5G: A Tutorial Overview of Standards, Trials, Challenges, Deployment, and Practice
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paper_title: 5G-Enabled Tactile Internet

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The long-term ambition of the Tactile Internet is to enable a democratization of skill, and how it is being delivered globally. An integral part of this is to be able to transmit touch in perceived real-time, which is enabled by suitable robotics and haptics equipment at the edges, along with an unprecedented communications network. The fifth generation (5G) mobile communications systems will underpin this emerging Internet at the wireless edge. This paper presents the most important technology concepts, which lay at the intersection of the larger Tactile Internet and the emerging 5G systems. The paper outlines the key technical requirements and architectural approaches for the Tactile Internet, pertaining to wireless access protocols, radio resource management aspects, next generation core networking capabilities, edge-cloud, and edge-AI capabilities. The paper also highlights the economic impact of the Tactile Internet as well as a major shift in business models for the traditional telecommunications ecosystem.
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paper_title: Wideband Millimeter-Wave Propagation Measurements and Channel Models for Future Wireless Communication System Design

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The relatively unused millimeter-wave (mmWave) spectrum offers excellent opportunities to increase mobile capacity due to the enormous amount of available raw bandwidth. This paper presents experimental measurements and empirically-based propagation channel models for the 28, 38, 60, and 73 GHz mmWave bands, using a wideband sliding correlator channel sounder with steerable directional horn antennas at both the transmitter and receiver from 2011 to 2013. More than 15,000 power delay profiles were measured across the mmWave bands to yield directional and omnidirectional path loss models, temporal and spatial channel models, and outage probabilities. Models presented here offer side-by-side comparisons of propagation characteristics over a wide range of mmWave bands, and the results and models are useful for the research and standardization process of future mmWave systems. Directional and omnidirectional path loss models with respect to a 1 m close-in free space reference distance over a wide range of mmWave frequencies and scenarios using directional antennas in real-world environments are provided herein, and are shown to simplify mmWave path loss models, while allowing researchers to globally compare and standardize path loss parameters for emerging mmWave wireless networks. A new channel impulse response modeling framework, shown to agree with extensive mmWave measurements over several bands, is presented for use in link-layer simulations, using the observed fact that spatial lobes contain multipath energy that arrives at many different propagation time intervals. The results presented here may assist researchers in analyzing and simulating the performance of next-generation mmWave wireless networks that will rely on adaptive antennas and multiple-input and multiple-output (MIMO) antenna systems.
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paper_title: Understanding the Current Operation and Future Roles of Wireless Networks: Co-Existence, Competition and Co-Operation in the Unlicensed Spectrum Bands

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Technology and policy are coming together to enable a paradigmatic change to the most widely used mechanism, exclusive rights, which allows mobile telecommunications operators to use the radio spectrum. Although spectrum sharing is not a new idea, the limited supply of spectrum and the enormous demand for mobile broadband services are forcing spectrum authorities to look more closely into a range of tools that might accelerate its adoption. This paper seeks to understand how co-existence and co-operation of Wi-Fi and cellular networks in the unlicensed spectrum can increase the overall capacity of heterogeneous wireless networks. It also reveals the challenges posed by new uses, such as machine-to-machine communications and the Internet of Things. It also brings together two major proposed regulatory approaches, such as those by the U.K.’s Ofcom and the European Commission, which currently represent leading efforts to provide spectrum authorities with robust spectrum sharing frameworks, to discuss policy tools likely to be implemented.
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paper_title: Spectrum Sharing in mmWave Cellular Networks via Cell Association, Coordination, and Beamforming

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This paper investigates the extent to which spectrum sharing in millimeter-wave (mmWave) networks with multiple cellular operators is a viable alternative to traditional dedicated spectrum allocation. Specifically, we develop a general mathematical framework to characterize the performance gain that can be obtained when spectrum sharing is used, as a function of the underlying beamforming, operator coordination, bandwidth, and infrastructure sharing scenarios. The framework is based on joint beamforming and cell association optimization, with the objective of maximizing the long-term throughput of the users. Our asymptotic and non-asymptotic performance analyses reveal five key points: 1) spectrum sharing with light on-demand intra- and inter-operator coordination is feasible, especially at higher mmWave frequencies (for example, 73 GHz); 2) directional communications at the user equipment substantially alleviate the potential disadvantages of spectrum sharing (such as higher multiuser interference); 3) large numbers of antenna elements can reduce the need for coordination and simplify the implementation of spectrum sharing; 4) while inter-operator coordination can be neglected in the large-antenna regime, intra-operator coordination can still bring gains by balancing the network load; and 5) critical control signals among base stations, operators, and user equipment should be protected from the adverse effects of spectrum sharing, for example by means of exclusive resource allocation. The results of this paper, and their extensions obtained by relaxing some ideal assumptions, can provide important insights for future standardization and spectrum policy.
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paper_title: WiGiG: Multi-gigabit wireless communications in the 60 GHz band

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The Wireless Gigabit Alliance [1] - commonly called WiGig - is an industry consortium devoted to the development and promotion of wireless communications in the 60 GHz band. Recent advances in 60 GHz technology and demand for higher-speed wireless connections are key drivers for WiGig. Among the unlicensed frequency bands available for wireless networks, 60 GHz is uniquely suited for carrying extremely high data rates (multiple gigabits per second) over short distances. WiGig has developed a medium access control (MAC) layer, a physical (PHY) layer, and several protocol adaptation layers (PALs) to enable interoperable devices that take advantage of these extremely high data rates. WiGig is also working closely with standards bodies, including IEEE 802.11, and other industry groups, such as the Wi-Fi Alliance, to help enable certification of standards-compliant devices.
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paper_title: A study on the coexistence of fixed satellite service and cellular networks in a mmWave scenario

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The use of a larger bandwith in the millimeter wave (mmWave) spectrum is one of the key components of next generation cellular networks. Currently, part of this band is allocated on a co-primary basis to a number of other applications, such as the fixed satellite services (FSSs). In this paper, we investigate the coexistence between a cellular network and FSSs in a mmWave scenario. In light of the parameters recommended by the standard and the recent results presented in the literature on the mmWave channel model, we analyze different BSs deployments and different antenna configurations at the transmitters. Finally, we show how, exploiting the features of a mmWave scenario, the coexistence between cellular and satellite services is feasible and the interference at the FSS antenna can be kept below recommended levels.
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paper_title: Spectrum Pooling in MmWave Networks: Opportunities, Challenges, and Enablers

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Motivated by the specific characteristics of mmWave technologies, we discuss the possibility of an authorization regime that allows spectrum sharing between multiple operators, also referred to as ...
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paper_title: Massive MIMO for Next Generation Wireless Systems

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Multi-user MIMO offers big advantages over conventional point-to-point MIMO: it works with cheap single-antenna terminals, a rich scattering environment is not required, and resource allocation is simplified because every active terminal utilizes all of the time-frequency bins. However, multi-user MIMO, as originally envisioned, with roughly equal numbers of service antennas and terminals and frequency-division duplex operation, is not a scalable technology. Massive MIMO (also known as large-scale antenna systems, very large MIMO, hyper MIMO, full-dimension MIMO, and ARGOS) makes a clean break with current practice through the use of a large excess of service antennas over active terminals and time-division duplex operation. Extra antennas help by focusing energy into ever smaller regions of space to bring huge improvements in throughput and radiated energy efficiency. Other benefits of massive MIMO include extensive use of inexpensive low-power components, reduced latency, simplification of the MAC layer, and robustness against intentional jamming. The anticipated throughput depends on the propagation environment providing asymptotically orthogonal channels to the terminals, but so far experiments have not disclosed any limitations in this regard. While massive MIMO renders many traditional research problems irrelevant, it uncovers entirely new problems that urgently need attention: the challenge of making many low-cost low-precision components that work effectively together, acquisition and synchronization for newly joined terminals, the exploitation of extra degrees of freedom provided by the excess of service antennas, reducing internal power consumption to achieve total energy efficiency reductions, and finding new deployment scenarios. This article presents an overview of the massive MIMO concept and contemporary research on the topic.
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paper_title: Coordinated multipoint: Concepts, performance, and field trial results

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Coordinated multipoint or cooperative MIMO is one of the promising concepts to improve cell edge user data rate and spectral efficiency beyond what is possible with MIMOOFDM in the first versions of LTE or WiMAX. Interference can be exploited or mitigated by cooperation between sectors or different sites. Significant gains can be shown for both the uplink and downlink. A range of technical challenges were identified and partially addressed, such as backhaul traffic, synchronization and feedback design. This article also shows the principal feasibility of COMP in two field testbeds with multiple sites and different backhaul solutions between the sites. These activities have been carried out by a powerful consortium consisting of universities, chip manufacturers, equipment vendors, and network operators.
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paper_title: Scaling up MIMO: Opportunities and challenges with very large arrays

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Presents a list of articles published by the IEEE Signal Processing Society (SPS) that ranked among the top 100 most downloaded IEEE Xplore articles.
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paper_title: Interference coordination for dense wireless networks

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The promise of ubiquitous and super-fast connectivity for the upcoming years will be in large part fulfilled by the addition of base stations and spectral aggregation. The resulting very dense networks (DenseNets) will face a number of technical challenges. Among others, the interference emerges as an old acquaintance with new significance. As a matter of fact, the interference conditions and the role of aggressor and victim depend to a large extent on the density and the scenario. To illustrate this, downlink interference statistics for different 3GPP simulation scenarios and a more irregular and dense deployment in Tokyo are compared. Evolution to DenseNets offers new opportunities for further development of downlink interference cooperation techniques. Various mechanisms in LTE and LTE-Advanced are revisited. Some techniques try to anticipate the future in a proactive way, whereas others simply react to an identified interference problem. As an example, we propose two algorithms to apply time domain and frequency domain small cell interference coordination in a DenseNet.
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paper_title: Hybrid Digital and Analog Beamforming Design for Large-Scale Antenna Arrays

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The potential of using of millimeter wave (mmWave) frequency for future wireless cellular communication systems has motivated the study of large-scale antenna arrays for achieving highly directional beamforming. However, the conventional fully digital beamforming methods which require one radio frequency (RF) chain per antenna element is not viable for large-scale antenna arrays due to the high cost and high power consumption of RF chain components in high frequencies. To address the challenge of this hardware limitation, this paper considers a hybrid beamforming architecture in which the overall beamformer consists of a low-dimensional digital beamformer followed by an RF beamformer implemented using analog phase shifters. Our aim is to show that such an architecture can approach the performance of a fully digital scheme with much fewer number of RF chains. Specifically, this paper establishes that if the number of RF chains is twice the total number of data streams, the hybrid beamforming structure can realize any fully digital beamformer exactly, regardless of the number of antenna elements. For cases with fewer number of RF chains, this paper further considers the hybrid beamforming design problem for both the transmission scenario of a point-to-point multiple-input multiple-output (MIMO) system and a downlink multi-user multiple-input single-output (MU-MISO) system. For each scenario, we propose a heuristic hybrid beamforming design that achieves a performance close to the performance of the fully digital beamforming baseline. Finally, the proposed algorithms are modified for the more practical setting in which only finite resolution phase shifters are available. Numerical simulations show that the proposed schemes are effective even when phase shifters with very low resolution are used.
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paper_title: Large-scale antenna systems with hybrid analog and digital beamforming for millimeter wave 5G

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With the severe spectrum shortage in conventional cellular bands, large-scale antenna systems in the mmWave bands can potentially help to meet the anticipated demands of mobile traffic in the 5G era. There are many challenging issues, however, regarding the implementation of digital beamforming in large-scale antenna systems: complexity, energy consumption, and cost. In a practical large-scale antenna deployment, hybrid analog and digital beamforming structures can be important alternative choices. In this article, optimal designs of hybrid beamforming structures are investigated, with the focus on an N (the number of transceivers) by M (the number of active antennas per transceiver) hybrid beamforming structure. Optimal analog and digital beamforming designs in a multi-user beamforming scenario are discussed. Also, the energy efficiency and spectrum efficiency of the N × M beamforming structure are analyzed, including their relationship at the green point (i.e., the point with the highest energy efficiency) on the energy efficiency-spectrum efficiency curve, the impact of N on the energy efficiency performance at a given spectrum efficiency value, and the impact of N on the green point energy efficiency. These results can be conveniently utilized to guide practical LSAS design for optimal energy/ spectrum efficiency trade-off. Finally, a reference signal design for the hybrid beamform structure is presented, which achieves better channel estimation performance than the method solely based on analog beamforming. It is expected that large-scale antenna systems with hybrid beamforming structures in the mmWave band can play an important role in 5G.
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paper_title: Downlink Cellular Network Analysis With Multi-Slope Path Loss Models

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Existing cellular network analyses, and even simulations, typically use the standard path loss model where received power decays like $\|x\|^{-\alpha}$ over a distance $\|x\|$. This standard path loss model is quite idealized, and in most scenarios the path loss exponent $\alpha$ is itself a function of $\|x\|$, typically an increasing one. Enforcing a single path loss exponent can lead to orders of magnitude differences in average received and interference powers versus the true values. In this paper we study \emph{multi-slope} path loss models, where different distance ranges are subject to different path loss exponents. We focus on the dual-slope path loss function, which is a piece-wise power law and continuous and accurately approximates many practical scenarios. We derive the distributions of SIR, SNR, and finally SINR before finding the potential throughput scaling, which provides insight on the observed cell-splitting rate gain. The exact mathematical results show that the SIR monotonically decreases with network density, while the converse is true for SNR, and thus the network coverage probability in terms of SINR is maximized at some finite density. With ultra-densification (network density goes to infinity), there exists a \emph{phase transition} in the near-field path loss exponent $\alpha_0$: if $\alpha_0>1$ unbounded potential throughput can be achieved asymptotically; if $\alpha_0<1$, ultra-densification leads in the extreme case to zero throughput.
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paper_title: Large Scale Parameter for the WINNER II Channel Model at 2.53 GHz in Urban Macro Cell

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This paper presents results of wide band channel measurements at 2.53 GHz for a representative urban macro cell environment in Ilmenau, Germany. The extensive channel sounding campaign covered the MIMO radio links from 22 mobile tracks to 3 different base stations and 1 relay station. The results presented in this paper provide insight into the large scale parameter analysis of the power, delay domain, including the transmission loss and the statistical distributions of the shadow fading, narrowband k-factor and delay spread. Large scale parameters from angle domain (azimuth and elevation) are derived based on the high resolution multipath parameter estimation (RIMAX). Furthermore the cross correlation of these parameters are investigated and compared to state-of-the-art channel models as from the IST-WINNER.
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paper_title: Cluster-based analysis of 3D MIMO channel measurement in an urban environment

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Massive MIMO (multiple - input - multiple - output) and full-dimensional MIMO systems in next-generation cellular communications systems as well as high-data-rate military systems have garnered considerable attention recently. For the assessment of their performance, knowledge of the 3D propagation channel characteristics, i.e., azimuth and elevation of the multipath components (MPCs) is essential. In this paper, we present first results of a 3D outdoor propagation channel measurement campaign performed in an urban macro-cellular environment. The measurements were performed with a 20 MHz wideband polarimetric MIMO channel sounder centered at 2.53 GHz. At each measurement location, parameters of all MPCs observed were extracted with RIMAX, an iterative maximum likelihood high-resolution algorithm. It was observed that MPCs naturally grouped into clusters. We then present a cluster-based analysis of the propagation channel providing some results of the intra and inter cluster parameters and their relevant statistics. Correlation between all extracted cluster parameters are also provided. Parameters such as elevation and azimuth spread (at the base-station) in this work have been used as input to recent international channel model standardization.
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paper_title: The double-directional radio channel

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We introduce the concept of the double-directional mobile radio channel. It is called this because it includes angular information at both link ends, e.g., at the base station and at the mobile station. We show that this angular information can be obtained with synchronized antenna arrays at both link ends. In wideband high-resolution measurements, we use a switched linear array at the receiver and a virtual-cross array at the transmitter. We evaluate the raw measurement data with a technique that alternately used estimation and beamforming, and that relied on ESPRIT (estimation of signal parameters via rotational invariance techniques) to obtain superresolution in both angular domains and in the delay domain. In sample microcellular scenarios (open and closed courtyard, line-of-sight and obstructed line-of-sight), up to 50 individual propagation paths are determined. The major multipath components are matched precisely to the physical environment by geometrical considerations. Up to three reflection/scattering points per propagation path are identified and localized, lending insight into the multipath spreading properties in a microcell. The extracted multipath parameters allow unambiguous scatterer identification and channel characterization, independently of a specific antenna, its configuration (single/array), and its pattern. The measurement results demonstrate a considerable amount of power being carried via multiply reflected components, thus suggesting revisiting the popular single-bounce propagation models. It turns out that the wideband double-directional evaluation is a most complete method for separating multipath components. Due to its excellent spatial resolution, the double-directional concept provides accurate estimates of the channel's multipath-richness, which is the important parameter for the capacity of multiple-input multiple-output (MIMO) channels.
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paper_title: Massive MIMO Channel Modeling - Extension of the COST 2100 Model

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As massive MIMO is currently considered a leading 5G technology candidate, channel models that capture important massive MIMO channel characteristics are urgently needed. In this paper we present an attempt for massive MIMO channel modeling based on measurement campaigns at 2.6 GHz in both outdoor and indoor environments, using physically-large arrays and with closely-spaced users. The COST 2100 MIMO channel model is adopted as a general framework. We discuss modeling approaches and scopes for massive MIMO, based on which we suggest extensions to the COST 2100 model. The extensions include 3D propagation, polarization, cluster behavior at the base station side for physically-large arrays, and multi-path component gain functions for closely-spaced users. Model parameters for these extensions in massive MIMO scenarios are reported. Initial validation against the measurements are also performed, which shows that the model is capable of reproducing the channel statistics in terms of temporal behavior of the user separability, singular value spread and sum-rate/capacity. (Less)
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paper_title: Massive MIMO in Real Propagation Environments: Do All Antennas Contribute Equally?

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Massive MIMO can greatly increase both spectral and transmit-energy efficiency. This is achieved by allowing the number of antennas and RF chains to grow very large. However, the challenges include high system complexity and hardware energy consumption. Here we investigate the possibilities to reduce the required number of RF chains, by performing antenna selection. While this approach is not a very effective strategy for theoretical independent Rayleigh fading channels, a substantial reduction in the number of RF chains can be achieved for real massive MIMO channels, without significant performance loss. We evaluate antenna selection performance on measured channels at 2.6 GHz, using a linear and a cylindrical array, both having 128 elements. Sum-rate maximization is used as the criterion for antenna selection. A selection scheme based on convex optimization is nearly optimal and used as a benchmark. The achieved sum-rate is compared with that of a very simple scheme that selects the antennas with the highest received power. The power-based scheme gives performance close to the convex optimization scheme, for the measured channels. This observation indicates a potential for significant reductions of massive MIMO implementation complexity, by reducing the number of RF chains and performing antenna selection using simple algorithms.
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paper_title: Angular power distribution and mean effective gain of mobile antenna in different propagation environments

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We measured the elevation angle distribution and cross-polarization power ratio of the incident power at a mobile station in different radio propagation environments at 2.15 GHz frequency. A novel measurement technique was utilized, based on a wideband channel sounder and a spherical dual-polarized antenna array at the receiver. Data were collected over 9 km of continuous measurement routes, both indoor and outdoor. Our results show that in non-line-of-sight situations, the power distribution in elevation has a shape of a double-sided exponential function, with different slopes on the negative and positive sides of the peak. The slopes and the peak elevation angle depend on the environment and base-station antenna height. The cross-polarization power ratio varied within 6.6 and 11.4 dB, being lowest for indoor and highest for urban microcell environments. We applied the experimental data for analysis of the mean effective gain (MEG) of several mobile handset antenna configurations, with and without the user's head. The obtained MEG values varied from approximately -5 dBi in free space to less than -11 dBi beside the head model. These values are considerably lower than what is typically used in system specifications. The result shows that considering only the maximum gain or total efficiency of the antenna is not enough to describe its performance in practical operating conditions. For most antennas, the environment type has little effect on the MEG, but clear differences exist between antennas. The effect of the user's head on the MEG depends on the antenna type and on which side of the head the user holds the handset.
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paper_title: Elevation Angle Characteristics of Urban Wireless Propagation Environment at 3.5 GHz

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It is known that channel model of propagation characteristics are crucial in the research and evaluation of three-dimensional multiple-input multiple-output (3D-MIMO) technique, especially the elevation angle (EA) model. In this paper, the results of 3D-MIMO channel measurement in the urban macro-cell (UMA) scenario at 3.5 GHz are presented. Based on the measurement data, it is find that the mean value of elevation angle has a offset angle to line-of-sight (LOS) angle especially in not-line-of- sight (NLOS) situation. And the offset angle is depend on the azimuth distance between BS and MS which can be modeled as a function that use distance as a parameter. A novel way to model mean angle of elevation angle is proposed at the end of this paper. The relation between Circular angular spread (CAS) and distance is also studied, the measurement result shows that the effects caused by distance can be neglected.
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paper_title: Correlation Properties of Large Scale Parameters from 2.66 GHz Multi-Site Macro Cell Measurements

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Multi-site measurements for urban macro cells at 2.66 GHz are performed with three base stations and one mobile station. In order to analyze the correlation properties of large scale parameters, we split up the routes into subsets, where it can be assumed that wide-sense stationarity (WSS) applies. The autocorrelation distance and correlation properties of large scale parameters for each link are analyzed. By comparing these properties with the corresponding parameters from the COST 2100 and WINNER II models, we can see that the measured autocorrelation distance of the shadow fading as well the autocorrelation distance of delay spread have similar properties as in the two models. The shadow fading and delay spread from the same link are negatively correlated and match the two models well. Based on the WSS subsets, we can see that large scale parameters for different links can be correlated, also when two BSs are far away from each other. In those cases the correlation of different links tends to be positively correlated when both base stations are in the same direction compared to the movement of the MS, otherwise the two links are usually negatively correlated.
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paper_title: A Radio Channel Sounder for Mobile Millimeter-Wave Communications: System Implementation and Measurement Assessment

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We describe a state-of-the-art channel sounder to support channel-model development for mobile millimeterwave (mm-wave) communications. The system can measure the complex amplitude, delay, and angle of arrival of the multipath components of indoor and outdoor channels. Specifically, a custom multiplexer (MUX) records the channel impulse response across a 16-element receive (RX) antenna array in 65.5 μs, while the channel is static. The delay resolution of the system is 1 ns and, because the elements are oriented in a 3-D space, both azimuth and elevation angles can be extracted. The robust link budget, comprising high-gain directional RX antennas, enables indoor link measurement beyond 150 m in line-of-sight and 20 m in non-line-of-sight conditions. The RX array is mounted on a location-aware robot, which is battery operated. Combined with the speed of the MUX, untethered acquisition of mobile-channel data is possible. To the best of our knowledge, this paper contributes the first sounder that is capable of mobile measurements at mm-wave frequencies. The hardware implementation of a functional 83.5-GHz system is described in this paper, and some illustrative results, including small-scale statistics and Doppler, are presented.
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paper_title: Wireless Device-to-Device Caching Networks: Basic Principles and System Performance

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As wireless video is the fastest growing form of data traffic, methods for spectrally efficient on-demand wireless video streaming are essential to both service providers and users. A key property of video on-demand is the asynchronous content reuse , such that a few popular files account for a large part of the traffic but are viewed by users at different times. Caching of content on wireless devices in conjunction with device-to-device (D2D) communications allows to exploit this property, and provide a network throughput that is significantly in excess of both the conventional approach of unicasting from cellular base stations and the traditional D2D networks for “regular” data traffic. This paper presents in a tutorial and concise form some recent results on the throughput scaling laws of wireless networks with caching and asynchronous content reuse, contrasting the D2D approach with other alternative approaches such as conventional unicasting, harmonic broadcasting , and a novel coded multicasting approach based on caching in the user devices and network-coded transmission from the cellular base station only. Somehow surprisingly, the D2D scheme with spatial reuse and simple decentralized random caching achieves the same near-optimal throughput scaling law as coded multicasting. Both schemes achieve an unbounded throughput gain (in terms of scaling law) with respect to conventional unicasting and harmonic broadcasting, in the relevant regime where the number of video files in the library is smaller than the total size of the distributed cache capacity in the network. To better understand the relative merits of these competing approaches, we consider a holistic D2D system design incorporating traditional microwave (2 GHz) and millimeter-wave (mm-wave) D2D links; the direct connections to the base station can be used to provide those rare video requests that cannot be found in local caches. We provide extensive simulation results under a variety of system settings and compare our scheme with the systems that exploit transmission from the base station only. We show that, also in realistic conditions and nonasymptotic regimes, the proposed D2D approach offers very significant throughput gains.
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paper_title: Environment Induced Shadowing of Urban Millimeter-Wave Access Links

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In this letter, we investigate how environment induced shadowing influences a 28.5 GHz access link in an urban open square scenario. Shadowing is caused by cars, buses, lorries, and pedestrians passing between the small cell base station and the mobile user terminal. We isolate typical single shadowing events and provide a model for their description. We also present statistical evaluations that can be used as input to 5G access channel models.
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paper_title: Wideband Millimeter-Wave Propagation Measurements and Channel Models for Future Wireless Communication System Design

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The relatively unused millimeter-wave (mmWave) spectrum offers excellent opportunities to increase mobile capacity due to the enormous amount of available raw bandwidth. This paper presents experimental measurements and empirically-based propagation channel models for the 28, 38, 60, and 73 GHz mmWave bands, using a wideband sliding correlator channel sounder with steerable directional horn antennas at both the transmitter and receiver from 2011 to 2013. More than 15,000 power delay profiles were measured across the mmWave bands to yield directional and omnidirectional path loss models, temporal and spatial channel models, and outage probabilities. Models presented here offer side-by-side comparisons of propagation characteristics over a wide range of mmWave bands, and the results and models are useful for the research and standardization process of future mmWave systems. Directional and omnidirectional path loss models with respect to a 1 m close-in free space reference distance over a wide range of mmWave frequencies and scenarios using directional antennas in real-world environments are provided herein, and are shown to simplify mmWave path loss models, while allowing researchers to globally compare and standardize path loss parameters for emerging mmWave wireless networks. A new channel impulse response modeling framework, shown to agree with extensive mmWave measurements over several bands, is presented for use in link-layer simulations, using the observed fact that spatial lobes contain multipath energy that arrives at many different propagation time intervals. The results presented here may assist researchers in analyzing and simulating the performance of next-generation mmWave wireless networks that will rely on adaptive antennas and multiple-input and multiple-output (MIMO) antenna systems.
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paper_title: A statistical model for the shadowing induced by human bodies in the proximity of a mmWaves radio link

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5G technology is a broad concept that describes the envisaged disruptive evolution of communication technology in the near future, with a dramatic increase in the network data-rate and capacity to support a variety of innovative services. The exploitation of the new spectrum available at mmWaves represents a key enabler for 5G. mmWaves are expected to revolutionize the indoor wireless connectivity providing a very large capacity at very high data rates. It is well known that mmWaves radio links are strongly influenced by human bodies and this issue is very relevant in indoor environments. Several models are available for ray-tracing investigations and line-of-sight blockage, whereas statistical models enabling tractable analytical studies and simulations of mmWaves wireless systems, accounting for people in the link's proximity, are still lacking. In this paper, measurements of 60 GHz channel impulse responses in static but “evolutionary” office scenarios that involve one, two and three individuals are presented. Regression fits are applied to the experimental responses to obtain an accurate characterization of human-induced shadowing events in both proximity and blockage situations. Tractable statistical models are provided for different scenarios and for eight carrier frequencies spanning the bands from 54 to 59 GHz and from 61 to 66 GHz.
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paper_title: Exploiting directionality for millimeter-wave wireless system improvement

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This paper presents directional and omnidirectional RMS delay spread statistics obtained from 28 GHz and 73 GHz ultrawideband propagation measurements carried out in New York City using a 400 Megachips per second broadband sliding correlator channel sounder and highly directional steerable horn antennas. The 28 GHz measurements did not systematically seek the optimum antenna pointing angles and resulted in 33% outage for 39 T-R separation distances within 200 m. The 73 GHz measurements systematically found the best antenna pointing angles and resulted in 14.3% outage for 35 T-R separation distances within 200 m, all for mobile height receivers. Pointing the antennas to yield the strongest received power is shown to significantly reduce RMS delay spreads in line-of-sight (LOS) environments. A new term, distance extension exponent (DEE) is defined, and used to mathematically describe the increase in coverage distance that results by combining beams from angles with the strongest received power at a given location. These results suggest that employing directionality in millimeter-wave communications systems will reduce inter-symbol interference, improve link margin at cell edges, and enhance overall system performance.
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paper_title: Directional multipath propagation characteristics based on 28GHz outdoor channel measurements

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Propagation characteristics of millimeter-wave are being widely studied for utilization of the fifth generation (5G) mobile communication systems. To overcome the severe path loss on the high frequency band, highly directive antennas or beamforming techniques using large array antennas can be used to establish a reliable communication link between a transmitter and a receiver. In this paper, a recently conducted outdoor channel measurement using directional horn antenna in the 28GHz frequency band is introduced. The measurement campaign has been conducted in typical urban low-rise and very high-rise environments in Korea. To investigate directional multipath characteristics, large-scale parameters such as root mean square (RMS) delay spread and angular spread of arrival corresponding to the beamwidth of antenna are extracted. Finally, we derived empirical models based on our measurement results to estimate these large-scale parameters.
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paper_title: Spatially consistent pathloss modeling for millimeter-wave channels in urban environments

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This paper consider a fundamental issue of pathloss modeling in urban environments, namely the spatial consistency of the model as the mobile station (MS) moves along a trajectory through street canyons. We show that the traditional model of power law pathloss plus lognormally distributed variations can provide misleading results that can have serious implications for system simulations. Rather, the pathloss coefficient has to be modeled as a random variable that changes from street to street, and is also a function of the street orientation. Variations of the channel gain, taken over the ensemble of the whole cell (or multiple cells) thus consist of the compound effect of these pathloss coefficient variations together with traditional shadowing variations along the trajectory of movement. Ray tracing results demonstrate that ignoring this effect can lead to a severe overestimation of shadowing standard deviation. While the effect is irrelevant for “drop-based” simulations, it can have critical impact on system simulations that require spatial consistency for large-scale movement, such as most mm-wave systems.
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paper_title: An updated model for millimeter wave propagation in moist air

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A practical atmospheric Millimeter-Wave Propagation Model (MPM) is formulated that predicts attenuation. delay, and noise properties of moist air for frequencies up to 1000 GHz. Input variables are height distributions (0-30 km) of pressure, temperature, humidity, and suspended droplet concentration along an anticipated radio path. Spectroscopic data consist of more than 450 parameters describing local O2 and H2O absorption lines complemented by continuum spectra for dry air, water vapor, and hydrosols. For a model (MPM*) limited to frequencies below 300 GHz, the number of spectroscopic parameters can be reduced to less than 200. Recent laboratory measurements by us at 138 GHz of absolute attenuation rates for simulated air with water vapor pressures up to saturation allow the formulation of an improved, though empirical water vapor continuum. Model predictions are compared with selected (2.5-430 GHz) data from both laboratory and field experiments. In general, good agreement is obtained.
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paper_title: Wireless Communications

paper_content:

"Professor Andreas F. Molisch, renowned researcher and educator, has put together the comprehensive book, Wireless Communications. The second edition, which includes a wealth of new material on important topics, ensures the role of the text as the key resource for every student, researcher, and practitioner in the field."Professor Moe Win, MIT, USAWireless communications has grown rapidly over the past decade from a niche market into one of the most important, fast moving industries. Fully updated to incorporate the latest research and developments, Wireless Communications, Second Edition provides an authoritative overview of the principles and applications of mobile communication technology.The author provides an in-depth analysis of current treatment of the area, addressing both the traditional elements, such as Rayleigh fading, BER in flat fading channels, and equalisation, and more recently emerging topics such as multi-user detection in CDMA systems, MIMO systems, and cognitive radio. The dominant wireless standards; including cellular, cordless and wireless LANs; are discussed.Topics featured include: wireless propagation channels, transceivers and signal processing, multiple access and advanced transceiver schemes, and standardised wireless systems.Combines mathematical descriptions with intuitive explanations of the physical facts, enabling readers to acquire a deep understanding of the subject.Includes new chapters on cognitive radio, cooperative communications and relaying, video coding, 3GPP Long Term Evolution, and WiMax; plus significant new sections on multi-user MIMO, 802.11n, and information theory.Companion website featuring: supplementary material on 'DECT', solutions manual and presentation slides for instructors, appendices, list of abbreviations and other useful resources.
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paper_title: Wideband characterisation of the propagation channel for outdoors at 60 GHz

paper_content:

This article presents a model the analysis of the propagation channel mobile communications in urban streets at the millimetrewave band. A ray-tracing tool, based on geometrical optics, is used to calculate the impulse response at the mobile, when it moves along the street; the scenario is defined by the ground and walls surfaces, and may have obstructing objects within it. A comparison of the statistical distributions of the usual impulse response parameters resulting from the simulations with measured ones shows a good agreement. Further simulations show that the dimensions of the street and the roughness of its walls, the existence of trees, and others factors, influence the average impulse response.
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paper_title: An Empirical Study of Urban Macro Propagation at 10, 18 and 28 GHz

paper_content:

This paper investigates the propagation characteristics of the urban macro cells at centimeter-wave (cmWave) frequencies, in particular at 10, 18 and 28 GHz. The measurements are performed at several transmitter (Tx) locations and heights, in both line-of-sight (LOS) and non line-of-sight (NLOS) conditions, and with distances up to 1,400 m. The distancedependent mean path loss and shadow fading standard deviation (std) are extracted for all cases based on a single-slope path loss model, and offered here for quick determination of link budget and system capacity. The results show the potential usage of the cmWave band for mobile cellular services in the years to come: the NLOS path loss slopes at 10 and 18 GHz are not much different from the 2 GHz reference, and the corresponding offsets are in the order of 20-23 dB for 25 m Tx height. This gap is expected to be overcome by the usage of high-gain miniaturized steerable antennas, which is feasible due to the reduced antenna aperture size at the cmWave band. Similar to the 2 GHz band, the NLOS shadow fading std for cmWave is within 6 dB. The effect of Tx height is clearly shown in the NLOS scenario: at 10 GHz, for example, 7.5 dB reduction in attenuation could be achieved by raising the Tx antenna from 15 m (below average roof-top) to 25 m (above roof-top), or 23.4 dB if the Tx height is elevated to 54 m.
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paper_title: Quasi-deterministic millimeter-wave channel models in MiWEBA

paper_content:

This article introduces a quasi-deterministic channel model and a link level-focused channel model, developed with a focus on millimeter-wave outdoor access channels. Channel measurements in an open square scenario at 60 GHz are introduced as a basis for the development of the model and its parameterization. The modeling approaches are explained, and their specific area of application is investigated.
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paper_title: Proposal on Millimeter-Wave Channel Modeling for 5G Cellular System

paper_content:

This paper presents 28 GHz wideband propagation channel characteristics for millimeter wave (mmWave) urban cellular communication systems. The mmWave spectrum is considered as a key-enabling feature of 5G cellular communication systems to provide an enormous capacity increment; however, mmWave channel models are lacking today. The paper compares measurements conducted with a spherical scanning 28 GHz channel sounder system in the urban street-canyon environments of Daejeon, Korea and NYU campus, Manhattan, with ray-tracing simulations made for the same areas. Since such scanning measurements are very costly and time-intensive, only a relatively small number of channel samples can be obtained. The measurements are thus used to quantify the accuracy of a ray-tracer; the ray-tracer is subsequently used to obtain a large number of channel samples to fill gaps in the measurements. A set of mmWave radio propagation parameters is presented based on both the measurement results and ray-tracing, and the corresponding channel models following the 3GPP spatial channel model (SCM) methodology are also described.
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paper_title: Directional Analysis of Measured 60 GHz Indoor Radio Channels Using SAGE

paper_content:

Directional properties of the radio channel are of high importance for the development of reliable wireless systems operating in the 60 GHz frequency band. Using transfer functions measured from 61 to 65 GHz in a conference room we have extracted estimates of the multi-path component parameters using the SAGE algorithm. In the paper we compare results for line-of-sight (LOS) scenarios and the corresponding non-line-of-sight (NLOS) scenarios and present values of the direction spread at the Tx and the Rx.
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paper_title: Propagation in ROF road-vehicle communication system using millimeter wave

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Radio on fiber (ROF) technology has received attention due to advantages such as multimode service for intelligent transport systems (ITS) road-vehicle- communication. For this system, using millimeter wave has the advantages of its large capacity and multi-transmission capability. We measured the effect of shadowing by another vehicle in case of road vehicle communication system and the effect of position of mobile station antennas in 36 GHz band.
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paper_title: 5G 3GPP-like Channel Models for Outdoor Urban Microcellular and Macrocellular Environments

paper_content:

For the development of new 5G systems to operate in bands up to 100 GHz, there is a need for accurate radio propagation models at these bands that currently are not addressed by existing channel models developed for bands below 6 GHz. This document presents a preliminary overview of 5G channel models for bands up to 100 GHz. These have been derived based on extensive measurement and ray tracing results across a multitude of frequencies from 6 GHz to 100 GHz, and this document describes an initial 3D channel model which includes: 1) typical deployment scenarios for urban microcells (UMi) and urban macrocells (UMa), and 2) a baseline model for incorporating path loss, shadow fading, line of sight probability, penetration and blockage models for the typical scenarios. Various processing methodologies such as clustering and antenna decoupling algorithms are also presented.
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paper_title: Investigation of Prediction Accuracy, Sensitivity, and Parameter Stability of Large-Scale Propagation Path Loss Models for 5G Wireless Communications

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This paper compares three candidate large-scale propagation path loss models for use over the entire microwave and millimeter-wave (mmWave) radio spectrum: the alpha-betagamma (ABG) model, the close-in (CI) free-space reference distance model, and the CI model with a frequency-weighted path loss exponent (CIF). Each of these models has been recently studied for use in standards bodies such as 3rd Generation Partnership Project (3GPP) and for use in the design of fifth-generation wireless systems in urban macrocell, urban microcell, and indoor office and shopping mall scenarios. Here, we compare the accuracy and sensitivity of these models using measured data from 30 propagation measurement data sets from 2 to 73 GHz over distances ranging from 4 to 1238 m. A series of sensitivity analyses of the three models shows that the physically based two-parameter CI model and three-parameter CIF model offer computational simplicity, have very similar goodness of fit (i.e., the shadow fading standard deviation), exhibit more stable model parameter behavior across frequencies and distances, and yield smaller prediction error in sensitivity tests across distances and frequencies, when compared to the four-parameter ABG model. Results show the CI model with a 1-m reference distance is suitable for outdoor environments, while the CIF model is more appropriate for indoor modeling. The CI and CIF models are easily implemented in existing 3GPP models by making a very subtle modification — by replacing a floating non-physically based constant with a frequency-dependent constant that represents free-space path loss in the first meter of propagation. This paper shows this subtle change does not change the mathematical form of existing ITU/3GPP models and offers much easier analysis, intuitive appeal, better model parameter stability, and better accuracy in sensitivity tests over a vast range of microwave and mmWave frequencies, scenarios, and distances, while using a simpler model with fewer parameters. This material is based upon work supported by the NYU WIRELESS Industrial Affiliates: ATT tsr@nyu.edu). T. A. Thomas and A. Ghosh are with Nokia, Arlington Heights, IL 60004, USA (e-mail: timothy.thomas@nokia.com; amitava.ghosh@nokia.com). H. Nguyen and I. Rodriguez are with Aalborg University, Aalborg 9220, Denmark (e-mail: huan.nguyen.ext@nokia.com; irl@es.aau.dk). I. Z. Kovacs is with Nokia, Aalborg 9220, Denmark (e-mail: istvan.kovacs@nokia.com). O. Koymen and A. Partyka are with Qualcomm RD apartyka@qti.qualcomm.com).
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paper_title: Propagation measurements and simulations for millimeter-wave mobile access in a busy urban environment

paper_content:

The authors have performed 60 GHz wideband channel measurements in Berlin to gain knowledge on millimeter-wave outdoor propagation in dense urban environments. In this paper first results for a street canyon are presented, focusing on path loss analysis of the line-of-sight-dominant small-cell access channel. They reveal that the local area path loss is very close to free-space propagation. Reflected paths do not contribute significantly to the received power as long as the line of sight (LOS) is unobstructed. However, they are usually strong enough to maintain a link if the LOS is blocked. The dominant multipath contributions can also be predicted by ray tracing simulations, though further calibration is needed to accurately determine their strength.
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paper_title: Wideband spatial channel model in an urban cellular environments at 28 GHz

paper_content:

This paper presents channel propagation measurements and analysis of the channel characteristics of millimeter wave (mmWave) transmission for urban cellular communication systems, in particular in the promising 28 GHz band. For channel propagation analysis, the urban measurement campaign was conducted with a synchronously spherical scanning 28 GHz channel sounder system, from which omni-like channel measurements are obtained for channel modeling. From the measurements, we analyze the spatio-temporal channel characteristics such as multipath delay, angular statistics, and pathloss. The clustering analysis has been done including its power distribution. Then, a set of millimeter wave radio propagation parameters is presented, and the corresponding channel models based on the 3GPP spatial channel model (SCM) are also described.
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paper_title: Frequency-Agile Pathloss Models for Urban Street Canyons

paper_content:

Frequency-agile pathloss models for urban street canyons are discussed in this paper. The models are floating intercept (FI), fixed reference (FR), and ITU-R M.2135 urban microcellular (UMi) line-of-sight (LOS) and Manhattan-grid non-LOS (NLOS) models. These models are parameterized based on channel sounding campaigns in three cities covering radio frequencies ranging from 0.8 to 60 GHz. Fitting the models with measured pathloss reveals that the models are usable to cover the considered frequency range. The FI and FR models are equally simple and robust, with a slight advantage of the FI model in accuracy because of the larger number of model parameters. The original M.2135 LOS model is based on a two-ray model that includes a break point (BP). The model is extended for a better fit with measurements by including new model parameters such as a pathloss offset and a BP scaling factor that represent local scattering conditions of surrounding environments. The new model parameters are found frequency dependent in many cases. The original M.2135 model is furthermore simplified in NLOS scenarios while maintaining the model accuracy. The model parameters are derived using maximum likelihood estimation, which also showed that the modified M.2135 model offers up to 50% better accuracy compared to the FI and FR models in terms of the employed log-likelihood function (LLF). The improvement in accuracy is particularly remarkable in NLOS scenarios. A full set of parameters is provided for the models, allowing a choice for any given requirements on accuracy and complexity. Finally, applicability of the proposed models to other street canyons is discussed using independent pathloss measurements.
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paper_title: 73 GHz Wideband Millimeter-Wave Foliage and Ground Reflection Measurements and Models

paper_content:

This paper presents 73 GHz wideband outdoor foliage and ground reflection measurements. Propagation measurements were made with a 400 Megachips-per-second sliding correlator channel sounder, with rotatable 27 dBi (7° half-power beamwidth) horn antennas at both the transmitter and receiver, to study foliage-induced scattering and de-polarization effects, to assist in developing future wireless systems that will use adaptive array antennas. Signal attenuation through foliage was measured to be 0.4 dB/m for both co- and cross-polarized antenna configurations. Measured ground reflection coefficients for dirt and gravel ranged from 0.02 to 0.34, for incident angles ranging from 60° to 81° (with respect to the normal incidence of the surface). These data are useful for link budget design and site-specific (ray-tracing) models for future millimeter-wave communications systems.
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paper_title: Centimeter and millimeter wave attenuation and brightness temperature due to atmospheric oxygen and water vapor

paper_content:

Calculated atmospheric absorption and emission of radiation for several atmospheric conditions and elevation angles are given for frequencies of 1 to 340 GHz. The emission curves are those accepted in 1982 by the International Radio Consultative Committee (CCIR). The absorption values from these calculations are compared with others appearing in the literature or in use elsewhere.
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paper_title: Study of the local multipoint distribution service radio channel

paper_content:

Millimeter wave communication systems in the 21.5 to 29.5 GHz band are being developed in the United States and Canada for use in a local multipoint distribution service (LMDS). This paper summarizes radiowave propagation impairments for the LMDS and reports measurement data for small cells. Results include area coverage estimates over a range of basic transmission losses for 0.5-, 1.0- and 2.0-km suburban cells with foliated trees. Multipath, signal attenuation, depolarization, and cell to cell coverage also are discussed. Data indicates a high probability of non-line-of-sight paths due to trees which can cause signal attenuation and signal variability when wind is present. Signal variability was studied using k factors and compared to the Rician cumulative distribution function. Depolarization caused by vegetation and other signal scatterers was found to be an order of magnitude greater than rain-induced depolarization. A simple tapped delay line model is presented to describe multipath for three channel states.
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paper_title: 28 GHz channel measurements and modeling in a ski resort town in pyeongchang for 5G cellular network systems

paper_content:

In this paper, radio propagation measurements and analysis are presented to investigate the channel characteristics of millimeter wave (mmWave) transmission for outdoor cellular communication systems. The measurement campaign is performed in a ski resort town placed in Pyeongchang, South Korea, which is the venue of the coming Winter Olympics in 2018. A 28 GHz synchronous channel sounder system is used in conducting the channel measurements. From the measurements, the spatio-temporal channel characteristics such as path loss, multipath delay, angular statistics are analyzed. The mmWave radio propagation parameters and the corresponding channel models based on the 3GPP spatial channel model (SCM) are also presented. Since a trial service is planned during the 2018 Pyeongchang Winter Olympic Games, this research is significant to demonstrate the emerging 5G technologies. Additionally, blocking tests, which measure the path loss variation when there are some obstacles, are discussed in this paper.
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paper_title: A Statistical Spatio-Temporal Radio Channel Model for Large Indoor Environments at 60 and 70 GHz

paper_content:

Millimeter-wave radios operating at unlicensed 60 GHz and licensed 70 GHz bands are attractive solutions to realize short-range backhaul links for flexible wireless network deployment. We present a measurement-based spatio-temporal statistical channel model for short-range millimeter-wave links in large office rooms, shopping mall, and station scenarios. Channel sounding in these scenarios at 60 and 70 GHz revealed that spatio-temporal channel characteristics of the two frequencies are similar, making it possible to use an identical channel model framework to cover the radio frequencies and scenarios. The sounding also revealed dominance of a line-of-sight and specular propagation paths over diffuse scattering because of weak reverberation of propagating energy in the scenarios. The main difference between 60 and 70 GHz channels lies in power levels of the specular propagation paths and diffuse scattering which affect their visibility over the noise level in the measurements, and the speed of power decay as the propagation delay increases. Having defined the channel model framework, a set of model parameters has been derived for each scenario at the two radio frequencies. After specifying the implementation recipe of the proposed channel model, channel model outputs are compared with the measurements to show validity of the channel model framework and implementation. Validity was demonstrated through objective parameters, i.e., pathloss and root-mean-square delay spread, which were not used as defining parameters of the channel model.
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paper_title: On mm-Wave Multipath Clustering and Channel Modeling

paper_content:

Efficient and realistic mm-wave channel models are of vital importance for the development of novel mm-wave wireless technologies. Though many of the current 60 GHz channel models are based on the useful concept of multipath clusters, only a limited number of 60 GHz channel measurements have been reported in the literature for this purpose. Therefore, there is still a need for further measurement based analyses of multipath clustering in the 60 GHz band. This paper presents clustering results for a double-directional 60 GHz MIMO channel model. Based on these results, we derive a model which is validated with measured data. Statistical cluster parameters are evaluated and compared with existing channel models. It is shown that the cluster angular characteristics are closely related to the room geometry and environment, making it infeasible to model the delay and angular domains independently. We also show that when using ray tracing to model the channel, it is insufficient to only consider walls, ceiling, floor and tables; finer structures such as ceiling lamps, chairs and bookshelves need to be taken into account as well.
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paper_title: Indoor mm-Wave Channel Measurements: Comparative Study of 2.9 GHz and 29 GHz

paper_content:

The millimeter-wave (mm-Wave) frequency band ~30-300 GHz has received significant attention lately as a prospective band for 5G systems. Millimeter-wave frequencies have traditionally been used for backhaul, satellite and other fixed services. While these bands offer substantial amount of bandwidth and opportunity for spatial multiplexing, the propagation characteristics for terrestrial mobile usage need to be fully understood prior to system design. Towards this end, this paper presents preliminary indoor measurement results obtained using a channel sounder equipped with omni- and directional antennas at 2.9 GHz and 29 GHz as a comparative study of the two bands. The measurements are made within a Qualcomm building in Bridgewater, NJ, USA, for two separate floors, each representing a different yet representative type of office plan. We present measurements and estimated parameters for path loss, excess delay, RMS delay and analyze the power profile of received paths. In addition, we present several spherical scans of particular links to illustrate the 3-D angular spread of the received paths. This work represents initial results of an ongoing effort for comprehensive indoor and outdoor channel measurements. The measurements presented here, along with cited references, offer interesting insights into propagation conditions (e.g. loss, delay/angular spread etc.), coverage and robustness for mobile use of millimeter-wave bands. We believe additional extensive measurement campaigns in diverse settings by academia and industry would help facilitate the generation of usable channel models.
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paper_title: Indoor 5G 3GPP-like Channel Models for Office and Shopping Mall Environments

paper_content:

Future mobile communications systems are likely to be very different to those of today with new service innovations driven by increasing data traffic demand, increasing processing power of smart devices and new innovative applications. To meet these service demands the telecommunications industry is converging on a common set of 5G requirements which includes network speeds as high as 10 Gbps, cell edge rate greater than 100 Mbps, and latency of less than 1 msec. To reach these 5G requirements the industry is looking at new spectrum bands in the range up to 100 GHz where there is spectrum availability for wide bandwidth channels. For the development of new 5G systems to operate in bands up to 100 GHz there is a need for accurate radio propagation models which are not addressed by existing channel models developed for bands below 6 GHz. This paper presents a preliminary overview of the 5G channel models for bands up to 100 GHz in indoor offices and shopping malls, derived from extensive measurements across a multitude of bands. These studies have found some extensibility of the existing 3GPP models (e.g. 3GPP TR36.873) to the higher frequency bands up to 100 GHz. The measurements indicate that the smaller wavelengths introduce an increased sensitivity of the propagation models to the scale of the environment and show some frequency dependence of the path loss as well as increased occurrence of blockage. Further, the penetration loss is highly dependent on the material and tends to increase with frequency. The small-scale characteristics of the channel such as delay spread and angular spread and the multipath richness is somewhat similar over the frequency range, which is encouraging for extending the existing 3GPP models to the wider frequency range. Further work will be carried out to complete these models, but this paper presents the first steps for an initial basis for the model development.
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paper_title: Experimental investigations of 60 GHz WLAN systems in office environment

paper_content:

This paper presents the results of an experimental investigation of 60 GHz wireless local area network (WLAN) systems in an office environment. The measurement setup with highly directional mechanically steerable antennas and 800 MHz bandwidth was developed and experiments were performed for conference room and cubicle environments. Measurement results demonstrate that the 60 GHz propagation channel is quasioptical in nature and received signal power is obtained through line of sight (LOS) and reflected signal paths of the first and second orders. The 60 GHz WLAN system prototype using steerable directional antennas with 18 dB gain was able to achieve about 30 dB baseband SNR for LOS transmission, about 15-20 dB for communications through the first-order reflected path, and 2-6 dB SNR when using second-order reflection for the office environments. The intra cluster statistical parameters of the propagation channel were evaluated and a statistical model for reflected clusters is proposed. Experimental results demonstrating strong polarization impact on the characteristics of the propagation channel are presented. Cross-polarization discrimination (XPD) of the propagation channel was estimated as approximately 20 dB for LOS transmission and 10-20 dB for NLOS reflected paths.
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paper_title: Experimental Multipath-Cluster Characteristics of 28-GHz Propagation Channel

paper_content:

In this paper, a channel measurement campaign is introduced, which utilizes direction-scan-sounding to capture the spatial characteristics of 28-GHz wave propagation channels with 500-MHz sounding bandwidth in office environments. Both line-of-sight and non-line-of-sight scenarios were considered. Measurements were performed by fixing a transmit pyramidal horn antenna, and rotating another one in the receiver site at 10° steps in azimuth. The antenna outputs are viewed as array signals, and a space-alternating generalized expectation-maximization (SAGE) algorithm is applied to estimate delay and angular parameters of multipath components. Benefiting from high resolution achieved by using the SAGE and deembedding of antenna radiation pattern and system responses, more multipath clusters with less spreads in delay and azimuth are found per channel compared with existing works on 28-GHz propagation. The statistics of channel parameters extracted here constitute a preliminary stochastic multipath-cluster spatial channel model.
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paper_title: Wideband channel sounder with measurements and model for the 60 GHz indoor radio channel

paper_content:

A wideband channel sounder and measurement results for the short range indoor 60 GHz channel are presented. The channel sounder is based on a 1 gigasamples/s dual channel arbitrary waveform generator and A/D converter/software demodulator, which synthesize and detect a baseband PN sequence with 500 MHz bandwidth. A heterodyne transmitter and receiver translate the baseband PN sequence to and from the 60 GHz band. Ten channel measurements taken across the 59 GHz to 64 GHz range are concatenated to provide a continuous channel measurement covering 5 GHz of bandwidth, resulting in 0.2 ns time domain channel impulse response resolution. The dynamic range and maximum sensitivity performance of the channel sounder are discussed in detail. Comparisons of results with a vector network analyzer based system are shown to verify the accuracy of the sounder. In an extensive measurement campaign with vertically polarized omnidirectional antennas, several different rooms (offices, labs, conference rooms and others) in four different buildings have been investigated. Over 700 channel measurements are the basis for a comprehensive characterization of the short range 60 GHz indoor radio channel with omnidirectional antennas. Finally, a simple stochastic static multipath channel model is derived from the measurement results.
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paper_title: Frequency-domain measurement of 60 GHz indoor channels: a measurement setup, literature data, and analysis

paper_content:

Despite the unique capability of 60-GHz technology to offer a multi-gigabit rate and a huge unlicensed bandwidth (up to 7 GHz), a number of technical challenges need to be overcome before its full deployment. The system performance of capacity, coverage, and throughput need to be well understood. All of these are based on characterizing the propagation channel and establishing realistic channel models of wireless systems [1]. Many researchers have reported on propagation studies of indoor channels at 60 GHz using frequency-domain measurements. However, details are scarce in the literature on whether different indoor environments and different frequency-domain measurement setups affect the measurement results in the millimeter-wave frequency band. This article explains the setup details of time resolution, spatial resolution, and windowing, then summarizes and analyzes frequency-domain measurement results selected from important research [2]-[9]. The mean path loss model and the average cumulative distribution function of a root mean squared (rms) delay spread are proposed and compared using measured results from the literature to describe the complete channel characteristics of indoor environments at 60 GHz.
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paper_title: 60 GHz channel directional characterization using extreme size virtual antenna array

paper_content:

In order to provide reliable knowledge about highly resolved directional properties of the radio propagation channel, straight forward beam-forming has been used in this study. Accurate measurement data based on an extreme size virtual antenna array (25×25×25 = 15625 elements) have been provided for an indoor scenario in both line of sight and non-line if sight conditions. The results indicate that the distinct spikes observed in the power-delay profile are caused mainly by specular reflections. There is however also a significant diffuse contribution due to scattering caused by the many smaller objects of the environment. The diffuse paths are spread out in essentially all azimuth and elevation directions except for the empty parts of the floor.
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paper_title: Statistical Characterization of 60-GHz Indoor Radio Channels

paper_content:

An extensive review of the statistical characterization of 60-GHz indoor radio channels is provided from a large number of published measurement and modeling results. First, the most prominent driver applications for 60 GHz are considered in order to identify those environment types that need to be characterized most urgently. Large-scale fading is addressed yielding path-loss parameter values for a generic 60-GHz indoor channel model as well as for the office environment in particular. In addition, the small-scale channel behavior is reviewed including the modeling of time-of-arrival and angle-of-arrival details and statistical parameters related to delay spread, angular spread and Doppler spread. Finally, some research directions for future channel characterization are given.
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paper_title: Indoor Office Wideband Millimeter-Wave Propagation Measurements and Channel Models at 28 and 73 GHz for Ultra-Dense 5G Wireless Networks

paper_content:

Ultra-wideband millimeter-wave (mmWave) propagation measurements were conducted in the 28- and 73-GHz frequency bands in a typical indoor office environment in downtown Brooklyn, New York, on the campus of New York University. The measurements provide large-scale path loss and temporal statistics that will be useful for ultra-dense indoor wireless networks for future mmWave bands. This paper presents the details of measurements that employed a 400 Megachips-per-second broadband sliding correlator channel sounder, using rotatable highly directional horn antennas for both co-polarized and crosspolarized antenna configurations. The measurement environment was a closed-plan in-building scenario that included a line-of-sight and non-line-of-sight corridor, a hallway, a cubicle farm, and adjacent-room communication links. Well-known and new single-frequency and multi-frequency directional and omnidirectional large-scale path loss models are presented and evaluated based on more than 14 000 directional power delay profiles acquired from unique transmitter and receiver antenna pointing angle combinations. Omnidirectional path loss models, synthesized from the directional measurements, are provided for the case of arbitrary polarization coupling, aswell as for the specific cases of co-polarized and cross-polarized antenna orientations. The results show that novel large-scale path loss models provided here are simpler and more physically based compared to previous 3GPP and ITU indoor propagation models that require more model parameters and offer very little additional accuracy and lack a physical basis. Multipath time dispersion statistics formmWave systems using directional antennas are presented for co-polarization, crosspolarization, and combined-polarization scenarios, and show that the multipath root mean square delay spread can be reduced when using transmitter and receiver antenna pointing angles that result in the strongest received power. Raw omnidirectional path loss data and closed-form optimization formulas for all path loss models are given in the Appendices.
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paper_title: The COST259 Directional Channel Model-Part I: Overview and Methodology

paper_content:

This paper describes a model for mobile radio channels that includes consideration of directions of arrival and is thus suitable for simulations of the performance of wireless systems that use smart antennas. The model is specified for 13 different types of environments, covering macro- micro- and picocells. In this paper, a hierarchy of modeling concepts is described, as well as implementation aspects that are valid for all environments. The model is based on the specification of directional channel impulse response functions, from which the impulse response functions at all antenna elements can be obtained. A layered approach, which distinguishes between external (fixed), large-scale-, and small-scale- parameters allows an efficient parameterization. Different implementation methods, based on either a tapped-delay line or a geometrical model, are described. The paper also derives the transformation between those two approaches. Finally, the concepts of clusters and visibility regions are used to account for large delay and angular spreads that have been measured. In two companion papers, the environment-specific values of the model parameters are explained and justified
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paper_title: The COST259 Directional Channel Model-Part I: Overview and Methodology

paper_content:

This paper describes a model for mobile radio channels that includes consideration of directions of arrival and is thus suitable for simulations of the performance of wireless systems that use smart antennas. The model is specified for 13 different types of environments, covering macro- micro- and picocells. In this paper, a hierarchy of modeling concepts is described, as well as implementation aspects that are valid for all environments. The model is based on the specification of directional channel impulse response functions, from which the impulse response functions at all antenna elements can be obtained. A layered approach, which distinguishes between external (fixed), large-scale-, and small-scale- parameters allows an efficient parameterization. Different implementation methods, based on either a tapped-delay line or a geometrical model, are described. The paper also derives the transformation between those two approaches. Finally, the concepts of clusters and visibility regions are used to account for large delay and angular spreads that have been measured. In two companion papers, the environment-specific values of the model parameters are explained and justified
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paper_title: A twin-cluster MIMO channel model

paper_content:

The paper presents a new approach to geometry-based stochastic channel models that can be used for simulating MIMO systems. We use twin-clusters to represent multiply reflected or diffracted multipath components. The location of the two twins can be chosen independently, in order to correctly reflect DoAs, DoDs, and delays. The model is thus more accurate than existing single-scatterer approaches. Simulation results, using a publicly available version of our model, show a very realistic behavior of our model.
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paper_title: "Virtual Cell Deployment Areas" and "Cluster Tracing" - new methods for directional channel modeling in microcells

paper_content:

We propose a new method for the simulation of microcellular propagation channels, which includes both deterministic and stochastic aspects. First, the geometrical layouts of "typical" deployment areas, called "Virtual Cell Deployment Areas" (VCDA), as well as routes of the mobile station (MS) through the VCDA, are defined. Then, the delays and directions-of-arrival of multipath clusters can be traced by geometrical means. Finally, the small-scale fading within the clusters is modelled stochastically. This approach was adopted by the COST259 DCM, the standard model for directional wireless channels.
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paper_title: Quasi-deterministic millimeter-wave channel models in MiWEBA

paper_content:

This article introduces a quasi-deterministic channel model and a link level-focused channel model, developed with a focus on millimeter-wave outdoor access channels. Channel measurements in an open square scenario at 60 GHz are introduced as a basis for the development of the model and its parameterization. The modeling approaches are explained, and their specific area of application is investigated.
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paper_title: An ultra-wideband space-variant multipath indoor radio channel model

paper_content:

We present a generic ultra-wideband (UWB) multipath radio channel model that has been designed for link-level simulations and is suited for indoor environments, e.g. for WPAN applications. It covers the FCC frequency band [FCC, Feb. 2002] and is capable of producing space-variant impulse responses (IR) and transfer functions. The model is of a hybrid type and combines a statistical approach to model dense multipath and a simple quasi-deterministic method to model strong individual echoes. These echoes are important to obtain a realistic spatial (Doppler) behavior. The architecture of the model facilitates the application to UWB MIMO analyses. For a few scenarios (e.g., intra-office and inter-office transmission), site-specific parameter profiles are given that have been derived from measured data.
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paper_title: Scaling up MIMO: Opportunities and challenges with very large arrays

paper_content:

Presents a list of articles published by the IEEE Signal Processing Society (SPS) that ranked among the top 100 most downloaded IEEE Xplore articles.
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paper_title: Enabling technologies and architectures for 5G wireless

paper_content:

The proliferation of smart devices and the resulting exponential growth in data traffic has increased the need for higher capacity wireless networks. The cellular systems industry is envisioning an increase in network capacity by a factor of 1000 over the next decade to meet this traffic demand. In addition, with the emergence of Internet of Things (IoT), billions of devices will be connected and managed by wireless networks. Future networks must satisfy the above mentioned requirements with high energy efficiency and at low cost. Hence, the industry attention is now shifting towards the next set of innovations in architecture and technologies that will address capacity and service demands envisioned for 2020, which cannot be met only with the evolution of 4G systems. These innovations are expected to form the so called fifth generation wireless communications systems, or 5G. Candidate 5G solutions include i) higher densification of heterogeneous networks with massive deployment of small base stations supporting various Radio Access Technologies (RATs), ii) use of very large Multiple Input Multiple Output (MIMO) arrays, iii) use of millimeter Wave spectrum where larger wider frequency bands are available, iv) direct device to device (D2D) communication, and v) simultaneous transmission and reception, among others. In this paper, we present the main 5G technologies. We also discuss the network and device evolution towards 5G.
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paper_title: Millimeter Wave Mobile Communications for 5G Cellular: It Will Work!

paper_content:

The global bandwidth shortage facing wireless carriers has motivated the exploration of the underutilized millimeter wave (mm-wave) frequency spectrum for future broadband cellular communication networks. There is, however, little knowledge about cellular mm-wave propagation in densely populated indoor and outdoor environments. Obtaining this information is vital for the design and operation of future fifth generation cellular networks that use the mm-wave spectrum. In this paper, we present the motivation for new mm-wave cellular systems, methodology, and hardware for measurements and offer a variety of measurement results that show 28 and 38 GHz frequencies can be used when employing steerable directional antennas at base stations and mobile devices.
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paper_title: Multi-gigabit millimeter wave wireless communications for 5G: from fixed access to cellular networks

paper_content:

With the formidable growth of various booming wireless communication services that require ever increasing data throughputs, the conventional microwave band below 10 GHz, which is currently used by almost all mobile communication systems, is going to reach its saturation point within just a few years. Therefore, the attention of radio system designers has been pushed toward ever higher segments of the frequency spectrum in a quest for increased capacity. In this article we investigate the feasibility, advantages, and challenges of future wireless communications over the Eband frequencies. We start with a brief review of the history of the E-band spectrum and its light licensing policy as well as benefits/challenges. Then we introduce the propagation characteristics of E-band signals, based on which some potential fixed and mobile applications at the E-band are investigated. In particular, we analyze the achievability of a nontrivial multiplexing gain in fixed point-to-point E-band links, and propose an E-band mobile broadband (EMB) system as a candidate for the next generation mobile communication networks. The channelization and frame structure of the EMB system are discussed in detail.
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paper_title: Massive MIMO in the UL/DL of Cellular Networks: How Many Antennas Do We Need?

paper_content:

We consider the uplink (UL) and downlink (DL) of non-cooperative multi-cellular time-division duplexing (TDD) systems, assuming that the number N of antennas per base station (BS) and the number K of user terminals (UTs) per cell are large. Our system model accounts for channel estimation, pilot contamination, and an arbitrary path loss and antenna correlation for each link. We derive approximations of achievable rates with several linear precoders and detectors which are proven to be asymptotically tight, but accurate for realistic system dimensions, as shown by simulations. It is known from previous work assuming uncorrelated channels, that as N→∞ while K is fixed, the system performance is limited by pilot contamination, the simplest precoders/detectors, i.e., eigenbeamforming (BF) and matched filter (MF), are optimal, and the transmit power can be made arbitrarily small. We analyze to which extent these conclusions hold in the more realistic setting where N is not extremely large compared to K. In particular, we derive how many antennas per UT are needed to achieve η% of the ultimate performance limit with infinitely many antennas and how many more antennas are needed with MF and BF to achieve the performance of minimum mean-square error (MMSE) detection and regularized zero-forcing (RZF), respectively.
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paper_title: Five Disruptive Technology Directions for 5G

paper_content:

New research directions will lead to fundamental changes in the design of future 5th generation (5G) cellular networks. This paper describes five technologies that could lead to both architectural and component disruptive design changes: device-centric architectures, millimeter Wave, Massive-MIMO, smarter devices, and native support to machine-2-machine. The key ideas for each technology are described, along with their potential impact on 5G and the research challenges that remain.
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paper_title: Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas

paper_content:

A cellular base station serves a multiplicity of single-antenna terminals over the same time-frequency interval. Time-division duplex operation combined with reverse-link pilots enables the base station to estimate the reciprocal forward- and reverse-link channels. The conjugate-transpose of the channel estimates are used as a linear precoder and combiner respectively on the forward and reverse links. Propagation, unknown to both terminals and base station, comprises fast fading, log-normal shadow fading, and geometric attenuation. In the limit of an infinite number of antennas a complete multi-cellular analysis, which accounts for inter-cellular interference and the overhead and errors associated with channel-state information, yields a number of mathematically exact conclusions and points to a desirable direction towards which cellular wireless could evolve. In particular the effects of uncorrelated noise and fast fading vanish, throughput and the number of terminals are independent of the size of the cells, spectral efficiency is independent of bandwidth, and the required transmitted energy per bit vanishes. The only remaining impairment is inter-cellular interference caused by re-use of the pilot sequences in other cells (pilot contamination) which does not vanish with unlimited number of antennas.
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paper_title: Evolution Towards 5G Multi-tier Cellular Wireless Networks: An Interference Management Perspective

paper_content:

The evolving fifth generation (5G) cellular wireless networks are envisioned to overcome the fundamental challenges of existing cellular networks, for example, higher data rates, excellent end-to-end performance, and user-coverage in hot-spots and crowded areas with lower latency, energy consumption, and cost per information transfer. To address these challenges, 5G systems will adopt a multi-tier architecture consisting of macrocells, different types of licensed small cells, relays, and device-to-device (D2D) networks to serve users with different quality-of-service (QoS) requirements in a spectrum and energy-efficient manner. Starting with the visions and requirements of 5G multi-tier networks, this article outlines the challenges of interference management (e.g. power control, cell association) in these networks with shared spectrum access (i.e. when the different network tiers share the same licensed spectrum). It is argued that the existing interference management schemes will not be able to address the interference management problem in prioritized 5G multi-tier networks where users in different tiers have different priorities for channel access. In this context a survey and qualitative comparison of the existing cell association and power control schemes is provided to demonstrate their limitations for interference management in 5G networks. Open challenges are highlighted and guidelines are provided to modify the existing schemes in order to overcome these limitations and make them suitable for the emerging 5G systems.
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paper_title: Emerging Technologies and Research Challenges for 5G Wireless Networks

paper_content:

As the take-up of Long Term Evolution (LTE)/4G cellular accelerates, there is increasing interest in technologies that will define the next generation (5G) telecommunication standard. This article identifies several emerging technologies which will change and define the future generations of telecommunication standards. Some of these technologies are already making their way into standards such as 3GPP LTE, while others are still in development. Additionally, we will look at some of the research problems that these new technologies pose.
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paper_title: Millimeter-wave access and backhauling: the solution to the exponential data traffic increase in 5G mobile communications systems?

paper_content:

The exponential increase of mobile data traffic requires disrupting approaches for the realization of future 5G systems. In this article, we overview the technologies that will pave the way for a novel cellular architecture that integrates high-data-rate access and backhaul networks based on millimeter-wave frequencies (57-66, 71-76, and 81-86 GHz). We evaluate the feasibility of short- and medium-distance links at these frequencies and analyze the requirements from the transceiver architecture and technology, antennas, and modulation scheme points of view. Technical challenges are discussed, and design options highlighted; finally, a performance evaluation quantifies the benefits of millimeter- wave systems with respect to current cellular technologies.
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paper_title: Large-scale antenna systems with hybrid analog and digital beamforming for millimeter wave 5G

paper_content:

With the severe spectrum shortage in conventional cellular bands, large-scale antenna systems in the mmWave bands can potentially help to meet the anticipated demands of mobile traffic in the 5G era. There are many challenging issues, however, regarding the implementation of digital beamforming in large-scale antenna systems: complexity, energy consumption, and cost. In a practical large-scale antenna deployment, hybrid analog and digital beamforming structures can be important alternative choices. In this article, optimal designs of hybrid beamforming structures are investigated, with the focus on an N (the number of transceivers) by M (the number of active antennas per transceiver) hybrid beamforming structure. Optimal analog and digital beamforming designs in a multi-user beamforming scenario are discussed. Also, the energy efficiency and spectrum efficiency of the N × M beamforming structure are analyzed, including their relationship at the green point (i.e., the point with the highest energy efficiency) on the energy efficiency-spectrum efficiency curve, the impact of N on the energy efficiency performance at a given spectrum efficiency value, and the impact of N on the green point energy efficiency. These results can be conveniently utilized to guide practical LSAS design for optimal energy/ spectrum efficiency trade-off. Finally, a reference signal design for the hybrid beamform structure is presented, which achieves better channel estimation performance than the method solely based on analog beamforming. It is expected that large-scale antenna systems with hybrid beamforming structures in the mmWave band can play an important role in 5G.
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paper_title: Millimeter-wave beamforming as an enabling technology for 5G cellular communications: theoretical feasibility and prototype results

paper_content:

The ever growing traffic explosion in mobile communications has recently drawn increased attention to the large amount of underutilized spectrum in the millimeter-wave frequency bands as a potentially viable solution for achieving tens to hundreds of times more capacity compared to current 4G cellular networks. Historically, mmWave bands were ruled out for cellular usage mainly due to concerns regarding short-range and non-line-of-sight coverage issues. In this article, we present recent results from channel measurement campaigns and the development of advanced algorithms and a prototype, which clearly demonstrate that the mmWave band may indeed be a worthy candidate for next generation (5G) cellular systems. The results of channel measurements carried out in both the United States and Korea are summarized along with the actual free space propagation measurements in an anechoic chamber. Then a novel hybrid beamforming scheme and its link- and system-level simulation results are presented. Finally, recent results from our mmWave prototyping efforts along with indoor and outdoor test results are described to assert the feasibility of mmWave bands for cellular usage.
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paper_title: What Will 5G Be?

paper_content:

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

paper_content:

The wireless research community aspires to conceive full duplex operation by supporting concurrent transmission and reception in a single time/frequency channel for the sake of improving the attainable spectral efficiency by a factor of two as compared to the family of conventional half duplex wireless systems. The main challenge encountered in implementing FD wireless devices is that of finding techniques for mitigating the performance degradation imposed by self-interference. In this article, we investigate the potential FD techniques, including passive suppression, active analog cancellation, and active digital cancellation, and highlight their pros and cons. Furthermore, the troubles of FD medium access control protocol design are discussed for addressing the problems such as the resultant end-to-end delay and network congestion. Additionally, an opportunistic decode-andforward- based relay selection scheme is analyzed in underlay cognitive networks communicating over independent and identically distributed Rayleigh and Nakagami-m fading channels in the context of FD relaying. We demonstrate that the outage probability of multi-relay cooperative communication links can be substantially reduced. Finally, we discuss the challenges imposed by the aforementioned techniques and a range of critical issues associated with practical FD implementations. It is shown that numerous open challenges, such as efficient SI suppression, high-performance FD MAC-layer protocol design, low power consumption, and hybrid FD/HD designs, have to be tackled before successfully implementing FD-based systems.
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paper_title: Device-to-device communication in 5G cellular networks: challenges, solutions, and future directions

paper_content:

In a conventional cellular system, devices are not allowed to directly communicate with each other in the licensed cellular bandwidth and all communications take place through the base stations. In this article, we envision a two-tier cellular network that involves a macrocell tier (i.e., BS-to-device communications) and a device tier (i.e., device-to-device communications). Device terminal relaying makes it possible for devices in a network to function as transmission relays for each other and realize a massive ad hoc mesh network. This is obviously a dramatic departure from the conventional cellular architecture and brings unique technical challenges. In such a two-tier cellular system, since the user data is routed through other users? devices, security must be maintained for privacy. To ensure minimal impact on the performance of existing macrocell BSs, the two-tier network needs to be designed with smart interference management strategies and appropriate resource allocation schemes. Furthermore, novel pricing models should be designed to tempt devices to participate in this type of communication. Our article provides an overview of these major challenges in two-tier networks and proposes some pricing schemes for different types of device relaying.
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paper_title: Enabling technologies and architectures for 5G wireless

paper_content:

The proliferation of smart devices and the resulting exponential growth in data traffic has increased the need for higher capacity wireless networks. The cellular systems industry is envisioning an increase in network capacity by a factor of 1000 over the next decade to meet this traffic demand. In addition, with the emergence of Internet of Things (IoT), billions of devices will be connected and managed by wireless networks. Future networks must satisfy the above mentioned requirements with high energy efficiency and at low cost. Hence, the industry attention is now shifting towards the next set of innovations in architecture and technologies that will address capacity and service demands envisioned for 2020, which cannot be met only with the evolution of 4G systems. These innovations are expected to form the so called fifth generation wireless communications systems, or 5G. Candidate 5G solutions include i) higher densification of heterogeneous networks with massive deployment of small base stations supporting various Radio Access Technologies (RATs), ii) use of very large Multiple Input Multiple Output (MIMO) arrays, iii) use of millimeter Wave spectrum where larger wider frequency bands are available, iv) direct device to device (D2D) communication, and v) simultaneous transmission and reception, among others. In this paper, we present the main 5G technologies. We also discuss the network and device evolution towards 5G.
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paper_title: The requirements, challenges, and technologies for 5G of terrestrial mobile telecommunication

paper_content:

In this article, we summarize the 5G mobile communication requirements and challenges. First, essential requirements for 5G are pointed out, including higher traffic volume, indoor or hotspot traffic, and spectrum, energy, and cost efficiency. Along with these changes of requirements, we present a potential step change for the evolution toward 5G, which shows that macro-local coexisting and coordinating paths will replace one macro-dominated path as in 4G and before. We hereafter discuss emerging technologies for 5G within international mobile telecommunications. Challenges and directions in hardware, including integrated circuits and passive components, are also discussed. Finally, a whole picture for the evolution to 5G is predicted and presented.
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paper_title: Cellular architecture and key technologies for 5G wireless communication networks

paper_content:

The fourth generation wireless communication systems have been deployed or are soon to be deployed in many countries. However, with an explosion of wireless mobile devices and services, there are still some challenges that cannot be accommodated even by 4G, such as the spectrum crisis and high energy consumption. Wireless system designers have been facing the continuously increasing demand for high data rates and mobility required by new wireless applications and therefore have started research on fifth generation wireless systems that are expected to be deployed beyond 2020. In this article, we propose a potential cellular architecture that separates indoor and outdoor scenarios, and discuss various promising technologies for 5G wireless communication systems, such as massive MIMO, energy-efficient communications, cognitive radio networks, and visible light communications. Future challenges facing these potential technologies are also discussed.
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paper_title: Five Disruptive Technology Directions for 5G

paper_content:

New research directions will lead to fundamental changes in the design of future 5th generation (5G) cellular networks. This paper describes five technologies that could lead to both architectural and component disruptive design changes: device-centric architectures, millimeter Wave, Massive-MIMO, smarter devices, and native support to machine-2-machine. The key ideas for each technology are described, along with their potential impact on 5G and the research challenges that remain.
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paper_title: Enabling device-to-device communications in millimeter-wave 5G cellular networks

paper_content:

Millimeter-wave communication is a promising technology for future 5G cellular networks to provide very high data rate (multi-gigabits-persecond) for mobile devices. Enabling D2D communications over directional mmWave networks is of critical importance to efficiently use the large bandwidth to increase network capacity. In this article, the propagation features of mmWave communication and the associated impacts on 5G cellular networks are discussed. We introduce an mmWave+4G system architecture with TDMA-based MAC structure as a candidate for 5G cellular networks. We propose an effective resource sharing scheme by allowing non-interfering D2D links to operate concurrently. We also discuss neighbor discovery for frequent handoffs in 5G cellular networks.
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paper_title: Toward 5G densenets: architectural advances for effective machine-type communications over femtocells

paper_content:

Ubiquitous, reliable and low-latency machinetype communication, MTC, systems are considered to be value-adds of emerging 5G cellular networks. To meet the technical and economical requirements for exponentially growing MTC traffic, we advocate the use of small cells to handle the massive and dense MTC rollout. We introduce a novel 3GPP-compliant architecture that absorbs the MTC traffic via home evolved NodeBs, allowing us to significantly reduce congestion and overloading of radio access and core networks. A major design challenge has been to deal with the interference to human-type traffic and the large degree of freedom of the system, due to the unplanned deployments of small cells and the enormous amount of MTC devices. Simulation results in terms of MTC access delay, energy consumption, and delivery rate corroborate the superiority of the proposed working architecture.
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paper_title: Emerging Technologies and Research Challenges for 5G Wireless Networks

paper_content:

As the take-up of Long Term Evolution (LTE)/4G cellular accelerates, there is increasing interest in technologies that will define the next generation (5G) telecommunication standard. This article identifies several emerging technologies which will change and define the future generations of telecommunication standards. Some of these technologies are already making their way into standards such as 3GPP LTE, while others are still in development. Additionally, we will look at some of the research problems that these new technologies pose.
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paper_title: Cognitive radio in 5G: a perspective on energy-spectral efficiency trade-off

paper_content:

A cognitive cellular network, which integrates conventional licensed cellular radio and cognitive radio into a holistic system, is a promising paradigm for the fifth generation mobile communication systems. Understanding the trade-off between energy efficiency, EE, and spectral efficiency, SE, in cognitive cellular networks is of fundamental importance for system design and optimization. This article presents recent research progress on the EE-SE trade-off of cognitive cellular networks. We show how EE-SE trade-off studies can be performed systematically with respect to different architectures, levels of analysis, and capacity metrics. Three representative examples are given to illustrate how EE-SE trade-off analysis can lead to important insights and useful design guidelines for future cognitive cellular networks.
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paper_title: Applications of self-interference cancellation in 5G and beyond

paper_content:

Self-interference cancellation invalidates a long-held fundamental assumption in wireless network design that radios can only operate in half duplex mode on the same channel. Beyond enabling true in-band full duplex, which effectively doubles spectral efficiency, self-interference cancellation tremendously simplifies spectrum management. Not only does it render entire ecosystems like TD-LTE obsolete, it enables future networks to leverage fragmented spectrum, a pressing global issue that will continue to worsen in 5G networks. Self-interference cancellation offers the potential to complement and sustain the evolution of 5G technologies toward denser heterogeneous networks and can be utilized in wireless communication systems in multiple ways, including increased link capacity, spectrum virtualization, any-division duplexing (ADD), novel relay solutions, and enhanced interference coordination. By virtue of its fundamental nature, self-interference cancellation will have a tremendous impact on 5G networks and beyond.
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paper_title: Scaling up MIMO: Opportunities and challenges with very large arrays

paper_content:

Presents a list of articles published by the IEEE Signal Processing Society (SPS) that ranked among the top 100 most downloaded IEEE Xplore articles.
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paper_title: Massive MIMO performance evaluation based on measured propagation data

paper_content:

Massive MIMO, also known as very-large MIMO or large-scale antenna systems, is a new technique that potentially can offer large network capacities in multi-user scenarios. With a massive MIMO system, we consider the case where a base station equipped with a large number of antenna elements simultaneously serves multiple single-antenna users in the same time-frequency resource. So far, investigations are mostly based on theoretical channels with independent and identically distributed (i.i.d.) complex Gaussian coefficients, i.e., i.i.d. Rayleigh channels. Here, we investigate how massive MIMO performs in channels measured in real propagation environments. Channel measurements were performed at 2.6 GHz using a virtual uniform linear array (ULA) which has a physically large aperture, and a practical uniform cylindrical array (UCA) which is more compact in size, both having 128 antenna ports. Based on measurement data, we illustrate channel behavior of massive MIMO in three representative propagation conditions, and evaluate the corresponding performance. The investigation shows that the measured channels, for both array types, allow us to achieve performance close to that in i.i.d. Rayleigh channels. It is concluded that in real propagation environments we have characteristics that can allow for efficient use of massive MIMO, i.e., the theoretical advantages of this new technology can also be harvested in real channels.
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paper_title: Dynamic Subarrays for Hybrid Precoding in Wideband mmWave MIMO Systems

paper_content:

Hybrid analog/digital precoding architectures can address the trade-off between achievable spectral efficiency and power consumption in large-scale MIMO systems. This makes it a promising candidate for millimeter wave systems, which require deploying large antenna arrays at both the transmitter and receiver to guarantee sufficient received signal power. Most prior work on hybrid precoding focused on narrowband channels and assumed fully-connected hybrid architectures. MmWave systems, though, are expected to be wideband with frequency selectivity. In this paper, a closed-form solution for fully-connected OFDM-based hybrid analog/digital precoding is developed for frequency selective mmWave systems. This solution is then extended to partially-connected but fixed architectures in which each RF chain is connected to a specific subset of the antennas. The derived solutions give insights into how the hybrid subarray structures should be designed. Based on them, a novel technique that dynamically constructs the hybrid subarrays based on the long-term channel characteristics is developed. Simulation results show that the proposed hybrid precoding solutions achieve spectral efficiencies close to that obtained with fully-digital architectures in wideband mmWave channels. Further, the results indicate that the developed dynamic subarray solution outperforms the fixed hybrid subarray structures in various system and channel conditions.
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paper_title: Variable-phase-shift-based RF-baseband codesign for MIMO antenna selection

paper_content:

We introduce a novel soft antenna selection approach for multiple antenna systems through a joint design of both RF (radio frequency) and baseband signal processing. When only a limited number of frequency converters are available, conventional antenna selection schemes show severe performance degradation in most fading channels. To alleviate those degradations, we propose to adopt a transformation of the signals in the RF domain that requires only simple, variable phase shifters and combiners to reduce the number of RF chains. The constrained optimum design of these shifters, adapting to the channel state, is given in analytical form, which requires no search or iterations. The resulting system shows a significant performance advantage for both correlated and uncorrelated channels. The technique works for both transmitter and receiver design, which leads to the joint transceiver antenna selection. When only a single information stream is transmitted through the channel, the new design can achieve the same SNR gain as the full-complexity system while requiring, at most, two RF chains. With multiple information streams transmitted, it is demonstrated by computer experiments that the capacity performance is close to optimum.
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paper_title: Massive MIMO in the UL/DL of Cellular Networks: How Many Antennas Do We Need?

paper_content:

We consider the uplink (UL) and downlink (DL) of non-cooperative multi-cellular time-division duplexing (TDD) systems, assuming that the number N of antennas per base station (BS) and the number K of user terminals (UTs) per cell are large. Our system model accounts for channel estimation, pilot contamination, and an arbitrary path loss and antenna correlation for each link. We derive approximations of achievable rates with several linear precoders and detectors which are proven to be asymptotically tight, but accurate for realistic system dimensions, as shown by simulations. It is known from previous work assuming uncorrelated channels, that as N→∞ while K is fixed, the system performance is limited by pilot contamination, the simplest precoders/detectors, i.e., eigenbeamforming (BF) and matched filter (MF), are optimal, and the transmit power can be made arbitrarily small. We analyze to which extent these conclusions hold in the more realistic setting where N is not extremely large compared to K. In particular, we derive how many antennas per UT are needed to achieve η% of the ultimate performance limit with infinitely many antennas and how many more antennas are needed with MF and BF to achieve the performance of minimum mean-square error (MMSE) detection and regularized zero-forcing (RZF), respectively.
---
paper_title: Five Disruptive Technology Directions for 5G

paper_content:

New research directions will lead to fundamental changes in the design of future 5th generation (5G) cellular networks. This paper describes five technologies that could lead to both architectural and component disruptive design changes: device-centric architectures, millimeter Wave, Massive-MIMO, smarter devices, and native support to machine-2-machine. The key ideas for each technology are described, along with their potential impact on 5G and the research challenges that remain.
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paper_title: Full-dimension MIMO (FD-MIMO) for next generation cellular technology

paper_content:

This article considers a practical implementation of massive MIMO systems [1]. Although the best performance can be achieved when a large number of active antennas are placed only in the horizontal domain, BS form factor limitation often makes horizontal array placement infeasible. To cope with this limitation, this article introduces full-dimension MIMO (FD-MIMO) cellular wireless communication system, where active antennas are placed in a 2D grid at BSs. For analysis of the FD-MIMO systems, a 3D spatial channel model is introduced, on which system-level simulations are conducted. The simulation results show that the proposed FD-MIMO system with 32 antenna ports achieves 2-3.6 times cell average throughput gain and 1.5-5 times cell edge throughput gain compared to the 4G LTE system of two antenna ports at the BS.
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paper_title: Near-Optimal Hybrid Processing for Massive MIMO Systems via Matrix Decomposition

paper_content:

For the practical implementation of massive multiple-input multiple-output (MIMO) systems, the hybrid processing (precoding/combining) structure is promising to reduce the high cost rendered by large number of RF chains of the traditional processing structure. The hybrid processing is performed through low-dimensional digital baseband processing combined with analog RF processing enabled by phase shifters. We propose to design hybrid RF and baseband precoders/combiners for multi-stream transmission in point-to-point massive MIMO systems, by directly decomposing the pre-designed unconstrained digital precoder/combiner of a large dimension. The constant amplitude constraint of analog RF processing results in the matrix decomposition problem non-convex. Based on an alternate optimization technique, the non-convex matrix decomposition problem can be decoupled into a series of convex sub-problems and effectively solved by restricting the phase increment of each entry in the RF precoder/combiner within a small vicinity of its preceding iterate. A singular value decomposition based technique is proposed to secure an initial point sufficiently close to the global solution of the original non-convex problem. Through simulation, the convergence of the alternate optimization for such a matrix decomposition based hybrid processing (MD-HP) scheme is examined, and the performance of the MD-HP scheme is demonstrated to be near-optimal.
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paper_title: Spatially Sparse Precoding in Millimeter Wave MIMO Systems

paper_content:

Millimeter wave (mmWave) signals experience orders-of-magnitude more pathloss than the microwave signals currently used in most wireless applications. MmWave systems must therefore leverage large antenna arrays, made possible by the decrease in wavelength, to combat pathloss with beamforming gain. Beamforming with multiple data streams, known as precoding, can be used to further improve mmWave spectral efficiency. Both beamforming and precoding are done digitally at baseband in traditional multi-antenna systems. The high cost and power consumption of mixed-signal devices in mmWave systems, however, make analog processing in the RF domain more attractive. This hardware limitation restricts the feasible set of precoders and combiners that can be applied by practical mmWave transceivers. In this paper, we consider transmit precoding and receiver combining in mmWave systems with large antenna arrays. We exploit the spatial structure of mmWave channels to formulate the precoding/combining problem as a sparse reconstruction problem. Using the principle of basis pursuit, we develop algorithms that accurately approximate optimal unconstrained precoders and combiners such that they can be implemented in low-cost RF hardware. We present numerical results on the performance of the proposed algorithms and show that they allow mmWave systems to approach their unconstrained performance limits, even when transceiver hardware constraints are considered.
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paper_title: Alternating beamforming methods for hybrid analog and digital MIMO transmission

paper_content:

Hybrid beamforming has drawn attention with the increasing concern on high frequency band communication and the appearance of flexible structures of base stations. This paper considers the practical transmitter structure that each antenna is only connected to a unique RF chain and optimizes the analog and digital beamforming matrices to maximize the achievable rate with transmit power constraint. Following alternating optimization principle, closed-form relationships between analog and digital precoders are obtained when both amplitude and phase or only phase can be adjusted to form analog beams. Simulation results demonstrate the advantage of our proposed methods over the existing beam steering method in terms of achievable rate with different scale antenna array. This reveals that the method can be applied to both low and high frequency band communications. When increasing the number of propagation paths, contrast to the traditional beam steering method, the performance of proposed methods becomes better while the complexity almost keeps at an acceptable constant level.
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paper_title: Hybrid Digital and Analog Beamforming Design for Large-Scale Antenna Arrays

paper_content:

The potential of using of millimeter wave (mmWave) frequency for future wireless cellular communication systems has motivated the study of large-scale antenna arrays for achieving highly directional beamforming. However, the conventional fully digital beamforming methods which require one radio frequency (RF) chain per antenna element is not viable for large-scale antenna arrays due to the high cost and high power consumption of RF chain components in high frequencies. To address the challenge of this hardware limitation, this paper considers a hybrid beamforming architecture in which the overall beamformer consists of a low-dimensional digital beamformer followed by an RF beamformer implemented using analog phase shifters. Our aim is to show that such an architecture can approach the performance of a fully digital scheme with much fewer number of RF chains. Specifically, this paper establishes that if the number of RF chains is twice the total number of data streams, the hybrid beamforming structure can realize any fully digital beamformer exactly, regardless of the number of antenna elements. For cases with fewer number of RF chains, this paper further considers the hybrid beamforming design problem for both the transmission scenario of a point-to-point multiple-input multiple-output (MIMO) system and a downlink multi-user multiple-input single-output (MU-MISO) system. For each scenario, we propose a heuristic hybrid beamforming design that achieves a performance close to the performance of the fully digital beamforming baseline. Finally, the proposed algorithms are modified for the more practical setting in which only finite resolution phase shifters are available. Numerical simulations show that the proposed schemes are effective even when phase shifters with very low resolution are used.
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paper_title: Joint Spatial Division and Multiplexing—The Large-Scale Array Regime

paper_content:

We propose joint spatial division and multiplexing (JSDM), an approach to multiuser MIMO downlink that exploits the structure of the correlation of the channel vectors in order to allow for a large number of antennas at the base station while requiring reduced-dimensional channel state information at the transmitter (CSIT). JSDM achieves significant savings both in the downlink training and in the CSIT uplink feedback, thus making the use of large antenna arrays at the base station potentially suitable also for frequency division duplexing (FDD) systems, for which uplink/downlink channel reciprocity cannot be exploited. In the proposed scheme, the multiuser MIMO downlink precoder is obtained by concatenating a prebeamforming matrix, which depends only on the channel second-order statistics, with a classical multiuser precoder, based on the instantaneous knowledge of the resulting reduced dimensional “effective” channel matrix. We prove a simple condition under which JSDM incurs no loss of optimality with respect to the full CSIT case. For linear uniformly spaced arrays, we show that such condition is approached in the large number of antennas limit. For this case, we use Szego's asymptotic theory of Toeplitz matrices to show that a DFT-based prebeamforming matrix is near-optimal, requiring only coarse information about the users angles of arrival and angular spread. Finally, we extend these ideas to the case of a 2-D base station antenna array, with 3-D beamforming, including multiple beams in the elevation angle direction. We provide guidelines for the prebeamforming optimization and calculate the system spectral efficiency under proportional fairness and max-min fairness criteria, showing extremely attractive performance. Our numerical results are obtained via asymptotic random matrix theory, avoiding lengthy Monte Carlo simulations and providing accurate results for realistic (finite) number of antennas and users.
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paper_title: Large-scale antenna systems with hybrid analog and digital beamforming for millimeter wave 5G

paper_content:

With the severe spectrum shortage in conventional cellular bands, large-scale antenna systems in the mmWave bands can potentially help to meet the anticipated demands of mobile traffic in the 5G era. There are many challenging issues, however, regarding the implementation of digital beamforming in large-scale antenna systems: complexity, energy consumption, and cost. In a practical large-scale antenna deployment, hybrid analog and digital beamforming structures can be important alternative choices. In this article, optimal designs of hybrid beamforming structures are investigated, with the focus on an N (the number of transceivers) by M (the number of active antennas per transceiver) hybrid beamforming structure. Optimal analog and digital beamforming designs in a multi-user beamforming scenario are discussed. Also, the energy efficiency and spectrum efficiency of the N × M beamforming structure are analyzed, including their relationship at the green point (i.e., the point with the highest energy efficiency) on the energy efficiency-spectrum efficiency curve, the impact of N on the energy efficiency performance at a given spectrum efficiency value, and the impact of N on the green point energy efficiency. These results can be conveniently utilized to guide practical LSAS design for optimal energy/ spectrum efficiency trade-off. Finally, a reference signal design for the hybrid beamform structure is presented, which achieves better channel estimation performance than the method solely based on analog beamforming. It is expected that large-scale antenna systems with hybrid beamforming structures in the mmWave band can play an important role in 5G.
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paper_title: Massive MIMO: An Introduction

paper_content:

Demand for wireless throughput, both mobile and fixed, will always increase. One can anticipate that, in five or ten years, millions of augmented reality users in a large city will want to transmit and receive 3D personal high-definition video more or less continuously, say 100 megabits per second per user in each direction. Massive MIMO-also called Large-Scale Antenna Systems-is a promising candidate technology for meeting this demand. Fifty-fold or greater spectral efficiency improvements over fourth generation (4G) technology are frequently mentioned. A multiplicity of physically small, individually controlled antennas performs aggressive multiplexing/demultiplexing for all active users, utilizing directly measured channel characteristics. Unlike today's Point-to-Point MIMO, by leveraging time-division duplexing (TDD), Massive MIMO is scalable to any desired degree with respect to the number of service antennas. Adding more antennas is always beneficial for increased throughput, reduced radiated power, uniformly great service everywhere in the cell, and greater simplicity in signal processing. Massive MIMO is a brand new technology that has yet to be reduced to practice. Notwithstanding, its principles of operation are well understood, and surprisingly simple to elucidate.
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paper_title: Channel Estimation and Hybrid Precoding for Millimeter Wave Cellular Systems

paper_content:

Millimeter wave (mmWave) cellular systems will enable gigabit-per-second data rates thanks to the large bandwidth available at mmWave frequencies. To realize sufficient link margin, mmWave systems will employ directional beamforming with large antenna arrays at both the transmitter and receiver. Due to the high cost and power consumption of gigasample mixed-signal devices, mmWave precoding will likely be divided among the analog and digital domains. The large number of antennas and the presence of analog beamforming requires the development of mmWave-specific channel estimation and precoding algorithms. This paper develops an adaptive algorithm to estimate the mmWave channel parameters that exploits the poor scattering nature of the channel. To enable the efficient operation of this algorithm, a novel hierarchical multi-resolution codebook is designed to construct training beamforming vectors with different beamwidths. For single-path channels, an upper bound on the estimation error probability using the proposed algorithm is derived, and some insights into the efficient allocation of the training power among the adaptive stages of the algorithm are obtained. The adaptive channel estimation algorithm is then extended to the multi-path case relying on the sparse nature of the channel. Using the estimated channel, this paper proposes a new hybrid analog/digital precoding algorithm that overcomes the hardware constraints on the analog-only beamforming, and approaches the performance of digital solutions. Simulation results show that the proposed low-complexity channel estimation algorithm achieves comparable precoding gains compared to exhaustive channel training algorithms. The results illustrate that the proposed channel estimation and precoding algorithms can approach the coverage probability achieved by perfect channel knowledge even in the presence of interference.
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paper_title: What Will 5G Be?

paper_content:

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

paper_content:

In this paper, we take a proposed low-complexity Vertical Bell Laboratories Layered Space Time receiver based on maximal ratio combining and further simplify the algorithm by replacing the channel norm ordering by a power based ordering. The receiver operates in an uplink massive multiple-input-multiple-output deployment with distributed single-antenna users and a large base station (BS) array. The novel receiver is compared with other more complex detection schemes such as linear zero forcing. Moreover, an explicit closed form analysis for error probability for both co-located and distributed BSs is provided. It is shown that the error performance of the distributed scenario is well approximated by a modified version of a co-located scenario. The simulation study demonstrates the performance of the proposed scheme and confirms the accuracy of analytical results.
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paper_title: Scaling up MIMO: Opportunities and challenges with very large arrays

paper_content:

Presents a list of articles published by the IEEE Signal Processing Society (SPS) that ranked among the top 100 most downloaded IEEE Xplore articles.
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paper_title: Massive MIMO in the UL/DL of Cellular Networks: How Many Antennas Do We Need?

paper_content:

We consider the uplink (UL) and downlink (DL) of non-cooperative multi-cellular time-division duplexing (TDD) systems, assuming that the number N of antennas per base station (BS) and the number K of user terminals (UTs) per cell are large. Our system model accounts for channel estimation, pilot contamination, and an arbitrary path loss and antenna correlation for each link. We derive approximations of achievable rates with several linear precoders and detectors which are proven to be asymptotically tight, but accurate for realistic system dimensions, as shown by simulations. It is known from previous work assuming uncorrelated channels, that as N→∞ while K is fixed, the system performance is limited by pilot contamination, the simplest precoders/detectors, i.e., eigenbeamforming (BF) and matched filter (MF), are optimal, and the transmit power can be made arbitrarily small. We analyze to which extent these conclusions hold in the more realistic setting where N is not extremely large compared to K. In particular, we derive how many antennas per UT are needed to achieve η% of the ultimate performance limit with infinitely many antennas and how many more antennas are needed with MF and BF to achieve the performance of minimum mean-square error (MMSE) detection and regularized zero-forcing (RZF), respectively.
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paper_title: Five Disruptive Technology Directions for 5G

paper_content:

New research directions will lead to fundamental changes in the design of future 5th generation (5G) cellular networks. This paper describes five technologies that could lead to both architectural and component disruptive design changes: device-centric architectures, millimeter Wave, Massive-MIMO, smarter devices, and native support to machine-2-machine. The key ideas for each technology are described, along with their potential impact on 5G and the research challenges that remain.
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paper_title: Network densification: the dominant theme for wireless evolution into 5G

paper_content:

This article explores network densification as the key mechanism for wireless evolution over the next decade. Network densification includes densification over space (e.g, dense deployment of small cells) and frequency (utilizing larger portions of radio spectrum in diverse bands). Large-scale cost-effective spatial densification is facilitated by self-organizing networks and intercell interference management. Full benefits of network densification can be realized only if it is complemented by backhaul densification, and advanced receivers capable of interference cancellation.
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paper_title: Evolution Towards 5G Multi-tier Cellular Wireless Networks: An Interference Management Perspective

paper_content:

The evolving fifth generation (5G) cellular wireless networks are envisioned to overcome the fundamental challenges of existing cellular networks, for example, higher data rates, excellent end-to-end performance, and user-coverage in hot-spots and crowded areas with lower latency, energy consumption, and cost per information transfer. To address these challenges, 5G systems will adopt a multi-tier architecture consisting of macrocells, different types of licensed small cells, relays, and device-to-device (D2D) networks to serve users with different quality-of-service (QoS) requirements in a spectrum and energy-efficient manner. Starting with the visions and requirements of 5G multi-tier networks, this article outlines the challenges of interference management (e.g. power control, cell association) in these networks with shared spectrum access (i.e. when the different network tiers share the same licensed spectrum). It is argued that the existing interference management schemes will not be able to address the interference management problem in prioritized 5G multi-tier networks where users in different tiers have different priorities for channel access. In this context a survey and qualitative comparison of the existing cell association and power control schemes is provided to demonstrate their limitations for interference management in 5G networks. Open challenges are highlighted and guidelines are provided to modify the existing schemes in order to overcome these limitations and make them suitable for the emerging 5G systems.
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paper_title: Orthogonal Frequency Division Multiplexing With Index Modulation

paper_content:

In this paper, a novel orthogonal frequency division multiplexing (OFDM) scheme, which is called OFDM with index modulation (OFDM-IM), is proposed for frequency-selective fading channels. In this scheme, inspiring from the recently introduced spatial modulation concept for multiple-input multiple-output (MIMO) channels, the information is conveyed not only by M-ary signal constellations as in classical OFDM, but also by the indices of the subcarriers, which are activated according to the incoming bit stream. Different transceiver structures are proposed and a theoretical error performance analysis is provided for the new scheme. It is shown via computer simulations that the proposed scheme achieves significantly better error performance than classical OFDM due to the information bits carried in the spatial domain by the indices of OFDM subcarriers.
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paper_title: Spatial Modulation for Generalized MIMO: Challenges, Opportunities, and Implementation

paper_content:

A key challenge of future mobile communication research is to strike an attractive compromise between wire- less network's area spectral efficiency and energy efficiency. This necessitates a clean-slate approach to wireless system design, embracing the rich body of existing knowledge, espe- cially on multiple-input-multiple-ouput (MIMO) technologies. This motivates the proposal of an emerging wireless commu- nications concept conceived for single-radio-frequency (RF) large-scale MIMO communications, which is termed as SM. The concept of SM has established itself as a beneficial transmission paradigm, subsuming numerous members of the MIMO system family. The research of SM has reached sufficient maturity to motivate its comparison to state-of-the-art MIMO communica- tions, as well as to inspire its application to other emerging wireless systems such as relay-aided, cooperative, small-cell, optical wireless, and power-efficient communications. Further- more, it has received sufficient research attention to be im- plemented in testbeds, and it holds the promise of stimulating further vigorous interdisciplinary research in the years to come. This tutorial paper is intended to offer a comprehensive state-of-the-art survey on SM-MIMO research, to provide a critical appraisal of its potential advantages, and to promote the discussion of its beneficial application areas and their research challenges leading to the analysis of the technological issues associated with the implementation of SM-MIMO. The paper is concluded with the description of the world's first experimental activities in this vibrant research field.
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paper_title: Massive MIMO with 1-bit ADC

paper_content:

We investigate massive multiple-input-multiple output (MIMO) uplink systems with 1-bit analog-to-digital converters (ADCs) on each receiver antenna. Receivers that rely on 1-bit ADC do not need energy-consuming interfaces such as automatic gain control (AGC). This decreases both ADC building and operational costs. Our design is based on maximal ratio combining (MRC), zero-forcing (ZF), and least squares (LS) detection, taking into account the effects of the 1-bit ADC on channel estimation. Through numerical results, we show good performance of the system in terms of mutual information and symbol error rate (SER). Furthermore, we provide an analytical approach to calculate the mutual information and SER of the MRC receiver. The analytical approach reduces complexity in the sense that a symbol and channel noise vectors Monte Carlo simulation is avoided.
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paper_title: On the comparison between code-index modulation and spatial modulation techniques

paper_content:

Recently, two promising modulation techniques have been developed aiming to increase data rate and save energy while being simple to implement. These modulation schemes belong to two different communication methods, however, they share the common structure of using an index as an additional parameter to convey information. The first scheme known as spatial modulation (SM), is a scheme that uses multiple antennas at the transmitter side where just one antenna is activated at a time and its index is used as means to convey information. The second is known as code-index modulation (CIM), a system that uses multiple spreading codes, where a certain code is selected and its index is used as a mechanism to ferry data. In this paper, we present these two modulation techniques and we discuss the associated set of challenges for each scheme. Moreover, in order to evaluate the advantages and disadvantages of each technique, we compare the energy efficiency, the system complexity, and the bit error rate performance of the SM and CIM schemes.
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paper_title: MMSE precoder for massive MIMO using 1-bit quantization

paper_content:

We propose a novel linear minimum-mean-squared-error (MMSE) precoder design for a downlink (DL) massive multiple-input-multiple-output (MIMO) scenario. For economical and computational efficiency reasons low resolution 1-bit digital-to-analog (DAC) and analog-to-digital (ADC) converters are used. This comes at the cost of performance gain that can be recovered by the large number of antennas deployed at the base station (BS) and an appropiate pre-coder design to mitigate the distortions due to the coarse quantization. The proposed precoder takes the quantization non-linearities into account and is split into a digital precoder and an analog precoder. We formulate the two-stage precoding problem such that the MSE of the users is minimized under the 1-bit constraint. In the simulations, we compare the new optimized precoding scheme with previously proposed linear precoders in terms of uncoded bit error ratio (BER).
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paper_title: On one-bit quantized ZF precoding for the multiuser massive MIMO downlink

paper_content:

We study low complexity precoding for a downlink massive MIMO multiuser system assuming a base station that employs one-bit digital-to-analog converters (DACs) in order to mitigate power usage. The use of one-bit DACs is equivalent to constraining the transmit signal to be drawn from a QPSK alphabet. While the precoding problem can be formulated using a standard maximum likelihood (ML) encoder, the implementation cost is prohibitive for massive numbers of antennas, even if a sphere encoding approach is used. Instead, we study the performance of a one-bit quantized zero-forcing precoder, and we show that it asymptotically provides the desired downlink vector with low complexity. Simulations show that the quantized ZF precoder can actually outperform the ML encoder for low to moderate signal-to-noise ratios.
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paper_title: User selection and power schedule for downlink non-orthogonal multiple access (NOMA) system

paper_content:

As a promising multiple access techniques for 5th generation (5G) wireless systems, non-orthogonal multiple access (NOMA) has received many attentions recently. In this paper, we study the NOMA based downlink multi-user beamforming system, where the base station (BS) tries to transmit information to multiple user clusters and each beam serves one user cluster compromising of two users simultaneously. User selection algorithm is proposed to reduce the interference and improve the system information rate. Moreover, we also provide the user power schedule scheme to guarantee the advantage of the proposed NOMA downlink multi-user beamforming system. Simulation results show that the proposed user selection algorithm and the power schedule scheme can improve the sum-rate of NOMA based downlink multi-user system.
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paper_title: On the Performance of Non-Orthogonal Multiple Access in 5G Systems with Randomly Deployed Users

paper_content:

In this letter, the performance of non-orthogonal multiple access (NOMA) is investigated in a cellular downlink scenario with randomly deployed users. The developed analytical results show that NOMA can achieve superior performance in terms of ergodic sum rates; however, the outage performance of NOMA depends critically on the choices of the users' targeted data rates and allocated power. In particular, a wrong choice of the targeted data rates and allocated power can lead to a situation in which the user's outage probability is always one, i.e. the user's targeted quality of service will never be met.
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paper_title: Pseudo-random phase precoded spatial modulation and precoder index modulation

paper_content:

Spatial modulation (SM) is a transmission scheme that uses multiple transmit antennas but only one transmit RF chain. At each time instant, only one among the transmit antennas will be active and the others remain silent. The index of the active transmit antenna will also convey information bits in addition to the information bits conveyed through modulation symbols. Pseudo-random phase precoding (PRPP) is a technique that can achieve high diversity orders even in single antenna systems without the need for channel state information at the transmitter and transmit power control. In this paper, we exploit the advantages of both SM and PRPP simultaneously. We propose a pseudo-random phase precoded SM (PRPP-SM) scheme, where both the modulation bits and the antenna index bits are precoded by pseudo-random phases. The proposed PRPP-SM system gives significant performance gains over SM system without PRPP and PRPP system without SM. Since maximum likelihood (ML) detection becomes exponentially complex in large dimensions, we propose a low complexity local search based detection (LSD) algorithm suited for PRPP-SM systems with large precoder sizes. Our simulation results show that with 4 transmit antennas, 1 receive antenna, 5 × 20 pseudo-random phase precoder matrix and BPSK modulation, the performance of PRPP-SM using ML detection is better than SM without PRPP with ML detection by about 9 dB at 102 BER. This performance advantage gets even better for large precoding sizes. We also propose a precoder index modulation (PIM) scheme, which conveys additional information bits through the choice of a precoding matrix among a set of pre-determined PRPP matrices. Finally, combining the PIM and PRPP-SM schemes, we propose a PIM-SM scheme which conveys bits through both antenna index as well as precoder index.
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paper_title: Cellular architecture and key technologies for 5G wireless communication networks

paper_content:

The fourth generation wireless communication systems have been deployed or are soon to be deployed in many countries. However, with an explosion of wireless mobile devices and services, there are still some challenges that cannot be accommodated even by 4G, such as the spectrum crisis and high energy consumption. Wireless system designers have been facing the continuously increasing demand for high data rates and mobility required by new wireless applications and therefore have started research on fifth generation wireless systems that are expected to be deployed beyond 2020. In this article, we propose a potential cellular architecture that separates indoor and outdoor scenarios, and discuss various promising technologies for 5G wireless communication systems, such as massive MIMO, energy-efficient communications, cognitive radio networks, and visible light communications. Future challenges facing these potential technologies are also discussed.
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paper_title: Emerging Technologies and Research Challenges for 5G Wireless Networks

paper_content:

As the take-up of Long Term Evolution (LTE)/4G cellular accelerates, there is increasing interest in technologies that will define the next generation (5G) telecommunication standard. This article identifies several emerging technologies which will change and define the future generations of telecommunication standards. Some of these technologies are already making their way into standards such as 3GPP LTE, while others are still in development. Additionally, we will look at some of the research problems that these new technologies pose.
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paper_title: Cognitive radio in 5G: a perspective on energy-spectral efficiency trade-off

paper_content:

A cognitive cellular network, which integrates conventional licensed cellular radio and cognitive radio into a holistic system, is a promising paradigm for the fifth generation mobile communication systems. Understanding the trade-off between energy efficiency, EE, and spectral efficiency, SE, in cognitive cellular networks is of fundamental importance for system design and optimization. This article presents recent research progress on the EE-SE trade-off of cognitive cellular networks. We show how EE-SE trade-off studies can be performed systematically with respect to different architectures, levels of analysis, and capacity metrics. Three representative examples are given to illustrate how EE-SE trade-off analysis can lead to important insights and useful design guidelines for future cognitive cellular networks.
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paper_title: Full duplex techniques for 5G networks: self-interference cancellation, protocol design, and relay selection

paper_content:

The wireless research community aspires to conceive full duplex operation by supporting concurrent transmission and reception in a single time/frequency channel for the sake of improving the attainable spectral efficiency by a factor of two as compared to the family of conventional half duplex wireless systems. The main challenge encountered in implementing FD wireless devices is that of finding techniques for mitigating the performance degradation imposed by self-interference. In this article, we investigate the potential FD techniques, including passive suppression, active analog cancellation, and active digital cancellation, and highlight their pros and cons. Furthermore, the troubles of FD medium access control protocol design are discussed for addressing the problems such as the resultant end-to-end delay and network congestion. Additionally, an opportunistic decode-andforward- based relay selection scheme is analyzed in underlay cognitive networks communicating over independent and identically distributed Rayleigh and Nakagami-m fading channels in the context of FD relaying. We demonstrate that the outage probability of multi-relay cooperative communication links can be substantially reduced. Finally, we discuss the challenges imposed by the aforementioned techniques and a range of critical issues associated with practical FD implementations. It is shown that numerous open challenges, such as efficient SI suppression, high-performance FD MAC-layer protocol design, low power consumption, and hybrid FD/HD designs, have to be tackled before successfully implementing FD-based systems.
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paper_title: Applications of self-interference cancellation in 5G and beyond

paper_content:

Self-interference cancellation invalidates a long-held fundamental assumption in wireless network design that radios can only operate in half duplex mode on the same channel. Beyond enabling true in-band full duplex, which effectively doubles spectral efficiency, self-interference cancellation tremendously simplifies spectrum management. Not only does it render entire ecosystems like TD-LTE obsolete, it enables future networks to leverage fragmented spectrum, a pressing global issue that will continue to worsen in 5G networks. Self-interference cancellation offers the potential to complement and sustain the evolution of 5G technologies toward denser heterogeneous networks and can be utilized in wireless communication systems in multiple ways, including increased link capacity, spectrum virtualization, any-division duplexing (ADD), novel relay solutions, and enhanced interference coordination. By virtue of its fundamental nature, self-interference cancellation will have a tremendous impact on 5G networks and beyond.
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paper_title: Massive MIMO performance evaluation based on measured propagation data

paper_content:

Massive MIMO, also known as very-large MIMO or large-scale antenna systems, is a new technique that potentially can offer large network capacities in multi-user scenarios. With a massive MIMO system, we consider the case where a base station equipped with a large number of antenna elements simultaneously serves multiple single-antenna users in the same time-frequency resource. So far, investigations are mostly based on theoretical channels with independent and identically distributed (i.i.d.) complex Gaussian coefficients, i.e., i.i.d. Rayleigh channels. Here, we investigate how massive MIMO performs in channels measured in real propagation environments. Channel measurements were performed at 2.6 GHz using a virtual uniform linear array (ULA) which has a physically large aperture, and a practical uniform cylindrical array (UCA) which is more compact in size, both having 128 antenna ports. Based on measurement data, we illustrate channel behavior of massive MIMO in three representative propagation conditions, and evaluate the corresponding performance. The investigation shows that the measured channels, for both array types, allow us to achieve performance close to that in i.i.d. Rayleigh channels. It is concluded that in real propagation environments we have characteristics that can allow for efficient use of massive MIMO, i.e., the theoretical advantages of this new technology can also be harvested in real channels.
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paper_title: Wideband Millimeter-Wave Propagation Measurements and Channel Models for Future Wireless Communication System Design

paper_content:

The relatively unused millimeter-wave (mmWave) spectrum offers excellent opportunities to increase mobile capacity due to the enormous amount of available raw bandwidth. This paper presents experimental measurements and empirically-based propagation channel models for the 28, 38, 60, and 73 GHz mmWave bands, using a wideband sliding correlator channel sounder with steerable directional horn antennas at both the transmitter and receiver from 2011 to 2013. More than 15,000 power delay profiles were measured across the mmWave bands to yield directional and omnidirectional path loss models, temporal and spatial channel models, and outage probabilities. Models presented here offer side-by-side comparisons of propagation characteristics over a wide range of mmWave bands, and the results and models are useful for the research and standardization process of future mmWave systems. Directional and omnidirectional path loss models with respect to a 1 m close-in free space reference distance over a wide range of mmWave frequencies and scenarios using directional antennas in real-world environments are provided herein, and are shown to simplify mmWave path loss models, while allowing researchers to globally compare and standardize path loss parameters for emerging mmWave wireless networks. A new channel impulse response modeling framework, shown to agree with extensive mmWave measurements over several bands, is presented for use in link-layer simulations, using the observed fact that spatial lobes contain multipath energy that arrives at many different propagation time intervals. The results presented here may assist researchers in analyzing and simulating the performance of next-generation mmWave wireless networks that will rely on adaptive antennas and multiple-input and multiple-output (MIMO) antenna systems.
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paper_title: Large scale antenna arrays with increasing antennas in limited physical space

paper_content:

Large Scale multiple input multiple output (MIMO) systems have recently emerged as a promising technology for 5G communications. While they have been shown to offer significant performance benefits in theoretical studies, the large scale MIMO transmitters will have to be deployed in the limited physical space of today's base stations (BSs). Accordingly, this paper examines effects of deploying increasing numbers of antennas in fxed physical space, by reducing the antenna spacing. We focus on the resulting performance of large-scale MIMO transmitters using low complexity closed form precoding techniques. In particular, we investigate the combined effect of reducing the distance between the antenna elements with increasing the number of elements in a fxed transmitter space. This gives rise to two contradicting phenomena: the reduction of spatial diversity due to reducing the separation between antennas and the increase in transmit diversity by increasing the number of elements. To quantify this tradeoff, we investigate densely deployed uniform antenna arrays modelled by detailed electromagnetic simulation. Our results show the somewhat surprising result that, by reducing the separations between the antennas to signifcantly less than the transmit wavelength to ft more antennas, the resulting system performance improves.
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paper_title: Towards Very Large Aperture Massive MIMO: a measurement based study

paper_content:

Massive MIMO is a new technique for wireless communications that claims to offer very high system throughput and energy efficiency in multi-user scenarios. The cost is to add a very large number of antennas at the base station. Theoretical research has probed these benefits, but very few measurements have showed the potential of Massive MIMO in practice. We investigate the properties of measured Massive MIMO channels in a large indoor venue. We describe a measurement campaign using 3 arrays having different shape and aperture, with 64 antennas and 8 users with 2 antennas each. We focus on the impact of the array aperture which is the main limiting factor in the degrees of freedom available in the multiple antenna channel. We find that performance is improved as the aperture increases, with an impact mostly visible in crowded scenarios where the users are closely spaced. We also test MIMO capability within a same user device with user proximity effect. We see a good channel resolvability with confirmation of the strong effect of the user hand grip. At last, we highlight that propagation conditions where line-of-sight is dominant can be favorable.
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paper_title: Five Disruptive Technology Directions for 5G

paper_content:

New research directions will lead to fundamental changes in the design of future 5th generation (5G) cellular networks. This paper describes five technologies that could lead to both architectural and component disruptive design changes: device-centric architectures, millimeter Wave, Massive-MIMO, smarter devices, and native support to machine-2-machine. The key ideas for each technology are described, along with their potential impact on 5G and the research challenges that remain.
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paper_title: Channel measurements and analysis for very large array systems at 2.6 GHz

paper_content:

Very large MIMO is a technique that potentially can offer large network capacities in multi-user scenarios where the users are equipped only with single antennas. In this paper we are investigating channel properties for a realistic, though somewhat extreme, outdoor base station scenario using a large array. We present measurement results using a 128 element linear array base station and 26 different user position in line-of-sight (LOS) and 10 different user position in non line-of-sight (NLOS). We analyze the Ricean K-factor, received power levels over the array, antenna correlation and eigenvalue distributions. We show that the statistical properties of the received signal vary significantly over the large array. Near field effects and the non-stationarities over the array help decorrelating the channel for different users, thereby providing a favorable channel conditions with stable channels and low interference for the considered single antenna users.
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paper_title: Multidimensional high-resolution channel sounding in mobile radio

paper_content:

Multidimensional high resolution channel sounding is a method of wave propagation analysis, channel modeling, and link performance evaluation in mobile radio. For this application antenna array architecture design, calibration, and a maximum likelihood channel parameter estimation framework (RIMAX) is described. Depending on the available measured dimensions, the algorithm estimates the four coefficients of the polymetric path weight matrix, the direction of arrival, the direction of departure, the time delay of arrival, and the Doppler-shift of the specular paths. Moreover, the parameters of a statistical model of the dense multipath components resulting from distributed diffuse scattering are estimated. Additional reliability measures are calculated to enhance the robustness of the estimate.
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paper_title: Base station polarization diversity reception for mobile radio

paper_content:

Base station polarization diversity reception in which signals are received by dual polarization (ex. ±45° polarization) is discussed. A theoretical analysis is presented on correlation coefficient ρ between diversity branches, and received signal level decrease L caused by polarization difference at the base and mobile station. The generalized expressions of ρ and L are then derived. Measurements were also carried out at 900 MHz in an urban area. Consequently, it was found that the ρ and L values are expressed by three factors, ρ is lower than 0.6 and L is smaller than 2.5 dB. It is concluded that this polarization diversity reception can be used as an effective diversity reception.
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paper_title: Wireless Communication for Factory Automation: an opportunity for LTE and 5G systems

paper_content:

The evolution of wireless communication from 4G toward 5G is driven by application demands and business models envisioned for 2020 and beyond. This requires network support for novel use cases in addition to classical mobile broadband services. Wireless factory automation is an application area with highly demanding communication requirements. We classify these requirements and identify the opportunities for the current LTE air interface for factory automation applications. Moreover, we give an outlook on the relevant design considerations to be addressed by 5G communication systems.
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paper_title: 5GNOW: non-orthogonal, asynchronous waveforms for future mobile applications

paper_content:

This article provides some fundamental indications about wireless communications beyond LTE/LTE-A (5G), representing the key findings of the European research project 5GNOW. We start with identifying the drivers for making the transition to 5G networks. Just to name one, the advent of the Internet of Things and its integration with conventional human-initiated transmissions creates a need for a fundamental system redesign. Then we make clear that the strict paradigm of synchronism and orthogonality as applied in LTE prevents efficiency and scalability. We challenge this paradigm and propose new key PHY layer technology components such as a unified frame structure, multicarrier waveform design including a filtering functionality, sparse signal processing mechanisms, a robustness framework, and transmissions with very short latency. These components enable indeed an efficient and scalable air interface supporting the highly varying set of requirements originating from the 5G drivers.
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paper_title: FANTASTIC-5G: flexible air interface for scalable service delivery within wireless communication networks of the 5th generation

paper_content:

5th generation mobile networks will have to cope with a high degree of heterogeneity in terms of services, mobility, number of devices and so on. Thus, diverse and often contradicting key performance indicators need to be supported, but having multiple radio access technologies for multi-service support below 6i¾źGHz will be too costly. FANTASTIC-5G will develop a new multi-service air interface through a modular design. To allow the system to adapt to the anticipated heterogeneity, some properties need to be pursued, like simplicity, flexibility, scalability, versatility, efficiency and future proofness. Based on these properties, a selected set of use cases and link and network design will be presented. The paper will also comprise validation and system level simulations through some indicative results and will conclude with the overall impact to 5G standardisation. Copyright © 2016 John Wiley & Sons, Ltd.
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paper_title: Understanding the Current Operation and Future Roles of Wireless Networks: Co-Existence, Competition and Co-Operation in the Unlicensed Spectrum Bands

paper_content:

Technology and policy are coming together to enable a paradigmatic change to the most widely used mechanism, exclusive rights, which allows mobile telecommunications operators to use the radio spectrum. Although spectrum sharing is not a new idea, the limited supply of spectrum and the enormous demand for mobile broadband services are forcing spectrum authorities to look more closely into a range of tools that might accelerate its adoption. This paper seeks to understand how co-existence and co-operation of Wi-Fi and cellular networks in the unlicensed spectrum can increase the overall capacity of heterogeneous wireless networks. It also reveals the challenges posed by new uses, such as machine-to-machine communications and the Internet of Things. It also brings together two major proposed regulatory approaches, such as those by the U.K.’s Ofcom and the European Commission, which currently represent leading efforts to provide spectrum authorities with robust spectrum sharing frameworks, to discuss policy tools likely to be implemented.
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paper_title: Filtered-OFDM - Enabler for Flexible Waveform in the 5th Generation Cellular Networks

paper_content:

The underlying waveform has always been a shaping factor for each generation of the cellular networks, such as orthogonal frequency division multiplexing (OFDM) for the 4th generation cellular networks (4G). To meet the diversified and pronounced expectations upon the upcoming 5G cellular networks, here we present an enabler for flexible waveform configuration, named as filtered-OFDM (f-OFDM). With the conventional OFDM, a unified numerology is applied across the bandwidth provided, balancing among the channel characteristics and the service requirements, and the spectrum efficiency is limited by the compromise we made. In contrast, with f-OFDM, the assigned bandwidth is split up into several subbands, and different types of services are accommodated in different subbands with the most suitable waveform and numerology, leading to an improved spectrum utilization. After outlining the general framework of f-OFDM, several important design aspects are also discussed, including filter design and guard tone arrangement. In addition, an extensive comparison among the existing 5G waveform candidates is also included to illustrate the advantages of f-OFDM. Our simulations indicate that, in a specific scenario with four distinct types of services, f-OFDM provides up to 46% of throughput gains over the conventional OFDM scheme.
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paper_title: Relaxed synchronization support of universal filtered multi-carrier including autonomous timing advance

paper_content:

5G wireless systems may benefit by waveforms supporting relaxed synchronization, as this enables reduced energy consumption, better support of low-end devices and reduction of signaling overhead. In this paper we evaluate UFMC (Universal Filtered Multi-Carrier), also known as UF-OFDM (universal filtered OFDM) — the recently appeared waveform option for 5G — with respect to its performance in scenarios with relaxed synchronization. Both carrier frequency offset, e.g. due to low-cost oscillators used in low-end devices, and relative fractional delay, e.g. due to the absence of an energy consuming closed-loop ranging mechanism, is considered. We introduce a concept called autonomous timing advance (ATA) improving the overall system performance. With ATA the system can operate purely based on open-loop synchronization. For comparing UFMC with CP-OFDM, we evaluate the mean squared error (MSE) in the receiver after frequency conversion. With applying a limit regarding the tolerable amount of distortion, we calculate the supported link distance for a system applying either UFMC or CP-OFDM for LTE-like settings. With applying UFMC, higher link distances are supported than with CP-OFDM, if the system applies open-loop synchronization.
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paper_title: Spectrum Sharing in mmWave Cellular Networks via Cell Association, Coordination, and Beamforming

paper_content:

This paper investigates the extent to which spectrum sharing in millimeter-wave (mmWave) networks with multiple cellular operators is a viable alternative to traditional dedicated spectrum allocation. Specifically, we develop a general mathematical framework to characterize the performance gain that can be obtained when spectrum sharing is used, as a function of the underlying beamforming, operator coordination, bandwidth, and infrastructure sharing scenarios. The framework is based on joint beamforming and cell association optimization, with the objective of maximizing the long-term throughput of the users. Our asymptotic and non-asymptotic performance analyses reveal five key points: 1) spectrum sharing with light on-demand intra- and inter-operator coordination is feasible, especially at higher mmWave frequencies (for example, 73 GHz); 2) directional communications at the user equipment substantially alleviate the potential disadvantages of spectrum sharing (such as higher multiuser interference); 3) large numbers of antenna elements can reduce the need for coordination and simplify the implementation of spectrum sharing; 4) while inter-operator coordination can be neglected in the large-antenna regime, intra-operator coordination can still bring gains by balancing the network load; and 5) critical control signals among base stations, operators, and user equipment should be protected from the adverse effects of spectrum sharing, for example by means of exclusive resource allocation. The results of this paper, and their extensions obtained by relaxing some ideal assumptions, can provide important insights for future standardization and spectrum policy.
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paper_title: Pulse shaped OFDM for asynchronous uplink access

paper_content:

This paper considers the scenario of massive machine type communication (MTC) in cellular uplink for short packet traffic. For this traffic type, the closed loop Timing Advance (TA) adjustment process applied in current LTE uplink introduces significant overhead in signaling and energy consumption. We propose to pulse shape the OFDM signals using time-frequency localized prototype filters. This improves the robustness of the waveform against timing offsets, so that the uplink transmission procedure can be simplified. Building on that property, an asynchronous uplink access scheme is developed. In the performance evaluation, we compare with both DFTs-OFDM and CP-OFDM schemes and demonstrate that higher spectral efficiency can be achieved by the pulse shaped OFDM scheme, highlighting the benefits in properly designing the pulse shapes of a multicarrier waveform.
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paper_title: Orthogonal Time Frequency Space Modulation

paper_content:

A new two-dimensional modulation technique called Orthogonal Time Frequency Space (OTFS) modulation designed in the delay-Doppler domain is introduced. Through this design, which exploits full diversity over time and frequency, OTFS coupled with equalization converts the fading, time-varying wireless channel experienced by modulated signals such as OFDM into a time-independent channel with a complex channel gain that is roughly constant for all symbols. Thus, transmitter adaptation is not needed. This extraction of the full channel diversity allows OTFS to greatly simplify system operation and significantly improves performance, particular in systems with high Doppler, short packets, and large antenna arrays. Simulation results indicate at least several dB of block error rate performance improvement for OTFS over OFDM in all of these settings. In addition these results show that even at very high Dopplers (500 km#x002F;h), OTFS approaches channel capacity through linear scaling of throughput with the MIMO order, whereas the performance of OFDM under typical design parameters breaks down completely.
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paper_title: FBMC receiver for multi-user asynchronous transmission on fragmented spectrum

paper_content:

Relaxed synchronization and access to fragmented spectrum are considered for future generations of wireless networks. Frequency division multiple access for filter bank multicarrier (FBMC) modulation provides promising performance without strict synchronization requirements contrary to conventional orthogonal frequency division multiplexing (OFDM). The architecture of a FBMC receiver suitable for this scenario is considered. Carrier frequency offset (CFO) compensation is combined with intercarrier interference (ICI) cancellation and performs well under very large frequency offsets. Channel estimation and interpolation had to be adapted and proved effective even for heavily fragmented spectrum usage. Channel equalization can sustain large delay spread. Because all the receiver baseband signal processing functionalities are proposed in the frequency domain, the overall architecture is suitable for multiuser asynchronous transmission on fragmented spectrum.
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paper_title: Flexible Configured OFDM for 5G Air Interface

paper_content:

A flexible orthogonal frequency division multiplex (OFDM)-based modulation scheme is proposed under the name of Flexible Configured OFDM (FC-OFDM). It enables a flexible subband configuration and targets a multi-service scenario, which is envisioned for future 5G networks. The proposed FC-OFDM scheme provides a good compromise between the filter bank multi-carrier with offset quadrature amplitude modulation and the classical cyclic prefix-based OFDM system. The detailed system structure is illustrated in this paper, together with efficiency evaluations.
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paper_title: Zero-tail DFT-spread-OFDM signals

paper_content:

In the existing scheduled radio standards using Orthogonal Frequency Division Multiplexing (OFDM) or Discrete Fourier Transform-spread-OFDM (DFT-s-OFDM) modulation, the Cyclic Prefix (CP) duration is usually hard-coded and set as a compromise between the expected channel characteristics and the necessity of fitting a predefined frame duration. This may lead to system inefficiencies as well as bad coexistence with networks using different CP settings. In this paper, we propose the usage of zero-tail DFT-s-OFDM signals as a solution for decoupling the radio numerology from the expected channel characteristics. Zero-tail DFT-s-OFDM modulation allows to adapt the overhead to the estimated delay spread/propagation delay. Moreover, it enables networks operating over channels with different characteristics to adopt the same numerology, thus improving their coexistence. An analytical description of the zero-tail DFT-s-OFDM signals is provided, as well as a numerical performance evaluation with Monte Carlo simulations. Zero-tail DFT-s-OFDM signals are shown to have approximately the same Block Error Rate (BLER) performance of traditional OFDM, with the further benefit of lower out-of-band (OOB) emissions.
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paper_title: A study on the coexistence of fixed satellite service and cellular networks in a mmWave scenario

paper_content:

The use of a larger bandwith in the millimeter wave (mmWave) spectrum is one of the key components of next generation cellular networks. Currently, part of this band is allocated on a co-primary basis to a number of other applications, such as the fixed satellite services (FSSs). In this paper, we investigate the coexistence between a cellular network and FSSs in a mmWave scenario. In light of the parameters recommended by the standard and the recent results presented in the literature on the mmWave channel model, we analyze different BSs deployments and different antenna configurations at the transmitters. Finally, we show how, exploiting the features of a mmWave scenario, the coexistence between cellular and satellite services is feasible and the interference at the FSS antenna can be kept below recommended levels.
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paper_title: QAM-FBMC: A New Multi-Carrier System for Post-OFDM Wireless Communications

paper_content:

Recently, as asynchronous heterogeneous network scenario becoming one of the key features for next generation wireless communications, superior spectrum confinement as well as higher spectral efficiency compared to cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM) has been taken into consideration for future radio access technologies. In this paper, we propose a new quadrature amplitude modulation based filter-bank multi-carrier (QAM-FBMC) system that provides with an inherent spectral efficiency gain against CP-OFDM, which comes from reduction of the redundancies such as cyclic prefix (CP) and guard-band, while keeping the symbol rate same as in CP-less OFDM. We propose a new transceiver structure consisting of at least two different filter-bank bases at both transmitter and receiver sides. Practical algorithms including channel estimation and equalization to mitigate multi-path fading channel without CP are proposed. Various evaluation results show that the proposed system performs comparable to the CP-OFDM system even without CP and guard-band reduction is also available from the well-confined spectrum.
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paper_title: FS-FBMC: A flexible robust scheme for efficient multicarrier broadband wireless access

paper_content:

An alternative implementation of the filter bank multicarrier (FBMC) concept is introduced. It is based on an FFT whose size is the length of the prototype filter. The approach clarifies the connection with OFDM and its main benefit is in the receiver, where high performance sub-channel equalization and timing offset compensation are achieved in a straightforward manner without additional delay. The scheme is particularly appropriate for broadband wireless access, to cope with fragmented frequency bands and to optimize the utilization of the spectrum, for example with the help of water-filling based sub-channel loading algorithms. The context of TV white spaces is taken for illustration. An issue with the proposed scheme is the computational complexity in the receiver and an approach having the potential for substantial savings is mentioned.
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paper_title: Investigating Spectrum Sharing between 5G Millimeter Wave Networks and Fixed Satellite Systems

paper_content:

In this paper we study coexistence of 5G small cells with fixed satellite systems (FSSs) in a scenario where both systems operate co-channel in the large spectrum bandwidth available around 28 GHz. Such studies are of great importance to inform the research community,industry and regulators which are currently investigating spectrum requirements and technology options for 5G systems. Focusing on the FSS uplink scenario, we use realistic FSS parameters and radiation pattern, combined with very recent channel from the literature, we analyzed the impact of interference resulting from FSS radiation on the achievable capacity and throughput of 5G small cells considering various multiple antenna configurations at the base stations (BSs) and different deployments of the mobile transmitters when no cooperation is allowed between the BSs. Starting from the lower bound, represented by an omnidirectional configuration of the transmitters, we extend our work to the analysis of large antenna arrays that will be used in the new generation of mobile cellular systems. Our results indicate that by exploiting a large number of antennas at the BSs and properly setting the protection distance between FSS and cellular BS, co-channel deployment of 5G small cells with FSS earth stations is possible, in the sense that adequate user data rates could be provided to the majority of mobile users.
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paper_title: A flexible 100-antenna testbed for Massive MIMO

paper_content:

Massive multiple-input multiple-output (MIMO) is one of the main candidates to be included in the fifth generation (5G) cellular systems. For further system development it is desirable to have real-time testbeds showing possibilities and limitations of the technology. In this paper we describe the Lund University Massive MIMO testbed — LuMaMi. It is a flexible testbed where the base station operates with up to 100 coherent radio-frequency transceiver chains based on software radio technology. Orthogonal Frequency Division Multiplex (OFDM) based signaling is used for each of the 10 simultaneous users served in the 20 MHz bandwidth. Real time MIMO precoding and decoding is distributed across 50 Xilinx Kintex-7 FPGAs with PCI-Express interconnects. The unique features of this system are: (i) high throughput processing of 384 Gbps of real time baseband data in both the transmit and receive directions, (ii) low-latency architecture with channel estimate to precoder turnaround of less than 500 micro seconds, and (iii) a flexible extension up to 128 antennas. We detail the design goals of the testbed, discuss the signaling and system architecture, and show initial measured results for a uplink Massive MIMO over-the-air transmission from four single-antenna UEs to 100 BS antennas.
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paper_title: Filtered OFDM: A new waveform for future wireless systems

paper_content:

A spectrally-localized waveform is proposed based on filtered orthogonal frequency division multiplexing (f-OFDM). By allowing the filter length to exceed the cyclic prefix (CP) length of OFDM and designing the filter appropriately, the proposed f-OFDM waveform can achieve a desirable frequency localization for bandwidths as narrow as a few tens of subcarriers, while keeping the inter-symbol interference/inter-carrier interference (ISI/ICI) within an acceptable limit. Enabled by the proposed f-OFDM, an asynchronous filtered orthogonal frequency division multiple access (f-OFDMA)/filtered discrete-Fourier transform-spread OFDMA (f-DFT-S-OFDMA) scheme is introduced, which uses the spectrum shaping filter at each transmitter for side lobe leakage elimination and a bank of filters at the receiver for inter-user interference rejection. Per-user downsampling and short fast Fourier transform (FFT) are used at the receiver to ensure a reasonable complexity of implementation. The proposed scheme removes the inter-user time-synchronization overhead required in the synchronous OFDMA/DFT-S-OFDMA. The performance of the asynchronous f-OFDMA is evaluated and compared with that of the universal-filtered OFDM (UF-OFDM), proposed in [1], [2].
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paper_title: 5G air interface design based on Universal Filtered (UF-)OFDM

paper_content:

In this paper we discuss 5G air interface design with respect to waveforms, multiple access and frame structure. We start by introducing the 5G system level requirements and expected scenarios. 5G will be driven by supporting very heterogeneous service and device classes. A unified frame structure for handling those heterogeneous traffic types is presented. Multiple access for 5G will make use of strict synchronicity where it is justifiable and will drop it where signalling overhead and energy consumption will demand for. In order to serve this unified frame structure best, the choice of the underlying waveform is discussed. CP-OFDM has its limitations in spectral properties and in conjunction with relaxed time-frequency alignment. The most discussed contender so far is Filter-Bank based Multi-Carrier (FBMC), with better spectral properties but new drawbacks introduced by offset-QAM and long filter lengths. Hence, a new alternative is required: Universal-Filtered OFDM (UF-OFDM), also known as Universal Filtered Multi-Carrier (UFMC), is a recent technology close to OFDM. UF-OFDM, according to encouraging results so far, summarized in this paper, fits best to the 5G system requirements. A further feature of the Unified Frame Structure is the usage of multiple signal layers. Here, users can be separated e.g. based on their interleavers, as done in Interleave-Division Multiple-Access (IDMA). This will introduce an additional degree of freedom for the system, improve robustness against crosstalk and helps to exploit the capacity of the multiple access channel (MAC). Altogether, the proposed new concepts offer an emboldening approach for dealing with the new challenges, faced by 5G wireless system designers.
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paper_title: Filtered-OFDM - Enabler for Flexible Waveform in the 5th Generation Cellular Networks

paper_content:

The underlying waveform has always been a shaping factor for each generation of the cellular networks, such as orthogonal frequency division multiplexing (OFDM) for the 4th generation cellular networks (4G). To meet the diversified and pronounced expectations upon the upcoming 5G cellular networks, here we present an enabler for flexible waveform configuration, named as filtered-OFDM (f-OFDM). With the conventional OFDM, a unified numerology is applied across the bandwidth provided, balancing among the channel characteristics and the service requirements, and the spectrum efficiency is limited by the compromise we made. In contrast, with f-OFDM, the assigned bandwidth is split up into several subbands, and different types of services are accommodated in different subbands with the most suitable waveform and numerology, leading to an improved spectrum utilization. After outlining the general framework of f-OFDM, several important design aspects are also discussed, including filter design and guard tone arrangement. In addition, an extensive comparison among the existing 5G waveform candidates is also included to illustrate the advantages of f-OFDM. Our simulations indicate that, in a specific scenario with four distinct types of services, f-OFDM provides up to 46% of throughput gains over the conventional OFDM scheme.
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paper_title: A survey: Several technologies of non-orthogonal transmission for 5G

paper_content:

One key advantage of 4G OFDM system is the relatively simple receiver implementation due to the orthogonal resource allocation. However, from sum-capacity and spectral efficiency points of view, orthogonal systems are never the achieving schemes. With the rapid development of mobile communication systems, a novel concept of non-orthogonal transmission for 5G mobile communications has attracted researches all around the world. In this trend, many new multiple access schemes and waveform modulation technologies were proposed. In this paper, some promising ones of them were discussed which include Non-orthogonal Multiple Access (NOMA), Sparse Code Multiple Access (SCMA), Multi-user Shared Access (MUSA), Pattern Division Multiple Access (PDMA) and some main new waveforms including Filter-bank based Multicarrier (FBMC), Universal Filtered Multi-Carrier (UFMC), Generalized Frequency Division Multiplexing (GFDM). By analyzing and comparing features of these technologies, a research direction of guiding on future 5G multiple access and waveform are given.
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paper_title: Pattern division multiple access (PDMA) for cellular future radio access

paper_content:

This paper presents a pattern division multiple access (PDMA) concept for cellular future radio access (FRA) towards the 2020s information society. Different from the current LTE radio access scheme (until Release 11), PDMA is a novel non-orthogonal multiple access technology based on the total optimization of multiple user communication system. It considers joint design from both transmitter and receiver. At the receiver, multiple users are detected by successive interference cancellation (SIC) detection method. Numerical results show that the PDMA system based on SIC improve the average sum rate of users over the orthogonal system with affordable complexity.
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paper_title: FB-OFDM: A novel multicarrier scheme for 5G

paper_content:

In this paper, a novel multicarrier waveform called filter bank OFDM (FB-OFDM) is proposed for use in 5G. FB-OFDM adopts complex modulation and filtering on subcarrier level. It is compatible to CP-OFDM utilized by LTE and its transceiver can be implemented by simple polyphase filter. Moreover, FB-OFDM system can support asynchrony between subcarriers and choose pulse function adaptively under different scenarios. A pulse function that has good locality in both time and frequency domain is also constructed based on root raised cosine function. Simulation results are provided to validate the performance of FB-OFDM and the proposed pulse function.
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paper_title: Argos: practical many-antenna base stations

paper_content:

Multi-user multiple-input multiple-output theory predicts manyfold capacity gains by leveraging many antennas on wireless base stations to serve multiple clients simultaneously through multi-user beamforming (MUBF). However, realizing a base station with a large number antennas is non-trivial, and has yet to be achieved in the real-world. We present the design, realization, and evaluation of Argos, the first reported base station architecture that is capable of serving many terminals simultaneously through MUBF with a large number of antennas (M >> 10). Designed for extreme flexibility and scalability, Argos exploits hierarchical and modular design principles, properly partitions baseband processing, and holistically considers real-time requirements of MUBF. Argos employs a novel, completely distributed, beamforming technique, as well as an internal calibration procedure to enable implicit beamforming with channel estimation cost independent of the number of base station antennas. We report an Argos prototype with 64 antennas and capable of serving 15 clients simultaneously. We experimentally demonstrate that by scaling from 1 to 64 antennas the prototype can achieve up to 6.7 fold capacity gains while using a mere 1/64th of the transmission power.
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Title: 5G: A Tutorial Overview of Standards, Trials, Challenges, Deployment, and Practice

Section 1: INTRODUCTION
Description 1: Introduce the evolution and necessity of 5G technology, including its usage scenarios and key performance parameters.

Section 2: 5G Requirements
Description 2: Discuss the minimum technical performance requirements for 5G and relevant key performance parameters.

Section 3: Spectrum Regulation
Description 3: Explain the frequency bands approved for 5G, including existing and new mm-wave bands, spectrum management, and coexistence.

Section 4: Standardisation for 5G
Description 4: Provide details on the standardization efforts by ITU-R and 3GPP, including the phased standardization process and key decisions.

Section 5: Key Technologies for RF Interface(s) of 5G
Description 5: Outline the significant technologies contributing to the 1000 times gain in capacity, including increased bandwidth, massive MIMO, network densification, and new waveforms.

Section 6: Channel Characteristics
Description 6: Summarize the propagation channel characteristics for 5G, including massive MIMO, distributed systems, and mm-wave channels.

Section 7: Signal Processing Techniques for 5G
Description 7: Discuss the major signal processing methods for 5G, including linear precoding, hybrid precoding, MU-MIMO receiver processing, and other important approaches.

Section 8: Antenna Layouts
Description 8: Detail the design and arrangement of large antenna arrays for 5G systems, including uniform linear arrays, rectangular arrays, and circular arrays.

Section 9: 5G New Waveforms and Channel Access
Description 9: Describe the waveforms and multi-user access schemes proposed for 5G, emphasizing flexibility and various requirements to support diverse use cases.

Section 10: Trials, Test-Beds, and Deployment
Description 10: Provide an overview of current 5G prototypes, test-beds, experimental trials, and announced early commercial deployments.

Section 11: 5G Core Network and Cloud RAN Architectures
Description 11: Explain the significant challenges and innovations in the core network and Cloud RAN architectures for 5G, focusing on SDN, NFV, network slicing, and fronthaul solutions.

Section 12: Deployment
Description 12: Discuss the deployment challenges for 5G networks, including transport and core network issues, beamforming strategies, and the implications of different deployment architectures.

Section 13: SUMMARY AND CHALLENGES
Description 13: Summarize the progression towards 5G commercialization, the technical challenges ahead, and key questions relating to its deployment.