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Collaborative beamforming is a powerful technique to increase communication energy efficiency and range in an energy-constrained network. To achieve high performance, collaborative beamforming requires accurate knowledge of channel state information (CSI) at the transmitters (collaborative nodes). In practice, however, such exact knowledge of CSI is not available. A robust transmitter design based on partial CSI is required to mitigate the effects of CSI mismatches. / This thesis focuses on the design and evaluation of a beamforming scheme that is robust to CSI mismatches for collaborative multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) wireless systems. Using a max-min robust design approach, the robust beamformer is designed to maximize the minimum (worst-case) received signal-to-noise ratio (SNR) within a predefined uncertainty region at each OFDM subcarrier. In addition, several subcarrier power allocation strategies are investigated to further improve the robustness of collaborative systems. Numerical simulation results show that the robust beamformer offers improved performance over the nonrobust beamformers and the use of power allocation strategies among subcarriers further improves the system performance.

Telecommunications in the current information age is increasingly relying on the wireless link. This is because wireless communication has made possible a variety of services ranging from voice to data and now to multimedia. Consequently, demand for new wireless capacity is growing rapidly at a very alarming rate. In a bid to cope with challenges of increasing demand for higher data rate, better quality of service, and higher network capacity, there is a migration from Single Input Single Output (SISO) antenna technology to a more promising Multiple Input Multiple Output (MIMO) antenna technology. On the other hand, Orthogonal Frequency Division Multiplexing (OFDM) technique has emerged as a very popular multi-carrier modulation technique to combat the problems associated with physical properties of the wireless channels such as multipath fading, dispersion, and interference. The combination of MIMO technology with OFDM techniques, known as MIMO-OFDM Systems, is considered as a promising solution to enhance the data rate of future broadband wireless communication Systems. This thesis addresses a major area of challenge to both SISO-OFDM and MIMO-OFDM Systems; estimation of accurate channel state information (CSI) in order to make possible coherent detection of the transmitted signal at the receiver end of the system. Hence, the first novel contribution of this thesis is the development of a low complexity adaptive algorithm that is robust against both slow and fast fading channel scenarios, in comparison with other algorithms employed in literature, to implement soft iterative channel estimator for turbo equalizer-based receiver for single antenna communication Systems. Subsequently, a Fast Data Projection Method (FDPM) subspace tracking algorithm is adapted to derive Channel Impulse Response Estimator for implementation of Decision Directed Channel Estimation (DDCE) for Single Input Single Output - Orthogonal Frequency Division Multiplexing (SISO-OFDM) Systems. This is implemented in the context of a more realistic Fractionally Spaced-Channel Impulse Response (FS-CIR) channel model, as against the channel characterized by a Sample Spaced-Channel Impulse Response (SS)-CIR widely assumed by other authors. In addition, a fast convergence Variable Step Size Normalized Least Mean Square (VSSNLMS)-based predictor, with low computational complexity in comparison with others in literatures, is derived for the implementation of the CIR predictor module of the DDCE scheme. A novel iterative receiver structure for the FDPM-based Decision Directed Channel Estimation scheme is also designed for SISO-OFDM Systems. The iterative idea is based on Turbo iterative principle. It is shown that improvement in the performance can be achieved with the iterative DDCE scheme for OFDM system in comparison with the non iterative scheme. Lastly, an iterative receiver structure for FDPM-based DDCE scheme earlier designed for SISO OFDM is extended to MIMO-OFDM Systems. In addition, Variable Step Size Normalized Least Mean Square (VSSNLMS)-based channel transfer function estimator is derived in the context of MIMO Channel for the implementation of the CTF estimator module of the iterative Decision Directed Channel Estimation scheme for MIMO-OFDM Systems in place of linear minimum mean square error (MMSE) criterion. The VSSNLMS-based channel transfer function estimator is found to show improved MSE performance of about -4 MSE (dB) at SNR of 5dB in comparison with linear MMSE-based channel transfer function estimator. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2010.

Multiple-input multiple-output (MIMO) systems have been extensively studied in the past decade. The attractiveness of MIMO systems is due to the fact that they drastically reduce the deleterious e ects of multipath fading leading to high system capacity and low error rates. In situations where wireless devices are restrained by their size and hardware complexity, such as mobile phones, transmit diversity is not achievable. A new paradigm called cooperative communication is a viable solution. In a cooperative scenario, a single-antenna device is assisted by another single-antenna device to relay its message to the destination or base station. This creates a virtual multiple-input multiple-output (MIMO) system. There exist two cooperative strategies: amplify-and-forward (AF) and decode-and-forward (DF). In the former, the relay ampli es the noisy signal received from the source before forwarding it to the destination. No form of demodulation is required. In the latter, the relay rst decodes the source signal before transmitting an estimate to the destination. In this work, focus is on the DF method. A drawback of an uncoded DF cooperative strategy is error propagation at the relay. To avoid error propagation in DF, various relay selection schemes can be used. Coded cooperation can also be used to avoid error propagation at the relay. Various error correcting codes such as convolutional codes or turbo codes can be used in a cooperative scenario. The rst part of this work studies a variation of the turbo codes in cooperative diversity, that further reduces error propagation at the relay, hence lowering the end-to-end error rate. The union bounds on the bit-error rate (BER) of the proposed scheme are derived using the pairwise error probability via the transfer bounds and limit-before-average techniques. In addition, the outage analysis of the proposed scheme is presented. Simulation results of the bit error and outage probabilities are presented to corroborate the analytical work. In the case of outage probability, the computer simulation results are in good agreement with the the analytical framework presented in this chapter. Recently, most studies have focused on cross-layer design of cooperative diversity at the physical layer and truncated automatic-repeat request (ARQ) at the data-link layer using the system throughput as the performance metric. Various throughput optimization strategies have been investigated. In this work, a cross-relay selection approach that maximizes the system throughput is presented. The cooperative network is comprised of a set of relays and the reliable relay(s) that maximize the throughput at the data-link layer are selected to assist the source. It can be shown through simulation that this novel scheme outperforms from a throughput point of view, a system throughput where the all the reliable relays always participate in forwarding the source packet. A power optimization of the best relay uncoded DF cooperative diversity is investigated. This optimization aims at maximizing the system throughput. Because of the non-concavity and non-convexity of the throughput expression, it is intractable to derive a closed-form expression of the optimal power through the system throughput. However, this can be done via the symbol-error rate (SER) optimization, since it is shown that minimizing the SER of the cooperative system is equivalent to maximizing the system throughput. The SER of the retransmission scheme at high signal-to-noise ratio (SNR) was obtained and it was noted that the derived SER is in perfect agreement with the simulated SER at high SNR. Moreover, the optimal power allocation obtained under a general optimization problem, yields a throughput performance that is superior to non-optimized power values from moderate to high SNRs. The last part of the work considers the throughput maximization of the multi-relay adaptive DF over independent and non-identically distributed (i.n.i.d.) Rayleigh fading channels, that integrates ARQ at the link layer. The aim of this chapter is to maximize the system throughput via power optimization and it is shown that this can be done by minimizing the SER of the retransmission. Firstly, the closed-form expressions for the exact SER of the multi-relay adaptive DF are derived as well as their corresponding asymptotic bounds. Results showed that the optimal power distribution yields maximum throughput. Furthermore, the power allocated at a relay is greatly dependent of its location relative to the source and destination. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2013.

Future wireless communication systems promise to offer a variety of multimedia services which require reliable transmission at high data rates over wireless links. Multiple input multiple output (MIMO) systems have received a great deal of attention because they provide very high data rates for such links. Theoretical studies have shown that the quality provided by MIMO systems can be increased by using space-time codes. Space-time codes combine both space (antenna) and time diversity in the transmitter to increase the efficiency of MIMO system. The three primary approaches, layered spacetime architecture, space-time trellis coding (STTC) and space-time block coding (STBC) represent a way to investigate transmitter-based signal processing for diversity exploitation and interference suppression. The advantages of STBC (i.e. low decoding complexity) and STTC (i.e. TCM encoder structure) can be used to design a high rate space-time trellis coded modulation (HR-STTCM). Most space-time codes designs are based on the assumption of perfect channel state information at the receiver so as to make coherent decoding possible. However, accurate channel estimation requires a long training sequence that lowers spectral efficiency. Part of this dissertation focuses on the performance of HR-STTCM under non-coherent detection where there is imperfect channel state information and also in environment where the channel experiences rapid fading. Prior work on space-time codes with particular reference to STBC systems in multiuser environment has not adequately addressed the performance of the decoupled user signalto-noise ratio. Part of this thesis enumerates from a signal-to-noise ratio point of view the performance of the STBC systems in multiuser environment and also the performance of the HR-STTCM in such environment. The bit/frame error performance of space-time codes in fading channels can be evaluated using different approaches. The Chemoff upper-bound combined with the pair state generalized transfer function bound approach or the modified state transition diagram transfer function bound approach has been widely used in literature. However, although readily detennined, this bound can be too loose over nonnal signal-to-noise ranges of interest. Other approaches, based on the exact calculation of the pairwise error probabilities, are often too cumbersome. A simple exact numerical technique, for calculating, within any desired degree of accuracy, of the pairwise error probability of the HR-STTCM scheme over Rayleigh fading channel is proposed in this dissertation. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.

Multiple-input multiple-output (MIMO) technology has emerged as a popular technique for enhancing the reliability and capacity of wireless communication systems. In this dissertation, we analyze the spatial modulation (SM) MIMO technique and investigate possible extensions to this scheme. To date, there has been no literature reporting on the theoretical performance of M-ary quadrature amplitude modulation (M-QAM) SM with maximum likelihood (ML) based detection. The first objective of this dissertation is to present an asymptotic bound to quantify the average bit error rate (BER) of M-QAM SM with ML detection over independent and identically distributed (i.i.d) Rayleigh flat fading channels. The analytical frameworks are validated by Monte Carlo simulation results, which show the derived bounds to be tight for high signal-to-noise ratio (SNR) values. The ML based SM detector is optimal, since it offers the best detection performance. However, this technique is not practical due to its high computational complexity. The second objective of this dissertation is to introduce a novel SM detection scheme, termed multiple-stage (MS) detection. Performance and complexity comparisons with existing SM detectors show two main benefits of MS detection: near optimal BER performance and up to a 35% reduction in receiver complexity as compared to the ML based detector. Conventional SM schemes are unable to exploit the transmit diversity gains provided by the MIMO channel. The third objective of this dissertation is to propose Alamouti coded spatial modulation (ACSM), a novel SM based scheme with transmit diversity. The ACSM technique combines SM with Alamouti space-time block coding (STBC), thereby improving the diversity aspect and overall system performance of conventional SM. A closed form expression for the average BER of real constellation ACSM over i.i.d Rayleigh flat fading channels is derived and Monte Carlo simulations are used to verify the accuracy of this analytical expression. The BER performance of ACSM is compared to that of SM and Alamouti STBC. Simulation results show that the new scheme outperforms SM and Alamouti STBC by approximately 5.5 dB and 1.5 dB respectively, albeit at the cost of increased receiver complexity. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2010.

Lattice-Reduction has become a popular way of improving the performance of MIMO detectors. However, developing an efficient high-throughput VLSI implementation of LR has been a major challenge in the literature. This thesis proposes a hardware-optimized version of the popular LLL algorithm that reduces its complexity by 70% and achieves a fixed runtime while maintaining ML diversity. The proposed algorithm is implemented for 4x4 MIMO systems and uses a novel pipelined architecture that achieves a fixed low processing latency of 40 cycles, resulting in a fixed throughput that is independent of the channel correlation. The proposed LR core, fabricated in 0.13um CMOS, is the first fabricated and tested LR ASIC implementation in the literature. Test results show that the LR core achieves a maximum clock rate of 204 MHz, yielding a throughput of 510 Mbps, thus satisfying the aggressive throughput requirements of emerging 4G wireless standards, such as IEEE-802.16m and LTE-Advanced.

Lattice-Reduction has become a popular way of improving the performance of MIMO detectors. However, developing an efficient high-throughput VLSI implementation of LR has been a major challenge in the literature. This thesis proposes a hardware-optimized version of the popular LLL algorithm that reduces its complexity by 70% and achieves a fixed runtime while maintaining ML diversity. The proposed algorithm is implemented for 4x4 MIMO systems and uses a novel pipelined architecture that achieves a fixed low processing latency of 40 cycles, resulting in a fixed throughput that is independent of the channel correlation. The proposed LR core, fabricated in 0.13um CMOS, is the first fabricated and tested LR ASIC implementation in the literature. Test results show that the LR core achieves a maximum clock rate of 204 MHz, yielding a throughput of 510 Mbps, thus satisfying the aggressive throughput requirements of emerging 4G wireless standards, such as IEEE-802.16m and LTE-Advanced.

Thesis (Ph. D.)--University of Hawaii at Manoa, 2005. / Includes bibliographical references (leaves 84-89). / Also available by subscription via World Wide Web / xi, 89 leaves, bound ill. 29 cm

Thesis (Ph. D.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 103-110). Also available on the World Wide Web.