The full spatial covariance matrix of the multiple input multiple-output (MIMO) channel is an important quantity in channel modeling, communication system signal processing, and performance analysis, and therefore this matrix forms the heart of the research outlined in this dissertation. The work begins with an investigation of a generalized framework for computing the full MIMO spatial covariance based on the power angular spectrum (PAS) of the multipath field and the transmit and receive antenna element radiation patterns. For the case of uniform linear arrays and when the PAS clusters satisfy uniform, truncated Gaussian, or truncated Laplacian distributions, a series expansion is used to allow analytic evaluation of the required integrals in the formulation. The study also demonstrates the validity of some simplifying assumptions used to reduce the complexity of the covariance computation by applying the technique to ray tracing data as well as considers an analysis of the convergence properties of the series when computed using a finite number of terms. The insights and tools obtained from this covariance analysis are then used to develop a general approach for constructing MIMO transmit and receive beamforming vectors based on the full spatial covariance. While transmit and receive beamforming for the MIMO channel is a well-studied topic, when the transmit precoding is based on channel covariance information, developing near-optimal transmit and receive beamformers when the receiver is constrained to use linear processing remains an unsolved problem. This iterative beamforming algorithm presented here can accommodate different types of available channel information and receiver capabilities as well as either a sum power constraint or a per-antenna power constraint. While the latter is more realistic, construction of the optimal transmit precoder is less understood for this constraint. Simulation results based on measured channels demonstrate that the approach generates beamformer solutions whose performance rivals that achieved for an optimal nonlinear receiver architecture.

One critical step to successfully integrate wireless data networks to the high-speed wired backbone is the design of network control policies that efficiently utilize resources to provide Quality of Service (QoS) to the users in the integrated networks. Such a design has remained a challenge since wireless networks are time-varying in nature, not only in terms of user/packet arrivals but also in terms of physical channel conditions and access opportunities. In this thesis, we study the stochastic control of time-varying networks to design efficient scheduling and resource allocation policies. In particular, in Chapter 3, we focus on a broad class of control policies that work based on a pick-and-compare principle for networks with time-varying channels. By trading the throughput for complexity and memory requirement, these policies require less complexity compared to the well-investigated throughput-optimal Generalized Maximum Weight Matching (GMWM) policy and also require only linear-memory storage with the number of data-flows. Through Lyapunov analysis tools, we characterize the stability region and delay performance of the studied policies and show how they vary in response to the channel variations. In Chapter 4, we go into further detail and consider the problem of network control from a new perspective through which we carefully incorporate the time-efficiency of underlying scheduling algorithms. Specifically, we develop a policy that dynamically adjusts the time given to the available scheduling algorithms according to queue-backlog and channel correlations. We study the resulting stability region of developed policy and show that the region is at least as large as the one for any static policy. Finally, motivated by the current under-utilization of wireless spectrum, in Chapter 5, we investigate the control of cognitive radio networks as a special example of networks that provide time-varying access opportunities. We assume that users dynamically join and leave the network and may have different utility functions, or could collaborate for a common purpose. We develop a policy that performs joint admission and resource control and works for any user load, either inside or outside the capacity region. Through Lyapunov Optimization techniques, we show that the developed policy can achieve a utility performance arbitrarily close to the optimality with a tradeoff in the average service delay of admitted users.

One critical step to successfully integrate wireless data networks to the high-speed wired backbone is the design of network control policies that efficiently utilize resources to provide Quality of Service (QoS) to the users in the integrated networks. Such a design has remained a challenge since wireless networks are time-varying in nature, not only in terms of user/packet arrivals but also in terms of physical channel conditions and access opportunities. In this thesis, we study the stochastic control of time-varying networks to design efficient scheduling and resource allocation policies. In particular, in Chapter 3, we focus on a broad class of control policies that work based on a pick-and-compare principle for networks with time-varying channels. By trading the throughput for complexity and memory requirement, these policies require less complexity compared to the well-investigated throughput-optimal Generalized Maximum Weight Matching (GMWM) policy and also require only linear-memory storage with the number of data-flows. Through Lyapunov analysis tools, we characterize the stability region and delay performance of the studied policies and show how they vary in response to the channel variations. In Chapter 4, we go into further detail and consider the problem of network control from a new perspective through which we carefully incorporate the time-efficiency of underlying scheduling algorithms. Specifically, we develop a policy that dynamically adjusts the time given to the available scheduling algorithms according to queue-backlog and channel correlations. We study the resulting stability region of developed policy and show that the region is at least as large as the one for any static policy. Finally, motivated by the current under-utilization of wireless spectrum, in Chapter 5, we investigate the control of cognitive radio networks as a special example of networks that provide time-varying access opportunities. We assume that users dynamically join and leave the network and may have different utility functions, or could collaborate for a common purpose. We develop a policy that performs joint admission and resource control and works for any user load, either inside or outside the capacity region. Through Lyapunov Optimization techniques, we show that the developed policy can achieve a utility performance arbitrarily close to the optimality with a tradeoff in the average service delay of admitted users.

[pt] Este trabalho apresenta uma análise detalhada de uma recente proposta de combinação da transmissão em blocos com portadora única e a técnica CDMA, referida como CS-CDMA (Chip Spread CDMA). Uma característica marcante desta técnica de transmissão é que diferentemente do que ocorre nos sistemas DS-CDMA, a ortogonalidade entre os códigos dos diferentes usuários é mantida mesmo quando a transmissão é feita através de um canal multipercurso seletivo em frequência, suposto invariante no tempo, permitindo assim que os usuários possam ser identicamente desacoplados na recepção. Além desta vantagem, resultados de desempenho indicaram uma significativa superioridade deste sistema sobre o tradicional DS-CDMA. Entretanto comparações existentes, consideraram apenas o up-link, do sistema e adotaram algumas premissas, que incluem, a utilização de códigos ortogonais pelos diversos usuários e a suposição de canais de transmissão invariantes no tempo. O presente trabalho apresenta uma análise detalhada da técnica (CS-CDMA e uma análise comparativa dos sistemas em condições menos favoráveis. Os sistemas operam em ambientes invariante e variante no tempo, com códigos ortogonais e não ortogonais e em dois cenários diferentes, down- link e up-link. Os resultados consideram recepção com equalização no domínio da frequência utilizando equalizadores do tipo ZF (Zero Forcing) e MMSE (Minimum Mean Squared Error). Simulações foram realizadas no intuito de se avaliar o desempenho dos dois sistemas considerados. Curvas de probabilidade de erro foram obtidas e ilustram e comparam tais desempenhos em diferentes situações e cenários de interesse. / [en] This dissertation proposes detailed analysis of a recent combined mechanism for transmission in blocks with a single carrier and CDMA technique known as Chip Spread CDMA (CS-CDMA). An important feature of this transmission technique is that unlike what happens in Direct Sequence (DS-CDMA) systems, the orthogonality between codes of different users is maintained even when the transmission, considered time-invariant, is made through a selective multipath channel frequency, which ideally allows users to be uncoupled in reception. However, existing comparisons only consider the up-link transmission and adopt certain assumptions, which include orthogonal codes for different users and time invariant channels. This technique have shown a significant superiority as compared with the traditional DS-CDMA. A comparison of (CS-CDMA systems in more realistic conditions are presented in this work. Various environments were tested in the presence of BPSK modulation systems, as well as invariant and time-varying transmission. Comparison between scenarios down-link and up-link are also presented. The results consider equalized reception in the frequency domain using the ZF (Zero Forcing) equalizers and MMSE (Minimum Mean Squared Error). Simulations were carried out in order to evaluate the performance of the two systems considered. Error probability curves were obtained to illustrate and compare the performances in different situations and scenarios.

[pt] Este trabalho tem como finalidade analisar diferentes estratégias de detecção passíveis de aplicação em sistemas de transmissão OFDM (Orthogonal Frequency Division Multiplexing) operando em canais que variam rapidamente no tempo. Além dos métodos clássicos de detecção lineares tais como filtro casado, Zero Forcing e MMSE (Minimum Mean-Square Error), outras duas técnicas são estudadas, abrangendo também combinações entre elas. A primeira é a técnica de cancelamento paralelo de interferência (PIC - Parallel Interference Cancellation), a segunda é a detecção por busca por verossimilhança ascendente (LAS – Likelihood Ascent Search), ambas empregadas em conjunção com o filtro casado. Esse trabalho apresenta também um estudo dos efeitos de uma estimativa imperfeita do canal, no desempenho dos esquemas de detecção aqui enfocados. Os resultados dos experimentos são analisados em termos da taxa de erro de bit (BER) e custo computacional (complexidade)associado a estes esquemas. / [en] This work analyzing different detection strategies that can be applied in OFDM (Orthogonal Frequency Division Multiplexing) transmission systems over fast time-varying channels. In addition to classical linear methods of detection such as a Matched Filter, Zero Forcing and MMSE, two other techniques are studied, also encompassing combinations of them. The first is the Parallel Interference Cancellation (PIC) technique, the second is Likelihood Ascent Search (LAS), both used in conjunction with the Matched Filter. This work also presents a study of the effects of imperfect channel estimation on the performance of the detection schemes studied here. The results of the experiments are analyzed in terms of bit error rate (BER) and computational cost (complexity) associated with these schemes.

The technique of cooperative communications is finding its way in the next generations of many wireless communication applications. Due to the distributed nature of cooperative networks, acquiring fading channels information for coherent detection is more challenging than in the traditional point-to-point communications. To bypass the requirement of channel information, differential modulation together with non-coherent detection can be deployed. This thesis is concerned with various issues related to differential modulation and non-coherent detection in cooperative networks. Specifically, the thesis examines the behavior and robustness of non-coherent detection in mobile environments (i.e., time-varying channels). The amount of channel variation is related to the normalized Doppler shift which is a function of user’s mobility. The Doppler shift is used to distinguish between slow time-varying (slow-fading) and rapid time-varying (fast-fading) channels. The performance of several important relay topologies, including single-branch and multi-branch dual-hop relaying with/without a direct link that employ amplify-and-forward relaying and two-symbol non-coherent detection, is analyzed. For this purpose, a time-series model is developed for characterizing the time-varying nature of the cascaded channel encountered in amplify-and-forward relaying. Also, for single-branch and multi-branch dual-hop relaying without a direct link, multiple-symbol differential detection is developed. First, for a single-branch dual-hop relaying without a direct link, the performance of two-symbol differential detection in time-varying Rayleigh fading channels is evaluated. It is seen that the performance degrades in rapid time-varying channels. Then, a multiple-symbol differential detection is developed and analyzed to improve the system performance in fast-fading channels. Next, a multi-branch dual-hop relaying with a direct link is considered. The performance of this relay topology using a linear combining method and two-symbol differential detection is examined in time-varying Rayleigh fading channels. New combining weights are proposed and shown to improve the system performance in fast-fading channels. The performance of the simpler selection combining at the destination is also investigated in general time-varying channels. It is illustrated that the selection combining method performs very close to that of the linear combining method. Finally, differential distributed space-time coding is studied for a multi-branch dual-hop relaying network without a direct link. The performance of this network using two-symbol differential detection in terms of diversity over time-varying channels is evaluated. It is seen that the achieved diversity is severely affected by the channel variation. Moreover, a multiple-symbol differential detection is designed to improve the performance of the differential distributed space-time coding in fast-fading channels.