays receive information from a common source. Computer simulations show that the mean square error (MSE) performance of the proposed estimator is close to the Cramer-Rao lower bound for small carrier frequency synchronization errors at the relays.
---
paper_title: Frequency Offset Estimation in I/Q Mismatched OFDM Receivers

paper_content:

The direct-conversion architecture is a promising solution for the design of low-cost mobile terminals. However, it introduces extra RF impairments that greatly complicate fundamental receiver functions, including the synchronization task. This work deals with pilot-aided carrier frequency offset (CFO) recovery in an OFDM direct-conversion receiver plagued by frequency-selective I/Q imbalance. Since the exact maximum likelihood (ML) solution of this problem requires a complete search over the frequency uncertainty range, we propose a simpler scheme that dispenses from any peak-search procedure. Numerical simulations and theoretical analysis indicate that the proposed scheme attains the relevant Cramer-Rao bound at all signal-to-noise ratios of practical interest.
---
paper_title: Joint estimation of I/Q imbalance, CFO and channel response for MIMO OFDM systems

paper_content:

In this paper, we study the joint estimation of inphase and quadrature-phase (I/Q) imbalance, carrier frequency offset (CFO), and channel response for multiple-input multipleoutput (MIMO) orthogonal frequency division multiplexing (OFDM) systems using training sequences. A new concept called channel residual energy (CRE) is introduced. We show that by minimizing the CRE, we can jointly estimate the I/Q imbalance and CFO without knowing the channel response. The proposed method needs only one OFDM block for training and the training symbols can be arbitrary. Moreover when the training block consists of two repeated sequences, a low complexity two-step approach is proposed to solve the joint estimation problem. Simulation results show that the mean-squared error (MSE) of the proposed method is close to the Cramer-Rao bound (CRB).
---
paper_title: Low-complexity joint regularised equalisation and carrier frequency offsets compensation scheme for single-carrier frequency division multiple access system

paper_content:

Since the conventional zero-forcing receiver does not operate satisfactorily in interference-limited environments, because of its noise amplification; the statistics of the transmitted data and the additive noise are required for the minimum-mean-square error receiver; the potential of the regularised receiver is proposed and investigated in this study to cope with these problems. In this study, the authors introduce an efficient low-complexity joint regularised equalisation and carrier frequency offset compensation (LJREC) scheme for single-carrier frequency division multiple access system. The proposed LJREC scheme avoids the noise amplification problem and the estimation of the signal-to-noise ratio and the interference matrices of other users. From the obtained simulation results, the proposed scheme enhances the system performance with lower complexity and sufficient robustness to the estimation errors.
---
paper_title: Carrier Frequency Offset Estimation in OFDMA using Digital Filtering

paper_content:

This letter deals with the frequency synchronization problem in the uplink of OFDMA communications systems with interleaved or generalized subcarrier allocation. An algorithm to estimate the carrier frequency offsets (CFOs) of all the active users is presented. The estimator relies upon finding the zeros of a suitable designed filter. The design of the filter boils down to the solution of a least squares (LS) problem and thus entails relatively low complexity. While under low subcarrier load conditions the estimation root mean squared error (RMSE) is higher than that of other existing methods, under heavy or full load achieves comparable and even superior performance.
---
paper_title: Carrier Frequency Offset Estimation for Uplink OFDMA Using Partial FFT Demodulation

paper_content:

Fast and accurate Carrier Frequency Offset (CFO) estimation is a problem of significance in many multi-carrier modulation based systems, especially in uplink Orthogonal Frequency Division Multiple Access (OFDMA) where the presence of multiple users exacerbates the inter-carrier interference (ICI) and results in multi-user interference (MUI). In this paper, a new technique called partial FFT demodulation is proposed. Estimators for the CFO are derived by considering an approximated matched filter for each user, implemented efficiently using several FFTs operating on sub-intervals of an OFDM block. Through simulations, the feasibility and performance of the proposed estimators are demonstrated. Associated trade-offs are discussed.
---
paper_title: Blind CFO Estimation for Linearly Precoded OFDMA Uplink

paper_content:

This paper presents a novel method for blind carrier-frequency-offset (CFO) estimation in linearly precoded OFDMA uplink. Our investigation starts from the single-user equivalent (SUE) scenario presented in where active users in the network are categorized into a number of reference (synchronized) users (RUs) and a new (asynchronous) user (NU). The major idea is to take advantage of time correlation induced by the linear precoder, which offers a second-order moments-based blind CFO estimation. The precoder design is carefully performed in terms of CFO identifiability, estimation accuracy and overall system performance. In the multiuser scenario, where all users can be misaligned in frequency domain, the proposed CFO estimator is capable of mitigating a considerable portion of interference from neighboring users through exploitation of a novel time-frequency multiuser data-mapping (MU-DM) scheme. To demonstrate the multiuser interference (MUI)-resilience feature of proposed scheme, theoretical analysis is performed through derivation of approximate minimum-mean-square error (MMSE) in both SUE and multiuser scenarios. It is shown that by exploitation of the proposed MU-DM scheme, the approximate multiuser MMSE is very close to that of SUE scenario. Simulation results show that the proposed approach outperforms state-of-the-art approaches in both SUE and multiuser scenarios.
---
paper_title: Generalised grouped minimum mean-squared errorbased multi-stage interference cancellation scheme for orthogonal frequency division multiple access uplink systems with carrier frequency offsets

paper_content:

In uplink orthogonal frequency division multiple access (OFDMA) systems with carrier frequency offsets (CFOs), there always be a dilemma that high performance and low complexity cannot be obtained simultaneously. In this study, in order to achieve better trade-off between performance and complexity, the authors propose a grouped minimum mean squared error (G-MMSE)-based multi-stage interference cancellation (MIC) scheme. The first stage of the proposed scheme is a G-MMSE detector, where the signal is detected group by group using banks of partial MMSE filters. The signal group can be either user based or subcarrier based. Multiple novel interference cancellation (IC) units are serially concatenated with the G-MMSE detector. Reusing the filters in the G-MMSE detector significantly reduces the computational complexity in the subsequent IC units as shown by the complexity analysis. The performance of the proposed G-MMSE-MIC schemes are evaluated by theoretical analysis and simulation. The results show that the proposed schemes outperform other existing schemes with considerably low complexity.
---
paper_title: Joint CFO and Channel Estimation for OFDM-Based Two-Way Relay Networks

paper_content:

Joint estimation of the carrier frequency offset (CFO) and the channel is developed for a two-way relay network (TWRN) that comprises two source terminals and an amplify-and-forward (AF) relay. The terminals use orthogonal frequency division multiplexing (OFDM). New zero-padding (ZP) and cyclic-prefix (CP) transmission protocols, which maintain the carrier orthogonality and ensure low estimation and detection complexity, are proposed. Both protocols lead to the same estimation problem which can be solved by the nulling-based least square (LS) algorithm and perform identically when the block length is large. We present detailed performance analysis by proving the unbiasedness of the LS estimators at high signal-to-noise ratio (SNR) and by deriving the closed-form expression of the mean-square-error (MSE). Simulation results are provided to corroborate our findings.
---
paper_title: A parallel ICI cancellation technique for OFDM systems

paper_content:

Carrier frequency offset, time variations due to Doppler shift or phase noise lead to a loss in the orthogonality between subcarriers of Orthogonal frequency division multiplexing (OFDM) systems and results in inter-carrier interference (ICI). Further developing the parallel cancellation (PC) scheme to mitigate the ICI of OFDM systems, we investigate the characteristics of this two-branch OFDM symbol-based PC scheme in details. Moreover, with known channel state information, we show that this PC scheme is the same as the optimal maximal ratio combining (MRC) technique for transmitter diversity, but with ICI mitigation capability. Additionally, we integrate this PC scheme into a space-time (ST) coded system to form a simple space-time parallel cancellation (STPC) scheme. This STPC scheme not only inherits advantages of the conventional PC scheme, such as backward compatibility with the existing OFDM systems, low receiver complexity, and two-branch diversity, but also provides better bit error rate (BER) performance with lower error floor, especially in slow and fast frequency selective fading channels at a high signal-noise-ratio (SNR) without increasing computational load.
---
paper_title: A Novel Frequency Offset Tracking Algorithm for Space-Time Block Coded OFDM Systems

paper_content:

A novel frequency offset tracking algorithm for Space-Time Block Coded (STBC) Orthogonal Frequency Division Multiplexing (OFDM) systems is proposed in this work. Tracking of a frequency offset between the transmitter and the receiver is often aided by transmitting pilots embedded in the data payload. The proposed algorithm mainly exploits the specific construction of the OFDM symbol in STBC-OFDM systems, which does not need any additional pilots or sequences in the data field, providing high efficiency in spectrum. The estimator is derived on the basis of the maximum likelihood theory. Simulation results show that in a 2×2 multiple input multiple output (MIMO) system, under the assumption that the antennas are uncorrelated to each other, this method can provide a significant performance improvement in terms of the estimation accuracy of the frequency offset.
---
paper_title: Robust Synchronization for 3GPP LTE System

paper_content:

This paper addresses problems related to time and frequency synchronization as well as blind CP (cyclic prefix) type identification in 3GPP LTE system. We start with a general framework based on maximum likelihood (ML) parameter estimation. Then, simplified practical algorithms for estimating time and frequency offset as well as blind CP length are described. Finally, a robust hierarchical scheme is proposed. Specifically, the CP based lagged auto-correlation is first employed to identify the CP type, estimate the carrier frequency offset (CFO), and grossly determine the symbol timing, and then the pilot (i.e. the LTE primary synchronization signal) based cross-correlation is used to more accurately determine the symbol and frame timing. A solution is given to the problem with unequal-length symbols, such as the symbols with the normal CP specified in the LTE. The proposed hierarchical scheme has both low complexity and high accuracy and can be realized without hardware support.
---
paper_title: Iterative Joint Detection, ICI Cancelation and Estimation of Multiple CFOs and Channels for DVB-T2 in MISO Transmission Mode

paper_content:

When DVB-T2 uses the option of (distributed) multiple-input single-output transmission mode, a pair of Alamouti-encoded orthogonal frequency division multiplex signals is transmitted simultaneously from two spatially separated transmitters in a single frequency network. Since both transmitters have their own local oscillators, two distinct carrier frequency offsets (CFOs), one for each transmitter-receiver link, occur in the received signal due to the frequency mismatch between the local oscillators. Unfortunately, the multiple CFOs cannot be compensated simultaneously by merely adjusting the carrier frequency at the receiver side, so intercarrier interference (ICI) always exists. In this paper, we present an iterative receiver design to combat multiple CFOs for DVB-T2 application. To estimate both multiple CFOs and channels jointly without sacrificing the spectral efficiency, the joint maximum likelihood (ML) estimation based on the soft information from the channel decoder is used. The proposed receiver performs the iterative joint processing of the data detection, ICI cancelation, and joint ML estimation of the multiple CFOs and channels by exchanging the soft information. We also show that the complexity of the joint ML estimation can be reduced significantly by transforming a huge matrix pseudo-inversion into two sub-matrix pseudo-inversions. The performances are evaluated via a full DVB-T2 simulator. The numerical results show that the mean-squared error performance of the joint ML estimation is closely matched with the Cramer-Rao bound. Furthermore, the resulting bit error rate performance is enhanced in a progressive manner and is able to approach the ideal CFOs-free performance within a few iterations.
---
paper_title: Transceiver Design for Distributed STBC Based AF Cooperative Networks in the Presence of Timing and Frequency Offsets

paper_content:

In multi-relay cooperative systems, the signal at the destination is affected by impairments such as multiple channel gains, multiple timing offsets (MTOs), and multiple carrier frequency offsets (MCFOs). In this paper we account for all these impairments and propose a new transceiver structure at the relays and a novel receiver design at the destination in distributed space-time block code (DSTBC) based amplify-and-forward (AF) cooperative networks. The Cramer-Rao lower bounds and a least squares (LS) estimator for the multi-parameter estimation problem are derived. In order to significantly reduce the receiver complexity at the destination, a differential evolution (DE) based estimation algorithm is applied and the initialization and constraints for the convergence of the proposed DE algorithm are investigated. In order to detect the signal from multiple relays in the presence of unknown channels, MTOs, and MCFOs, novel optimal and sub-optimal minimum mean-square error receiver designs at the destination node are proposed. Simulation results show that the proposed estimation and compensation methods achieve full diversity gain in the presence of channel and synchronization impairments in multi-relay AF cooperative networks.
---
paper_title: A Synchronization Design for UWB-Based Wireless Multimedia Systems

paper_content:

Multi-band orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) technology offers large throughput, low latency and has been adopted in wireless audio/video (AV) network products. The complexity and power consumption, however, are still major hurdles for the technology to be widely adopted. In this paper, we propose a unified synchronizer design targeted for MB-OFDM transceiver that achieves high performance with low implementation complexity. The key component of the proposed synchronizer is a parallel auto-correlator structure in which multiple ACF units are instantiated and their outputs are shared by functional blocks in the synchronizer, including preamble signal detection, time-frequency code identification, symbol timing, carrier frequency offset estimation and frame synchronization. This common structure not only reduces the hardware cost but also minimizes the number of operations in the functional blocks in the synchronizer as the results of a large portion of computation can be shared among different functional blocks. To mitigate the effect of narrowband interference (NBI) on UWB systems, we also propose a low-complexity ACF-based frequency detector to facilitate the design of (adaptive) notch filter in analog/digital domain. The theoretical analysis and simulation show that the performance of the proposed design is close to optimal, while the complexity is significantly reduced compared to existing work.
---
paper_title: Training Signal Design and Tradeoffs for Spectrally-Efficient Multi-User MIMO-OFDM Systems

paper_content:

In this paper, we design MMSE-optimal training sequences for multi-user MIMO-OFDM systems with an arbitrary number of transmit antennas and an arbitrary number of training symbols. It addresses spectrally-efficient uplink transmission scenarios where the users overlap in time and frequency and are separated using spatial processing at the base station. The robustness of the proposed training sequences to residual carrier frequency offset and phase noise is evaluated. This analysis reveals an interesting design tradeoff between the peak-to-average power ratio of a training sequence and the increase in channel estimation mean squared error over the ideal case when these two impairments are not present.
---
paper_title: Beamforming Based Receiver Scheme for DVB-T2 System in High Speed Train Environment

paper_content:

In this paper, the received signal from the different base stations (BSs) of the second generation of Terrestrial Digital Video Broadcasting (DVB-T2) in the high-speed-train (HST) scenario is modeled as a fast time-varying signal with the multiple Doppler frequency offsets. The interference caused by the multiple Doppler frequency offsets and the channel variations, and the signal to interference plus noise ratio of the received signal, are derived for the DVB-T2 receiver. The results of the theoretical analysis show that the interference greatly degraded the performance of the DVB-T2 system. To suppress the interference, we proposed a beamforming based receiver scheme for DVB-T2 system. By using the new signal processing scheme for the received signal vector from the antenna array, one can separate the received signal with the multiple Doppler frequency offsets into the multiple signals, each of which is with a single Doppler frequency offset. The separated signals are compensated by the corresponding Doppler frequency offsets and equalized by the estimated channel responses respectively, then combined into a signal to be demodulated. The results of the simulation show that the proposed scheme can effectively suppress the interference and greatly improve the performance of the DVB-T2 system in the HST environment.
---
paper_title: Low-Complexity Sequential Searcher for Robust Symbol Synchronization in OFDM Systems

paper_content:

Based on the frequency-domain analog-to-digital conversion (FD ADC), this work builds a low-complexity sequential searcher for robust symbol synchronization in a 4 × 4 FD multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) modem. The proposed scheme adopts a symbol-rate sequential search with simple cross-correlation metric to recover symbol timing over the frequency domain. Simulation results show that the detection error is less than 2% at signal-to-noise ratio (SNR) ≤5 dB. Performance loss is not significant when carrier frequency offset (CFO) ≤100 ppm. Using an in-house 65-nm CMOS technology, the proposed solution occupies 84.881 k gates and consumes 5.2 mW at 1.0 V supply voltage. This work makes the FD ADC more attractive to be adopted in high throughput OFDM systems.
---
paper_title: Software Defined Radio Implementation of SMSE Based Overlay Cognitive Radio in High Mobility Environment

paper_content:

A spectrally modulated spectrally encoded (SMSE) based overlay cognitive radio has been implemented and demonstrated in [1] via GNU software define radio (SDR). However, like most of the current cognitive radio implementations and demonstrations, this work does not consider the mobility between cognitive radio nodes. In a high mobility environment, the frequency offset introduced by Doppler shift leads to loss of the orthogonality among subcarriers. As a direct result, severe inter-carrier interference (ICI) and performance degradation is observed. In our previous work, we have proposed a new ICI cancellation method (namely Total ICI Cancellation) for OFDM [2] and MC-CDMA [3] mobile communication systems, which eliminates the ICI without lowering the transmission rate nor reducing the bandwidth efficiency. In this paper, we apply the total ICI cancellation algorithm onto the SMSE base overlay cognitive radio to demonstrate a high performance cognitive radio in high mobility environment. Specifically, we demonstrate an SMSE based overlay cognitive radio that is capable of detecting primary users in real time and adaptively adjusting its transmission parameters to avoid interference to (and from) primary users. When the primary user transmission changes, the cognitive radio dynamically adjusts its transmission accordingly. Additionally, this cognitive radio maintains seamless real time video transmission between the cognitive radio pair even when large frequency offset is introduced by mobility between CR transmitter and receiver.
---
paper_title: Estimation of Time and Frequency Offsets in LTE Coordinated Multi-Point Transmission

paper_content:

We address the impact, estimation and compensation of time and frequency offsets in LTE CoMP. In LTE CoMP, transmissions may come from a different transmission point in every subframe of one millisecond. Due to propagation delay differences and time-frequency synchronization imperfections between the transmission points, the user equipment (UE) receiver is thus exposed to different time- frequency offsets in every subframe. In this paper we illustrate both analytically and numerically the impact of these time and frequency offsets on channel estimation performance and finally on LTE CoMP link-level demodulation performance. Furthermore, we study the applicability of existing LTE reference signals to the time-frequency offset estimation problem. In particular we compare two approaches in which the UE is either aware or unaware of the exact transmission point. Finally, we show with LTE link-level simulations that using the proposed approaches the impacts of time-frequency offsets can be almost perfectly compensated.
---
paper_title: On the true Cramer-Rao lower bound for data-aided carrier-phase-independent frequency offset and symbol timing estimation

paper_content:

In this letter we present new and simple closed form expressions for the true Cramer-Rao lower bound (CRB) for data-aided (DA) joint and individual carrier frequency offset and symbol timing estimation from a linearly modulated waveform transmitted over an AWGN channel. The bounds are derived under a carrier-phase-independent (CPI) estimation strategy wherein the carrier phase is viewed as a nuisance parameter and assumed to have a worst-case noninformative uniform distribution over [-?, ?]. The computation of these CRBs requires only a single numerical integration. In addition, computationally simpler yet highly accurate asymptotic lower bounds are presented. As particularizations, new bounds for individual CPI frequency estimation with known symbol timing from M-PSK and continuous wave (CW) signals are also reported.
---
paper_title: Iterative ICI Cancellation for OFDM Receiver with Residual Carrier Frequency Offset

paper_content:

For mobile OFDM receiver, both Doppler spread and residual Carrier Frequency Offset (CFO) can cause inter-carrier interference (ICI) therefore performance degradation. In this paper, both the effects of Doppler spread and residual CFO (RCFO) are jointly considered to improve the receiver performance. A low complexity channel estimation is proposed to allow ICI cancellation at the receiver. We demonstrate through simulation that the receiver can achieve desirable performance even in fast fading with the presence of high residual CFO.
---
paper_title: Closed Form SER Expressions for QPSK OFDM Systems With Frequency Offset in Rayleigh Fading Channels

paper_content:

In this letter, the performance of QPSK OFDM systems with frequency offsets is investigated. Closed form expressions for the symbol error rate (SER) that are valid for any frequency offset in Rayleigh fading channels are derived. To achieve this, a modification to the result of the integral of the product of two Q-functions over the Rayleigh distribution is provided. The modification makes the result of this integral valid in any case whether the arguments of the Q-functions are positive or negative. Simulation results are provided to demonstrate the accuracy of the derived expressions.
---
paper_title: Inter-subcarrier interference compensation in the frequency-hopped single-carrier frequency division multiple access communication system

paper_content:

Single-carrier frequency division multiple access (SC-FDMA) system having very good characteristic on peak-to-average power ratio is promising in communication area. But the performance of SC-FDMA system is very sensitive to inter-subcarrier interference (ICI), and existing phase noise suppression algorithm has reduced just phase noise only. Therefore the authors propose effective method, phase noise and frequency offset suppression (PNFS) algorithm, using comb-type pilot. PNFS algorithm can suppress phase noise, carrier frequency offset and even Doppler effect simultaneously in frequency domain. Through PNFS algorithm, SC-FDMA system has robust characteristic against ICI of performance degradation. Moreover, in these days, most electronic communication systems are weak to jamming. So, frequency-hopped (FH) scheme, which is one kind of spread spectrum techniques as a means of making performance better to supplement a disadvantage of SC-FDMA system on jamming attack, is considered. The authors would like to analyse ICI and jamming in SC-FDMA system and employ FH technique in SC-FDMA system because existing studies have not considered the situation regarding ICI and jamming exist simultaneously in mobile channel. So, the authors analyse degradation factors in FH-SC-FDMA system about ICI and jamming signal categorised as partial band jamming and multi-tone jamming. Then, the proposed FH-SC-FDMA system using PNFS algorithm has strong performances about ICI and jamming.
---
paper_title: Frequency Offset and Channel Estimation in Co-Relay Cooperative OFDM Systems

paper_content:

Frequency offset and channel estimation in cooperative orthogonal frequency division multiplexing (OFDM) systems is studied in this paper. We consider the scenario of two or more source nodes sharing the same relay, i.e., corelay cooperative communications, and a new preamble, which is central-symmetric in time-domain, is proposed to perform the frequency offset and channel estimation. The non-zero samples in the proposed preamble are sparsely distributed with two neighboring non-zero samples being separated by μ > 1 zeros. As long as μ > 2L - 1 is satisfied, the multipath interference can be effectively eliminated, where L stands for the channel order. Unlike [1], the proposed preamble has a much lower Peak-to-Average Power Ratio (PAPR). The interference among the multiple source nodes can also be eliminated by using a backoff modulation scheme on the proposed preamble in each source node, and the mean-square error (MSE) of the proposed Least-Square (LS) channel estimator can be minimized by ensuring the orthogonality among the source nodes. The Pairwise Error Probability (PEP) performance of the proposed system by considering both the frequency offset and channel estimation errors is also derived in this paper. For a given Signalto-Noise-Ratio (SNR), by keeping the total power consumption to the source nodes and the relay to be constant, the PEP can be minimized by adjusting the ratio between the power allocated to the source nodes and the total power.
---
paper_title: Blind Maximum-Likelihood Carrier-Frequency-Offset Estimation for Interleaved OFDMA Uplink Systems

paper_content:

Blind maximum-likelihood (ML) carrier-frequency-offset (CFO) estimation is considered to be difficult in interleaved orthogonal frequency-division multiple-access (OFDMA) uplink systems. This is because multiple CFOs have to be simultaneously estimated (each corresponding to a user's carrier), and an exhaustive multidimensional search is required. The computational complexity of the search may be prohibitively high. Methods such as the multiple signal classification and the estimation of signal parameters via the rotational invariance technique have been proposed as alternatives. However, these methods cannot maximize the likelihood function, and the performance is not optimal. In this paper, we propose a new method to solve the problem. With our formulation, the likelihood function can be maximized, and the optimum solution can be obtained by solving a polynomial function. Compared with the exhausted search, the computational complexity can be reduced dramatically. Simulations show that the performance of the proposed method can approach that of the Cramer-Rao lower bound.
---
paper_title: A Composite PN-Correlation Based Synchronizer for TDS-OFDM Receiver

paper_content:

In this paper, we presents a novel synchronizer dealing with carrier frequency offset (CFO), sampling frequency offset (SFO), as well as frame timing offset (FTO) in TDS-OFDM receiver. The proposed schemes are based on tracking the output waveform of a composite PN-correlator (CPC), which provides sufficient correlative gains to detect its peaks even in the presence of large CFOs. From the correlation peaks, we can extract useful information for estimating the synchronization offsets. CFO is recovered by a multi-stage CPC scheme, of which the parameters are adjustable for meeting the system's demands on the tracking range and accuracy. According to the inter-frame variations of correlation waveform, we estimate SFO for a large scale and correct SFO through an interpolator. Meanwhile, frame timing is investigated in this paper, and the analysis indicates a very fast and robust timing scheme is possible for TDS-OFDM receiver. The developed synchronizer is quite robust against a large CFO even in very adverse fading channels, and it is shown by computer simulation that the residual synchronization error has little effect on the performance of TDS-OFDM receiver.
---
paper_title: A Simplified MMSE Equalizer for Distributed TR-STBC Systems with Multiple CFOs

paper_content:

In distributed wireless systems, the traditional carrier frequency offset (CFO) compensation methods may not be applied due to the existence of multiple CFOs. In this paper, we address the equalization issue for distributed time-reversal space-time block coded (TR-STBC) systems when multiple CFOs are presented. A simplified minimum mean-square error (MMSE) equalizer is proposed, which exploits the nearly-banded structure of the channel matrices and utilizes the {LDL}^H factorization to reduce the computational complexity. Simulation results show that the proposed equalizer can achieve the similar performance as the traditional MMSE equalizer while possessing much less complexity.
---
paper_title: A Bayesian Algorithm for Joint Symbol Timing Synchronization and Channel Estimation in Two-Way Relay Networks

paper_content:

This work investigates joint estimation of symbol timing synchronization and channel response in two-way relay networks (TWRN) that utilize amplify-and-forward (AF) relay strategy. With unknown relay channel gains and unknown timing offset, the optimum maximum likelihood (ML) algorithm for joint timing recovery and channel estimation can be overly complex. We develop a new Bayesian based Markov chain Monte Carlo (MCMC) algorithm in order to facilitate joint symbol timing recovery and effective channel estimation. In particular, we present a basic Metropolis-Hastings algorithm (BMH) and a Metropolis-Hastings-ML (MH-ML) algorithm for this purpose. We also derive the Cramer-Rao lower bound (CRLB) to establish a performance benchmark. Our test results of ML, BMH, and MH-ML estimation illustrate near-optimum performance in terms of mean-square errors (MSE) and estimation bias. We further present bit error rate (BER) performance results.
---
paper_title: A Practical Double Peak Detection Coarse Timing for OFDM in Multipath Channels

paper_content:

For orthogonal frequency division multiplexing (OFDM) systems in multipath channels, the detection of the first path is desirable to avoid inter-symbol-interference (ISI). However, the conventional peak detection of the cyclic prefix (CP) correlation curve can detect the strongest path but not the first path, while the largest slope detection is sensitive to background noise. This paper presents a novel coarse timing algorithm using double peak detection of CP correlation, which is robust to noise and simple to implement in practice. Considering practical hardware impairments like sampling frequency offset (SFO), the performance of the proposed method is verified by simulations.
---
paper_title: Low-cost integer frequency offset estimation for OFDM-based DRM receiver

paper_content:

In this paper, we propose an improved and low-cost integer frequency offset (IFO) estimation method by partitioning pilot symbols effectively in an orthogonal frequency division multiplexing based digital radio mondiale (DRM) system. To this end, the time reference cell (TRC) symbol for frequency estimation is grouped into a number of pilot clusters, so that the TRC subcarriers in each cluster are closely spaced to show approximately frequency-nonselective characteristics. The performance of the proposed IFO estimator is compared with that of the conventional estimator, and shows that the proposed technique can effectively achieve lower estimation errors in frequency offset estimation as well as can be implemented with reduced computational complexity.
---
paper_title: Blind spectrum sensing in cognitive radio over fading channels and frequency offsets

paper_content:

This paper deals with the problem of spectrum sensing in cognitive radio. We consider a stochastic system model where the Primary User (PU) transmits a periodic signal over fading channels. The effect of frequency offsets due to oscillator mismatch, and Doppler offset is studied. We show that for this case the Likelihood Ratio Test (LRT) cannot be evaluated pointwise. We present a novel approach to approximate the marginilisation of the frequency offset using a single point estimate. This is obtained via a low complexity Constrained Adaptive Notch Filter (CANF) to estimate the frequency offset. Performance is evaluated via numerical simulations and it is shown that the proposed spectrum sensing scheme can achieve the same performance as the “near-optimal” scheme, that is based on a bank of matched filters, using only a fraction of the complexity required.
---
paper_title: Timing and Frequency Offsets Compensation in Relay Transmission for 3GPP LTE Uplink

paper_content:

Relays can be used between the source and the destination to improve network coverage or reliability. In the relay based distributed space-time block-coded (DSTBC) single-carrier (SC) transmission scheme, asynchronism in time and frequency causes degradation in system performance because of the interblock interference, non-coherent combining and multiuser interference. The simultaneous presence of timing offset and carrier frequency offset also destroy the orthogonal structure of DSTBC. In this paper, we study the combined effect of timing offset and carrier frequency offset and propose a two stage equalization technique for this system at the destination terminal. Technique is based on interference cancellation and cyclic prefix reconstruction. The proposed equalization technique maintains the orthogonal structure of DSTBC and allows the use of low complexity one-tap frequency-domain equalizer. The technique significantly alleviates the effect of time offsets and frequency offsets at the destination without increasing the complexity of the receiver or disturbing the 3rd generation partnership project-long term evolution (3GPP LTE) uplink frame structure or the data rate over frequency selective channels.
---
paper_title: Modified Symbol Timing Offset Estimation for OFDM over Frequency Selective Channels

paper_content:

This paper deals with the symbol timing issue of an OFDM system in frequency selective fading scenarios. A modified symbol timing offset estimator based on an energy transition metric is proposed. This metric can also be applied to estimate the maximum channel delay spread. Benefiting from the knowledge of the maximum channel delay spread, the modified estimator shows its advantage in a relatively low SNR regime. Compared to the original, the modified algorithm is able to acquire the symbol timing correctly in a relatively low SNR region.
---
paper_title: Carrier Frequency Offset Tracking in the IEEE 802.16e OFDMA Uplink

paper_content:

The IEEE 802.16e standard for nomadic wireless metropolitan area networks adopts orthogonal frequency-division multiple-access (OFDMA) as an air interface. In these systems, residual carrier frequency offsets (CFOs) between the uplink signals and the base station local oscillator give rise to interchannel interference (ICI) as well as multiple access interference (MAI). Accurate CFO estimation and compensation is thus necessary to avoid a serious degradation of the error-rate performance. In this work, we address the problem of CFO tracking in the IEEE 802.16e uplink and present a closed-loop solution based on the least-squares (LS) principle. In doing so, we exploit a set of pilot tones that are available in each user's subchannel. The resulting scheme can be implemented with affordable complexity and is able to reliably track the CFOs of all active users. When used in conjunction with a frequency offset compensator, it can effectively mitigate ICI and MAI, thereby allowing channel equalization and data detection to follow directly. Numerical simulations are used to demonstrate the effectiveness of the proposed solution in the presence of residual time-varying frequency offsets.
---
paper_title: Carrier frequency offset estimation for non-contiguous OFDM receiver in cognitive radio systems

paper_content:

For non-contiguous (NC) OFDM based cognitive radio (CR) systems, schemes have been developed in literature to acquire spectrum synchronization information (SSI) with perfect carrier frequency offset (CFO) synchronization. However, OFDM is extremely sensitive to the CFO in practice, which leads to inter-carrier interference (ICI), hence degrading the spectrum synchronization performance for existing schemes. An accurate CFO estimation is therefore required before setting up the SSI. In this paper, we present a novel scheme based on the maximum likelihood (ML) algorithm to estimate the CFO for the NC-OFDM receiver when the SSI is unknown. A corresponding Cramer-Rao lower bound (CRB) with the ideal SSI is derived to demonstrate the efficiency of the proposed scheme. Simulation results show that the proposed scheme is robust against interference and achieves a satisfactory accuracy of estimation, which is close to the relevant CRB.
---
paper_title: On the effects of carrier frequency offset on cyclic prefix based OFDM and filter bank based multicarrier systems

paper_content:

Being sensitive to carrier frequency offset (CFO) is known to be one of the main drawbacks of multicarrier systems. In this paper, the effects of CFO on a filter bank based multicarrier system (FBMC) in a multipath fading channel are discussed, where an ideal root-raised cosine (RRC) filter with roll-off factor 1 is used as the prototype filter which enables analytical derivations of the interference caused by the CFO. Based on these results, an approximation on the SNR degradation with very small CFO is also given. Numerical experiments as well as Monte Carlo simulations are done to verify the analysis and the accuracy of the approximation in FBMC systems. A comparison with the SNR degradations in cyclic prefix based orthogonal frequency division multiplexing (CP-OFDM) systems has indicated an advantage of FBMC systems as being more robust to frequency misalignments.
---
paper_title: An Optimized Iterative (Turbo) Receiver for OFDM Systems with Type-I Hybrid ARQ: Clipping and CFO Cases

paper_content:

An optimized iterative (Turbo) receiver for Orthogonal Frequency Division Multiplexing (OFDM) transmission is proposed for improved performance when used with type-I hybrid Automatic Repeat Request (ARQ) protocols. The proposed receiver is a Maximum Aposteriori Expectation-Maximization (MAP-EM) Turbo processor that exploits information from all available transmissions of a packet, including failed and new ones. Two distinct practical problems associated with OFDM are considered: (i) Clipping due to non-linearities in the transmit amplifier, and (ii) Carrier frequency offset (CFO). In the optimized scheme, failed transmissions are combined with the new ones to form an iterative receiver that jointly estimates the channel, compensates for the distortion (clipping, or CFO) and decodes the message. The modulation employed is M-ary Phase Shift Keying (MPSK). The system uses an embedded pilot structure for initial channel estimation of the channel, which is assumed doubly-selective. It is shown that gains on the order of 3 dB are achieved in the packet rejection probability with the proposed system. The performance of the optimized system is close to that of the ideal system, i.e. one that performs packet combining with perfect channel knowledge, no clipping, and no CFO.
---
paper_title: Joint Carrier Synchronization and Equalization Algorithm for Packet-Based OFDM Systems Over the Multipath Fading Channel

paper_content:

In this paper, a joint carrier synchronization and equalization algorithm is presented for orthogonal frequency-division multiplexing (OFDM) systems in the tracking stage. Based on the minimum mean-square-error (MMSE) criterion, the cost function of the joint algorithm is proposed to minimize the mean square error (MSE), namely, the uncoded bit error rate (BER), on each subchannel and to further lower the carrier frequency jitter concurrently. The carrier synchronization scheme with multirate processing is a dual-loop structure, which is composed of outer and inner loops. The outer loop is a frequency-tracking loop that deals with the phase offset, which is induced by the carrier frequency offset (CFO), in the time domain. The inner loop is a phase-tracking loop to cope with the phase distortions, which are caused by the carrier frequency error and the channel phase variation, on each subcarrier in the frequency domain. There is a gain equalization loop to compensate the magnitude distortion on each subchannel. Furthermore, the closed-loop stability of the carrier synchronization loop is particularly explored for the loop delay induced by hardware realization. Many simulations are done for the additive white Gaussian noise (AWGN) and the multipath frequency-selective fading channels to show that the joint algorithm not only accurately estimates and compensates the CFO and the channel impairment but also provides the cost-effective feature compared with the considered algorithms.
---
paper_title: Reduction of the Peak Interference to Carrier Ratio of OFDM Signals

paper_content:

In this paper, we provide a geometric interpretation of the Peak Interference to Carrier Ratio (PICR) of OFDM signals. Based on this interpretation, we propose a simple approach to reduce the sensitivity of OFMA system to carrier frequency offset (CFO). The main idea is to choose a well understood code and to rotate each coordinate of the code by a fixed phase shift such that the maximum inter-carrier interference (ICI) taken over all sub-carriers is minimized. This approach enjoys the twin benefits of interference reduction and error correction and is easy to implement in practice. Simulation results show that a reduction of 7 dB can freely be obtained.
---
paper_title: I/Q imbalance and CFO in OFDM/OQAM systems: Interference analysis and compensation

paper_content:

Offset-QAM (OQAM) based OFDM has been considered as a promising technique for future wireless networks due to its higher resilience to narrow-band interference and doubly dispersive channels. In this paper, we investigate the effects of the RX imperfections, namely the I/Q imbalance (IQI) and the carrier frequency offset (CFO), on OFDM/OQAM downlink performance coupled with the imperfect knowledge of the frequency-selective channel. The influence of each impairment is characterized by the interference analysis of the demodulated signal that reveals interesting insight into the distortion origination. Then, we analytically investigate the performance loss as a function of IQI, CFO and channel distortions in a realistic receiver with channel estimation error. To cope with impaired reception, a joint maximum-likelihood estimation method with repetitive training signals is employed. Simulation results demonstrate the relative sensitivity of OFDM/OQAM receivers to the impairments and that the performance loss due to the imperfections can be recovered by the compensation technique.
---
paper_title: Inter-carrier interference-free Alamouti-coded OFDM for cooperative systems with frequency offsets in non-selective fading environments

paper_content:

A modified Alamouti-coded orthogonal frequency division multiplexing (OFDM) scheme is proposed for cooperative systems in non-selective fading environments. Even with the frequency offset between two distributed transmit antennas, the proposed scheme achieves ideal performance and full rate of Alamouti code. By switching subcarriers in the second OFDM symbol of each Alamouti-coded OFDM symbol pair after a simple-phase rotation in the first OFDM symbol, inter-carrier interference terms can be perfectly cancelled after a simple linear combining with the processing overhead of two times down-conversions and discrete Fourier transform (DFT) operations at each OFDM symbol.
---
paper_title: Optimal Frequency Offsets with Doppler Spreads in Mobile OFDM System

paper_content:

In highly mobile OFDM systems, the carrier frequency offsets (CFO) with Doppler spreads for downlink detection can be considerably large, which degrades the frequency alignment for uplink transmission, particularly in employing directional antennas for inter-carrier interference (ICI) reduction. In prior works, the directional antenna was investigated with appropriate frequency alignment in receiver's local oscillator to efficiently reduce ICI in fast time varying OFDM systems. To resolve the optimal frequency offsets problem with Doppler spreads, this paper develops a simple scheme to capture instant Doppler power spectrum density (PSD) through moving directional antennas with arbitrary gain patterns. Thus, the optimal aligning frequency is derived as the center of gravity of the Doppler PSD. Simulations show our approach acquires the highest carrier to interference (C/I) ratio and the lowest bit error rates (BER) compared with other approaches.
---
paper_title: Improving Range Accuracy of IEEE 802.15.4a Radios In the Presence of Clock Frequency Offsets

paper_content:

Two-way time-of-arrival (TW-ToA) is a ranging protocol that provides distance between two devices in absence of synchronization, but it suffers from range estimation errors when clock frequency offset is present. In this work, we provide a timing counter management scheme for TW-ToA that suppresses ranging errors induced by any clock frequency offset between a transmitter and receiver pair. The suggested scheme is shown to be superior both theoretically and empirically to the one that is recommended in the IEEE 802.15.4a standard.
---
paper_title: Joint Carrier Frequency Offset and fast time-varying channel estimation for MIMO-OFDM systems

paper_content:

In this paper, a novel pilot-aided iterative algorithm is developed for MIMO-OFDM systems operating in fast time-varying environment. An L-path channel model with known path delays is considered to jointly estimate the multi-path Rayleigh channel complex gains and Carrier Frequency Offset (CFO). Each complex gain time-variation within one OFDM symbol is approximated by a Basis Expansion Model (BEM) representation. An auto-regressive (AR) model is built for the parameters to be estimated. The algorithm performs recursive estimation using Extended Kalman Filtering. Hence, the channel matrix is easily computed and the data symbol is estimated with free inter-sub-carrier-interference (ICI) when the channel matrix is QR-decomposed. It is shown that only one iteration is sufficient to approach the performance of the ideal case for which the knowledge of the channel response and CFO is available.
---
paper_title: An Efficient Blind Estimation of Carrier Frequency Offset in OFDM Systems

paper_content:

In this paper, we propose a low-complexity blind carrier frequency offset (CFO) estimation scheme for constant modulus (CM)-signaling-based orthogonal frequency-division multiplexing (OFDM) systems. Provided that the channel can be assumed to be slowly time-varying, subcarriers having the same indexes in two consecutive OFDM symbols will experience nearly the same channel effect. This assumption enables us to derive a cost function that is determined by the sum of the products of the signal amplitudes on each pair of equivalent subcarriers from two successive OFDM symbols. The maximization process of this cost function makes it possible to find an appropriate estimate of the CFO. Over frequency-selective Rayleigh fading channels, the proposed CFO estimation method provides improved performance over existing techniques. Moreover, in the context of narrow-band noise and signal gain variations, the simulations demonstrate the robustness and immunity of our scheme.
---
paper_title: Training-Based Synchronization and Channel Estimation in AF Two-Way Relaying Networks

paper_content:

Two-way relaying networks (TWRNs) allow for more bandwidth efficient use of the available spectrum since they allow for simultaneous information exchange between two users with the assistance of an intermediate relay node. However, due to superposition of signals at the relay node, the received signal at the user terminals is affected by multiple impairments, i.e., channel gains, timing offsets, and carrier frequency offsets, that need to be jointly estimated and compensated. This paper presents a training-based system model for amplify-and-forward (AF) TWRNs in the presence of multiple impairments and proposes maximum likelihood and differential evolution based algorithms for joint estimation of these impairments. The Cramer-Rao lower bounds (CRLBs) for the joint estimation of multiple impairments are derived. A minimum mean-square error based receiver is then proposed to compensate the effect of multiple impairments and decode each user's signal. Simulation results show that the performance of the proposed estimators is very close to the derived CRLBs at moderate-to-high signal-to-noise-ratios. It is also shown that the bit-error rate performance of the overall AF TWRN is close to a TWRN that is based on assumption of perfect knowledge of the synchronization parameters.
---
paper_title: Sequence Designs for Robust Consistent Frequency-Offset Estimation in OFDM Systems

paper_content:

In this paper, we derive the average pairwise error probability (PEP) of an integer carrier frequency-offset (CFO) estimator with consistent pilots in orthogonal frequency-division multiplexing (OFDM) systems and address several issues based on PEP analysis. In particular, the relationship between the PEP and consistent pilots is established in terms of a diversity gain and a shift gain. Based on the observations, we present new criteria for sequence designs. Simulation results show that the sequence developed from these criteria yields much reduced outliers compared with conventional sequences for consistent CFO estimation in frequency-selective fading channels.
---
paper_title: Efficient Sequential Integer CFO and Sector Identity Detection for LTE Cell Search

paper_content:

In this letter, we propose an efficient algorithm for the detection of the integer carrier frequency offset (CFO) and sector identity used in the LTE cell search. For the conventional LTE cell search, the integer CFO and sector identity are jointly detected by utilizing the primary synchronization signal (PSS). By exploiting the symmetric property of the PSS, the integer CFO can be solely detected without the knowledge of sector identity, and thus, the joint detection of the integer CFO and sector identity is decoupled. Additionally, the proposed sequential integer CFO and sector identity detection (SISID) removes the effect of the channel frequency responses such that the detection accuracy is enhanced. We also design the SISID hardware architecture where the parts of the signals are processed in the polar coordinate so that all the multiplications are realized by additions/subtractions. The simulation results demonstrate that the proposed SISID achieves better detection accuracy than the conventional methods with only one third of the computational complexity.
---
paper_title: Digital Baseband IC Design of OFDM PHY for a 60GHz Proximity Communication System

paper_content:

This paper presents a digital baseband IC design based on OFDM PHY for a 60GHz proximity communication system. We propose a low computational complexity OFDM demodulator with a carrier frequency offset estimation method in polar coordinates suitable for high-speed parallel architecture. The proposed architecture is implemented in 65nm CMOS technology, and is experimentally verified to achieve the PHY data rate above 2.2Gbps. The digital baseband IC includes a complete functionality of OFDM transceiver with error correcting codecs and MAC.
---
paper_title: Performance Improvement of MDPSK Signal Reception in the Presence of Carrier Frequency Offset

paper_content:

Performance of mobile systems is significantly influenced by the carrier frequency offset caused by a Doppler shift, which leads to deployment of differential modulation schemes instead of coherent detection-based schemes. In this paper, we propose a novel M-ary differential phase-shift keying (MDPSK) receiver that performs better than the receiver with MDPSK differential detection and is very close to the performance of a coherent MDPSK detector. The symbol error probability of the proposed MDPSK receiver is approximately constant within the practical frequency offset range. The proposed receiver may be used in the systems that work in fading and with high-frequency offset environments.
---
paper_title: Iterative Sampling Frequency Offset Estimation for MB-OFDM UWB Systems With Long Transmission Packet

paper_content:

A multiband orthogonal frequency-division multiplexing (MB-OFDM) system, which is one of the effective modulation techniques that have been adopted for high-speed ultrawideband (UWB) systems, is very sensitive to sampling frequency offset (SFO) due to the mismatch between local oscillators at the transmitter and the receiver. In this paper, we propose an iterative SFO estimation method for a high-data-rate MB-OFDM UWB system to improve its SFO estimation accuracy in the case of a long transmission packet. The proposed method is an iterative process of 2-D SFO estimation across pilot subcarriers and consecutive OFDM symbols, together with joint channel estimation. Furthermore, we derive the Cramer-Rao lower bound (CRLB) for the proposed SFO estimation method. This CRLB can be used as a guide for algorithm design and to explore the theoretical limit. Our performance analysis and simulation results show that the proposed iterative SFO estimation method is both practical and effective.
---
paper_title: A Blind Fine Synchronization Scheme for SC-FDE Systems

paper_content:

This work presents a blind fine synchronization scheme, which estimates and compensates residual carrier-frequency offset (RCFO) and symbol timing offset (STO) , for single-carrier frequency-domain equalization (SC-FDE) systems. Existing fine synchronization schemes for SC-FDE systems rely on time-domain unique words (UW) sequences as reference signals to assure the estimation accuracy, at the cost of decreased system throughput. The proposed technique, named simplified weighted least-square method for single-carrier systems (SWLS-SC), combines the decision feedback structure and SWLS estimator for OFDM systems. Together with specifically derived weighting factors, it has much better estimation accuracy than the well-known linear least-square (LLS) method for SC-FDE systems, and its BER performance can approach that of the ideal synchronization condition. The proposed technique is more effective than existing techniques, in terms of both performance and throughput. Theoretical estimation bounds are also derived to verify the effectiveness of the proposed method.
---
paper_title: Optimizing Wideband Cyclostationary Spectrum Sensing Under Receiver Impairments

paper_content:

In the context of Cognitive Radios (CRs), cyclostationary detection of primary users (PUs) is regarded as a common method for spectrum sensing. Cyclostationary detectors rely on the knowledge of the signal's symbol rate, carrier frequency, and modulation class in order to detect the present cyclic features. Cyclic frequency and sampling clock offsets are the two receiver impairments considered in this work. Cyclic frequency offsets, which occur due to oscillator frequency offsets, Doppler shifts, or imperfect knowledge of the cyclic frequencies, result in computing the test statistic at an offset from the true cyclic frequency. In this paper, we analyze the effect of cyclic frequency offsets on conventional cyclostationary detection, and propose a new multi-frame test statistic that reduces the degradation due to cyclic frequency offsets. Due to the multi-frame processing of the proposed statistic, non-coherent integration might occur across frames. Through an optimization framework developed in this work that can be performed offline, we determine the best frame length that maximizes the average detection performance of the proposed cyclostationary detection method given the statistical distributions of the receiver impairments. As a result of the optimization, the proposed detectors is shown to achieve the performance gains over conventional detectors given the constrained sensing time. We derive the proposed detector's theoretical average detection performance, and compare it to the performance of the conventional cyclostationary detector. Our analysis shows that gains in average detection performance using the proposed method can be achieved when the effect of sampling clock offset is less severe than that of the cyclic frequency offset. The analysis given in this paper can be used as a design guideline for practical implementation of cyclostationary spectrum sensors.
---
paper_title: Low-complexity pilot-aided compensation for carrier frequency offset and I/Q imbalance

paper_content:

We propose a novel pilot-aided compensation scheme for carrier frequency offset (CFO) and I/Q imbalance. The proposed scheme comprises a generalized periodic pilot and a low-complexity acquisition algorithm, where the CFO and the coefficients for I/Q imbalance compensation can be obtained in explicit closed-form.
---
paper_title: Towards practical channel reciprocity exploitation: Relative calibration in the presence of frequency offset

paper_content:

Relative calibration has been proposed as a simple way to practically exploit the reciprocity of the wireless channel. It is based on a simple convolutional model for the relationship between the channel impulse responses in both directions, and accounts for the discrepancies between transmit and receive radiofrequency components, without the need for specific calibration hardware. However, the relative calibration methods developed so far have been shown to lack robustness with respect to frequency offset between the devices on both sides of the considered channel. In this article, we introduce a relative calibration algorithm that properly deals with the presence of frequency offset. We verify its robustness and assess its performance through simulations, and validate experimentally the proposed model on measured channels.
---
paper_title: Blind Channel Shortening for Asynchronous SC-IFDMA Systems with CFOs

paper_content:

This paper proposes a blind channel shortening algorithm for uplink reception of a single-carrier interleaved frequency-division multiple-access (SC-IFDMA) system transmitting over a highly-dispersive channel, which is affected by both timing offsets (TOs) and frequency offsets (CFOs). When the length of the cyclic prefix (CP) is insufficient to compensate for channel dispersion and TOs, a common strategy is to shorten the channel by means of time-domain equalization, in order to restore CP properties and ease signal reception. The proposed receiver exhibits a three-stage structure: the first stage performs blind shortening of all the user channel impulse responses (CIRs), by adopting the minimum mean-output energy criterion, without requiring neither a priori knowledge of the CIRs to be shortened, nor preliminary compensation of the CFOs; the second stage performs joint compensation of the CFOs; finally, to alleviate noise amplification effects, possibly arising from CFO compensation, the third stage implements per-user signal-to-noise ratio (SNR) maximization, without requiring knowledge of the shortened CIRs. A theoretical analysis is carried out to assess the effectiveness of the proposed shortening algorithm in the high-SNR regime; moreover, the performances of the overall receiver are validated and compared with those of existing methods by extensive Monte Carlo computer simulations.
---
paper_title: Joint Low-Complexity Equalization and Carrier Frequency Offsets Compensation Scheme for MIMO SC-FDMA Systems

paper_content:

Due to their noise amplification, conventional Zero-Forcing (ZF) equalizers are not suited for interference-limited environments such as the Single-Carrier Frequency Division Multiple Access (SC-FDMA) in the presence of Carrier-Frequency Offsets (CFOs) . Moreover, they suffer increasing complexity with the number of subcarriers and in particular with Multiple-Input Multiple-Output (MIMO) systems. In this letter, we propose a Joint Low-Complexity Regularized ZF (JLRZF) equalizer for MIMO SC-FDMA systems to cope with these problems. The main objective of this equalizer is to avoid the direct matrix inversion by performing it in two steps to reduce the complexity. We add a regularization term in the second step to avoid the noise amplification. From the obtained simulation results, the proposed scheme is able to enhance the system performance with lower complexity and sufficient robustness to estimation errors.
---
paper_title: A Scheme to Support Concurrent Transmissions in OFDMA Based Ad Hoc Networks

paper_content:

In this paper, we propose a novel system architecture to realize OFDMA in ad hoc networks. A partial time synchronization strategy is presented based on the proposed system model. This proposed scheme can support concurrent transmission without global clock synchronization. We also propose a null subcarrier based frequency synchronization scheme to estimate and compensate frequency offsets in a multiple user environment. The simulation results show a good performance of our proposed synchronization scheme in terms of frequency offset estimation error and variance.
---
paper_title: Low-Complexity Cell Search With Fast PSS Identification in LTE

paper_content:

Cell search and synchronization in the Third-Generation Partnership Project (3GPP) Long-Term Evolution (LTE) system is performed in each user equipment (UE) by using the primary synchronization signal (PSS) and secondary synchronization signal (SSS). The overall synchronization performance is heavily dominated by robust PSS detection, which can be achieved in the conventional noncoherent detector by exploiting the essential near-perfect autocorrelation and cross-correlation properties of Zadoff-Chu (ZC) sequences. However, a relatively high computational complexity is observed in conventional algorithms. As compared with them, two new detectors, i.e., almost-half-complexity (AHC) and central-self-correlation (CSC) detectors, are proposed in this paper to achieve reliable PSS detection with much lower complexity by exploiting the central-symmetric property of ZC sequences. The complexity of the proposed CSC detector is only 50% that of the AHC detector, which achieves exactly the same PSS detection accuracy as that of the conventional detector with one half of complexity being saved. An improvement of CSC, i.e., CSCIns, is also proposed in this paper to combat the large-frequency offset. The performance of the CSCIns detector is independent of the frequency offset, and numerical results show that the 90% PSS acquisition time of CSCIns can be well within an 80-ms duration, even in a heavy intercell-interference environment with a signal-to-noise ratio (SNR) of -10 dB.
---
paper_title: Iterative Blind OFDM Parameter Estimation and Synchronization for Cognitive Radio Systems

paper_content:

An iterative design method for Orthogonal Frequency Division Multiplexing (OFDM) system parameter estimation and synchronization under a blind scenario for cognitive radio systems is proposed in this paper. A novel envelope spectrumbased arbitrary oversampling ratio estimator is presented first, based on which the algorithms are then developed to provide the identification of other OFDM parameters (number of subcarriers, cyclic prefix (CP) length). Carrier frequency offset (CFO) and timing offset are estimated for the purpose of synchronization with the help of the identified parameters. An iterative scheme is employed to increase the estimation accuracy. To validate the proposed design, the performance is evaluated under an experimental propagation environment and the results show that the proposed design is capable of adapting blind parameter estimation and synchronization for cognitive radio with improved performances.
---
paper_title: Semi-analytic selection of sub-carrier allocation schemes in uplink orthogonal frequency division multiple access

paper_content:

The authors propose a generalised framework that analytically compares different sub-carrier allocation (SA) or sub-channelisation schemes (such as interleaved SA (ISA), localised SA (LSA) or hybrid schemes) in uplink (UL) orthogonal frequency division multiple access (OFDMA). The ultimate goal of the proposed framework is to systematically determine the best SA scheme among the considered candidates for a given condition of inter-user frequency offsets (IUFOs) and the target bit error rate (BER). As an illustration, two typical SA schemes are considered, that is, ISA and LSA for comparison. First, based on the well-known fact that multiple access interference by IUFO and frequency diversity gain are both dictated by the employed SA scheme, the authors propose a semi-analytic approach to derive coded BER curve formula as a function of frequency offset bound (FOB) for each scheme. Then, the signal-to-noise ratio gain of ISA over LSA for various target BERs and FOBs is derived. Finally, the cutoff FOB over which ISA obtains worse than LSA is obtained and its functional relationship with the target BER is investigated. By following the overall procedure in the proposed framework, the best SA scheme among the various SA candidates for the general UL OFDMA systems with arbitrary system parameters can be chosen.
---
paper_title: Iterative Frequency Domain Equalization and Carrier Synchronization for Multi-Resolution Constellations

paper_content:

Broadband broadcast and multicast wireless systems usually employ OFDM modulations (Orthogonal Frequency Division Multiplexing) combined with non-uniform hierarchical constellations. However, these schemes are very prone to nonlinear distortion effects and have high carrier synchronization requirements. SC-FDE (Single-Carrier with Frequency-Domain Equalization) is an attractive alternative for OFDM, especially when an efficient power amplification is intended. In this paper we consider the use of SC-FDE schemes combined with non-uniform hierarchical constellations in broadband broadcast and multicast wireless systems. We study the impact of residual CFO (Carrier Frequency Offset) on the performance of multi-resolution schemes and we propose iterative frequency domain receivers with joint detection and carrier synchronization to cope with residual CFO estimation errors (a coarse CFO estimation and compensation is assumed before the equalization procedure). Our results show that while a very high carrier synchronization accuracy is required for the least protected bits, the most protected bits are relatively robust to the CFO. By employing the proposed receiver we increase significantly the robustness to residual CFO estimation errors.
---
paper_title: Iterative Synchronization-Assisted Detection of OFDM Signals in Cognitive Radio Systems

paper_content:

Despite many attractive features of an orthogonal frequency-division multiplexing (OFDM) system, the signal detection in an OFDM system over multipath fading channels remains a challenging issue, particularly in a relatively low signal-to-noise ratio (SNR) scenario. This paper presents an iterative synchronization-assisted OFDM signal detection scheme for cognitive radio (CR) applications over multipath channels in low-SNR regions. To detect an OFDM signal, a log-likelihood ratio (LLR) test is employed without additional pilot symbols using a cyclic prefix (CP). Analytical results indicate that the LLR of received samples at a low SNR can be approximated by their log-likelihood (LL) functions, thus allowing us to estimate synchronization parameters for signal detection. The LL function is complex and depends on various parameters, including correlation coefficient, carrier frequency offset (CFO), symbol timing offset, and channel length. Decomposing a synchronization problem into several relatively simple parameter estimation subproblems eliminates a multidimensional grid search. An iterative scheme is also devised to implement a synchronization process. Simulation results confirm the effectiveness of the proposed detector.
---
paper_title: Feedback Generation for CoMP Transmission in Unsynchronized Networks with Timing Offset

paper_content:

Coordinated multipoint (CoMP) transmission is a promising technique in long term evolution (LTE) systems to provide coverage and broadband communication for cell edge user equipments (UEs). However, as signals from multiple transmitters may reach the UE at different times, CoMP networks might experience high time offsets, leading to significant performance loss in closed-loop transmission. In this paper we show that spacing between reference signals (RSs) imposes a phase offset on the transmit covariance matrix in presence of timing offset (TO), affecting the feedback generation. The promised advantages of closed-loop CoMP transmission vanish in presence of TO due to improper precoding matrix index (PMI) selection. Keeping the phase shift close to zero, reliable PMI selection can be guaranteed and as a result near optimum performance is achieved for closed-loop CoMP transmission in unsynchronized networks with TO present.
---
paper_title: Analysis of carrier frequency offset estimation with multiple pilot block sequences

paper_content:

As is well known, dividing the pilot symbols into multiple blocks and separating these blocks at some distances away from each other can significantly improve the frequency estimation accuracy. However, the mean square error (MSE) performance at moderate and low SNR, especially the SNR threshold effect, with multiple pilot block sequences has not been investigated properly. Thus, a numerical calculation method is proposed in this paper to derive the approximate MSE of the ML estimator with multiple pilot block sequences at all SNRs and the proposed calculation method obtains the MSEs coinciding with the simulation results and the calculated approximate MSEs can tell exactly where the SNR thresholds are.
---
paper_title: Joint Frequency Synchronization and Spectrum Occupancy Characterization in OFDM-Based Cognitive Radio Systems

paper_content:

OFDM-based cognitive radio (CR) systems are shown to be an effective solution for increasing spectrum efficiency by activating the certain group of subcarriers (subbands) in locally available spectrum. However, each CR receiver should synchronize itself to appropriate carrier frequency and to identify currently activated subbands. Moreover, energy and bandwidth efficiency of CR systems can be improved if each CR could provide additional characterization of the local spectral content. In this paper a novel joint frequency synchronization and spectrum occupancy characterization method for OFDM-based CR systems is proposed. The synchronization preamble structure is appropriately modified in order to efficiently perform frequency offset estimation, to identify occupied subbands, and, finally, to provide SNR and interference power estimates as reliable quantitative indicators of spectrum occupancy. The performance of proposed method is evaluated for different amounts of spectrum occupancy and interference levels.
---
paper_title: An Eigenvalue Based Carrier Frequency Offset Estimator for OFDM Systems

paper_content:

Orthogonal frequency division multiplexing (OFDM) is sensitive to frequency synchronization errors. This letter proposes a novel data-aided carrier frequency offset (CFO) estimator. We show that the eigenvalues of the inter-carrier interference (ICI) coefficient matrix are the elements of a geometric series distributed on the unit circle of the complex plane. Then, we prove that estimating the CFO is equivalent to finding the eigenvalues of a two-dimensional ICI coefficient matrix. As a result, by transmitting the corresponding eigenvectors, an estimate of the CFO value can be found. In addition to its simplicity, the proposed estimator is proven to be a maximum likelihood estimator. Simulation results are presented to demonstrate the high accuracy of the proposed estimator in presence of channel noise and fading.
---
paper_title: An efficient synchronization signal structure for OFDM-based cellular systems

paper_content:

OFDM has been a widely accepted technology in high rate and multimedia data service systems such as long term evolution (LTE) in the 3rd generation partnership project (3GPP). In this paper, we investigate a synchronization signal structure and corresponding cell search algorithm in the LTE system where two, primary and secondary synchronization signals are employed. We focus on the secondary synchronization signal which possesses two layered scrambling sequences in addition to basic sequences. These scrambling sequences minimize performance degradation in cell search, but incur a high complexity to a mobile station receiver. In this paper, we propose a new secondary synchronization signal structure which does not require additional scrambling sequences while maintaining almost the same performance as the current LTE scheme. We evaluate the performance of the proposed scheme under various channel environments by examining the impacts of multipath fading, frequency offset, and vehicular speed. We also compare the complexity of the proposed scheme with the LTE scheme.
---
paper_title: Low-Complexity Iterative Carrier Frequency Offset Estimation with ICI Elimination for OFDM Systems

paper_content:

In OFDM systems, carrier frequency offset (CFO) between the transmitter and the receiver destroys the orthogonality among subcarriers and induces strong inter-carrier interference (ICI). For the CFO estimation schemes based on frequency-domain pilot signals, the estimation performance is degraded and bounded by the ICI. In this work we propose a novel CFO estimation scheme - PTA-based with ICI elimination (PTA-IE), to improve the CFO estimation performance. By applying an iterative process, the CFO estimation performance can be progressively enhanced, attaining the best achievable performance without ICI. Our proposed scheme can benefit not only the CFO estimation performance but also the demodulation performance on data subcarriers. Moreover, the proposed scheme has low computational complexity without the need of matrix inversion.
---
paper_title: Efficient carrier frequency offset estimation for orthogonal frequency-division multiple access uplink with an arbitrary number of subscriber stations

paper_content:

An efficient method is proposed to estimate the carrier frequency offsets (CFOs) in the orthogonal frequency-division multiple access (OFDMA) uplink. The conventional alternating projection method is accelerated by utilising the inherited properties of the matrices involved. The multiplication of large sparse projection matrices can be elegantly transformed to a series of products involving small dense matrices, and the inverse operation of these large matrices can be substituted by direct computations. Hence, the computational cost is significantly reduced without compromising the accuracy of the CFO estimation.
---
paper_title: Signal Design for Reduced Complexity and Accurate Cell Search/Synchronization in OFDM-Based Cellular Systems

paper_content:

This paper proposes a variant of the frequency-domain synchronization structure specified in the long-term evolution (LTE) standard. In the proposed scheme, the primary synchronization signal used in step-1 cell search is the concatenation of a Zadoff-Chu (ZC) sequence and its conjugate (as opposed to only the ZC sequence in LTE). For step-2 cell search, we propose a complex scrambling sequence requiring no descrambling and a new remapped short secondary synchronization signal that randomizes the intercell interference (as opposed to the first/second scrambling sequence and swapped short signals in LTE). Through a combination of analysis and simulation, we demonstrate that the proposed synchronization signals lead to lower searcher complexity than LTE, a lower detection error rate, a shorter mean cell search time, and immunity toward a frequency offset.
---
paper_title: Network-Wide Distributed Carrier Frequency Offsets Estimation and Compensation via Belief Propagation

paper_content:

In this paper, we propose a fully distributed algorithm for frequency offsets estimation in decentralized systems. With the proposed algorithm, each node estimates its frequency offsets by local computations and limited exchange of information with its direct neighbors. Such algorithm does not require any centralized information processing or knowledge of global network topology. It is shown analytically that the proposed algorithm always converges to the optimal estimates regardless of network topology. Simulation results demonstrate the fast convergence of the algorithm and show that estimation mean-squared-error at each node touches the centralized Cram\'{e}r-Rao bound within a few {iterations of message exchange}. Therefore, the proposed method has low overhead and is scalable with network size.
---
paper_title: Implementation and Co-Simulation of Hybrid Pilot-Aided Channel Estimation With Decision Feedback Equalizer for OFDM Systems

paper_content:

This paper introduces novel hybrid pilot-aided channel estimation with decision feedback equalizer(DFE) for OFDM systems and its corresponding hardware co-simulation platform. This pilot-aided channel estimation algorithm consists of two parts: coarse estimation and fine estimation. In the coarse estimation, the combined classical channel estimation methods including carrier frequency offset (CFO) and channel impulse response (CIR) estimation are used. Based on the received training sequence and pilot tones in the frequency domain, the major CFO, sampling clock frequency offset (SFO) and CIR effect coefficients are derived. In the fine estimation, the pilot-aided polynomial interpolation estimation combined with a new decision feedback equalizer scheme based on minimum mean squared error (MMSE) criteria is proposed to reduce the residual effect caused by imperfect CIR equalizer, SFO and CFO. At the same time, for the purpose of speeding up the whole development and verification process, a new architecture of co-simulation platform which combines software and hardware is introduced. The simulation results on the co-simulation platform indicate that the proposed hybrid channel estimation scheme can enhance the receiver performance by 6 dB in terms of error vector magnitude (EVM) over large ranges of CFO and SFO and BER performance by 7 dB for SNR range over 15 dB.
---
paper_title: Blind Timing and Carrier Synchronization in Decode and Forward Cooperative Systems

paper_content:

Synchronization in Decode and Forward (DF) cooperative communication systems is a complex and challenging task requiring estimation of many independent timing and carrier offsets at each relay in the broadcasting phase and multiple timing and carrier offsets at the destination in the relaying phase. This paper presents a scheme for blind channel, timing and carrier offset estimation in a DF cooperative system with one source, M relays and one destination equipped with N antennas. In particular, we exploit blind source separation at the destination to convert the difficult problem of jointly estimating multiple synchronization parameters in the relaying phase into more tractable sub-problems of estimating many individual timing and carrier offsets for the independent relays. We also modify and propose a criteria for best relay selection at the destination. Simulation results demonstrate the excellent end-to-end Bit Error Rate (BER) performance of the proposed blind scheme with relay selection, which is shown to achieve the maximum diversity order with M = 4 relays using N = 5 antennas at the destination. The presented work is a complete solution to blind synchronization and channel estimation in DF cooperative communication systems.
---
paper_title: CRB for Carrier Frequency Offset Estimation with Pilot and Virtual Subcarriers

paper_content:

The Cramer Rao bound (CRB) for the carrier frequency offset (CFO) estimation with OFDM blocks consisting of virtual, pilot and data subcarriers is derived in this paper. From the CRB derived in the general case that all the three kinds of subcarriers exist, the CRB for the special cases that one or two kinds of the subcarriers are absent can be easily deduced. Further more, the derived CRB is a tight lower bound as well designed estimators, e.g. the maximum likelihood (ML) estimator, can achieve it at high signal to noise ratio (SNR). Thus, the derived CRB is a good benchmark for evaluating many different CFO estimators.
---
paper_title: Fine carrier and sampling frequency synchronization in OFDM systems

paper_content:

This paper investigates the joint pilot-assisted estimation of the residual carrier frequency offset (RCFO) and sampling frequency offset (SFO) in an orthogonal frequency division multiplexing (OFDM) system. As it is known, the exact maximum-likelihood (ML) solution to this problem involves a bidimensional grid-search that cannot be pursued in practice. After introducing an enlarged set of auxiliary unknown parameters, however, the RCFO and SFO recovery tasks can be decoupled and the bidimensional search is thus replaced with a simpler mono-dimensional search. This results into an estimation algorithm of reasonable complexity which is suitable for practical implementation. To further reduce the processing load, we also present an alternative scheme yielding frequency estimates in closed-form. Numerical simulations indicate that the proposed methods outperform existing estimators available in the literature in terms of both estimation accuracy and error-rate performance.
---
paper_title: Iterative Decision-Directed Joint Frequency Offset and Channel Estimation for KSP-OFDM

paper_content:

We propose an iterative decision-directed joint frequency offset (FO) and channel estimation algorithm in a known symbol padding (KSP) orthogonal frequency division multiplexing (OFDM) system, where the guard interval is filled with pilot symbols. Besides those time domain pilot symbols, some additional pilot symbols are transmitted on the pilot carriers. The decision-directed algorithm is initialized by pilot-aided FO estimation without channel knowledge. We propose a possible initialization algorithm that operates in the frequency domain (FD). After the initialization phase, the iterative decision-directed estimation algorithm is applied. For the channel estimation step, an existing pilot-aided channel estimation algorithm is extended to a decision-directed algorithm which uses the Fast Fourier Transform outputs at both the pilot and data carrier positions. For the uncoded case, the proposed iterative decision-directed joint FO and channel estimation algorithm reaches the bit error rate performance of a receiver with perfect synchronization and perfect channel knowledge. For a coded system, there is small loss in performance of less than 1 dB when our proposed algorithm is applied compared to a receiver with perfect knowledge about the FO and the channel.
---
paper_title: Spectrum Sensing for OFDM Signals Using Pilot Induced Auto-Correlations

paper_content:

Orthogonal frequency division multiplex (OFDM) has been widely used in various wireless communications systems. Thus the detection of OFDM signals is of significant importance in cognitive radio and other spectrum sharing systems. A common feature of OFDM in many popular standards is that some pilot subcarriers repeat periodically after certain OFDM blocks. In this paper, sensing methods for OFDM signals are proposed by using such repetition structure of the pilots. Firstly, special properties for the auto-correlation (AC) of the received signals are identified, from which the optimal likelihood ratio test (LRT) is derived. However, this method requires the knowledge of channel information, carrier frequency offset (CFO) and noise power. To make the LRT method practical, we then propose an approximated LRT (ALRT) method that does not rely on the channel information and noise power, thus the CFO is the only remaining obstacle to the ALRT. To handle the problem, we propose a method to estimate the composite CFO and compensate its effect in the AC using multiple taps of ACs of the received signals. Computer simulations have shown that the proposed sensing methods are robust to frequency offset, noise power uncertainty, time delay uncertainty, and frequency selectiveness of the channel.
---
paper_title: About the use of different processing domains for synchronization in non-contiguous FBMC systems

paper_content:

The problem of synchronization in non-contiguous OFDM (NC-OFDM) in the presence of other in-band systems is addressed by means of a comparison between time-and frequency-based signal processing approaches. The filterbank multicarrier scheme with its spectral-efficient offset-QAM (OQAM) OFDM implementation is thereby a good choice for an NC-OFDM system design due to its well-localized spectral signal shape. In this work, we present and discuss two NC-OFDM synchronization methods for OQAM-OFDM, employing the time domain and the already demodulated frequency domain signal for synchronization and evaluate them in terms of sustainable bit-error rate over signal-to-interference ratio and variance of time and frequency offset estimation in different resource allocation scenarios. Additionally, their computational complexity is taken into account to estimate the costs of implementation on hardware. The results show that the task of synchronization can be achieved with comparable performance in both domains with an advantage of frequency domain processing over time domain processing in terms of efficiency in preamble design and complexity.
---
paper_title: Single Carrier Modulation With Nonlinear Frequency Domain Equalization: An Idea Whose Time Has Come—Again

paper_content:

In recent years single carrier modulation (SCM) has again become an interesting and complementary alternative to multicarrier modulations such as orthogonal frequency division multiplexing (OFDM). This has been largely due to the use of nonlinear equalizer structures implemented in part in the frequency domain by means of fast Fourier transforms, bringing the complexity close to that of OFDM. Here a nonlinear equalizer is formed with a linear filter to remove part of intersymbol interference, followed by a canceler of remaining interference by using previous detected data. Moreover, the capacity of SCM is similar to that of OFDM in highly dispersive channels only if a nonlinear equalizer is adopted at the receiver. Indeed, the study of efficient nonlinear frequency domain equalization techniques has further pushed the adoption of SCM in various standards. This tutorial paper aims at providing an overview of nonlinear equalization methods as a key ingredient in receivers of SCM for wideband transmission. We review both hybrid (with filters implemented both in time and frequency domain) and all-frequency-domain iterative structures. Application of nonlinear frequency domain equalizers to a multiple input multiple output scenario is also investigated, with a comparison of two architectures for interference reduction. We also present methods for channel estimation and alternatives for pilot insertion. The impact on SCM transmission of impairments such as phase noise, frequency offset and saturation due to high power amplifiers is also assessed. The comparison among the considered frequency domain equalization techniques is based both on complexity and performance, in terms of bit error rate or throughput.
---
paper_title: Pilot Subset Partitioning Based Integer Frequency Offset Estimation for OFDM Systems With Cyclic Delay Diversity

paper_content:

Cyclic delay diversity (CDD) is a simple transmit diversity technique for coded OFDM systems with multiple transmit antennas. However, high frequency selectivity caused by CDD degrades the performance of post- FFT estimation, i.e., integer frequency offset (IFO). This paper suggests a simple way of improving the performance of the IFO estimator based on the pilot subset partitioning which is designed to reduce the effect of frequency selective fading by adopting the CDD. By partitioning uncorrelated pilot subcarriers into subsets to satisfy high correlation, and performing frequency estimation for each pilot subset, a robust IFO estimation scheme is derived. The simulation results show that the proposed method can provide benefit to the overall system performance .
---
paper_title: A practical equalizer for cooperative delay diversity with multiple carrier frequency Offsets

paper_content:

Cooperative transmission in wireless networks provides diversity gain in multipath fading environments. Among all the proposed cooperative transmission schemes, delay diversity has the advantage of needing less coordination and higher spectrum efficiency. However, in a distributed network, the asynchrony comes from both carrier frequency and symbol timing between the cooperating relays. In this paper, a minimum mean square error fractionally spaced decision feedback equalizer (MMSE-FS-DFE) is developed to extract the diversity with large multiple carrier frequency offsets, its performance approaches the case without multiple carrier frequency offsets. The front end design for the receiver in this scenario is discussed, and a practical frame structure is designed for carrier frequency offsets and channel estimation. A subblock-wise decision-directed adaptive least squares (LS) estimation method is developed to solve the problem caused by error in frequency offset estimation. The purpose of this paper is to provide a practical design for cooperative transmission (CT) with the delay diversity scheme.
---
paper_title: FADAC-OFDM: Frequency-Asynchronous Distributed Alamouti-Coded OFDM

paper_content:

We propose frequency-asynchronous distributed Alamouti-coded orthogonal frequency-division multiplexing (FADAC-OFDM). The proposed scheme effectively mitigates the intercarrier interference (ICI) due to frequency offset (FO) between two distributed antennas. The transmitter side of the proposed scheme transmits each of the two subslots in Alamouti code through two remote subcarriers symmetric to the center frequency and is referred to as space–frequency reversal schemes by Wang et al. or Choi. The receiver side of the proposed scheme is significantly different from the conventional scheme or the scheme proposed by Wang et al. in that it performs two discrete Fourier transform (DFT), each of which is synchronized to each transmit antenna's carrier frequency. The decision variables are generated by performing a simple linear combining with the same complexity as that of the conventional Alamouti code. The derivation shows that in flat-fading channels, the dominant ICI components due to FO cancel each other during the combining process. The proposed scheme achieves almost the same performance as the ideal Alamouti scheme, even with large FO. To use this ICI self-cancellation property for selective fading channels or in cases with timing offset (TO) between two transmit antennas, the total subcarriers are divided into several subblocks, and the proposed scheme is applied to each subblock. For mildly selective channels or cases with practically small TO, the proposed scheme achieves significantly improved performance compared with the conventional space–frequency Alamouti-coded OFDM.
---
paper_title: Carrier frequency synchronization in the downlink of 3GPP LTE

paper_content:

In this paper, we investigate carrier frequency synchronization in the downlink of 3GPP Long Term Evolution (LTE). A complete carrier frequency offset estimation and compensation scheme based on standardized synchronization signals and reference symbols is presented. The estimation performance in terms of mean square error is derived analytically and compared to simulation results. The impact of estimation error on the system performance is shown in terms of uncoded bit error ratio and physical layer coded throughput. Compared to perfect synchronization, the presented maximum likelihood estimator shows hardly any performance loss, even when the most sophisticated MIMO schemes of LTE are employed.
---
paper_title: Maximum likelihood algorithms for joint estimation of synchronisation impairments and channel in multiple input multiple output–orthogonal frequency division multiplexing system

paper_content:

Maximum likelihood (ML) algorithms, for the joint estimation of synchronisation impairments and channel in multiple input multiple output–orthogonal frequency division multiplexing (MIMO–OFDM) system, are investigated in this work. A system model that takes into account the effects of carrier frequency offset, sampling frequency offset, symbol timing error and channel impulse response is formulated. Cramer–Rao lower bounds for the estimation of continuous parameters are derived, which show the coupling effect among different impairments and the significance of the joint estimation. The authors propose an ML algorithm for the estimation of synchronisation impairments and channel together, using the grid search method. To reduce the complexity of the joint grid search in the ML algorithm, a modified ML (MML) algorithm with multiple one-dimensional searches is also proposed. Further, a stage-wise ML (SML) algorithm using existing algorithms, which estimate less number of parameters, is also proposed. Performance of the estimation algorithms is studied through numerical simulations and it is found that the proposed ML and MML algorithms exhibit better performance than SML algorithm.
---
paper_title: Joint AOD and CFO estimation in wireless sensor networks localization system

paper_content:

Self-localization system of wireless sensor networks based on angle of departure (AOD) is studied in this paper. In AOD model, anchor node with multi-antenna transmits orthogonal pilot signals to sensor node with single antenna, which entitles the sensor node to function as equipping multi-antenna. Given that the limited number of antenna at anchor nodes will result in degree of freedom (DOF) deficiency at sensor node when dealing with more multipath components (MPC), we adopt a novel method without requiring extra antennas. The proposed algorithm simultaneously includes the oscillator mismatch between anchor node and sensor node, i.e., carrier frequency offset (CFO). With the aid of anchor's movement and CFO, the equivalent antenna array at sensor node is expended by synthetic aperture procedure to a much larger one, which subsequently improve the estimation ability to MPCs. In addition, the close-form solutions of CFO and AOD are also derived. The effectiveness and performance of proposed algorithm are demonstrated by numerical simulations.
---
paper_title: Classification of Space-Time Block Codes Based on Second-Order Cyclostationarity with Transmission Impairments

paper_content:

Signal classification is important in various commercial and military applications. Multiple antenna systems complicate the signal classification problem since there is now the issue of estimating the number and configuration of transmit antennas. The novel blind classification algorithm proposed in this paper exploits the cyclostationarity property of space-time block codes (STBCs) for the classification of multiple antenna systems in the presence of possible transmission impairments. Analytical expressions for the second-order cyclic statistics used as the basis of the algorithm are derived, and the computational cost of the proposed algorithm is considered. This algorithm avoids the need for a priori knowledge of the channel coefficients, modulation, carrier phase, and timing offsets. Moreover, it does not need accurate information about the transmission data rate and carrier frequency offset. Monte Carlo simulation results demonstrate a good classification performance with low sensitivity to phase noise and channel effects, including frequency-selective fading and Doppler shift.
---
paper_title: A Novel Algebraic Carrier Frequency Offset Estimator for ASTC-MIMO-OFDM Systems Over a Correlated Frequency-Selective Channel

paper_content:

This paper presents a new algebraic carrier frequency offset (CFO) estimation technique for multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) system, to overcome the sensitivity of algebraic space-time codes (ASTCs) to frequency synchronization in quasi-static correlated Rayleigh frequency-selective fading channels. The technique uses a preamble and is thus particularly suitable for burst-mode communications. The preamble consists of orthogonal training sequences that are simultaneously transmitted from the various transmit antennas. The proposed system exploits all subcarriers in the frequency domain, which provides a remarkable performance improvement, and reaches the Cramer-Rao lower bound (CRLB) at a high signal-to-noise ratio. The proposed method is compared with three known CFO estimators in the literature: Cyclic-Prefix-based (CP), Moose, and Classen techniques that show clear advantages.
---
paper_title: Blind carrier frequency offset estimator for multi-input multi-output-orthogonal frequency division multiplexing systems over frequency-selective fading channels

paper_content:

This study presents a new blind carrier frequency offset (CFO) estimation technique for multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems employing space-time coding (STC). CFO estimation is crucial for OFDM systems to avoid the performance degradation because of the inter-carrier interference that results when the CFO is not estimated and compensated accurately. Based on the assumptions that the data symbols are selected from a constant modulus constellation and the channel is varying slowly over time, a new blind CFO estimator is proposed by minimising the power difference between all subcarriers in two consecutive STC blocks. Therefore the proposed system exploits all subcarriers in time and frequency domain, which provides a remarkable performance improvement over other techniques reported in the literature. The complexity of the proposed estimator is substantially reduced by approximating the cost function by a sinusoid that can be minimised using direct closed-form computations within one OFDM symbol period. Monte Carlo simulations are used to assess the performance of the proposed system by means of mean squared error (MSE) in both static and time-varying frequency-selective fading channels. The simulation results demonstrate that the proposed estimator can eliminate the MSE error floors that usually appear at moderate and high signal-to-noise ratios for the estimators that work only in frequency domain.
---
paper_title: A Compact Preamble Design for Synchronization in Distributed MIMO OFDM Systems

paper_content:

In distributed multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems, signals arrive at the receiver on different timing and are characterized by distinct carrier frequency offsets (CFOs), which makes synchronization rather challenging than that associated with centralized MIMO systems. Current solutions to this problem are mainly based on special preamble designs, where different training sequences are cascaded and then separately used to assist timing synchronization and CFO estimation. Such preamble designs not only increase system overhead but also burden the receivers with independent algorithms for timing synchronization and CFO estimation. In this paper, we propose a low-overhead (compact) preamble having the same length as one OFDM symbol, along with a unified algorithm for both timing synchronization and CFO estimation. Furthermore, the CFO estimation range can be flexibly extended to cope with larger CFOs in the proposed approach. Under the same training overhead and power consumption, simulation results indicate that the proposed approach outperforms a timing synchronization scheme that based on unequal period synchronization patterns.
---
paper_title: Semi-blind MIMO OFDM systems with precoding aided CFO estimation and ICA based equalization

paper_content:

We propose a semi-blind multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system, with a precoding aided carrier frequency offset (CFO) estimation approach, and an independent component analysis (ICA) based equalization structure. A number of reference data sequences are carefully designed offline and are superimposed to source data via a non-redundant linear precoding process, which can kill two birds with one stone, without introducing any extra total transmit power and spectral overhead. First, the reference data sequences are selected from a pool of carefully designed orthogonal sequences. The CFO estimation is to minimize the sum cross-correlation between the CFO compensated signals and the rest orthogonal sequences in the pool. Second, the same reference data enable elimination of the permutation and quadrant ambiguity in the ICA equalized signals by maximizing the cross-correlation between the ICA equalized signals and the reference data. Simulation results show that, without extra bandwidth and power needed, the proposed semi-blind system achieves a bit error rate (BER) performance close to the ideal case with perfect channel state information (CSI) and no CFO. Also, the precoding aided CFO estimation outperforms the constant amplitude zero autocorrelation (CAZAC) sequences based CFO estimation approach, with no spectral overhead.
---
paper_title: Linear Least Squares CFO Estimation and Kalman Filtering Based I/Q Imbalance Compensation in MIMO SC-FDE Systems

paper_content:

This paper investigates carrier frequency offset (CFO) estimation and inphase/quadrature (I/Q) imbalance compensation in time-varying frequency-selective channels. We first propose a linear least squares (LLS) CFO estimation approach which has a lower complexity and a higher accuracy than the previous nonlinear CFO estimation methods. We then propose a Kalman filtering based I/Q imbalance compensation approach in the presence of CFO, which demonstrates a good ability to track the channel time variations with a fast convergence speed, by nulling the cyclic prefix (CP) and including the CFO in the state vector of the equivalent channel model. The proposed Kalman filtering based I/Q imbalance compensation approach with associated equalization tracks the time variation with a fast convergence speed. Simulation results show that the proposed compensation approach for CFO and I/Q imbalance provides a bit error rate (BER) performance close to the ideal case with perfect channel state information (CSI), no CFO and no I/Q imbalance.
---
paper_title: Reduced-complexity baseband compensation of joint Tx/Rx I/Q imbalance in mobile MIMO-OFDM

paper_content:

Direct-conversion multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) transceivers enjoy high data rates and reliability at practical implementation complexity. However, analog front-end impairments such as I/Q imbalance and high mobility requirements of next-generation broadband wireless standards result in performance-limiting inter-carrier interference (ICI). In this paper, we study the effects of ICI due to these impairments for OFDM with space frequency block codes and spatial multiplexing, derive a generalized linear model and propose a non-iterative reduced-complexity digital baseband joint compensation scheme. Furthermore, we present a pilot scheme for joint estimation of the channel and the I/Q imbalance parameters and evaluate its performance through simulations. Our proposed scheme is effective in estimating and compensating for frequency-independent and frequency-dependent transmit and receive I/Q imbalances even in the presence of a residual frequency offset.
---
paper_title: Iterative Receiver Design With Joint Doubly Selective Channel and CFO Estimation for Coded MIMO-OFDM Transmissions

paper_content:

This paper is concerned with the problem of turbo (iterative) processing for joint channel and carrier frequency offset (CFO) estimation and soft decoding in coded multiple-input-multiple-output (MIMO) orthogonal frequency-division-multiplexing (OFDM) systems over time- and frequency-selective (doubly selective) channels. In doubly selective channel modeling, a basis expansion model (BEM) is deployed as a fitting parametric model to reduce the number of channel parameters to be estimated. Under pilot-aided Bayesian estimation, CFO and BEM coefficients are treated as random variables to be estimated by the maximum a posteriori technique. To attain better estimation performance without sacrificing spectral efficiency, soft bit information from a soft-input-soft-output (SISO) decoder is exploited in computing soft estimates of data symbols to function as pilots. These additional pilot signals, together with the original signals, can help to enhance the accuracy of channel and CFO estimates for the next iteration of SISO decoding. The resulting turbo estimation and decoding performance is enhanced in a progressive manner by benefiting from the iterative extrinsic information exchange in the receiver. Both extrinsic information transfer chart analysis and numerical results show that the iterative receiver performance is able to converge fast and close to the ideal performance using perfect CFO and channel estimates.
---
paper_title: An Efficient Time Synchronization Scheme for Broadband Two-Way Relaying Networks Based on Physical-Layer Network Coding

paper_content:

We present an efficient time synchronization scheme for broadband two-way relaying networks based on two-phase physical layer network coding. Especially, a preamble structure is proposed in this letter for the synchronization. The synchronization approach exploits the preamble in frequency domain and time domain to effectively separate the mixed signals, and jointly estimate timing-offsets and channel parameters, respectively. Numerical results confirm that the suggested method is superior to the conventional scheme, and is very suitable for the synchronization in broadband two-way relaying networks based on two-phase physical layer network coding.
---
paper_title: One-Shot Blind CFO and Channel Estimation for OFDM With Multi-Antenna Receiver

paper_content:

In this paper, we design a new blind joint carrier frequency offset (CFO) and channel estimation method for orthogonal frequency-division multiplexing (OFDM) with multiantenna receiver. The proposed algorithm requires only one received OFDM block and thus belongs to the category of one-shot estimation methods. Other advantages of the proposed algorithm include 1) it supports fully loaded data carriers and is thus spectral efficient; 2) the channel from the transmitter to each receive antenna can be blindly estimated with only a scaling ambiguity; and 3) the algorithms outperforms the existing methods. Moreover, we derive the Cramer-Rao Bounds (CRB) of joint CFO and channel estimation in closed forms. Numeral results not only show the effectiveness of the proposed algorithm but also demonstrate its closed performance to CRB.
---
paper_title: Fast Kalman Equalization for Time-Frequency Asynchronous Cooperative Relay Networks With Distributed Space-Time Codes

paper_content:

Cooperative relay networks are inherently time and frequency asynchronous due to their distributed nature. In this correspondence, we propose a transceiver scheme to combat both time and frequency offsets for cooperative relay networks with multiple relay nodes. At the relay nodes, a distributed linear convolutive space-time coding is adopted, which has the following advantages: 1) Full cooperative diversity can be achieved using a minimum mean square error (MMSE) or MMSE decision feedback equalizer (MMSE-DFE) detector, instead of a maximum-likelihood receiver when only time asynchronism exists. 2) The resultant equivalent channel possesses some special structure, which can be exploited to reduce the equalization complexity at the destination node. By taking full advantage of such a special structure, fast Kalman equalizations based on linear MMSE and MMSE-DFE are proposed for the receiver, where the estimation of the state vector (information symbols) can be recursively taken and become very computationally efficient, compared with direct equalization. The proposed scheme can achieve considerable diversity gain with both time and frequency offsets and applies to frequency-selective fading channels.
---
paper_title: Improved CIR-Based Receiver Design for DVB-T2 System in Large Delay Spread Channels: Synchronization and Equalization

paper_content:

This paper proposes to implement an improved orthogonal frequency division multiplexing (OFDM) receiver by utilizing a channel impulse response (CIR)-based synchronization and sparse equalization for DVB-T2 system operating in both the single-input single-output (SISO) and multi-input single-output (MISO) transmission modes. First, the proposed OFDM receiver performs a pilot-aided CIR estimation after a coarse symbol timing recovery (STR). Then, the proposed CIR-based fine STR compensates for a false symbol timing offset (STO). In particular, the fine STR resolves an ambiguity effect of CIR, which is the main problem caused by a false coarse STO in exploiting the CIR. Upon the completion of the fine synchronization, the proposed CIR-based sparse equalization is performed in order to minimize the noise and interference effects by shifting or selecting a basic frequency interpolation (FI) filter according to an echo delay (phase) or maximum delay spread, respectively. Performance evaluations are accomplished in large delay spread channels in which the maximum delay spread is less or longer than a guard interval (GI). It is shown that the proposed receiver is not only capable of estimating the fine STO but also minimizing effectively the noise effects. In particular, the performance gain in a single pre-echo channel being longer than GI is remarkable as compared with a conventional receiver.
---
paper_title: Joint maximum-likelihood estimation of frequency offset and channel coefficients in multiple-input multiple-output orthogonal frequency-division multiplexing systems with timing ambiguity

paper_content:

In this study, the authors consider a multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing system operating over unknown frequency-selective fading channels. Based on two different signal models, two maximum-likelihood schemes for joint carrier frequency offset (CFO) and MIMO channel estimation under timing ambiguity have been derived. The first estimator jointly estimates the timing offset, CFO and channel coefficients, but the complexity is high owing to the need of two-dimensional searches. The second estimator has a reduced complexity, but its performance slightly degrades compared with the first one. The performance of proposed estimators was benchmarked with Cramer–Rao bounds (CRB), and investigated by computer simulations. The simulation results show that the proposed estimators achieve almost ideal performances compared with the CRB.
---
paper_title: Semiblind Iterative Receiver for Coded MIMO-OFDM Systems

paper_content:

In this paper, a semiblind iterative receiver is proposed for coded multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) systems. A novel iterative extended soft-recursive least square (IES-RLS) estimator for joint channel and frequency offset estimation is introduced. Extrinsic bit information obtained from the channel decoder and expected symbol decisions obtained from the demodulator are simultaneously exploited by the IES-RLS. The proposed receiver combines the MIMO data demodulator, the proposed channel estimator, and the channel decoder in an iterative manner to yield an improved bit error rate (BER) performance. To arrive at a feasible algorithm, the first-order linearization of the received vector signal with respect to the frequency offset is used in the IES-RLS channel estimator. The BER performance, a constellation-constrained mutual information analysis, and an EXIT chart analysis are used to verify the effectiveness of the proposed receiver. Simulation results show the superiority of the proposed semiblind receiver, compared with conventional semiblind receivers.
---
paper_title: Fractional timing offset and channel estimation for MIMO OFDM systems over flat fading channels

paper_content:

This paper addresses the problem of fractional timing offset and channel estimation in Multiple input Multiple output orthogonal frequency division multiplexing (MIMO OFDM) systems. The estimators have been derived assuming a flat fading channel and using the maximum likelihood criterion. Closed form Cramer Rao bound (CRB) expressions for fractional timing offset and channel response are also derived. Simulation results have been used to cross-check the accuracy of the proposed estimation algorithm.
---
paper_title: Parameter Estimation and Tracking in Physical Layer Network Coding

paper_content:

In this paper, we present an algorithm for joint decoding of the modulo-2 sum of the bits transmitted from two unsynchronized transmitters using Physical Layer Network Coding (PLNC). We address the problems that arise when the boundaries of the signals do not align with each other and when the channel parameters are slowly varying and are not known to the receiver at the relay node. Our approach first estimates jointly the timing and fading gains of both the signals, and uses a state-based Viterbi decoding scheme that takes into account the timing offsets between the interfering signals. We also track the amplitude and phase of the channel which may be slowly varying. Simulation results demonstrate the sensitivity of the detection performance at the relay node to the relative offset of the timings of the two user's signals as well as the advantage of our algorithm over previously published algorithms.
---
paper_title: Blind timing and carrier synchronisation in distributed multiple input multiple output communication systems

paper_content:

This study addresses the problem of joint blind timing and carrier synchronisation in a (distributed-M ) × N antenna system where the objective is to estimate the M carrier offsets, the M timing offsets and to recover the transmitted symbols for each of the M users given only the measured signal at the N antennas of the receiver. The authors propose a modular receiver structure that exploits blind source separation to reduce the problem into more tractable sub-problems of estimating individual timing and carrier offsets for multiple users. This leads to a robust solution of low complexity. The authors investigate the performance of the estimators analytically using modified Cramer- Rao bounds and computer simulations. The results show that the proposed receiver exhibits robust performance over a wide range of parameter values, even with worst-case Doppler of 200- 300 Hz and frame size as small as 400 symbols. This work is relevant to future wireless networks and is a complete solution to the problem of estimating multiple timing and carrier offsets in distributed multiple input multiple output (MIMO) communication systems.
---
paper_title: Preamble Based Joint CFO, Frequency-Selective I/Q-Imbalance and Channel Estimation and Compensation in MIMO OFDM Systems

paper_content:

A very promising technical approach for future wireless communication systems is to combine MIMO OFDM and Direct (up/down) Conversion Architecture (DCA). However, while OFDM is sensitive to Carrier Frequency Offset (CFO), DCA is sensitive to I/Q-imbalance. Such RF impairments can seriously degrade the system performance. For the compensation of these impairments, a preamble-based scheme is proposed in this paper for the joint estimation of CFO, transmitter (Tx) and receiver (Rx) frequency-selective I/Q-imbalance and the MIMO channel. This preamble is constructed both in time- and frequency domain and requires much less overhead than the existing designs. Moreover, Closed-Form Estimators (CLFE) are allowed, enabling efficient implementation. The advantages and effectiveness of the proposed scheme have been verified by numerical simulations.
---
paper_title: Iterative Joint Estimation Procedure for Channel and Frequency Offset in Multi-Antenna OFDM Systems With an Insufficient Cyclic Prefix

paper_content:

This paper addresses a strategy to improve the joint channel and frequency offset (FO) estimation in multi-antenna systems, widely known as multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM), in the presence of intersymbol interference (ISI) and intercarrier interference (ICI) occasioned by an insufficient cyclic prefix (CP). The enhancement is attained by the use of an iterative joint estimation procedure (IJEP) that successively cancels the interferences located in the preamble of the OFDM frame, which is used for the joint estimation and initially contains the interferences due to a CP shorter than the channel length. The IJEP requires at certain steps a proper iterative interference cancellation algorithm, which makes use of an initial FO compensation and channel estimation obtained due to the use of a symmetric sequence in the preamble. After the iterative cancellation of interferences, the procedure performs an additional joint channel and FO estimation whose mean square error converges to the Cramér-Rao bound (CRB). Later on, this subsequent joint estimation permits the removal of the interferences in the data part of the frame, which are also due to an insufficient CP, in the same iterative fashion but saving iterations compared with the use of other estimation strategies. The appraisal of the procedure has been performed by assessing the convergence of the simulated estimators to the CRB as a function of the number of iterations. Additionally, simulations for the evaluation of the bit error rate (BER) have been carried out to probe how the utilization of the proposed IJEP clearly improves the performance of the system. It is concluded that, with a reduced number of iterations in the preamble, the IJEP converges to the theoretical bounds, thus reducing the disturbances caused by a hard wireless channel or a deliberately insufficient CP. Therefore, this eases the interference cancellation in the data part, leading to an improvement in the BER that approximates to the ideal case of a sufficient CP and, consequently, an improvement in the computational cost of the whole procedure that has been analyzed.
---
paper_title: Joint CFO and Channel Estimation for Asynchronous Cooperative Communication Systems

paper_content:

This letter addresses the joint maximum likelihood carrier frequency offset (CFO) and channel estimation for asynchronous cooperative communication systems. We first present a space-alternating generalized expectation-maximization (SAGE) based iterative estimator (SAGE-IE). Then a low-complexity approximate SAGE-IE (A-SAGE-IE) is developed. Our proposed algorithms decouple the multi-dimensional optimization problem into many one-dimensional optimization problems where the CFO and channel coefficients of each relay-destination link can be determined separately. Simulations indicate that, even though timing offsets are present, the proposed estimators can asymptotically achieve the Cramer-Rao bround (CRB) for the perfectly timing synchronized case.
---
paper_title: Integer frequency offset estimation by pilot subset selection for OFDM system with CDD

paper_content:

Cyclic delay diversity (CDD) is a simple transmit diversity technique for an OFDM system using multiple transmit antennas. However, the performance of post-FFT estimation, i.e. integer frequency offset (IFO) is deteriorated by high frequency selectivity introduced by CDD. Proposed is an IFO estimation scheme for an OFDM system with CDD. Based on the pilot subset partitioning, the proposed IFO estimation scheme reduces the effect of frequency selective fading by adopting the CDD.
---
paper_title: Joint estimation of Carrier and Sampling Frequency Offset, phase noise, IQ Offset and MIMO channel for LTE Advanced UL MIMO

paper_content:

In LTE Advanced Uplink MIMO the pilot symbols on a subcarrier and OFDM symbol are not transmitted exclusively by one layer. If the pilots are transmitted exclusively, like in LTE Advanced Downlink or Mobile WiMAX, the estimation of Carrier Frequency Offset (CFO) and Sampling Frequency Offset (SFO) can be based on correlating two pilot symbols at different OFDM symbols. In addition the estimation of CFO / SFO can be performed separately from IQ Offset and channel estimation. In LTE Advanced Uplink (UL) the pilot symbols on a subcarrier and OFDM symbol are transmitted by all layers simultaneously. As the received symbol consists of the sum of all transmitted pilot symbols, the CFO / SFO estimation approaches used for SISO seem not applicable anymore. This paper introduces a joint estimation of carrier and sampling frequency offset, phase noise, IQ offset and MIMO channel for LTE Advanced UL MIMO.
---
paper_title: E2KF based joint multiple CFOs and channel estimate for MIMO-OFDM systems over high mobility scenarios

paper_content:

An enhanced extended Kalman filtering (E2KF) algorithm is proposed in this paper to cope with the joint multiple carrier frequency offsets (CFOs) and time-variant channel estimate for MIMO-OFDM systems over high mobility scenarios. It is unveiled that, the auto-regressive (AR) model not only provides an effective method to capture the dynamics of the channel parameters, which enables the prediction capability in the EKF algorithm, but also suggests an method to incorporate multiple successive pilot symbols for the improved measurement update.
---
paper_title: Channel Equalization and Symbol Detection for Single-Carrier MIMO Systems in the Presence of Multiple Carrier Frequency Offsets

paper_content:

A new frequency-domain channel equalization and symbol detection scheme is proposed for multiple-input-multiple-output (MIMO) single-carrier broadband wireless systems in the presence of severely frequency-selective channel fading and multiple unknown carrier-frequency offsets (CFOs). Multiple CFOs cause severe phase distortion in the equalized data for large block lengths and/or constellation sizes, thus yielding poor detection performance. Instead of explicitly estimating the CFOs and then compensating them, the proposed scheme estimates the rotated phases (not frequencies) caused by multiple unknown CFOs and then removes the phase rotations from the equalized data before symbol detection. The estimation accuracy of the phase rotation is improved by utilizing a groupwise method rather than symbol-by-symbol methods. This paper differs from other related work in orthogonal frequency division multiplexing (OFDM) studies in that it can combat multiple CFOs that are time varying within each block. Numerical examples for 4 × 2 and 8 × 4 single-carrier systems with quaternary phase-shift keying (QPSK) and eight-phase-shift keying (8PSK) modulation illustrate the effectiveness of the proposed scheme in terms of scatter plots of constellation, mean square error (MSE), and bit error rate (BER).
---
paper_title: Joint Carrier Frequency Offset and Channel Estimation for MIMO-OFDM Systems Using Extended H_{∞} Filter

paper_content:

We address the problem of joint carrier frequency offset (CFO) and channel estimation for multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems over time-varying channels. The CFOs between different pairs of transmit and receive antennas are considered to be different. The method is derived based on the extended H∞ filter (EHF). Compared to the conventional extended Kalman filter (EKF)-based method, the EHF-based method does not require any knowledge of the noise distributions, so it is more flexible and robust to the unknown disturbances. Simulation results are provided to illustrate the performance of this method.
---
paper_title: Diversity analysis of distributed linear convolutive space-time codes for time-frequency asynchronous cooperative networks

paper_content:

This study analyses the achievable cooperative diversity order of the distributed linear convolutional space-time coding (DLC-STC) scheme for time–frequency asynchronous cooperative networks. The authors first prove that perfect time or frequency synchronisation is impractical for cooperative networks with multiple relays serving multiple destinations even when the relays know all accurate time delays and frequency offsets. Then the DLC-STC scheme, in which the exact time synchronisation at the relay nodes is unnecessary, is introduced into this type of cooperative networks. This study proves that the achievable time–frequency asynchronous cooperative diversity order of the DLC-STC scheme with maximum-likelihood receivers is equal to the number of relays. Simulation results verify the analysis.
---
paper_title: An efficient algorithm for space-time block code classification

paper_content:

This paper proposes a novel and efficient algorithm for space-time block code (STBC) classification, when a single antenna is employed at the receiver. The algorithm exploits the discriminating features provided by the discrete Fourier transform (DFT) of the fourth-order lag products (FOLPs) of the received signal. It does not require estimation of the channel, signal-to-noise ratio (SNR), and modulation of the transmitted signal. Computer simulations are conducted to evaluate the performance of the proposed algorithm. The results show the validity of the algorithm, its robustness to carrier frequency offset, and low sensitivity to timing offset.
---
paper_title: Timing Estimation and Resynchronization for Amplify-and-Forward Communication Systems

paper_content:

This paper proposes a general framework to effectively estimate the unknown timing and channel parameters, as well as design efficient timing resynchronization algorithms for asynchronous amplify-and-forward (AF) cooperative communication systems. In order to obtain reliable timing and channel parameters, a least squares (LS) estimator is proposed for initial estimation and an iterative maximum-likelihood (ML) estimator is derived to refine the LS estimates. Furthermore, a timing and channel uncertainty analysis based on the Crame?r-Rao bounds (CRB) is presented to provide insights into the system uncertainties resulted from estimation. Using the parameter estimates and uncertainty information in our analysis, timing resynchronization algorithms that are robust to estimation errors are designed jointly at the relays and the destination. The proposed framework is developed for different AF systems with varying degrees of timing misalignment and channel uncertainties and is numerically shown to provide excellent performances that approach the synchronized case with perfect channel information.
---
paper_title: Joint estimation and suppression of phase noise and carrier frequency offset in multiple-input multiple-output single carrier frequency division multiple access with single-carrier space frequency block coding

paper_content:

Carrier frequency offset (CFO) and phase noise are challenging problems in single carrier frequency division multiple access (SC-FDMA) system. In this study, the authors have studied single-carrier space frequency block coding (SC-SFBC) to reduce the peak to average power ratio (PAPR) of the multiple-input multiple-output (MIMO) SC-FDMA signal, in which Alamouti SFBC will change the signal spectrum structure, break the single-carrier property and increase PAPR. Also, the authors propose a joint algorithm to suppress the inter-carrier interference (ICI) caused by phase noise and CFO. Conventional methods are to estimate phase noise and CFO in other algorithms, which are pretty difficult and complicated to obtain accurate estimation results. Unlike the conventional works, the novelty of the authors proposed algorithm is that it directly calculates the interference components and then reconstructs the ICI matrix. Thus, it avoids the degrading interactions between phase noise and CFO estimations. The proposed algorithm exploits block-type pilot, which is a common pilot pattern in SC-FDMA communications and is used in other wireless communication standards. Simulation results show that the suppression performance keeps smooth while phase noise and CFO varies, and BER performance degradation can be significantly reduced in 3 dB.
---
paper_title: Full Diversity Space-Frequency Codes for Frequency Asynchronous Cooperative Relay Networks with Linear Receivers

paper_content:

In a previous work, we presented a technique that allows verifying the conformance between Java implementations and UML class diagrams, using design tests. While it allowed verifying conformance it does so in a sense that does not deal adequately with some design patterns. In such scenarios, there are semantic constraints among the involved elements that UML does not allow our technique to recognize them. Therefore, if one evolves the implementation violating the design pattern, the generated design tests will no longer be capable of detecting the design violation. To address this problem, we propose in this paper an approach based on: (1) UML profiles to explicitly tag UML incorporating design patterns; and (2) a set of design test templates able to recognize the appropriate implementation of these design patterns on Java code. We also present a prototype capable of automatically generating the design tests to verify the design patterns explicated by the UML profile.
---
paper_title: Blind Maximum Likelihood Carrier Frequency Offset Estimation for OFDM With Multi-Antenna Receiver

paper_content:

In this paper, based on the maximum likelihood (ML) criterion, we propose a blind carrier frequency offset (CFO) estimation method for orthogonal frequency division multiplexing (OFDM) with multi-antenna receiver. We find that the blind ML solution in this situation is quite different from the case of single antenna receiver. As compared to the conventional MUSIC-like CFO searching algorithm, our proposed method not only has the advantage of being applicable to fully loaded systems, but also can achieve much better performance in the presence of null subcarriers. It is demonstrated that the proposed method also outperforms several existing estimators designed for multi-antenna receivers. The theoretical performance analysis and numerical results are provided, both of which demonstrate that the proposed method can achieve the Cramer-Rao bound (CRB) under the high signal-to-noise ratio (SNR) region.
---
paper_title: Joint Semi-Blind Channel Estimation and Synchronization in Two-Way Relay Networks

paper_content:

In this paper, we propose a synchronization and channel estimation method for amplify-and-forward two-way relay networks (AF-TWRNs) based on a low-complexity maximum-likelihood (LCML) algorithm and a joint synchronization and channel estimation (JSCE) algorithm. For synchronous AF-TWRNs, the LCML algorithm blindly estimates general nonreciprocal flat-fading channels. We formulate the channel estimation as a convex optimization problem and obtain a closed-form channel estimator. Based on the mean square error (MSE) analysis of the LCML algorithm, we propose a generalized LCML (GLCML) algorithm to perform channel estimation in the presence of the timing offset. Based on the approximation of the LCML algorithm, the JSCE algorithm is proposed to estimate jointly the timing offset and channel parameters. The theoretical analysis shows that the closed-form LCML channel estimator is consistent and unbiased. The analytical MSE expression shows that the estimation error approaches zero in scenarios with either a high signal-to-noise ratio (SNR) or a large frame length. Monte Carlo simulations are employed to verify the theoretical MSE analysis of the LCML algorithm. In the absence of perfect timing synchronization, the GLCML algorithm selects an estimation sample, which produces the optimal channel estimation, according to the MSE analysis. Simulation results also demonstrate that the JSCE algorithm is able to achieve accurate timing offset estimation.
---
paper_title: Differential modulation for amplify-and-forward two-way relaying with carrier offsets

paper_content:

In this paper, differential modulation (DM) schemes, including single differential and double differential, are proposed for amplify-and-forward two-way relaying (TWR) networks with unknown channel state information (CSI) and carrier frequency offsets. Most existing work in TWR assumes perfect channel knowledge at all nodes and no carrier offsets. However, accurate CSI can be difficult to obtain for fast varying channels while increases computational complexity in channel estimation, and commonly existing carrier offsets can greatly degrade the system performance. Therefore, we propose two schemes to remove the effect of unknown frequency offsets for TWR networks, when neither the sources nor the relay has any knowledge of CSI. Simulation results show that the proposed differential modulation schemes are both effective in overcoming the impact of carrier offsets with linear computational complexity.
---
paper_title: Effective Symbol Timing Recovery Based on Pilot-Aided Channel Estimation for MISO Transmission Mode of DVB-T2 System

paper_content:

This paper proposes an effective symbol timing recovery (STR) based on a pilot-aided channel impulse response (CIR) estimation for multi-input single-output (MISO) transmission mode of DVB-T2 system. In particular, this paper focuses on fine STR capable of resolving an ambiguity effect of the CIR which is caused by an inaccurate coarse symbol timing offset (STO). In the proposed fine STR, the CIR of the MISO channel is estimated after performing coarse STR. Then, the ambiguity of the CIR is investigated by categorizing it into four regions under the assumption of an inaccurate STO. Finally, accurate STO is estimated by changing the fast Fourier transform (FFT) window with respect to the ambiguity categorization. Performance evaluations are accomplished by comparing the proposed STR with the conventional STR in large delay channels.
---
paper_title: Blind Identification of Spatial Multiplexing and Alamouti Space-Time Block Code via Kolmogorov-Smirnov (K-S) Test

paper_content:

A novel algorithm for blind identification of spatial multiplexing and Alamouti space-time block code is proposed in this paper. It relies on the Kolmogrov-Smirnov test, and employs the maximum distance between the empirical cumulative distribution functions of two statistics derived from the received signal. The proposed algorithm does not require estimation of the channel coefficients, noise statistics and modulation type, and is robust to the carrier frequency offset and impulsive noise. Additionally, it outperforms the algorithms in the literature under a variety of transmission impairments.
---
paper_title: Physical-Layer Network Coding Using FSK Modulation under Frequency Offset

paper_content:

Physical-layer network coding is a protocol capable of increasing throughput over conventional relaying in the two- way relay channel, but is sensitive to phase and frequency offsets among transmitted signals. Modulation techniques which require no phase synchronization such as noncoherent FSK can compensate for phase offset, however, the relay receiver must still compensate for frequency offset. In this work, a soft- output noncoherent detector for the relay is derived, under the assumption that the source oscillators generating FSK tones lack frequency synchronization. The derived detector is shown through simulation to improve error rate performance over a conventional detector which does not model offset, for offset values on the order of a few hundredths of a fraction of FSK tone spacing.
---
paper_title: A New ML Detector for SIMO Systems with Imperfect Channel and Carrier Frequency Offset Estimation

paper_content:

The objective of this paper is to develop a new detection algorithm for single input multiple output (SIMO) systems, using maximum likelihood (ML) scheme. The proposed method takes into account both channel and carrier frequency offset (CFO) estimation errors for detection of the transmitted data. Simulation results show that the new algorithm improve performance in the presence of multiple estimation error variances as compared to the conventional method for different modulation schemes.
---
paper_title: Estimation, Training, and Effect of Timing Offsets in Distributed Cooperative Networks

paper_content:

Successful collaboration in cooperative networks require accurate estimation of multiple timing offsets. When combined with signal processing algorithms the estimated timing offsets can be applied to mitigate the resulting inter-symbol interference (ISI). This paper seeks to address timing synchronization in distributed multi-relay amplify-and-forward (AF) and decode-and-forward (DF) relaying networks, where timing offset estimation using a training sequence is analyzed. First, training sequence design guidelines are presented that are shown to result in improved estimation performance. Next, two iterative estimators are derived that can determine multiple timing offsets at the destination. The proposed estimators have a considerably lower computational complexity while numerical results demonstrate that they are accurate and reach or approach the Cramer-Rao lower bound (CRLB).
---
paper_title: Timing and Carrier Synchronization With Channel Estimation in Multi-Relay Cooperative Networks

paper_content:

Multiple distributed nodes in cooperative networks generally are subject to multiple carrier frequency offsets (MCFOs) and multiple timing offsets (MTOs), which result in time varying channels and erroneous decoding. This paper seeks to develop estimation and detection algorithms that enable cooperative communications for both decode-and-forward (DF) and amplify-and-forward (AF) relaying networks in the presence of MCFOs, MTOs, and unknown channel gains. A novel transceiver structure at the relays for achieving synchronization in AF-relaying networks is proposed. New exact closed-form expressions for the Cramer-Rao lower bounds (CRLBs) for the multi-parameter estimation problem are derived. Next, two iterative algorithms based on the expectation conditional maximization (ECM) and space-alternating generalized expectation-maximization (SAGE) algorithms are proposed for jointly estimating MCFOs, MTOs, and channel gains at the destination. Though the global convergence of the proposed ECM and SAGE estimators cannot be shown analytically, numerical simulations indicate that through appropriate initialization the proposed algorithms can estimate channel and synchronization impairments in a few iterations. Finally, a maximum likelihood (ML) decoder is devised for decoding the received signal at the destination in the presence of MCFOs and MTOs. Simulation results show that through the application of the proposed estimation and decoding methods, cooperative systems result in significant performance gains even in presence of impairments.
---
paper_title: Bounds and Algorithms for Multiple Frequency Offset Estimation in Cooperative Networks

paper_content:

The distributed nature of cooperative networks may result in multiple carrier frequency offsets (CFOs), which make the channels time varying and overshadow the diversity gains promised by collaborative communications. This paper seeks to address multiple CFO estimation using training sequences in space-division multiple access (SDMA) cooperative networks. The system model and CFO estimation problem for cases of both decode-and-forward (DF) and amplify-and-forward (AF) relaying are formulated and new closed-form expressions for the Cramer-Rao lower bound (CRLB) for both protocols are derived. The CRLBs are then applied in a novel way to formulate training sequence design guidelines and determine the effect of network protocol and topology on CFO estimation. Next, two computationally efficient iterative estimators are proposed that determine the CFOs from multiple simultaneously relaying nodes. The proposed algorithms reduce multiple CFO estimation complexity without sacrificing bandwidth and training performance. Unlike existing multiple CFO estimators, the proposed estimators are also accurate for both large and small CFO values. Numerical results show that the new methods outperform existing algorithms and reach or approach the CRLB at mid-to-high signal-to-noise ratio (SNR). When applied to system compensation, simulation results show that the proposed estimators significantly reduce average-bit-error-rate (ABER).
---
paper_title: H-inf channel estimation for MIMO-OFDM systems in the presence of carrier frequency offset

paper_content:

An H-infinity (H-inf) channel estimation algorithm is proposed for estimating the channels in MIMO-OFDM systems in the presence of carrier frequency offset (CFO). The goal is to contribute to an algorithm with low complexity, good estimate performance and better suppression to CFO. For this purpose, the H-inf with simplified objective function is first developed, and then, its computational load is reduced by using the iterative expectation maximization (EM) process. To resistant the CFO, we derive a precise equivalent signal model (ESM) to identify the channels. It is observed that the H-inf estimator could be regarded as a substitute for optimal maximum a posteriori (MAP) estimator, but with much less complexity. At the same time, by using ESM, a remarkable improvement for the performance degradation caused by CFO will appear.
---
paper_title: Practical analysis of codebook design and frequency offset estimation for virtual-multiple-input-multipleoutput systems

paper_content:

A virtual-multiple-input-multiple-output (MIMO) wireless system using the receiver-side cooperation with the compress-and-forward (CF) protocol, is an alternative to a point-to-point MIMO system, when a single receiver is not equipped with multiple antennas. It is evident that the practicality of CF cooperation will be greatly enhanced if an efficient source coding technique can be used at the relay. It is even more desirable that CF cooperation should not be unduly sensitive to carrier frequency offsets (CFOs). This study presents a practical study of these two issues. Firstly, codebook designs of the Voronoi vector quantisation (VQ) and the tree-structure VQ (TSVQ) to enable CF cooperation at the relay are described. A comparison in terms of the codebook design and encoding complexity is analysed. It is shown that the TSVQ is much simpler to design and operate, and can achieve a favourable performance-complexity tradeoff. Furthermore, this study demonstrates that CFO can lead to significant performance degradation for the virtual-MIMO system. To overcome this, it is proposed to maintain clock synchronisation and jointly estimate the CFO between the relay and the destination. This approach is shown to provide a significant performance improvement.
---
paper_title: DSTBC based DF cooperative networks in the presence of timing and frequency offsets

paper_content:

In decode-and-forward (DF) relaying networks, the received signal at the destination may be affected by multiple impairments such as multiple channel gains, multiple timing offsets (MTOs), and multiple carrier frequency offsets (MCFOs). This paper proposes novel optimal and sub-optimal minimum mean-square error (MMSE) receiver designs at the destination node to detect the signal in the presence of these impairments. Distributed space-time block codes (DSTBCs) are used at the relays to achieve spatial diversity. The proposed sub-optimal receiver uses the estimated values of multiple channel gains, MTOs, and MCFOs, while the optimal receiver assumes perfect knowledge of these impairments at the destination and serves as a benchmark performance measure. To achieve robustness to estimation errors, the estimates statistical properties are exploited at the destination. Simulation results show that the proposed optimal and sub-optimal MMSE compensation receivers achieve full diversity gain in the presence of channel and synchronization impairments in DSTBC based DF cooperative networks.
---
paper_title: Comments on "Timing Estimation and Resynchronization for Amplify-and-Forward Communication Systems"

paper_content:

This comment first shows that the Cramer-Rao lower bound (CRLB) derivations in the above paper are not exact. In addition, contrary to the claims in the above paper, the assumptions of perfect timing offset estimation and matched-filtering at the relays affect the generality of the analytical results and cannot be justified.
---
paper_title: Optimal Training Sequences for Joint Timing Synchronization and Channel Estimation in Distributed Communication Networks

paper_content:

For distributed multi-user and multi-relay cooperative networks, the received signal may be affected by multiple timing offsets (MTOs) and multiple channels that need to be jointly estimated for successful decoding at the receiver. This paper addresses the design of optimal training sequences for efficient estimation of MTOs and multiple channel parameters. A new hybrid Cramer-Rao lower bound (HCRB) for joint estimation of MTOs and channels is derived. Subsequently, by minimizing the derived HCRB as a function of training sequences, three training sequence design guidelines are derived and according to these guidelines, two training sequences are proposed. In order to show that the proposed design guidelines also improve estimation accuracy, the conditional Cramer-Rao lower bound (ECRB), which is a tighter lower bound on the estimation accuracy compared to the HCRB, is also derived. Numerical results show that the proposed training sequence design guidelines not only lower the HCRB, but they also lower the ECRB and the mean-square error of the proposed maximum a posteriori estimator. Moreover, extensive simulations demonstrate that application of the proposed training sequences significantly lowers the bit-error rate performance of multi-relay cooperative networks when compared to training sequences that violate these design guidelines.
---
paper_title: Time and frequency offset estimation for distributed multiple-input multiple-output orthogonal frequency division multiplexing systems

paper_content:

This study addresses the problem of time and frequency offset estimation in the case of the delays, and the frequency offsets of all the transmitter/receiver pairs are different for distributed multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. A new training structure is designed and an efficient time and frequency offset estimator is proposed. With the symmetric conjugate property of the preamble, the timing metric proposed has an impulse shape, making the time offset estimation accurate. Using the repetition property of the preamble, iterative frequency offset estimation algorithm proposed can acquire a high precision and a wide estimation range. To distinguish different transmit antennas, the preamble is weighted by pseudo-noise (PN) sequences with low cross-correlations. The analysis and the simulation results show that the proposed estimator offers an accurate estimation on the different time delays and frequency offsets caused by the distributed transmitters of the MIMO-OFDM system. This is a situation that the conventional estimators cannot handle as far.
---
paper_title: Joint estimation of I/Q imbalance, CFO and channel response for MIMO OFDM systems

paper_content:

In this paper, we study the joint estimation of inphase and quadrature-phase (I/Q) imbalance, carrier frequency offset (CFO), and channel response for multiple-input multipleoutput (MIMO) orthogonal frequency division multiplexing (OFDM) systems using training sequences. A new concept called channel residual energy (CRE) is introduced. We show that by minimizing the CRE, we can jointly estimate the I/Q imbalance and CFO without knowing the channel response. The proposed method needs only one OFDM block for training and the training symbols can be arbitrary. Moreover when the training block consists of two repeated sequences, a low complexity two-step approach is proposed to solve the joint estimation problem. Simulation results show that the mean-squared error (MSE) of the proposed method is close to the Cramer-Rao bound (CRB).
---
paper_title: A Novel Frequency Offset Tracking Algorithm for Space-Time Block Coded OFDM Systems

paper_content:

A novel frequency offset tracking algorithm for Space-Time Block Coded (STBC) Orthogonal Frequency Division Multiplexing (OFDM) systems is proposed in this work. Tracking of a frequency offset between the transmitter and the receiver is often aided by transmitting pilots embedded in the data payload. The proposed algorithm mainly exploits the specific construction of the OFDM symbol in STBC-OFDM systems, which does not need any additional pilots or sequences in the data field, providing high efficiency in spectrum. The estimator is derived on the basis of the maximum likelihood theory. Simulation results show that in a 2×2 multiple input multiple output (MIMO) system, under the assumption that the antennas are uncorrelated to each other, this method can provide a significant performance improvement in terms of the estimation accuracy of the frequency offset.
---
paper_title: Quantize and forward cooperative communication: Joint channel and frequency offset estimation

paper_content:

Cooperative communication systems can effectively be used to combat fading. A cooperative protocol that can be used with half-duplex terminals is the quantize and forward (QF) protocol, in which the relay quantizes the information received from the source before forwarding it to the destination. Most studies on the QF protocol are carried out under the assumption of perfect channel state information (CSI) at the destination, which is not often the case in real-life systems. Therefore, in the present contribution, the effect of incomplete CSI is analyzed for flat Rayleigh fading channels with a frequency offset. To limit the complexity of the estimation, the destination terminal assumes that the relay operates in an amplify and forward (AF) mode. By using the expectation maximization (EM) algorithm to refine the initial pilot-based estimates, the resulting error performance can be made very close to that of a system with perfect CSI.
---
paper_title: Iterative Joint Detection, ICI Cancelation and Estimation of Multiple CFOs and Channels for DVB-T2 in MISO Transmission Mode

paper_content:

When DVB-T2 uses the option of (distributed) multiple-input single-output transmission mode, a pair of Alamouti-encoded orthogonal frequency division multiplex signals is transmitted simultaneously from two spatially separated transmitters in a single frequency network. Since both transmitters have their own local oscillators, two distinct carrier frequency offsets (CFOs), one for each transmitter-receiver link, occur in the received signal due to the frequency mismatch between the local oscillators. Unfortunately, the multiple CFOs cannot be compensated simultaneously by merely adjusting the carrier frequency at the receiver side, so intercarrier interference (ICI) always exists. In this paper, we present an iterative receiver design to combat multiple CFOs for DVB-T2 application. To estimate both multiple CFOs and channels jointly without sacrificing the spectral efficiency, the joint maximum likelihood (ML) estimation based on the soft information from the channel decoder is used. The proposed receiver performs the iterative joint processing of the data detection, ICI cancelation, and joint ML estimation of the multiple CFOs and channels by exchanging the soft information. We also show that the complexity of the joint ML estimation can be reduced significantly by transforming a huge matrix pseudo-inversion into two sub-matrix pseudo-inversions. The performances are evaluated via a full DVB-T2 simulator. The numerical results show that the mean-squared error performance of the joint ML estimation is closely matched with the Cramer-Rao bound. Furthermore, the resulting bit error rate performance is enhanced in a progressive manner and is able to approach the ideal CFOs-free performance within a few iterations.
---
paper_title: Transceiver Design for Distributed STBC Based AF Cooperative Networks in the Presence of Timing and Frequency Offsets

paper_content:

In multi-relay cooperative systems, the signal at the destination is affected by impairments such as multiple channel gains, multiple timing offsets (MTOs), and multiple carrier frequency offsets (MCFOs). In this paper we account for all these impairments and propose a new transceiver structure at the relays and a novel receiver design at the destination in distributed space-time block code (DSTBC) based amplify-and-forward (AF) cooperative networks. The Cramer-Rao lower bounds and a least squares (LS) estimator for the multi-parameter estimation problem are derived. In order to significantly reduce the receiver complexity at the destination, a differential evolution (DE) based estimation algorithm is applied and the initialization and constraints for the convergence of the proposed DE algorithm are investigated. In order to detect the signal from multiple relays in the presence of unknown channels, MTOs, and MCFOs, novel optimal and sub-optimal minimum mean-square error receiver designs at the destination node are proposed. Simulation results show that the proposed estimation and compensation methods achieve full diversity gain in the presence of channel and synchronization impairments in multi-relay AF cooperative networks.
---
paper_title: Low-Complexity Sequential Searcher for Robust Symbol Synchronization in OFDM Systems

paper_content:

Based on the frequency-domain analog-to-digital conversion (FD ADC), this work builds a low-complexity sequential searcher for robust symbol synchronization in a 4 × 4 FD multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) modem. The proposed scheme adopts a symbol-rate sequential search with simple cross-correlation metric to recover symbol timing over the frequency domain. Simulation results show that the detection error is less than 2% at signal-to-noise ratio (SNR) ≤5 dB. Performance loss is not significant when carrier frequency offset (CFO) ≤100 ppm. Using an in-house 65-nm CMOS technology, the proposed solution occupies 84.881 k gates and consumes 5.2 mW at 1.0 V supply voltage. This work makes the FD ADC more attractive to be adopted in high throughput OFDM systems.
---
paper_title: A Bayesian Algorithm for Joint Symbol Timing Synchronization and Channel Estimation in Two-Way Relay Networks

paper_content:

This work investigates joint estimation of symbol timing synchronization and channel response in two-way relay networks (TWRN) that utilize amplify-and-forward (AF) relay strategy. With unknown relay channel gains and unknown timing offset, the optimum maximum likelihood (ML) algorithm for joint timing recovery and channel estimation can be overly complex. We develop a new Bayesian based Markov chain Monte Carlo (MCMC) algorithm in order to facilitate joint symbol timing recovery and effective channel estimation. In particular, we present a basic Metropolis-Hastings algorithm (BMH) and a Metropolis-Hastings-ML (MH-ML) algorithm for this purpose. We also derive the Cramer-Rao lower bound (CRLB) to establish a performance benchmark. Our test results of ML, BMH, and MH-ML estimation illustrate near-optimum performance in terms of mean-square errors (MSE) and estimation bias. We further present bit error rate (BER) performance results.
---
paper_title: Timing and Frequency Offsets Compensation in Relay Transmission for 3GPP LTE Uplink

paper_content:

Relays can be used between the source and the destination to improve network coverage or reliability. In the relay based distributed space-time block-coded (DSTBC) single-carrier (SC) transmission scheme, asynchronism in time and frequency causes degradation in system performance because of the interblock interference, non-coherent combining and multiuser interference. The simultaneous presence of timing offset and carrier frequency offset also destroy the orthogonal structure of DSTBC. In this paper, we study the combined effect of timing offset and carrier frequency offset and propose a two stage equalization technique for this system at the destination terminal. Technique is based on interference cancellation and cyclic prefix reconstruction. The proposed equalization technique maintains the orthogonal structure of DSTBC and allows the use of low complexity one-tap frequency-domain equalizer. The technique significantly alleviates the effect of time offsets and frequency offsets at the destination without increasing the complexity of the receiver or disturbing the 3rd generation partnership project-long term evolution (3GPP LTE) uplink frame structure or the data rate over frequency selective channels.
---
paper_title: Inter-carrier interference-free Alamouti-coded OFDM for cooperative systems with frequency offsets in non-selective fading environments

paper_content:

A modified Alamouti-coded orthogonal frequency division multiplexing (OFDM) scheme is proposed for cooperative systems in non-selective fading environments. Even with the frequency offset between two distributed transmit antennas, the proposed scheme achieves ideal performance and full rate of Alamouti code. By switching subcarriers in the second OFDM symbol of each Alamouti-coded OFDM symbol pair after a simple-phase rotation in the first OFDM symbol, inter-carrier interference terms can be perfectly cancelled after a simple linear combining with the processing overhead of two times down-conversions and discrete Fourier transform (DFT) operations at each OFDM symbol.
---
paper_title: Joint Carrier Frequency Offset and fast time-varying channel estimation for MIMO-OFDM systems

paper_content:

In this paper, a novel pilot-aided iterative algorithm is developed for MIMO-OFDM systems operating in fast time-varying environment. An L-path channel model with known path delays is considered to jointly estimate the multi-path Rayleigh channel complex gains and Carrier Frequency Offset (CFO). Each complex gain time-variation within one OFDM symbol is approximated by a Basis Expansion Model (BEM) representation. An auto-regressive (AR) model is built for the parameters to be estimated. The algorithm performs recursive estimation using Extended Kalman Filtering. Hence, the channel matrix is easily computed and the data symbol is estimated with free inter-sub-carrier-interference (ICI) when the channel matrix is QR-decomposed. It is shown that only one iteration is sufficient to approach the performance of the ideal case for which the knowledge of the channel response and CFO is available.
---
paper_title: Training-Based Synchronization and Channel Estimation in AF Two-Way Relaying Networks

paper_content:

Two-way relaying networks (TWRNs) allow for more bandwidth efficient use of the available spectrum since they allow for simultaneous information exchange between two users with the assistance of an intermediate relay node. However, due to superposition of signals at the relay node, the received signal at the user terminals is affected by multiple impairments, i.e., channel gains, timing offsets, and carrier frequency offsets, that need to be jointly estimated and compensated. This paper presents a training-based system model for amplify-and-forward (AF) TWRNs in the presence of multiple impairments and proposes maximum likelihood and differential evolution based algorithms for joint estimation of these impairments. The Cramer-Rao lower bounds (CRLBs) for the joint estimation of multiple impairments are derived. A minimum mean-square error based receiver is then proposed to compensate the effect of multiple impairments and decode each user's signal. Simulation results show that the performance of the proposed estimators is very close to the derived CRLBs at moderate-to-high signal-to-noise-ratios. It is also shown that the bit-error rate performance of the overall AF TWRN is close to a TWRN that is based on assumption of perfect knowledge of the synchronization parameters.
---
paper_title: Network-Wide Distributed Carrier Frequency Offsets Estimation and Compensation via Belief Propagation

paper_content:

In this paper, we propose a fully distributed algorithm for frequency offsets estimation in decentralized systems. With the proposed algorithm, each node estimates its frequency offsets by local computations and limited exchange of information with its direct neighbors. Such algorithm does not require any centralized information processing or knowledge of global network topology. It is shown analytically that the proposed algorithm always converges to the optimal estimates regardless of network topology. Simulation results demonstrate the fast convergence of the algorithm and show that estimation mean-squared-error at each node touches the centralized Cram\'{e}r-Rao bound within a few {iterations of message exchange}. Therefore, the proposed method has low overhead and is scalable with network size.
---
paper_title: Blind Timing and Carrier Synchronization in Decode and Forward Cooperative Systems

paper_content:

Synchronization in Decode and Forward (DF) cooperative communication systems is a complex and challenging task requiring estimation of many independent timing and carrier offsets at each relay in the broadcasting phase and multiple timing and carrier offsets at the destination in the relaying phase. This paper presents a scheme for blind channel, timing and carrier offset estimation in a DF cooperative system with one source, M relays and one destination equipped with N antennas. In particular, we exploit blind source separation at the destination to convert the difficult problem of jointly estimating multiple synchronization parameters in the relaying phase into more tractable sub-problems of estimating many individual timing and carrier offsets for the independent relays. We also modify and propose a criteria for best relay selection at the destination. Simulation results demonstrate the excellent end-to-end Bit Error Rate (BER) performance of the proposed blind scheme with relay selection, which is shown to achieve the maximum diversity order with M = 4 relays using N = 5 antennas at the destination. The presented work is a complete solution to blind synchronization and channel estimation in DF cooperative communication systems.
---
paper_title: Alamouti Coding Scheme for AF Relaying With Doppler Shifts

paper_content:

In this paper, we propose an Alamouti-code-based relaying scheme for frequency asynchronous amplify-and-forward (AF) relay networks. Both the oscillator frequency offsets and the Doppler shifts among the distributed nodes are considered in our design. We employ orthogonal frequency-division multiplexing (OFDM) modulation at the source node and let the two relay nodes implement only simple operations, such as time reversal, conjugation, and amplification. We show that without Doppler shifts, the multiple carrier frequency offsets (CFOs) can be directly compensated at the destination, and the received signals exhibit an Alamouti-like structure. We further prove that full spatial diversity can be achieved by the fast symbol-wise detection when the oscillator frequency offset between the relay nodes is smaller than a certain threshold, which yields lower decoding complexity compared with the existing schemes. In the case with Doppler shifts, where the direct CFO compensation becomes impossible, we develop a repetition-aided Alamouti coding approach, by which full diversity can be nearly achieved from the fast symbol-wise detection. Numerical results are provided to corroborate the proposed studies.
---
paper_title: Space–Frequency Convolutional Coding for Frequency-Asynchronous AF Relay Networks

paper_content:

In this paper, we design a space-frequency (SF) convolutional coding scheme for amplify-and-forward (AF) relay networks that contain multiple distributed relay nodes. The frequency-asynchronous nature of the distributed system is considered in our design. Orthogonal frequency-division multiplexing (OFDM) modulation is adopted, which is robust to certain timing errors. We exploit the signal space diversity technique and employ an extended cyclic prefix (CP) at the source node. The relay nodes need to perform only simple operations, e.g., convolution and amplification, and they need no information about the channels and the frequency offsets. Attributed to the extended CP, the multiple frequency offsets can directly be compensated at the destination. We further prove that both spatial and multipath diversity can be achieved by the proposed scheme. Numerical results are provided to corroborate the proposed studies.
---
paper_title: Multiple CFO Mitigation in Amplify-and-Forward Cooperative OFDM Transmission

paper_content:

In cooperative orthogonal frequency division multiplexing (OFDM) systems, accurate frequency synchronization is critical to achieving any potential gains brought by the cooperative operation. The carrier frequency offsets (CFOs) present among multiple nodes (source, relays and destination) are more difficult to tackle than the single CFO problem in point-to-point systems. Multiple CFOs cause phase drift, inter-carrier interference (ICI) and inter-block interference (IBI) in the received signal. This paper deals with the CFO induced interference mitigation problem in distributed space time block coded (STBC) amplify-and-forward (AF) cooperative OFDM systems. We propose a two step approach to recover the phase distortion and suppress the ICI and IBI using low complexity methods to achieve high performance. The first step is time domain (TD) compensation and the second step is frequency domain (FD) decoding. Two TD compensation schemes are proposed, i.e., IBI-removal and ICI-removal. The IBI-removal scheme decouples the two blocks of one STBC codeword completely and then decodes the ICI degraded blocks individually. The ICI-removal scheme removes ICI first and the subsequent decoding requires joint decoding of the two blocks. Simulation results show that the IBI-removal scheme which is of lower complexity performs well with small CFO. For large CFO, the ICI-removal with modified iterative joint maximum likelihood decoding (MIJMLD) outperforms other schemes.
---
paper_title: Joint channel, phase noise, and carrier frequency offset estimation in cooperative OFDM systems

paper_content:

Cooperative communication systems employ cooperation among nodes in a wireless network to increase data throughput and robustness to signal fading. However, such advantages are only possible if there exist perfect synchronization among all nodes. Impairments like channel multipath, time varying phase noise (PHN) and carrier frequency offset (CFO) result in the loss of synchronization and diversity performance of cooperative communication systems. Joint estimation of these multiple impairments is necessary in order to correctly decode the received signal in cooperative systems. In this paper, we propose an iterative pilot-aided algorithm based on expectation conditional maximization (ECM) for joint estimation of multipath channels, Wiener PHNs, and CFOs in amplify-and-forward (AF) based cooperative orthogonal frequency division multiplexing (OFDM) system. Numerical results show that the proposed estimator achieves mean square error performance close to the derived hybrid Cramer-Rao lower bound (HCRB) for different PHN variances.
---
paper_title: Decentralised ranging method for orthogonal frequency division multiple access systems with amplify-and-forward relays

paper_content:

In this study, a decentralised ranging method for uplink orthogonal frequency division multiple access (OFDMA) systems with half-duplex (HD) amplify-and-forward (AF) relay stations (RSs) is proposed. In the OFDMA systems with HD AF RSs, twice more resources and delays are required as ranging without RS. To reduce the required resources and delays for ranging, the authors propose a two-phase ranging scheme based on the decentralised timing-offset estimation at each ranging mobile station (MS). At the first phase, RS occasionally broadcasts timing reference signal, and at the second phase RS retransmits the collected ranging signals from the MSs. Then, each ranging MSs can individually estimate its own timing offset from the received signals. In the proposed ranging method, the base station does not need to send a timing-adjustment message, and the overhead associated with ranging in the downlink resources, and computational complexity can be significantly reduced without degrading the timing-offset-estimation performance. Moreover, the delay associated with ranging can be maintained as same as ranging without RS.
---
paper_title: Compensation of multiple carrier frequency offsets in amplify-and-forward cooperative networks

paper_content:

In this paper, we propose a method to find an optimal correction value for multiple carrier frequency offsets (CFO) compensation, where two received signals have different path gains, in orthogonal frequency division multiplexing (OFDM) systems. Multiple CFOs are occurred when two spatially-separated transmitters are used to transmit the same signal simultaneously. In this case, the sidelobes of the subcarrier spectrums of the undesired signals have similar magnitude with opposite signs. Based on this fact, we propose a self-cancellation method of intercarrier interference caused by multiple CFOs. In the proposed scheme, we compensate the multiple CFOs for the signal to intercarrier interference power ratio of the received signal to be maximized, instead only the signal power is maximized in the ordinary methods.
---
paper_title: Frequency Offset and Channel Estimation in Co-Relay Cooperative OFDM Systems

paper_content:

Frequency offset and channel estimation in cooperative orthogonal frequency division multiplexing (OFDM) systems is studied in this paper. We consider the scenario of two or more source nodes sharing the same relay, i.e., corelay cooperative communications, and a new preamble, which is central-symmetric in time-domain, is proposed to perform the frequency offset and channel estimation. The non-zero samples in the proposed preamble are sparsely distributed with two neighboring non-zero samples being separated by μ > 1 zeros. As long as μ > 2L - 1 is satisfied, the multipath interference can be effectively eliminated, where L stands for the channel order. Unlike [1], the proposed preamble has a much lower Peak-to-Average Power Ratio (PAPR). The interference among the multiple source nodes can also be eliminated by using a backoff modulation scheme on the proposed preamble in each source node, and the mean-square error (MSE) of the proposed Least-Square (LS) channel estimator can be minimized by ensuring the orthogonality among the source nodes. The Pairwise Error Probability (PEP) performance of the proposed system by considering both the frequency offset and channel estimation errors is also derived in this paper. For a given Signalto-Noise-Ratio (SNR), by keeping the total power consumption to the source nodes and the relay to be constant, the PEP can be minimized by adjusting the ratio between the power allocated to the source nodes and the total power.
---
paper_title: OFDM Transmission scheme for asynchronous two-way multi-relay cooperative networks with analog network coding

paper_content:

For two-way relaying assisted by analog network coding, most investigation so far is based on perfect synchronization assumption. In contrast, in this paper we consider the more practical asynchronism assumption, and develop a new OFDM transmission scheme that is robust to the lack of synchronization in both timing and carrier frequency. In our scheme, the relays' signals are constructed by fusing several OFDM symbols received from the source nodes transmissions. The source node receivers can successfully demodulate the received OFDM signals after mitigating effectively multiple carrier frequency offsets and multiple timing phase offsets. Simulations are conducted to demonstrate its superior performance. This scheme has the same bandwidth efficiency as the conventional OFDM transmission, and can achieve the same relaying gain as the existing multiple relay transmissions. By relieving the stringent synchronization requirement, this scheme leads to simplified relay design, which makes it more practical to exploit multiple relays in two-way relaying networks.
---
paper_title: Carrier Frequency Offset Estimation for Two-Way Relaying: Optimal Preamble and Estimator Design

paper_content:

We consider the problem of carrier frequency offset (CFO) estimation for a two-way relaying system based on the amplify-and-forward (AF) protocol. Our contributions are in designing an optimal preamble, and the corresponding estimator, to closely achieve the minimum Cramer-Rao bound (CRB) for the CFO. This optimality is asserted with respect to the novel class of preambles, referred to as the block-rotated preambles (BRPs). This class includes the periodic preamble that is used widely in practice, yet it provides an additional degree of design freedom via a block rotation angle. We first identify the catastrophic scenario of an arbitrarily large CRB when a conventional periodic preamble is used. We next resolve this problem by using a BRP with a non-zero block rotation angle. This angle creates, in effect, an artificial frequency offset that separates the desired relayed signal from the self-interference that is introduced in the AF protocol. With appropriate optimization, the CRB incurs only marginal loss from one-way relaying under practical channel conditions. To facilitate implementation, a specific low-complexity class of estimators is examined, and conditions for the estimators to achieve the optimized CRB is established. Numerical results are given which corroborate with theoretical findings.
---
paper_title: Joint CFO and Channel Estimation for OFDM-Based Two-Way Relay Networks

paper_content:

Joint estimation of the carrier frequency offset (CFO) and the channel is developed for a two-way relay network (TWRN) that comprises two source terminals and an amplify-and-forward (AF) relay. The terminals use orthogonal frequency division multiplexing (OFDM). New zero-padding (ZP) and cyclic-prefix (CP) transmission protocols, which maintain the carrier orthogonality and ensure low estimation and detection complexity, are proposed. Both protocols lead to the same estimation problem which can be solved by the nulling-based least square (LS) algorithm and perform identically when the block length is large. We present detailed performance analysis by proving the unbiasedness of the LS estimators at high signal-to-noise ratio (SNR) and by deriving the closed-form expression of the mean-square-error (MSE). Simulation results are provided to corroborate our findings.
---
paper_title: Localized or Interleaved? A Tradeoff between Diversity and CFO Interference in Multipath Channels

paper_content:

Carrier frequency offset (CFO) damages the orthogonality between sub-carriers and thus causes multiuser interference in uplink OFDMA/SC-FDMA systems. For a given CFO, such multiuser interference is mainly dictated by channel (sub-carrier) allocation, which also specifies the diversity gain of one user over multi-path channels. In particular, the positions of one user's sub-channels will determine its diversity gain, while the distances between sub-channels of the concerned user and those of others will govern the CFO interference. Two popular channel allocation methods are the localized and interleaved (distributed) schemes where the former has less CFO interference but the latter achieves more diversity gain. In this paper, we will consider the channel allocation scheme for uplink LTE systems by investigating the effects of channel allocation on both of the diversity gain and CFO interference. By combining these two effects, we will propose a semi-interleaved scheme, which achieves full diversity gain with minimum CFO interference.
---
paper_title: Lte - The Umts Long Term Evolution: From Theory to Practice

paper_content:

Where this book is exceptional is that the reader will not just learn how LTE works but why it works Adrian Scrase, ETSI Vice-President, International Partnership Projects LTE - The UMTS Long Term Evolution: From Theory to Practice provides the reader with a comprehensive system-level understanding of LTE, built on explanations of the theories which underlie it. The book is the product of a collaborative effort of key experts representing a wide range of companies actively participating in the development of LTE, as well as academia. This gives the book a broad, balanced and reliable perspective on this important technology. Lucid yet thorough, the book devotes particular effort to explaining the theoretical concepts in an accessible way, while retaining scientific rigour. It highlights practical implications and draws comparisons with the well-known WCDMA/HSPA standards. The authors not only pay special attention to the physical layer, giving insight into the fundamental concepts of OFDMA, SC-FDMA and MIMO, but also cover the higher protocol layers and system architecture to enable the reader to gain an overall understanding of the system. Key Features: Draws on the breadth of experience of a wide range of key experts from both industry and academia, giving the book a balanced and broad perspective on LTE Provides a detailed description and analysis of the complete LTE system, especially the ground-breaking new physical layer Offers a solid treatment of the underlying advances in fundamental communications and information theory on which LTE is based Addresses practical issues and implementation challenges related to the deployment of LTE as a cellular system Includes an accompanying website containing a complete list of acronyms related to LTE, with a brief description of each (http://www.wiley.com/go/sesia_theumts) This book is an invaluable reference for all research and development engineers involved in LTE implementation, as well as graduate and PhD students in wireless communications. Network operators, service providers and R&D managers will also find this book insightful.
---
paper_title: An interference self-cancellation technique for SC-FDMA systems

paper_content:

A new interference self-cancellation (ISC) method for Single Carrier-FDMA (SC-FDMA) systems is proposed to mitigate the inter-user interference caused by frequency offset or Doppler effect. By transmitting a compensation symbol at the first symbol location in each resource block, the energy leakage can be significantly suppressed. With little bandwidth and power sacrifice, the proposed method can greatly improve the system robustness against frequency offset. Simulation results show that the signal-to-interference ratio (SIR) can be improved by 7 dB on average for the entire system band, and up to 11.7 dB for an individual user.
---
paper_title: Adaptive schemes and analysis for blind beamforming with insufficient cyclic prefix in single carrierfrequency division multiple access systems

paper_content:

When the duration of the cyclic prefix (CP) is shorter than that of the channel impulse response in single carrier-frequency division multiple access systems, inter-symbol interference and inter-carrier interference will degrade the system performance. Previously, one solution of this problem while considering the effect of carrier frequency offsets (CFOs) and the co-channel interference is a blind received beamforming scheme based on eigenanalysis in a batch mode. As the capability in suppressing the multipath signal with the delay larger than the CP length has not previously been analysed theoretically for the scheme, the theoretical analysis regarding the capability in suppressing the long-delayed multipath signal is provided in this study. The analysis provided in this study is also utilised to design an adaptive processing scheme. The adaptive algorithm is developed to find the beamforming weight vector updated on a per symbol basis without using reference signals. The proposed adaptive algorithm reduces the computational complexity of and shows competitive performance under the insufficient CP, the CFOs, the co-channel interference and the time-varying scenarios. The simulation results reveal that the proposed adaptive algorithm provides better performance than the previously proposed algorithm.
---
paper_title: Uplink single-carrier frequency division multiple access system with joint equalisation and carrier frequency offsets compensation

paper_content:

Similar to the orthogonal frequency division multiple access (OFDMA) system, the single-carrier frequency division multiple access (SC-FDMA) system also suffers from frequency mismatches between the transmitter and the receiver. As a result, in this system, the carrier frequency offsets (CFOs) disrupt the orthogonality between subcarriers and give rise to inter-carrier interference (ICI) and multiple access interference (MAI) among users. The authors present a new minimum mean square error (MMSE) equaliser, which jointly performs equalisation and carrier frequency offsets (CFOs) compensation. The mathematical expression of this equaliser has been derived taking into account the MAI and the channel noise. A low complexity implementation of the proposed equalisation scheme using a banded matrix approximation is presented here. From the obtained simulation results, the proposed equalisation scheme is able to enhance the performance of the SC-FDMA system, even in the presence of estimation errors.
---
paper_title: Adaptive Frequency-Domain RLS DFE for Uplink MIMO SC-FDMA

paper_content:

It is well known that, in the case of highly frequency-selective fading channels, the linear equalizer (LE) can suffer significant performance degradation compared with the decision feedback equalizer (DFE). In this paper, we develop a low-complexity adaptive frequency-domain DFE (AFD-DFE) for single-carrier frequency-division multiple-access (SC-FDMA) systems, where both the feedforward and feedback filters operate in the frequency domain and are adapted using the well-known block recursive least squares (RLS) algorithm. Since this DFE operates entirely in the frequency domain, the complexity of the block RLS algorithm can be substantially reduced when compared with its time-domain counterpart by exploiting a matrix structure in the frequency domain. Furthermore, we extend our formulation to multiple-input–multiple-output (MIMO) SC-FDMA systems, where we show that the AFD-DFE enjoys a significant reduction in computational complexity when compared with the frequency-domain nonadaptive DFE. Finally, extensive simulations are carried out to demonstrate the robustness of our proposed AFD-DFE to high Doppler and carrier frequency offset (CFO).
---
paper_title: Combined MMSE-FDE and Interference Cancellation for Uplink SC-FDMA with Carrier Frequency Offsets

paper_content:

Due to its lower peak-to-average power ratio (PAPR) compared with orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA) has been recently accepted as the uplink multiple access scheme in the Long Term Evolution (LTE) of cellular systems by the Third Generation Partnership Project (3GPP). However, similar to OFDMA, carrier frequency offset (CFO) can destroy the orthogonality among subcarriers and degrade the performance of SC-FDMA. To mitigate the effect of CFOs, we propose a combined minimum mean square error frequencydomain equalization (MMSE-FDE) and interference cancellation scheme. In this scheme, joint FDE with CFO compensation (JFC) is utilized to obtain the initial estimation for each user. In contrast to previous schemes, where the FDE and CFO compensation are done separately, in JFC, the MMSE FDE is designed to suppress the MUI after CFO compensation. To further eliminate the MUI, we combine JFC with parallel interference cancellation (PIC). In particular, we iteratively design the MMSE FDE equalizer to suppress the remaining MUI at each stage and obtain better estimation. Simulation results show that the proposed scheme can significantly improve the system performance.
---
paper_title: CFO Estimation and Compensation in SC-IFDMA Systems

paper_content:

Single carrier interleaved frequency division multiple access (SC-IFDMA) has been recently receiving much attention for uplink multiuser access in the next generation mobile systems because of its lower peak-to-average transmit power ratio (PAPR). In this paper, we investigate the effect of carrier frequency offset (CFO) on SC-IFDMA and propose a new low-complexity time domain linear CFO compensation (TD-LCC) scheme. The TD-LCC scheme can be combined with successive interference cancellation (SIC) to further improve the system performance. The combined method will be referred to as TD-CC-SIC. We shall study the use of user equipment (UE) ordering algorithms in our TD-CC-SIC scheme and propose both optimal and suboptimal ordering algorithms in the MMSE sense. We also analyze both the output SINR and the BER performance of the proposed TD-LCC and TD-CC-SIC schemes. Simulation results along with theoretical SINR and BER results will show that the proposed TD-LCC and TD-CC-SIC schemes greatly reduce the CFO effect on SC-IFDMA. We also propose a new blind CFO estimation scheme for SC-IFDMA systems when the numbers of subcarrier sets allocated to different UEs are not the same due to their traffic requirements. Compared to the conventional blind CFO estimation schemes, it is shown that by using a virtual UE concept, the proposed scheme does not have the CFO ambiguity problem, and in some cases can improve the throughput efficiency since it does not need to increase the length of cyclic prefix (CP).
---
paper_title: Suppression of ICI and MAI in SC-FDMA communication system with carrier frequency offsets

paper_content:

Similar to other orthogonal frequency division multiplexing (OFDM)-based systems, carrier frequency offset (CFO) is a challenging problem in uplink communications of single carrier frequency division multiple access (SC-FDMA) system. It must be noticed that CFO, which is mainly due to oscillator instability and/or Doppler shift, would generate inter carrier interference (ICI) as well as multi-access interference (MAI) to disturb the received signals and seriously degrade system performance. Frequency synchronization in uplink communications is difficult because different users always experience different CFOs and one user's CFO correction would misalign other users. In this paper, we proposed a suppression method to overcome the multi CFOs problem. To implement this algorithm, block type pilots would be exploited, which is also utilized in LTE uplink standard. The proposed algorithm is based on the following two assumptions. Firstly the proposed algorithm is applied in this scenario, where different users start to communicate with the base station at different symbol periods, and secondly, during the pilot block and the following data blocks, the CFO of each user is quasi static, which is feasible since CFO is slow varying. Compared with other interference suppression methods, the proposed method could directly estimate the interference components from the inverse pilot matrix, thus it does not need to do the CFO estimation. Further more, since block type pilots is a common pilot pattern in wireless communication system, this algorithm can be easily extended to other communication system. Simulation results show that the proposed suppression algorithm can significantly improved system performance.
---
paper_title: Low-complexity joint regularised equalisation and carrier frequency offsets compensation scheme for single-carrier frequency division multiple access system

paper_content:

Since the conventional zero-forcing receiver does not operate satisfactorily in interference-limited environments, because of its noise amplification; the statistics of the transmitted data and the additive noise are required for the minimum-mean-square error receiver; the potential of the regularised receiver is proposed and investigated in this study to cope with these problems. In this study, the authors introduce an efficient low-complexity joint regularised equalisation and carrier frequency offset compensation (LJREC) scheme for single-carrier frequency division multiple access system. The proposed LJREC scheme avoids the noise amplification problem and the estimation of the signal-to-noise ratio and the interference matrices of other users. From the obtained simulation results, the proposed scheme enhances the system performance with lower complexity and sufficient robustness to the estimation errors.
---
paper_title: Blind Channel Shortening for Asynchronous SC-IFDMA Systems with CFOs

paper_content:

This paper proposes a blind channel shortening algorithm for uplink reception of a single-carrier interleaved frequency-division multiple-access (SC-IFDMA) system transmitting over a highly-dispersive channel, which is affected by both timing offsets (TOs) and frequency offsets (CFOs). When the length of the cyclic prefix (CP) is insufficient to compensate for channel dispersion and TOs, a common strategy is to shorten the channel by means of time-domain equalization, in order to restore CP properties and ease signal reception. The proposed receiver exhibits a three-stage structure: the first stage performs blind shortening of all the user channel impulse responses (CIRs), by adopting the minimum mean-output energy criterion, without requiring neither a priori knowledge of the CIRs to be shortened, nor preliminary compensation of the CFOs; the second stage performs joint compensation of the CFOs; finally, to alleviate noise amplification effects, possibly arising from CFO compensation, the third stage implements per-user signal-to-noise ratio (SNR) maximization, without requiring knowledge of the shortened CIRs. A theoretical analysis is carried out to assess the effectiveness of the proposed shortening algorithm in the high-SNR regime; moreover, the performances of the overall receiver are validated and compared with those of existing methods by extensive Monte Carlo computer simulations.
---
paper_title: Joint Low-Complexity Equalization and Carrier Frequency Offsets Compensation Scheme for MIMO SC-FDMA Systems

paper_content:

Due to their noise amplification, conventional Zero-Forcing (ZF) equalizers are not suited for interference-limited environments such as the Single-Carrier Frequency Division Multiple Access (SC-FDMA) in the presence of Carrier-Frequency Offsets (CFOs) . Moreover, they suffer increasing complexity with the number of subcarriers and in particular with Multiple-Input Multiple-Output (MIMO) systems. In this letter, we propose a Joint Low-Complexity Regularized ZF (JLRZF) equalizer for MIMO SC-FDMA systems to cope with these problems. The main objective of this equalizer is to avoid the direct matrix inversion by performing it in two steps to reduce the complexity. We add a regularization term in the second step to avoid the noise amplification. From the obtained simulation results, the proposed scheme is able to enhance the system performance with lower complexity and sufficient robustness to estimation errors.
---
paper_title: Improved Detection of Uplink OFDM-IDMA Signals with Carrier Frequency Offsets

paper_content:

This letter proposes an improved detection scheme to mitigate the influence of carrier frequency offsets (CFOs) in uplink orthogonal frequency division multiplexing-interleave division multiple access (OFDM-IDMA) systems. The basic principle is to iteratively estimate and cancel the combined interference from multiple users and CFOs at the receiver so that the additional interference due to the residual CFOs from other users can be suppressed. Simulation results show that the proposed scheme can effectively eliminate the interference and significantly improve the system performance in the presence of CFOs.
---
paper_title: MMSE Solution for OFDMA Systems with Carrier Frequency Offset Correction

paper_content:

The multi-access orthogonal frequency division multiplexing (OFDMA) technology has drawn a lot of attention in next generation wireless mobile communications. It is well-known that carrier frequency offsets in OFDMA systems can destroy the orthogonality among subcarriers and produce intercarrier interference (ICI) and multiuser interference (MUI). In our previous works, we proposed a common carrier frequency offset (CCFO) correction scheme at the OFDMA receiver, which can reduce the ICI/MUI effect and the bit error rate. In the scheme, the CFO correction can be performed by adaptively converging the MSE between the demodulated output and the decision feedback data. This paper studies the minimum MSE solution for the CCFO value, and the result is exploited to verify the adaptive CCFO estimation algorithm by means of the decision feedback. Simulation results show that the adaptive decision feedback scheme for CCFO estimation is effective and the minimum MSE performance is well achieved.
---
paper_title: Multi-User Interference Cancellation Schemes for Carrier Frequency Offset Compensation in Uplink OFDMA

paper_content:

Each user in the uplink of an Division Multiple Access (OFDMA) system may experience a different carrier frequency offset (CFO). These uncorrected CFOs destroy the orthogonality among subcarriers, causing inter-carrier interference and multi-user interference, which degrade the system performance severely. In this paper, novel time-domain multi-user interference cancellation schemes for OFDMA uplink are proposed. They employ an architecture with multiple OFDMA-demodulators to compensate for the impacts of multi-user CFOs at the base station's side. Analytical and numerical evaluations show that the proposed schemes achieve a significant performance gain compared to the conventional receiver and a reference frequency-domain multi-user interference cancellation scheme. In a particular scenario, a maximum CFO of up to 40% of the subcarrier spacing can be tolerated, and the CFO-free performance is maintained in the OFDMA uplink.
---
paper_title: Frequency Synchronization for Multiuser MIMO-OFDM System Using Bayesian Approach

paper_content:

This paper addresses the problem of frequency synchronization in multiuser multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) systems. Different from existing work, a Bayesian approach is used in the parameter estimation problem. In this paper, the Bayes estimator for carrier frequency offset (CFO) estimation is proposed and the Bayesian Cram'er-Rao bound (BCRB) is also derived in closed form. Direct implementation of the resultant estimation scheme with conventional methods is challenging since a high degree of mathematical sophistication is always required. To solve this problem, the Gibbs sampler is exploited with an efficient sample generation method. Simulation results illustrate the effectiveness of the proposed estimation scheme.
---
paper_title: Joint Carrier Frequency Offset and Channel Estimation for Uplink MIMO-OFDMA Systems Using Parallel Schmidt Rao-Blackwellized Particle Filters

paper_content:

Joint carrier frequency offset (CFO) and channel estimation for uplink MIMO-OFDMA systems over time-varying channels is investigated. To cope with the prohibitive computational complexity involved in estimating multiple CFOs and channels, pilot-assisted and semi-blind schemes comprised of parallel Schmidt Extended Kalman filters (SEKFs) and Schmidt-Kalman Approximate Particle Filters (SK-APF) are proposed. In the SK-APF, a Rao-Blackwellized particle filter (RBPF) is developed to first estimate the nonlinear state variable, i.e. the desired user's CFO, through the sampling-importance-resampling (SIRS) technique. The individual user channel responses are then updated via a bank of Kalman filters conditioned on the CFO sample trajectories. Simulation results indicate that the proposed schemes can achieve highly accurate CFO/channel estimates, and that the particle filtering approach in the SK-APF outperforms the more conventional Schmidt Extended Kalman Filter.
---
paper_title: A Novel Subspace Decomposition-Based Detection Scheme with Soft Interference Cancellation for OFDMA Uplink

paper_content:

Abstract-In this paper we propose a novel subspace decomposition-based detection scheme with the assistance of soft interference cancellation in the uplink of interleaved orthogonal frequency division multiple access (OFDMA) systems. By utilizing the inherent data structure, the interference is first separated with the desired symbol and then further decomposed into the one caused by the residues of decision errors and the other one by the undetected symbols in the successive interference cancellation (SIC) process.With such an ingenious interference decomposition along with the soft processing scheme, the new receiver can render more thorough interference cancellation, which in turn entails enhanced system performance. Moreover, for practical implementations, a low-complexity version, which only deal with the principal components of inter-carrier interference (ICI), is also addressed. Conducted simulations show that the developed receiver and its low-complexity implementation can provide superior performance compared with pervious works and is resilient to the presence of carrier-frequency offsets (CFOs). The low complexity implementation, in particular, requires substantially lower computational overhead with only slight performance loss.
---
paper_title: An Improved Frequency Offset Estimation Based on Companion Matrix in Multi-User Uplink Interleaved OFDMA Systems

paper_content:

In this letter, we consider a multiuser uplink orthogonal frequency-division multiple access (OFDMA) system. To estimate the carrier frequency offset (CFO) efficiently, a modified pilot structure has been proposed to allow sufficient frequency separation between the subcarriers allocated to any two users. The proposed structure can reduce the ambiguity problem caused by the multiple signal classification (MUSIC) based algorithm when a CFO of one user is close to that of a different user. We present a CFO estimation method based on the companion matrix obtained using the received signal from the proposed pilot structure. Simulation results show that the proposed CFO estimator performs better than the conventional estimator and maintains its performance as the CFO range increases.
---
paper_title: Low complexity scheme for carrier frequency offset estimation in orthogonal frequency division multiple access uplink

paper_content:

Maximum likelihood (ML) carrier-frequency offset estimation for orthogonal frequency-division multiple access uplink is a complex multi-parameter estimation problem. The ML approach is a global optima search problem, which is prohibitive for practical applications because of the requirement of multidimensional exhaustive search for a large number of users. There are a few attempts to reduce the complexity of ML search by applying evolutionary optimisation algorithms. In this study, the authors propose a novel canonical particle swarm optimisation (CPSO)-based scheme, to reduce the computational complexity without compromising the performance and premature convergence. The proposed technique is a two-step process, where, in the first step, low resolution alternating projection frequency estimation (APFE) is used to generate a single better positioned particle for CPSO, followed by an actual CPSO procedure in second step. The mean square error performance of the proposed scheme is compared with existing low complexity algorithms namely APFE and linear particle swarm optimisation with mutation. Simulation results presented in this study show that the new scheme completely avoids premature convergence for a large number of users as high as 32.
---
paper_title: Joint Carrier Frequency Offset and Direction of Arrival Estimation via Hierarchical ESPRIT for Interleaved OFDMA/SDMA Uplink Systems

paper_content:

In this paper, we propose an efficient algorithm to jointly estimate the directions of arrival (DOAs) and carrier frequency offsets (CFOs) in interleaved orthogonal frequency division multiple access / space division multiple access (OFDMA/SDMA) uplink networks. The algorithm makes use of the signal structure by estimating the CFOs and DOAs in a hierarchical tree structure, in which two CFO estimations and one DOA estimation are employed alternatively. One special feature in the proposed algorithm is that the algorithm proceeds in a coarse-fine manner with temporal filtering or spatial beamforming being invoked between the parameter estimations to decompose the signals progressively into subgroups so as to enhance the estimation accuracy and lower the computational overhead. Simulations show that the proposed algorithm can provide satisfactory performance with increased channel capacity.
---
paper_title: MMSE-Based CFO Compensation for Uplink OFDMA Systems with Conjugate Gradient

paper_content:

In this paper, we present a low-complexity carrier frequency offset (CFO) compensation algorithm based on the minimum mean square error (MMSE) criterion for uplink orthogonal frequency division multiple access systems. CFO compensation with an MMSE filter generally requires an inverse operation on an interference matrix whose size equals the number of subcarriers. Thus, the computational complexity becomes prohibitively high when the number of subcarriers is large. To reduce the complexity, we employ the conjugate gradient (CG) method which iteratively finds the MMSE solution without the inverse operation. To demonstrate the efficacy of the CG method for our problem, we analyze the interference matrix and present several observations which provide insight on the iteration number required for convergence. The analysis indicates that for an interleaved carrier assignment scheme, the maximum iteration number for computing an exact solution is at most the same as the number of users. Moreover, for a general carrier assignment scheme, we show that the CG method can find a solution with far fewer iterations than the number of subcarriers. In addition, we propose a preconditioning technique which speeds up the convergence of the CG method at the expense of slightly increased complexity for each iteration. As a result, we show that the CFO can be compensated with substantially reduced computational complexity by applying the CG method.
---
paper_title: Blind Carrier Frequency Offset Estimation for Interleaved OFDMA Uplink

paper_content:

In this paper, we develop two novel blind carrier frequency offset (CFO) estimators for interleaved orthogonal frequency division multiple access (OFDMA) uplink transmission in a multiantenna system. The first estimator is the subspace-based one and could blindly estimate multiple CFOs from a rank reduction approach. The second estimator is based on the maximum likelihood (ML) approach and improves the performance as compared to the first one. The higher computational complexity of the ML estimator is alleviated by the alternating projection (AP) method. Both the proposed estimators support fully loaded data transmissions, i.e., all subcarriers being occupied, which provides higher bandwidth efficiency as compared to the existing schemes. The numerical results are then provided to corroborate the proposed studies.
---
paper_title: Low complexity LS and MMSE based CFO compensation techniques for the uplink of OFDMA systems

paper_content:

Orthogonal frequency division multiple access (OFDMA), where different subcarriers are allocated to different users, has been adopted for the uplink of several standards and has attracted a great deal of attention as a result. However, OFDMA is highly sensitive to carrier frequency offset (CFO) between the transmitter and receiver. In the uplink, different carrier frequency offsets due to different users can adversely affect subcarrier orthogonality. We propose a low complexity CFO compensation approach that addresses this problem while maintaining optimal performance. This approach is based on the least squares and minimum mean square error criteria applicable to interleaved and block interleaved carrier assignment schemes. The proposed algorithms use the special block circulant property of the interference matrix. In contrast to existing CFO compensation techniques, our algorithms do not rely on iterations or approximations. We present our approach in this paper and describe how a considerable reduction in computational complexity can be achieved by adopting it.
---
paper_title: A Time Domain Inverse Matrix Receiver for CFO Suppression in WIMAX Uplink System

paper_content:

In orthogonal frequency division multiple access (OFDMA) uplink system, orthogonal multiple subcarriers are assigned to different users for parallel high data rate communications. However, carrier frequency offset (CFO), which is mainly due to oscillator mismatches and/or Doppler shift, will destroy the orthogonality among subcarriers and introduce intercarrier interference (ICI) as well as multiple-access interference (MAI) in uplink scenario. Thus, system performance will be seriously degraded. To overcome this problem, it is of great importance to do research on suppression of the interferences caused by CFO. In this paper, we proposed a novel time domain inverse matrix receiver to suppress the interference of multiple CFOs. Compared with the conventional frequency domain direct ZF inverse matrix method, which has high complexity in obtaining the ICI matrix and its inverse matrix, the proposed method has very low complexity in obtaining the interference matrix. Furthermore, the signal after interference suppression is a frequency domain signal. Thus the receiver complexity can be simplified. Simulation results show that this algorithm has almost the same performance to the frequency domain direct ZF inverse matrix method.
---
paper_title: Blind carrier frequency offset estimation for tile-based orthogonal frequency division multiple access uplink with multi-antenna receiver

paper_content:

In this study, the authors propose a blind carrier frequency offset (CFO) estimation method for the tile structure orthogonal frequency division multiple access (OFDMA) uplink with multi-antenna receiver. They employ an iterative approach to extract the signal component for each user gradually and propose a carefully designed CFO estimator to update the CFO estimate during the iterative procedure. The key ingredient of the proposed method is using few subcarriers on both sides within each tile as the ‘guard subcarriers’, which can greatly mitigate the effect of multi-user interference. The proposed method supports not only fully loaded transmissions, but also the generalised assignment scheme that provides the flexibility for dynamical resource allocation. The numerical results are provided, which indicate that the proposed method can almost converge to the analytical lower bound within a few iterative cycles. It is seen that the proposed method also outperforms the existing competitor with multi-antenna receiver in terms of estimation performance, especially with few adopted blocks.
---
paper_title: Low Complexity Pilot Assisted Carrier Frequency Offset Estimation for OFDMA Uplink Systems

paper_content:

In this letter, we propose a low complexity pilot aided carrier frequency offset (CFO) estimation algorithm for orthogonal frequency division multiplexing access (OFDMA) uplink systems based on two consecutive received OFDMA symbols. Assuming that the channels and the CFOs are static over the two consecutive symbols, we express the second received OFDMA symbol in terms of the CFOs and the first OFDMA symbol. Based on this signal model, a new estimation algorithm which obtains the CFOs by minimizing the mean square distance between the received OFDMA symbol and its regenerated signal is provided. Also, we implement the proposed algorithm via fast Fourier transform (FFT) operations by utilizing the block matrix inversion lemma and the conjugate gradient method. Simulation results show that the proposed algorithm approaches the average Cramer Rao bound for moderate and high signal to noise ratio (SNR) regions. Moreover, the algorithm can be applied for any carrier assignment schemes with low complexity.
---
paper_title: Trade off between Frequency Diversity and Robustness to Carrier Frequency Offset in Uplink OFDMA System

paper_content:

In this paper, we investigate the effect of subcarrier allocation on the CFO for uplink OFDMA systems. Carriers are allocated to the users in order to get maximum throughput by making use of the channel frequency diversity. But in systems with CFO, while allocating the carriers to the users, attention must be paid to the ICI resulting due to CFO. In this paper we propose a carrier allocation scheme that provides a good compromise between the throughput maximization and robustness to the CFO induced ICI for systems with and without channel state information (CSI).
---
paper_title: Frequency Offset Estimation in 3G LTE

paper_content:

3G Long Term Evolution (LTE) technology aims at addressing the increasing demand for mobile multimedia services in high user density areas while maintaining good performance in extreme channel conditions such as high mobility of High Speed Trains. This paper focuses on the latter aspect and compares different algorithms for the uplink frequency offset estimation in LTE Base Stations (eNodeB). A frequency-domain maximum-likelihood based solution is proposed, taking profit of the available interference-free OFDM symbols de-mapped (or de-multiplexed) at the output of the FFT of an OFDMA multi-user receiver. It is shown to outperform the state-of-the-art CP correlation approach on both link-level performance and complexity aspects.
---
paper_title: ML Detection with Successive Group Interference Cancellation for Interleaved OFDMA Uplink

paper_content:

To mitigate the interference caused by multiple carrier frequency offsets (CFO) of distributed users, a maximum likelihood (ML) detection with group-wise successive interference cancellation (GSIC) is proposed for interleaved Orthogonal Frequency Division Multiplexing Access (OFDMA) uplink. By exploiting the interference characteristics and the finite alphabet property of transmitted symbols, the proposed scheme first extracts the block diagonal of the frequency domain channel matrix and then employs ML detection via sphere decoding in each block. The decisions obtained from the detected blocks are used to mitigate the interference to the residual blocks. Numerical results show that the proposed scheme outperforms both the minimum mean square error (MMSE) detection and parallel interference cancellation (PIC) with affordable computational complexity.
---
paper_title: Frequency Synchronization for the OFDMA Uplink Based on the Tile Structure of IEEE 802.16e

paper_content:

The multiple carrier frequency offsets (CFOs) of multiple users make frequency synchronization a challenging task in the orthogonal frequency-division multiple-access (OFDMA) uplink. In this paper, a computationally efficient iterative CFO estimation and compensation method for the OFDMA uplink with the generalized subcarrier allocation scheme (CAS) based on the tile structure of IEEE 802.16e is proposed. The proposed method only needs a few iteration cycles for its convergence. It greatly lowers the computational cost for the existing methods and can achieve better CFO estimation and compensation performance.
---
paper_title: Carrier Frequency Offset Estimation in OFDMA using Digital Filtering

paper_content:

This letter deals with the frequency synchronization problem in the uplink of OFDMA communications systems with interleaved or generalized subcarrier allocation. An algorithm to estimate the carrier frequency offsets (CFOs) of all the active users is presented. The estimator relies upon finding the zeros of a suitable designed filter. The design of the filter boils down to the solution of a least squares (LS) problem and thus entails relatively low complexity. While under low subcarrier load conditions the estimation root mean squared error (RMSE) is higher than that of other existing methods, under heavy or full load achieves comparable and even superior performance.
---
paper_title: Carrier Frequency Offset Estimation for Uplink OFDMA Using Partial FFT Demodulation

paper_content:

Fast and accurate Carrier Frequency Offset (CFO) estimation is a problem of significance in many multi-carrier modulation based systems, especially in uplink Orthogonal Frequency Division Multiple Access (OFDMA) where the presence of multiple users exacerbates the inter-carrier interference (ICI) and results in multi-user interference (MUI). In this paper, a new technique called partial FFT demodulation is proposed. Estimators for the CFO are derived by considering an approximated matched filter for each user, implemented efficiently using several FFTs operating on sub-intervals of an OFDM block. Through simulations, the feasibility and performance of the proposed estimators are demonstrated. Associated trade-offs are discussed.
---
paper_title: Generalised grouped minimum mean-squared errorbased multi-stage interference cancellation scheme for orthogonal frequency division multiple access uplink systems with carrier frequency offsets

paper_content:

In uplink orthogonal frequency division multiple access (OFDMA) systems with carrier frequency offsets (CFOs), there always be a dilemma that high performance and low complexity cannot be obtained simultaneously. In this study, in order to achieve better trade-off between performance and complexity, the authors propose a grouped minimum mean squared error (G-MMSE)-based multi-stage interference cancellation (MIC) scheme. The first stage of the proposed scheme is a G-MMSE detector, where the signal is detected group by group using banks of partial MMSE filters. The signal group can be either user based or subcarrier based. Multiple novel interference cancellation (IC) units are serially concatenated with the G-MMSE detector. Reusing the filters in the G-MMSE detector significantly reduces the computational complexity in the subsequent IC units as shown by the complexity analysis. The performance of the proposed G-MMSE-MIC schemes are evaluated by theoretical analysis and simulation. The results show that the proposed schemes outperform other existing schemes with considerably low complexity.
---
paper_title: Blind Maximum-Likelihood Carrier-Frequency-Offset Estimation for Interleaved OFDMA Uplink Systems

paper_content:

Blind maximum-likelihood (ML) carrier-frequency-offset (CFO) estimation is considered to be difficult in interleaved orthogonal frequency-division multiple-access (OFDMA) uplink systems. This is because multiple CFOs have to be simultaneously estimated (each corresponding to a user's carrier), and an exhaustive multidimensional search is required. The computational complexity of the search may be prohibitively high. Methods such as the multiple signal classification and the estimation of signal parameters via the rotational invariance technique have been proposed as alternatives. However, these methods cannot maximize the likelihood function, and the performance is not optimal. In this paper, we propose a new method to solve the problem. With our formulation, the likelihood function can be maximized, and the optimum solution can be obtained by solving a polynomial function. Compared with the exhausted search, the computational complexity can be reduced dramatically. Simulations show that the performance of the proposed method can approach that of the Cramer-Rao lower bound.
---
paper_title: Carrier Frequency Offset Tracking in the IEEE 802.16e OFDMA Uplink

paper_content:

The IEEE 802.16e standard for nomadic wireless metropolitan area networks adopts orthogonal frequency-division multiple-access (OFDMA) as an air interface. In these systems, residual carrier frequency offsets (CFOs) between the uplink signals and the base station local oscillator give rise to interchannel interference (ICI) as well as multiple access interference (MAI). Accurate CFO estimation and compensation is thus necessary to avoid a serious degradation of the error-rate performance. In this work, we address the problem of CFO tracking in the IEEE 802.16e uplink and present a closed-loop solution based on the least-squares (LS) principle. In doing so, we exploit a set of pilot tones that are available in each user's subchannel. The resulting scheme can be implemented with affordable complexity and is able to reliably track the CFOs of all active users. When used in conjunction with a frequency offset compensator, it can effectively mitigate ICI and MAI, thereby allowing channel equalization and data detection to follow directly. Numerical simulations are used to demonstrate the effectiveness of the proposed solution in the presence of residual time-varying frequency offsets.
---
paper_title: A Scheme to Support Concurrent Transmissions in OFDMA Based Ad Hoc Networks

paper_content:

In this paper, we propose a novel system architecture to realize OFDMA in ad hoc networks. A partial time synchronization strategy is presented based on the proposed system model. This proposed scheme can support concurrent transmission without global clock synchronization. We also propose a null subcarrier based frequency synchronization scheme to estimate and compensate frequency offsets in a multiple user environment. The simulation results show a good performance of our proposed synchronization scheme in terms of frequency offset estimation error and variance.
---
paper_title: Efficient carrier frequency offset estimation for orthogonal frequency-division multiple access uplink with an arbitrary number of subscriber stations

paper_content:

An efficient method is proposed to estimate the carrier frequency offsets (CFOs) in the orthogonal frequency-division multiple access (OFDMA) uplink. The conventional alternating projection method is accelerated by utilising the inherited properties of the matrices involved. The multiplication of large sparse projection matrices can be elegantly transformed to a series of products involving small dense matrices, and the inverse operation of these large matrices can be substituted by direct computations. Hence, the computational cost is significantly reduced without compromising the accuracy of the CFO estimation.
---
paper_title: MCMOE-Based CFO Estimator Aided With the Correlation Matrix Approach for Alamouti's STBC MC-CDMA Downlink Systems

paper_content:

This paper addresses the estimation problem of carrier frequency offset (CFO) in the downlink transmission of space-time block-coded multicarrier code-division multiple-access (STBC MC-CDMA) systems over multipath fading. This study proposes a multiply constrained minimum output energy (MCMOE)-based blind CFO estimator, which is simply assisted by the presented correlation matrix approach to efficiently achieve the CFO estimate. We formulate a two-level CFO estimator by optimizing the receiver output power, as well as the data correlation matrix. At the first level, all possible CFO candidates are found by evaluating the well-defined estimated merit figure. Then, exploiting multiple constraints in the design of the MCMOE receiver, a criterion is used in level two to determine the exact CFO estimate. Numerical results are presented to verify that both precise CFO estimation and reliable error performance can be achieved, even when the channel is dominated by noise because the impact of CFO on the output signal-to-interference-plus-noise ratio (SINR) and bit error rate (BER) are effectively removed by the proposed CFO estimator.
---
paper_title: SINR Lower Bound Based Multiuser Detector for Uplink MC-CDMA Systems with Residual Frequency Offset

paper_content:

For uplink multi-carrier code-division multiple access (MC-CDMA) systems, we propose a multiuser detector that is robust to a small residual frequency offset existing after frequency offset estimation and compensation. In this paper, when the residual frequency offset is normalized by subcarrier spacing, it is called a normalized residual frequency offset (NRFO). In the proposed scheme, we first derive a lower bound of the signal-to-interference plus noise ratio (SINR) of the desired user when the NRFO of the desired user is bounded by a small value and the value is known to the receiver. We then design a detection filter to maximize the SINR lower bound. Simulation results show that the proposed scheme has better SINR and bit error rate (BER) performances in a high signal-to-noise ratio (SNR) region than a conventional minimum mean square error (MMSE) receiver that ignores the NRFO.
---
paper_title: Widely Linear MVDR Beamformers for Noncircular Signals Based on Time-Averaged Second-Order Noncircularity Coefficient Estimation

paper_content:

The optimal widely linear (WL) minimum variance distortionless response (MVDR) beamformer, which has a powerful performance for the reception of a noncircular signal, was proposed by Chevalier in 2009. Nevertheless, in spectrum monitoring or passive listening, the optimal WL MVDR beamformer is hard to implement due to an unknown second-order (SO) noncircularity coefficient. This paper aims at estimating the time-averaged SO noncircularity coefficient of a desired noncircular signal, whose waveform is unknown but whose steering vector is known, in the context of the optimal WL MVDR beamformer. The proposed noncircularity coefficient estimator can process 2N - 1 rectilinear signals at most using an array of N sensors. Moreover, a frequency-shift WL MVDR beamforming algorithm is proposed for a noncircular signal having a nonnull frequency offset or carrier residue, jointly with the estimation of the frequency offset of the rectilinear signal. Due to the inevitable estimation error of the time-averaged SO noncircularity coefficient, a diagonal loading technique is used to enhance the robustness of the optimal WL beamformers. Simulations are shown to verify the effectiveness of the proposed algorithms.
---
paper_title: Iterative frequency-domain fractionally spaced receiver for zero-padded multi-carrier code division multiple access systems

paper_content:

In this study, the authors propose an improved frequency-domain fractionally spaced (FDFS) minimum mean square error (MMSE) receiver for zero-padded multi-carrier code division multiple access (MC-CDMA) systems when the guard interval is not enough to avoid the inter-symbol interference (ISI) caused by the multipath channel. The proposed novel iterative FDFS-based receivers firstly reconstruct the received symbol to reduce the ISI and then followed by the FDFS-based equalisers to minimise the effect of ISI and inter-carrier interference (ICI) caused by carrier frequency offset (CFO) and Doppler shifts. A few iterations are performed to achieve the expected bit error rate (BER) performance. To reduce the receiver complexity, the novel simplified diagonal FDFS-based receivers with a fixed noise variance are developed with slight performance degradation. The proposed iterative receivers have never been studied in the existing literature. Simulation results show that the proposed iterative FDFS-based receivers can significantly improve the BER performance of the conventional FDFS-MMSE receiver in severe multiple interferences environments caused by multipath, CFO and Doppler shift.
---
paper_title: Multi-tone CDMA design for arbitrary frequency offsets using orthogonal code multiplexing at the transmitter and a tunable receiver

paper_content:

The authors propose a new multi-tone (MT) code division multiple access (CDMA) design which has a superior bit error rate (BER) performance than conventional MT CDMA in the presence of frequency offset. The design involves multiplexing of Walsh codes onto the sub-carriers in conjunction with double differential modulation. To exploit the full potential of the design a partial correlation receiver has been proposed. Depending on the signal-to-noise ratio (SNR) and frequency offset it is possible to tune this receiver for the best possible performance. The simulated BER performance of the proposed system has been found to be better than MT CDMA for small as well as large frequency offsets for both single and multi-user systems in additive white Gaussian noise (AWGN) and Rayleigh fading channels.
---
paper_title: Simplified Multiaccess Interference Reduction for MC-CDMA With Carrier Frequency Offsets

paper_content:

Multicarrier code-division multiple-access (MC-CDMA) system performance can severely be degraded by multiaccess interference (MAI) due to the carrier frequency offset (CFO). We argue that MAI can more easily be reduced by employing complex carrier interferometry (CI) codes. We consider the scenario with spread gain N, multipath length L, and N users, i.e., a fully loaded system. It is proved that, when CI codes are used, each user only needs to combat 2(L - 1) (rather than N - 1) interferers, even in the presence of CFO. It is shown that this property of MC-CDMA with CI codes in a CFO channel can be exploited to simplify three multiuser detectors, namely, parallel interference cancellation (PIC), maximum-likelihood, and decorrelating multiuser detectors. The bit-error probability (BEP) for MC-CDMA with binary phase-shift keying (BPSK) modulation and single-stage PIC and an upper bound for the minimum error probability are derived. Finally, simulation results are given to corroborate theoretical results.
---
paper_title: MUI-Reducing Spreading Code Design for BS-CDMA in the Presence of Carrier Frequency Offset

paper_content:

Using mutually shift-orthogonal spreading codes, block-spread code-division multiple access (BS-CDMA) systems have been shown to achieve multiuser interference (MUI)-free reception when synchronization between the base station and the subscriber stations is achieved. In practice, when carrier frequency offset (CFO) is present, orthogonality among users is destroyed, and MUI occurs. This paper presents three methods of designing the spreading and despreading codes for uplink BS-CDMA systems to reduce MUI due to CFO. We show through rigorous derivation that all three codes can completely eliminate MUI due to CFO. In particular, by fixing the spreading codes at the transmitter, despreading codes that were obtained by minimizing the interference power or their cross correlation with the spreading codes are shown to yield a performance close to a synchronous system. The advantages and disadvantages of the BS-CDMA systems using the three proposed codes are discussed.
---
paper_title: Total Inter-Carrier Interference Cancellation for MC-CDMA System in Mobile Environment

paper_content:

Multi-carrier code division multiple access (MC-CDMA)has been considered as a strong candidate for next generation wireless communication system due to its excellent performance in multi-path fading channel and simple receiver structure. However, like all the multi-carrier transmission technologies such as OFDM, the inter-carrier interference (ICI) produced by the frequency offset between the transmitter and receiver local oscillators or by Doppler shift due to high mobility causes significant BER (bit error rate) performance degradation in MC-CDMA system. Many ICI cancellation methods such as windowing and frequency domain coding have been proposed in the literature to cancel ICI and improve the BER performance for multi-carrier transmission technologies. However, existing ICI cancellation methods do not cancel ICI entirely and the BER performance after ICI cancellation is still much worse than the BER performance of original system without ICI. Moreover, popular ICI cancellation methods like ICI self-cancellation reduce ICI at the price of lowering the transmission rate and reducing the bandwidth efficiency. Other frequency-domain coding methods do not reduce the data rate, but produce less reduction in ICI as well. In this paper, we propose a novel ICI cancellation scheme that can eliminate the ICI entirely and offer a MC-CDMA mobile system with the same BER performance of a MC-CDMA system without ICI. More importantly, the proposed ICI cancellation scheme (namely Total ICI Cancellation) does not lower the transmission rate or reduce the bandwidth efficiency. Specifically, by exploiting frequency offset quantization, the proposed scheme takes advantage of the orthogonality of the ICI matrix and offers perfect ICI cancellation and significant BER improvement at linearly growing cost. Simulation results in AWGN channel and multi-path fading channel confirm the excellent performance of the proposed Total ICI Cancellation scheme in the presence of frequency offset or time variations in the channel, outperforming existing ICI cancellation methods.
---
paper_title: Cluster-Based Differential Energy Detection for Spectrum Sensing in Multi-Carrier Systems

paper_content:

This paper presents a novel differential energy detection scheme for multi-carrier systems, which can form fast and reliable decision of spectrum availability even in very low signal-to-noise ratio (SNR) environment. For example, the proposed scheme can reach 90% in probability of detection (PD) and 10% in probability of false alarm (PFA) for the SNRs as low as -21 dB, while the observation length is equivalent to 2 multi-carrier symbol duration. The underlying initiative of the proposed scheme is applying order statistics on the clustered differential energy-spectral-density (ESD) in order to exploit the channel frequency diversity inherent in high data-rate communications. Specifically, to enjoy a good frequency diversity, the clustering operation is utilized to group uncorrelated subcarriers, while, the differential operation applied onto each cluster can effectively remove the noise floor and consequently overcome the impact of noise uncertainty while exploiting the frequency diversity. More importantly, the proposed scheme is designed to allow robustness in terms of both, time and frequency offsets. In order to analytically evaluate the proposed scheme, PFA and PD for Rayleigh fading channel are derived. The closed-form expressions show a clear relationship between the sensing performance and the cluster size, which is an indicator of the diversity gain. Moreover, we are able to observe up to 10 dB gain in the performance compared to the state-of-the-art spectrum sensing schemes.
---
paper_title: Secondary Transceiver Design in the Presence of Frequency Offset between Primary and Secondary Systems

paper_content:

When both primary and secondary systems are orthogonal frequency division multiplexing modulated and are non-cooperative, carrier frequency offset between the systems is inevitable to cause harmful interference. In this paper, we jointly optimize secondary transceivers assuming that the frequency offset between the secondary transmitter (ST) and the primary receiver (PR) and different channel information from the ST to the PR are known at the ST. We first derive unified interference constraints and obtain the secondary transceivers minimizing the mean square error through convex optimization techniques. We then derive closed-form transceivers for several special cases to reveal the impact of the frequency offset on the secondary transceivers. We show that when there is no frequency offset between the ST and the PR, the optimal processing at the ST is power allocation. Otherwise, both power allocation and precoding are necessary. The impact of the frequency offset on the performance of both systems increases as the interference constraints become tighter and the bandwidth of the primary system becomes smaller. When the proposed transceivers are used, the performance of the secondary system is robust to the frequency offset and the performance of the primary system degrades little due to the remanent frequency offset.
---
paper_title: Performance Analysis of Arbitrarily-Shaped Underlay Cognitive Networks: Effects of Secondary User Activity Protocols

paper_content:

This paper analyzes the performance of the primary users (PUs) and secondary users (SUs) in an arbitrarily-shaped underlay cognitive network. In order to meet the interference threshold requirement for a primary receiver at an arbitrary location, we consider different SU activity protocols that limit the number of active SUs. We propose a framework, based on the moment-generating function of the interference due to a random SU, to analytically compute the outage probability in the primary network, as well as the average number of active SUs in the secondary network. We also propose a cooperation-based SU activity protocol in the underlay cognitive network that includes the existing threshold-based protocol as a special case. We study the average number of active SUs for the different SU activity protocols, subject to a given outage probability constraint at the PU, and we employ it as an analytical approach to compare the effect of different SU activity protocols on the performance of the primary and secondary networks.
---
paper_title: Breaking Spectrum Gridlock With Cognitive Radios: An Information Theoretic Perspective

paper_content:

Cognitive radios hold tremendous promise for increasing spectral efficiency in wireless systems. This paper surveys the fundamental capacity limits and associated trans- mission techniques for different wireless network design paradigms based on this promising technology. These para- digms are unified by the definition of a cognitive radio as an intelligent wireless communication device that exploits side information about its environment to improve spectrum utilization. This side information typically comprises knowl- edge about the activity, channels, codebooks, and/or messages of other nodes with which the cognitive node shares the spectrum. Based on the nature of the available side information as well as a priori rules about spectrum usage, cognitive radio systems seek to underlay, overlay, or interweave the cognitive radios' signals with the transmissions of noncognitive nodes. We provide a comprehensive summary of the known capacity characterizations in terms of upper and lower bounds for each of these three approaches. The increase in system degrees of freedom obtained through cognitive radios is also illuminated. This information-theoretic survey provides guidelines for the spectral efficiency gains possible through cognitive radios, as well as practical design ideas to mitigate the coexistence challenges in today's crowded spectrum.
---
paper_title: A Low-Delay Low-Complexity EKF Design for Joint Channel and CFO Estimation in Multi-User Cognitive Communications

paper_content:

Parameter estimation in cognitive communications can be formulated as a multi-user estimation problem, which is solvable under maximum likelihood solution but involves high computational complexity. This paper presents a time-sharing and interference mitigation based EKF (Extended Kalman Filter) design for joint CFO (carrier frequency offset) and channel estimation at multiple cognitive users. The key objective is to realize low implementation complexity by decomposing high-dimensional parameters into multiple separate low-dimensional estimation problems, which can be solved in a time- shared manner via pipelining operation. We first present a basic EKF design that estimates the parameters from one TX user to one RX antenna. Then such basic design is time-shared and reused to estimate parameters from multiple TX users to multiple RX antennas. Meanwhile, we use interference mitigation module to cancel the co-channel interference at each RX sample. In addition, we further propose adaptive noise variance tracking module to improve the estimation performance. The proposed design enjoys low delay and low buffer size (because of its online real-time processing), as well as low implementation complexity (because of time-sharing and pipeling design). Its estimation performance is verified to be close to Cramer-Rao bound.
---
paper_title: Sensing orthogonal frequency division multiplexing systems for cognitive radio with cyclic prefix and pilot tones

paper_content:

The detection of orthogonal frequency division multiplexing (OFDM) for cognitive radio is considered in this paper. A frequency-selective fading channel is considered and the receiving process is modeled with timing and frequency offsets. Firstly, the authors propose a new decision statistic based on time-domain cross-correlation of the cyclic prefix (CP) embedded in OFDM signals. The probability distribution functions (PDFs) of the statistics under both hypotheses of primary signal absence and presence are derived. Estimation of the timing and frequency offset is obtained through the maximum likelihood method and the received signals are modified. Then another new decision statistic based on frequency-domain cross-correlation of the pilot tones (PTs) is proposed whose PDF is also analyzed. Then, through the likelihood ratio test, the authors utilize CP and PT jointly and propose a global test statistic. The theoretical probabilities of false alarm (PFA) and detection are derived, and the theoretical threshold for any given PFA is proposed. The simulation results show that the proposed spectrum-sensing scheme has excellent performance, especially under very low signal-to-noise ratio (SNR).
---
paper_title: Second-Order Cyclostationarity of Mobile WiMAX and LTE OFDM Signals and Application to Spectrum Awareness in Cognitive Radio Systems

paper_content:

Spectrum sensing and awareness are challenging requirements in cognitive radio (CR). To adequately adapt to the changing radio environment, it is necessary for the CR to detect the presence and classify the on-the-air signals. The wireless industry has shown great interest in orthogonal frequency division multiplexing (OFDM) technology. Hence, classification of OFDM signals has been intensively researched recently. Generic signals have been mainly considered, and there is a need to investigate OFDM standard signals, and their specific discriminating features for classification. In this paper, realistic and comprehensive mathematical models of the OFDM-based mobile Worldwide Interoperability for Microwave Access (WiMAX) and third-Generation Partnership Project Long Term Evolution (3GPP LTE) signals are developed, and their second-order cyclostationarity is studied. Closed-from expressions for the cyclic autocorrelation function (CAF) and cycle frequencies (CFs) of both signal types are derived, based on which an algorithm is proposed for their classification. The proposed algorithm does not require carrier, waveform, and symbol timing recovery, and is immune to phase, frequency, and timing offsets. The classification performance of the algorithm is investigated versus signal-to-noise ratio (SNR), for diverse observation intervals and channel conditions. In addition, the computational complexity is explored versus the signal type. Simulation results show the efficiency of the algorithm is terms of classification performance, and the complexity study proves the real time applicability of the algorithm.
---
paper_title: Eigenvalue-Based Spectrum Sensing of Orthogonal Space-Time Block Coded Signals

paper_content:

We consider spectrum sensing of signals encoded with an orthogonal space-time block code (OSTBC). We propose a CFAR detector based on knowledge of the eigenvalue multiplicities of the covariance matrix which are inherent owing to the OSTBC and derive theoretical performance bounds. In addition, we show that the proposed detector is robust to a carrier frequency offset, and propose a detector that deals with timing synchronization using the detector for the synchronized case as a building block. The proposed detectors are shown numerically to perform well.
---
paper_title: Software Defined Radio Implementation of SMSE Based Overlay Cognitive Radio in High Mobility Environment

paper_content:

A spectrally modulated spectrally encoded (SMSE) based overlay cognitive radio has been implemented and demonstrated in [1] via GNU software define radio (SDR). However, like most of the current cognitive radio implementations and demonstrations, this work does not consider the mobility between cognitive radio nodes. In a high mobility environment, the frequency offset introduced by Doppler shift leads to loss of the orthogonality among subcarriers. As a direct result, severe inter-carrier interference (ICI) and performance degradation is observed. In our previous work, we have proposed a new ICI cancellation method (namely Total ICI Cancellation) for OFDM [2] and MC-CDMA [3] mobile communication systems, which eliminates the ICI without lowering the transmission rate nor reducing the bandwidth efficiency. In this paper, we apply the total ICI cancellation algorithm onto the SMSE base overlay cognitive radio to demonstrate a high performance cognitive radio in high mobility environment. Specifically, we demonstrate an SMSE based overlay cognitive radio that is capable of detecting primary users in real time and adaptively adjusting its transmission parameters to avoid interference to (and from) primary users. When the primary user transmission changes, the cognitive radio dynamically adjusts its transmission accordingly. Additionally, this cognitive radio maintains seamless real time video transmission between the cognitive radio pair even when large frequency offset is introduced by mobility between CR transmitter and receiver.
---
paper_title: Blind spectrum sensing in cognitive radio over fading channels and frequency offsets

paper_content:

This paper deals with the problem of spectrum sensing in cognitive radio. We consider a stochastic system model where the Primary User (PU) transmits a periodic signal over fading channels. The effect of frequency offsets due to oscillator mismatch, and Doppler offset is studied. We show that for this case the Likelihood Ratio Test (LRT) cannot be evaluated pointwise. We present a novel approach to approximate the marginilisation of the frequency offset using a single point estimate. This is obtained via a low complexity Constrained Adaptive Notch Filter (CANF) to estimate the frequency offset. Performance is evaluated via numerical simulations and it is shown that the proposed spectrum sensing scheme can achieve the same performance as the “near-optimal” scheme, that is based on a bank of matched filters, using only a fraction of the complexity required.
---
paper_title: Carrier frequency offset estimation for non-contiguous OFDM receiver in cognitive radio systems

paper_content:

For non-contiguous (NC) OFDM based cognitive radio (CR) systems, schemes have been developed in literature to acquire spectrum synchronization information (SSI) with perfect carrier frequency offset (CFO) synchronization. However, OFDM is extremely sensitive to the CFO in practice, which leads to inter-carrier interference (ICI), hence degrading the spectrum synchronization performance for existing schemes. An accurate CFO estimation is therefore required before setting up the SSI. In this paper, we present a novel scheme based on the maximum likelihood (ML) algorithm to estimate the CFO for the NC-OFDM receiver when the SSI is unknown. A corresponding Cramer-Rao lower bound (CRB) with the ideal SSI is derived to demonstrate the efficiency of the proposed scheme. Simulation results show that the proposed scheme is robust against interference and achieves a satisfactory accuracy of estimation, which is close to the relevant CRB.
---
paper_title: Optimizing Wideband Cyclostationary Spectrum Sensing Under Receiver Impairments

paper_content:

In the context of Cognitive Radios (CRs), cyclostationary detection of primary users (PUs) is regarded as a common method for spectrum sensing. Cyclostationary detectors rely on the knowledge of the signal's symbol rate, carrier frequency, and modulation class in order to detect the present cyclic features. Cyclic frequency and sampling clock offsets are the two receiver impairments considered in this work. Cyclic frequency offsets, which occur due to oscillator frequency offsets, Doppler shifts, or imperfect knowledge of the cyclic frequencies, result in computing the test statistic at an offset from the true cyclic frequency. In this paper, we analyze the effect of cyclic frequency offsets on conventional cyclostationary detection, and propose a new multi-frame test statistic that reduces the degradation due to cyclic frequency offsets. Due to the multi-frame processing of the proposed statistic, non-coherent integration might occur across frames. Through an optimization framework developed in this work that can be performed offline, we determine the best frame length that maximizes the average detection performance of the proposed cyclostationary detection method given the statistical distributions of the receiver impairments. As a result of the optimization, the proposed detectors is shown to achieve the performance gains over conventional detectors given the constrained sensing time. We derive the proposed detector's theoretical average detection performance, and compare it to the performance of the conventional cyclostationary detector. Our analysis shows that gains in average detection performance using the proposed method can be achieved when the effect of sampling clock offset is less severe than that of the cyclic frequency offset. The analysis given in this paper can be used as a design guideline for practical implementation of cyclostationary spectrum sensors.
---
paper_title: Iterative Blind OFDM Parameter Estimation and Synchronization for Cognitive Radio Systems

paper_content:

An iterative design method for Orthogonal Frequency Division Multiplexing (OFDM) system parameter estimation and synchronization under a blind scenario for cognitive radio systems is proposed in this paper. A novel envelope spectrumbased arbitrary oversampling ratio estimator is presented first, based on which the algorithms are then developed to provide the identification of other OFDM parameters (number of subcarriers, cyclic prefix (CP) length). Carrier frequency offset (CFO) and timing offset are estimated for the purpose of synchronization with the help of the identified parameters. An iterative scheme is employed to increase the estimation accuracy. To validate the proposed design, the performance is evaluated under an experimental propagation environment and the results show that the proposed design is capable of adapting blind parameter estimation and synchronization for cognitive radio with improved performances.
---
paper_title: Joint Frequency Synchronization and Spectrum Occupancy Characterization in OFDM-Based Cognitive Radio Systems

paper_content:

OFDM-based cognitive radio (CR) systems are shown to be an effective solution for increasing spectrum efficiency by activating the certain group of subcarriers (subbands) in locally available spectrum. However, each CR receiver should synchronize itself to appropriate carrier frequency and to identify currently activated subbands. Moreover, energy and bandwidth efficiency of CR systems can be improved if each CR could provide additional characterization of the local spectral content. In this paper a novel joint frequency synchronization and spectrum occupancy characterization method for OFDM-based CR systems is proposed. The synchronization preamble structure is appropriately modified in order to efficiently perform frequency offset estimation, to identify occupied subbands, and, finally, to provide SNR and interference power estimates as reliable quantitative indicators of spectrum occupancy. The performance of proposed method is evaluated for different amounts of spectrum occupancy and interference levels.
---
paper_title: Spectrum Sensing for OFDM Signals Using Pilot Induced Auto-Correlations

paper_content:

Orthogonal frequency division multiplex (OFDM) has been widely used in various wireless communications systems. Thus the detection of OFDM signals is of significant importance in cognitive radio and other spectrum sharing systems. A common feature of OFDM in many popular standards is that some pilot subcarriers repeat periodically after certain OFDM blocks. In this paper, sensing methods for OFDM signals are proposed by using such repetition structure of the pilots. Firstly, special properties for the auto-correlation (AC) of the received signals are identified, from which the optimal likelihood ratio test (LRT) is derived. However, this method requires the knowledge of channel information, carrier frequency offset (CFO) and noise power. To make the LRT method practical, we then propose an approximated LRT (ALRT) method that does not rely on the channel information and noise power, thus the CFO is the only remaining obstacle to the ALRT. To handle the problem, we propose a method to estimate the composite CFO and compensate its effect in the AC using multiple taps of ACs of the received signals. Computer simulations have shown that the proposed sensing methods are robust to frequency offset, noise power uncertainty, time delay uncertainty, and frequency selectiveness of the channel.
---
paper_title: Interference mitigation techniques for asynchronous multiple access communications in SIMO FBMC systems

paper_content:

In this paper we derive linear equalizers for FBMC systems. We focus on the multiple access channel where signals transmitted by different users may have different carrier frequency offsets and time delays. Aiming at reducing the bandwidth requirements of the periodic ranging messages we formulate two SIMO solutions that are tolerant to time and frequency misalignments. Simulation-based results show that the same performance can be achieved in the BER range [10−2, 10−4] in comparison to an OFDM multi-user minimum mean square error receiver. Considering a guard interval between users, the BER range in which FBMC and OFDM perform equally can be broadened. However, the OFDM solution requires a complexity 8.6 times higher and its spectral efficiency is reduced by 0.72 b/s/Hz due to the cyclic prefix.
---
paper_title: Cooperative Space-Time Coded OFDM with Timing Errors and Carrier Frequency Offsets

paper_content:

The use of distributed space-time codes in cooperative communications promises to increase the rate and reliability of data transmission. These gains were mostly demonstrated for ideal scenarios, where all nodes are perfectly synchronized. Considering a cooperative uplink scenario with asynchronous nodes, the system suffers from two effects: timing errors and individual carrier frequency offsets. In effect, timing errors can completely cancel the advantages introduced by space-time codes, while individual carrier frequency offsets provide a great challenge to receivers. Indeed, frequency offsets are perceived as a time-variant channel (even if the individual links are static) in distributed cooperative communications. We show that using OFDM, space-time codes (STCs) become tolerant to timing errors. Channel estimation and tracking takes care of frequency offsets. Our simulations demonstrate that the bit error rate performance improves by an order of magnitude, when using a cooperative system design, which takes these two effects into account.
---
paper_title: A Novel Initial Cell Search Scheme in TD-LTE

paper_content:

In LTE system, in order to access the network, user equipment (UE) should detect the primary synchronization signal (PSS) and secondary synchronization signal (SSS) in downlink (DL) signal from the surrounding base stations (BS). This paper presents a novel initial cell search (ICS) scheme in TD-LTE system that contains two steps. In the first step, modified normalization based PSS detection is proposed to combat carrier frequency offset (CFO) and uplink (UL) interference. After CFO estimation and compensation, coherent SSS detection is adopted in frequency domain in the second period. Furthermore, in order to combat channel fading and noise, a method of flexible combination of PSS and SSS within several frames is proposed. Simulation results demonstrate that the proposed scheme is more robust and effective than conventional approaches in TDD system.
---
paper_title: A Simplified MMSE Equalizer for Distributed TR-STBC Systems with Multiple CFOs

paper_content:

In distributed wireless systems, the traditional carrier frequency offset (CFO) compensation methods may not be applied due to the existence of multiple CFOs. In this paper, we address the equalization issue for distributed time-reversal space-time block coded (TR-STBC) systems when multiple CFOs are presented. A simplified minimum mean-square error (MMSE) equalizer is proposed, which exploits the nearly-banded structure of the channel matrices and utilizes the {LDL}^H factorization to reduce the computational complexity. Simulation results show that the proposed equalizer can achieve the similar performance as the traditional MMSE equalizer while possessing much less complexity.
---
paper_title: Simultaneous Multiple Carrier Frequency Offsets Estimation for Coordinated Multi-Point Transmission in OFDM Systems

paper_content:

Orthogonal frequency division multiplexing (OFDM) combined with the coordinated multi-point (CoMP) transmission technique has been proposed to improve performance of the receivers located at the cell border. However, the inevitable carrier frequency offset (CFO) will destroy the orthogonality between subcarriers and induce strong inter-carrier interference (ICI) in OFDM systems. In a CoMP-OFDM system, the impact of CFO is more severe because of the mismatch in carrier frequencies among multiple transmitters. To reduce performance degradation, CFO estimation and compensation is essential. For simultaneous estimation of multiple CFOs, the performance of conventional CFO estimation schemes is significantly degraded by the mutual interference among the signals from different transmitters. In this work, our goal is to propose an effective approach that can simultaneously estimate multiple CFOs in the downlink by using the composite signal coming from multiple base stations corresponding to CoMP transmission. Based on the Zadoff-Chu sequences, we design an optimal set of training sequences, which minimizes the mutual interference and is robust to the variations in multiple CFOs. Then, we propose a maximum likelihood (ML)-based estimator, the robust multi-CFO estimation (RMCE) scheme, for simultaneous estimation of multiple CFOs. In addition, by incorporating iterative interference cancellation into the RMCE scheme, we propose an iterative scheme to further improve the estimation performance. According to the simulations, our scheme can eliminate the mutual interference effectively, approaching the Cramer-Rao bound performance.
---
paper_title: A Joint Channel and Frequency Offset Estimator for the Downlink of Coordinated MIMO-OFDM Systems

paper_content:

The issues of frequency offset and channel estimation are considered for the downlink of coordinated multiple-input multiple-output orthogonal frequency-division multiplexing systems. Multiple carrier frequency offsets exist in this coordinated system. Without implementing an appropriate compensation for these offsets, both inter-carrier and inter-cell interference will degrade the system performance. Here, we adopt a parallel interference cancelation strategy to iteratively mitigate inter-cell interference, and propose a frequency offset estimator, approximated by a Hadamard product and a Taylor series expansion, to eliminate the inter-carrier interference. Our scheme is significantly less complex than existing methods. Furthermore, the proposed channel estimator is robust to frequency offsets and performs comparably well to these conventional approaches.
---
paper_title: An Enhanced Signal-Timing-Offset Compensation Algorithm for Coordinated Multipoint-to-Multiuser Systems

paper_content:

In coordinated multipoint-to-multiuser systems, since coordinated base stations (BSs) may transmit their signals simultaneously to multiple user equipments (UEs) and a UE may receive signals with different signal timing offsets (STOs) from different BSs at given time-slots and subcarriers, it is impossible for BSs to pre-compensate or for UEs to post-compensate the STOs as they do it in multipoint-to-user or point-to-multiuser systems, respectively. In this letter, we convince by demonstrations that the STOs cannot be completely eliminated by any compensation algorithm. In addition, we propose a novel STO compensation algorithm associated with tolerant residual STOs to minimize the STOs, and discuss in details how to calculate the pre-compensation vector at BSs. Simulation results indicate that the proposed algorithm is capable of mitigating the STOs effectively to tolerable values, and outperforms state of the art approaches.
---
paper_title: Channel Estimation and Equalization for Asynchronous Single Frequency Networks

paper_content:

Single carrier frequency-domain equalization (SC-FDE) modulations are known to be suitable for broadband wireless communications due to their robustness against severe time-dispersion effects and the relatively low envelope fluctuations of the transmitted signals. In this paper, we consider the use of SC-FDE schemes in broadcasting systems. A single frequency network transmission is assumed, and we study the impact of distinct carrier frequency offset (CFO) between the local oscillator at each transmitter and the local oscillator at the receiver. We propose an efficient method for estimating the channel frequency response and CFO associated to each transmitter and propose receiver structures able to compensate the equivalent channel variations due to different CFO for different transmitters. Our performance results show that we can have excellent performance, even when transmitters have substantially different frequency offsets.
---
paper_title: Universal-filtered multi-carrier technique for wireless systems beyond LTE

paper_content:

In this paper, we propose a multi-carrier transmission scheme to overcome the problem of intercarrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems. In the proposed scheme, called universal-filtered multi-carrier (UFMC), a filtering operation is applied to a group of consecutive subcarriers (e.g. a given allocation of a single user) in order to reduce out-of-band sidelobe levels and subsequently minimize the potential ICI between adjacent users in case of asynchronous transmissions. We consider a coordinated multi-point (CoMP) reception technique, where a number of base stations (BSs) send the received signals from user equipments (UEs) to a CoMP central unit (CCU) for joint detection and processing. We examine the impact of carrier frequency offset (CFO) on the performance of the proposed scheme and compare the results with the performance of cyclic prefix based orthogonal frequency division multiplexing (CP-OFDM) systems. We use computer experiments to illustrate the efficiency of the proposed multi-carrier scheme. The results indicate that the UFMC scheme outperforms the OFDM for both perfect and non-perfect frequency synchronization between the UEs and BSs.
---
paper_title: Space-time coding for time and frequency asynchronous CoMP transmissions

paper_content:

This paper deals with time and frequency offset in coordinated multipoint (CoMP) transmission/reception networks by using distributed linear convolutional space-time coding (DLC-STC). We first prove that perfect time synchronization is impractical for CoMP transmission/reception networks. Then the DLC-STC scheme, in which exact time synchronization at the relay nodes is unnecessary, is proposed for the CoMP joint processing mode (CoMP-JP). Finally, we show the detecting method by minimum mean-squared error decision-feedback equalizer (MMSE-DFE) receivers with any frequency offsets. Simulation results show that with MMSE-DFE receivers, the proposed DLC-STC scheme outperforms the delay diversity scheme and the MMSE-DFE receivers can achieve the same diversity orders as the maximum likelihood sequence detection (MLSD) receivers.
---
paper_title: Coordinated Multi-Cell Systems: Carrier Frequency Offset Estimation and Correction

paper_content:

We consider a coordinated multi-cell (CMC) system and the associated problem of independent carrier frequency offsets (CFOs) at the basestations (BSs). These BS CFOs cause accumulated phase errors that compromise downlink beamforming accuracy, and consequently degrade the spectral efficiency of the CMC system. Since the inherent structure of coordinated downlink beamforming techniques makes it impossible to correct for the BS CFOs at the mobile subscriber (MS), our topic is estimation and correction of the BS CFOs at the BSs. Our method begins with the formation of MS-side estimates of the BS CFOs, which are then fed back to the coordinated BSs. We then derive an optimum maximum likelihood (ML) estimator for the BS CFOs that uses independent MS-side CFO estimates and average channel signal-to-noise ratios. However, it is demonstrated that the CFOs of the MSs themselves introduce a bias to the optimal BS CFO estimator. To compensate for this bias, a joint BS and MS CFO estimator is derived, but shown both to require high computation and have rank deficiency. This motivates an improved technique that removes the rank problem and employs successive estimation to reduce computation. It is demonstrated to overcome the MS CFO bias and efficiently solve the joint BS and MS CFO problem in systems that have low to moderate shadowing. We term the full BS CFO estimation and correction procedure "BS CFO tightening".
---
paper_title: Low-Complexity Semiblind Multi-CFO Estimation and ICA-Based Equalization for CoMP OFDM Systems

paper_content:

We propose a low-complexity semiblind structure with multiple-carrier-frequency-offset (CFO) estimation and independent component analysis (ICA)-based equalization for multiuser coordinated multipoint (CoMP) orthogonal frequency-division-multiplexing (OFDM) systems. A short pilot is carefully designed for each user and has a twofold advantage. On the one hand, using the pilot structure, a complex multidimensional search for multiple CFOs is divided into a number of low-complexity monodimensional searches. On the other hand, the cross correlations between the transmitted and the received pilots are explored to allow simultaneous elimination of permutation ambiguity and quadrant ambiguity in the ICA equalized signals. Simulation results show that with a low training overhead of 1.6%, the proposed semiblind system not only outperforms the existing multi-CFO estimation schemes in terms of bit error rate (BER) and mean square error (MSE) of multi-CFO estimation but achieves a BER performance close to the ideal case with perfect channel state information (CSI) and no CFO at the receiver as well.
---
paper_title: Estimation of Time and Frequency Offsets in LTE Coordinated Multi-Point Transmission

paper_content:

We address the impact, estimation and compensation of time and frequency offsets in LTE CoMP. In LTE CoMP, transmissions may come from a different transmission point in every subframe of one millisecond. Due to propagation delay differences and time-frequency synchronization imperfections between the transmission points, the user equipment (UE) receiver is thus exposed to different time- frequency offsets in every subframe. In this paper we illustrate both analytically and numerically the impact of these time and frequency offsets on channel estimation performance and finally on LTE CoMP link-level demodulation performance. Furthermore, we study the applicability of existing LTE reference signals to the time-frequency offset estimation problem. In particular we compare two approaches in which the UE is either aware or unaware of the exact transmission point. Finally, we show with LTE link-level simulations that using the proposed approaches the impacts of time-frequency offsets can be almost perfectly compensated.
---
paper_title: Coordinated multipoint transmission and reception in LTE-advanced: deployment scenarios and operational challenges

paper_content:

3GPP has completed a study on coordinated multipoint transmission and reception techniques to facilitate cooperative communications across multiple transmission and reception points (e.g., cells) for the LTE-Advanced system. In CoMP operation, multiple points coordinate with each other in such a way that the transmission signals from/to other points do not incur serious interference or even can be exploited as a meaningful signal. The goal of the study is to evaluate the potential performance benefits of CoMP techniques and the implementation aspects including the complexity of the standards support for CoMP. This article discusses some of the deployment scenarios in which CoMP techniques will likely be most beneficial and provides an overview of CoMP schemes that might be supported in LTE-Advanced given the modern silicon/DSP technologies and backhaul designs available today. In addition, practical implementation and operational challenges are discussed. We also assess the performance benefits of CoMP in these deployment scenarios with traffic varying from low to high load.
---
paper_title: Feedback Generation for CoMP Transmission in Unsynchronized Networks with Timing Offset

paper_content:

Coordinated multipoint (CoMP) transmission is a promising technique in long term evolution (LTE) systems to provide coverage and broadband communication for cell edge user equipments (UEs). However, as signals from multiple transmitters may reach the UE at different times, CoMP networks might experience high time offsets, leading to significant performance loss in closed-loop transmission. In this paper we show that spacing between reference signals (RSs) imposes a phase offset on the transmit covariance matrix in presence of timing offset (TO), affecting the feedback generation. The promised advantages of closed-loop CoMP transmission vanish in presence of TO due to improper precoding matrix index (PMI) selection. Keeping the phase shift close to zero, reliable PMI selection can be guaranteed and as a result near optimum performance is achieved for closed-loop CoMP transmission in unsynchronized networks with TO present.
---
paper_title: Efficient Phase-Error Suppression for Multiband OFDM-Based UWB Systems

paper_content:

This paper proposes an efficient phase-error suppression scheme for multiband (MB) orthogonal frequency-division multiplexing (OFDM)-based ultrawideband (UWB) communication systems. The proposed scheme consists of a clock-recovery loop and a common phase-error (CPE) tracking loop. The clock-recovery loop performs estimation of the sampling frequency offset (SFO) and its 2-D (time and frequency) compensation, while the CPE tracking loop estimates and corrects the phase errors caused by residual carrier frequency offset (CFO), residual SFO, and phase noise (PHN). The SFO and CPE estimators employ pilot-tone-based and channel-frequency-response (CFR)-weighted low-complexity approaches, each of which uses a robust error-reduction scheme without using angle calculations or divisions. Analytical results and numerical examples show the effectiveness of the proposed scheme in different multipath fading scenarios and signal-to-noise ratio (SNR) regimes.
---
paper_title: A non-coherent neighbor cell search scheme for LTE/LTE-A systems

paper_content:

A new neighbor cell search algorithm for LTE/LTE-A systems is presented in this paper. To improve the interference problem in channel estimation for coherent SSS detection in the conventional neighbor cell search approaches, we propose a non-coherent scheme that takes advantage of the similarity of channel responses at adjacent subcarriers. The proposed neighbor cell search procedure not only includes both PSS and SSS detection, but also can combat different carrier frequency offsets that the home cell signal and the neighbor cell signal may suffer. The removal of the home cell synchronization signals in our algorithm converts the neighbor cell PSS and SSS into new sequences for recognition, respectively. By examining the cross-correlation properties of the new sequences, we show that partial correlation can well detect the neighbor cell sector ID and group ID through the new sequences. From simulation results, it is also clear that the proposed algorithm has good detection results and outperforms the conventional coherent approaches.
---
paper_title: Sequence Designs for Interference Mitigation in Multi-Cell Networks

paper_content:

We propose a training sequence that can be used at the handshaking stage for multi-cell networks. The proposed sequence is theoretically proved to enjoy several nice properties including constant amplitude, zero autocorrelation, and orthogonality in multipath channels. Moreover, the analytical results show that the proposed sequence can greatly reduce the multi-cell interference (MCI) induced by carrier frequency offset (CFO) to a negligible level. Therefore, the CFO estimation algorithms designed for single-user or single-cell environments can be slightly modified, and applied in multi-cell environments; an example is given for showing how to modify the estimation algorithms. Consequently, the computational complexity can be dramatically reduced. Simulation results show that the proposed sequences and the CFO estimation algorithms outperform conventional schemes in multi-cell environments.
---
paper_title: Resource Block Basis MMSE Beamforming for Interference Suppression in LTE Uplink

paper_content:

This paper proposes a new method to suppress interference using antenna array for LTE uplink. The proposed method does not require knowledge on resource allocation of either interfering or communicating mobile stations, and thus it has a significant advantage of ease of implementation. An additional advantage of this method is parallelization and scalability for multi-core processing. This paper also proposes a novel iterative timing offset compensation to enable effective interference suppression. The proposed method has been successfully implemented on System-on-Chip consisting of multi-core DSP and ARM microprocessors and verified that it successfully suppresses interference in real time.
---
paper_title: Robust carrier frequency offset and channel estimation for orthogonal frequency division multiple access downlink systems in the presence of severe adjacent-cell interference

paper_content:

This study investigates joint estimation carrier frequency offset (CFO) and channel impulse response (CIR) in the problematic orthogonal frequency division multiple access (OFDMA) downlink scenario at the cell boundary, where a maximum supposition of three adjacent base stations preambles can be received. A novel scheme is proposed for estimating the CFOs and CIRs of different cells. First, the highly efficient joint maximum-likelihood (JML) algorithm is used to enable robust CFO estimation by exploiting the idempotent property of the projection matrix. Next, in order to estimate the precise CIR, the composite CFO-based and circularly shifted preamble signatures are proposed by applying the constrained minimum variance (CMV) algorithm to suppress adjacent-cell interference. Finally, the CFO and CIR estimations are enhanced by an iterative cancellation scheme. To the best of the authors research, there is no publication study proposed joint JML-CFO and CMV-CIR estimators for OFDMA downlink systems with adjacent-cell interference. Simulation results show that the proposed algorithms provide better performance than the conventional estimators and approach the theoretical Cramer–Rao lower bound at the cell boundary over frequency-selective fading channels.
---
paper_title: Synchronization, Channel Estimation, and Equalization in MB-OFDM Systems

paper_content:

This paper addresses preamble-based low complexity synchronization, channel estimation and equalization for Zero-padded (ZP) MB-OFDM based UWB systems. The proposed synchronization method consists of sync detection, coarse timing estimation, fine timing estimation, and oscillator frequency offset estimation. The distinctive features of MB-OFDM systems and the interplay between the timing and carrier frequency hopping at the receiver are judiciously incorporated in the proposed synchronization method. In order to apply the low complexity one-tap frequency-domain equalizer, the required circular convolution property of the received signal is obtained by means of an overlap-add method after the frequency offset compensation. The proposed low complexity channel estimator for each band is developed by first averaging the over-lapadded received preamble symbols within the same band and then applying time-domain least-squares method followed by the discrete Fourier transform. We develop an MMSE equalizer and its approximate version with low complexity. We also derive the probability density functions of the UWB channel path delays, and using them we present several optimization criteria for our proposed synchronization, channel estimation, and equalization. The effectiveness of our proposed methods and optimization criteria are confirmed by computer simulation results.
---
paper_title: Training-Based Synchronization and Demodulation With Low Complexity for UWB Signals

paper_content:

In this paper, we propose a low-complexity data-aided (DA) synchronization and efficient demodulation technique for an ultrawideband (UWB) impulse radio system. Depending on the autocorrelation property of a judiciously chosen training sequence, a redundance-included demodulation template (RDT) can be extracted from the received signal by separating, shifting, and realigning two connected portions in the observation window. After constructing the RDT, two receiver designs are available. One approach is to demodulate transmitted symbols by correlating the RDT in a straightforward manner, which does not require the explicit timing acquisition and, thus, considerably reduces the complexity of the receiver. An alternative approach is accomplished with the assistance of a non-RDT (NRDT). The NRDT-based receiver is able to remove the redundant noisy component of the RDT by acquiring timing offset via a simple synchronization scheme, therefore achieving a better bit error rate (BER) performance. Both the schemes can counteract the effects of interframe interference (IFI) and unknown multipath channels. Furthermore, analytical performance evaluations of the RDT- and NRDT-based receivers are provided. Simulations verify the realistic performance of the proposed receivers in the presence of multiuser interference (MUI) and timing errors.
---
paper_title: A generalized RAKE receiver for interference suppression

paper_content:

Currently, a global third-generation cellular system based on code-division multiple-access (CDMA) is being developed with a wider bandwidth than existing second-generation systems. The wider bandwidth provides increased multipath resolution in a time-dispersive channel, leading to higher frequency-selectivity. A generalized RAKE receiver for interference suppression and multipath mitigation is proposed. The receiver exploits the fact that time dispersion significantly distorts the interference spectrum from each base station in the downlink of a wideband CDMA system. Compared to the conventional RAKE receiver, this generalized RAKE receiver may have more fingers and different combining weights. The weights are derived from a maximum likelihood formulation, modeling the intracell interference as colored Gaussian noise. This low-complexity detector is especially useful for systems with orthogonal downlink spreading codes, as orthogonality between own cell signals cannot be maintained in a frequency-selective channel. The performance of the proposed receiver is quantified via analysis and simulation for different dispersive channels, including Rayleigh fading channels. Gains on the order of 1-3.5 dB are achieved, depending on the dispersiveness of the channel, with only a modest increase in the number of fingers. For a wideband CDMA (WCDMA) system and a realistic mobile radio channel, this translates to capacity gains of the order of 100%.
---
paper_title: Traffic-reduced precise ranging protocol for asynchronous UWB positioning networks

paper_content:

This letter proposes a precise two-way ranging (TWR) protocol toward low traffic for asynchronous UWB positioning networks. The proposed TWR protocol pursuing instantaneous ranging update enables the estimation of clock frequency offset to achieve high ranging accuracy. Theoretical analysis and simulation results verify the performance.
---
paper_title: Robust, Low-Complexity, and Energy Efficient Downlink Baseband Receiver Design for MB-OFDM UWB System

paper_content:

This paper presents optimized synchronization algorithms and architecture designs of a downlink baseband receiver for multiband orthogonal frequency division multiplexing ultra wideband (MB-OFDM UWB). The receiver system targets at low complexity and low power under the premise of good performance. At algorithm level, dual-threshold (DT) detection method is proposed for robust detection performance in timing synchronization; multipartite table method (MTM) is employed to implement arctangent and sin/cos functions in coarse frequency synchronization. MTM outperforms other state-of-the-art methods in power and area. A highly simplified phase tracking method is proposed with better performance in fine frequency synchronization. At architecture level, we focus on optimizing matched filter of packet detector, carrier frequency offset (CFO) corrector and FFT output reorder buffer. The proposed downlink baseband receiver is implemented with 0.13 /mi CMOS technology. The core area of layout is 2.66 × 0.94 mm2, which saves 45.1% hardware cost due to the low-complexity synchronization algorithms and architecture optimization. The postlayout power consumption is 170 mW at 132 MHz clock frequency, which is equivalent to 88 pJ/b energy efficiency at 480 Mbps data rate.
---
paper_title: Blind frequency-offset tracking scheme for multiband orthogonal frequency division multiplexing using time-domain spreading

paper_content:

A blind scheme for estimating the residual carrier-frequency offset of multiband orthogonal frequency division multiplexing (MB-OFDM)-based ultra-wideband (UWB) systems is proposed. In the MB-OFDM UWB system, time-domain spreading (TDS) is used by transmitting the same information across two adjacent OFDM symbols. By using the TDS structure, the proposed frequency estimation scheme does not require the use of pilot symbols. To demonstrate the usefulness of the proposed estimator, analytical expression of the mean square error is derived and the performance is compared with a conventional pilot-assisted estimator.
---
paper_title: Data-Aided Timing Synchronization for FM-DCSK UWB Communication Systems

paper_content:

Frequency-modulated differential chaos shift keying (FM-DCSK) ultrawideband (UWB) communication systems convey information by transmitting ultrashort chaotic pulses (in the nanosecond scale). Since such pulses are ultrashort, timing offset may severely degrade the bit error rate (BER) performance. In this paper, a fast data-aided timing synchronization algorithm with low complexity is proposed for FM-DCSK UWB systems, which capitalizes on the excellent correlation characteristic of chaotic signals. Simulation results show that the BER performance of such systems is fairly close to that of perfect timing thanks to the proposed new algorithm. Moreover, the new algorithm requires less synchronization searching time and lower computational complexity than the conventional one for transmitted reference UWB systems existing in the current literature.
---
paper_title: Improved fine CFO synchronization for MB-OFDM UWB

paper_content:

Proposed is an improved blind carrier frequency offset (CFO) estimator suitable for Multi-Band OFDM Ultra Wideband (MB-OFDM UWB) system. By exploiting the conjugate symmetry of the physical layer convergence protocol (PLCP) the need for training symbols can be avoided and estimation performance is improved as well. Computer simulations show that the proposed method achieves better estimation performance than existing method.
---
paper_title: Schmidl-Cox-like Frequency Offset Estimation in Time-Hopping UWB

paper_content:

This paper presents a time hopping ultra wide band (TH-UWB) receiver design targeted to high performance and low complexity. The classical Schmidl and Cox idea of extracting the frequency offset from the phase of a correlation measure between two identical transmitted signals is here extended to a TH format. The algorithm exploits the low duty cycle of the time hopping access, and combines received samples in order to strengthen the signal-to-noise ratio. A clever selection of which samples to use in the correlation measure (first and last third of the received signal) is proved to perform 0.5 dB from the Cramer-Rao lower bound, thus providing an improvement of 0.75 dB with respect to the standard approach of the literature (correlating the first and second halves of the received signal). A modified algorithm that suitably weights received samples is also proposed for achieving robustness in impulsive multiple user access scenarios.
---
paper_title: Ultrawideband radio design: the promise of high-speed, short-range wireless connectivity

paper_content:

The paper provides a tutorial overview of ultrawideband (UWB) radio technology for high-speed wireless connectivity. Subsequent to establishing a historical and technological context, it describes the new impetus for UWB systems development and standardization resulting from the FCC's recent decision to permit unlicensed operation in the 3.1-10.6 GHz band subject to modified Part 15 rules and indicates the potential new applications that may result. Thereafter, the paper provides a system architect's perspectives on the various issues and challenges involved in the design of link layer subsystems. Specifically, we outline current developments in UWB system design concepts that are oriented to high-speed applications and describe some of the design tradeoffs involved.
---
paper_title: Maximum Likelihood Frequency Offset Estimation in Multiple Access Time-Hopping UWB

paper_content:

Frequency offset estimation for time-hopping (TH) ultra-wide-band (UWB) is addressed in the literature by relying on an AWGN assumption and by exploiting a periodic preamble appended to each packet. In this paper we generalize these techniques with two aims. First, we identify a solution which does not rely on any periodic structure, but can be implemented with a generic TH format. Second, we identify a solution which is robust to multiple access interference (MAI) by assuming a Gaussian mixture (GM) model for MAI. In fact, GMs have recently been identified as good descriptors of UWB interference, and they provide closed form and limited complexity results. With these ideas in mind, we build a data aided maximum likelihood (ML) estimator. The proposed ML solution shows quasi optimum performance in the Cramer-Rao bound sense, and proves to be robust in meaningful multiple user scenarios.
---
paper_title: Resource Efficient Implementation of Low Power MB-OFDM PHY Baseband Modem With Highly Parallel Architecture

paper_content:

The multi-band orthogonal frequency-division multiplexing modem needs to process large amount of computations in short time for support of high data rates, i.e., up to 480 Mbps. In order to satisfy the performance requirement while reducing power consumption, a multi-way parallel architecture has been proposed. But the use of the high degree parallel architecture would increase chip resource significantly, thus a resource efficient design is essential. In this paper, we introduce several novel optimization techniques for resource efficient implementation of the baseband modem which has highly, i.e., 8-way, parallel architecture, such as new processing structures for a (de)interleaver and a packet synchronizer and algorithm reconstruction for a carrier frequency offset compensator. Also, we describe how to efficiently design several other components. The detailed analysis shows that our optimization technique could reduce the gate count by 27.6% on average, while none of techniques degraded the overall system performance. With 0.18-μm CMOS process, the gate count and power consumption of the entire baseband modem were about 785 kgates and less than 381 mW at 66 MHz clock rate, respectively.
---
paper_title: Maximum Likelihood Frequency Estimation and Preamble Identification in OFDMA-based WiMAX Systems

paper_content:

In multi-cellular WiMAX systems based on orthogonal frequency-division multiple-access (OFDMA), the training preamble is chosen from a set of known sequences so as to univocally identify the transmitting base station. Therefore, in addition to timing and frequency synchronization, preamble index identification is another fundamental task that a mobile terminal must successfully complete before establishing a communication link with the base station. In this work we investigate the joint maximum likelihood (ML) estimation of the carrier frequency offset (CFO) and preamble index in a multicarrier system compliant with the WiMAX specifications, and derive a novel expression of the relevant Cramer-Rao bound (CRB). Since the exact ML solution is prohibitively complex in its general formulation, suboptimal algorithms are developed which can provide a reasonable trade-off between estimation accuracy and processing load. Specifically, we show that the fractional CFO can be recovered by combining the ML estimator with an existing algorithm that attains the CRB in all practical scenarios. The integral CFO and preamble index are subsequently retrieved by a suitable approximation of their joint ML estimator. Compared to existing alternatives, the resulting scheme exhibits improved accuracy and reduced sensitivity to residual timing errors. The price for these advantages is a certain increase of the system complexity.
---
paper_title: Improved frequency offset estimation scheme for UWB systems with cyclic delay diversity

paper_content:

This paper proposes an improved pilot-based algorithm for joint estimation of carrier frequency offset (CFO) and sampling frequency offset (SFO) in ultrawideband orthogonal frequency division multiplexing (UWB-OFDM) systems with cyclic delay diversity. By proper selection of cyclic delay time and a frequency band with maximum channel power, a joint estimation of CFO and SFO is derived. Via computer simulation and analysis, the proposed frequency estimator is shown to benefit from the properly selected delay parameter and frequency band.
---
paper_title: Ultra-Wideband TOA Estimation in the Presence of Clock Frequency Offset

paper_content:

The paper is concerned with the impact of clock frequency offsets on the accuracy of ranging systems based on time of arrival (TOA) measurements. It is shown that large TOA errors are incurred if the transmitter and receiver clocks are mistuned by more than just one part per million (ppm). This represents a serious obstacle to the use of commercial low-cost quartz oscillators, as they exhibit frequency drifts in the range of ± 10 ppm and more. A solution is to estimate first the transmitter clock frequency relative to the receiver's and then compensate for the difference by acting on the receiver clock tuning. An algorithm is proposed that estimates the transmitter clock frequency with an accuracy better than 0.1 ppm. Computer simulations indicate that its use in ranging systems makes TOA measurements as good as those obtained with perfectly synchronous clocks.
---
paper_title: Receiver Design for Single-Frequency Networks with Fast-Varying Channels

paper_content:

SC-FDE (Single Carrier Frequency-Domain Equalization) modulations are known to be suitable for broadband wireless communications due to their robustness against severe time-dispersion effects and the relatively low envelope fluctuations of the transmitted signals. In this paper we consider the use of SC-FDE schemes in broadcast and multicast systems with SFN (Single Frequency Network) operation where we do not have perfect carrier synchronization between different transmitters. We study the impact of different CFO (Carrier Frequency Offset) between the local oscillator at the receiver and the local oscillator at each transmitter. We also propose receiver structures able to reduce the performance degradation caused by different CFO at different transmitters. Our receivers can be regarded as modified turbo equalizer implemented in the frequency-domain, where a frequency offset compensation is performed before the iterative receiver.
---
paper_title: A Synchronization Design for UWB-Based Wireless Multimedia Systems

paper_content:

Multi-band orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) technology offers large throughput, low latency and has been adopted in wireless audio/video (AV) network products. The complexity and power consumption, however, are still major hurdles for the technology to be widely adopted. In this paper, we propose a unified synchronizer design targeted for MB-OFDM transceiver that achieves high performance with low implementation complexity. The key component of the proposed synchronizer is a parallel auto-correlator structure in which multiple ACF units are instantiated and their outputs are shared by functional blocks in the synchronizer, including preamble signal detection, time-frequency code identification, symbol timing, carrier frequency offset estimation and frame synchronization. This common structure not only reduces the hardware cost but also minimizes the number of operations in the functional blocks in the synchronizer as the results of a large portion of computation can be shared among different functional blocks. To mitigate the effect of narrowband interference (NBI) on UWB systems, we also propose a low-complexity ACF-based frequency detector to facilitate the design of (adaptive) notch filter in analog/digital domain. The theoretical analysis and simulation show that the performance of the proposed design is close to optimal, while the complexity is significantly reduced compared to existing work.
---
paper_title: Beamforming Based Receiver Scheme for DVB-T2 System in High Speed Train Environment

paper_content:

In this paper, the received signal from the different base stations (BSs) of the second generation of Terrestrial Digital Video Broadcasting (DVB-T2) in the high-speed-train (HST) scenario is modeled as a fast time-varying signal with the multiple Doppler frequency offsets. The interference caused by the multiple Doppler frequency offsets and the channel variations, and the signal to interference plus noise ratio of the received signal, are derived for the DVB-T2 receiver. The results of the theoretical analysis show that the interference greatly degraded the performance of the DVB-T2 system. To suppress the interference, we proposed a beamforming based receiver scheme for DVB-T2 system. By using the new signal processing scheme for the received signal vector from the antenna array, one can separate the received signal with the multiple Doppler frequency offsets into the multiple signals, each of which is with a single Doppler frequency offset. The separated signals are compensated by the corresponding Doppler frequency offsets and equalized by the estimated channel responses respectively, then combined into a signal to be demodulated. The results of the simulation show that the proposed scheme can effectively suppress the interference and greatly improve the performance of the DVB-T2 system in the HST environment.
---
paper_title: Iterative Sampling Frequency Offset Estimation for MB-OFDM UWB Systems With Long Transmission Packet

paper_content:

A multiband orthogonal frequency-division multiplexing (MB-OFDM) system, which is one of the effective modulation techniques that have been adopted for high-speed ultrawideband (UWB) systems, is very sensitive to sampling frequency offset (SFO) due to the mismatch between local oscillators at the transmitter and the receiver. In this paper, we propose an iterative SFO estimation method for a high-data-rate MB-OFDM UWB system to improve its SFO estimation accuracy in the case of a long transmission packet. The proposed method is an iterative process of 2-D SFO estimation across pilot subcarriers and consecutive OFDM symbols, together with joint channel estimation. Furthermore, we derive the Cramer-Rao lower bound (CRLB) for the proposed SFO estimation method. This CRLB can be used as a guide for algorithm design and to explore the theoretical limit. Our performance analysis and simulation results show that the proposed iterative SFO estimation method is both practical and effective.
---
paper_title: Signal Design for Reduced Complexity and Accurate Cell Search/Synchronization in OFDM-Based Cellular Systems

paper_content:

This paper proposes a variant of the frequency-domain synchronization structure specified in the long-term evolution (LTE) standard. In the proposed scheme, the primary synchronization signal used in step-1 cell search is the concatenation of a Zadoff-Chu (ZC) sequence and its conjugate (as opposed to only the ZC sequence in LTE). For step-2 cell search, we propose a complex scrambling sequence requiring no descrambling and a new remapped short secondary synchronization signal that randomizes the intercell interference (as opposed to the first/second scrambling sequence and swapped short signals in LTE). Through a combination of analysis and simulation, we demonstrate that the proposed synchronization signals lead to lower searcher complexity than LTE, a lower detection error rate, a shorter mean cell search time, and immunity toward a frequency offset.
---
paper_title: A Subspace-Based Two-Way Ranging System Using a Chirp Spread Spectrum Modem, Robust to Frequency Offset

paper_content:

Herein, we propose and implement a subspace-based two-way ranging system for high resolution indoor ranging that is robust to frequency offset. Due to the frequency offset between wireless nodes, issues about sampling frequency offset (SFO) and carrier frequency offset (CFO) arise in range estimation. Although the problem of SFO is resolved by adopting the symmetric double-sided two-way ranging (SDS-TWR) protocol, the CFO of the received signals impacts the time-of-arrival (TOA) estimation, obtained by conventional subspace-based algorithms such as ESPRIT and MUSIC. Nevertheless, the CFO issue has not been considered with subspace-based TOA estimation algorithms.Our proposed subspace-based algorithm, developed for the robust TOA estimation to CFO, is based on the chirp spread spectrum (CSS) signals. Our subspace-based ranging system is implemented in FPGA with CSS modem using a hardware/software co-design methodology. Simulations and experimental results show that the proposed method can achieve robust ranging between the CSS nodes in an indoor environment with frequency offset.
---
paper_title: Frequency-division spread-spectrum makes frequency synchronisation easy

paper_content:

Frequency division spread spectrum system (FD/S3) has been recently proposed, which uses frequency domain spreading codes and can allow frequency offsets between users. Motivated by the conventional time-domain code acquisition, we propose a frequency (F)-domain code acquisition method using time (T)-domain integrator, which permits frequency offset, which leads to a new T-domain code acquisition using F-domain integrator. Recently proposed Gabor division (GD)/S3 system permits us to use both F- and T-domain code acquisitions separately and cooperatively*.
---
paper_title: Self-encoded multi-carrier spread spectrum with iterative despreading for random residual frequency offset

paper_content:

In this study, we investigate the multi-carrier spread spectrum (MCSS) communication system which adopts the self-encoded spread spectrum in a downlink synchronous channel. It is very difficult to completely eliminate the frequency offset in practical channel scenarios. We demonstrate that the self-encoded MCSS (SE-MCSS) with iterative despreading manifests a remarkable immunity to residual frequency offset. The SE-MCSS can be an excellent candidate for the future generation of wireless services.
---
paper_title: A Fast Time-Delay Estimator of PN Signals

paper_content:

This work proposes an effective time-delay estimator for PN satellite signals, exploiting a fast triangular interpolator, running on three estimated ambiguity samples in the neighborhood of the coarse estimate. Performance analysis (theory and simulation) is carried out in comparison with conventional approaches based on the interpolation, usually carried out by (time-consuming) narrow-band over-sampling or (fast) fitting of few samples of a smoothed function of the ambiguity function around its maximum. The theoretical results, substantiated by computer simulations, have evidenced that the devised method outperforms the conventional estimator for all the timing offsets and is well suited for satellite spread-spectrum communications.
---
paper_title: Gabor Division/Spread Spectrum System Is Separable in Time and Frequency Synchronization

paper_content:

Recently proposed new Time-Domain (TD) synchronization using frequency integration and TD Spread Spectrum (SS) codes has been shown to be robust to frequency offset, that has its dual Frequency-Domain (FD) synchronization using time integration and FD SS codes which is robust to timing offset. Separable Property (SP) is defined for time-frequency synchronization under the condition containing time and frequency deviations to be performed separately and cooperatively. The SP compels us to design phase correction on SS codes and transmitted data.
---
paper_title: Wireless Networks With RF Energy Harvesting: A Contemporary Survey

paper_content:

Radio frequency (RF) energy transfer and harvesting techniques have recently become alternative methods to power the next generation wireless networks. As this emerging technology enables proactive energy replenishment of wireless devices, it is advantageous in supporting applications with quality of service (QoS) requirement. In this paper, we present an extensive literature review on the research progresses in wireless networks with RF energy harvesting capability, referred to as RF energy harvesting networks (RF-EHNs). First, we present an overview of the RF-EHNs including system architecture, RF energy harvesting techniques and existing applications. Then, we present the background in circuit design as well as the state-of-the-art circuitry implementations, and review the communication protocols specially designed for RF-EHNs. We also explore various key design issues in the development of RF-EHNs according to the network types, i.e., single-hop network, multi-antenna network, relay network and cognitive radio network. Finally, we envision some open research directions.
---
paper_title: Relaying Protocols for Wireless Energy Harvesting and Information Processing

paper_content:

An emerging solution for prolonging the lifetime of energy constrained relay nodes in wireless networks is to avail the ambient radio-frequency (RF) signal and to simultaneously harvest energy and process information. In this paper, an amplify-and-forward (AF) relaying network is considered, where an energy constrained relay node harvests energy from the received RF signal and uses that harvested energy to forward the source information to the destination. Based on the time switching and power splitting receiver architectures, two relaying protocols, namely, i) time switching-based relaying (TSR) protocol and ii) power splitting-based relaying (PSR) protocol are proposed to enable energy harvesting and information processing at the relay. In order to determine the throughput, analytical expressions for the outage probability and the ergodic capacity are derived for delay-limited and delay-tolerant transmission modes, respectively. The numerical analysis provides practical insights into the effect of various system parameters, such as energy harvesting time, power splitting ratio, source transmission rate, source to relay distance, noise power, and energy harvesting efficiency, on the performance of wireless energy harvesting and information processing using AF relay nodes. In particular, the TSR protocol outperforms the PSR protocol in terms of throughput at relatively low signal-to-noise-ratios and high transmission rate.
---
paper_title: Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas

paper_content:

A cellular base station serves a multiplicity of single-antenna terminals over the same time-frequency interval. Time-division duplex operation combined with reverse-link pilots enables the base station to estimate the reciprocal forward- and reverse-link channels. The conjugate-transpose of the channel estimates are used as a linear precoder and combiner respectively on the forward and reverse links. Propagation, unknown to both terminals and base station, comprises fast fading, log-normal shadow fading, and geometric attenuation. In the limit of an infinite number of antennas a complete multi-cellular analysis, which accounts for inter-cellular interference and the overhead and errors associated with channel-state information, yields a number of mathematically exact conclusions and points to a desirable direction towards which cellular wireless could evolve. In particular the effects of uncorrelated noise and fast fading vanish, throughput and the number of terminals are independent of the size of the cells, spectral efficiency is independent of bandwidth, and the required transmitted energy per bit vanishes. The only remaining impairment is inter-cellular interference caused by re-use of the pilot sequences in other cells (pilot contamination) which does not vanish with unlimited number of antennas.
---
paper_title: In-Band Full-Duplex Relaying: A Survey, Research Issues and Challenges

paper_content:

Recent advances in self-interference cancellation techniques enable in-band full-duplex wireless systems, which transmit and receive simultaneously in the same frequency band with high spectrum efficiency. As a typical application of in-band full-duplex wireless, in-band full-duplex relaying (FDR) is a promising technology to integrate the merits of in-band full-duplex wireless and relaying technology. However, several significant research challenges remain to be addressed before its widespread deployment, including small-size full-duplex device design, channel modeling and estimation, cross-layer/joint resource management, interference management, security, etc. In this paper, we provide a brief survey on some of the works that have already been done for in-band FDR, and discuss the related research issues and challenges. We identify several important aspects of in-band FDR: basics, enabling technologies, information-theoretical performance analysis, key design issues and challenges. Finally, we also explore some broader perspectives for in-band FDR.
---
paper_title: Frequency synchronization and phase offset tracking in a real-time 60-GHz CS-OFDM MIMO system

paper_content:

The performance of an Orthogonal Frequency-Division-Multiplexing (OFDM)-based 60-GHz system can be strongly degraded due to carrier frequency impairment and Phase Noise (PN). In this paper we present a practical approach to the design of a frequency synchronization and phase offset tracking scheme for a 60-GHz, Non-Line-of-Sight (NLOS) capable wireless communication system. We first analyse the architecture of the 60-GHz system and propose a simple algorithm for Carrier Frequency Offset (CFO) estimation on the basis of numerical investigations. Then, we explore pilot based and blind tracking methods for mitigation of Residual Frequency Offset (RFO) and Common Phase Error (CPE). Provided are also analysis and implementation results on an Altera Startix III FPGA.
---
paper_title: MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer

paper_content:

Wireless power transfer (WPT) is a promising new solution to provide convenient and perpetual energy supplies to wireless networks. In practice, WPT is implementable by various technologies such as inductive coupling, magnetic resonate coupling, and electromagnetic (EM) radiation, for short-/mid-/long-range applications, respectively. In this paper, we consider the EM or radio signal enabled WPT in particular. Since radio signals can carry energy as well as information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) is pursued. Specifically, this paper studies a multiple-input multiple-output (MIMO) wireless broadcast system consisting of three nodes, where one receiver harvests energy and another receiver decodes information separately from the signals sent by a common transmitter, and all the transmitter and receivers may be equipped with multiple antennas. Two scenarios are examined, in which the information receiver and energy receiver are separated and see different MIMO channels from the transmitter, or co-located and see the identical MIMO channel from the transmitter. For the case of separated receivers, we derive the optimal transmission strategy to achieve different tradeoffs for maximal information rate versus energy transfer, which are characterized by the boundary of a so-called rate-energy (R-E) region. For the case of co-located receivers, we show an outer bound for the achievable R-E region due to the potential limitation that practical energy harvesting receivers are not yet able to decode information directly. Under this constraint, we investigate two practical designs for the co-located receiver case, namely time switching and power splitting, and characterize their achievable R-E regions in comparison to the outer bound.
---
paper_title: Improving Bandwidth Efficiency in E-band Communication Systems

paper_content:

The allocation of a large amount of bandwidth by regulating bodies in the 70/80 GHz band, that is, the E-band, has opened up new potentials and challenges for providing affordable and reliable gigabit-per-second wireless point-to-point links. This article first reviews the available bandwidth and licensing regulations in the E-band. Subsequently, different propagation models (e.g., the ITU-R and Cane models) are compared against measurement results, and it is concluded that to meet specific availability requirements, E-band wireless systems may need to be designed with larger fade margins compared to microwave systems. A similar comparison is carried out between measurements and models for oscillator phase noise. It is confirmed that phase noise characteristics, which are neglected by the models used for narrowband systems, need to be taken into account for the wideband systems deployed in the E-band. Next, a new MIMO transceiver design, termed continuous aperture phased (CAP)-MIMO, is presented. Simulations show that CAP-MIMO enables E-band systems to achieve fiber-optic-like throughputs. Finally, it is argued that full-duplex relaying can be used to greatly enhance the coverage of E-band systems without sacrificing throughput, thus facilitating their application in establishing the backhaul of heterogeneous networks.
---
paper_title: In-Band Full-Duplex Wireless: Challenges and Opportunities

paper_content:

In-band full-duplex (IBFD) operation has emerged as an attractive solution for increasing the throughput of wireless communication systems and networks. With IBFD, a wireless terminal is allowed to transmit and receive simultaneously in the same frequency band. This tutorial paper reviews the main concepts of IBFD wireless. One of the biggest practical impediments to IBFD operation is the presence of self-interference, i.e., the interference that the modem's transmitter causes to its own receiver. This tutorial surveys a wide range of IBFD self-interference mitigation techniques. Also discussed are numerous other research challenges and opportunities in the design and analysis of IBFD wireless systems.
---
paper_title: Enhanced List-based Group-wise overloaded receiver with application to satellite reception

paper_content:

The market trends towards the use of smaller dish antennas for TV satellite receivers, as well as the growing density of broadcasting satellites in orbit require the application of robust adjacent satellite interference (ASI) cancellation algorithms at the receivers. The wider beamwidth of a small size dish and the growing number of satellites in orbit impose an overloaded scenario, i.e., a scenario where the number of transmitting satellites exceeds the number of receiving antennas. For such a scenario, we present a two stage receiver to enhance signal detection from the satellite of interest, i.e., the satellite that the dish is pointing to, while reducing interference from neighboring satellites. Towards this objective, we propose an enhanced List-based Group-wise Search Detection (LGSD) receiver architecture that takes into account the spatially correlated additive noise and uses the signal-to-interference-plus-noise ratio (SINR) maximization criterion to improve detection performance. Simulations show that the proposed receiver structure enhances the performance of satellite systems in the presence of ASI when compared to existing methods.
---
paper_title: Heterogeneous Vehicular Networking: A Survey on Architecture, Challenges, and Solutions

paper_content:

With the rapid development of the Intelligent Transportation System (ITS), vehicular communication networks have been widely studied in recent years. Dedicated Short Range Communication (DSRC) can provide efficient real-time information exchange among vehicles without the need of pervasive roadside communication infrastructure. Although mobile cellular networks are capable of providing wide coverage for vehicular users, the requirements of services that require stringent real-time safety cannot always be guaranteed by cellular networks. Therefore, the Heterogeneous Vehicular NETwork (HetVNET), which integrates cellular networks with DSRC, is a potential solution for meeting the communication requirements of the ITS. Although there are a plethora of reported studies on either DSRC or cellular networks, joint research of these two areas is still at its infancy. This paper provides a comprehensive survey on recent wireless networks techniques applied to HetVNETs. Firstly, the requirements and use cases of safety and non-safety services are summarized and compared. Consequently, a HetVNET framework that utilizes a variety of wireless networking techniques is presented, followed by the descriptions of various applications for some typical scenarios. Building such HetVNETs requires a deep understanding of heterogeneity and its associated challenges. Thus, major challenges and solutions that are related to both the Medium Access Control (MAC) and network layers in HetVNETs are studied and discussed in detail. Finally, we outline open issues that help to identify new research directions in HetVNETs.
---
paper_title: Spectrum Sensing of OFDM Signals in the Presence of Carrier Frequency Offset

paper_content:

This paper addresses the important issue of detecting orthogonal frequency-division multiplexing (OFDM) signals in the presence of carrier frequency offset (CFO). The proposed algorithm utilizes the characteristics of the covariance matrix of the discrete Fourier transform of the input signal to the detector to determine the presence of the primary user's signal. This algorithm can be exploited to differentiate OFDM signals from the noise through the proposal of a new decision metric, which measures the off-diagonal elements of the input signal's covariance matrix. The decision threshold subject to a given probability of false alarm is derived, whereas performance analysis is carried out to demonstrate the potential of the proposed algorithm. Finally, simulation results are presented to validate the effectiveness of the proposed sensing method in comparison with other existing approaches.
---
paper_title: Digital Baseband IC Design of OFDM PHY for a 60GHz Proximity Communication System

paper_content:

This paper presents a digital baseband IC design based on OFDM PHY for a 60GHz proximity communication system. We propose a low computational complexity OFDM demodulator with a carrier frequency offset estimation method in polar coordinates suitable for high-speed parallel architecture. The proposed architecture is implemented in 65nm CMOS technology, and is experimentally verified to achieve the PHY data rate above 2.2Gbps. The digital baseband IC includes a complete functionality of OFDM transceiver with error correcting codecs and MAC.
---
paper_title: Indoor Millimeter Wave MIMO: Feasibility and Performance

paper_content:

In this paper, we investigate spatial multiplexing at millimeter (mm) wave carrier frequencies for short-range indoor applications by quantifying fundamental limits in line-of-sight (LOS) environments and then investigating performance in the presence of multipath and LOS blockage. Our contributions are summarized as follows. For linear arrays with constrained form factor, an asymptotic analysis based on the properties of prolate spheroidal wave functions shows that a sparse array producing a spatially uncorrelated channel matrix effectively provides the maximum number of spatial degrees of freedom in a LOS environment, although substantial beamforming gains can be obtained by using denser arrays. This motivates our proposed mm-wave MIMO architecture, which utilizes arrays of subarrays to provide both directivity and spatial multiplexing gains. System performance is evaluated in a simulated indoor environment using a ray-tracing model that incorporates multipath effects and potential LOS blockage. Eigenmode transmission with waterfilling power allocation serves as a performance benchmark, and is compared to the simpler scheme of beamsteering transmission with MMSE reception and a fixed signal constellation. Our numerical results provide insight into the spatial variations of attainable capacity within a room, and the combinations of beamsteering and spatial multiplexing used in different scenarios.
---

Title: Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last Five Years

Section 1: INTRODUCTION
Description 1: Write about the motivation, background, scope, and methodology of the survey, as well as the organization of the paper.

Section 2: SISO SYSTEMS
Description 2: Discuss the system model, synchronization challenges, literature review, and summary of timing and carrier synchronization in single-input single-output (SISO) systems.

Section 3: MULTI-ANTENNA SYSTEMS
Description 3: Explore the system model, synchronization challenges, literature review, and summary of timing and carrier synchronization in multi-antenna systems, including single-carrier and multi-carrier systems.

Section 4: COOPERATIVE RELAYING
Description 4: Review the system model, synchronization challenges, literature review, and summary of timing and carrier synchronization in cooperative relaying systems, distinguishing between decode-and-forward and amplify-and-forward modes.

Section 5: MULTICELL/MULTIUSER COMMUNICATION SYSTEMS
Description 5: Analyze the synchronization challenges, literature review, and summary in multiuser/multicell communication systems, including SC-FDMA, OFDMA uplink communication, CDMA communication, cognitive radio-based communication, distributed multiuser communication, CoMP-based communication, and multicell interference-based communication systems.

Section 6: UWB AND NON-CDMA BASED SPREAD SPECTRUM COMMUNICATION SYSTEMS
Description 6: Detail the system models, synchronization challenges, literature review, and summary for ultra-wideband (UWB) communication systems and non-CDMA based spread spectrum communication systems.

Section 7: FUTURE DIRECTIONS
Description 7: Discuss anticipated trends and promising directions for future research in synchronization for wireless communication systems, such as millimeter-wave and terahertz frequencies, massive MIMO, full-duplex communications, RF energy harvesting, vehicular communications, cognitive radio networks, and satellite systems.

Section 8: CONCLUSIONS
Description 8: Summarize the main findings of the survey, emphasize key contributions such as the classification tables, and outline future research directions.