Multiple-Input, Multiple-Output (MIMO) processing techniques for wireless communication are of interest for next-generation systems because of their potential to dramatically improve capacity in some propagation environments. When used in applications such as wireless LAN and cellular telephony, the MIMO processing methods must be adapted for the situation where a base station is communicating with many users simultaneously. This dissertation focuses on the downlink of such a channel, where the base station and all of the users have antenna arrays. If the transmitter has advance knowledge of the users' channel transfer functions, it can use that information to minimize the interuser interference due to the signals that are simultaneously transmitted to other users. If the transmitter assumes that all receivers treat the interference as noise, finding a solution that optimizes the use of resources is very difficult. This work proposes two classes of solutions to this problem. First, by forcing some or all of the interference to zero, it is possible to achieve a sub-optimal solution in closed-form. Second, a class of iterative solutions can be derived by extending optimal algorithms for multi-user downlink beamforming to accommodate receivers with multiple antennas. The closed-form solutions generally require less computation, but the iterative solutions offer improved performance are more robust to channel estimation errors, and thus may be more useful in practical applications. The performance of these algorithms were tested under realistic channel conditions by testing them on channels derived from both measurement data and a statistical model of an indoor propagation environment. These tests demonstrated both the ability of the channel to support multiple users, and the expected amount of channel estimation error due to movement of the users, with promising results. The success of any multi-user MIMO processing algorithm is ultimately dependent on the degree of correlation between the users' channels. If a base station is required to support a large number of users, one way to ensure minimal correlation between users' channels is to select groups of users whose channels are most compatible. The globally optimal solution to this problem is not possible without an exhaustive search, so a channel allocation algorithm is proposed that attempts to intelligently select groups of users at a more reasonable computational cost.

<p>This thesis considers wireless multi-input multi-output (MIMO) communication systems in block flat-fading environments. It develops novel designs of transmission and reception schemes for single-user and multi-user systems. The designs are developed under different models for the information about the communication channel that is available at the transmitter.</p> <p>For single-user systems, the thesis studies the class of non-linear MIMO transceivers that implement sequential interference (pre-) subtraction, namely transceivers with Tomlinson-Harashima precoding (THP) and transceivers with decision feedback equalization (DFE). For these transceivers a novel design framework is developed to unify the design of these two dual systems when channel state information (CSI) is available at both the transmitter and the receiver. The framework encompasses a broad range of performance criteria, and generates closed-form expressions for the optimal designs under these criteria. The framework reveals that a single transceiver design is optimal for a large subclass of these performance criteria and shows that this unique optimal design is (strictly) superior to the corresponding optimal linear transceiver for the same performance criterion. The framework also characterizes another class of design criteria for which the optimal non-linear transceiver reduces to the optimal linear transceiver for the same criterion. This novel design framework brings the design of non-linear MIMO transceivers to a level of maturity similar to the linear counterparts, and will impact the design of practical wireless communication systems that implement these interference subtraction schemes. The framework is then generalized to the case of DFE transceivers that satisfy an additional zero-forcing (ZF) constraint and operate in a "limited feedback" regime in which CSI is available only to the receiver and there is a limited rate feedback channel between the receiver and the transmitter. The proposed limited feedback system is the first that involves a "precoded" DFE transceiver.</p> <p> The multi-user part of the thesis develops multi-user transceivers that are robust to uncertainties in the available information about the users' channels. These uncertainties are inevitable in most practical multi-user communication systems, and can result in significant performance degradation.</p> <p> The first component of the multi-user part develops robust broadcast channel transceivers with quality of service (QoS) requirements for communication scenarios with bounded channel uncertainty at the transmitter. It formulates design problems for QoS requirements that can be expressed as constraints on the signal-to-interference-plus-noise-ratio (SINR) of each user, or as constraints on the mean square error (MSE) each user's received signal. For both formulations, convex and efficiently solvable design approaches are proposed. These design approaches are used to derive solutions to other related design problems, such as robust counterparts of the fair broadcasting problem.</p> <p> The second component of the multi-user part develops robust designs for multiuser transceivers that minimize different MSE criteria subject to a power constraint. The designs are obtained for different models of channel uncertainty: stochastic uncertainty models and bounded uncertainty models. For each channel uncertainty model, the robust multi-user designs are developed for both linear and non-linear MIMO transceivers, for both broadcast channels (BC) and multiple access channels (MAC).</p> <p>Simulation studies demonstrate the impact of the proposed robust designs on the performance of multi-user systems, and show that by incorporating robustness in the design one can significantly reduce the sensitivity of these systems to channel uncertainty and mitigate its deleterious effects.</p> / Thesis / Doctor of Philosophy (PhD)

<p> We consider the design of the precoders for a multi-input multi-output (MIMO) communication system equipped with a decision feedback equalizer (DFE) receiver. For such design problems, perfect knowledge of the channel state information (CSI) at both the transmitter and the receiver is usually required. However, in the environment of wireless communications, it is often difficult to provide sufficiently timely and accurate feedback of CSI from the receiver to the transmitter for such designs to be practically viable.</p> <p> In this thesis, we consider the optimum precoder designs for a wireless communication link having M transmitter antennas and N receiver antennas (M < N), in which the channels are assumed to be flat fading and may be correlated. We assume that full knowledge of CSI is available at the receiver. At the transmitter, however, only the first- and second-order statistics of the channels are available. Our first goal is to come up with an efficient design of the optimal precoder for such a MIMO system by minimizing the average arithmetic mean-squared error (MSE) of zero-forcing (ZF) decision feedback detection subject to a constraint on the total transmission power. Applying some of the properties of the matrix parameters, this non-convex optimization problem can be transformed into a convex geometrical programming problem which can then be efficiently solved using an interior point method. The performance of the MIMO system equipped with this optimum precoder and a ZF-DFE has also been found to be comparable, and in some cases, superior to that of V-BLAST which necessitates optimally ordered successive interference cancellation based on the largest post-detection signal-to-noise ratio (SNR). In terms of trade-off between performance and implementation simplicity, the proposed system is certainly an attractive alternative.</p> <p> In addition, we also utilize these important properties of our system parameters to investigate an "inverse problem" of our first design. That is, we design another precoding matrix by minimizing the total transmission power of the MIMO communication system subject to a constraint on the average MSE. Also, a closed-form solution is derived when the channels are uncorrelated while simulation results for the minimum power precoder designs is given at the end of this thesis.</p> / Thesis / Master of Applied Science (MASc)

La synchronisation temporelle des systèmes MIMO a été abondamment étudiée dans les quinze dernières années, mais la plupart des techniques existantes supposent que le bruit est blanc temporellement et spatialement, ce qui ne permet pas de modéliser la présence d'interférence. Nous considérons donc le cas de bruits blancs temporellement mais pas spatialement, dont la matrice de covariance spatiale est inconnue. En formulant le problème de l'estimation de l'instant de synchronisation comme un test d'hypothèses, nous aboutissons au test du rapport de vraisemblance généralisé (GLRT) qui donne lieu à la comparaison avec un seuil d'une statistique de test eta_GLRT. Cependant, pour des raisons de complexité, l'utilisation de cette statistique n'est pas toujours considérée comme réaliste. La première partie de ce travail a donc été consacrée à mettre en évidence des tests alternatifs moins complexes à mettre en œuvre, tout en ayant des performances similaires. Une analyse comparative exhaustive, prenant en considération le bruit et l'interférence, le type de canal, le nombre d'antennes en émission et en réception, et l'orthogonalité de la séquence de synchronisation est réalisée. Enfin, nous étudions le problème de l'optimisation du nombre d'antennes en émission K pour la synchronisation temporelle, montrant que pour un RSB élevé, les performances augmentent avec K dès que le produit de K avec le nombre d'antennes de réception M n'est pas supérieur à 8.Le deuxième aspect de ce travail est une analyse statistique de eta_GLRT dans le cas où la taille de la séquence d'apprentissage N est du même ordre de grandeur que M, ce qui conduit naturellement à étudier le comportement de eta_GLRT dans le régime asymptotique des grands systèmes M tend vers l'infini, N tend l'infini de telle sorte que M/N tende vers une constante non nulle. Nous considérons le cadre applicatif d'un système muni d'une unique antenne d'émission et d'un canal à trajets multiples, qui est formellement identique à celui d'un système MIMO dont le nombre d'antennes d'émissions correspondrait au nombre de trajets. Lorsque le nombre de trajets L est beaucoup plus faible que N et M, nous établissons que eta_GLRT a un comportement gaussien avec l'espérance asymptotique L log (1 / (1-M/N)) et la variance (L/N)*(M/N)/(1-M/N). Ceci est en contraste avec le régime asymptotique standard quand N tend vers l'infini et M et L fixe où eta_GLRT a un comportement chi2. Sous l'hypothèse H_1, eta_GLRT a aussi un comportement gaussien. Nous considérons également le cas où le nombre de trajets L tend vers l'infini à la même vitesse que M et N. Nous utilisons des résultats connus concernant le comportement des statistiques linéaires des valeurs propres des grandes F matrices, et déduisons que dans le régime où L,M,N tendent vers l'infini à la même vitesse, eta_GLRT a encore un comportement gaussien sous H_0, mais avec une espérance et variance différentes. L'analyse de eta_GLRT sous H_1 lorsque L,M,L convergent vers l'infini nécessite l'établissement d'un théorème central limite pour les statistiques linéaires des valeurs propres de matrices F de moyennes non-nulles, une tâche difficile. Motivé par les résultats obtenus dans le cas où L reste fini, nous proposons d'approximer la distribution asymptotique par une distribution gaussienne dont l'espérance et la variance sont la somme de l'espérance et la variance asymptotique sous H_0quand L tend vers l'infini avec l'espérance et la variance asymptotique sous H_1 dans le régime classique N tend vers l'infini et M fixé. Des simulations numériques permettent de comparer les courbes ROC des différents approximant avec des courbes ROC empiriques. Les résultats montrent que nos approximant de grandes dimensions fournissent de meilleurs résultats quand M/N augmente, tout en permettant de capturer la performance réelle pour les petites valeurs de M/N / Time synchronization of MIMO systems have been strongly studied in the last fifteen years, but most of the existing techniques assume a spatially and temporally white noise, which does not allow modeling the presence of interference. We consider thus a temporally white but spatially colored noise, with an unknown covariance matrix. Formulating the estimation problem as a hypothesis testing problem, we obtain a Generalized likelihood ratio test (GLRT), which gives us a synchronization statistics eta_GLRT. However, for complexity reasons, it is not always considered realistic for practical situations. A part of this work has thus been devoted to showing that there exist non-GLRT statistics that are less complex to implement than theet a_GLRT, while having similar performance. Furthermore, we perform a comparative parameter analysis, taking into consideration the noise type, channel type, the number of transmit and receive antennas, and the orthogonality of the synchronization sequence. Lastly, the problem of optimization of the number of transmit antennas K for time synchronization has been investigated. showing, for high SNR, increasing performance with K as long as the product KM is not larger than 8, where M is the number of receive antennas. The second aspect of MIMO synchronization studied in thesis is asymptotic analysis of the same GLRT, but for large M. In this context, the synchronization sequence length N is the same order of magnitude as M, and this leads us naturally to the study of the the behavior of eta_GLRT in the asymptotic regime where M,N go towards infinity such that M/N go towards a non-zero constant. We consider the case of a single transmit antenna in a multi-path channel, which formally is equivalent to the MIMO system where the transmit antennas correspond to the number of paths. We address the case When the number of paths L does not scale with M and N, we establish that eta_GLRT has a Gaussian behavior with asymptotic mean L log (1/ (1 - M/N))and variance (L/N)*(M/N)/(1-M/N).This is in contrast with the standard asymptotic regime N goes to infinity and M fixed where eta_GLRT has a chi^2 behaviour. Under hypothesis H_1, eta_GLRT still has a Gaussian behaviour. The corresponding asymptotic mean and variance are obtained as the sum of the asymptotic mean and variance in the standard regime N goes to infinity and M fixed, and L log(1/(1-/M/N))L log (1 / (1-M/N)) and (L/N)*(M/N)/(1-M/N)respectively, i.e. the asymptotic mean and variance under H_0.We also consider the case where the number of paths L converges towards infinity at the same rate as M and N. Using known results of concerning the behaviour of linear statistics of the eigenvalues of large F-matrices, we deduce that in the regime where L,M,N converge to infinity at the same rate, eta_GLRT still has a Gaussian behaviour under H_0, but with a different mean and variance. The analysis of eta_GLRT under H_1 whenL,M,N converge to infinity needs to establish a central limit theorem for linear statistics of the eigenvalues of large non zero-mean F-matrices, a difficult ask. Motivated by the results obtained in the case where L remains finite, we propose to approximate the asymptotic distribution of eta_GLRT by a Gaussian distribution whose mean and variance are the sum of the asymptotic mean and variance under H_0when L goes to infinity with the asymptotic mean and variance under H_1 in the standard regime N goes to infinity and M fixed. Numerical simulations allow to compare the ROC curves obtained with the different approximations with the empirical ROC curves. The results show that the large-system approximations provide better results when M/N increases, while also allowing to capture the actual performance for small values of M/N

In this dissertation, channel estimation and channel prediction are studied for wireless communication systems. Wireless communication for time-variant channels becomes more important by the fast development of intelligent transportation systems which motivates us to propose a reduced rank channel estimator for time-variant frequency-selective high-speed railway (HSR) systems and a reduced rank channel predictor for fast time-variant flat fading channels. Moreover, the potential availability of large bandwidth channels at mm-wave frequencies and the small wavelength of the mm-waves, offer the mm-wave massive multiple-input multiple-output (MIMO) communication as a promising technology for 5G cellular networks. The high fabrication cost and power consumption of the radio frequency (RF) units at mm-wave frequencies motivates us to propose a low-power hybrid channel estimator for mm-wave MIMO orthogonal frequency-division multiplexing (OFDM) systems. The work on HSR channel estimation takes advantage of the channel's restriction to low dimensional subspaces due to the time, frequency and spatial correlation of the channel and presents a low complexity linear minimum mean square error (LMMSE) estimator for MIMO-OFDM HSR channels. The channel estimator utilizes a four-dimensional (4D) basis expansion channel model obtained from band-limited generalized discrete prolate spheroidal (GDPS) sequences. Exploiting the channel's band-limitation property, the proposed channel estimator outperforms the conventional interpolation based least square (LS) and MMSE estimators in terms of estimation accuracy and computational complexity, respectively. Simulation results demonstrate the robust performance of the proposed estimator for different delay, Doppler and angular spreads. Channel state information (CSI) is required at the transmitter for improving the performance gain of the spatial multiplexing MIMO systems through linear precoding. In order to avoid the high data rate feedback lines, which are required in fast time-variant channels for updating the transmitter with the rapidly changing CSI, a subframe-wise channel tracking scheme is presented. The proposed channel predictor is based on an assumed DPS basis expansion model (DPS-BEM) for exploiting the variation of the channel coefficients inside each sub-frame and an autoregressive (AR) model of the basis coefficients over each transmitted frame. The proposed predictor properly exploits the channel's restriction to low dimensional subspaces for reducing the prediction error and the computational complexity. Simulation results demonstrate that the proposed channel predictor out-performs the DPS based minimum energy (ME) predictor for different ranges of normalized Doppler frequencies and has better performance than the conventional Wiener predictor for slower time-variant channels and almost the similar performance to it for very fast time-variant channels with the reduced amount of computational complexity. The work on the hybrid mm-wave channel estimator considers the sparse nature of the mm-wave channel in angular domain and leverages the compressed sensing (CS) tools for recovering the angular support of the MIMO-OFDM mm-wave channel. The angular channel is treated in a continuous framework which resolves the limited angular resolution of the discrete sparse channel models used in the previous CS based channel estimators. The power leakage problem is also addressed by modeling the continuous angular channel as a multi-band signal with the bandwidth of each sub-band being proportional to the amount of power leakage. The RF combiner is designed to be implemented using a network of low-power switches for antenna subset selection based on a multi-coset sampling pattern. Simulation results validate the effectiveness of the proposed hybrid channel estimator both in terms of the estimation accuracy and the RF power consumption. / Graduate

This work deals with the design of MT transmit, MR receive antenna MIMO (Multiple Input Multiple Output) communication system where the transmitter performs a linear operation on data. This linear precoding model includes systems which involve signal shaping for achieving higher data rates, uncoded MIMO Multicarrier and Single-Carrier systems and, the more recent, MIMO-OFDM (Orthogonal Frequency Division Multiplexing) systems employing full diversity Space-Frequency codes. The objective of this work is to design diversity centric and rate centric linear precoded MIMO systems whose performance is better than the existing designs. In particular, we consider MIMO-OFDM systems, Zero Padded MIMO systems and MIMO systems with limited rate feedback. Design of full diversity MIMO-OFDM systems of rate symbol per channel use (1 s/ pcu) : In literature, MIMO-OFDM systems exploiting full diversity at a rate of 1 s/ pcu are based on a few specific Space-Frequency (SF)/ Space-Time-Frequency (STF) codes. In this work, we devise a general parameterized framework for the design of MIMO-OFDM systems employing full diversity STF codes of rate 1 s/ pcu. This framework unifies all existing designs and provides tools for the design of new systems with interesting properties and superior performance. Apart from rate and diversity, the parameters of the framework are designed for a low complexity receiver. The parameters of the framework usually depend on the channel characteristics (number of multipath, Delay Profile (DP)). When channel characteristics are available at the transmitter, a procedure to optimize the performance of STF codes is provided. The resulting codes are termed as DP optimized codes. Designs obtained using the optimization are illustrated and their performance is shown to be better than the existing ones. To cater to the scenarios where channel characteristics are not available at the transmitter, a complete characterization of a class of full diversity DP Independent (DPI) STF codes is provided. These codes exploit full diversity on channels with a given number of multipath irrespective of their characteristics. Design of DP optimized STF codes and DPI codes from the same framework highlights the flexibility of the framework. Design of Zero Padded (ZP) MIMO systems : While the MIMO-OFDM transmitter needs to precode data for exploiting channel induced multipath diversity, ZP MIMO systems with ML receivers are shown to exploit multipath diversity without any precoding. However, the receiver complexity of such systems is enormous and hence a study ZP MIMO system with linear receivers is undertaken. Central to this study involves devising low complexity receivers and deriving the diversity gain of linear receivers. Reduced complexity receiver implementations are presented for two classes of precoding schemes. An upper bound on the diversity gain of linear receivers is evaluated for certain precoding schemes. For uncoded systems operating on a channel of length L, this bound is shown to be MRL_MT +1 for uncoded transmissions, i.e, such systems tend to exploit receiver and multipath diversities. On the other hand, MIMO-OFDM systems designed earlier have to trade diversity with receiver complexity. These observations motivate us to use ZP MIMO systems with linear receivers for channels with large delay spread when receiver complexity is at a premium. Design examples highlighting the attractiveness of ZP systems when employed on channels with large delay spread are also presented. Efficient design of MIMO systems with limited feedback : Literature presents a number of works that consider the design of MIMO systems with partial feedback. The works that consider feedback of complete CSI, however, do not provide for an efficient system design. In this work, we consider two schemes, Correlation matrix feedback and Channel information feedback that convey complete CSI to the transmitter. This CSI is perturbed due to various impairments. A perturbation analysis is carried out to study the variations in mutual information for each of the proposed schemes. For ergodic channels, this analysis is used to design a MIMO system with a limited rate feedback. Using a codebook based approach, vector quantizers are designed to minimize the loss in ergodic capacity for each of the proposed schemes. The efficiency of the design stems from the ability to obtain closed-form expression for centroids during the iterative vector quantizer design. The performance of designed vector quantizers compare favorably with the existing designs. The vector quantizer design for channel information feedback is robust in the sense that the same codebook can be used across all operating SNR. Use of vector quantizers for improving the outage performance is also presented.