Since the introduction of fifth-generation new radio (5G-NR) in Third Generation Partnership Project (3GPP) Release 15, swift progress has been made to evolve 5G with 3GPP Release 18 emerging. A critical aspect is the design of massive multiple-input multiple-output (MIMO) technology. In this line, this paper makes several important contributions: We provide a comprehensive overview of the evolution of standardized massive MIMO features from 3GPP Release 15 to 17 for both time/frequency-division duplex operation across frequency range in (FR)-1 and FR 2-1. We analyze the progress on channel state information (CSI) frameworks, beam management frameworks and present enhancements for uplink CSI. We shed light on emerging 3GPP Release 18 problems requiring imminent attention. These include advanced codebook design and sounding reference signal design for coherent joint transmission (CJT) with multiple transmission/reception points (multi TRPs). We discuss advancements in uplink demodulation reference signal design, enhancements for mobility to provide accurate CSI estimates, and unified transmission configuration indicator framework tailored for FR 2–1 bands. For each concept, we provide system level simulation results to highlight their performance benefits. Via field trials in an outdoor environment at Shanghai Jiaotong University, we demonstrate the gains of multi-TRP CJT relative to single TRP at 3.7 GHz.

Millimeter-wave channel measurements for a meeting room, lecture room and open plan office floor were used to analyse and model the spatial consistency of channel clusters. Particularly, how the angles of arrival and delays of extracted multi-path components in each cluster varied with small changes in receiver location. We extended the KPowerMeans algorithm, for classifying captured multi-path components into clusters from the delay/angular domains, to include the location domain to allow the spatial consistency of the clusters between locations to be analysed. The observed spatial consistency in measurements was then used to validate whether the 3GPP spatial consistency procedure (3GPP SC-I) reflects real world spatial consistency. The 3GPP spatial consistency procedure for ensuring spatial consistency of cluster parameters during mobile user simulations was then applied for each environment, allowing for comparison between the ‘predicted’ cluster parameters and the measured cluster parameters. The 3GPP model/procedure showed a good fit for all environments but highlighted the impact of the environment on the amount of accuracy of the procedure.

We provide a simple, yet general and accurate framework for analyzing zero-forcing (ZF) precoding performance for a full-dimensional massive multiple-input multiple-output system. Exploiting an order two Neumann series, our framework approximates the expected (average) ZF signal-to-noise ratio and ergodic sum spectral efficiency, while catering for a finite multipath propagation model, as well as correlated and uncorrelated user equipment (UE) positions in both azimuth and elevation domains. The analysis provides clear insights on the influence of propagation and system parameters on ZF performance. We identify how far UEs must lie in azimuth and elevation for the ZF precoder to approach uncorrelated channels. Our framework is useful for predicting the ZF performance degradation arising from channel correlation. For optimal performance, UEs could be separated by around 30° in azimuth and 15° in elevation - conditions which are difficult to meet in reality.

Mobile communications have been undergoing a generational change every ten years or so. However, the time difference between the so-called “G's” is also decreasing. While fifth-generation (5G) systems are becoming a commercial reality, there is already significant interest in systems beyond 5G, which we refer to as the sixth generation (6G) of wireless systems. In contrast to the already published papers on the topic, we take a top-down approach to 6G. More precisely, we present a holistic discussion of 6G systems beginning with lifestyle and societal changes driving the need for next-generation networks. This is followed by a discussion into the technical requirements needed to enable 6G applications, based on which we dissect key challenges and possibilities for practically realizable system solutions across all layers of the Open Systems Interconnection stack (i.e., from applications to the physical layer). Since many of the 6G applications will need access to an order-of-magnitude more spectrum, utilization of frequencies between 100 GHz and 1 THz becomes of paramount importance. As such, the 6G ecosystem will feature a diverse range of frequency bands, ranging from below 6 GHz up to 1 THz. We comprehensively characterize the limitations that must be overcome to realize working systems in these bands and provide a unique perspective on the physical and higher layer challenges relating to the design of next-generation core networks, new modulation and coding methods, novel multiple-access techniques, antenna arrays, wave propagation, radio frequency transceiver design, and real-time signal processing. We rigorously discuss the fundamental changes required in the core networks of the future, such as the redesign or significant reduction of the transport architecture that serves as a major source of latency for time-sensitive applications. This is in sharp contrast to the present hierarchical network architectures that are not suitable to realize many of the anticipated 6G services. While evaluating the strengths and weaknesses of key candidate 6G technologies, we differentiate what may be practically achievable over the next decade, relative to what is possible in theory. Keeping this in mind, we present concrete research challenges for each of the discussed system aspects, providing inspiration for what follows.

Fifth-generation (5G) new radio (NR) deployments are being rolled out in both the C-band (3.3 - 5.0 GHz) and millimeter-wave (mmWave) band (24.5 - 29.5 GHz). For outdoor scenarios, the C-band is expected to provide wide area coverage and throughput uniformity, whereas the mmWave band is expected to provide ultra-high throughput to dedicated areas within the C-band coverage. Due to the differences in the frequency bands, both systems are expected to be designed with different transmit and receive parameters, naturally resulting in performance variations proportional to the chosen parameters. Unlike many previous works, this paper presents measurement evaluations in central Auckland, New Zealand, from a pre-commercial deployment of a single-user, single-cell 5G-NR system operating in both bands. The net throughput, coverage reliability, and channel rank are analyzed across the two bands with baseband and analog beamforming. Our results show that the C-band coverage is considerably better than mmWave, with a consistently higher channel rank. Furthermore, the spatial stationarity region (SSR) for the azimuth angles-of-departure (AODs) is characterized, and a model derived from the measured beam identities is presented. The SSR of azimuth AODs is seen to closely follow a gamma distribution.

Millimeter-wave channel models are fundamental for evaluating the performance of technologies for 5G. In this paper, we present two kinds of millimeter-wave channel measurements, fixed point measurements and virtual measurements, in an indoor open office. A correlation sounder with a bandwidth of 600 MHz is used to carry out these measurements. Then, channel parameters, e.g., the path loss, shadow fading, root mean square (RMS) delay spread (DS), K-factor, delay-cluster number and angular spread, are extracted and modelled. These extracted models will enable the generation of an impulse response for the indoor hotspot environment. Differences between our measurement-based channel model parameters and the International Telecommunication Union (ITU) channel model are discussed. The impact of differences on capacity are investigated by simulations. We find that the ITU channel model is optimistic about capacity. These results can help researchers to use millimeter-wave channel models for indoor office scenarios when evaluating 5G performance.

Multi-cell wireless systems usually encounter both intra-cell and inter-cell interference, which can be mitigated via coordinated multipoint (CoMP) transmission. Previous works on multi-cell analysis in the microwave band generally consider fully digital beamforming, requiring a complete radio-frequency chain behind each antenna. This is practically infeasible for millimeter-wave (mmWave) systems where large amounts of antennas are necessary to provide sufficient gain and to enable transmission/reception of multiple streams to/from a user. This paper provides a general methodology to analytically compute the expected per-cell spectral efficiency of an mmWave multi-cell single-stream system using phase-shifter-based analog beamforming and regularized zero-forcing digital beamforming. Four analog-digital hybrid beamforming techniques for multi-cell multi-stream mmWave communication are proposed, assuming that base stations in different cells can share channel state information to cooperatively transmit signals to their home-cell users. Spectral efficiency of the proposed hybrid beamforming approaches is investigated and compared using two channel models suitable for fifth-generation cellular systems, namely the 3rd Generation Partnership Project model and the NYUSIM model. Numerical results show that the benefits of base station coordination (as compared with the non-CoMP case) are governed by the underlying propagation model, and the aggregate interference levels proportional to the cell radius and number of users per cell. We show that in sparse channels, non-CoMP approaches exceed CoMP (coordinated beamforming) performance.

Results of recent measurement campaigns demonstrate a significant increase in the degrees of freedom introduced by the elevation domain, and consequently, a large capacity gap as predicted by 3D and 2D MIMO channel models. To gain further insight into the effects of the elevation domain on MIMO system performance, we derive new spatial correlation (SC) expressions for 3D and 2D channel models, given rectangular arrays of cross-polarized antennas. The main result relates 3D SC to the 2D counterpart, allowing us to quantify the effects of the elevation domain on SC. Furthermore, our result allows us to derive the following three simple relationships between 2D and 3D correlations. We show that for a small elevation angle spread, the difference between 2D and 3D SCs grows quadratically in both vertical separation and angle spread. For the case of small vertical spacings, we derive a bound on the above correlation gap. Furthermore, we show that the elevation effects on correlation can be accurately decoupled, allowing us to express the 3D SC matrix as a Hadamard product of a 2D SC matrix and an elevation component. Collectively, our results demonstrate the need for accurate channel modeling when evaluating the performance of 3D MIMO systems.

The range of candidate bands considered for 5G wireless systems extends from 6-100 GHz. In this paper we examine the impact of key parameters of millimeter-wave (mmWave) and microwave channel models on various system performance metrics, e.g., spectral efficiency, eigenvalue structure and convergence to massive multiple-input multiple-output (MIMO) properties. We consider 6 spatial channel model (SCM) based scenarios, including 3GPP SCM at 2.6 GHz and those derived from recent measurement campaigns at 6 GHz, 28 GHz and 73 GHz. We define and evaluate an effective degrees of freedom metric (EDOF), demonstrating a dramatic EDOF reduction in mmWave bands compared to microwave, unless serving multiple users, where the antenna separation increases the effective rank of the composite channel. Furthermore, we analytically derive the covariance matrix of two SCM structures showing the impact of modelling choice on the channel correlations.

Channel models describe how wireless channel parameters behave in a given scenario, and help evaluate link- and system-level performance. A proper channel model should be able to faithfully reproduce the channel parameters obtained in field measurements and accurately predict the spatial and temporal channel impulse response along with large-scale fading. This paper compares two popular channel models for next generation wireless communications: the 3rd Generation Partnership Project (3GPP) TR 38.900 Release 14 channel model and the statistical spatial channel model NYUSIM developed by New York University (NYU). The two channel models employ different modeling approaches in many aspects, such as the line-of-sight probability, path loss, and clustering methodology. Simulations are performed using the two channel models to analyze the channel eigenvalue distribution and spectral efficiency leveraging the analog/digital hybrid beamforming methods found in the literature. Simulation results show that the 3GPP model produces different eigenvalue and spectral efficiency distributions for mmWave bands, as compared to the outcome from NYUSIM that is based on massive amounts of real-world measured data in New York City. This work shows NYUSIM is more accurate for realistic simulations than 3GPP in urban environments.

There is considerable pressure to define the key requirements of 5G, develop 5G standards, and perform technology trials as quickly as possible. Normally, these activities are best done in series but there is a desire to complete these tasks in parallel so that commercial deployments of 5G can begin by 2020. 5G will not be an incremental improvement over its predecessors; it aims to be a revolutionary leap forward in terms of data rates, latency, massive connectivity, network reliability, and energy efficiency. These capabilities are targeted at realizing high-speed connectivity, the Internet of Things, augmented virtual reality, the tactile internet, and so on. The requirements of 5G are expected to be met by new spectrum in the microwave bands (3.3-4.2 GHz), and utilizing large bandwidths available in mm-wave bands, increasing spatial degrees of freedom via large antenna arrays and 3-D MIMO, network densification, and new waveforms that provide scalability and flexibility to meet the varying demands of 5G services. Unlike the one size fits all 4G core networks, the 5G core network must be flexible and adaptable and is expected to simultaneously provide optimized support for the diverse 5G use case categories. In this paper, we provide an overview of 5G research, standardization trials, and deployment challenges. Due to the enormous scope of 5G systems, it is necessary to provide some direction in a tutorial article, and in this overview, the focus is largely user centric, rather than device centric. In addition to surveying the state of play in the area, we identify leading technologies, evaluating their strengths and weaknesses, and outline the key challenges ahead, with research test beds delivering promising performance but pre-commercial trials lagging behind the desired 5G targets.

There are rapid developments towards the deployment of 5G and hardly a day goes by without the release of a major announcement concerning 5G. Many of these developments are happening in parallel as there is a race against time for 5G deployment. Various test beds have been established, laboratory trials and proof of concept trials of individual building blocks of 5G are already underway or have been completed, large scale system trials are also happening or are planned and standardisation efforts in the ITU, 3GPP, IEEE, etc. are also in progress. In the case of ITU/3GPP, the standards are expected to be completed by 2019. Many candidate bands for 5G in the centimetric and mm wave range were identified by the World Radio Conference (WRC) 2015 and are expected to be finalized by WRC 2019, while coexistence studies of 5G and existing systems in the new candidate bands are underway.

This is the second part of the papers in this special issue on deployment issues and performance challenges for 5G. The papers have been divided into five categories:•Massive MIMO•Co existence•Signal Processing and New Waveforms•Cloud RAN•Field Trials

A general framework for the analysis of expected per-user signal-to-interference-plus-noise-ratio (SINR) of a multiuser multiple-input-multiple-output system is presented. Our analysis assumes spatially correlated Rayleigh fading channels with regularized zero-forcing precoding on the downlink. Unlike previous works, our analytical expressions are averaged over the eigenvalue densities of the complex Wishart distributed channel correlation matrix. To aid the derivation of the expected per-user SINR, we derive a closed-form expression for the joint density of two arbitrary eigenvalues of the complex Wishart matrix. In the high signal-to-noise-ratio (SNR) regime, with zero-forcing precoding, we derive analytical expressions to approximate the instantaneous per-user SNR and show that it is approximately gamma distributed. The generality of the approximations is validated with numerical results over a wide range of system dimensions, spatial correlation and SNR levels.

Analytical expressions to approximate the expected per-user signal-to-interference-plus-noise-ratio (SINR) and ergodic sum-rate of a multiuser multiple-input-multiple-output system are presented. Our analysis assumes uncorrelated Ricean fading channels with regularized zero-forcing precoding on the downlink. The derived expressions are averaged with respect to the previously unknown arbitrary eigenvalue densities of the complex non-central Wishart distributed channel correlation matrix. To aid the derivation of the expected SINR, we derive analytical expressions for the joint density of two arbitrary eigenvalues of the complex non-central Wishart matrix. Unlike previous studies, our model caters to the presence of a unique Rice factor for each user terminal, making it suitable for analysis of modern systems, such as small cells and millimeter-wave. Our findings suggest that while the presence of strong line-of-sight has an adverse effect on the expected SINR and ergodic sum-rates, increasing the variability of Rice factors enhances the peak rate performance of the system. Our analysis can be applied to arbitrary system dimensions and is seen to remain tight across the signal-to-noise-ratio range considered.

In this study, the authors investigate the impact of cooperation in an interference limited downlink cellular network. The authors identify the areas in which the transmissions of adjacent cells overlap in a 19 cell network for urban and suburban environments. They assume that the cooperative users are located in these overlapping areas and thus can receive channel information from neighbouring base stations (BSs) to cancel intercell interference. They also study the performance of such systems with various types of precoding techniques. They demonstrate that they can improve the system performance employing precoders that maximise signal‐to‐leakage‐noise ratios for each user. They also consider imperfect cross channel state information (CCSI) between cooperative users and interfering BSs. Furthermore, they analytically derive the additional intercell interference that is caused by imperfect CCSI at the receivers. The results demonstrate that the system can achieve better sum rates with cooperation, however this improvement is susceptible to the accuracy of the CCSI. They show that imperfect CCSI degrades the cooperation gains drastically.

Approximate expressions for the spatial correlation of cylindrical and uniform rectangular arrays (URA) are derived using measured distributions of angles of departure (AOD) for both the azimuth and zenith domains. We examine massive multiple-input-multiple-output (MIMO) convergence properties of the correlated channels by considering a number of convergence metrics. The per-user matched filter (MF) signal-to-interference-plus-noise ratio (SINR) performance and convergence rate, to respective limiting values, of the two antenna topologies is also explored.

We present the results of a 3 dimensional (3D) multiple-input multiple-output (MIMO) channel impulse response (CIR) measurement conducted in China and New Zealand (NZ) for an outdoor-to-indoor (O2I) urban macro environment. Key channel parameters are computed from the measured data using the space-alternating generalized expectation-maximization (SAGE) algorithm, and are used to re-construct the 16 × 16 3D MIMO CIR according to the guidelines given in the 3GPP 3D MIMO channel model. The CIR is then used to evaluate the channel capacity for four different antenna configurations. These different topologies, along with different antenna spacings, allow us to study the characteristics of the MIMO channel with different spatial correlation structures.

In this paper we examine convergence properties of massive MIMO systems with the aim of determining the number of antennas required for massive MIMO gains. We consider three characteristics of a channel matrix and study their asymptotic behaviour. Furthermore, we derive ZF SNR and MF SINR for a scenario of unequal receive powers. In our results we include the effects of spatial correlation. We show that the rate of convergence of channel metrics is much slower than that of the ZF/MF precoder properties.

We propose a novel combined receiver which applies zero forcing (ZF) to null the dominant interferers, followed by minimum-mean-square-error (MMSE) processing to reduce the effect of the remaining interference and noise. The receiver is used in conjunction with an interference alignment (IA) scheme in a multicell environment. Simulation results considering a range of interferer profiles show that the combined receiver outperforms an MMSE receiver, due to its ability to eliminate the dominant interferers. The complete nulling of many dominant interferers in a multi-user, multicell case requires a large number of receive antennas. If the antennas are closely located in a compact array, then spatial correlation is important and its impact on the IA scheme is also considered.

In this paper, we present a differential codebook design by modifying the Grassmannian codebook for a single-cell multiuser (MU) multiple-input single-output (MISO) system operating under spatially and temporally correlated channels. The differential codebook design involves scaling and rotation methods that help a codebook to track the slow varying channel. We propose an adaptive scaling technique that improves the performance of the system in correlated channels without any additional feedback information. Monte Carlo simulations show that the proposed differential codebook reduces quantization errors and improves sum-rate performance as compared to other differential codebooks designed for spatially and temporally correlated MISO channels.

We investigate the efficiency of interference alignment for 2 × 2 × 2 interference networks formed by concatenation of two user interference channels under the assumption of imperfect channel state information. We show that, as a result of cumulative noise in zero forcing receivers, aligned interference neutralisation is not practical when perfect channel state information is not available. We give analytical SNR and SINR expressions at relays and receivers and also provide some simulation results using a channel estimation error model.

Interference modeling is central to the analysis of many interference limited systems. The aggregate interference due to many individual interferers is often very difficult to characterize exactly, especially when lognormal shadowing and path loss is considered. Some results are available for homogeneous sources where the individual components have the same distribution, but many more detailed studies involve heterogeneous sources. Hence, in this paper we build upon previous models for homogeneous sources to develop three general purpose approaches to modeling the aggregate interference from heterogeneous sources which include both shadowing and path-loss. These models are the inverse gamma, inverse generalized gamma and extreme value distributions. These models are motivated by prior work as well as by three distinct interference scenarios relating to cognitive radio systems, digital television and femtocells. The models are fitted to these wide ranging examples and numerical results show good agreement, both for interference and SINR distributions.

In this paper, we evaluate the capacity gains offered by MIMO systems relative to SISO by using two metrics: the ratio between the expected value of the two capacities and a weighted difference between the two capacities. We present results for these metrics for the case of a general channel, where fading could be iid, correlated Rayleigh, or any other type. We derive limiting values of the capacity metrics for low and high SNR and show via simulation their behavior at other values of SNR. These simulations provide valuable insight into the variation of the capacity metrics in the important SNR range from -5 dB to 20 dB which is commonly observed in cellular systems.

Multi-user downlink MIMO communication has the potential to increase throughput using efficient beamforming. However, the precoding criteria assume perfect knowledge of the beamforming matrices at the base station (BS). This requires a user to transmit its complete channel information to the BS, a task that entails significant overhead. Limited feedback precoding, a part of LTE standardization now, needs only limited information be fed back to the BS. This paper studies the capacity loss incurred by using LTE codebooks, under different power allocation schemes with varying degrees of optimality. The sub-optimal power allocation schemes also ensure a better throughput-fairness trade-off. It is observed that using LTE codebooks results in a significant loss and that there is a need to develop codebooks which offer better adaptation to channel variations.

We investigate the performance of femto cell networks in both closed and open access regimes. Specifically, we analyse the typical user capacity as well as the sum capacity of all users in a macro cell with a Poisson field of femto base stations. The closed access results demonstrate that while the system sum capacity initially increases with the density of femto cells, this gain comes with a significant performance penalty suffered by an individual macro user. In open access mode, we show via analytical derivations that macro users only benefit from access to the available femto base stations for impractically high densities. Furthermore, we derive limiting sum capacity results showing that while the mean sum capacity initially increases linearly with femto density, sum capacity will eventually decay to zero in the limiting case as the femto density approaches infinity.

In limited feedback multiple-input multiple-output (MIMO) systems, it is important that the codebooks used provide a good approximation to the channel. Any mismatch between the codebook entries and the actual channel creates a non zero chordal distance (or subspace angle) between the codebook element and the corresponding columns of the right singular matrix of the channel. As a result, the system exhibits a capacity loss. We evaluate and quantify the amount of this capacity loss with long term evolution (LTE) and Grassmannian codebooks in the case of Layer 1 (L1) and Layer 2 (L2) transmission using linear receivers in three different channel models. We evaluate the distribution of the minimum distance for LTE and Grassmannian codebooks via simulations. We also derive capacity loss approximations for zero forcing (ZF), minimum mean-squared error (MMSE) and singular value decomposition (SVD) receivers. We find that the capacity loss approximations for L1 transmission are accurate for all receivers and lead to an extremely simple relationship between capacity loss and minimum distance.

Limited feedback precoding is part of the LTE standard. Despite standardization, important fundamental questions, especially relating to the performance due to the use of codebooks and receiver processing techniques, remain to be explored. In order to understand these questions, we consider a single user in a single cell employing single or multi-stream transmission using a variety of codebooks and a choice of different receiver types. We derive expressions for capacity loss relative to perfect feedback due to the limited size of codebook. When multistream transmission is deployed, we show that codebook feedback manifests itself as inter-stream interference resulting in a capacity loss for all receiver types. In the case of SVD receivers, this interference results in a capacity floor. We define a precoding matrix index (PMI) coherence time and bandwidth and show how these parameters are respectively related to channel coherence time and bandwidth. The PMI coherence parameters shed new light on how often feedback is required, both in time and frequency domains, and help us to determine the capacity penalty if feedback is delayed beyond the coherence parameters. Finally, we show the distribution of PMI and also show that this is environment dependent. This supports the need for codebooks that are adaptable to different environments and are not strictly tied to i.i.d. channels.

We examine the impact of channel knowledge on the secondary user (SU) in a cognitive radio system. Under a minimum signal-to-interference-and-noise ratio (SINR) constraint for the primary user (PU) receiver, we determine the SU capacity under four channel knowledge scenarios. We derive analytical expressions for the capacity cumulative distribution functions which are verified by means of simulations. We show that the lack of exact knowledge of the PU-PU channel gain by the SU-Tx either prohibits SU transmission or necessitates high interference level at the PU. We also show that the lack of exact knowledge of the SU-Tx to PU-Rx link has little or no impact on SU capacity.

Cognitive radio design aims to increase spectrum utilization by allowing the secondary users (SUs) to coexist with the primary users (PUs) as long as the interference caused by the SUs to each PU is properly regulated. At the SU, channel state information (CSI) between its transmitter and the PU receiver is used to calculate the maximum allowable SU transmit power to limit the interference. We assume that this PU-SU CSI is imperfect as it is the practical case. In addition to a peak received interference power constraint, an upper limit to the SU transmit power constraint is also considered. We derive a closed-form expression for the mean SU capacity under this scenario. Due to imperfect CSI, the SU can not always satisfy the received peak interference power constraint at the PU and has to back off its transmit power. The resulting capacity loss for the SU is quantified using the cumulative distribution function of the interference at the PU. Additionally, we investigate the impact of CSI quantization. Our results are confirmed through comparison with simulations.

In this paper we describe a model for calculating the aggregate interference encountered by primary receivers in the presence of randomly placed cognitive radios (CRs). We show that incorporating the impact of distance attenuation and lognormal fading on each constituent interferer in the aggregate, leads to a composite interference that cannot be satisfactorily modeled by a lognormal. Using the interference statistics we determine a number of key parameters needed for the deployment of CRs. Examples of these are the exclusion zone radius, needed to protect the primary receiver under different types of fading environments and acceptable interference levels, and the numbers of CRs that can be deployed. We further show that if the CRs have apriori knowledge of the radio environment map (REM), then a much larger number of CRs can be deployed especially in a high density environment. Given REM information, we also look at the CR numbers achieved by two different types of techniques to process the scheduling information.

Cognitive radios are expected to work in bands below about 3.5 GHz and may be used for a variety of applications, e.g., broadband fixed wireless access, mobile and nomadic access, etc. Cognitive radio system designers must have access to a wide range of channel models covering a wide span of operating frequencies, carrier bandwidths, deployment conditions, and environments. This paper provides a comprehensive overview of the propagation channel models that will be used for the design of cognitive radio systems. We start with classical models for signal loss versus distance and discuss their dependence on the physical properties of the environment and operating frequency. Here we also introduce the concept of log-normal shadowing resulting from signal blockage by man-made and natural features. Next, we discuss the time-varying nature of the wireless channel, introduced as a result of the motion of objects in the channel. This is followed by a discussion on the dispersion of the signal caused by various effects of propagation, especially in the time and frequency domains. Angular dispersion, which is discussed next, is important because cognitive radios may be based on modems that exploit the spatial domain. Lastly, we summarize channel models that have been standardized for fixed and mobile systems.

Single-user, multiuser, and network MIMO performance is evaluated for downlink cellular networks with 12 antennas per site, sectorization, universal frequency reuse, scheduled packet-data, and a dense population of stationary users. Compared to a single-user MIMO baseline system with 3 sectors per site, network MIMO coordination is found to increase throughput by a factor of 1.8 with intra-site coordination among antennas belonging to the same cell site. Intra-site coordination performs almost as well as a highly sectorized system with 12 sectors per site. Increasing the coordination cluster size from 1 to 7 sites increases the throughput gain factor to 2.5.

Cognitive radio technology is an innovative radio design concept which aims to increase spectrum utilization by exploiting unused spectrum in dynamically changing environments. By extending previous results, we investigate the capacity gains achievable with this dynamic spectrum approach in asymmetric fading channels. More specifically, we allow the secondary-to- primary and secondary-to-secondary user channels to undergo Rayleigh or Rician fading, with arbitrary link power. In order to compute the capacity, we derive the distributions of ratios of Rayleigh and Rician variables. Compared to the symmetric fading scenario, our results indicate several interesting features of the capacity behaviour under both average and peak received power constraints. Finally, the impact of multiple primary users on the capacity under asymmetric fading has also been studied.

The 17 papers in this special issue focus on MIMO systems and applications: field experience, practical aspects, limitations and challenges. The papers are summarized here.

In this paper we develop a multiple input multiple output (MIMO) channel model and derive its spatial and temporal correlation properties. We present a generalized methodology to derive the spatial correlation when the angles of arrival (AoA) and angles of departure (AoD) are either independent or partly correlated. Our model therefore spans the full range from well-established single ring models, where AoA and AoD are fully correlated to complex industrial channel models where they are uncorrelated. It is shown that first order and second order approximations to the channel give rise to a single- Kroneckermodel and a sum-Kroneckermodel respectively. We compare our model to other MIMO channel models in terms of correlation structure and the ergodic mutual information (EMI) and study the effect of the non-Kronecker correlation structure.

Singular Value Decomposition (SVD) techniques are well-known for providing an elegant transceiver architecture, that can realize the intrinsic spatial links of a MIMO channel. Such a scheme is also referred to as eigen-beamforming or eigenmode transmission. The system performance is, however, closely related to the quality of the channel information at both ends of the link. Hence, we focus on the effect of feedback time delay, which causes the transmitter to use outdated channel information in timevarying fading channels. In this paper, we derive an analytical expression for the instantaneous SINR of eigenmode transmission with a feedback time delay T. Furthermore, this expression implies a novel metric that gauges the system performance sensitivity to time-variations of the steering vectors (eigenvectors of the channel correlation matrix) at the transmitter.

Many channel models for MIMO systems have appeared in the literature. However, with the exception of a few recent results, they are largely focussed on two dimensional (2D) propagation, i.e., propagation in the horizontal plane, and the impact of elevation angle is not considered. The assumption of 2D propagation breaks down when in some propagation environments the elevation angle distribution is significant. Consequently, the estimation of ergodic capacity assuming a 2D channel coefficient alone can lead to erroneous results. In this paper, for cross polarized channels, we define a composite channel model and channel coefficient that takes into account both 2D and 3D propagation. Using this composite channel coefficient we assess the ergodic channel capacity and discuss its sensitivity to a variety of different azimuth and elevation power distributions and other system parameters.

Fourth-generation (4G) systems are expected to support data rates of the order of 100 Mb/s in the outdoor environment and 1 Gb/s in the indoor/stationary environment. In order to support such large payloads, the radio physical layer must employ receiver algorithms that provide a significant increase in spectrum efficiency (and, hence, capacity) over current wireless systems. Recently, an explosion of multiple-input-multiple-output (MIMO) studies have appeared with many journals presenting special issues on this subject. This has occurred due to the potential of MIMO to provide a linear increase in capacity with antenna numbers. Environmental considerations and tower loads will often restrict the placing of large antenna spans on base stations (BSs). Similarly, customer device form factors also place a limit on the antenna numbers that can be placed with a mutual spacing of 0.5 wavelength. The use of cross-polarized antennas is widely used in modern cellular installations as it reduces spacing needs and tower loads on BSs. Hence, this approach is also receiving considerable attention in MIMO systems. In order to study and compare various receiver architectures that are based on MIMO techniques, one needs to have an accurate knowledge of the MIMO channel. However, very few studies have appeared that characterize the cross-polarized MIMO channel. Recently, the third-generation partnership standards bodies (3GPP/3GPP2) have defined a cross-polarized channel model for MIMO systems but this model neglects the elevation spectrum. In this paper, we provide a deeper understanding of the channel model for cross-polarized systems for different environments and propose a composite channel impulse model for the cross-polarized channel that takes into account both azimuth and elevation spectrum. We use the resulting channel impulse response to derive closed-form expressions for the spatial correlation. We also present models to describe the dependence of cross-polarization discrimination (XPD) on distance, azimuth and elevation and delay spread. In addition, we study the impact of array width, signal-to-noise ratio, and antenna slant angle on the mutual information (MI) of the system. In particular, we present an analytical model for large system mean mutual information values and consider the impact of elevation spectrum on MI. Finally, the impact of multipath delays on XPD and MI is also explored.

This paper presents asymptotic bounds and limits for the mean channel capacity of MIMO systems under Ricean channel conditions. It is shown that the mean capacity per dimension decreases as the K factor increases in value and approaches a value equal to that of the underlying scattering channel when the number of antennas are large and the specular matrix has unit rank. The accuracy of the bounds is verified by simulations. In addition, a variety of results for the MIMO Ricean channel are brought together to give an overview of the current knowledge in this area. We also show that the variance of the capacity for a Ricean channel approaches that of the scattering channel for large numbers of antennas

In this paper we present an approximate closed form solution for dimensioning mixed service 3G wireless networks for defined service and quality constraints. Both Web-browsing services alone, as well as a mixture of Web and circuit voice services are considered. Dimensioning is achieved through knowledge of the distribution of the latency delay experienced by end users of the 3G network. The parameters defining the distribution are shown to be dependent on a range of network and service parameters. This allows the dimensioning of 3G wireless networks under a wide range of quality and service conditions.

In this paper we derive a generalized method to compute the error probabilities of singular value decomposition-based receivers for a MIMO system with uncoded transmission. The method can be used for a wide class of flat fading environments, including i.i.d. and semi-correlated Rayleigh and i.i.d. Ricean channels. Although we apply the method to equal-power BPSK, it can easily be extended to higher order M-PSK and M-QAM signal constellations and adaptive "water-filling" schemes. The error probability curves derived from closed-form formulas and simulations demonstrate very close agreement. We also compare the error performances of CI, MMSE and ZF receivers with the SVD receiver.

We propose a new low-complexity MAC solution for multimedia CDMA cellular networks considering packet reservation and scheduling jointly. The proposed MAC, which we call joint PRMA and packet scheduling (JPPS), treats real-time traffic and non-real-time traffic separately. Once a real-time traffic flow (a continuous packet burst) is admitted to the system, its code channel(s) will be implicitly reserved until its last packet is transmitted, as in PRMA. Real-time traffic has delay priority over non-real-time, and we also prioritize within various non-real-time traffic flows. Packet scheduling techniques are then used to schedule all the real-time and non-real-time packets. Furthermore, an optimal channel access scheme based on a simple linear program is proposed. Simulation results comparing the performance of JPPS versus the well-known MAC protocol WISPER demonstrate lower delay for real time traffic and lower overall packet loss ratio for JPPS

This paper presents asymptotic bounds and limits for the ergodic channel capacity of MIMO systems under Ricean channel conditions. It is shown that the ergodic capacity per dimension decreases as the K factor increases in value and approaches a value equal to that of the underlying scattering channel when the number of antennas is large. The accuracy of the hounds is verified by simulations. In addition, a variety of results for the MIMO Ricean channel are brought together to give an overview of the current knowledge in this area.

In recent years there have been a proliferation of channel models suggested for MIMO systems. In this paper we take a look at the impact of model complexity on channel behaviour and on performance measures of systems operating in these channels. In particular we investigate whether large increases in channel complexity are justified or if simple models will lead to similar results and conclusions. We take the simplest channel model, which gives both spatial and temporal correlation, and compare it to a complex industrial model developed by the 3GPP. We compare the two models in terms of the spatial and temporal correlation of the channel. In addition, we look at the temporal correlation level crossing rate and average fade duration of the link capacity in these channels. This gives us an opportunity to assess the impact of channel differences on a performance measure of interest.

In this letter, we derive the exact variance of the capacity of a multiple-input multiple-output (MIMO) system. This enables an investigation of the accuracy of a Gaussian approximation to the capacity foreshadowed by various central limit theorems. We confirm recent results which state that the capacity variance appears to converge to a limit independent of absolute antenna numbers, but dependent on the ratio of the numbers of receive to transmit antennas. The Gaussian approximation itself is surprisingly good, even in the worst cases giving satisfactory results.

It is well known that MIMO systems offer the promise of achieving very high spectrum efficiencies (many tens of bits/Hz) in a mobile environment. The gains in MIMO capacity are sensitive to the presence of spatial and temporal correlation introduced by the radio environment. In this paper we examine how MIMO capacity is influenced by a number of factors, e.g.: a) temporal correlation, b) various combinations of low/high spatial correlations at either end, c) combined spatial and temporal correlations, In all cases we compare the channel capacity that would be achievable under independent fading. We investigate the behaviour of "capacity fades", examine how often the capacity experiences the fades, develop a method to determine level crossing rates and average data durations and relate these to antenna numbers.

This paper presents an overview of progress in the area of multiple input multiple output (MIMO) space-time coded wireless systems. After some background on the research leading to the discovery of the enormous potential of MIMO wireless links, we highlight the different classes of techniques and algorithms proposed which attempt to realize the various benefits of MIMO including spatial multiplexing and space-time coding schemes. These algorithms are often derived and analyzed under ideal independent fading conditions. We present the state of the art in channel modeling and measurements, leading to a better understanding of actual MIMO gains. Finally, the paper addresses current questions regarding the integration of MIMO links in practical wireless systems and standards.

The primary contribution of this work lies in the derivation of the exact characteristic function (and hence, the mean and variance) of the capacity of multiple-input multiple-output (MIMO) systems for semicorrelated flat-fading channels. A Gaussian approximation to the exact capacity results is suggested and evaluated for its accuracy. We show that over a range of correlation levels this approximation is adequate even for moderate numbers of transmit and receive antennas.

We consider the capacity of a single user t /spl rarr/ /spl tau/ MIMO wireless system in a Rayleigh or Ricean fading environment. It is known that a certain central limit theorem exists which states (under certain conditions) that the distribution of the standardized capacity is asymptotically Gaussian as r /spl rarr/ /spl infin/, t /spl rarr/ /spl infin/ and /spl tau//t /spl rarr/ y for some constant y. However we demonstrate the surprising accuracy of a Gaussian approximation to the capacity for virtually all values of /spl tau/ and t. In order to investigate the accuracy of the Gaussian fit we derive the variance of the capacity in the Rayleigh fading case.

Importance sampling (IS) is a simulation technique which aims to reduce the variance (or other cost function) of a given simulation estimator. In communication systems, this usually, but not always, means attempting to reduce the variance of the bit error rate (BER) estimator. By reducing the variance, IS estimators can achieve a given precision from shorter simulation runs; hence the term "quick simulation." The idea behind IS is that certain values of the input random variables in a simulation have more impact on the parameter being estimated than others. If these "important" values are emphasized by sampling more frequently, then the estimator variance can be reduced. Hence, the basic methodology in IS is to choose a distribution which encourages the important values. This use of a "biased" distribution will, of course, result in a biased estimator if applied directly in the simulation. However, there is a simple procedure whereby the simulation outputs are weighted to correct for the use of the biased distribution, and this ensures that the new IS estimator is unbiased. Hence, the "art" of designing quick simulations via IS is entirely dependent on the choice of biased distribution. Over the last 50 years, IS techniques have flourished, but it is only in the last decade that coherent design methods have emerged. The outcome of these developments is that at the expense of increasing technical content, modern techniques can offer substantial run-time saving for a very broad range of problems. We present a comprehensive history and survey of IS methods. In addition, we offer a guide to the strengths and weaknesses of the techniques, and hence indicate which techniques are suitable for various types of communications systems. We stress that simple approaches can still yield useful savings, and so the simulation practitioner as well as the technical researcher should consider IS as a possible simulation tool.

Wireless communications are expected to be the dominant mode of access technology in the next century. Besides voice, a new range of services such as multimedia, high-speed data, etc. are being offered for delivery over wireless networks. Mobility will be seamless, realizing the concept of persons being in contact anywhere, at any time. Two developments are likely to have a substantial impact on the mobile communications systems deployed in the twenty-first century: the adoption of International Mobile Telecommunications-2000 and wireless asynchronous transfer mode (ATM). They are two different but cooperative approaches to providing high-speed wireless access. The limitations of the radio-frequency spectrum and radio channel propagation impose special constraints on the technologies of systems to be deployed. To make future mobile systems globally acceptable, standardization efforts are underway in the International Telecommunications Union and the ATM Forum. This paper reviews the international standardization efforts, the challenges to the technologies imposed by the radio spectrum limitations, radio channel propagation-induced distortions, and possible solutions. Evolution, migration, and architecture issues also are discussed.

Study Group XIII of the ITU-TS has recently approved a new performance recommendation G.826 which the ITU-R has recommended as a standard for satellite systems in recommendation S.1062. G.826 establishes guidelines on the performance levels of digital systems operating at or above primary rate. These performance levels are stated in terms of block error rates. We discuss the impact of the block-based error performance objectives on the performance of transport systems. However, the impact of G.826 is difficult to evaluate directly since the performance recommendations are not given in terms of bit-error rate (BER) values. Hence, we present a procedure to convect block error rates to BER. This conversion is based on the statistical modeling of the error process and includes an analysis of bursty errors as well as random errors. Rather than using the equivalent BER values as simple thresholds, we produce BER against time distributions which satisfy G.826 and can be used as masks for a comparison with measured data and also for route design. It is shown that these masks are a crucial tool in any study of G.826 and contain useful information for system design and planning. Indeed it is shown that false conclusions can be drawn if the masks are not used. Finally, the impact of G.826 is discussed with particular reference to apportionment strategies, system margins, signal processing, and error correction schemes.

We review the work on optical heterodyne binary-DPSK systems. In particular we show why the study of these systems has been so challenging and discuss the techniques which have been developed to analyse the standard DPSK receiver. Next we describe the methods used to develop and analyse improved receiver structures and comment on the impact of improved performance and lowered error rate floors. We discuss the work on suboptimal receivers and outline the current state of the search for a truly optimal receiver. This work include a considerable discussion of the modeling and analysis of laser phase noise since it is this noise which makes the study of coherent optical systems so complex. We describe the historical development of this work from early approximate techniques to the accurate approaches currently emerging in the literature. We also evaluate the contributions made and outline their strengths, weaknesses and areas of use.>

Knowledge of the probability density function (pdf) of intersymbol interference (ISI) can be used in the computation of system error probabilities and to help identify the cause of the errors. Exact computation of the probability density is only practicable in a few special cases. Several algorithms exist for computing approximate pdfs of ISI but these only work with certain signal constellations and transmitted symbols that are mutually independent or coded by partial response. This paper considers an algorithm for the computation of ISI pdfs for trellis-coded modulation with general 1D and 2D constellations. The results suggest that symbol dependencies introduced into the transmitted signal by symmetric trellis-codes have negligible effect on the pdf of the ISI. Although the ideas presented are general in nature, examples are given for ISI pdfs on a line-of-sight radio channel.>

A statistical method for evaluating the performance measures of the base to mobile link of a multiple-cell CDMA system is presented. Specifically, techniques and expressions for estimating the short term average bit-error-rate (BER), service reliability and link availability are developed. The link performance is estimated for a mobile at the vertex of multiple adjacent cells. In the analysis, the system is assumed to employ coherent BPSK modulation and direct sequence spreading and the received signals are assumed to undergo Rayleigh fading, log-normal shadowing and frequency selective fading. The effects of power control and error correction are also investigated.>

The performance of trellis-coded modulation (TCM) on additive white Gaussian noise channels is well understood, and tight analytical bounds exist on the probability of the Viterbi decoder making a decision error. When a channel is also time-dispersive, the performance of TCM systems has been studied mainly by simulation. However, simulation is limited to symbol error probabilities greater than 10/sup -6/ and is not a particularly useful tool for designing codes. Tight analytical bounds on the error probability of TCM on time-dispersive channels are required for a more thorough study of performance. Moreover, the design of good codes and optimum metrics for time-dispersive channels requires tight analytical bounds. In this paper we derive analytical upper bounds, which, although requiring numerical techniques for tractable evaluation, are tight for a wide range of time-dispersive channel conditions. The bounds are based on a union bound of error events that leads to a summation of pairwise error probabilities, which are themselves upper bounded.>

The paper analyzes and quantifies the performance of a space diversity combining receiver operating in a digital cellular radio environment with quaternary phase shift keying (QPSK), frequency-selective Rayleigh multipath fading, and co-channel interference (CCI). The receiver has an infinite length filter in each branch of the diversity combiner, and the filters are jointly optimized according to the minimum mean-square error (MMSE) criterion. The link bit-error-rate (BER) is accurately estimated using Metzger's algorithm (Metzger, 1987) which approximates the probability density function of the combined intersymbol interference (ISI) and CCI. The authors present numerical performance results showing the influence of the diversity order, the number of dominant co-channel interferers, the multipath channel's delay spectrum, and the root-mean square delay spread. The results show that, under certain conditions, the optimum linear receiver can almost completely eliminate all ISI and dominant CCI while providing near-optimum noise filtering.>

A study of minimum mean-square error (MMSE) diversity combining for wide-band digital cellular radio, designed to combat intersymbol interference (ISI) caused by frequency selective fading is outlined. The systems analyzed use binary phase-shift keying (BPSK), quarternary phase-shift keying (QPSK) or 16-level quadrature amplitude modulation (16-QAM) with cosine rolloff spectral shaping, and space diversity with selection, maximal ratio or MMSE combining. A set of performance curves is presented for selected combining schemes showing the influence of the following system parameters: the diversity order (1 to 4); the cosine rolloff factor; the power delay spectrum (with its associated delay spread); the signal-to-interference ratio; and the number of modulation levels (2, 4 and 16).>

The application of trellis-coded modulation to reducing the effects of residual interference levels encountered on multipath fading channels after adaptive equalization in high capacity digital microwave radio systems is investigated. Simulations used to derive error performance curves and outage probabilities are discussed. The results indicate that trellis-coded modulation offers performance improvements in addition to those offered by adaptive equalization.>

A description is given of the results of a 140 Mb/s 64 QAM digital microwave radio field trial to characterize a space diversity channel model with maximum power combining. Measurements were taken on a 60.8-km overwater route. A Rummler model has been fitted to the spectral data collected from the field trial. It is shown that the model can also be fitted when the effects of maximize power combining are considered. The PDFs of the nondiversity and diversity channel model parameters are estimated from the measured data.>

The results are described of a 140 Mb/s, 16 QAM digital microwave radio field trial to characterize multipath fading on a 60-km overwater route in New Zealand. Propagation data have been collected from the path between September 1985 and September 1986, and include one fading season of the Southern Hemisphere. The authors specifically apply and extend work on multipath fading by W.D. Rummler and others (1979, 1981, 1986). To the spectral data collected from the trial, a Rummler-type model (echo delay 6.35 ns) has been fitted. Distributions for the parameters a, b, and f/sub o/ ar observed to be similar to those presented by Rummler (1979) for the 26.4-mile Atlanta to Palmetto, Georgia path. Data which cannot be characterized well by the Rummler model include samples which show the evidence of a ray with equivalent long delay (>or=10 ns). This ray is either conjectured to be a sea reflection (however sea reflections are blocked at an Earth radius factor of k=4/3), or is indicative of the presence of more than one secondary ray.>

The performance of a high-capacity digital radio system is evaluated in the presence of ground-induced reflections and atmospheric fading. Results indicate that ground reflections significantly contribute to outage even when their amplitude is 20-25 dB below the main signal. A three-path channel model is used to simulate reflections from the terrain and atmosphere. With this model, the group delay suffers extremely rapid phase transitions and the amplitude pattern shows the presence of minima deeper than that produced by either ray alone.>

A general three ray channel model for microwave radio applications is simulated with the constituent amplitudes and delays as random variables. The amplitudes are assumed to follow a Gaussian or Rayleigh distribution and the delays follow a Gaussian distribution. The mean values and standard deviations of the amplitudes/delays may be related to path parameters and other measurables (ray parameter statistics). Results show that single frequency powers follow a Gamma distribution and this property may be used to derive a simple measure of dispersion induced outage.

Channel distortions due to multipath fading are a principal source of outage for high-capacity digital radio. Time domain equalization is increasingly becoming an effective means to compensate for fading induced ISI. This paper compares the performance of transversal and decision feedback equalizers used for 16 QAM and 64 QAM digital radio systems. The effect of tap spacing is examined by comparing the results of baud period spacing with fractionally spaced equalizers. The performance measure used is peak distortion. This measure is plotted against various fade parameters for the different equalizers studied, the two modulation schemes, forms of timing recovery, and the effects of joint operation of demodulation and symbol synchronization circuitry. Results clearly indicate, in terms of better signature curves, the superiority of fractionally spaced equalizers over synchronous equalizers. An additional advantage is the robustness of FSE's to timing phase errors. A decision feedback equalizer with fractional spacing on the feedforward part offers the best performance. It is shown that the performance of a DFE synchronized to the first precursor is significantly improved as compared to the conventional case of synchronizing with the main sample of the received pulse. With the new position of reference (first precursor), fractional DFE's benefit more than theirT-spaced counterpart.

A theoretical analysis of the problem of estimating frequency selective fade model parameters from measurements on existing analog links is presented. These measurements essentially consist of obtaining records of AGC and continuity pilot level variations from which a first approximation ofa, \alpha, \omega_{0}, and τ (fade model parameters) can be obtained. Field results obtained on a working link in New Zealand are reported.