Due to its significant advantage in spectral efficiency, multiple-antenna communication technology will undoubtedly be a major component in future wireless system implementations. However, the full exploitation of this technology also requires perfect feedback of channel state information (CSI) to the transmitter-- something that is not practically feasible. This motivates the study of limited feedback systems, where CSI feedback is rate limited. This thesis focuses on the optimal design of limited feedback systems for three types of communication channels: the relay channel, the single-user point-to-point channel, and the multiuser broadcast channel. For the relay channel, we prove the efficiency of the Grassmannian codebooks as the source and relay beamforming codebooks, and propose a method for CSI exchange between the relay and the destination when global CSI is not available at destination. For the single-user point-to-point channel, we study the joint power control and beamforming problem and address the channel magnitude and direction quantization codebook design problem. It is shown that uniform quantization of the channel magnitude (in dB scale) is asymptotically optimal regardless of the channel distribution. The analysis further derives the optimal split of feedback bandwidth between the magnitude and direction quantization codebooks. For the multiuser broadcast channel, we first prove the sufficiency of a product magnitude-direction quantization codebook for managing the multiuser interference. We then derive the optimal split of feedback bandwidth across the users and their magnitude and direction codebooks. The optimization results reveal an inherent structural difference between the single-user and multiuser quantization codebooks: a multiuser codebook should have a finer direction quantization resolution as compared to a single-user codebook. It is further shown that the users expecting higher rates and requiring more reliable communication should provide a finer quantization of their CSI. Finally, we determine the minimum required total feedback rate based on users' quality-of-service constraints and derive the scaling of the system performance with the total feedback rate.

Code Division Multiple Access (CDMA) is the technology used in all third generation cellular communications networks, and it is a promising candidate for the definition of fourth generation standards. The wireless mobile channel is usually frequency-selective causing interference among the users in one CDMA cell. Multiuser Transmission (MUT) algorithms for the downlink can increase the number of supportable users per cell, or decrease the necessary transmit power to guarantee a certain quality-of-service. Transmitter-based algorithms exploiting the channel knowledge in the transmitter are also motivated by information theoretic results like the Writing-on-Dirty-Paper theorem. The signal-to-noise ratio (SNR) is a reasonable performance criterion for noise-dominated scenarios. Using linear filters in the transmitter and the receiver, the SNR can be maximized with the proposed Eigenprecoder. Using multiple transmit and receive antennas, the performance can be significantly improved. The Generalized Selection Combining (GSC) MIMO Eigenprecoder concept enables reduced complexity transceivers. Methods eliminating the interference completely or minimizing the mean squared error exist for both the transmitter and the receiver. The maximum likelihood sequence detector in the receiver minimizes the bit error rate (BER), but it has no direct transmitter counterpart. The proposed Minimum Bit Error Rate Multiuser Transmission (TxMinBer) minimizes the BER at the detectors by transmit signal processing. This nonlinear approach uses the knowledge of the transmit data symbols and the wireless channel to calculate a transmit signal optimizing the BER with a transmit power constraint by nonlinear optimization methods like sequential quadratic programming (SQP). The performance of linear and nonlinear MUT algorithms with linear receivers is compared at the example of the TD-SCDMA standard. The interference problem can be solved with all MUT algorithms, but the TxMinBer approach requires less transmit power to support a certain number of users. The high computational complexity of MUT algorithms is also an important issue for their practical real-time application. The exploitation of structural properties of the system matrix reduces the complexity of the linear MUT mthods significantly. Several efficient methods to invert the ystem matrix are shown and compared. Proposals to reduce the omplexity of the Minimum Bit Error Rate Multiuser Transmission mehod are made, including a method avoiding the constraint by pase-only optimization. The complexity of the nonlinear methods i still some magnitudes higher than that of the linear MUT lgorithms, but further research on this topic and the increasing processing power of integrated circuits will eventually allow to exploit their better performance. / Der codegeteilte Mehrfachzugriff (CDMA) wird bei allen zellularen Mobilfunksystemen der dritten Generation verwendet und ist ein aussichtsreicher Kandidat für zukünftige Technologien. Die Netzkapazität, also die Anzahl der Nutzer je Funkzelle, ist durch auftretende Interferenzen zwischen den Nutzern begrenzt. Für die Aufwärtsstrecke von den mobilen Endgeräten zur Basisstation können die Interferenzen durch Verfahren der Mehrnutzerdetektion im Empfänger verringert werden. Für die Abwärtsstrecke, die höhere Datenraten bei Multimedia-Anwendungen transportiert, kann das Sendesignal im Sender so vorverzerrt werden, dass der Einfluß der Interferenzen minimiert wird. Die informationstheoretische Motivation liefert dazu das Writing-on-Dirty-Paper Theorem. Das Signal-zu-Rausch-Verhältnis ist ein geeignetes Kriterium für die Performanz in rauschdominierten Szenarien. Mit Sende- und Empfangsfiltern kann das SNR durch den vorgeschlagenen Eigenprecoder maximiert werden. Durch den Einsatz von Mehrfachantennen im Sender und Empfänger kann die Performanz signifikant erhöht werden. Mit dem Generalized Selection MIMO Eigenprecoder können Transceiver mit reduzierter Komplexität ermöglicht werden. Sowohl für den Empfänger als auch für den Sender existieren Methoden, die Interferenzen vollständig zu eliminieren, oder den mittleren quadratischen Fehler zu minimieren. Der Maximum-Likelihood-Empfänger minimiert die Bitfehlerwahrscheinlichkeit (BER), hat jedoch kein entsprechendes Gegenstück im Sender. Die in dieser Arbeit vorgeschlagene Minimum Bit Error Rate Multiuser Transmission (TxMinBer) minimiert die BER am Detektor durch Sendesignalverarbeitung. Dieses nichtlineare Verfahren nutzt die Kenntnis der Datensymbole und des Mobilfunkkanals, um ein Sendesignal zu generieren, dass die BER unter Berücksichtigung einer Sendeleistungsnebenbedingung minimiert. Dabei werden nichtlineare Optimierungsverfahren wie Sequentielle Quadratische Programmierung (SQP) verwendet. Die Performanz linearer und nichtlinearer MUT-Verfahren MUT-Algorithmen mit linearen Empfängern wird am Beispiel des TD-SCDMA-Standards verglichen. Das Problem der Interferenzen kann mit allen untersuchten Verfahren gelöst werden, die TxMinBer-Methode benötigt jedoch die geringste Sendeleistung, um eine bestimmt Anzahl von Nutzern zu unterstützen. Die hohe Rechenkomplexität der MUT-Algorithmen ist ein wichtiges Problem bei der Implementierung in Real-Zeit-Systemen. Durch die Ausnutzung von Struktureigenschaften der Systemmatrizen kann die Komplexität der linearen MUT-Verfahren signifikant reduziert werden. Verschiedene Verfahren zur Invertierung der Systemmatrizen werden aufgezeigt und verglichen. Es werden Vorschläge gemacht, die Komplexität der Minimum Bit Error Rate Multiuser Transmission zu reduzieren, u.a. durch Vermeidung der Sendeleistungsnebenbedingung durch eine Beschränkung der Optimierung auf die Phasen des Sendesignalvektors. Die Komplexität der nichtlinearen Methoden ist um einige Größenordungen höher als die der linearen Verfahren. Weitere Forschungsanstrengungen an diesem Thema sowie die wachsende Rechenleistung von integrierten Halbleitern werden künftig die Ausnutzung der besseren Leistungsfähigkeit der nichtlinearen MUT-Verfahren erlauben.

Multi-antenna wireless systems have become very popular due to their theoretically predicted higher spectral efficiencies and improved performance compared to single-antenna systems. Large-scale multiple-input multiple-output (MIMO) systems refer to wireless systems where communication terminals employ tens to hundreds of antennas to achieve in-creased spectral efficiencies/sum rates, reliability, and power efficiency. Large-scale multi-antenna systems are attractive to meet the increasing wireless data rate requirements, without compromising on the bandwidth. This thesis addresses key signal processing issues in large-scale MIMO systems. Specifically, the thesis investigates efficient algorithms for signal detection and channel estimation in large-scale MIMO systems. It also investigates ‘spatial modulation,’ a multi-antenna modulation scheme that can reduce the number of transmit radio frequency (RF) chains, without compromising much on the spectral efficiency. The work reported in this thesis is comprised of the following two parts: 1 investigation of low-complexity receiver algorithms based on Markov chain Monte Carlo (MCMC) technique, tabu search, and belief propagation for large-scale uplink multiuser MIMO systems, and 2 investigation of achievable rates and signal detection in generalized spatial modulation. 1. Receiver algorithms for large-scale multiuser MIMO systems on the uplink In this part of the thesis, we propose low-complexity algorithms based on MCMC techniques, Gaussian sampling based lattice decoding (GSLD), reactive tabu search (RTS), and factor graph based belief propagation (BP) for signal detection on the uplink in large-scale multiuser MIMO systems. We also propose an efficient channel estimation scheme based on Gaussian sampling. Markov chain Monte Carlo (MCMC) sampling: We propose a novel MCMC based detection algorithm, which achieves near-optimal performance in large dimensions at low complexities by the joint use of a mixed Gibbs sampling (MGS) strategy and a multiple restart strategy with an efficient restart criterion. The proposed mixed Gibbs sampling distribution is a weighted mixture of the target distribution and uniform distribution. The presence of the uniform component in the sampling distribution allows the algorithm to exit from local traps quickly and alleviate the stalling problem encountered in conventional Gibbs sampling. We present an analysis for the optimum choice of the mixing ratio. The analysis approach is to define an absorbing Markov chain and use its property regarding the expected number of iterations needed to reach the global minima for the first time. We also propose an MCMC based algorithm which exploits the sparsity in uplink multiuser MIMO transmissions, where not all users are active simultaneously. Gaussian sampling based lattice decoding: Next, we investigate the problem of searching the closest lattice point in large dimensional lattices and its use in signal detection in large-scale MIMO systems. Specifically, we propose a Gaussian sampling based lattice decoding (GSLD) algorithm. The novelty of this algorithm is that, instead of sampling from a discrete distribution as in Gibbs sampling, the algorithm iteratively generates samples from a continuous Gaussian distribution, whose parameters are obtained analytically. This makes the complexity of the proposed algorithm to be independent of the size of the modulation alpha-bet. Also, the algorithm is able to achieve near-optimal performance for different antenna and modulation alphabet settings at low complexities. Random restart reactive tabu search (R3TS): Next, we study receiver algorithms based on reactive tabu search (RTS) technique in large-scale MIMO systems. We propose a multiple random restarts based reactive tabu search (R3TS) algorithm that achieves near-optimal performance in large-scale MIMO systems. A key feature of the proposed R3TS algorithm is its performance based restart criterion, which gives very good performance-complexity tradeoff in large-dimension systems. Lower bound on maximum likelihood (ML) bit error rate (BER) performance: We propose an approach to obtain lower bounds on the ML performance of large-scale MIMO systems using RTS simulation. In the proposed approach, we run the RTS algorithm using the transmitted vector as the initial vector, along with a suitable neighborhood definition, and find a lower bound on number of errors in ML solution. We demonstrate that the proposed bound is tight (within about 0.5 dB of the optimal performance in a 16×16MIMO system) at moderate to high SNRs. Factor graph using Gaussian approximation of interference (FG-GAI): Multiuser MIMO channels can be represented by graphical models that are fully/densely connected (loopy graphs), where conventional belief propagation yields suboptimal performance and requires high complexity. We propose a solution to this problem that uses a simple, yet effective, Gaussian approximation of interference (GAI) approach that carries out a linear per-symbol complexity message passing on a factor graph (FG) based graphical model. The proposed algorithm achieves near-optimal performance in large dimensions in frequency-flat as well as frequency-selective channels. Gaussian sampling based channel estimation: Next, we propose a Gaussian sampling based channel estimation technique for large-scale time-division duplex (TDD) MIMO systems. The proposed algorithm refines the initial estimate of the channel by iteratively detecting the data block and using that knowledge to improve the estimated channel knowledge using a Gaussian sampling based technique. We demonstrate that this algorithm achieves near-optimal performance both in terms of mean square error of the channel estimates and BER of detected data in both frequency-flat and frequency-selective channels. 2. Generalized spatial modulation In the second part of the thesis, we investigate generalized spatial modulation (GSM) in point-to point MIMO systems. GSM is attractive because of its ability to work with less number of transmit RF chains compared to traditional spatial multiplexing, without com-promising much on spectral efficiency. In this work, we show that, by using an optimum combination of number of transmit antennas and number of transmit RF chains, GSM can achieve better throughput and/or BER than spatial multiplexing. We compute tight bounds on the maximum achievable rate in a GSM system, and quantify the percentage savings in the number of transmit RF chains as well as the percentage increase in the rate achieved in GSM compared to spatial multiplexing. We also propose a Gibbs sampling based algorithm suited to detect GSM signals, which yields impressive BER performance and complexity results.

Code Division Multiple Access (CDMA) is the technology used in all third generation cellular communications networks, and it is a promising candidate for the definition of fourth generation standards. The wireless mobile channel is usually frequency-selective causing interference among the users in one CDMA cell. Multiuser Transmission (MUT) algorithms for the downlink can increase the number of supportable users per cell, or decrease the necessary transmit power to guarantee a certain quality-of-service. Transmitter-based algorithms exploiting the channel knowledge in the transmitter are also motivated by information theoretic results like the Writing-on-Dirty-Paper theorem. The signal-to-noise ratio (SNR) is a reasonable performance criterion for noise-dominated scenarios. Using linear filters in the transmitter and the receiver, the SNR can be maximized with the proposed Eigenprecoder. Using multiple transmit and receive antennas, the performance can be significantly improved. The Generalized Selection Combining (GSC) MIMO Eigenprecoder concept enables reduced complexity transceivers. Methods eliminating the interference completely or minimizing the mean squared error exist for both the transmitter and the receiver. The maximum likelihood sequence detector in the receiver minimizes the bit error rate (BER), but it has no direct transmitter counterpart. The proposed Minimum Bit Error Rate Multiuser Transmission (TxMinBer) minimizes the BER at the detectors by transmit signal processing. This nonlinear approach uses the knowledge of the transmit data symbols and the wireless channel to calculate a transmit signal optimizing the BER with a transmit power constraint by nonlinear optimization methods like sequential quadratic programming (SQP). The performance of linear and nonlinear MUT algorithms with linear receivers is compared at the example of the TD-SCDMA standard. The interference problem can be solved with all MUT algorithms, but the TxMinBer approach requires less transmit power to support a certain number of users. The high computational complexity of MUT algorithms is also an important issue for their practical real-time application. The exploitation of structural properties of the system matrix reduces the complexity of the linear MUT mthods significantly. Several efficient methods to invert the ystem matrix are shown and compared. Proposals to reduce the omplexity of the Minimum Bit Error Rate Multiuser Transmission mehod are made, including a method avoiding the constraint by pase-only optimization. The complexity of the nonlinear methods i still some magnitudes higher than that of the linear MUT lgorithms, but further research on this topic and the increasing processing power of integrated circuits will eventually allow to exploit their better performance. / Der codegeteilte Mehrfachzugriff (CDMA) wird bei allen zellularen Mobilfunksystemen der dritten Generation verwendet und ist ein aussichtsreicher Kandidat für zukünftige Technologien. Die Netzkapazität, also die Anzahl der Nutzer je Funkzelle, ist durch auftretende Interferenzen zwischen den Nutzern begrenzt. Für die Aufwärtsstrecke von den mobilen Endgeräten zur Basisstation können die Interferenzen durch Verfahren der Mehrnutzerdetektion im Empfänger verringert werden. Für die Abwärtsstrecke, die höhere Datenraten bei Multimedia-Anwendungen transportiert, kann das Sendesignal im Sender so vorverzerrt werden, dass der Einfluß der Interferenzen minimiert wird. Die informationstheoretische Motivation liefert dazu das Writing-on-Dirty-Paper Theorem. Das Signal-zu-Rausch-Verhältnis ist ein geeignetes Kriterium für die Performanz in rauschdominierten Szenarien. Mit Sende- und Empfangsfiltern kann das SNR durch den vorgeschlagenen Eigenprecoder maximiert werden. Durch den Einsatz von Mehrfachantennen im Sender und Empfänger kann die Performanz signifikant erhöht werden. Mit dem Generalized Selection MIMO Eigenprecoder können Transceiver mit reduzierter Komplexität ermöglicht werden. Sowohl für den Empfänger als auch für den Sender existieren Methoden, die Interferenzen vollständig zu eliminieren, oder den mittleren quadratischen Fehler zu minimieren. Der Maximum-Likelihood-Empfänger minimiert die Bitfehlerwahrscheinlichkeit (BER), hat jedoch kein entsprechendes Gegenstück im Sender. Die in dieser Arbeit vorgeschlagene Minimum Bit Error Rate Multiuser Transmission (TxMinBer) minimiert die BER am Detektor durch Sendesignalverarbeitung. Dieses nichtlineare Verfahren nutzt die Kenntnis der Datensymbole und des Mobilfunkkanals, um ein Sendesignal zu generieren, dass die BER unter Berücksichtigung einer Sendeleistungsnebenbedingung minimiert. Dabei werden nichtlineare Optimierungsverfahren wie Sequentielle Quadratische Programmierung (SQP) verwendet. Die Performanz linearer und nichtlinearer MUT-Verfahren MUT-Algorithmen mit linearen Empfängern wird am Beispiel des TD-SCDMA-Standards verglichen. Das Problem der Interferenzen kann mit allen untersuchten Verfahren gelöst werden, die TxMinBer-Methode benötigt jedoch die geringste Sendeleistung, um eine bestimmt Anzahl von Nutzern zu unterstützen. Die hohe Rechenkomplexität der MUT-Algorithmen ist ein wichtiges Problem bei der Implementierung in Real-Zeit-Systemen. Durch die Ausnutzung von Struktureigenschaften der Systemmatrizen kann die Komplexität der linearen MUT-Verfahren signifikant reduziert werden. Verschiedene Verfahren zur Invertierung der Systemmatrizen werden aufgezeigt und verglichen. Es werden Vorschläge gemacht, die Komplexität der Minimum Bit Error Rate Multiuser Transmission zu reduzieren, u.a. durch Vermeidung der Sendeleistungsnebenbedingung durch eine Beschränkung der Optimierung auf die Phasen des Sendesignalvektors. Die Komplexität der nichtlinearen Methoden ist um einige Größenordungen höher als die der linearen Verfahren. Weitere Forschungsanstrengungen an diesem Thema sowie die wachsende Rechenleistung von integrierten Halbleitern werden künftig die Ausnutzung der besseren Leistungsfähigkeit der nichtlinearen MUT-Verfahren erlauben.

Esse trabalho de investigação visa a realização de uma análise sistemática, integrada e iterativa da utilização de algoritmos heurísticos aplicados aos problemas de estimativa de parâmetros e detecção multiusuário, sob o ponto de vista do compromisso desempenho × complexidade. O sistema considera topologias do tipo CDMA com exploração de diversidade multidimensional, ou seja, que utilizam uma ou mais técnicas de diversidade, considerando a diversidade de código, tempo, frequência e espaço, entre outras, sujeitos a desvanecimentos multipercurso. A solução integrada para os problemas de estimativa de parâmetros e detecção multiusuário consiste no uso recorrente de técnicas heurísticas. Além disso, estabelece-se uma análise comparada e sistêmica de convergência e de complexidade computacional da técnica de detecção proposta com alguns outros métodos, heurísticos ou determinísticos, relatados na literatura, considerando como métrica de desempenho o número de operações computacionais que cada estratégia requer para a detecção simultânea da informação de todos os usuários ativos no sistema. Por fim, e mais importante, considera-se como a principal contribuição deste trabalho a sistematização da utilização dos algoritmos heurísticos no processo de otimização dos problemas já citados, caracterização de limiares de desempenho e análise de complexidade destas técnicas, trazendo à comunidade científica parâmetros suficientes que devem ser respeitados na configuração dos algoritmos para garantia de resultados satisfatórios quando da utilização destes métodos em problemas de detecção multiusuário com diversidade multidimensional e estimativa de parâmetros. / This work will perform a systematic, integrated and iterative research of heuristic algorithms applied to parameter estimation and multiuser detection problems, considering the performance × complexity tradeoff. The CDMA systems with multidimensional diversity exploitation, i.e., with one or more diversity techniques, code diversity, frequency, time and space, among other, in multipath fading channel scenarios are considered. The integrated solution for parameter estimation and multiuser detection problem uses heuristic techniques in recurrent form. In addition, we intend to establish a systemic and comparative analysis of convergence and computational complexity of the proposal detection technique with some other methods, heuristic or deterministic, reported in the literature, considering the number of computational operations that each strategy requires for simultaneous detection from all active users as a performance metrics. Finally, and most importantly, this work systematizes the heuristic algorithms approach in the optimization problems process already mentioned, considering the thresholds for performance and complexity of these techniques, bringing the scientific community enough configuration parameters that must be respected in the setup algorithms step to guarantee satisfactory results when using these methods to multiuser detection with multidimensional diversity and parameter estimation problems.

Esse trabalho de investigação visa a realização de uma análise sistemática, integrada e iterativa da utilização de algoritmos heurísticos aplicados aos problemas de estimativa de parâmetros e detecção multiusuário, sob o ponto de vista do compromisso desempenho × complexidade. O sistema considera topologias do tipo CDMA com exploração de diversidade multidimensional, ou seja, que utilizam uma ou mais técnicas de diversidade, considerando a diversidade de código, tempo, frequência e espaço, entre outras, sujeitos a desvanecimentos multipercurso. A solução integrada para os problemas de estimativa de parâmetros e detecção multiusuário consiste no uso recorrente de técnicas heurísticas. Além disso, estabelece-se uma análise comparada e sistêmica de convergência e de complexidade computacional da técnica de detecção proposta com alguns outros métodos, heurísticos ou determinísticos, relatados na literatura, considerando como métrica de desempenho o número de operações computacionais que cada estratégia requer para a detecção simultânea da informação de todos os usuários ativos no sistema. Por fim, e mais importante, considera-se como a principal contribuição deste trabalho a sistematização da utilização dos algoritmos heurísticos no processo de otimização dos problemas já citados, caracterização de limiares de desempenho e análise de complexidade destas técnicas, trazendo à comunidade científica parâmetros suficientes que devem ser respeitados na configuração dos algoritmos para garantia de resultados satisfatórios quando da utilização destes métodos em problemas de detecção multiusuário com diversidade multidimensional e estimativa de parâmetros. / This work will perform a systematic, integrated and iterative research of heuristic algorithms applied to parameter estimation and multiuser detection problems, considering the performance × complexity tradeoff. The CDMA systems with multidimensional diversity exploitation, i.e., with one or more diversity techniques, code diversity, frequency, time and space, among other, in multipath fading channel scenarios are considered. The integrated solution for parameter estimation and multiuser detection problem uses heuristic techniques in recurrent form. In addition, we intend to establish a systemic and comparative analysis of convergence and computational complexity of the proposal detection technique with some other methods, heuristic or deterministic, reported in the literature, considering the number of computational operations that each strategy requires for simultaneous detection from all active users as a performance metrics. Finally, and most importantly, this work systematizes the heuristic algorithms approach in the optimization problems process already mentioned, considering the thresholds for performance and complexity of these techniques, bringing the scientific community enough configuration parameters that must be respected in the setup algorithms step to guarantee satisfactory results when using these methods to multiuser detection with multidimensional diversity and parameter estimation problems.

Future wireless networks will have applications that require many devices to be connected to the network. Non-orthogonal multiple access (NOMA) is a promising multiple access scheme that allows more users to simultaneously transmit in a common channel than orthogonal signaling techniques. This overloading allows for high spectral efficiencies which can support the high demand for wireless access. One notable NOMA scheme is low-density spreading (LDS), which is a code domain multiple access scheme. Low density spreading operates like code division multiple access (CDMA) in the sense that users use a spreading sequence to spread their data, but the spreading sequences have a low number of nonzero chips, hence the term low-density. The message passing algorithm (MPA) is typically used for multi-user detection (MUD) of LDS systems. The MPA detector has complexity that is exponential to the number of users contributing to each chip. LDS systems suffer from two inherent problems: high computational complexity, and vulnerability to multipath channels. In this thesis, these two problems are addressed. A lower complexity MUD technique is presented, which offers complexity that is proportional to the number of users squared. The proposed detector is based on minimum mean square error (MMSE) and parallel interference cancellation (PIC) detectors. Simulation results show the proposed MUD technique achieves reductions in multiplications and additions by 81.84% and 67.87% with a loss of about 0.25 dB with overloading at 150%. In addition, a precoding scheme designed to mitigate the effects of the multipath channel is also presented. This precoding scheme applies an inverse channel response to the input signal before transmission. This allows for the received signal to eliminate the multipath effects that destroy the low-density structure.

The study of the spatial aspects of multipath in wireless communications environments is an increasingly important addition to the study of the temporal aspects in the search for ways to increase the utilization of the available wireless channel capacity. Traditionally, multipath has been viewed as an encumbrance in wireless communications, two of the major impairments being signal fading and intersymbol interference. However, recently the potential advantages of the diversity offered by multipath rich environments in multiuser communications have been recognised. Space time coding, for example, is a recent technique which relies on a rich scattering environment to create many practically uncorrelated signal transmission channels. Most often, statistical models have been used to describe the multipath environments in such applications. This approach has met with reasonable success but is limited when the statistical nature of a field is not easily determined or is not readily described by a known distribution.¶ Our primary aim in this thesis is to probe further into the nature of multipath environments in order to gain a greater understanding of their characteristics and diversity potential. We highlight the shortcomings of beamforming in a multipath multiuser access environment. We show that the ability of a beamformer to resolve two or more signals in angle directly limits its achievable capacity.¶ We test the probity of multipath as a source of spatial diversity, the limiting case of which is co-located users. We introduce the concept of separability to define the fundamental limits of a receiver to extract the signal of a desired user from interfering users’ signals and noise. We consider the separability performances of the minimum mean square error (MMSE), decorrelating (DEC) and matched filter (MF) detectors as we bring the positions of a desired and an interfering user closer together. We show that both the MMSE and DEC detectors are able to achieve acceptable levels of separability with the users as close as λ/10.¶ In seeking a better understanding of the nature of multipath fields themselves, we take two approaches. In the first we take a path oriented approach. The effects on the variation of the field power of the relative values of parameters such as amplitude and propagation direction are considered for a two path field. The results are applied to a theoretical analysis of the behaviour of linear detectors in multipath fields. This approach is insightful for fields with small numbers of multipaths, but quickly becomes mathematically complex.¶ In a more general approach, we take a field oriented view, seeking to quantify the complexity of arbitrary fields. We find that a multipath field has an intrinsic dimensionality of (πe)R/λ≈8.54R/λ, for a field in a two dimensional circular region, increasing only linearly with the radius R of the region. This result implies that there is no such thing as an arbitrarily complicated multipath field. That is, a field generated by any number of nearfield and farfield, specular and diffuse multipath reflections is no more complicated than a field generated by a limited number of plane waves. As such, there are limits on how rich multipath can be. This result has significant implications including means: i) to determine a parsimonious parameterization for arbitrary multipath fields and ii) of synthesizing arbitrary multipath fields with arbitrarily located nearfield or farfield, spatially discrete or continuous sources. The theoretical results are corroborated by examples of multipath field analysis and synthesis.

This thesis examines various signal processing techniques that are required for establishing efficient (near optimal) communications in multiuser multiple-input multiple-output (MIMO) environments. The central part of this thesis is dedicated to acquisition of information about the MIMO channel state - at both the receiver and the transmitter. This information is required to organize a communication set up which utilizes all the available channel resources. Realistic channel model, i.e., the spatial channel model (SCM), has been used in this study, together with modern long-term evolution (LTE) standard. The work consists of three major themes: (a) estimation of the channel at the receiver, also known as tracking; (b) quantization of the channel information and its feedback from receiver to the transmitter (feedback quantization); and (c) reconstruction of the channel knowledge at the transmitter, and its use for data precoding during communication transmission. / Communications

Future cellular networks are expected to support extremely high data rates and user capacities. This thesis investigates the downlink of a wireless cellular system that takes advantage of multiple antennas at base stations and mobile stations, frequency reuse across all cells, and cooperation among base stations. We identify asynchronous interference resulting from multi-cell communication as a key challenge, prove the existence of a downlink/uplink duality in that case, and present a linear precoding scheme that exploits this duality. Since this result is not directly extendable to orthogonal frequency division multiplexing (OFDM), we propose a `hybrid' algorithm for two cooperating base stations, which combines linear and nonlinear precoding. This algorithm minimizes the sum mean squared error of the system and is extendable to OFDM. Finally, we consider the problem of user selection for multiuser precoding in OFDM-based systems. We extend an available single-cell user selection scheme to multiple cooperating cells.