A Survey on High Mobility Wireless Communications: Challenges, Opportunities and Solutions
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paper_title: TDD-based mobile communication solutions for high-speed railway scenarios

paper_content:

Mobile communication solutions for highspeed railway scenarios aim at providing seamless coverage, high system capacity and a high data rate for end users, which in turn face the challenges of Doppler frequency shift, penetration loss, mobility management and system capacity planning. Because the scarcity of frequency resources and increasing wireless broadband requirements have become very serious problems worldwide, increasing attention has been focused on time-division duplexing based technology, on which we focus in this article. Time-division synchronous code-division multiple access based and time-division Long Term Evolution based solutions are introduced with appropriate technology analysis. The experimental results of TD-SCDMA-based solutions in the Shanghai maglev high-speed network environment and the simulation results of TD-LTE-based solutions are presented. Finally, to improve performance, we discuss an enhanced solution based on mobile relays with performance evaluation, which is considered as a more comprehensive solution for high-speed railway scenarios.
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paper_title: Key techniques for 5G wireless communications: network architecture, physical layer, and MAC layer perspectives

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The fourth generation (4G) mobile communication systems are offering service worldwide steadily. Although 4G systems could be loaded with much more services and data than previous systems, there is still a dramatic gap between the people’s practical requirements and what can be offered by the 4G technologies. Consequently, the research and development for the fifth generation (5G) systems have already been started. This article presents an overview of potential network architecture and highlights several promising techniques which could be employed in the future 5G systems. These techniques include non-orthogonal multiple access (NOMA), massive multiple input and multiple output (MIMO), cooperative communications and network coding, full duplex (FD), device-to-device (D2D) communications, millimeter wave communications, automated network organization, cognitive radio (CR), and green communications. The state-of-art and implementation issue of these techniques are also addressed.
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paper_title: TDD-based mobile communication solutions for high-speed railway scenarios

paper_content:

Mobile communication solutions for highspeed railway scenarios aim at providing seamless coverage, high system capacity and a high data rate for end users, which in turn face the challenges of Doppler frequency shift, penetration loss, mobility management and system capacity planning. Because the scarcity of frequency resources and increasing wireless broadband requirements have become very serious problems worldwide, increasing attention has been focused on time-division duplexing based technology, on which we focus in this article. Time-division synchronous code-division multiple access based and time-division Long Term Evolution based solutions are introduced with appropriate technology analysis. The experimental results of TD-SCDMA-based solutions in the Shanghai maglev high-speed network environment and the simulation results of TD-LTE-based solutions are presented. Finally, to improve performance, we discuss an enhanced solution based on mobile relays with performance evaluation, which is considered as a more comprehensive solution for high-speed railway scenarios.
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paper_title: Finite state Markov modelling for high speed railway wireless communication channel

paper_content:

How to provide reliable, cost-effective wireless services for high-speed railway (HSR) users attracts increasing attention due to the fast deployment of HSRs worldwide. A key issue is to develop reasonably accurate and mathematically tractable models for HSR wireless communication channels. Finite-state Markov chains (FSMCs) have been extensively investigated to describe wireless channels. However, different from traditional wireless communication channels, HSR communication channels have the unique features such as very high speed, deterministic mobility pattern and frequent handoff events, which are not described by the existing FSMC models. In this paper, based on the Winner II physical layer channel model parameters, we propose a novel FSMC channel model for HSR communication systems, considering the path loss, fast fading and shadowing with high mobility. Extensive simulation results are given, which validate the accuracy of the proposed FSMC channel model. The model is not only ready for performance analysis, protocol design and optimization for HSR communication systems, but also provides an effective tool for faster HSR communication network simulation.
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paper_title: Basis Expansion Models and Diversity Techniques for Blind Identification and Equalization of Time-Varying Channels

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The time-varying impulse response of rapidly fading mobile communication channels is expanded over a basis of complex exponentials that arise due to Doppler effects encountered with multipath propagation. Blind methods are reviewed for estimating the bases' parameters and the model orders. Existing second-order methods are critiqued and novel algorithms are developed for blind identification, direct, zero-forcing equalization and minimum mean square error (MMSE) equalization by combining channel diversity with temporal (fractional sampling) and/or spatial diversity which becomes available with multiple receivers. Illustrative simulations are also presented.
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paper_title: New deterministic and stochastic simulation models for non-isotropic scattering mobile-to-mobile Rayleigh fading channels †

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For the practical simulation and performance evaluation of mobile-to-mobile (M2M) communication systems, it is desirable to develop accurate M2M channel simulation models for more realistic scenarios of non-isotropic scattering. In this paper, by using a ‘double-ring’ concept to describe M2M non-isotropic scattering environments, we propose new deterministic and stochastic sum-of-sinusoids (SoS) based simulation models. The proposed simulation models extensively consider the distributions of the angle of arrival (AoA) and the angle of departure (AoD), and thus provide a good approximation to the desired statistical properties of the reference model. Copyright © 2009 John Wiley & Sons, Ltd. ::: ::: (This paper was presented in part at VehiCom'09, co-located with IWCMC'09, Leipzig, Germany, 21–24 June 2009.)
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paper_title: A statistical theory of mobile-radio reception

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The statistical characteristics of the fields and signals in the reception of radio frequencies by a moving vehicle are deduced from a scattering propagation model. The model assumes that the field incident on the receiver antenna is composed of randomly phased azimuthal plane waves of arbitrary azimuth angles. Amplitude and phase distributions and spatial correlations of fields and signals are deduced, and a simple direct relationship is established between the signal amplitude spectrum and the product of the incident plane waves' angular distribution and the azimuthal antenna gain. The coherence of two mobile-radio signals of different frequencies is shown to depend on the statistical distribution of the relative time delays in the arrival of the component waves, and the coherent bandwidth is shown to be the inverse of the spread in time delays. Wherever possible theoretical predictions are compared with the experimental results. There is sufficient agreement to indicate the validity of the approach. Agreement improves if allowance is made for the nonstationary character of mobile-radio signals.
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paper_title: An Improved Parameter Computation Method for a MIMO V2V Rayleigh Fading Channel Simulator Under Non-Isotropic Scattering Environments

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For simulations of multiple-input multiple-output (MIMO) vehicle-to-vehicle (V2V) Rayleigh fading channels under more realistic non-isotropic scattering scenarios, we propose a new parameter computation method for the design of sum-of-sinusoids (SoS) simulation models. Compared with the existing relevant methods, the proposed method has the same simulation efficiency but provides a better approximation to the desired statistical properties of the theoretical reference model.
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paper_title: A generalized simulation model for Rayleigh fading channels with accurate second-order statistics

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A new sum-of-sinusoids statistical simulation model is proposed for Rayleigh fading channels. The new model employs random path gain, random initial phase, and conditional random Doppler frequency for all individual sinusoids. It is shown that the autocorrelations and cross-correlations of the quadrature components, the autocorrelation of the complex envelope of the new simulator match the desired ones exactly even if the number of sinusoids is as small as a single-digit integer Furthermore, the probability density functions of the fading envelope and phase, the level crossing rate and the average fade duration of the new simulator asymptotically approach the desired ones as the number of sinusoids approaches infinity, while good convergence is achieved when the number of sinusoids is small. Statistical properties of the new simulator are evaluated by numerical results, finding good agreement in all cases.
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paper_title: Simulation models with correct statistical properties for Rayleigh fading channels

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In this paper, new sum-of-sinusoids statistical simulation models are proposed for Rayleigh fading channels. These new models employ random path gain, random initial phase, and conditional random Doppler frequency for all individual sinusoids. It is shown that the autocorrelations and cross correlations of the quadrature components, and the autocorrelation of the complex envelope of the new simulators match the desired ones exactly, even if the number of sinusoids is as small as a single-digit integer. Moreover, the probability density functions of the envelope and phase, the level crossing rate, the average fade duration, and the autocorrelation of the squared fading envelope which contains fourth-order statistics of the new simulators, asymptotically approach the correct ones as the number of sinusoids approaches infinity, while good convergence is achieved even when the number of sinusoids is as small as eight. The new simulators can be directly used to generate multiple uncorrelated fading waveforms for frequency selective fading channels, multiple-input multiple-output channels, and diversity combining scenarios. Statistical properties of one of the new simulators are evaluated by numerical results, finding good agreements.
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paper_title: Karhunen-Loeve expansion of a fast Rayleigh fading process

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An approximate Karhunen-Loeve expansion is given for a random fading process characterised by an autocorrelation function Rḡ(τ) = J0(2πfdτ), J0(x) is the zeroth order Bessel function and fd is the process fading rate.
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paper_title: Time-variant channel estimation using discrete prolate spheroidal sequences

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We propose and analyze a low-complexity channel estimator for a multiuser multicarrier code division multiple access (MC-CDMA) downlink in a time-variant frequency-selective channel. MC-CDMA is based on orthogonal frequency division multiplexing (OFDM). The time-variant channel is estimated individually for every flat-fading subcarrier, assuming small intercarrier interference. The temporal variation of every subcarrier over the duration of a data block is upper bounded by the Doppler bandwidth determined by the maximum velocity of the users. Slepian showed that time-limited snapshots of bandlimited sequences span a low-dimensional subspace. This subspace is also spanned by discrete prolate spheroidal (DPS) sequences. We expand the time-variant subcarrier coefficients in terms of orthogonal DPS sequences we call Slepian basis expansion. This enables a time-variant channel description that avoids the frequency leakage effect of the Fourier basis expansion. The square bias of the Slepian basis expansion per subcarrier is three magnitudes smaller than the square bias of the Fourier basis expansion. We show simulation results for a fully loaded MC-CDMA downlink with classic linear minimum mean square error multiuser detection. The users are moving with 19.4 m/s. Using the Slepian basis expansion channel estimator and a pilot ratio of only 2%, we achieve a bit error rate performance as with perfect channel knowledge.
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paper_title: Iterative Time-Variant Channel Estimation for 802.11p Using Generalized Discrete Prolate Spheroidal Sequences

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This paper deals with channel estimation for orthogonal frequency-division multiplexing (OFDM) in time-variant wireless propagation channels. We particularly consider the challenges of the IEEE 802.11p standard, which is the worldwide dominant system for vehicular communications. For historic reasons, 802.11p uses a pilot pattern that is identical to the pattern used in 802.11a, which was initially designed for the estimation of indoor channels with little or no time variations. Therefore, this pilot pattern violates the sampling theorem for channels with both large delay spread and large Doppler spread, as often occurs in vehicular communications. To remedy this problem, we design a robust iterative channel estimator based on a 2-D subspace spanned by generalized discrete prolate spheroidal sequences. Due to the tight subspace design, the iterative receiver is able to converge to the same bit error rate (BER) as a receiver with perfect channel knowledge. Furthermore, we propose a backward compatible modification of the 802.11p pilot pattern such that the number of iterations sufficient for convergence can be reduced by a factor of 2-3, strongly reducing implementation complexity.
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paper_title: Kalman channel estimation based on oversampled polynomial model for OFDM over doubly-selective channels

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To take intercarrier interference (ICI) attributed to time variations of the channel into consideration, we propose to parameterize the doubly-selective channel by the oversampled polynomial model (OPM). The OPM outperforms the over-sampled basis expansion model (OBEM). On top of that, we also propose to incorporate the effective ICI suppressing, reduced-complexity Viterbi-type maximum likelihood (RML) equalizer into Kalman filter for recursive channel tracking and equalization. An enhancement in the channel tracking ability and a significant improvement in BER performance are validated by numerical simulations.
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paper_title: Joint data QR-detection and Kalman estimation for OFDM time-varying Rayleigh channel complex gains

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This paper deals with the case of a high speed mobile receiver operating in an orthogonal-frequency-division-multiplexing (OFDM) communication system. Assuming the knowledge of delay-related information, we propose an iterative algorithm for joint multi-path Rayleigh channel complex gains estimation and data recovery in fast fading environments. Each complex gain time-variation, within one OFDM symbol, is approximated by a polynomial representation. Based on the Jakes process, an auto-regressive (AR) model of the polynomial coefficients dynamics is built, making it possible to employ the Kalman filter estimator for the polynomial coefficients. Hence, the channel matrix is easily computed, and the data symbol is estimated with free inter-sub-carrier-interference (ICI) thanks to the use of a QR-decomposition of the channel matrix. Our claims are supported by theoretical analysis and simulation results, which are obtained considering Jakes' channels with high Doppler spreads.
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paper_title: Finite state Markov modelling for high speed railway wireless communication channel

paper_content:

How to provide reliable, cost-effective wireless services for high-speed railway (HSR) users attracts increasing attention due to the fast deployment of HSRs worldwide. A key issue is to develop reasonably accurate and mathematically tractable models for HSR wireless communication channels. Finite-state Markov chains (FSMCs) have been extensively investigated to describe wireless channels. However, different from traditional wireless communication channels, HSR communication channels have the unique features such as very high speed, deterministic mobility pattern and frequent handoff events, which are not described by the existing FSMC models. In this paper, based on the Winner II physical layer channel model parameters, we propose a novel FSMC channel model for HSR communication systems, considering the path loss, fast fading and shadowing with high mobility. Extensive simulation results are given, which validate the accuracy of the proposed FSMC channel model. The model is not only ready for performance analysis, protocol design and optimization for HSR communication systems, but also provides an effective tool for faster HSR communication network simulation.
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paper_title: Finite-State Markov Model for the Aeronautical Channel

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A Finite-State Markov Channel model is proposed in this paper to model the aeronautical channel. A class of aeronautical channel models is introduced, featuring parking and taxi environments, takeoff and landing situations, and en-route scenarios for ground-air and air-air links. Rayleigh and Ricean fading are fitted to these scenarios. The parameters of the model are obtained from theoretical analysis. The transition probabilities and the steady-state probabilities are calculated by SNR and LCR of channel model. Simulation and results indicate that Finite-State Markov Channel model sufficiently characterizes aeronautical channel behavior.
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paper_title: The dynamic evolution of multipath components in High-Speed Railway in viaduct scenarios: From the birth-death process point of view

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Based on the realistic channel measurement on High-Speed Railway (HSR) in viaduct scenarios at 2.35 GHz, the dynamic evolution of multipath components is investigated from the birth-death process point of view. Due to the distinction in the amount of resolvable multipath signals, the channel is divided into five segments and can be completely parameterized by several sets of statistical parameters associated with the type of environment and scenario. Then the four-state Markov chain, describing the birth-death number variation of the detected propagation waves, is employed to specialize the temporal stochastic properties. Furthermore, the steady probabilities and transition probabilities are provided which will facilitate the development and evaluation of wireless communication systems under HSR.
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paper_title: Vehicle-to-vehicle channel modeling and measurements: recent advances and future challenges

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Vehicle-to-vehicle communications have recently received much attention due to some new applications, such as wireless mobile ad hoc networks, relay-based cellular networks, and intelligent transportation systems for dedicated short range communications. The underlying V2V channels, as a foundation for the understanding and design of V2V communication systems, have not yet been sufficiently investigated. This article aims to review the state-of-the-art in V2V channel measurements and modeling. Some important V2V channel measurement campaigns and models are briefly described and classified. Finally, some challenges of V2V channel measurements and modeling are addressed for future studies.
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paper_title: Channel Measurements and Models for High-Speed Train Communication Systems: A Survey

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The recent development of high-speed trains (HSTs) as an emerging high mobility transportation system, and the growing demands of broadband services for HST users, introduce new challenges to wireless communication systems for HSTs. Accurate and efficient channel models considering both large-scale and non-stationary small-scale fading characteristics are crucial for the design, performance evaluation, and parameter optimization of HST wireless communication systems. However, the characteristics of the underlying HST channels have not yet been sufficiently investigated. This paper first provides a comprehensive review of the measurement campaigns conducted in different HST scenarios and then addresses the recent advances in HST channel models. Finally, key challenges of HST channel measurements and models are discussed and several research directions in this area are outlined.
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paper_title: A novel MIMO channel model for high speed railway system

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This paper focuses on the Geometry based Stochastic Channel Model (GSCM) and its application in high speed railway (HSR) multiple-input multiple-output (MIMO) systems. Different probability distribution functions (PDFs) of scatterers are studied and simulation results show that scatterers in Gaussian distribution make the best approximation to the realistic power delay profile (PDP) and power azimuth spectra (PAS). Additionally, the impacts of different scattering area shapes on the PDP and PAS in high speed railway scenario are simulated. Based on the conclusions above and existing measurement data, a novel channel model is established with local and far scatterer clusters for high speed railway scenario. The simulation results verify that our model is realistic.
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paper_title: Semi-Deterministic Path-Loss Modeling for Viaduct and Cutting Scenarios of High-Speed Railway

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Semi-deterministic modeling with low data resolution requirement and low computation time is always of interest. By conjunctively utilizing the extended Hata model and the Deygout model, this letter presents a hybrid model for viaduct and cutting scenarios of high-speed railway. The proposed model achieves higher accuracy than empirical and statistical models, but uses totally free sources. It can be easily implemented for the network planning, and therefore, it meets the demand for fast development of high-speed railway.
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paper_title: Channel modeling based on random propagation graphs for high speed railway scenarios

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In this paper, a channel modeling method based on random-propagation-graph is elaborated, validated, and applied to characterizing time-variant channels observed in typical environments for high-speed railway wireless communications. The advantage of the proposed method is that the frequency-tempo-spatial channel coefficients, as well as the multi-dimensional channel impulse responses in delay, Doppler frequency, direction of arrival (i.e. azimuth and elevation of arrival) and direction of departure are calculated analytically for specific environments. The validation of the proposed method is performed by comparing the statistics of two large-scale parameters obtained with those described in the well-established standards. Finally, stochastic geometry-based models in the same format as the well-known spatial channel model enhanced (SCME) are generated by using the proposed method for the high-speed scenarios in the rural, urban, and suburban environments.
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paper_title: A Nonstationary Wideband MIMO Channel Model for High-Mobility Intelligent Transportation Systems

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The recent development of high-speed trains (HSTs), as a high-mobility intelligent transportation system, and the growing demands of broad-band services for HST users, introduce new challenges to wireless communication systems for HSTs. The deployment of mobile relay stations on top of the train carriages is one of the promising solutions for HST wireless systems. For a proper design and evaluation of HST wireless communication systems, we need accurate channel models that can mimic the underlying channel characteristics for different HST scenarios. In this paper, a novel nonstationary geometry-based stochastic model (GBSM) is proposed for wideband multiple-input multiple-output HST channels in rural macrocell scenarios. The corresponding simulation model is then developed with angle parameters calculated by the modified method of equal areas. Both channel models can also be used to model nonstationary vehicle-to-infrastructure channels in vehicular communication networks. The system functions and statistical properties of the proposed channel models are investigated based on a theoretical framework that describes nonstationary channels. Numerical and simulation results demonstrate that the proposed channel models have the capability to characterize the nonstationarity of HST channels. The statistical properties of the simulation model, verified by the simulation results, can match those of the proposed theoretical GBSM. An excellent agreement is achieved between the stationary intervals of the proposed simulation model and those of relevant measurement data, demonstrating the utility of the proposed channel models.
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paper_title: Channel Analysis for an OFDM-MISO Train Communications System Using Different Antennas

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For the design of an OFDM train communications system it is essential to characterise and consider the channel parameters. The transmission channel of a high-speed train scenario is frequency selective as well as time variant. Thus, delay spread and Doppler spread are investigated as crucial parameters for the OFDM system performance. Using a ray-tracing tool realistic impulse responses of the transmission channels are simulated. The investigated system includes three base stations operating in common frequency mode along a railway track and one mobile station situated on a high-speed train. For the mobile station different antenna patterns are included in the simulation model. The results are compared and assessed with respect to delay spread, Doppler spread and receive power. When using directional antennas a distinct reduction in Doppler spread is achieved.
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paper_title: Ray optical modeling of wireless communications in high-speed railway tunnels

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A new deterministic approach for wave propagation modeling in high-speed train tunnels is presented. The model is based on a new ray launching method and results in the polarimetric and complex channel impulse response as well as the Doppler diagram for radio links between on-train stations and tunnel-fixed stations. Different channel simulations under certain propagation conditions are presented.
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paper_title: Geometry-Based Stochastic Modeling for MIMO Channel in High-Speed Mobile Scenario

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The geometry-based stochastic channel models are proposed in this paper for the terrain cutting, suburb, and urban scenarios in high-speed railway. First, the special scenarios in high-speed railway are described. And the channel models based on the geometry scenarios are introduced. Some channel parameters are based on measurement data. Then, the space-time correlation functions in analytical form are obtained in suburb and urban scenarios. Finally, the space correlation characteristics in three scenarios are compared.
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paper_title: Empirical Geometry-Based Random-Cluster Model for High-Speed-Train Channels in UMTS Networks

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In this paper, a recently conducted measurement campaign for high-speed-train (HST) channels is introduced, where the downlink signals of an in-service Universal Mobile Terrestrial System (UMTS) deployed along an HST railway between Beijing and Shanghai were acquired. The channel impulse responses (CIRs) are extracted from the data received in the common pilot channels (CPICHs). Within 1318 km, 144 base stations (BSs) were detected. Multipath components (MPCs) estimated from the CIRs are clustered and associated across the time slots. The results show that, limited by the sounding bandwidth of 3.84 MHz, most of the channels contain a single line-of-sight (LoS) cluster, and the rest consists of several LoS clusters due to distributed antennas, leaking cable, or neighboring BSs sharing the same CPICH. A new geometry-based random-cluster model is established for the clusters' behavior in delay and Doppler domains. Different from conventional models, the time-evolving behaviors of clusters are characterized by random geometrical parameters, i.e., the relative position of BS to railway, and the train speed. The distributions of these parameters, and the per-cluster path loss, shadowing, delay, and Doppler spreads, are extracted from the measurement data.
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paper_title: An adaptive geometry-based stochastic model for non-isotropic MIMO mobile-to-mobile channels

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In this paper, a generic and adaptive geometrybased stochastic model (GBSM) is proposed for non-isotropic multiple-input multiple-output (MIMO) mobile-to-mobile (M2M) Ricean fading channels. The proposed model employs a combined two-ring model and ellipse model, where the received signal is constructed as a sum of the line-of-sight, single-, and doublebounced rays with different energies. This makes the model sufficiently generic and adaptable to a variety of M2M scenarios (macro-, micro-, and pico-cells). More importantly, our model is the first GBSM that has the ability to study the impact of the vehicular traffic density on channel characteristics. From the proposed model, the space-time-frequency correlation function and the corresponding space-Doppler-frequency power spectral density (PSD) of any two sub-channels are derived for a non-isotropic scattering environment. Based on the detailed investigation of correlations and PSDs, some interesting observations and useful conclusions are obtained. These observations and conclusions can be considered as a guidance for setting important parameters of our model appropriately and building up more purposeful measurement campaigns in the future. Finally, close agreement is achieved between the theoretical results and measured data, demonstrating the utility of the proposed model.
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paper_title: Three-dimensional modeling and simulation of wideband MIMO mobile-to-mobile channels

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A three-dimensional (3-D) geometrical propagation model for wideband multiple-input multiple-output (MIMO) mobile-to-mobile (M-to-M) communications is proposed. Based on the geometrical model, a 3-D parametric reference model for wideband MIMO M-to-M multipath fading channels is developed. From the reference model, the corresponding space-time-frequency correlation function is derived for a 3-D non-isotropic scattering environment. It is shown that the time and frequency dispersion of a wide sense stationary uncorrelated scattering channel cannot be treated independently, contrary to common practice. From the space-time-frequency correlation function, the space-Doppler power spectral density and the power space-delay spectrum are derived and compared with measured data. Finally, a new sum-of-sinusoids based simulation model for wideband MIMO M-to-M Ricean fading channels is proposed. The statistics of the simulation model are verified by simulation. The results show that the simulation model is a good approximation of the reference model.
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paper_title: 3D Polarized Channel Modeling and Performance Comparison of MIMO Antenna Configurations With Different Polarizations

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We propose a three-dimensional (3D) polarized MIMO channel model, which takes into account 3D power angular spectrum and comprehensive propagation characteristics of electromagnetic waves excited by polarized antennas. Based on the model, we derive a close form expression of the spatial correlation as a function of the physical parameters representing both characteristics of arbitrary antennas and propagation environment in 3D space. The spatial correlation expression allows to use the Von Mises Fisher (VMF) distribution, resulting in a more accurate and general channel model. Through simulation, we evaluate and compare performance, in terms of the spatial correlation and capacity, of 2 × 2 MIMO configurations with different polarizations, i.e., V/V, V/H, and slanted ±45° polarizations, as a function of critical input parameters including elevation angle, antenna orientation, antenna spacing, cross-polarization discrimination (XPD), and signal-to-noise ratio (SNR). The effect of the parameters on the performance is analyzed, and verified in certain cases through the literature.
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paper_title: A Geometry-Based Stochastic Model for Wideband MIMO Mobile-to-Mobile Channels

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In this paper, based on the tapped delay line (TDL) structure, we propose a geometry-based stochastic model (GBSM) for wideband multiple-input multiple-output (MIMO) mobile-to-mobile (M2M) Ricean fading channels. The proposed wideband model is the first GBSM that has the ability to study the impact of the vehicular traffic density (VTD) on channel statistics for different time delays, i.e., for every tap in our model. From the proposed model, the space-time (ST) correlation function (CF) and the corresponding space-Doppler (SD) power spectral density (PSD) are derived. Excellent agreement is achieved between the theoretical Doppler PSDs and measured data, demonstrating the utility of the proposed model.
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paper_title: Wideband Channel Modeling and Intercarrier Interference Cancellation for Vehicle-to-Vehicle Communication Systems

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In this paper, we propose a new regular-shaped geometry-based stochastic model (RS-GBSM) for non-isotropic scattering wideband multiple-input multiple-output vehicle-to-vehicle (V2V) Ricean fading channels. By correcting the unrealistic assumption widely used in current RS-GBSMs, the proposed model can more practically study the impact of the vehicular traffic density on channel statistics for different time delays. From the proposed model, we derive the Doppler power spectral density (PSD) and find that highly dynamic Doppler spectrum appears for V2V channels. Excellent agreement is achieved between the derived Doppler PSD and measured data, demonstrating the utility of the proposed model. To combat the intercarrier interference (ICI) caused by highly dynamic Doppler spectrum in real orthogonal frequency division multiplexing based V2V systems, this paper proposes a new type of ICI cancellation scheme, named as precoding based cancellation (PBC) scheme. The proposed scheme can be easily implemented into real V2V systems with the same ICI mitigation performance as the current best ICI cancellation scheme that has high complexity. To further improve the performance of the proposed PBC scheme, a new phase rotation aided (PRA) method, namely constant PRA (CPRA) method, is proposed. Compared with the existing PRA method, the CPRA method has better performance and much less implementation complexity. Therefore, the proposed PBC scheme with the CPRA method is the best ICI cancellation scheme for real V2V systems.
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paper_title: Space-Time Correlated Mobile-to-Mobile Channels: Modelling and Simulation

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A single- and double-bounced two-ring parametric reference model is proposed for multiple-input multiple-output (MIMO) mobile-to-mobile (M-to-M) Ricean fading channels. From this model, a closed-form joint space-time correlation function and a space-Doppler power spectrum are derived for a two-dimensional (2D) non-isotropic scattering environment. Also, space-time correlation functions for the in-phase and quadrature components of the complex faded envelope are derived, assuming a 2D isotropic scattering environment. Finally, two new sum-of-sinusoids based simulation models for MIMO M-to-M Ricean fading channels are proposed. The statistics of the simulation models are verified by simulation. The results show that the simulation models are a good approximation of the reference model and that they outperform existing simulation models.
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paper_title: A statistical model of mobile-to-mobile land communication channel

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A statistical model applicable to a local area has been developed for mobile-to-mobile urban and suburban land communication channels. Using the model, the probability distributions of the received envelope and phase, spatial-time correlation function, and the power spectral density of the complex envelope have been developed.
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paper_title: Novel 3D Geometry-Based Stochastic Models for Non-Isotropic MIMO Vehicle-to-Vehicle Channels

paper_content:

This paper proposes a novel three-dimensional (3D) theoretical regular-shaped geometry-based stochastic model (RS-GBSM) and the corresponding sum-of-sinusoids (SoS) simulation model for non-isotropic multiple-input multiple-output (MIMO) vehicle-to-vehicle (V2V) Ricean fading channels. The proposed RS-GBSM, combining line-of-sight (LoS) components, a two-sphere model, and an elliptic-cylinder model, has the ability to study the impact of the vehicular traffic density (VTD) on channel statistics, and jointly considers the azimuth and elevation angles by using the von Mises Fisher distribution. Moreover, a novel parameter computation method is proposed for jointly calculating the azimuth and elevation angles in the SoS channel simulator. Based on the proposed 3D theoretical RS-GBSM and its SoS simulation model, statistical properties are derived and thoroughly investigated. The impact of the elevation angle in the 3D model on key statistical properties is investigated by comparing with those of the corresponding two-dimensional (2D) model. It is demonstrated that the 3D model is more accurate to characterize real V2V channels, in particular for pico cell scenarios. Finally, close agreement is achieved between the theoretical model, SoS simulation model, and simulation results, demonstrating the utility of the proposed models.
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paper_title: Simulation Models for MIMO Mobile-to-Mobile Channels

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Two new sum-of-sinusoids (SoS) based simulation models for multi-input multi-output (MIMO) mobile-to-mobile (M-to-M) Rayleigh fading channels are proposed. First, an ergodic statistical (deterministic) SoS simulation model is proposed, and its statistical properties are derived and verified by simulations. The deterministic model is shown to well approximate the reference model and, compared to existing simulation models, requires smaller number of scatterers. By allowing all three parameter sets (amplitudes, phases, and Doppler frequencies) to be random variables, the deterministic model is modified to yield a statistical simulation model. Compared to the deterministic and existing simulation models, the statistical model better matches the reference model over a wider range of normalized time delays while using a smaller number of scatterers.
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paper_title: Envelope Level Crossing Rate and Average Fade Duration of Nonisotropic Vehicle-to-Vehicle Ricean Fading Channels

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This paper proposes a generic geometry-based stochastic model for nonisotropic scattering vehicle-to-vehicle (V2V) Ricean fading channels. With the proposed model, the level crossing rate (LCR) and average fade duration (AFD) are derived. The resultant expressions are sufficiently general and subsume many well-known existing LCRs and AFDs as special cases. The derived LCR and AFD are further investigated in terms of some important parameters, e.g., the shape of the scattering region (two-ring or ellipse), mean angle, angle spread, and directions of movement of the Tx and Rx (same or opposite direction). More importantly, in this paper, the impact of the vehicular traffic density on the LCR and AFD for nonisotropic scattering V2V Ricean fading channels is investigated for the first time. Excellent agreement is observed between the theoretical LCRs/AFDs and corresponding measured data, thus demonstrating the validity and utility of the proposed model.
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paper_title: Air-ground channels & models: Comprehensive review and considerations for unmanned aircraft systems

paper_content:

Use of unmanned aircraft systems (UASs) for multiple applications is expected to grow dramatically in the coming decades; this fact has motivated this paper's focus on fundamental physical layer characteristics relevant to UAS communications. In the past, for aeronautical communications with high transmitted power levels, narrow signal bandwidths, elevated ground site antennas in open areas, and low duty cycle transmissions, simple models for channel attenuation sufficed. In the future, when UAS ground stations may not all be in cleared areas with elevated antennas, higher data rates (wider bandwidths) are required, and small UASs with stringent power limitations still require high reliability, more comprehensive air-ground (AG) channel characteristics will be required in order to ensure robust signal designs for high-reliability AG links. We have found that no accurate, validated wideband models exist for the AG channel, particularly not in the Land C-bands that are being proposed for UASs. Airframe shadowing models also do not yet exist. We thus provide a comprehensive review of past work on the AG channel, and follow this with a brief description of plans for an AG channel measurement and modeling campaign. Resulting AG channel models will subsequently be used in the evaluation of candidate air interfaces for UAS control and non-payload communications (CNPC). The air interface must operate in the presence of both delay and Doppler spreads, and shadowing. It should also be spectrally efficient, low-latency, and reasonably robust to interference. We discuss these AG air interface considerations, and also show some initial modeling results based on both analysis and measurements.
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paper_title: Real-time aeronautical channel simulator

paper_content:

Air-to-Ground (A/G) channel model is quite important for verification of the key technologies in the A/G communications, especially the schemes in physical layers, such as the modulation and coding etc. The aeronautical A/G channel has three important features:1) Due to the high speed of the aircraft, the Doppler spread will be very serious; 2) In some scenarios, the radio wave will reflet many times and the multiple signals are abundant, such as a mountain valley; 3) As the transmission scenario is very sophisticated, there is not fispc eccihannel model for A/G communications for the spfiecci location; 4) The frequency band that can be used is discrete; 5) Some electromagnetic interferences are quite strong, Establishing statistical model needs large experimental data of channel measurement, which is really costly and time-exhausted. In this letter, a method of real-time A/G aeronautical communications channel modeling is proposed with ray-tracing algorithm. This model can directly produce the real-time simulation result of the channel with computer. Moreover, the result can be further employed to build up the statistical model, which is really economic compared with the traditional experimental method.
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paper_title: A geometric air-to-ground radio channel model

paper_content:

Presents a geometric channel model for simulation and analysis of air-to-ground radio communications. The model represents an airborne station communicating with a ground station in a cluttered environment (e.g., ground-based, mobile transceiver in an urban or village region). The model call be used to produce multipath component delay, strength, and direction of arrival (DOA) information, and it is therefore well suited for simulating vector channels for smart antenna arrays and antenna diversity systems. This air-to-ground channel model is based on a three-dimensional ellipsoidal geometry and uses principles similar to those used by the geometrically based single-bounce elliptical (GBSBE) and geometrically based single-bounce circular (GBSBC) models. However, instead of assuming two ground-based stations, this geometric air-to,ground model uses altitude and range information. The result is the ability to simulate multipath air-to-ground channels with a geometry more appropriate for the physical configuration of the stations.
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paper_title: Iterative Detection for Channels With Memory

paper_content:

In this paper, we present an overview on the design of algorithms for iterative detection over channels with memory. The starting point for all the algorithms is the implementation of soft-input soft-ouput maximum a posteriori (MAP) symbol detection strategies for transmissions over channels encompassing unknown parameters, either stochastic or deterministic. The proposed solutions represent effective ways to reach this goal. The described algorithms are grouped into three categories: i) we first introduce algorithms for adaptive iterative detection, where the unknown channel parameters are explicitly estimated; ii) then, we consider finite-memory iterative detection algorithms, based on ad hoc truncation of the channel memory and often interpretable as based on an implicit estimation of the channel parameters; and iii) finally, we present a general detection-theoretic approach to derive optimal detection algorithms with polynomial complexity. A few illustrative numerical results are also presented.
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paper_title: Semi-Blind Data Detection for Unitary Space-Time Modulation in MIMO Communications Systems

paper_content:

We propose an iterative algorithm based on expectation maximization (EM) to jointly estimate the system parameters and decode the data in a MIMO system using unitary space-time block codes. It is assumed that the receiver is unaware of the channel coefficients and their distribution, the average energy of the received signal, the prior probability of each signal in the constellation, and the noise power. The algorithm works for arbitrary modulation schemes and any channel model including Rayleigh and Rician fading. The complexity of the proposed receiver is computed and it is shown to be significantly lower than previously published methods, as it does not require any matrix inversion or trellis search. The performance of the proposed receiver is evaluated by simulations in terms of symbol error rate (SER) vs. signal-to-noise ratio (SNR) and it is shown that with only a few iterations of the algorithm it achieves a performance close to that of the receiver which knows all the parameters.
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paper_title: Adaptive Bayesian and EM-Based Detectors for Frequency-Selective Fading Channels

paper_content:

The problems of adaptive maximum a posteriori (MAP) symbol detection for uncoded transmission and of adaptive soft-input soft-output (SISO) demodulation for coded transmission of data symbols over time-varying frequency-selective channels are explored within the framework of the expectation-maximization (EM) algorithm. In particular, several recursive forms of the classical Baum-Welch (BW) algorithm and its Bayesian counterpart (often referred to a Bayesian EM algorithm) are derived in an unified way. In contrast to earlier developments of the BW and BEM algorithms, these formulations lead to computationally attractive algorithms which avoid matrix inversions while using sequential processing over the time and trellis branch indices. Moreover, it is shown how these recursive versions of the BW and BEM algorithms can be integrated with the well-known forward-backward processing SISO algorithms resulting in adaptive SISOs with embedded soft decision directed (SDD) channel estimators. An application of the proposed algorithms to iterative "turbo-processing" receivers illustrates how these SDD channel estimators can efficiently exploit the extrinsic information obtained as feedback from the SISO decoder in order to enhance their estimation accuracy.
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paper_title: Optimal Joint Detection/Estimation in Fading Channels With Polynomial Complexity

paper_content:

The problem of sequence detection in frequency-nonselective/time-selective fading channels, when channel state information (CSI) is not available at the transmitter and receiver, is considered in this paper. The traditional belief is that exact maximum-likelihood sequence detection (MLSD) of an uncoded sequence over this channel has exponential complexity in the channel coherence time. Thus, for slowly varying channels, i.e., channels having coherence time on the order of the sequence length, the complexity appears to be exponential in the sequence length. In the first part of this work, it is shown that exact MLSD can be computed with only polynomial worst case complexity in the sequence length regardless of the operating signal-to-noise ratio (SNR) for equal-energy signal constellations. By establishing a relationship between the aforementioned complexity and the rank of the correlation matrix of the fading process, an understanding of how complexity of the optimal MLSD receiver varies as the channel dynamics change is provided. In the second part of this paper, the problem of decoding turbo-like codes in frequency-nonselective/time-selective fading channels without receiver CSI is examined. Using arguments similar to the ones used for the MLSD case, it is shown that the exact symbol-by-symbol soft-decision metrics (SbSSDMs) implied by the min-sum algorithm can be evaluated with polynomial worst case complexity in the sequence length regardless of SNR for equal-energy signal constellations. Finally, by simplifying some key steps in the polynomial-complexity algorithm, a family of fast, approximate algorithms is derived, which yield near-optimal performance
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paper_title: Semi-Blind Channel Estimation Using the EM Algorithm in Iterative MIMO APP Detectors

paper_content:

We consider channel estimation in multiple-input multiple-output (MIMO) systems using iterative detection at the receiver. Space-time bit-interleaved coded modulation (BICM) and soft-input soft-output maximum a posteriori (MAP) symbol detection and decoding are considered. Channel coefficients are updated at each iteration of the detector using a semi-blind estimation approach based on the expectation maximization (EM) algorithm. We first show that a "classical" and non-optimized EM implementation, as already proposed in some previous works, gives a biased estimate of the channel coefficients. We then try to optimize the EM implementation and propose a modification to it that provides an unbiased channel estimate and leads to a better convergence of the iterative detector. We show that considerable improvement in the receiver performance can be obtained by using our proposed modified unbiased (MU) EM algorithm, especially for large number of transmit antennas and short training sequences. We also show that when MIMO signal detection is strongly asymmetric in the sense of too few receive antennas, the EM-based channel estimation may be of little interest. Moreover, we consider a simple semi-blind estimation scheme, based on hard decisions on reliable decoded data bits, and compare its performance with the EM based estimation methods
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paper_title: Adaptive Channel Estimation for OFDM Systems with Doppler spread

paper_content:

In this paper, we propose adaptive channel estimation for Orthogonal Frequency Division Multiplexing (OFDM) in fast time-varying (TV) channels. A Basis Expansion Model (BEM) approach is used to capture the time variation of the channel within each OFDM block, and to reduce the estimator dimensionality. Capitalizing on the BEM structure and on a frequency domain training, two adaptive approaches are proposed, based on Kalman filtering and Recursive Least Squares (LS) methods, which exploit the time correlation of the channel between successive blocks and do not require any a-priori knowledge of the channel statistics. Simulation results show that, compared to classical Least Squares and statistically-aided Linear Minimum Mean Squared Error (LMMSE) approaches, the two proposed techniques effectively estimate the channel, adapt fast to its non stationary changes, thus enabling efficient TV channel equalization of the inter-carrier interference (ICI) induced in OFDM systems by high Doppler spreads.
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paper_title: Modified clustered comb pilot-aided fast time-varying channel estimation for OFDM system

paper_content:

As high-speed railway is booming worldwide, the communication system with fast-time varying channel has drawn great attention. The comb pilot based linear minimum mean square error (LMMSE) channel estimator is proved to be an effective method for fast time-varying channel estimation. In this paper, the clustered comb pilot-aided channel estimation for orthogonal frequency-division multiplexing (OFDM) system is discussed, where the time varying channel is approximated by a basis expansion model (BEM). A modified clustered comb pilot structure is proposed and justified to improve the estimation performance compared with the clustered comb pilot proposed by Tang. Based on the complex-exponential BEM (CE-BEM) model, a suboptimal-pilot structure is proposed. In addition, optimal pilot length is analyzed and simulated with a predefined total number of pilots. The simulation results show that the modified clustered comb pilot can greatly reduce the estimation error especially with high Doppler spread. The suboptimal-pilot structure with guard pilot approximation is proven to be competitive. Optimal nonzero pilot lengths for different Doppler spread are obtained by simulation with a predefined channel order and fixed pilot subcarriers.
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paper_title: Subspace-based blind channel estimation for OFDM by exploiting virtual carriers

paper_content:

Reliable channel estimation is indispensable for orthogonal frequency-division multiplexing (OFDM) systems employing coherent detection and adaptive loading in order to achieve high data rate communications. Several options exist in practical OFDM systems-including training symbols, cyclic prefix, virtual carriers, pilot tones, and receiver diversity-to facilitate channel estimation. In this paper, a subspace blind channel estimation method based on exploiting the presence of virtual carriers is proposed for OFDM systems over a time-dispersive channel. The method can be applied to conventional OFDM systems with cyclic prefix as well as OFDM systems with no cyclic prefix. The reduction/elimination of cyclic prefix thereby provides the OFDM systems the potential to achieve higher channel utilization than most previously reported cyclic prefix based estimators. Sufficient channel identifiability condition is developed as well. Comparison with two other recently reported subspace methods is presented via computer simulations to support the effectiveness of the proposed method.
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paper_title: Subspace-based blind and semi-blind channel estimation for OFDM systems

paper_content:

This paper proposes a new blind channel estimation method for orthogonal frequency division multiplexing (OFDM) systems. The algorithm makes use of the redundancy introduced by the cyclic prefix to identify the channel based on a subspace approach. Thus, the proposed method does not require any modification of the transmitter and applies to most existing OFDM systems. Semi-blind procedures taking advantage of training data are also proposed. These can be training symbols or pilot tones, the latter being used for solving the intrinsic indetermination of blind channel estimation. Identifiability results are provided, showing that in the (theoretical) situation where channel zeros are located on subcarriers, the algorithm does not ensure uniqueness of the channel estimation, unless the full noise subspace is considered. Simulations comparing the proposed method with a decision-directed channel estimator finally illustrates the performance of the proposed algorithm.
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paper_title: Linear prediction error method for blind identification of periodically time-varying channels

paper_content:

Blind channel estimation for single-input multiple-output (SIMO) periodically time-varying channels is considered using only the second-order statistics of the data. The time-varying channel is assumed to be described by a complex exponential basis expansion model (CE-BEM). The linear prediction error method for blind identification of time-invariant channels is extended to time-varying channels represented by a CE-BEM. Sufficient conditions for identifiability are investigated. The cyclostationary nature of the received signal is exploited to consistently estimate the time-varying correlation function of the data from a single observation record. The proposed method requires the knowledge of the active basis functions but not the channel length (an upper bound suffices). Several existing methods require precise knowledge of the channel length. Equalization of the time-varying channel, given the estimated channel, is investigated. Computer simulation examples are presented to illustrate the approach and to compare it with two existing approaches.
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paper_title: Data-efficient blind OFDM channel estimation using receiver diversity

paper_content:

We investigate non data-aided channel estimation for cyclically prefixed orthogonal frequency division multiplexing (OFDM) systems. By exploiting channel diversity using only two receive antennas, a blind deterministic algorithm is proposed. Identifiability conditions are derived that guarantee the perfect channel retrieval in the absence of noise. In the presence of noise, the proposed method has the desired property of being data efficient-only a single OFDM block is needed to achieve good estimation performance for a wide range of SNR values. The algorithm is also robust to input symbols as it does not have any restriction on the input symbols with regard to their constellation or their statistical properties. In addition, this diversity-based algorithm is computationally efficient, and its performance compares favorably to most existing blind algorithms.
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paper_title: Maximizing Spectral Efficiency for High Mobility Systems with Imperfect Channel State Information

paper_content:

This paper studies the optimum system design that can maximize the spectral efficiency of high mobility wireless communication systems with imperfect channel state information (CSI). The fast time-varying fading in high mobility systems can be tracked with pilot-assisted channel estimation. The percentage of pilot symbols in the transmitted symbols plays a critical role on the system performance: a higher pilot percentage yields a more accurate channel estimation, but also more overhead. The effects of pilot percentage are quantified through the derivation of the channel estimation mean squared error (MSE), which is expressed as a closed-form expression of various system parameters through asymptotic analysis. It is discovered that, if the pilots sample the channel above its Nyquist rate, then the estimation of the channel coefficients of data symbols through temporal interpolation yields the same asymptotic MSE as the direct estimation of the channel coefficients of the pilot symbols. Based on the statistical properties of the channel estimation error, we quantify the impacts of the imperfect CSI on the system performance by developing the analytical symbol error rate (SER) and a spectral efficiency lower bound of the communication system. The optimum pilot percentage that can maximize the spectral efficiency lower bound is identified through both analytical and simulation results.
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paper_title: Optimum designs of high mobility wireless systems with channel estimation errors

paper_content:

This research studies the optimum system designs of high mobility wireless communication systems with imperfect channel state information. In high mobility systems, the channel estimation error is usually inevitable and might have significant impacts on system performance. The impacts of channel estimation errors on system performance are quantified through the development of a spectral efficiency lower bound. With the help of the asymptotic analysis of the channel estimation mean squared error, the spectral efficiency lower bound is expressed as explicit functions of a number of system parameters, such as the maximum Doppler frequency, the percentage of pilots in the transmitted symbols, and the ratio of energy allocated between data symbols and pilot symbols. The optimum pilot percentage and energy allocation factor that jointly maximise the spectral efficiency lower bound are identified through analytical studies. It is discovered that, in term of maximising spectral efficiency, the optimum performance can be achieved by optimising the energy allocation factor when the pilots sample the fast time-varying fading at its Nyquist rate.
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paper_title: Pilot-assisted wireless transmissions: general model, design criteria, and signal processing

paper_content:

Pilot-assisted transmission (PAT) multiplexes the known symbols with the information bearing data. These pilot symbols and the specific multiplexing scheme are known at the receiver and can be exploited for channel estimation, receiver adaptation, and optimal decoding. Even though PAT has been used for many practical reasons, there still remains a need for an optimal design. The theory and methodology for the design of an optimal PAT have emerged, but much still remains unknown. In this article, the author has presented an overview of PAT. A general PAT model is given as well as the review of the common design criteria. The information theoretic and signal processing issues were also discussed.
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paper_title: Minimum Error Transmissions with Imperfect Channel Information in High Mobility Systems

paper_content:

One of the main challenges faced by communications in high mobility systems, such as unmanned aerial vehicle (UAV), is the fast time-varying fading. This paper studies the optimum system design that can minimize the error probability of a high mobility wireless communication system with imperfect channel state information (CSI). We consider a system that employs pilot-assisted minimum mean square error (MMSE) channel estimation. The channel estimation error is usually non-negligible in high mobility systems and might have significant impacts on system performance. With asymptotic analysis, the channel estimation mean square error (MSE) is expressed as a closed-form expression of the percentage of pilot symbols in the transmitted symbols. An optimum receiver for systems with imperfect CSI and the corresponding symbol error rate (SER) are then derived by analyzing the statistical properties of the channel estimation error. The SER is expressed as an explicit function of a number of parameters, such as pilot percentage, maximum Doppler spread, and the signal-to-noise ratio (SNR), etc. The optimum pilot percentage that can minimize the SER is analytically identified, and the impacts of the imperfects CSI on the system performance are studied through both theoretical analysis and simulations.
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paper_title: Maximizing Spectral Efficiency for High Mobility Systems with Imperfect Channel State Information

paper_content:

This paper studies the optimum system design that can maximize the spectral efficiency of high mobility wireless communication systems with imperfect channel state information (CSI). The fast time-varying fading in high mobility systems can be tracked with pilot-assisted channel estimation. The percentage of pilot symbols in the transmitted symbols plays a critical role on the system performance: a higher pilot percentage yields a more accurate channel estimation, but also more overhead. The effects of pilot percentage are quantified through the derivation of the channel estimation mean squared error (MSE), which is expressed as a closed-form expression of various system parameters through asymptotic analysis. It is discovered that, if the pilots sample the channel above its Nyquist rate, then the estimation of the channel coefficients of data symbols through temporal interpolation yields the same asymptotic MSE as the direct estimation of the channel coefficients of the pilot symbols. Based on the statistical properties of the channel estimation error, we quantify the impacts of the imperfect CSI on the system performance by developing the analytical symbol error rate (SER) and a spectral efficiency lower bound of the communication system. The optimum pilot percentage that can maximize the spectral efficiency lower bound is identified through both analytical and simulation results.
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paper_title: Optimal design and placement of pilot symbols for channel estimation

paper_content:

The problem of designing and placing pilot symbols for the estimation of frequency-selective random channels is considered. The channel is assumed to be a block-fading model with finite impulse response (FIR). For both single-input single-output (SISO) and multiple-input multiple-output (MIMO) channels, under the assumption of independent and identical distributed channel taps, the Cramer-Rao bound (CRB) on the mean square error (MSE) of semi-blind channel estimators is derived and minimized with respect to pilot symbols and their placement. It is shown that the optimal strategy is to place pilot symbols satisfying certain orthogonality condition in such a way that data and pilot symbols with higher power are in the middle of the packet. The results also indicate that the optimal pilot placements are independent of channel probability distribution. For constant modulus symbols, we show that the quasi-periodic placement and its generalization in the multiuser case turn out to be optimal. We further consider estimating channels with correlated taps and show that the previous placement strategy is also optimal among orthogonal pilot sequences.
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paper_title: Optimum designs for high mobility systems with channel estimation errors

paper_content:

This paper studies the optimum system designs of high mobility wireless communication systems with channel estimation errors. In high mobility systems, the accurate estimation and tracking of the fast time-varying fading channel is nontrivial, and the residual channel estimation errors might have significant impacts on the system performance. The impacts of channel estimation error on system performance are quantified through the development of the analytical symbol error rate (SER) by analyzing the statistical properties of the estimated channel coefficients. With the help of the asymptotic analysis of the channel estimation mean squared error, the SER is expressed as an explicit function of a number of system parameters, such as the maximum Doppler frequency, the percentage of pilots in the transmitted symbols, and the ratio of energy allocated between data symbols and pilot symbols. The optimum pilot percentage and energy allocation factor that minimize the SER are identified through analytical and numerical studies.
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paper_title: Average-Rate Optimal PSAM Transmissions over Time-Selective Fading Channels

paper_content:

Enabling linear minimum-mean square error (LMMSE)-based estimation of random time-selective channels, pilot-symbol-assisted modulation (PSAM) has well-documented merits as a fading counter-measure boosting bit-error rate performance. We design average-rate optimal PSAM transmissions by maximizing a tight lower bound of the average channel capacity. Relying on a simple closed-form expression of this bound in terms of the LMMSE channel estimator variance, we obtain PSAM transmissions with optimal spacing of pilot symbols and optimal allocation of the transmit-power budget between pilot and information symbols. Equi-powered transmitted symbols, channels with special Doppler spectra, and analytical and simulation based comparisons of possible alternatives shed more light on information-theoretic aspects of PSAM-based transmissions.
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paper_title: Design and Analysis of MMSE Pilot-Aided Cyclic-Prefixed Block Transmissions for Doubly Selective Channels

paper_content:

This paper considers affine cyclic-prefixed block-based pilot-aided transmission (PAT) over the single-antenna doubly selective channel, where the channel is assumed to obey a complex-exponential basis expansion model. First, a tight lower bound on the mean-squared error (MSE) of pilot-aided channel estimates is derived, along with necessary and sufficient conditions on the pilot/data pattern that achieves this bound. From these conditions, novel minimum-MSE (MMSE) PAT schemes are proposed and upper/lower bounds on their ergodic achievable rates are derived. A pilot/data power allocation technique is also developed. A high-SNR asymptotic analysis of the ergodic achievable rate of affine MMSE-PAT is then performed which suggests that the channel's spreading parameters should be taken into account when choosing among affine MMSE-PAT schemes. Specifically, we establish that multicarrier MMSE-PAT achieves higher rates than single-carrier MMSE-PAT when the channel's delay-spread dominates its Doppler-spread, and vice versa.
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paper_title: Position-Based Compressed Channel Estimation and Pilot Design for High-Mobility OFDM Systems

paper_content:

With the development of high speed trains (HST) in many countries, providing broadband wireless services in HSTs is becoming crucial. Orthogonal frequency-division multiplexing (OFDM) has been widely adopted for broadband wireless communications due to its high spectral efficiency. However, OFDM is sensitive to the time selectivity caused by high-mobility channels, which costs large spectrum or time resources to obtain the accurate channel state information (CSI). Therefore, the channel estimation in high-mobility OFDM systems has been a long-standing challenge. In this paper, we first propose a new position-based high-mobility channel model,in which the HST's position information and Doppler shift are utilized to determine the positions of the dominant channel coefficients. %In this way, we can reduce the estimation complexity and to design the transmitted pilot.Then, we propose a joint pilot placement and pilot symbol design algorithm for compressed channel estimation. It aims to reduce the coherence between the pilot signal and the proposed channel model, and hence can improve the channel estimation accuracy. Simulation results demonstrate that the proposed method achieves better performances than existing channel estimation methods over high-mobility channels. Furthermore, we give an example of the designed pilot codebook to show the practical applicability of the proposed scheme.
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paper_title: Optimal Training for Block Transmissions over Doubly Selective Wireless Fading Channels

paper_content:

High data rates give rise to frequency-selective propagation, whereas carrier frequency-offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. To mitigate the resulting time- and frequency-selective (or doubly selective) channels, optimal training sequences have been designed only for special cases: pilot symbol assisted modulation (PSAM) for time-selective channels and pilot tone-assisted orthogonal frequency division multiplexing (OFDM) for frequency-selective channels. Relying on a basis expansion channel model, we design low-complexity optimal PSAM for block transmissions over doubly selective channels. The optimality in designing our PSAM parameters consists of maximizing a tight lower bound on the average channel capacity that is shown to be equivalent to the minimization of the minimum mean-square channel estimation error. Numerical results corroborate our theoretical designs.
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paper_title: On the Performance of Doubly-Selective Fading Estimations in High Mobility Systems

paper_content:

This paper studies the theoretical performance of channel estimations in high mobility wireless communication systems operating with doubly-selective (both time-selective and frequency-selective) fading. High mobility communications result in fast time-selective fading with a Doppler spread as high as a few kilo-Hertz. The fast fading variation can be estimated and tracked with pilot-assisted minimum mean squared error (MMSE) channel estimation. The percentage of pilot symbols among the transmitted symbols plays a critical role. A higher pilot percentage can lead to a better channel estimation, but also more overhead. The impacts of the pilot percentage on channel estimations are identified through the derivation of the asymptotic mean squared error (MSE) of channel estimations. The results are expressed as a closed-form expression of various system parameters, including the pilot percentage, the maximum Doppler spread, the signal-to-noise ratio (SNR), etc.
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paper_title: On Optimum Pilot Design for Comb-Type OFDM Transmission over Doubly-Selective Channels

paper_content:

We consider comb-type OFDM transmission over doubly-selective channels. Given a fixed number and total power of the pilot subcarriers, we show that the MMSE-optimum pilot design consists of identical equally-spaced clusters where each cluster is zero-correlation-zone sequence.
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paper_title: Modified clustered comb pilot-aided fast time-varying channel estimation for OFDM system

paper_content:

As high-speed railway is booming worldwide, the communication system with fast-time varying channel has drawn great attention. The comb pilot based linear minimum mean square error (LMMSE) channel estimator is proved to be an effective method for fast time-varying channel estimation. In this paper, the clustered comb pilot-aided channel estimation for orthogonal frequency-division multiplexing (OFDM) system is discussed, where the time varying channel is approximated by a basis expansion model (BEM). A modified clustered comb pilot structure is proposed and justified to improve the estimation performance compared with the clustered comb pilot proposed by Tang. Based on the complex-exponential BEM (CE-BEM) model, a suboptimal-pilot structure is proposed. In addition, optimal pilot length is analyzed and simulated with a predefined total number of pilots. The simulation results show that the modified clustered comb pilot can greatly reduce the estimation error especially with high Doppler spread. The suboptimal-pilot structure with guard pilot approximation is proven to be competitive. Optimal nonzero pilot lengths for different Doppler spread are obtained by simulation with a predefined channel order and fixed pilot subcarriers.
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paper_title: On the estimation of doubly-selective fading channels

paper_content:

Coherent communications over doubly-selective fading channels are attracting increasing research interests because of the performance advantage over their noncoherent counterparts. In this paper, we use a simple windowing and de-widowing technique to improve the accuracy of existing basis expansion model (BEM) and develop a windowed least-squares (WLS) estimator for doubly-selective fading channels. Our designs can considerably improve the channel estimation performance. We also design the optimum pilot patten for the WLS estimator. Simulations are provided to corroborate our theoretical analysis.
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paper_title: Capacity maximizing MMSE-optimal pilots for wireless OFDM over frequency-selective block Rayleigh-fading channels

paper_content:

The location, number, and power of pilot symbols embedded in multicarrier block transmissions over rapidly fading channels, are important design parameters affecting not only channel estimation performance, but also channel capacity. Considering orthogonal frequency-division multiplexing (OFDM) systems with decoupled information-bearing symbols from pilot symbols transmitted over wireless frequency-selective Rayleigh-fading channels, we show that equispaced and equipowered pilot symbols are optimal in terms of minimizing the mean-square channel estimation error. We also design the number of pilots, and the power distributed between information bearing and pilot symbols, using as criterion a lower bound on the average capacity. Numerical results corroborate our theoretical findings.
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paper_title: Pilot-Assisted Time-Varying Channel Estimation for OFDM Systems

paper_content:

In this paper, we deal with channel estimation for orthogonal frequency-division multiplexing (OFDM) systems. The channels are assumed to be time-varying (TV) and approximated by a basis expansion model (BEM). Due to the time-variation, the resulting channel matrix in the frequency domain is no longer diagonal, but approximately banded. Based on this observation, we propose novel channel estimators to combat both the noise and the out-of-band interference. In addition, the effect of a receiver window on channel estimation is also studied. Our claims are supported by simulation results, which are obtained considering Jakes' channels with fairly high Doppler spreads
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paper_title: On the estimation of doubly-selective fading channels

paper_content:

Coherent communications over doubly-selective fading channels are attracting increasing research interests because of the performance advantage over their noncoherent counterparts. In this paper, we use a simple windowing and de-widowing technique to improve the accuracy of existing basis expansion model (BEM) and develop a windowed least-squares (WLS) estimator for doubly-selective fading channels. Our designs can considerably improve the channel estimation performance. We also design the optimum pilot patten for the WLS estimator. Simulations are provided to corroborate our theoretical analysis.
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paper_title: Adaptive Channel Estimation Using Pilot-Embedded Data-Bearing Approach for MIMO-OFDM Systems

paper_content:

Multiple-input multiple-output (MIMO) orthogonal-frequency-division-multiplexing (OFDM) systems employing coherent receivers crucially require channel state information (CSI). Since the multipath delay profile of channels is arbitrary in the MIMO-OFDM systems, an effective channel estimator is needed. In this paper, we first develop a pilot-embedded data-bearing (PEDB) approach for joint channel estimation and data detection, in which PEDB least-square (LS) channel estimator and maximum-likelihood (ML) data detection are employed. Then, we propose an LS fast Fourier transform (FFT)-based channel estimator by employing the concept of FFT-based channel estimation to improve the PEDB-LS one via choosing a certain number of significant taps for constructing a channel frequency response. The effects of model mismatch error inherent in the proposed LS FFT-based estimator when considering noninteger multipath delay profiles and its performance analysis are investigated. The relationship between the mean-squared error (MSE) and the number of chosen significant taps is revealed, and hence, the optimal criterion for obtaining the optimum number of significant taps is explored. Under the framework of pilot embedding, we further propose an adaptive LS FFT-based channel estimator employing the optimum number of significant taps to compensate the model mismatch error as well as minimize the corresponding noise effect. Simulation results reveal that the adaptive LS FFT-based estimator is superior to the LS FFT-based and PEDB-LS estimators under quasi-static channels or low Doppler's shift regimes
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paper_title: Adaptive Reduced-Rank Estimation of Nonstationary Time-Variant Channels Using Subspace Selection

paper_content:

In this paper, we focus on adaptive time-variant channel estimation for vehicle-to-vehicle (V2V) communications in intelligent transportation systems (ITS) using the IEEE 802.11p physical layer. The IEEE 802.11p pilot pattern is identical to that in the well-known IEEE 802.11a/g (WiFi) standard, which was initially designed for indoor environments with little or no mobility. However, in a V2V drive-by situation, the channel impulse response rapidly changes due to the high relative velocity between transmitter and receiver, as well as the changes in the scattering environment. Hence, for such V2V channels, advanced decision directed channel estimation methods are needed to reach a frame error rate (FER) smaller than 10-1 . Even more importantly, the channels are nonstationary, which implies that the Doppler power spectral density (DSD) and the power delay profile (PDP) change on a timescale comparable with the frame length, which complicates the estimator design. In this paper, we develop an adaptation method for the channel estimation filter that is suitable for the following: 1) the short frame length in IEEE 802.11p; 2) the given pilot structure; and 3) the requirement of only a single received short frame for filter adaptation. We define a set of hypotheses on the support of the DSD and a second set of hypotheses on the support of the PDP. Each hypothesis is represented by a specific subspace spanned by orthogonal basis vectors. For basis vector calculation, we develop a numerically stable algorithm utilizing generalized discrete prolate spheroidal sequences. The adaptation algorithm chooses a hypothesis from both sets such that a probabilistic bound on the channel estimation error is minimized. We implement the hypothesis test by means of a novel subspace selection algorithm that allows utilizing correlated observations of a time- and frequency-selective (2-D) fading process. We validate the adaptive channel estimation scheme in an IEEE 802.11p compliant link level simulation for a relative velocity range from 0 to 111 m/s ≈ 400 km/h ≈ 249 mi/h. Adaptive filtering enables an up to fourfold reduction in the number of required iterations to reach an FER below 10-1 for an Eb/N0 = 12 dB.
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paper_title: Modified clustered comb pilot-aided fast time-varying channel estimation for OFDM system

paper_content:

As high-speed railway is booming worldwide, the communication system with fast-time varying channel has drawn great attention. The comb pilot based linear minimum mean square error (LMMSE) channel estimator is proved to be an effective method for fast time-varying channel estimation. In this paper, the clustered comb pilot-aided channel estimation for orthogonal frequency-division multiplexing (OFDM) system is discussed, where the time varying channel is approximated by a basis expansion model (BEM). A modified clustered comb pilot structure is proposed and justified to improve the estimation performance compared with the clustered comb pilot proposed by Tang. Based on the complex-exponential BEM (CE-BEM) model, a suboptimal-pilot structure is proposed. In addition, optimal pilot length is analyzed and simulated with a predefined total number of pilots. The simulation results show that the modified clustered comb pilot can greatly reduce the estimation error especially with high Doppler spread. The suboptimal-pilot structure with guard pilot approximation is proven to be competitive. Optimal nonzero pilot lengths for different Doppler spread are obtained by simulation with a predefined channel order and fixed pilot subcarriers.
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paper_title: Design and Analysis of MMSE Pilot-Aided Cyclic-Prefixed Block Transmissions for Doubly Selective Channels

paper_content:

This paper considers affine cyclic-prefixed block-based pilot-aided transmission (PAT) over the single-antenna doubly selective channel, where the channel is assumed to obey a complex-exponential basis expansion model. First, a tight lower bound on the mean-squared error (MSE) of pilot-aided channel estimates is derived, along with necessary and sufficient conditions on the pilot/data pattern that achieves this bound. From these conditions, novel minimum-MSE (MMSE) PAT schemes are proposed and upper/lower bounds on their ergodic achievable rates are derived. A pilot/data power allocation technique is also developed. A high-SNR asymptotic analysis of the ergodic achievable rate of affine MMSE-PAT is then performed which suggests that the channel's spreading parameters should be taken into account when choosing among affine MMSE-PAT schemes. Specifically, we establish that multicarrier MMSE-PAT achieves higher rates than single-carrier MMSE-PAT when the channel's delay-spread dominates its Doppler-spread, and vice versa.
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paper_title: OFDM channel estimation by singular value decomposition

paper_content:

A new approach to low-complexity channel estimation in orthogonal-frequency division multiplexing (OFDM) systems is proposed. A low-rank approximation is applied to a linear minimum mean-squared error (LMMSE) estimator that uses the frequency correlation of the channel. By using the singular value decomposition (SVD) an optimal low-rank estimator is derived, where performance is essentially preserved-even for low computational complexities. A fixed estimator, with nominal values for channel correlation and signal-to-noise ratio (SNR), is analysed. Analytical mean-squared error (MSE) and symbol-error rates (SER) are presented for a 16-QAM OFDM system.
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paper_title: A Model Reduction Approach for OFDM Channel Estimation Under High Mobility Conditions

paper_content:

Orthogonal frequency-division multiplexing (OFDM) combines the advantages of high performance and relatively low implementation complexity. However, for reliable coherent detection of the input signal, the OFDM receiver needs accurate channel information. When the channel exhibits fast time variation as it is the case with several recent OFDM-based mobile broadband wireless standards (e.g., WiMAX, LTE, DVB-H), channel estimation at the receiver becomes quite challenging for two main reasons: 1) the receiver needs to perform this estimation more frequently and 2) channel time-variations introduce intercarrier interference among the OFDM subcarriers which can degrade the performance of conventional channel estimation algorithms significantly. In this paper, we propose a new pilot-aided algorithm for the estimation of fast time-varying channels in OFDM transmission. Unlike many existing OFDM channel estimation algorithms in the literature, we propose to perform channel estimation in the frequency domain, to exploit the structure of the channel response (such as frequency and time correlations and bandedness), optimize the pilot group size and perform most of the computations offline resulting in high performance at substantial complexity reductions.
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paper_title: Minimum Error Transmissions with Imperfect Channel Information in High Mobility Systems

paper_content:

One of the main challenges faced by communications in high mobility systems, such as unmanned aerial vehicle (UAV), is the fast time-varying fading. This paper studies the optimum system design that can minimize the error probability of a high mobility wireless communication system with imperfect channel state information (CSI). We consider a system that employs pilot-assisted minimum mean square error (MMSE) channel estimation. The channel estimation error is usually non-negligible in high mobility systems and might have significant impacts on system performance. With asymptotic analysis, the channel estimation mean square error (MSE) is expressed as a closed-form expression of the percentage of pilot symbols in the transmitted symbols. An optimum receiver for systems with imperfect CSI and the corresponding symbol error rate (SER) are then derived by analyzing the statistical properties of the channel estimation error. The SER is expressed as an explicit function of a number of parameters, such as pilot percentage, maximum Doppler spread, and the signal-to-noise ratio (SNR), etc. The optimum pilot percentage that can minimize the SER is analytically identified, and the impacts of the imperfects CSI on the system performance are studied through both theoretical analysis and simulations.
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paper_title: Maximizing Spectral Efficiency for High Mobility Systems with Imperfect Channel State Information

paper_content:

This paper studies the optimum system design that can maximize the spectral efficiency of high mobility wireless communication systems with imperfect channel state information (CSI). The fast time-varying fading in high mobility systems can be tracked with pilot-assisted channel estimation. The percentage of pilot symbols in the transmitted symbols plays a critical role on the system performance: a higher pilot percentage yields a more accurate channel estimation, but also more overhead. The effects of pilot percentage are quantified through the derivation of the channel estimation mean squared error (MSE), which is expressed as a closed-form expression of various system parameters through asymptotic analysis. It is discovered that, if the pilots sample the channel above its Nyquist rate, then the estimation of the channel coefficients of data symbols through temporal interpolation yields the same asymptotic MSE as the direct estimation of the channel coefficients of the pilot symbols. Based on the statistical properties of the channel estimation error, we quantify the impacts of the imperfect CSI on the system performance by developing the analytical symbol error rate (SER) and a spectral efficiency lower bound of the communication system. The optimum pilot percentage that can maximize the spectral efficiency lower bound is identified through both analytical and simulation results.
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paper_title: Channel estimation in OFDM systems operating under high mobility using Wiener filter combined basis expansion model

paper_content:

In this article, we first thoroughly analyze Wiener filter combined least squares based channel estimation (WF-LS) and then illustrate its limitation in high-speed mobile environments. Based on the analysis, we propose to combine WF with basis expansion model (BEM) based channel estimation to deal with channel estimation in various mobile environments, especially in high-speed cases. The expression for Wiener filter combined BEM based channel estimation (WF-BEM) is derived and the result explicitly considers the effect of intercarrier interference (ICI) that occurs in orthogonal frequency division multiplexing (OFDM) systems when operating under high mobility. The simulation results demonstrate that our proposed WF-BEM is better than WF-LS in time-varying channels, and the performance improvement is significant especially in fast time-varying channels.
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paper_title: Robust channel estimation for OFDM systems with rapid dispersive fading channels

paper_content:

Orthogonal frequency division multiplexing (OFDM) modulation is a promising technique for achieving the high-bit-rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3 dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator to channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.
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paper_title: Pilot-Assisted Time-Varying Channel Estimation for OFDM Systems

paper_content:

In this paper, we deal with channel estimation for orthogonal frequency-division multiplexing (OFDM) systems. The channels are assumed to be time-varying (TV) and approximated by a basis expansion model (BEM). Due to the time-variation, the resulting channel matrix in the frequency domain is no longer diagonal, but approximately banded. Based on this observation, we propose novel channel estimators to combat both the noise and the out-of-band interference. In addition, the effect of a receiver window on channel estimation is also studied. Our claims are supported by simulation results, which are obtained considering Jakes' channels with fairly high Doppler spreads
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paper_title: On channel estimation and detection for multicarrier signals in fast and selective Rayleigh fading channels

paper_content:

Time-domain channel estimation and detection techniques are presented for multicarrier signals in a fast and frequency-selective Rayleigh fading channel. As a consequence of the time-varying channel, the orthogonality between subcarriers is destroyed in conventional frequency-domain approaches, resulting in interchannel interference, which increases an irreducible error floor in proportion to the normalized Doppler frequency. An important feature of the proposed technique is the ability to exploit the time-selective channel as a provider of time diversity. This enables us to achieve performance superior to any other structure without increasing bandwidth or incorporating redundancy, in order to reduce the complexity of the estimator, we apply the theory of optimal low rank approximation to a minimum mean squared error channel estimator and present a theoretical calculation of mean squared error and simulations to confirm that the estimator is robust to changes in channel characteristics.
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paper_title: Optimum designs for high mobility systems with channel estimation errors

paper_content:

This paper studies the optimum system designs of high mobility wireless communication systems with channel estimation errors. In high mobility systems, the accurate estimation and tracking of the fast time-varying fading channel is nontrivial, and the residual channel estimation errors might have significant impacts on the system performance. The impacts of channel estimation error on system performance are quantified through the development of the analytical symbol error rate (SER) by analyzing the statistical properties of the estimated channel coefficients. With the help of the asymptotic analysis of the channel estimation mean squared error, the SER is expressed as an explicit function of a number of system parameters, such as the maximum Doppler frequency, the percentage of pilots in the transmitted symbols, and the ratio of energy allocated between data symbols and pilot symbols. The optimum pilot percentage and energy allocation factor that minimize the SER are identified through analytical and numerical studies.
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paper_title: Low-complexity pilot-aided channel estimation for OFDM systems over doubly-selective channels

paper_content:

In this paper, we investigate channel estimation (CE) and data detection for OFDM systems over doubly-selective channels. We derive an oversampling basis expansion model (BEM) for doubly-selective channels and its statistical properties. The time diversity in the Doppler-induced inter-carrier-interference (ICI) and its relationship to the carrier frequency offset (CFO) induced ICI are illustrated using the BEM. We derive two low complexity linear minimum mean-square-error (LMMSE) channel estimators using the BEM. The sphere decoder (SD) is modified to equalize the ICI channel. A low-complexity iterative equalizer without matrix inversion is also proposed. Our proposed channel estimators have low complexity and achieve good performance. Furthermore, the low-complexity iterative equalizer performs close to SD.
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paper_title: Adaptive Channel Estimation for OFDM Systems with Doppler spread

paper_content:

In this paper, we propose adaptive channel estimation for Orthogonal Frequency Division Multiplexing (OFDM) in fast time-varying (TV) channels. A Basis Expansion Model (BEM) approach is used to capture the time variation of the channel within each OFDM block, and to reduce the estimator dimensionality. Capitalizing on the BEM structure and on a frequency domain training, two adaptive approaches are proposed, based on Kalman filtering and Recursive Least Squares (LS) methods, which exploit the time correlation of the channel between successive blocks and do not require any a-priori knowledge of the channel statistics. Simulation results show that, compared to classical Least Squares and statistically-aided Linear Minimum Mean Squared Error (LMMSE) approaches, the two proposed techniques effectively estimate the channel, adapt fast to its non stationary changes, thus enabling efficient TV channel equalization of the inter-carrier interference (ICI) induced in OFDM systems by high Doppler spreads.
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paper_title: Wireless Communications Over Rapidly Time-Varying Channels

paper_content:

As a result of higher frequencies and increased user mobility, researchers and systems designers are shifting their focus from time-invariant models to channels that vary within a block. This book explains the latest theoretical advances and practical methods to give an understanding of rapidly time varying channels, together with performance trade-offs and potential performance gains, providing the expertise to develop future wireless systems technology. As well as an overview of the issues of developing wireless systems using time-varying channels, the book gives extensive coverage to methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, as well as providing models and transceiver methods for time-varying ultra-wideband channels. *An introduction to time-varying channel models gives in a nutshell the important issues of developing wireless systems technology using time-varying channels*Extensive coverage of methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, enables development of high performance wireless systems*Chapters on transceiver design for OFDM and receiver algorithms for MIMO communication channels over time-varying channels, with an emphasis on modern iterative turbo-style architectures, demonstrates how these important technologies can optimize future wireless systems
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paper_title: Two-dimensional recursive least square adaptive channel estimation for OFDM systems

paper_content:

In orthogonal frequency division multiplexing (OFDM) system, time- and frequency-domain two-dimensional minimum mean square error (2D-MMSE) channel estimation is optimum. However, accurate channel statistics, which are often time-varying and unavailable in practice, are required to realize it. In this paper, we propose two-dimensional recursive least square (2D-RLS) adaptive channel estimation for OFDM system. 2D-RLS adaptive channel estimation does not require accurate channel statistics, and at the same time can make full use of time-and frequency-domain correlations of the frequency response of time-varying wireless channels. With properly chosen parameters, 2D-RLS adaptive channel estimation can converge into the steady state in only several OFDM symbols' time. Computer simulations demonstrate that 2D-RLS adaptive channel estimation is very effective and suitable for a broad range of channel conditions. Furthermore, with proper training sequences design on transmitter antennas, 2D-RLS adaptive channel estimation can be applied to multiple inputs multiple outputs (MIMO) OFDM system.
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paper_title: Doubly selective channel estimation using exponential basis models and subblock tracking

paper_content:

Three versions of a novel adaptive channel estimation approach, exploiting the over-sampled complex exponential basis expansion model (CE-BEM), is presented for doubly selective channels, where we track the BEM coefficients rather than the channel tap gains. Since the time-varying nature of the channel is well captured in the CE-BEM by the known exponential basis functions, the time variations of the (unknown) BEM coefficients are likely much slower than those of the channel, and thus more convenient to track. We propose a ?subblockwise? tracking scheme for the BEM coefficients using time-multiplexed (TM) periodically transmitted training symbols. Three adaptive algorithms, including a Kalman filtering scheme based on an assumed autoregressive (AR) model of the BEM coefficients, and two recursive least-squares (RLS) schemes not requiring any model for the BEM coefficients, are investigated for BEM coefficient tracking. Simulation examples illustrate the superior performance of our approach over several existing doubly selective channel estimators.
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paper_title: Space–Time Block Coding for Doubly-Selective Channels

paper_content:

In this paper, we present a new space-time block code for time- and frequency-selective (doubly-selective) channels. It can be interpreted as the extension of the Alamouti code to doubly-selective channels, and relies on a joint time-frequency reversal of the transmitted sequences. Under certain channel conditions, the proposed space-time block code belongs to the class that achieves full spatial, delay, and Doppler diversity using a maximum-likelihood (ML) receiver, as well as a linear zero-forcing (LZF) or linear minimum mean-squared error (LMMSE) receiver. For realistic doubly-selective channels, a real-valued linear data model is presented, for which different receiver structures can be developed.
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paper_title: Maximum-diversity transmissions over doubly selective wireless channels

paper_content:

High data rates and multipath propagation give rise to frequency-selectivity of wireless channels, while carrier frequency offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. The resulting time- and frequency-selective (or doubly selective) channels offer joint multipath-Doppler diversity gains. Relying on a basis expansion model of the doubly selective channel, we prove that the maximum achievable multipath-Doppler diversity order is determined by the rank of the correlation matrix of the channel's expansion coefficients, and is multiplicative in the effective degrees of freedom that the channel exhibits in the time and frequency dimensions. Interestingly, it turns out that time-frequency reception alone does not guarantee maximum diversity, unless the transmission is also designed judiciously. We design such block precoded transmissions. The corresponding designs for frequency-selective or time-selective channels follow as special cases, and thorough simulations are provided to corroborate our theoretical findings.
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paper_title: Space-time-Doppler block coding for correlated time-selective fading channels

paper_content:

Coping with time-selective fading channels is challenging but also rewarding, especially with multiantenna systems, where joint space-Doppler diversity and coding gains can be collected to enhance performance of wireless mobile links. These gains have not been quantified, and space-time coded systems maximizing joint space-Doppler benefits have not been designed. Based on a parsimonious basis expansion model for the underlying time-selective (and possibly correlated) channels, we quantify these gains in closed form. Furthermore, we develop space-time-Doppler coded systems that guarantee the maximum possible space-Doppler diversity, along with the largest coding gains within all linearly coded systems. Our three novel designs exploit knowledge of the maximum Doppler spread, and each offers a uniquely desirable tradeoff, including high spectral efficiency, low decoding complexity, and high performance. Our analytical results are confirmed by simulations and reveal the relative of merits of our three designs in comparison with an existing approach.
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paper_title: Maximum-diversity transmissions over time-selective wireless channels

paper_content:

Carrier frequency-offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. Doppler-RAKE receivers have been developed for collecting the resulting diversity gains only for spread-spectrum systems. Relying on a basis expansion model of time-selective channels, we find that the maximum achievable Doppler diversity is determined by the rank of the correlation matrix of the channel's expansion coefficients. We also prove that RAKE reception can not collect maximum diversity gains, unless the transmission is appropriately designed. Finally, we design such block precoded transmissions to ensure maximum diversity gains, and provide thorough simulations to corroborate our theoretical findings.
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paper_title: TDD-based mobile communication solutions for high-speed railway scenarios

paper_content:

Mobile communication solutions for highspeed railway scenarios aim at providing seamless coverage, high system capacity and a high data rate for end users, which in turn face the challenges of Doppler frequency shift, penetration loss, mobility management and system capacity planning. Because the scarcity of frequency resources and increasing wireless broadband requirements have become very serious problems worldwide, increasing attention has been focused on time-division duplexing based technology, on which we focus in this article. Time-division synchronous code-division multiple access based and time-division Long Term Evolution based solutions are introduced with appropriate technology analysis. The experimental results of TD-SCDMA-based solutions in the Shanghai maglev high-speed network environment and the simulation results of TD-LTE-based solutions are presented. Finally, to improve performance, we discuss an enhanced solution based on mobile relays with performance evaluation, which is considered as a more comprehensive solution for high-speed railway scenarios.
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paper_title: High Mobility Wireless Communications With Doppler Diversity: Fundamental Performance Limits

paper_content:

The objective of this paper is to quantify the fundamental tradeoff between Doppler diversity and channel estimation errors in high mobility systems. Fast fading variation in high mobility systems introduces Doppler diversity that can benefit system performance. On the other hand, it is more difficult to estimate and track a fast changing channel, thus channel estimation errors are non-negligible and they may seriously degrade system performance. Such a tradeoff relationship is studied by identifying the exact analytical expressions of two performance metrics: the maximum Doppler diversity order achievable with imperfect channel state information (CSI), and the loss in signal-to-noise ratio (SNR) caused by channel estimation errors. The analytical study is enabled by a simple repetition code at the transmitter, and a new optimum diversity receiver developed by analyzing the statistical properties of channel estimation errors. With the analytical results, the optimum allocation of transmission energy between pilot symbols and data symbols are obtained to simultaneously maximize the Doppler diversity order and minimize the SNR loss, thus achieve the optimum tradeoff between the two. The results reveal the fundamental performance limits of Doppler diversity systems operating in the presence of imperfect CSI, and they can be used to guide the design of practical systems.
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paper_title: Pilot-based estimation of time-varying multipath channels for coherent CDMA receivers

paper_content:

Reliable coherent wireless communication requires accurate estimation of the time-varying multipath channel. This paper addresses two issues in the context of direct-sequence code-division multiple access (CDMA) systems: (i) linear minimum-mean-squared-error (MMSE) channel estimation based on a pilot transmission and (ii) impact of channel estimation errors on coherent receiver performance. A simple characterization of the MMSE estimator in terms of a bank of filters is derived. A key channel characteristic controlling system performance is the normalized coherence time, which is approximately the number of symbols over which the channel remains strongly correlated. It is shown that the estimator performance is characterized by an effective signal-to-noise ratio (SNR)-the product of the pilot SNR and the normalized coherence time. A simple uniform averaging estimator is also proposed that is easy to implement and delivers near-optimal performance if properly designed. The receivers analyzed in this paper are based on a time-frequency RAKE structure that exploits joint multipath-Doppler diversity. It is shown that the overall receiver performance is controlled by two competing effects: shorter coherence times lead to degraded channel estimation but improved inherent receiver performance due to Doppler diversity, with opposite effects for longer coherence times. Our results demonstrate that exploiting Doppler diversity can significantly mitigate the error probability floors that plague conventional CDMA receivers under fast fading due to errors in channel estimation.
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paper_title: Exploiting cooperative multipath–Doppler diversity in relay-assisted high-speed train communications

paper_content:

Severe doubly selective channel fading, or the time- and frequency-selective fading, prevailing in high-speed train (HST) communications, has been a critical issue that hinders the improvement of system efficiency and reliability. In this paper, a relay-assisted HST communication system is considered, and a low-complexity but effective scheme is proposed to cooperatively exploit the joint multipath–Doppler diversity of both the direct link and the relay link and thus to mitigate the negative effect of channel fading and improve the system reliability. In particular, using a special precoding structure followed by two-dimensional Fourier transform, the transmit signal is generated which in effect parallelizes the doubly selective fading channel. As a result, in the transform domain, pointwise combining and equalization, which are of linear complexity, become feasible and diversity gain is obtained. The maximal diversity order is then analyzed under the amplify-and-forward relay settings, which reveals that by cooperative relaying the joint multipath–Doppler diversity of both links can be well exploited. Simulation results verified our analysis.
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paper_title: Space–Time Block Coding for Doubly-Selective Channels

paper_content:

In this paper, we present a new space-time block code for time- and frequency-selective (doubly-selective) channels. It can be interpreted as the extension of the Alamouti code to doubly-selective channels, and relies on a joint time-frequency reversal of the transmitted sequences. Under certain channel conditions, the proposed space-time block code belongs to the class that achieves full spatial, delay, and Doppler diversity using a maximum-likelihood (ML) receiver, as well as a linear zero-forcing (LZF) or linear minimum mean-squared error (LMMSE) receiver. For realistic doubly-selective channels, a real-valued linear data model is presented, for which different receiver structures can be developed.
---
paper_title: Maximum-diversity transmissions over doubly selective wireless channels

paper_content:

High data rates and multipath propagation give rise to frequency-selectivity of wireless channels, while carrier frequency offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. The resulting time- and frequency-selective (or doubly selective) channels offer joint multipath-Doppler diversity gains. Relying on a basis expansion model of the doubly selective channel, we prove that the maximum achievable multipath-Doppler diversity order is determined by the rank of the correlation matrix of the channel's expansion coefficients, and is multiplicative in the effective degrees of freedom that the channel exhibits in the time and frequency dimensions. Interestingly, it turns out that time-frequency reception alone does not guarantee maximum diversity, unless the transmission is also designed judiciously. We design such block precoded transmissions. The corresponding designs for frequency-selective or time-selective channels follow as special cases, and thorough simulations are provided to corroborate our theoretical findings.
---
paper_title: WLCp1-04: Exploring Maximum Doppler Diversity by Doppler Domain Multiplexing

paper_content:

Doppler effect induced by temporal fading variation provides Doppler diversity, which can be exploited to combat the impairments caused by multipath fading. In this paper, a Doppler domain multiplexing (DDM) communication structure is presented to explore the potential Doppler diversity. Data streams in DDM system are encoded and multiplexed at different Doppler shifts to achieve both Doppler diversity and high spectral efficiency. Theo- retical performances of both uncoded and coded DDM systems are analyzed. The analytical results show that DDM structure can ef- fectively exploit the potential Doppler diversity inherent in time- varying fading. A sub-optimum decoder with Doppler domain equalization and soft maximum a posteriori decoding is employed to recover the data at the communication receiver. Simulation re- sults show that significant performance gains can be achieved by DDM system over conventional time domain system.
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paper_title: Maximum-diversity transmissions over time-selective wireless channels

paper_content:

Carrier frequency-offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. Doppler-RAKE receivers have been developed for collecting the resulting diversity gains only for spread-spectrum systems. Relying on a basis expansion model of time-selective channels, we find that the maximum achievable Doppler diversity is determined by the rank of the correlation matrix of the channel's expansion coefficients. We also prove that RAKE reception can not collect maximum diversity gains, unless the transmission is appropriately designed. Finally, we design such block precoded transmissions to ensure maximum diversity gains, and provide thorough simulations to corroborate our theoretical findings.
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paper_title: Low-complexity equalization of time-varying channels with precoding

paper_content:

This correspondence deals with time-varying (TV) single-input multiple-output (SIMO) channels, which are both frequency selective (due to high data rate) and time selective (due to mobility). A complex exponential basis expansion model (CE-BEM) is used to model the channel. We consider a block transmission system, where on the transmit side a precoder is employed to enable the maximum available diversity for a CE-BEM channel. After direct decoding on the receive side, the resulting channel resembles a finite-impulse-response (FIR) filter on both block and symbol level. We therefore propose an equalizer that bears a structure analogous to the effective channel. In comparison with a standard block minimum mean-square error decision-feedback equalizer (BMMSE-DFE) that involves the inversion of a large-size matrix, the proposed parametric equalizer renders a similar performance but at a lower computational cost if there are multiple outputs present. Another contribution of this correspondence is a semiblind algorithm to estimate this equalizer when the channel state information is not available: the equalizer taps and the information symbol estimates are refined recursively by means of normalized least-mean-squares (NLMS) adaptation
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paper_title: Spectral Efficient Doppler Diversity Transmissions in High Mobility Systems with Channel Estimation Errors

paper_content:

This paper studies spectral efficient Doppler diversity transmissions in the presence of imperfect channel state information (CSI). Fast time-varying fading in high mobility communication systems introduces Doppler diversity that can benefit system performance. On the other hand, it is more difficult to estimate and track fast time-varying channel, thus channel estimation errors might seriously degrade system performance in high mobility systems. We propose a practical pilot- assisted system design that can balance the tradeoff between Doppler diversity and channel estimation errors. At the transmitter, the data symbols are precoded with a rate-1 Doppler domain multiplexing scheme to achieve maximum Doppler diversity without sacrificing spectral efficiency. At the receiver, the information is detected by using imperfect CSI and a block decision feedback equalizer (BDFE), which is developed by exploiting the statistical properties of channel estimation errors. The analytical pairwise error probability (PEP) and a bit error rate (BER) lower bound are developed by considering a large number of design parameters, such as the percentage of pilot symbols in the transmitted symbols, the energy allocation between pilot and data symbols, and the maximum Doppler spread of the channel. Both simulation and analytical results show that maximum Doppler diversity can be achieved through optimizations of the various design parameters in the presence of imperfect CSI.
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paper_title: Energy and spectral efficient Doppler diversity transmissions in high-mobility systems with imperfect channel estimation

paper_content:

This paper studies energy and spectral efficient Doppler diversity transmissions in the presence of imperfect channel state information (CSI). Fast time-varying fading in high-mobility communication systems introduces Doppler diversity that can benefit system performance. On the other hand, it is more difficult to estimate and track fast time-varying channel; thus, channel estimation errors might seriously degrade system performance in high-mobility systems. The trade-offs between channel estimation errors and Doppler diversity are studied by using two precoding schemes, a simple repetition code, and a rate 1 Doppler domain multiplexing (DDM) scheme. The repetition code can achieve the maximum Doppler diversity at the cost of a lower spectral efficiency, and the DDM scheme can achieve the energy and spectral efficient Doppler diversity transmissions. Unlike many other Doppler diversity systems that assume perfect CSI, we explicitly consider the impacts of imperfect CSI on the design and performance of both precoding schemes. Optimum and suboptimum receivers for both schemes are developed by studying the statistical properties of channel estimation errors. The analytical error probabilities of the receivers are expressed as functions of a number of system parameters, such as the maximum Doppler spread, the percentage of pilot symbols for channel estimation, the energy allocation between the pilot and data symbols, etc. The analytical and simulation results indicate that both precoding schemes can achieve the maximum Doppler diversity order through the optimization of the various parameters, even in the presence of imperfect CSI.
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paper_title: Pilot-based estimation of time-varying multipath channels for coherent CDMA receivers

paper_content:

Reliable coherent wireless communication requires accurate estimation of the time-varying multipath channel. This paper addresses two issues in the context of direct-sequence code-division multiple access (CDMA) systems: (i) linear minimum-mean-squared-error (MMSE) channel estimation based on a pilot transmission and (ii) impact of channel estimation errors on coherent receiver performance. A simple characterization of the MMSE estimator in terms of a bank of filters is derived. A key channel characteristic controlling system performance is the normalized coherence time, which is approximately the number of symbols over which the channel remains strongly correlated. It is shown that the estimator performance is characterized by an effective signal-to-noise ratio (SNR)-the product of the pilot SNR and the normalized coherence time. A simple uniform averaging estimator is also proposed that is easy to implement and delivers near-optimal performance if properly designed. The receivers analyzed in this paper are based on a time-frequency RAKE structure that exploits joint multipath-Doppler diversity. It is shown that the overall receiver performance is controlled by two competing effects: shorter coherence times lead to degraded channel estimation but improved inherent receiver performance due to Doppler diversity, with opposite effects for longer coherence times. Our results demonstrate that exploiting Doppler diversity can significantly mitigate the error probability floors that plague conventional CDMA receivers under fast fading due to errors in channel estimation.
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paper_title: Optimal diversity combining based on linear estimation of rician fading channels

paper_content:

Optimal receiver diversity combining employing linear channel estimation is examined. Based on the statistical properties of pilot-assisted least-squares (LS) and minimum mean square error (MMSE) channel estimation, an optimal diversity receiver for wireless systems employing practical linear channel estimation on Rician fading channels is proposed. Exact analytical expressions for the symbol error rates of LS and MMSE channel estimation aided optimal diversity combining are derived. It is shown that an MPSK wireless system with MMSE channel estimation has the same SER when the MMSE channel estimation is replaced by LS estimation. This is an interesting counter-example to the common perception that channel estimation with smaller mean square error leads to smaller SER. Extensive simulation results validate the theoretical results.
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paper_title: Conditional Power and Rate Adaptation for MQAM/OFDM Systems Under CFO With Perfect and Imperfect Channel Estimation Errors

paper_content:

In this paper, a new conditional power and rate adaptation scheme for orthogonal frequency-division multiplexing (OFDM) systems is proposed in the presence of carrier frequency offset (CFO), with perfect and imperfect channel state information (CSI). The conventional adaptive scheme is shown to be a special case of the conditionally adaptive scheme technique that enables the resulting nonconvex optimization problem, which is solved in a feasible way. It leads to a solution for optimal power adaptation that maximizes the spectral efficiency of an OFDM system using $M$ -ary quadrature amplitude modulation (MQAM) under average power and instantaneous bit error rate (BER) constraints. Closed-form expressions for the average spectral efficiency (ASE) of adaptive OFDM systems are derived for perfect and imperfect CSI cases. The theoretical results and computer simulations show that range of the conditional adaptation becomes narrow and the performance of constant power and continuous rate is very close to that of the conditionally adaptive power and continuous rate for higher CFO or high signal-to-noise ratio (SNR) values.
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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.
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paper_title: Decision-directed fine synchronization in OFDM systems

paper_content:

A new decision-directed (DD) synchronization scheme is proposed for joint estimation of carrier frequency offset (CFO) and sampling clock frequency offset (SFO) in orthogonal frequency-division multiplexing (OFDM) systems. By exploiting the hard decisions, we report accurate estimators of residual CFO and small SFO. The performance analysis and simulation results indicate that the proposed novel DD scheme achieves much better performance than the conventional pilot-based schemes in both additive white Gaussian noise and frequency-selective channels.
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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.
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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.
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paper_title: A carrier frequency offset estimator with minimum output variance for OFDM systems

paper_content:

In this letter, we propose a blind carrier frequency offset (CFO) estimator with high resolution and high bandwidth efficiency for orthogonal frequency-division multiplexing (OFDM) systems. The proposed estimator utilizes minimum output variance to estimate CFO. Simulation results show that the proposed estimator is highly accurate and reliable for OFDM systems.
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paper_title: Subspace method for blind CFO estimation for OFDM systems with constant modulus constellations

paper_content:

In this paper, we propose a novel subspace based approach for blind carrier frequency offset estimation in OFDM. Correlation in squared spectrum of the channel is exploited. Low rank signal model is thereby obtained without virtual subcarriers. The proposed estimator accomplishes frequency synchronization with a single OFDM block. No extensive time averaging is needed, which makes the approach very attractive for time and frequency selective channels where the offset may be time varying. Superior performance is achieved over existing algorithms exploiting virtual carriers or constant modulus criterion.
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paper_title: Carrier frequency offset estimation for OFDM systems using null subcarriers

paper_content:

Carrier frequency offset (CFO) in orthogonal frequency-division multiplexing(OFDM) systems, which can induce the loss of orthogonality among subcarriersand result in significant performance degradation, is critical to be estimatedand compensated for. In this paper, a suboptimal scheme is proposed to estimatethe CFO using null subcarriers. In the proposed scheme, the CFO is dividedinto the fractional part and the integer part, with the fractional CFO estimatedfollowed by the integer CFO estimation. Compared with previous work usingnull subcarriers and exhaustive search, the implementation complexity of theproposed scheme is significantly lower due to the fact that the main componentrequired is a simple correlator. Furthermore, with only one trainingOFDM symbol and proper null subcarrier allocation, the estimation range ofthe proposed scheme can be the inverse of the sampling duration. In contrast,conventional schemes with the same estimation range and complexity require two OFDM symbols. To achieve good performance,it will be shown that the null subcarrier allocation should be based uponsome specific binary sequences such as the proposed extended m-sequences. It willalso be shown by simulations that the same null subcarrier allocation criterionshould be employed for optimal maximum-likelihood CFO estimation.
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paper_title: Estimation of Channel Transfer Function and Carrier Frequency Offset for OFDM Systems With Phase Noise

paper_content:

The joint estimation of carrier frequency offset (CFO) and channel transfer function (CTF) for orthogonal frequency-division multiplexing (OFDM) systems with phase noise is discussed in this paper. A CFO estimation algorithm is developed by exploring the time-frequency structure of specially designed training symbols, and it provides a very accurate estimation of the CFO in the presence of both unknown frequency-selective fading and phase noise. Based on the estimated CFO, phase noise and frequency-selective fading are jointly estimated by employing the maximum a posteriori (MAP) criterion. Specifically, the fading channel is estimated in the form of the frequency-domain CTF. The estimation of the CTF eliminates the requirement of a priori knowledge of channel length, and it is simpler compared with the time-domain channel impulse response (CIR) estimation methods used in the literature. Theoretical analysis with the Cramer-Rao lower bound (CRLB) demonstrates that the proposed CFO and CTF estimation algorithms can achieve near-optimum performance.
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paper_title: Maximum Likelihood Timing and Carrier Frequency Offset Estimation for OFDM systems with Periodic Preambles

paper_content:

Timing offset (TO) and carrier frequency offset (CFO) are two common problems observed in OFDM systems. Maximum likelihood (ML) methods have been proposed for CFO estimation. The conventional ML estimates consider a pilot-assisted system, in which two identical preambles are repeatedly transmitted. In some applications, the preambles of an OFDM system are repeated more than two times. A typical example is the preambles of the IEEE802.11a/g wireless local area network (LAN) system. A ML CFO estimation method for the system was proposed recently; however, the method has to solve the roots of the derivative of the likelihood function, and the computational complexity is high. Also, the TO estimation problem is not considered. In this paper, new methods are developed for the TO and CFO estimation problems. Our method is simple and does not rely on the root-finding approach. Simulations show that while the performance of the proposed method is either equal to or better than the existing method, the computational complexity is much lower.
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paper_title: Joint estimation of channel response, frequency offset, and phase noise in OFDM

paper_content:

Accurate channel estimates are needed in orthogonal frequency-division multiplexing (OFDM), and easily obtained under the assumption of perfect phase and frequency synchronization. However, the practical receiver encounters nonnegligible phase noise (PHN) and carrier frequency offset (CFO), which create substantial intercarrier interference that a conventional OFDM channel estimator cannot account for. In this paper, we introduce an optimal (maximum a posteriori) joint estimator for the channel impulse response (CIR), CFO, and PHN, utilizing prior statistical knowledge of PHN that can be obtained from measurements or data sheets. In addition, in cases where a training symbol consists of two identical halves in the time domain, we propose a variant to Moose's CFO estimation algorithm that optimally removes the effect of PHN with lower complexity than with a nonrepeating training symbol. To further reduce the complexity of the proposed algorithms, simplified implementations based on the conjugate gradient method are also introduced such that the estimators studied in this paper can be realized efficiently using the fast Fourier transform with only minor performance degradation. EDICS: SPC-MULT, SPC-CEST, SPC-DETC
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paper_title: RLS-Based Estimation and Tracking of Frequency Offset and Channel Coefficients in MIMO-OFDM Systems

paper_content:

In this paper, we investigate the problem of carrier frequency-offset (CFO) synchronization and channel estimation in multiple-input multiple-output orthogonal frequency-division multiplexing systems operating over unknown frequency-selective fading channels. We first propose a novel joint CFO and channel estimator, assuming time-domain training blocks are available. The proposed joint estimator consists of two recursive least-square (RLS) algorithms which iterate their estimated CFO and CIR values. We then derive a more precise pilot-aided RLS algorithm to estimate the residual frequency synchronization errors or track small CFO changes. With this, the accuracy of channel estimation is also enhanced. The analysis and simulation results show that, the proposed estimation and tracking scheme which is fully compatible with the existing standards is able to attain fast convergence, high stability, and ideal performances as compared with relevant Cramer---Rao bounds in all ranges of signal-to-noise ratio. Moreover, it can work well for wide tracking range up to ±0.5 of the subcarrier spacing.
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paper_title: Doppler frequency offset estimation and diversity reception scheme of high-speed railway with multiple antennas on separated carriage

paper_content:

The challenges of severe Doppler effects in high-speed railway are considered. By building a cooperative antenna system, an algorithm of joint channel estimation and Doppler frequency offset (DFO) estimation is proposed based on Ricean channel model. First, a maximum likelihood estimation (MLE) algorithm for DFO is designed, showing that the Doppler estimation can be obtained by estimating moving velocity of the train and the path loss with the exploitation of pilots that are placed inside the frame. Then a joint detection algorithm for the receiver is proposed to exploit multi-antenna diversity gains. Last, the theoretical Crammer Rao bound (CRB) for joint channel estimation and DFO estimation is derived. The steady performance of the system is confirmed by numerical simulations. In particular, when the Ricean fading channel parameter equals 5 and the velocities of train are 100 m/s and 150 m/s, the estimation variances of DFO are very close to the theoretical results obtained by using CRB. Meanwhile, the corresponding signal to noise ratio loss is less than 1.5 dB when the bit error rate is 10−5 for 16QAM signals.
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paper_title: Joint weighted least-squares estimation of carrier-frequency offset and timing offset for OFDM systems over multipath fading channels

paper_content:

This work presents an algorithm for joint estimation of carrier-frequency offset and timing offset for orthogonal frequency-division multiplexing (OFDM) systems in the tracking mode. The proposed weighted least-squares algorithm derives its estimates based on phase differences in the received pilot subcarrier signals between two symbols. Moreover, the optimal weights in two different channel conditions are derived. Both analysis and simulation show that the weighted least-squares algorithm can effectively and accurately estimate the carrier-frequency offset as well as the timing offset of OFDM signals in multipath fading channels.
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paper_title: Low-complexity Kalman filter-based carrier frequency offset estimation and tracking for OFDM systems

paper_content:

In this paper, an iterative blind estimator for fractional carrier frequency offset (CFO) in orthogonal frequency division multiplexing (OFDM) systems is proposed. The estimator utilizes the null subcarriers transmitted at the edge of the spectrum and does not require any training. In addition, the proposed estimator does not require any prior knowledge of the frequency response of the channel. The problem is formulated using a state-space model, and an extended Kalman filter (EKF) is employed to estimate the CFO iteratively. Simulation results illustrate the enhanced ability of the proposed algorithm, relative to the existing approaches, to estimate and track the CFO even in the presence of high Doppler.
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paper_title: Class of cyclic-based estimators for frequency-offset estimation of OFDM systems

paper_content:

We present a new class of blind cyclic-based estimators for carrier frequency-offset and symbol-timing error estimation of orthogonal frequency-division multiplexing (OFDM) systems. The proposed approach exploits the properties of the cyclic prefix subset to reveal the synchronization parameters in the likelihood function of the received vector. A new likelihood function for the joint timing and frequency-offset estimation is derived, which globally characterizes the estimation problem. The resulting probabilistic measure is used to develop three classes of unbiased estimators, namely, maximum-likelihood, minimum variance unbiased, and moment estimator. In comparison to the previously proposed methods, the proposed estimators in this study are computationally and statistically efficient, which makes the estimators more attractive for real-time applications. The performance of the estimators is assessed by simulation for an OFDM system.
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paper_title: Recursive maximum likelihood estimation of time-varying carrier frequency offset for orthogonal frequency-division multiplexing systems

paper_content:

A recursive maximum likelihood carrier frequency offset (CFO) estimator is proposed in this work, where redundancy information contained in the cyclic prefix of multiple consecutive orthogonal frequency-division multiplexing (OFDM) symbols is exploited in an efficient recursive fashion. Because the estimator is based on multiple OFDM symbols, the time-varying CFO must be considered. We investigate the effect of time-varying CFO on the performance of the estimator and the trade-off between fast tracking ability and low estimation variance. We show that, without channel noise, the mean squared error (MSE) of estimation due to CFO estimation variation increases approximately quadratically with n, where n is the number of OFDM symbols used for CFO estimation (estimation window size), whereas the MSE due to channel noise decreases proportionally to 1/n (approximately) if the CFO is constant. A closed-form expression of the optimal estimation window size (approximately) is derived by minimizing the MSE caused by both time-varying CFO and channel noise. For wireless systems with time-varying rate of change for CFO, the proposed estimator can be implemented adaptively. In addition, typical optimal estimation window sizes for WiMAX, DVB-SH and MediaFLO systems are evaluated as an example. Copyright © 2011 John Wiley & Sons, Ltd.
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paper_title: Joint Frequency Offset and Channel Estimation for OFDM Systems Using Pilot Symbols and Virtual Carriers

paper_content:

We consider joint estimation of carrier frequency offset and channel impulse response (CIR) for orthogonal frequency division multiplexing (OFDM) systems with pilot symbols and virtual subcarriers (VCs). We derive the receive-signal correlation structure due to the pilots and VCs, give the evidence of joint multivariate Gaussian distribution of the received samples, and derive an approximate maximum likelihood (ML) frequency offset estimator. We also derive the asymptotic mean-square error (MSE) and an approximate Cramer-Rao bound (CRB) and establish the asymptotic unbiasedness. Without pilots, in high signal-to-noise ratio, our estimator is equivalent to Liu and Tureli's estimator with Nv virtual carriers. When the pilot number (Np) is greater than the channel length L, our estimator acts as a subspace-based estimator with Nv + Np - L virtual carriers. A decision-directed joint ML estimator is derived to iteratively update the estimates of frequency offset, data symbols and CIR. The optimal pilot and virtual carrier placement strategies are also discussed. The resulting decision-directed joint estimator performs within 0.8 dB of the ideal case even when the frequency offset is as large as 20%
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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.
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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.
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paper_title: ML estimation of time and frequency offset in OFDM systems

paper_content:

We present the joint maximum likelihood (ML) symbol-time and carrier-frequency offset estimator in orthogonal frequency-division multiplexing (OFDM) systems. Redundant information contained within the cyclic prefix enables this estimation without additional pilots. Simulations show that the frequency estimator may be used in a tracking mode and the time estimator in an acquisition mode.
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paper_title: A Consistent OFDM Carrier Frequency Offset Estimator Based on Distinctively Spaced Pilot Tones

paper_content:

A pilot-tone-based maximum likelihood estimator (PBMLE) for carrier frequency offset (CFO) in orthogonal frequency-division multiplexing (OFDM) systems is proposed. To obtain a consistent estimate of the CFO over a frequency-selective fading channel, the proposed method employs a preamble comprising distinctively spaced pilot tones. As a result of this preamble configuration, a large estimation range equal to the bandwidth of the OFDM signal can be achieved. Different from previous ad hoc pilot-tone-based CFO estimators, the PBMLE exploits the relationship between the CFO and the periodogram of the preamble. Analysis shows that the proposed PBMLE is asymptotically unbiased and efficient. To realize this PBMLE in practice, a suboptimal estimator is also introduced, in which a zero-padded fast Fourier transform is invoked and the CFO estimation is split into two phases: coarse and fine estimation. Coarse estimation is obtained through the correlation between the received preamble and its original pattern, whereas fine estimation is obtained by exploiting the magnitude attenuation in the vicinities of those CFO-shifted pilot tones. Both analytical investigations and computer simulations indicate that the accuracy of this simplified suboptimal estimator is proportional to the oversize ratio of zero padding. When the oversize ratio is sufficiently high, the performance of the suboptimal estimator approaches that of the proposed PBMLE.
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paper_title: Bounding Performance and Suppressing Intercarrier Interference in Wireless Mobile OFDM

paper_content:

While rapid variations of the fading channel cause intercarrier interference (ICI) in orthogonal frequency-division multiplexing (OFDM), thereby degrading its performance considerably, they also introduce temporal diversity, which can be exploited to improve performance. We first derive a matched-filter bound (MFB) for OFDM transmissions over doubly selective Rayleigh fading channels, which benchmarks the best possible performance if ICI is completely canceled without noise enhancement. We then derive universal performance bounds which show that the time-varying channel causes most of the symbol energy to be distributed over a few subcarriers, and that the ICI power on a subcarrier mainly comes from several neighboring subcarriers. Based on this fact, we develop low-complexity minimum mean-square error (MMSE) and decision-feedback equalizer (DFE) receivers for ICI suppression. Simulations show that the DFE receiver can collect significant gains of ICI-impaired OFDM with affordable complexity. In the relatively low Doppler frequency region, the bit-error rate of the DFE receiver is close to the MFB.
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paper_title: Oversampled Orthogonal Frequency Division Multiplexing in Doubly Selective Fading Channels

paper_content:

A new oversampled orthogonal frequency division multiplexing (OFDM) system operating in doubly selective fading environment is proposed in this paper. The oversampled OFDM structure employs oversampling in time domain and linear signal processing in frequency domain. The time-frequency processing enables a two dimensional Doppler-frequency grid that allows each data symbol be transmitted at multiple subcarriers and various Doppler spreads simultaneously, without sacrificing spectral efficiency. As a result, both Doppler diversity and multipath diversity inherent in doubly selective fading are achieved with simple linear operations. A Doppler domain block decision feedback equalizer is designed to suppress the intercarrier interference (ICI) caused by time varying fading, as well as to collect the diversity benefits enabled by the oversampled OFDM system. Simulation results show that the new system outperforms conventional OFDM system with the same spectral efficiency by as much as 8 dB.
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paper_title: Adaptive channel estimation and equalization for rapidly mobile communication channels

paper_content:

This paper presents a reduced-complexity maximum-likelihood sequence estimation receiver, based on the Viterbi algorithm (VA), suitable for rapidly fading mobile communication channels. The channel impulse response is expanded onto a set of basis sequences and time-invariant (TI) expansion parameters. The proposed receiver continuously estimates the TI expansion parameters directly within the metric calculation of the VA. At every time instant of the VA, the accumulated metrics of the survivor paths are compared, and the survivors whose metrics are lower than the average value of the accumulated survivor metrics are retained. The other survivors are discarded from the trellis. Then the sequences associated with the minimum survivor of the retained survivors are used to update the estimate of the TI expansion parameters. The convergence properties of the estimation algorithm are investigated, and the steady-state value of the mean square error of estimation is derived. The performance of the proposed receiver is evaluated in terms of the symbol-error probability and compared with other receivers. The effects of time offset and frequency offset on the performance of the receiver are studied.
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paper_title: Low-Complexity Block Turbo Equalization for OFDM Systems in Time-Varying Channels

paper_content:

We propose low-complexity block turbo equalizers for orthogonal frequency-division multiplexing (OFDM) systems in time-varying channels. The presented work is based on a soft minimum mean-squared error (MMSE) block linear equalizer (BLE) that exploits the banded structure of the frequency-domain channel matrix, as well as a receiver window that enforces this banded structure. This equalization approach allows us to implement the proposed designs with a complexity that is only linear in the number of subcarriers. Three block turbo equalizers are discussed: two are based on a biased MMSE criterion, while the third is based on the unbiased MMSE criterion. Simulation results show that the proposed iterative MMSE BLE achieves a better bit error rate (BER) performance than a previously proposed iterative MMSE serial linear equalizer (SLE). The proposed equalization algorithms are also tested in the presence of channel estimation errors.
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paper_title: Low-complexity equalization of OFDM in doubly selective channels

paper_content:

Orthogonal frequency division multiplexing (OFDM) systems may experience significant inter-carrier interference (ICI) when used in time- and frequency-selective, or doubly selective, channels. In such cases, the classical symbol estimation schemes, e.g., minimum mean-squared error (MMSE) and zero-forcing (ZF) estimation, require matrix inversion that is prohibitively complex for large symbol lengths. An analysis of the ICI generation mechanism leads us to propose a novel two-stage equalizer whose complexity (apart from the FFT) is linear in the OFDM symbol length. The first stage applies optimal linear preprocessing to restrict ICI support, and the second stage uses iterative MMSE estimation to estimate finite-alphabet frequency-domain symbols. Simulation results indicate that our equalizer has significant performance and complexity advantages over the classical linear MMSE estimator in doubly selective channels.
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paper_title: Iterative interference cancellation and channel estimation for mobile OFDM

paper_content:

In mobile reception, the reliability of orthogonal frequency division multiplexing (OFDM) is limited because of the time-varying nature of the channel. This causes intercarrier interference (ICI) and increases inaccuracies in channel tracking. We model the ICI using derivatives of the channel amplitude. This allows us to design a relatively simple receiver scheme that iteratively cancels the ICI. The design of the canceler aims at maximizing the signal-to-noise-plus-ICI ratio at the detector input. We also propose a new channel estimator, and we show that it achieves reliable mobile reception in practical situations that are relevant to terrestrial Digital Video Broadcasting (DVB-T). Extensive simulations for a receiver with one or two antennas show that a small number of iterations between ICI cancellation and channel estimation allow a reliable reception at vehicle speeds above 100 km/h.
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paper_title: Bounding Performance and Suppressing Intercarrier Interference in Wireless Mobile OFDM

paper_content:

While rapid variations of the fading channel cause intercarrier interference (ICI) in orthogonal frequency-division multiplexing (OFDM), thereby degrading its performance considerably, they also introduce temporal diversity, which can be exploited to improve performance. We first derive a matched-filter bound (MFB) for OFDM transmissions over doubly selective Rayleigh fading channels, which benchmarks the best possible performance if ICI is completely canceled without noise enhancement. We then derive universal performance bounds which show that the time-varying channel causes most of the symbol energy to be distributed over a few subcarriers, and that the ICI power on a subcarrier mainly comes from several neighboring subcarriers. Based on this fact, we develop low-complexity minimum mean-square error (MMSE) and decision-feedback equalizer (DFE) receivers for ICI suppression. Simulations show that the DFE receiver can collect significant gains of ICI-impaired OFDM with affordable complexity. In the relatively low Doppler frequency region, the bit-error rate of the DFE receiver is close to the MFB.
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paper_title: On the Diversity Order of BICM-OFDM in Doubly Selective Fading Channels

paper_content:

Bit-interleaved coded modulation with orthogonal frequency-division multiplexing (BICM-OFDM) is an attractive approach to achieve time and frequency diversity. Remarkable diversity gain can be obtained when the channel is doubly selective fading. In this paper, the asymptotic diversity orders of BICM-OFDM systems in doubly selective fading channels for both single-input–single-output (SISO) and multiple-input–multiple-output (MIMO) cases are derived. In addition, the system bit-error-rate (BER) behavior in practical situations with moderate signal-to-noise ratios (SNRs) is also investigated. In the SISO case, the diversity order depends on the rank of the channel correlation matrix. Therefore, the channel variations induced by fast fading contribute to improving diversity. In the MIMO case, the diversity order can be further increased when factors such as cyclic delays or phase rolls are introduced. Simulations are provided to verify the analysis.
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paper_title: Low-Complexity Iterative Equalization for Symbol-Reconstruction-Based OFDM Receivers Over Doubly Selective Channels

paper_content:

Time selectivity of multipath channels introduces significant inter-carrier interference (ICI) in OFDM systems demanding high levels of mobility and capacity. In this paper, a piecewise channel approximation is presented for improving the structure of the frequency-domain channel gain matrix, leading to a low-complexity iterative equalization scheme for OFDM receivers. The equalization is a sequential data detection based on a parallel-filtering architecture, avoiding matrix inversion adopted by classical equalizers with ICI cancellation. Meanwhile, information redundancy in the traditional cyclically prefixed OFDM is investigated for time-invariant channels, then a method of symbol reconstruction (SR) is proposed to enhance OFDM system performance, including compensation for SNR loss and improvements on the accuracy of parameter estimation. On the basis of the appropriate channel modeling in this work, the SR processing is proved to be effective for OFDM receivers over doubly selective channels. The iterative equalization, combined with the SR processing, is applied to the DVB-H receiver design. Numerical simulation indicates that the proposed equalizer significantly improves the immunity of OFDM systems to time selectivity with very little complexity increased, in contrast to the conventional one-tap equalizer without ICI cancellation.
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paper_title: Joint Doppler-Frequency Diversity for OFDM Systems Using Hybrid Interference Cancellation in Time-Varying Multipath Fading Channels

paper_content:

In time-varying fading channels, orthogonal frequency division multiplexing systems suffer severe performance degradation caused by interchannel interference (ICI). We utilize this ICI as a source of diversity by using a new ICI-cancellation algorithm and Wiener filter. The proposed algorithm can support joint Doppler-frequency diversity gain. In addition, both the computational complexity and total detection time are reduced.
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paper_title: Oversampled Orthogonal Frequency Division Multiplexing in Doubly Selective Fading Channels

paper_content:

A new oversampled orthogonal frequency division multiplexing (OFDM) system operating in doubly selective fading environment is proposed in this paper. The oversampled OFDM structure employs oversampling in time domain and linear signal processing in frequency domain. The time-frequency processing enables a two dimensional Doppler-frequency grid that allows each data symbol be transmitted at multiple subcarriers and various Doppler spreads simultaneously, without sacrificing spectral efficiency. As a result, both Doppler diversity and multipath diversity inherent in doubly selective fading are achieved with simple linear operations. A Doppler domain block decision feedback equalizer is designed to suppress the intercarrier interference (ICI) caused by time varying fading, as well as to collect the diversity benefits enabled by the oversampled OFDM system. Simulation results show that the new system outperforms conventional OFDM system with the same spectral efficiency by as much as 8 dB.
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paper_title: Low-Complexity ICI Mitigation Methods for High-Mobility SISO/MIMO-OFDM Systems

paper_content:

In orthogonal frequency-division multiplexing (OFDM) systems, it is generally assumed that the channel response is static in an OFDM symbol period. However, the assumption does not hold in high-mobility environments. As a result, intercarrier interference (ICI) is induced, and system performance is degraded. A simple remedy for this problem is the application of the zero-forcing (ZF) equalizer. Unfortunately, the direct ZF method requires the inversion of an N times N ICI matrix, where N is the number of subcarriers. When N is large, the computational complexity can become prohibitively high. In this paper, we first propose a low-complexity ZF method to solve the problem in single-input-single-output (SISO) OFDM systems. The main idea is to explore the special structure inherent in the ICI matrix and apply Newton's iteration for matrix inversion. With our formulation, fast Fourier transforms (FFTs) can be used in the iterative process, reducing the complexity from O (N3) to O (N log2 N). Another feature of the proposed algorithm is that it can converge very fast, typically in one or two iterations. We also analyze the convergence behavior of the proposed method and derive the theoretical output signal-to-interference-plus-noise ratio (SINR). For a multiple-input-multiple-output (MIMO) OFDM system, the complexity of the ZF method becomes more intractable. We then extend the method proposed for SISO-OFDM systems to MIMO-OFDM systems. It can be shown that the computational complexity can be reduced even more significantly. Simulations show that the proposed methods perform almost as well as the direct ZF method, while the required computational complexity is reduced dramatically.
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paper_title: EM-Based Iterative Receivers for OFDM and BICM/OFDM Systems in Doubly Selective Channels

paper_content:

In this paper, we resort to the expectation-maximization (EM) algorithm to tackle the inter-carrier interference (ICI) problem, caused by time-variant multipath channels, for both orthogonal frequency division multiplexing (OFDM) systems and bit-interleaved coded modulation (BICM)/OFDM systems. We first analyze the ICI in frequency domain with a reduced set of parameters, and following this analysis, we derive an EM algorithm for maximum likelihood (ML) data detection. An ML-EM receiver for OFDM systems and a TURBO-EM receiver for BICM/OFDM systems are then developed to reduce computational complexity of the EM algorithm and to exploit temporal diversity, the main idea of which is to integrate the proposed EM algorithm with a groupwise ICI cancellation method. Compared with the ML-EM receiver, the TURBO-EM receiver further employs a soft-output Viterbi algorithm (SOVA) decoder to exchange information with a maximum a posteriori (MAP) EM detector through the turbo principle. Computer simulation demonstrates that the two proposed receivers clearly outperform the conventional one-tap equalizer, and the performance of the TURBO-EM receiver is close to the matched-filter bound even at a normalized maximum Doppler frequency (MDF) up to 0.2.
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paper_title: Iterative frequency-domain channel estimation and equalization for single-carrier transmissions without cyclic-prefix

paper_content:

Compared to conventional time-domain equalization, frequency-domain equalization (FDE) presents a computationally efficient alternative for the reception of single carrier (SC) transmissions. In this paper, we consider iterative FDE (IFDE) with explicit frequency-domain channel estimation (FDCE) for non-cyclic-prefixed SC systems. First, an improved IFDE algorithm is presented based on soft iterative interference cancellation. Second, a new adaptive FDCE (AFDCE) algorithm based on per-tone Kalman filtering is proposed to track and predict the frequency-domain channel coefficients. The AFDCE algorithm employs across-tone noise reduction, exploits temporal correlation between successive blocks, and adaptively updates the auto-regressive model coefficients, bypassing the need for prior knowledge of channel statistics. Finally, block-overlapping is used to facilitate the joint operation of IFDE and AFDCE. Simulation results show that, compared to related IFDE and adaptive channel estimation schemes, the proposed schemes offer lower mean-square error (MSE) in channel prediction, lower bit error rate (BER) after decoding, and robustness to non-stationary channels.
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paper_title: Robust OFDM receivers for dispersive time-varying channels: equalization and channel acquisition

paper_content:

In orthogonal frequency-division multiplexing, time variations of a multipath channel lead to a loss of orthogonality between the subcarriers, and thereby limit the achievable throughput. This paper proposes a general framework for a controlled removal of intercarrier interference (ICI) and channel acquisition. The core idea behind our method is to use a finite power series expansion for the time-varying frequency response, along with the known statistical properties of mobile radio channels. Channel acquisition and ICI removal are accomplished in the frequency domain and allow for any desired tradeoff between the residual ICI level, the required training for channel acquisition, and processing complexity. The proposed approach enables a high spectral efficiency (64-quadrature amplitude modulation mode) of digital video broadcasting-terrestrial in highly mobile environments.
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paper_title: Low-Complexity MAP-Based Successive Data Detection for Coded OFDM Systems Over Highly Mobile Wireless Channels

paper_content:

This paper is concerned with the challenging and timely problem of data detection for coded orthogonal frequency-division multiplexing (OFDM) systems in the presence of frequency-selective and very rapidly time varying channels. New low-complexity maximum a posteriori probability (MAP) data detection algorithms are proposed based on sequential detection with optimal ordering (SDOO) and sequential detection with successive cancellation (SDSC). The received signal vector is optimally decomposed into reduced dimensional subobservations by exploiting the banded structure of the frequency-domain channel matrix whose bandwidth is a parameter to be adjusted according to the speed of the mobile terminal. The data symbols are then detected by the proposed algorithms in a computationally efficient way by means of the Markov chain Monte Carlo (MCMC) technique with Gibbs sampling. The impact of the imperfect channel state information (CSI) on the bit error rate (BER) performance of these algorithms is investigated analytically and by computer simulations. A detailed computational complexity investigation and simulation results indicate that, particularly, the algorithm based on SDSC has significant performance and complexity advantages and is very robust against channel estimation errors compared with existing suboptimal detection and equalization algorithms proposed earlier in the literature.
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paper_title: MLSE and MAP Equalization for Transmission Over Doubly Selective Channels

paper_content:

In this paper, equalization for transmission over doubly selective channels is discussed. The symbol-by-symbol maximum a posteriori probability (MAP) equalizer and the maximum-likelihood sequence estimation (MLSE) are discussed. The doubly selective channel is modeled using the basis expansion model (BEM). Using the BEM allows for an easy and low-complexity mechanism for constructing the channel trellis to implement the MLSE and the MAP equalizer. The MLSE and the MAP equalizer are implemented for single-carrier transmission and for multicarrier transmission implemented using orthogonal frequency-division multiplexing (OFDM). In this scenario, a complexity-diversity tradeoff can be observed. In addition, we propose a joint estimation and equalization technique for doubly selective channels. In this joint estimation and equalization technique, the channel state information (CSI) is obtained in an iterative manner. Simulation results show that the performance of the joint channel estimation and equalization approaches the performance when perfect CSI is available at the receiver.
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paper_title: Robust and Efficient Intercarrier Interference Mitigation for OFDM Systems in Time-Varying Fading Channels

paper_content:

Due to its spectral efficiency and robustness over multipath channels, orthogonal frequency-division multiplexing (OFDM) has served as one of the major modulation schemes for high-speed communication systems. In the future, wireless OFDM systems are expected to operate at high carrier frequencies, high speed, and high throughput for mobile reception, where fast time-varying fading channels are encountered. Channel variation destroys the orthogonality among the subcarriers and leads to intercarrier interference (ICI). ICI poses a significant limitation on wireless OFDM systems. The aim of this paper is to find an efficient method of providing reliable communications using OFDM in fast time-varying fading channels. It is observed that ICI power arises from a few adjacent subcarriers. This observation motivates us to design low-complexity -tap ICI equalizers. To employ these equalizers, channel state information is also required. In this paper, we also design a pilot-aided minimum mean square error (MMSE) channel estimation scheme for a time-varying wide-sense stationary uncorrelated scatters channel model. The MMSE channel estimator utilizes the statistical channel properties to achieve computational efficiency. Simulation results show that our proposed low-complexity ICI suppression scheme, which incorporates the -tap equalizer with the MMSE channel estimator, can significantly improve the performance of OFDM systems in fast time-varying fading channels.
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paper_title: Bit-interleaved time-frequency coded modulation for OFDM systems over time-varying channels

paper_content:

Orthogonal frequency-division multiplexing (OFDM) is a promising technology in broadband wireless communications, with its ability to transform a frequency-selective fading channel into multiple flat-fading channels. However, the time-varying characteristics of wireless channels induce the loss of orthogonality among OFDM subcarriers, which was generally considered harmful to system performance. In this paper, we propose a bit-interleaved time-frequency coded modulation (BITFCM) scheme for OFDM to achieve both the time and frequency diversity inherent in broadband time-varying channels. We will show that the time-varying characteristics of the channel are beneficial to system performance. Using the BITFCM scheme, and for relatively low maximum normalized Doppler frequency, a reduced-complexity maximum-likelihood decoding approach is proposed to achieve good performance with low complexity. For high maximum normalized Doppler frequency, the intercarrier interference (ICI) can be large, and an error floor will be induced. To solve this problem, we propose two ICI-mitigation schemes by taking advantage of the second-order channel statistics and the complete channel information, respectively. It will be shown that both schemes can reduce the ICI significantly.
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paper_title: ICI mitigation and diversity gain for OFDM systems in time‐varying multipath fading channels

paper_content:

In orthogonal frequency division multiplexing (OFDM) systems, channels' rapid variations cause intercarrier interference (ICI), resulting in performance degradation. Although ICI caused by channels' variations makes the reliable signal detection difficult, it also introduces Doppler diversity, which can be exploited to improve the system performance. In this paper, an ICI mitigation and Doppler-multipath diversity technique based on fractionally sampling at the receiver is proposed for OFDM systems over time-varying multipath fading channels. To reduce the computation burden of minimum mean-square error (MMSE) equaliser with successive detection (SD), an efficient algorithm is presented by using the property of matrix decomposition. The algorithm avoids direct or recursive computation of matrix inversion, thereby reducing the computation complexity and ensuring numerical stability. Simulations show that the proposed techniques can collect both Doppler and multipath diversity gains with moderate complexities. Copyright © 2010 John Wiley & Sons, Ltd.
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paper_title: Mobility adaptation in OFDM systems over rapidly time-varying fading channels

paper_content:

Due to the rapidly time-varying nature of wireless channels, it is not possible to obtain accurate instantaneous channel state information (CSI). However, predicting and obtaining the velocity of the moving terminals is not difficult, where the velocity (mobility) can be considered as a large scale metric. A new concept called mobility adaptation is proposed for OFDM systems under rapidly time-varying channels to combat intercarrier interference (ICI) by adjusting system parameters based on the estimated velocity of the terminals, without need to know instantaneous CSI. As examples of mobility adaptation, two kinds of adaptation based on velocity are presented. Our analysis and simulation results show that the proposed mobility schemes based on velocity are quite effective to improve the performance of OFDM systems over rapidly time-varying fading channels.
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paper_title: Variable-Rate Variable-Power MQAM for Fading Channels

paper_content:

We propose a variable-rate and variable-power MQAM modulation scheme for high-speed data transmission over fading channels. We first review results for the Shannon capacity of fading channels with channel side information, where capacity is achieved using adaptive transmission techniques. We then derive the spectral efficiency of our proposed modulation. We show that there is a constant power gap between the spectral efficiency of our proposed technique and the channel capacity, and this gap is a simple function of the required bit-error rate (BER). In addition, using just five or six different signal constellations, we achieve within 1-2 dB of the maximum efficiency using unrestricted constellation sets. We compute the rate at which the transmitter needs to update its power and rate as a function of the channel Doppler frequency for these constellation sets. We also obtain the exact efficiency loss for smaller constellation sets, which may be required if the transmitter adaptation rate is constrained by hardware limitations. Our modulation scheme exhibits a 5-10-dB power gain relative to variable-power fixed-rate transmission, and up to 20 dB of gain relative to nonadaptive transmission. We also determine the effect of channel estimation error and delay on the BER performance of our adaptive scheme. We conclude with a discussion of coding techniques and the relationship between our proposed modulation and Shannon capacity.
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paper_title: On the Spectral Efficiency of Rate and Subcarrier Bandwidth Adaptive OFDM Systems Over Very Fast Fading Channels

paper_content:

In this paper, rate and subcarrier bandwidth adaptation (RSBA) on the average spectral efficiency of orthogonal frequency-division multiplexing (OFDM) systems using $M$ -ary quadrature amplitude modulation ( $M$ -QAM) is investigated in the presence of frequency-selective and very rapidly time-varying fading channels. The closed-form expressions for rate and capacity with perfect channel state information (CSI) and imperfect CSI are derived. In this paper, imperfect CSI is caused jointly by channel estimation error and unavoidable delay between when CSI is estimated and when the estimation results are used for actual transmission. The capacity with perfect CSI is just a special case of imperfect CSI. Based on the closed-form expressions of the maximum capacity, optimal RSBA algorithms with perfect and imperfect CSI are proposed, respectively. Theoretical and numerical results show that the RSBA scheme can improve the system performance effectively under the perfect-CSI case. It can make a tradeoff between intercarrier interference (ICI) and transmission efficiency by considering the cyclic prefix. The results also show that the RSBA scheme is effective in repelling the impact of outdated CSI, as well as in reducing the impact of channel estimation error, but to a lesser extent.
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paper_title: Power Adaptation in OFDM Systems Based on Velocity Variation Under Rapidly Time-Varying Channels

paper_content:

A novel power adaptation scheme, based on the terminal velocity instead of channel state information (CSI), in OFDM systems over rapidly time-varying channels, is proposed. Simulation results show that the proposed power adaptation scheme based on the velocity variation is effective for improving the performance of OFDM systems in fast fading channels.
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paper_title: On channel estimation and detection for multicarrier signals in fast and selective Rayleigh fading channels

paper_content:

Time-domain channel estimation and detection techniques are presented for multicarrier signals in a fast and frequency-selective Rayleigh fading channel. As a consequence of the time-varying channel, the orthogonality between subcarriers is destroyed in conventional frequency-domain approaches, resulting in interchannel interference, which increases an irreducible error floor in proportion to the normalized Doppler frequency. An important feature of the proposed technique is the ability to exploit the time-selective channel as a provider of time diversity. This enables us to achieve performance superior to any other structure without increasing bandwidth or incorporating redundancy, in order to reduce the complexity of the estimator, we apply the theory of optimal low rank approximation to a minimum mean squared error channel estimator and present a theoretical calculation of mean squared error and simulations to confirm that the estimator is robust to changes in channel characteristics.
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paper_title: Wireless Communications Over Rapidly Time-Varying Channels

paper_content:

As a result of higher frequencies and increased user mobility, researchers and systems designers are shifting their focus from time-invariant models to channels that vary within a block. This book explains the latest theoretical advances and practical methods to give an understanding of rapidly time varying channels, together with performance trade-offs and potential performance gains, providing the expertise to develop future wireless systems technology. As well as an overview of the issues of developing wireless systems using time-varying channels, the book gives extensive coverage to methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, as well as providing models and transceiver methods for time-varying ultra-wideband channels. *An introduction to time-varying channel models gives in a nutshell the important issues of developing wireless systems technology using time-varying channels*Extensive coverage of methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, enables development of high performance wireless systems*Chapters on transceiver design for OFDM and receiver algorithms for MIMO communication channels over time-varying channels, with an emphasis on modern iterative turbo-style architectures, demonstrates how these important technologies can optimize future wireless systems
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paper_title: Capacity bounds for noncoherent fading channels with a peak constraint

paper_content:

A discrete-time single-user channel with temporally correlated Rayleigh fading is considered. Neither the transmitter nor the receiver has channel side information (CSI), and both peak and average power constraints are placed on the inputs. Two lower bounds to the capacity are presented. One is motivated by the technique of decision feedback decoding. The other is related to the information rate with side information present, minus a penalty term to account for the information about the channel that is learned at the receiver. The second lower bound is a slight variation of a bound of Shamai and Marzetta. The bounds are compared numerically to two upper bounds for a channel with Gauss Markov Rayleigh fading. One upper bound is the capacity for complete CSI, with the peak constraint ignored, and the other is based on the capacity per unit energy. In general, the gap between the upper and lower bounds depends on the channel memory, but is quite small for low SNR
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paper_title: Capacity of a Mobile Multiple-Antenna Communication Link in Rayleigh Flat Fading

paper_content:

We analyze a mobile wireless link comprising M transmitter and N receiver antennas operating in a Rayleigh flat-fading environment. The propagation coefficients between pairs of transmitter and receiver antennas are statistically independent and unknown; they remain constant for a coherence interval of T symbol periods, after which they change to new independent values which they maintain for another T symbol periods, and so on. Computing the link capacity, associated with channel coding over multiple fading intervals, requires an optimization over the joint density of T/spl middot/M complex transmitted signals. We prove that there is no point in making the number of transmitter antennas greater than the length of the coherence interval: the capacity for M>T is equal to the capacity for M=T. Capacity is achieved when the T/spl times/M transmitted signal matrix is equal to the product of two statistically independent matrices: a T/spl times/T isotropically distributed unitary matrix times a certain T/spl times/M random matrix that is diagonal, real, and nonnegative. This result enables us to determine capacity for many interesting cases. We conclude that, for a fixed number of antennas, as the length of the coherence interval increases, the capacity approaches the capacity obtained as if the receiver knew the propagation coefficients.
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paper_title: Oversampling Increases the Pre-Log of Noncoherent Rayleigh Fading Channels

paper_content:

We analyze the capacity of a continuous-time, time-selective, Rayleigh block-fading channel in the high signal-to-noise ratio (SNR) regime. The fading process is assumed stationary within each block and to change independently from block to block; furthermore, its realizations are not known a priori to the transmitter and the receiver (noncoherent setting). A common approach to analyzing the capacity of this channel is to assume that the receiver performs matched filtering followed by sampling at symbol rate (symbol matched filtering). This yields a discrete-time channel in which each transmitted symbol corresponds to one output sample. Liang&Veeravalli (2004) showed that the capacity of this discrete-time channel grows logarithmically with the SNR, with a capacity pre-log equal to $1-{Q}/{N}$. Here, $N$ is the number of symbols transmitted within one fading block, and $Q$ is the rank of the covariance matrix of the discrete-time channel gains within each fading block. In this paper, we show that symbol matched filtering is not a capacity-achieving strategy for the underlying continuous-time channel. Specifically, we analyze the capacity pre-log of the discrete-time channel obtained by oversampling the continuous-time channel output, i.e., by sampling it faster than at symbol rate. We prove that by oversampling by a factor two one gets a capacity pre-log that is at least as large as $1-1/N$. Since the capacity pre-log corresponding to symbol-rate sampling is $1-Q/N$, our result implies indeed that symbol matched filtering is not capacity achieving at high SNR.
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paper_title: Wireless Communications Over Rapidly Time-Varying Channels

paper_content:

As a result of higher frequencies and increased user mobility, researchers and systems designers are shifting their focus from time-invariant models to channels that vary within a block. This book explains the latest theoretical advances and practical methods to give an understanding of rapidly time varying channels, together with performance trade-offs and potential performance gains, providing the expertise to develop future wireless systems technology. As well as an overview of the issues of developing wireless systems using time-varying channels, the book gives extensive coverage to methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, as well as providing models and transceiver methods for time-varying ultra-wideband channels. *An introduction to time-varying channel models gives in a nutshell the important issues of developing wireless systems technology using time-varying channels*Extensive coverage of methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, enables development of high performance wireless systems*Chapters on transceiver design for OFDM and receiver algorithms for MIMO communication channels over time-varying channels, with an emphasis on modern iterative turbo-style architectures, demonstrates how these important technologies can optimize future wireless systems
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paper_title: Capacity bounds via duality with applications to multiple-antenna systems on flat-fading channels

paper_content:

A technique is proposed for the derivation of upper bounds on channel capacity. It is based on a dual expression for channel capacity where the maximization (of mutual information) over distributions on the channel input alphabet is replaced with a minimization (of average relative entropy) over distributions on the channel output alphabet. We also propose a technique for the analysis of the asymptotic capacity of cost-constrained channels. The technique is based on the observation that under fairly mild conditions capacity achieving input distributions "escape to infinity." The above techniques are applied to multiple-antenna flat-fading channels with memory where the realization of the fading process is unknown at the transmitter and unknown (or only partially known) at the receiver. It is demonstrated that, for high signal-to-noise ratio (SNR), the capacity of such channels typically grows only double-logarithmically in the SNR. To better understand this phenomenon and the rates at which it occurs, we introduce the fading number as the second-order term in the high-SNR asymptotic expansion of capacity, and derive estimates on its value for various systems. It is suggested that at rates that are significantly higher than the fading number, communication becomes extremely power inefficient, thus posing a practical limit on practically achievable rates. Upper and lower bounds on the fading number are also presented. For single-input-single-output (SISO) systems the bounds coincide, thus yielding a complete characterization of the fading number for general stationary and ergodic fading processes. We also demonstrate that for memoryless multiple-input single-output (MISO) channels, the fading number is achievable using beam-forming, and we derive an expression for the optimal beam direction. This direction depends on the fading law and is, in general, not the direction that maximizes the SNR on the induced SISO channel. Using a new closed-form expression for the expectation of the logarithm of a noncentral chi-square distributed random variable we provide some closed-form expressions for the fading number of some systems with Gaussian fading, including SISO systems with circularly symmetric stationary and ergodic Gaussian fading. The fading number of the latter is determined by the fading mean, fading variance, and the mean squared error in predicting the present fading from its past; it is not directly related to the Doppler spread. For the Rayleigh, Ricean, and multiple-antenna Rayleigh-fading channels we also present firm (nonasymptotic) upper and lower bounds on channel capacity. These bounds are asymptotically tight in the sense that their difference from capacity approaches zero at high SNR, and their ratio to capacity approaches one at low SNR.
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paper_title: Capacity of noncoherent time-selective Rayleigh-fading channels

paper_content:

The capacity of noncoherent time-selective Rayleigh-fading channels is studied under various models for the variations in time. The study includes both single-input and single-output (SISO) and multiple-input and multiple-output (MIMO) systems. A block-fading model is first considered where the channel changes correlatively over each block period of length T, and independently across blocks. The predictability of the channel is characterized through the rank Q of the correlation matrix of the vector of channel gains in each block. This model includes, as special cases, the standard block-fading model where the channel remains constant over block periods (Q=1), and models where the fading process has finite differential entropy rate (Q=T). The capacity is initially studied for long block lengths and some straightforward but interesting asymptotes are established. For the case where Q is kept fixed as T/spl rarr//spl infin/, it is shown that the noncoherent capacity converges to the coherent capacity. For the case where both T,Q/spl rarr//spl infin/, with Q/T being held constant, a bound on the capacity loss due to channel unpredictability is established. The more interesting scenario of large signal-to-noise ratio (SNR) is then explored in detail. For SISO systems, useful upper and lower bounds on the large SNR asymptotic capacity are derived, and it is shown that the capacity grows logarithmically with SNR with a slope of T-Q/spl rarr/T, for Q<T. Next, in order to facilitate the analysis of MIMO systems, the rank-Q block-fading model is specialized to the case where each T-symbol block consists of Q subblocks of length L, with the channel remaining constant over each subblock and changing correlatively across subblocks. For this model, it is shown that the log SNR growth behavior of the capacity is the same as that of the standard block-fading model with block length L. Finally, the SISO and MIMO channel models are generalized to allow the fading process to be correlated across blocks in a stationary and ergodic manner. It is shown that the log SNR growth behavior of the capacity is not affected by the correlation across blocks.
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paper_title: Unitary space-time modulation for multiple-antenna communications in Rayleigh flat fading

paper_content:

Motivated by information-theoretic considerations, we propose a signaling scheme, unitary space-time modulation, for multiple-antenna communication links. This modulation is ideally suited for Rayleigh fast-fading environments, since it does not require the receiver to know or learn the propagation coefficients. Unitary space-time modulation uses constellations of T/spl times/M space-time signals (/spl Phi//sub i/, l=1, ..., L), where T represents the coherence interval during which the fading is approximately constant, and M<T is the number of transmitter antennas. The columns of each /spl Phi//sub i/ are orthonormal. When the receiver does not know the propagation coefficients, which between pairs of transmitter and receiver antennas are modeled as statistically independent, this modulation performs very well either when the signal-to-noise ratio (SNR) is high or when T/spl Gt/M. We design some multiple-antenna signal constellations and simulate their effectiveness as measured by bit-error probability with maximum-likelihood decoding. We demonstrate that two antennas have a 6-dB diversity gain over one antenna at 15-dB SNR.
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paper_title: Basis Expansion Models and Diversity Techniques for Blind Identification and Equalization of Time-Varying Channels

paper_content:

The time-varying impulse response of rapidly fading mobile communication channels is expanded over a basis of complex exponentials that arise due to Doppler effects encountered with multipath propagation. Blind methods are reviewed for estimating the bases' parameters and the model orders. Existing second-order methods are critiqued and novel algorithms are developed for blind identification, direct, zero-forcing equalization and minimum mean square error (MMSE) equalization by combining channel diversity with temporal (fractional sampling) and/or spatial diversity which becomes available with multiple receivers. Illustrative simulations are also presented.
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paper_title: On the Capacity of Large-MIMO Block-Fading Channels

paper_content:

We characterize the capacity of Rayleigh block-fading multiple-input multiple-output (MIMO) channels in the noncoherent setting where transmitter and receiver have no a priori knowledge of the realizations of the fading channel. We prove that unitary space-time modulation (USTM) is not capacity-achieving in the high signal-to-noise ratio (SNR) regime when the total number of antennas exceeds the coherence time of the fading channel (expressed in multiples of the symbol duration), a situation that is relevant for MIMO systems with large antenna arrays (large-MIMO systems). This result settles a conjecture by Zheng & Tse (2002) in the affirmative. The capacity-achieving input signal, which we refer to as Beta-variate space-time modulation (BSTM), turns out to be the product of a unitary isotropically distributed random matrix, and a diagonal matrix whose nonzero entries are distributed as the square-root of the eigenvalues of a Beta-distributed random matrix of appropriate size. Numerical results illustrate that using BSTM instead of USTM in large-MIMO systems yields a rate gain as large as 13% for SNR values of practical interest.
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paper_title: Communication on the Grassmann Manifold: A Geometric Approach to the Noncoherent Multiple-Antenna Channel

paper_content:

We study the capacity of multiple-antenna fading channels. We focus on the scenario where the fading coefficients vary quickly; thus an accurate estimation of the coefficients is generally not available to either the transmitter or the receiver. We use a noncoherent block fading model proposed by Marzetta and Hochwald (see ibid. vol.45, p.139-57, 1999). The model does not assume any channel side information at the receiver or at the transmitter, but assumes that the coefficients remain constant for a coherence interval of length T symbol periods. We compute the asymptotic capacity of this channel at high signal-to-noise ratio (SNR) in terms of the coherence time T, the number of transmit antennas M, and the number of receive antennas N. While the capacity gain of the coherent multiple antenna channel is min{M, N} bits per second per Hertz for every 3-dB increase in SNR, the corresponding gain for the noncoherent channel turns out to be M* (1 - M*/T) bits per second per Hertz, where M*=min{M, N, [T/2]}. The capacity expression has a geometric interpretation as sphere packing in the Grassmann manifold.
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paper_title: Noncoherent Capacity of Underspread Fading Channels

paper_content:

We derive bounds on the noncoherent capacity of wide-sense stationary uncorrelated scattering (WSSUS) channels that are selective both in time and frequency, and are underspread, i.e., the product of the channel's delay spread and Doppler spread is small. For input signals that are peak constrained in time and frequency, we obtain upper and lower bounds on capacity that are explicit in the channel's scattering function, are accurate for a large range of bandwidth and allow to coarsely identify the capacity-optimal bandwidth as a function of the peak power and the channel's scattering function. We also obtain a closed-form expression for the first-order Taylor series expansion of capacity in the limit of large bandwidth, and show that our bounds are tight in the wideband regime. For input signals that are peak constrained in time only (and, hence, allowed to be peaky in frequency), we provide upper and lower bounds on the infinite-bandwidth capacity and find cases when the bounds coincide and the infinite-bandwidth capacity is characterized exactly. Our lower bound is closely related to a result by Viterbi (1967). The analysis in this paper is based on a discrete-time discrete-frequency approximation of WSSUS time- and frequency-selective channels. This discretization explicitly takes into account the underspread property, which is satisfied by virtually all wireless communication channels.
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paper_title: On the Sensitivity of Continuous-Time Noncoherent Fading Channel Capacity

paper_content:

The noncoherent capacity of stationary discrete-time fading channels is known to be very sensitive to the fine details of the channel model. More specifically, the measure of the support of the fading-process power spectral density (PSD) determines if noncoherent capacity grows logarithmically in SNR or slower than logarithmically. Such a result is unsatisfactory from an engineering point of view, as the support of the PSD cannot be determined through measurements. The aim of this paper is to assess whether, for general continuous-time Rayleigh-fading channels, this sensitivity has a noticeable impact on capacity at SNR values of practical interest. To this end, we consider the general class of band-limited continuous-time Rayleigh-fading channels that satisfy the wide-sense stationary uncorrelated-scattering (WSSUS) assumption and are, in addition, underspread. We show that, for all SNR values of practical interest, the noncoherent capacity of every channel in this class is close to the capacity of an AWGN channel with the same SNR and bandwidth, independently of the measure of the support of the scattering function (the two-dimensional channel PSD). Our result is based on a lower bound on noncoherent capacity, which is built on a discretization of the channel input-output relation induced by projecting onto Weyl-Heisenberg (WH) sets. This approach is interesting in its own right as it yields a mathematically tractable way of dealing with the mutual information between certain continuous-time random signals.
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paper_title: Maximum-likelihood differential detection of uncoded and trellis coded amplitude phase modulation over AWGN and fading channels-metrics and performance

paper_content:

This paper derives metrics for maximum-likelihood differential detection of uncoded and trellis coded MPSK and QAM transmitted over Rayleigh and Rician fading channels. Receiver structures based on these metrics are proposed and their error probability performance analyzed and/or simulated. The results represent a generalization of the notion of multiple symbol differential detection, previously introduced by the authors for MPSK over an AWGN, to the fading channel and other modulations. For the coded cases, ideal interleaving/deinterleaving is assumed and furthermore the presence or absence of channel state information. An interesting side result is that for a constant envelope modulation transmitted over a fading channel with unknown but rapidly-varying phase error (the other extreme to the slowly-varying phase error case normally assumed for differential detection), under certain practical assumptions, it is shown that the optimum receiver is of the limiter-discriminator type. >
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paper_title: Tree-Search Multiple-Symbol Differential Decoding for Unitary Space-Time Modulation

paper_content:

Differential space-time modulation (DSTM) using unitary-matrix signal constellations is an attractive solution for transmission over multiple-input multiple-output (MIMO) fading channels without requiring channel state information (CSI) at the receiver. To avoid a high error floor for DSTM in relatively fast MIMO fading channels, multiple-symbol differential detection (MSDD) has to be applied at the receiver. MSDD jointly processes blocks of several received matrix-symbols, and power efficiency improves as the blocksize increases. But since the search space of MSDD grows exponentially with the blocksize and also with the number of transmit antennas and the data rate, the complexity of MSDD quickly becomes prohibitive. In this paper, we investigate the application of tree-search algorithms to overcome the complexity limitation of MSDD. We devise a nested MSDD structure consisting of an outer and a number of inner tree-search decoders, which renders MSDD feasible for wide ranges of system parameters. Decoder designs tailored for diagonal and orthogonal DSTM codes are given, and a more power-efficient variant of MSDD, so-called subset MSDD, is proposed. Furthermore, we derive a tight symbol-error rate approximation for MSDD, which lends itself to efficient numerical evaluation. Numerical and simulation results for different DSTM constellations and fading channel scenarios show that the new tree-search MSDD achieves a significantly better performance-complexity tradeoff than benchmark decoders.
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paper_title: Performance of space-time modulation for a generalized time-varying Rician channel model

paper_content:

We analyze the performance of trained and differential space-time modulation for channels with a constant specular component and time-varying diffuse fading. We examine the case where the channel varies from sample to sample within a space-time symbol matrix according to a first-order time-varying model. We show that the effect of the time-varying diffuse channel can be described by an effective signal-to-noise ratio (SNR) that decreases with time. We derive pairwise probability of error expressions based on these effective SNR values that accurately describe performance for unitary modulation. We quantify the significant advantage that differential modulation provides at high SNR where the effect of the time-varying channel dominates. At low SNR where additive noise dominates, we note that trained modulation with perfect channel state information provides a 3-dB advantage over differential modulation, but decoding based on a maximum likelihood channel estimate yields worse performance than differential modulation at all SNR values. Simulation results are provided to support our analysis.
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paper_title: Error performance of multiple symbol differential detection of PSK signals transmitted over correlated Rayleigh fading channels

paper_content:

A study is made of the error performance of multiple-symbol differential detection of uncoded PSK (phase shift keying) signals transmitted over correlated flat Rayleigh fading channels. It is found that the optimal detector uses a decoding metric which is a quadratic form of Gaussian variates. By using the characteristic function/residue theorem approach, the authors are able to derive an exact expression for the pairwise error event probability for the optimal detector. Subsequently, it is found that multiple-symbol differential detection is a very effective strategy for eliminating the irreducible error floor commonly associated with conventional differential detection. It is also found that the error performances of these detectors are not very sensitive to the mismatch between the decoding metric and the channel fading statistics. >
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paper_title: Differential Modulation and Non-Coherent Detection in Wireless Relay Networks

paper_content:

The technique of cooperative communications is finding its way in the next generations of many wireless communication applications. Due to the distributed nature of cooperative networks, acquiring fading channels information for coherent detection is more challenging than in the traditional point-to-point communications. To bypass the requirement of channel information, differential modulation together with non-coherent detection can be deployed. This thesis is concerned with various issues related to differential modulation and non-coherent detection in cooperative networks. Specifically, the thesis examines the behaviour and robustness of non-coherent detection in mobile environments (i.e., time-varying channels). The amount of channel variation is related to the normalized Doppler shift which is a function of user's mobility. The Doppler shift is used to distinguish between slow time-varying (slow-fading) and rapid time-varying (fast-fading) channels. The performance of several important relay topologies, including single-branch and multi-branch dual-hop relaying with/without a direct link that employ amplify-and-forward relaying and two-symbol non-coherent detection, is analyzed. For this purpose, a time-series model is developed for characterizing the time-varying nature of the cascaded channel encountered in amplify-and-forward relaying.
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paper_title: Multiple-symbol differential sphere decoding

paper_content:

In multiple-symbol differential detection (MSDD) for power-efficient transmission over fading channels without channel state information, blocks of N received symbols are jointly processed to decide on N - 1 data symbols. The search space for the maximum-likelihood (ML) estimate is therefore (complex) (N - 1)-dimensional, and MSDD quickly becomes computationally intractable as N grows. Sphere decoding (SD) is a widely used approach to find the ML estimate in such high-dimensional spaces. In this paper, we devise the application of SD to accomplish MSDD, and we refer to the resulting technique as multiple-symbol differential sphere decoding (MSDSD). We present an efficient algorithm for MSDSD, whose excellent performance versus complexity trade-off is verified by various simulation results and by comparisons with other, suboptimum approaches known from the literature.
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paper_title: Differential space-time modulation

paper_content:

Space-time coding and modulation exploit the presence of multiple transmit antennas to improve the performance on multipath radio channels. Thus far, most work on space-time coding has assumed that perfect channel estimates are available at the receiver. In certain situations, however, it may be difficult or costly to estimate the channel accurately, in which case it is natural to consider the design of modulation techniques that do not require channel estimates at the transmitter or receiver. We propose a general approach to differential modulation for multiple transmit antennas based on group codes. This approach ran be applied to any number of transmit and receive antennas, and any signal constellation. We also derive low-complexity differential receivers, error bounds, and modulator design criteria, which we use to construct optimal differential modulation schemes for two transmit antennas. These schemes can be demodulated with or without channel estimates. This permits the receiver to exploit channel estimates when they are available. The performance degrades by approximately 3 dB when estimates are not available.
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paper_title: Capacity of a Mobile Multiple-Antenna Communication Link in Rayleigh Flat Fading

paper_content:

We analyze a mobile wireless link comprising M transmitter and N receiver antennas operating in a Rayleigh flat-fading environment. The propagation coefficients between pairs of transmitter and receiver antennas are statistically independent and unknown; they remain constant for a coherence interval of T symbol periods, after which they change to new independent values which they maintain for another T symbol periods, and so on. Computing the link capacity, associated with channel coding over multiple fading intervals, requires an optimization over the joint density of T/spl middot/M complex transmitted signals. We prove that there is no point in making the number of transmitter antennas greater than the length of the coherence interval: the capacity for M>T is equal to the capacity for M=T. Capacity is achieved when the T/spl times/M transmitted signal matrix is equal to the product of two statistically independent matrices: a T/spl times/T isotropically distributed unitary matrix times a certain T/spl times/M random matrix that is diagonal, real, and nonnegative. This result enables us to determine capacity for many interesting cases. We conclude that, for a fixed number of antennas, as the length of the coherence interval increases, the capacity approaches the capacity obtained as if the receiver knew the propagation coefficients.
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paper_title: Unitary space-time modulation for multiple-antenna communications in Rayleigh flat fading

paper_content:

Motivated by information-theoretic considerations, we propose a signaling scheme, unitary space-time modulation, for multiple-antenna communication links. This modulation is ideally suited for Rayleigh fast-fading environments, since it does not require the receiver to know or learn the propagation coefficients. Unitary space-time modulation uses constellations of T/spl times/M space-time signals (/spl Phi//sub i/, l=1, ..., L), where T represents the coherence interval during which the fading is approximately constant, and M<T is the number of transmitter antennas. The columns of each /spl Phi//sub i/ are orthonormal. When the receiver does not know the propagation coefficients, which between pairs of transmitter and receiver antennas are modeled as statistically independent, this modulation performs very well either when the signal-to-noise ratio (SNR) is high or when T/spl Gt/M. We design some multiple-antenna signal constellations and simulate their effectiveness as measured by bit-error probability with maximum-likelihood decoding. We demonstrate that two antennas have a 6-dB diversity gain over one antenna at 15-dB SNR.
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paper_title: On the Capacity of Large-MIMO Block-Fading Channels

paper_content:

We characterize the capacity of Rayleigh block-fading multiple-input multiple-output (MIMO) channels in the noncoherent setting where transmitter and receiver have no a priori knowledge of the realizations of the fading channel. We prove that unitary space-time modulation (USTM) is not capacity-achieving in the high signal-to-noise ratio (SNR) regime when the total number of antennas exceeds the coherence time of the fading channel (expressed in multiples of the symbol duration), a situation that is relevant for MIMO systems with large antenna arrays (large-MIMO systems). This result settles a conjecture by Zheng & Tse (2002) in the affirmative. The capacity-achieving input signal, which we refer to as Beta-variate space-time modulation (BSTM), turns out to be the product of a unitary isotropically distributed random matrix, and a diagonal matrix whose nonzero entries are distributed as the square-root of the eigenvalues of a Beta-distributed random matrix of appropriate size. Numerical results illustrate that using BSTM instead of USTM in large-MIMO systems yields a rate gain as large as 13% for SNR values of practical interest.
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paper_title: A Multi-Mode Multi-Band and Multi-System-Based Access Architecture for High-Speed Railways

paper_content:

With the rapid development of high-speed railways all over the world, it is very promising to deliver public broadband access to passengers aboard high-speed trains. However, it has been a challenge to support high speed (e.g. above 350 km/h) mobile users with broadband communications, because of rapidly fading radio channels and Doppler frequency shift. This paper presents a review of network architectures providing broadband access to high-speed trains. A novel scheme referred as multimode multi-band and multi-system-based access architecture is proposed in this paper. It consists of three parts: (1) train network part that relays data to large number of users in carriages without changing their own terminals, (2) ground network part that deals with traffic from different operators' networks, and (3) ground-to-train network part that connects the train network and ground network. Moreover, this architecture could support high mobility with broad bandwidth, seamless handover, scalable and cost-effective deployment.
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paper_title: An Adaptive Hard Handoff Algorithm for Mobile Cellular Communication Systems

paper_content:

In this letter, we propose an adaptive hard handoff algorithm with a dynamic hysteresis value based on the received signal strength from the serving base station, for mobile cellular communication systems. A discrete-time method is presented to evaluate handoff algorithms analytically. Performance is evaluated in terms of the average number of handoffs, the probability of link degradation, and the average handoff delay. Numerical results and simulations demonstrate that the proposed algorithm outperforms the handoff algorithm with fixed hysteresis values and the handoff algorithm using both threshold and hysteresis.
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paper_title: The cross layer RMPA handover: a reliable mobility pattern aware handover strategy for broadband wireless communication in a high-speed railway domain

paper_content:

Enhancing the handover process in broadband wireless communication deployment has traditionally motivated many research initiatives. In a high-speed railway domain, the challenge is even greater. Owing to the long distances covered, the mobile node gets involved in a compulsory sequence of handover processes. Consequently, poor performance during the execution of these handover processes significantly degrades the global end-to-end performance. This article proposes a new handover strategy for the railway domain: the RMPA handover, a Reliable Mobility Pattern Aware IEEE 802.16 handover strategy “customized” for a high-speed mobility scenario. The stringent high mobility feature is balanced with three other positive features in a high-speed context: mobility pattern awareness, different sources for location discovery techniques, and a previously known traffic data profile. To the best of the authors’ knowledge, there is no IEEE 802.16 handover scheme that simultaneously covers the optimization of the handover process itself and the efficient timing of the handover process. Our strategy covers both areas of research while providing a cost-effective and standards-based solution. To schedule the handover process efficiently, the RMPA strategy makes use of a context aware handover policy; that is, a handover policy based on the mobile node mobility pattern, the time required to perform the handover, the neighboring network conditions, the data traffic profile, the received power signal, and current location and speed information of the train. Our proposal merges all these variables in a cross layer interaction in the handover policy engine. It also enhances the handover process itself by establishing the values for the set of handover configuration parameters and mechanisms of the handover process. RMPA is a cost-effective strategy because compatibility with standards-based equipment is guaranteed. The major contributions of the RMPA handover are in areas that have been left open to the handover designer’s discretion. Our simulation analysis validates the RMPA handover decision rules and design choices. Our results supporting a high-demand video application in the uplink stream show a significant improvement in the end-to-end quality of service parameters, including end-to-end delay (22%) and jitter (80%), when compared with a policy based on signal-to-noise-ratio information.
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paper_title: A least squares path loss estimation approach to handover algorithms

paper_content:

In this paper a new class of linear handover algorithms is introduced and analyzed for two conflicting performance measures, namely the average number of outages and the average number of handovers, for a given trip of the mobile station. Assuming the distances between the mobile station and surrounding base stations are known, the new algorithm is based on the least squares estimate of path loss parameters of the various radio links. In a log-normal fading macrocellular environment, it is seen that for an average number of 1.2 handovers (the minimum value is one, given by the ideal reference case), the least squares algorithm reduces the average number of outages by more than 30% with respect to other linear handover algorithms. Furthermore, the above performance measures improve for longer estimation windows: the typical trade-off of averaging algorithms does not apply to the least squares method.
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paper_title: An Adaptive Handoff Triggering Mechanism for Vehicular Networks

paper_content:

Handoff plays an important role in vehicular networks due to high movement of vehicles. To provide seamless connectivity under Access Points (AP), this paper proposes an adaptive handoff triggering method to minimize communication time for a vehicle with an AP switch (i.e., whether and when to trigger a handoff process). In the proposed method, combined with an improved data transmission rate based trigger, handoff triggering decision is executed based on three different communication methods (called C-Dire, C-Relay and C-ALLRelay) to minimize the transmission delay when a vehicle moves from an AP to another. Transmission delay is derived through considering vehicle mobility and transmission rate diversity. The simulation results show that the proposed method is proven to be adaptive to vehicular networks.
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paper_title: Adaptive handoff method using mobile location information

paper_content:

The performance or handoff algorithm is critical to the overall system performance of cellular mobile communications. This paper presents a handoff algorithm that uses adaptive handoff values based on mobile location information and cell RF propagation statistics. Analysis is focused on the probability of hard/soft handoff and the probability of dropped calls/outage in the TDMA and CDMA systems. The simulation results shorn, that the probability of unnecessary handoff is reduced using the adaptive handoff algorithm with a minor increase in the dropped-call probability. This improvement is significant if the propagation signal varies rapidly in the cell.
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paper_title: Radio-over-Fiber Distributed Antenna System for WiMAX Bullet Train Field Trial

paper_content:

Wireless broadband access for mobile applications is an area of increasing growth. An international collaboration, led by the Taiwan Industrial Technology Research Institute (ITRI), has been working to implement a dedicated WiMAX network for a user field trial on the Taiwan High Speed Rail (THSR) bullet train system. Continuous wireless coverage at train speeds up to 300km/hr is challenging due to a number of critical issues, including handover at high speeds and coverage in challenging terrain such as tunnels. A Radio-over-Fiber (RoF) Distributed Antenna System (DAS) was developed by Corning Incorporated to address the critical issue of handover and increase the coverage for each WiMAX basestation. For a DAS deployment consisting of multiple discrete antennas, proper signal strength management is required because the maximum delay spread allowed by the WiMAX protocol is easily surpassed at realistic fiber ranges up to 9km. This paper reviews some of the initial results of the RoF DAS experiments in SongShan tunnel in Taipei. Error-free signal transmission was achieved when the difference in received power from two consecutive antennas was greater than 10dB.
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paper_title: Downlink performance and capacity of distributed antenna systems in a multicell environment

paper_content:

Distributed antenna systems (DAS) have been widely implemented in state-of-the art cellular communication systems to cover dead spots. Recent academic studies have shown that in addition to coverage improvements, DAS can also have potential advantages such as reduced power and increased system capacity in a single cell environment. This paper analytically quantifies downlink capacity of multicell DAS for two different transmission strategies: selection diversity (where just one or two of the distributed antennas are used) and blanket transmission (where all antennas in the cell broadcast data). Simple repeaters are a special case of our analysis. A generalized information theoretic analysis is provided to illuminate the fundamental limits of such systems in the cellular context. The results show that DAS reduces other-cell interference in a multicell environment and hence significantly improves capacity (by about 2x), with particularly large improvements for users near cell boundaries. Less obviously, from a communication theory standpoint, it is shown that selection diversity is preferable to blanket transmission in terms of achievable ergodic capacity. For blanket transmission, we show that the optimal transmission strategy is just phase steering due to the per antenna module power constraints in DAS
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paper_title: Broadband wireless communications on high speed trains

paper_content:

This tutorial paper provides a comprehensive overview on the recent development in broadband wireless communications for high speed trains. Starting with the introduction of the two-hop network structure, radio-over-fiber (RoF) based cell planning is described in detail. Moreover, based on the analysis of differences between conventional cellular systems and the one for high speed trains, promising techniques are recommended to improve the performance of handover, which is one of the main challenges in high speed train communications. Finally, in order to combat the fast fading caused by the high mobility, robust algorithms are needed in physical layer signal processing, including synchronization, channel estimation, modulation/demodulation, and so on.
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paper_title: Reliable broadband wireless communication for high speed trains using baseband cloud

paper_content:

We propose a novel high-speed train communication using baseband cloud (C-HSTC) system framework for providing continuous broadband services to highly mobile users. This framework is featured with a new virtualized single cell design which mitigates the impact of conventional handover failures and guarantees continuous communication services. Through exploiting the baseband units (BBU) cloud and the ful frequent frequency reuse in the virtualized single cell, we also proposed a highly efficient joint transmit beamforming algorithm targeting at compensating the inter-carrier interference (ICI) caused by severe Doppler frequency shift due to mobility. Numerical analysis shows that the new architecture and corresponding algorithms are suitable for high-speed train communication and can provide a continuous data rate of more than 100 megabits per second (Mbps) for passengers at a speed of 450 kilometers per hour (kmph). This would help to achieve satisfactory mobile broadband services for high speed train passengers.
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paper_title: An Effective Handover Scheme Based on Antenna Selection in Ground–Train Distributed Antenna Systems

paper_content:

Due to frequent handovers in broadband wireless communications for the high-speed railway system, communication interruption caused by handover failures could seriously degrade the users' experience. Aiming at reducing the handover failure probability, this paper proposes an effective handover scheme based on the distributed antenna system (DAS) and mobile-relay-aided two-link architecture, i.e., ground-relay link and relay-onboard link, where the mobile relay is mounted on top of the moving train. In addition, an antenna selection scheme with optimal power allocation is proposed to improve handover performance. Our simulation and analysis results show that the DAS structure with macrodiversity can reduce the unnecessary handovers, as well as reduce the handover failure and link outage probabilities compared with the traditional cell without DAS. In addition, the proposed DAS with an antenna selection scheme performs better in terms of handover failure and link outage probabilities than that of DAS with a equal power allocation (EPA) scheme.
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paper_title: Seamless Dual-Link Handover Scheme in Broadband Wireless Communication Systems for High-Speed Rail

paper_content:

Due to frequent handovers in broadband wireless communications in high-speed rail, communication interruption during handover could seriously degrade the experiences of passengers on the train. Aiming to reduce the interruption time, this paper proposes a seamless handover scheme based on a dual-layer and dual-link system architecture, where a Train Relay Station is employed to execute handover for all users in a train and two antennas are mounted at the front and rear of a train. In the proposed scheme, the front antenna executes handover while the rear antenna is still communicating with BS, so that the communication can keep non-interruptive throughout the handover. Moreover, bi-casting is adopted to eliminate the data forwarding delay between the serving BS and target BS. A complete handover protocol is designed and the performance of the proposed scheme is analyzed. It can be seen from analytical results that the handover failure probability decreases as cell overlap increases and the communication interruption probability decreases with the decrease of train handover location and the increase of cell overlap. The simulation results show that in the proposed scheme, the communication interruption probability is smaller than 1% when the handover location is properly selected and the system throughput is not affected by handover. In conclusion, both theoretical and simulation results show that the proposed scheme can efficiently perform seamless handover for high-speed rail with low implementation overhead.
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paper_title: On the performance of capacity integrated CoMP handover algorithm in LTE-Advanced

paper_content:

Coordinated multipoint (CoMP) transmission and reception is the key technique in LTE-Advanced to improve the cell-edge throughput and/or system throughput. Joint processing (JP) in CoMP technology provides multiple data transmission points for each user among multiple cooperated radio base stations. Handover is one of the key components in cellular network mobility management. A handover algorithm is needed for making a handover decision. In this paper, a capacity integrated CoMP handover algorithm supporting JP in LTE-A system is proposed. Capacity integrated CoMP handover algorithm aims to ensure the radio resources are efficiently used in the system in both capacity and channel quality domains while reducing unnecessary feedbacks. Simulation results show that capacity integrated CoMP handover algorithm not only can improve the system throughput but also minimize the system delay, packet loss, and less total number of handovers than open literature CoMP handover algorithm.
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paper_title: A CoMP soft handover scheme for LTE systems in high speed railway

paper_content:

With the development of high-speed railway and public growing demand on data traffic, people pay much more attention to provide high data rate and high reliable services under high mobility circumstance. Due to the higher data rate and lower system latency, long-term evolution (LTE) has been chosen as the next generation's evolution of railway mobile communication system by the International Union of Railways. However, there are still many problems to be solved in the high mobility applications of LTE, especially the higher handover failure probability, which seriously degrades the reliability of railway communication. This article proposes an optimized handover scheme, in which the coordinated multiple point transmission technology and dual vehicle station coordination mechanism are applied to improve the traditional hard handover performance of LTE. The scheme enables the high speed train to receive signals from both adjacent base stations and obtain diversity gain when it moves through the overlapping areas, so it improves the quality of the received signal and provides reliable communication between train and ground eNodeBs. Numerical analysis and simulation results show that the proposed scheme can decrease the outage probability remarkably during handover and guarantee the reliability of train to ground communication.
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paper_title: An overview of vertical handover decision strategies in heterogeneous wireless networks

paper_content:

In the next generation of wireless networks, mobile users can move between heterogeneous networks, using terminals with multiple access interfaces and non-real-time or real-time services. The most important issue in such environment is the Always Best Connected (ABC) concept allowing the best connectivity to applications anywhere at anytime. To answer ABC requirement, various vertical handover decision strategies have been proposed in the literature recently, using advanced tools and proven concepts. In this paper, we give an overview of the most interesting and recent strategies. We classify it into five categories for which we present their main characteristics. We also compare each one with the others in order to introduce our vertical handover decision approach.
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paper_title: Review: A review on mobility management and vertical handover solutions over heterogeneous wireless networks

paper_content:

In the nowadays heterogeneous wireless environment, a plethora of access networks have to be inter-connected in an optimal manner to meet the always best connected paradigm. This paper highlights some of the main technical challenges in heterogeneous wireless networks underlying seamless vertical handover making as a fundamental feature to all future networking endeavors. It provides a survey on the vertical mobility management process and mainly focuses on decision making mechanisms. After presenting the related standards, the main vertical handover approaches in the literature are analyzed and compared. The paper also points out the importance of multihoming in a such heterogeneous environment and provides an overview of the most known supporting mobility protocols. Finally, the main research trends and challenges are discussed.
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paper_title: Power-space functions in high speed railway wireless communications

paper_content:

To facilitate the base station planning in high speed railway communication systems, it is necessary to consider the functional relationships between the base station transmit power and space parameters such as train velocity and cell radius. Since these functions are able to present some inherent system properties determined by its spatial topology, they will be referred to as the power-space functions in this paper. In light of the fact that the line-of-sight path persists the most power of the received signal of each passing train, this paper considers the average transmission rate and bounds on power-space functions based on the additive white Gaussian noise channel (AWGN) model. As shown by Monte Carlo simulations, using AWGN channel instead of Rician channel introduces very small approximation errors, but a tractable mathematical framework and insightful results. Particularly, lower bounds and upper bounds on the average transmission rate, as well as transmit power as functions of train velocity and cell radius are presented in this paper. It is also proved that to maintain a fixed amount of service or a fixed average transmission rate, the transmit power of a base station needs to be increased exponentially, if the train velocity or cell radius is increased, respectively.
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paper_title: Optimal Scheduling for Wireless On-Demand Data Packet Delivery to High-Speed Trains

paper_content:

In this paper, we consider a cellular/infostation integrated network that supports on-demand data service delivery over a wireless link to users in a high-speed train. For the requests with different lifetimes and prices that they are willing to pay, we develop the optimal packet schedule that aims at earning the maximum revenue. To this end, we formulate the problem as an integer linear program (ILP). The problem cannot be solved efficiently by existing ILP solvers with guaranteed polynomial-time complexity in the worst case. By exploring the special structure of the formulated ILP, we propose a novel Checker algorithm and prove that this algorithm is guaranteed to find the offline optimal schedule in polynomial time. Based on the relevant insights, we further develop a class of online Checker algorithms that require only causal knowledge of service demands and wireless channel capacities. It is established that these online algorithms have a worst-case competitive ratio of 1/2, i.e., total revenue earned by them is at least half as much as the revenue earned by an offline optimal schedule that knows the complete a priori knowledge of future requests and channel conditions. Simulation results are provided to demonstrate the merit of the proposed algorithms.
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paper_title: Optimal Power Allocation With Delay Constraint for Signal Transmission From a Moving Train to Base Stations in High-Speed Railway Scenarios

paper_content:

Widespread deployment of high-speed railways in recent years has created huge demand for high-mobility broadband wireless communications. To provide broadband wireless access for passengers on the train, a well-acknowledged approach is to apply a two-hop architecture, under which passengers communicate with base stations (BSs) via an access point (AP) installed in the train. We consider the uplink transmission from the AP to the BSs on the ground along the railway. The key problem we discuss is how to match user-data arrival process and time-varying channel service process with a delay constraint at the AP. We first assume the constant-rate data arrival and the wireless channel affected only by large-scale fading in high-speed railway scenarios. We present the optimal power-allocation policy given the delay constraint. Based on the policy, we show that there exists a power–delay tradeoff in our system model, with the delay within a limited range. Besides, we find that the average transmit power and the train velocity have a tradeoff, which is similar to the power–delay tradeoff. Besides, given average transmit power constraint, the delay constraint is inversely proportional to the train velocity. Finally, when small-scale fading is considered, we modify the optimal power-allocation policy and provide a convenient approach to counteract Nakagami- $m$ fading in high signal-to-noise ratio (SNR) regimes. The modified power-allocation policy can be well adapted to different fading statistics in different terrestrial environments experienced by the high-speed train.
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paper_title: On the Uplink Capacity of High Speed Railway Communications with Massive MIMO Systems

paper_content:

This paper studies the performance and design of high speed railway communications that employ massive MIMO systems. The users onboard the train communicate with the BS by using antennas mounted on top of the train as gateways. Two antenna layouts on the train are considered, one has co-located antennas (CA) at the center of the train, and the other has distributed antennas (DA) uniformly placed along the train. The analytical uplink ergodic capacities for both layouts are derived by averaging over all the locations in a cell. For the CA layout, the average uplink cell capacity is concave in the number of antennas on the train, and the optimum number of antennas on the train is identified by maximizing the average uplink capacity. For the DA layout, the average cell capacity is upper and lower bounded by two concave functions, which are then used to find a sub-optimum value of the number of antennas on the train. Both analytical and numerical results demonstrate that the CA layout has a higher average cell capacity while the DA layout has less variations in terms of uplink capacity at different locations inside a cell.
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paper_title: Handover Scheme for 5G C/U Plane Split Heterogeneous Network in High-Speed Railway

paper_content:

Being a promising technology for fifth-generation (5G) communication systems, a novel railway communication system based on control/user (C/U) plane split heterogeneous networks can provide a high-quality broadband wireless service for passengers in high-speed railways with higher system capacity, better transmission reliability, and less cochannel interference. However, due to its special architecture where the C-plane and the U-plane must be split and supported by a macro Evolved Node B (eNB) and a phantom eNB, respectively, it would suffer more serious handover problem, particularly in intermacrocell handover, which directly degrades its applicability and availability in high-speed railways. Moreover, no technical specification has been released about this network architecture. Therefore, this paper focuses on redesigning and analyzing technical details and handover procedures based on Long-Term Evolution (LTE) specifications to guarantee the proposed system's practicability and generality and its analytical tractability. To resolve the handover problem, this paper proposes a handover trigger decision scheme based on GM(1, $n$) model of the grey system theory. By this scheme, the received signal quality from the $(N + hbox{1})$th measurement report can be predicted from the $N$th measurement period, and the predicted values can be then utilized to make the handover trigger decision. The simulation results show that the proposed scheme is capable of triggering handover in advance effectively and of enhancing handover success probability remarkably.
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paper_title: A novel architecture for LTE-B :C-plane/U-plane split and Phantom Cell concept

paper_content:

This paper introduces a novel approach in increasing the capacity of LTE cellular networks. The solution is based on massive deployment of small cells by leveraging high frequency reuse at high frequency bands in conjunction with a Macrocell. The presence, discovery and usage of the small cells are controlled dynamically by a Macrocell in a master-slave configuration hence they are called Phantom Cells. To realize this concept, a new method of managing the connections between mobile terminals and small cell nodes is introduced. It is achieved by splitting the Control and User (C/U) planes of the radio link. The combination of C/U-plane split and Phantom Cells can achieve high capacity enhancement using small cells at the same time taking into consideration mobility, scalability and flexibility requirements for massive deployment. The advantages of this approach as well as the implementation aspects are described in the paper. Simulations were also conducted to verify the concept and the results show some promising capacity enhancements. The rest of the paper describes the Phantom Cell concept as well as the challenges of deploying small cells in LTE networks.
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paper_title: Queue-Aware Joint Remote Radio Head Activation and Beamforming for Green Cloud Radio Access Networks

paper_content:

The cloud radio access network (C-RAN) is an emerging network architecture that holds the promise of coping with the explosive growth of mobile wireless data. In this paper, by considering the stochastic traffic arrivals and time-varying channel conditions, we address the stochastic optimization of joint remote radio head (RRH) activation and linear beamforming to maintain delay performance and minimize average network power consumption in downlink slotted C-RAN. We first formulate the joint optimization as a group sparse beamforming problem. Based on Lyapunov optimization technique, the stochastic optimization problem is then transformed into a queue-aware joint RRH activation and beamforming problem, which can be greedily solved at each slot. Finally, a low-complexity stationary algorithm with guaranteed convergence and closed-form expressions is proposed. The proposed algorithm can be implemented in a parallel manner, thus it is highly scalable to a large-sized C-RAN. Extensive numerical simulations validate the effectiveness of the proposed algorithm.
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paper_title: A Novel Multi-Service Small-Cell Cloud Radio Access Network for Mobile Backhaul and Computing Based on Radio-Over-Fiber Technologies

paper_content:

Small-cell systems based on cloud radio access network (cloud-RAN) architecture have been proposed as promising solutions to meet the capacity demand of the future wireless access networks in a cost-effective and power-efficient way. High-speed and scalable backhaul links between the centralized baseband processing units (BBUs) and the remote antenna units (RAUs) are very important to support the small-cell cloud-RAN. Conventionally, digital baseband I/Q samples are transmitted in the cloud-RAN backhaul, which puts stringent requirements on backhaul bandwidth, latency, and jitter. In this paper, we propose a novel multi-service small-cell wireless access architecture based on radio-over-fiber technologies (cloud-RoF access network). By utilizing analog radio frequency (RF) signal transmission in the optical backhaul links, high-speed data transmission can be achieved with highly simplified RAU design. In addition, by combing RoF with optical wavelength division multiplexing (WDM) techniques, multiple bands, multiple services and multiple operators can coexist in a shared optical infrastructure without interference. Two-operator coexistence in a shared small-cell cloud-RoF access network is demonstrated in an in-building testbed by using off-the-shelf optoelectronic components and commercialized WiMAX base stations and clients. In addition, the feasibility of delivering both conventional wireless services and future-proof millimeter-wave services is also demonstrated in the proposed multi-service small-cell cloud-RoF access system.
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Title: A Survey on High Mobility Wireless Communications: Challenges, Opportunities and Solutions

Section 1: INTRODUCTION
Description 1: Introduce the context and significance of high mobility wireless communications, including current developments and challenges.

Section 2: RAPIDLY TIME-VARYING CHANNEL MODELING
Description 2: Discuss the unique properties of high mobility channels and review different models, including statistical and geometry-based models.

Section 3: ESTIMATIONS OF FAST TIME-VARYING FADING CHANNELS
Description 3: Explore the methods for estimating fast time-varying channels, with a focus on pilot-assisted channel estimation algorithms.

Section 4: DOPPLER DIVERSITY TRANSMISSIONS
Description 4: Present theory and practical implementations of Doppler diversity in high mobility systems, considering both perfect and imperfect CSI scenarios.

Section 5: MULTI-CARRIER COMMUNICATIONS IN HIGH MOBILITY SYSTEMS
Description 5: Review the impacts of carrier frequency offsets and intercarrier interference in multi-carrier systems, and explore techniques for mobility adaptation.

Section 6: NON-COHERENT DETECTIONS
Description 6: Analyze non-coherent detection techniques suitable for high mobility systems, including information-theoretic perspectives and practical implementations like differential modulation.

Section 7: HANDOVER IN HIGH MOBILITY SYSTEMS
Description 7: Examine handover challenges and strategies in high mobility systems, including techniques for various network architectures like DAS, CoMP, and HetNet.

Section 8: ARCHITECTURE OF HIGH MOBILITY NETWORKS
Description 8: Discuss the network architectures specifically designed for high mobility communications, including systems for high-speed railways and mobile relays.

Section 9: CONCLUSIONS
Description 9: Summarize the survey, discuss the current progress, highlight future research directions, and address the key challenges remaining in high mobility communications.