Several efficient low complexity interference management techniques were developed for improving the performance of multiple-input multiple-output (MIMO) networks. Sub-optimal techniques involving optimal antenna selection-aided stream control were proposed for joint optimization of co-channel MIMO links in a space division multiple access (SDMA) network. Results indicated that the use of the SDMA scheme along with partial channel state information at the transmitters significantly reduces the signaling overhead with minimal loss in throughput performance. Next, a mean squared error (MSE) based antenna selection framework was presented for developing low complexity algorithms for finite complexity receivers. These selection algorithms were shown to provide reasonable bit-error rate performance while keeping the overall system complexity low. Furthermore, some new algebraic properties of linear orthogonal space-time block codes (OSTBCs) were utilized to develop a single-stage and minimum MSE optimal detector for two co-channel users employing unity rate real and derived rate-1/2 complex OSTBCs. A sub-optimal space-time interference cancellation (IC) technique was also developed for a spatial-multiplexing link subjected to Alamouti interference. The performance of proposed interference management techniques and their implications for future research are discussed.

[pt] As técnicas de múltiplo acesso mais cogitadas nas propostas para padrões de 3 geração de sistemas celulares envolvem alguma forma de CDMA ( Code Division Multiple Access)na qual os usuários podem transmitir simultaneamente na mesma faixa de freqüência sendo separáveis no receptor pela ortogonalidade, ou quase ortogonalidade, entre os sinais utilizados pelos diferentes usuários para transmissão de informação.Os receptores multiusuário, propostos para uso na 3 geração de sistemas celulares, realizam a detecção do sinal do usuário desejado conjuntamente com os sinais interferentes, resultando em um ganho de desempenho em relação aos receptores CDMA convencionais, que tratam os sinais interferentes como um simples ruído de fundo.Este trabalho tem por objetivo estudar a viabilidade de um receptor CDMA multiusuário hpibrido síncrono, que combina a eficiência do recepor Decorrelator no combate a interferência de múltiplo acesso (IMA) e sua simpicidade de implementação com a robustez e alto desempenho do receptor ótimo. Para evitar a complexidade inerente a estrutura de detecção ótima é sugerido que o conjunto de sinais na saída do estágio Decorrelator seja particionado em grupos e detectados de forma paralela e ótima. Esta técnica é conhecida como detecção ótima por grupos. Também é proposto e explorada a utilização de uma métrica para a formação dos grupos, baseada na minimização da probabilidade de erro de bit do usuário com pior desenpenho no sistema. Ao longo deste trabalho são exibidos resultados analíticos e de simulação, que comprovam a utilidade e exelente relação entre desempenho e complexidade da estrutura proposta em canais AWGN. Por fim, a estrutura proposta é avaliada para um canal com desvanecimento de Rayleigh plano. / [en] The most discussed multiple access techiniques for the 3G of mobile wireless systems involve some form of CDMA - Code Division Multiple Access- where the users can transmit simultaneously their signals in the same frequency band separable in the receiver by the orthogonality or, quasi- orthogonality, properties of the transmitted signals. Multiuser detectors MUD perform the detection of the desired user signal jointly with the signals transmitted by the remaining users, resulting in a better performance than the conventional CDMA receivers, that treat the interference created by the signals of other users simply as background noise.This work has the objective of investigating the viability of syncrhronous hybrid multiuser detector, wich combines the implementation simplicity and efficiency of the decorrelating multiuser detector in mitigating the multiple access interference with the robustness and high performance of the optimum multiuser detector.In order to avoid the complexity of the optimum multiuser detection scheme it is proposed to combine the signals from the decorrelator output into groups and perform optimal detection of the signals in each group. This approach is known as optimal group detection. Also in this work, a strategy on how to perform the grouping is proposed and explored. This strategy is based on the minimization the bit error probability of the user with the worst performance.Simulation and analytical results show the excellent trade-off between performance and complexity of the proposed optimum multiuser group detector in AWGN channels. The receiver performance is also evaluated in Rayleigh flat fading channels.

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The focus of this dissertation is the study of advanced processing techniques for multiuser interference cancellation in direct sequence code division multiple access communications. Emphasis is placed on the development of efficient techniques that are practical to implement. The work begins with a study of several sub-optimal multiuser detection techniques under a variety of conditions. Multistage parallel interference cancellation is identified as a practical and robust approach for mitigating multiple access interference. In order to reduce the effect of biased decision statistics inherent to parallel cancellation, a low-complexity modification to parallel interference cancellation that significantly improves performance is derived. Based on this approach, two real-time DSP implementations are devised, one fully coherent and one non-coherent. Multi-symbol differential detection is then explored as an alternative for improving the performance of the non-coherent approach. Additionally, dual-antenna diversity techniques are also investigated as a means for improving performance in multipath environments. / Ph. D.

This research investigates the application of multiuser detection to Code Division Multiple Access for cellular communications. This investigation focuses on the use of multiuser receivers at the base station of mobile radio systems. The first two chapters are dedicated to multiuser detection in general. An extensive literature survey is performed on the research concerning multiuser receivers to date. Six major receiver structures are chosen for extensive simulation studies. The bit error rate performance of these receivers is investigated in several system environments. Additionally, practical issues are considered such as computational complexity and robustness to code tracking errors. From this work, one receiver structure is identified for further study, namely multistage interference cancellation. The theoretical performance of this receiver is analyzed using a standard Gaussian Approximation and an Improved Gaussian Approximation for AWGN and fading environments. Additionally, the resistance of the receiver to interference energy levels is explored. Parameter estimation is an important issue for interference cancellation. Simple methods of improving parameter estimation are examined, as is the effect of parameter estimation error on system performance. A baseband hardware implementation is detailed and several design challenges are presented. Results are given for the performance of the implemented receiver and shown to match well with theory and computer simulation. Finally, the implications of this research are discussed. / Ph. D.

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.

Direct sequence code division multiple access (DS-CDMA) technique is one of the significant multiplexing technologies used in wireless communication services. In the DS-CDMA framework, all users have been assigned distinct signature code sequence to achieve multiple accesses within the same frequency band, and allow signal separating at the receiver. Under multipath fading environment with near-far effect, the current CDMA systems employed the RAKE receiver, to enhance the system performance. It is known that if training data is available the minimum mean squares error (MMSE) multiuser receiver, in which the average power of the receiver output is minimized subject to appropriate constraints, could be obtained by solving directly by the constrained Wiener estimation solution. However, if this is not the case, the blind multiuser receiver is an alternative approach to achieve desired performance closed to the one with the MMSE approach. In this thesis, based on the max/min criterion, the blind multiuser receiver, with linear constraints, is devised. Here constraint equations are written in parametric forms, which depend on the multipath structure of the signal of interest. Constraint parameters are jointly optimized with the parameters of the linear receiver to obtain the optimal parameters. In consequence, the sliding window linearly constrained RLS (SW-LC-RLS) algorithm is employed to implement the optimal blind receiver, with max/min approach. This new proposed scheme can be used to deal with multiple access interference (MAI) suppression for the environments, in which the narrow band interference (NBI) due to other systems is joined suddenly to the DS-CDMA systems, and having serious near-far effect. Under such circumstance, the channel character due to the NBI and near-far effect will become violent time varying, such that the conventional LC-RLS algorithm as well as LC-LMS algorithms could not perform well. Via computer simulation it confirms that our proposed scheme has better capability for MAI suppression in DS-CDMA systems than other existing schemes, and is more robust against the NBI and near-far problems.

Ultrawideband (UWB) is one of the promising technologies for future short-range high data rate communications (e.g. for wireless personal area networks) and longer range low data rate communications (e.g. wireless sensor networks).Despite its various advantages and potentials (e.g. low-cost circuitry, unlicensed reuse of licensed spectrum, precision ranging capability etc.), UWB also has its own challenges. The goal of this dissertation is to identify and address some of those challenges, and provide a framework for practical UWB transceiver design.In this dissertation, various modulation options for UWB systems are reviewed in terms of their bit error rate (BER) performances, spectral characteristics, modem and hardware complexities, and data rates. Time hopping (TH) code designs for both synchronous (introduced an adaptive code assignment technique) and asynchronous UWB impulse radio (IR) systems are studied. An adaptive assignment of two different multiple access parame ters (number of pulses per symbol and number of pulse positions per frame)is investigated again considering both synchronous and asynchronous scenarios, and a mathematical framework is developed using Gaussian approximations of interference statistics for different scenarios. Channel estimation algorithms for multiuser UWB communication systems using symbol-spaced (proposed a technique that decreases the training size), frame-spaced (proposed a pulse-discarding algorithm for enhanced estimationperformance), and chip-spaced (using least squares (LS) estimation) sampling are analyzed.A comprehensive review on multiple accessing andinterference avoidance/cancellation for IR-UWB systems is presented.BER performances of different UWB modulation schemes in the presence of timing jitter are evaluated and compared in static and multipath fading channels, and finger estimation error, effects of jitter distribution, and effects of pulse shape are investigated. A unified performance analysis app roach for different IR-UWB transceiver types (stored-reference, transmitted-reference, and energy detector) employing various modulation options and operating at sub-Nyquist sampling rates is presented. The time-of-arrival (TOA) estimation performance of different searchback schemesunder optimal and suboptimal threshold settings are analyzed both for additive white Gaussian noise (AWGN) and multipath channels.