This thesis investigates the application of an iterative leakage-based precoding algorithm to practical multiuser-MIMO systems. We consider the effect of practical impairments including imperfect channel state information, transmit antenna correlation, and time-varying channels. Solutions are derived which improve performance of the algorithm with imperfect channel state information at the transmitter by leveraging knowledge of the second-order statistics of the error. From this work we draw a number of conclusions on how imperfect channel state information may impact the system design including the importance of interference suppression at the receiver and the selection of the number of co-scheduled users. We also demonstrate an efficient approach to improve the convergence of the algorithm when using interference-rejection-combining receivers. Finally, we conduct simulations of an LTE-A system employing the improved algorithm to show its utility for modern communication systems. / Master of Science

Theoretic results have shown that the capacity of a channel does not decrease if the receiver observes the transmitted signal in the presence of interference, provided that the transmitter knows this interference non-causally. That is, if the transmitter has non-causal access to the interference, by using proper precoding this interference could be “avoided” (as if it were not present) under the same transmit power constraint. It indicates that lossless (in the sense of capacity) precoding is theoretically possible at any signal-to-noise-ratio (SNR). This is of special interest in digital watermarking, transmission for ISI channels as well as for MIMO broadcast channels. Recent research has elegantly demonstrated the (near) achievability of this “existence-type” result, while the complexity is notable. An interesting question is what one can do when very little extra complexity is permitted. This thesis treats such special cases of this problem in order to shed some light on this question. In the AWGN channel with additive interference, an optimum modulator is designed under the constraint of a binary signaling alphabet with binary interference.Tomlinson-Harashima precoding (THP), which is originally proposed for ISI channels, is improved by picking up optimized parameters and then taken as a benchmark. Simulation results show that the Optimum Modulator always outperforms the THP with optimized parameters. The difference in performance, in terms of mutual information between channel input and output as well as coded bit error rate with Turbo codes, is significant in many scenarios.

This thesis is targeted at precoding methods and resource allocation for the downlink of fixed multi-user multi-antenna broadband wireless systems. We explore different utilizations of precoders in transmission over frequency-selective channels. We first consider the weighted sum-rate (WSR) maximization problem for multi-carrier systems using linear precoding and propose a low complexity algorithm which exhibits near-optimal performance. Moreover, we offer a novel rate allocation method that utilizes the signalto- noise-ratio (SNR) gap to capacity concept to choose the rates to allocate to each data stream. We then study a single-carrier transmission scheme that overcomes known impairments associated with multi-carrier systems. The proposed scheme utilizes timereversal space-time block coding (TR-STBC) to orthogonalize the downlink receivers and performs the required pre-equalization using Tomlinson-Harashima precoding (THP).We finally discuss the strengths and weaknesses of the proposed method.

This thesis is targeted at precoding methods and resource allocation for the downlink of fixed multi-user multi-antenna broadband wireless systems. We explore different utilizations of precoders in transmission over frequency-selective channels. We first consider the weighted sum-rate (WSR) maximization problem for multi-carrier systems using linear precoding and propose a low complexity algorithm which exhibits near-optimal performance. Moreover, we offer a novel rate allocation method that utilizes the signalto- noise-ratio (SNR) gap to capacity concept to choose the rates to allocate to each data stream. We then study a single-carrier transmission scheme that overcomes known impairments associated with multi-carrier systems. The proposed scheme utilizes timereversal space-time block coding (TR-STBC) to orthogonalize the downlink receivers and performs the required pre-equalization using Tomlinson-Harashima precoding (THP).We finally discuss the strengths and weaknesses of the proposed method.

Les modulations à phase continue (CPMs) sont des méthodes de modulations robuste à la noncohérence du canal de propagation. Dans un contexte spatial, les CPM sont utilisées dans la chaîne de transmission de télémesure de la fusée. Depuis les années 70, la modulation la plus usitée dans les systèmes de télémesures est la modulation CPFSK continuous phase frequency shift keying filtrée. Historiquement, ce type de modulation est concaténée avec un code ReedSolomon (RS) afin d'améliorer le processus de décodage. Côté récepteur, les séquences CPM non-cohérentes sont démodulées par un détecteur Viterbi à sortie dure et un décodeur RS. Néanmoins, le gain du code RS n'est pas aussi satisfaisant que des techniques de codage moderne capables d'atteindre la limite de Shannon. Actualiser la chaîne de communication avec des codes atteignant la limite de Shannon tels que les codes en graphe creux, implique deremanier l’architecture du récepteur usuel pour un détecteur à sortie souple. Ainsi, on propose dans cette étude d' élaborer un détecteur treillis à sortie souple pour démoduler les séquences CPM non-cohérentes. Dans un deuxième temps, on concevra des schémas de pré-codages améliorant le comportement asymptotique du récepteur non-cohérent et dans une dernière étape on élabora des codes de parité à faible densité (LDPC) approchant la limite de Shannon. / Continuous phase modulations (CPM) are modulation methods robust to the non-coherency of propagation channels. In a space context, CPMs are used in the communication link between the rocket and the base stations. Since the 70's, the most popular telemetry modulation is the filtered continuous phase frequency shift keying (CPFSK). Traditionally, the CPFSK scheme isconcatenated with a Reed-Solomon (RS) code to enhance the decoding process. At the receiver side, the non-coherent CPM sequences are demodulated through a hard Viterbi detector and a RS decoder. However, the RS's coding gain is no more satisfactory when directly compared to modern coding schemes enable to reach the Shannon limit. Updating the communication link to capacity achieving codes, as sparse graph codes, implies to redesign the receiver architecture to soft detector. In that respect, we propose in this study to design a trellis-based soft detector to demodulate non-coherent CPM sequences. In a second part, we will elaborate precoding schemes to improve the asymptotic behaviour of the non-coherent receiver and in a last step we will build low density parity check codes approaching the Shannon limit.

This dissertation examines multi-antenna transceiver design over flat-fading wireless channels. Bit Interleaved Coded Modulation (BICM) and MultiLevel Coded Modulation (MLCM) transmitter structures are considered, as well as the used of an optional spatial precoder under slow and quasi-static fading conditions. At the receiver, MultiStage Decoder (MSD) and Iterative Detection and Decoding (IDD) strategies are applied. Precoder, mapper and subcode designs are optimized for different receiver structures over the different antenna and fading scenarios. Under slow and quasi-static channel conditions, fade resistant multi-antenna transmission is achieved through a combination of linear spatial precoding and non-linear multi-dimensional mapping. A time-varying random unitary precoder is proposed, with significant performance gains over spatial interleaving. The fade resistant properties of multidimensional random mapping are also analyzed. For MLCM architectures, a group random labelling strategy is proposed for large antenna systems. The use of complexity constrained receivers in BICM and MLCM transmissions is explored. Two multi-antenna detectors are proposed based on a group detection strategy, whose complexity can be adjusted through the group size parameter. These detectors show performance gains over the the Minimum Mean Squared Error (MMSE)detector in spatially multiplexed systems having an excess number of transmitter antennas. A class of irregular convolutional codes is proposed for use in BICM transmissions. An irregular convolutional code is formed by encoding fractions of bits with different puncture patterns and mother codes of different memory. The code profile is designed with the aid of extrinsic information transfer charts, based on the channel and mapping function characteristics. In multi-antenna applications, these codes outperform convolutional turbo codes under independent and quasi-static fading conditions. For finite length transmissions, MLCM-MSD performance is affected by the mapping function. Labelling schemes such as set partitioning and multidimensional random labelling generate a large spread of subcode rates. A class of generalized Low Density Parity Check (LDPC) codes is proposed, to improve low-rate subcode performance. For MLCM-MSD transmissions, the proposed generalized LDPC codes outperform conventional LDPC code construction over a wide range of channels and design rates.

Le trafic de données a connu une explosion durant les dernières décennies, en raison d’une augmentation de la demande liée à la vidéo en ligne, mais aussi de la voix sur IP et le peer to peer. L’émergence du cloud computing et du cloud gaming ainsi que la haute résolution des contenus vidéo tend à accroître encore cette demande de débit. Ainsi les normes de communication évoluent afin de fournir les utilisateurs avec le meilleur débit possible. De ce fait, les systèmes nécessitent d’embarquer des techniques d’accès multiples afin de séparer les utilisateurs et l’augmentation du nombre d’antennes devient indispensable. Par conséquent, ces équipements deviennent de plus en plus complexes et énergivores. Dans le but de réduire la complexité des systèmes et ainsi diminuer leur consommation, en permettant de plus de fonctionner avec de faibles puissances d’émission, le Retournement Temporel apparaît comme une solution envisageable. En effet, cette technique permet de focaliser des ondes électromagnétiques dans le temps et l’espace. Ainsi, grâce à ce procédé il devient possible de réaliser des communications multi-antennes à faible complexité et à faibles interférences entre symboles grâce à la propriété de focalisation temporelle, et à faibles interférences intérutilisateurs grâce à la propriété de focalisation spatiale. Cependant, afin de l’adapter aux normes de communication actuelles, il est nécessaire d’étudier la combinaison duRetournement Temporel avec les systèmes multi-porteuses présents dans les principales normes de communication par paquets. Le but de cette thèse est donc d’étudier le Retournement Temporel appliqué aux modulations multi-porteuses, et plus particulièrement à l’OFDM. Dans un premier temps, la manière de réaliser une telle combinaison est étudiée dans un contexte SISO (Single Input Single Output). Ensuite, les avantages au niveau de la synchronisation et de la réduction de l’intervalle de garde d’une telle association sont décrits et analysés. Par la suite, une extension aux systèmes MISO (Multiple Input Single Output) est présentée, ainsi que la combinaison avec les codes espace temps en bloc orthogonaux. Les performances de ces systèmes en termes de capacité et de taux d’erreurs binaire sont ensuite évaluées analytiquement et comparées avec celles des systèmes concurrents. Un algorithme de modulation adaptative est ensuite ajouté au système afin d’en améliorer les performances. Ensuite, la manière de combiner le Retournement Temporel avec un système OQAM est présentée. Il est montré que cette combinaison permet de réaliser simplement des systèmes MISO-OQAM, et d’exploiter la diversité spatiale. Il est également montré ici que la combinaison avec les codes espace temps en bloc orthogonaux est réalisable. Enfin, la robustesse du système combinant le Retournement Temporel et l’OFDM face aux erreurs d’estimation est évaluée à l’aide de deux algorithmes d’estimation de canal. / The data traffic has been exploding in the last decades, owing to the increasing demand in streaming video, just as voice over IP and peer to peer. The appearance of cloud computing and cloud gaming, as well as the high resolution of video contents tend to increase this data rate demand. Hence, the communication norms evolve to provide users with the best possible throughput. Consequently, systems necessitate embedding multiple accesstechniques in order to discriminate users and the increase of the number of antennas becomes mandatory. Hence, these equipments become more and more complex and power consuming. To reduce the systems complexity and then decrease their consumption, also allowing working with low transmitting power, Time Reversal appears as a conceivable solution. Indeed, this technique allows focusing electromagnetic waves in the time and in the space domain. Hence, thanks to this process, low complexity multi-antennas communication becomes feasible, and with low inter symbol interference thanks to the time focusing property, and with lowinter user interference thanks to the space focusing property. However, in order to adapt if to current communication norms, it is necessary to study the combination of Time Reversal with multi-carrier systems used in the main packet communication norms. The aim of this PhD thesis is to study Time Reversal whenapplied to multi-carrier modulations, and particularly with OFDM. At first, the way to combine the two techniques isshowed in a SISO (Single Input Single Output) context. Then, the benefits in the synchronization process and the guard interval reduction of such association are described and analyzed. Afterwards, an extension to MISO (Multiple Input Single Output) systems is presented, as well as the combination with orthogonal space time bloc codes. The performances of these systems in terms of capacity and bit error rate are then analytically calculated and compared with other systems. An adaptive modulation algorithm is then added to the system aiming to improve the performances. Then, the way to combine Time Reversal with an OQAM system is presented. It is shown that this combination allows realizing simple MISO-OQAM systems, and also allows exploiting thespatial diversity. It is also shown that the combination with space time orthogonal block codes is feasible.Finally, the robustness of the system combining Time Reversal and OFDM when experiencing channel estimation errors is evaluated, with two channel estimation logarithms for multicarrier systems.

Les modulations à phase continue (CPMs) sont des méthodes de modulations robuste à la noncohérence du canal de propagation. Dans un contexte spatial, les CPM sont utilisées dans la chaîne de transmission de télémesure de la fusée. Depuis les années 70, la modulation la plus usitée dans les systèmes de télémesures est la modulation CPFSK continuous phase frequency shift keying filtrée. Historiquement, ce type de modulation est concaténée avec un code ReedSolomon (RS) afin d'améliorer le processus de décodage. Côté récepteur, les séquences CPM non-cohérentes sont démodulées par un détecteur Viterbi à sortie dure et un décodeur RS. Néanmoins, le gain du code RS n'est pas aussi satisfaisant que des techniques de codage moderne capables d'atteindre la limite de Shannon. Actualiser la chaîne de communication avec des codes atteignant la limite de Shannon tels que les codes en graphe creux, implique deremanier l’architecture du récepteur usuel pour un détecteur à sortie souple. Ainsi, on propose dans cette étude d' élaborer un détecteur treillis à sortie souple pour démoduler les séquences CPM non-cohérentes. Dans un deuxième temps, on concevra des schémas de pré-codages améliorant le comportement asymptotique du récepteur non-cohérent et dans une dernière étape on élabora des codes de parité à faible densité (LDPC) approchant la limite de Shannon.

In this thesis we investigate the effects of the physical constraints such as antenna aperture size, antenna geometry and non-isotropic scattering distribution parameters (angle of arrival/departure and angular spread) on the performance of coherent and non-coherent space-time coded wireless communication systems. First, we derive analytical expressions for the exact pairwise error probability (PEP) and PEP upper-bound of coherent and non-coherent space-time coded systems operating over spatially correlated fading channels using a moment-generating function-based approach. These analytical expressions account for antenna spacing, antenna geometries and scattering distribution models. Using these new PEP expressions, the degree of the effect of antenna spacing, antenna geometry and angular spread is quantified on the diversity advantage (robustness) given by a space-time code. It is shown that the number of antennas that can be employed in a fixed antenna aperture without diminishing the diversity advantage of a space-time code is determined by the size of the antenna aperture, antenna geometry and the richness of the scattering environment. ¶ In realistic channel environments the performance of space-time coded multiple-input multiple output (MIMO) systems is significantly reduced due to non-ideal antenna placement and non-isotropic scattering. In this thesis, by exploiting the spatial dimension of a MIMO channel we introduce the novel use of linear spatial precoding (or power-loading) based on fixed and known parameters of MIMO channels to ameliorate the effects of non-ideal antenna placement on the performance of coherent and non-coherent space-time codes. The spatial precoder virtually arranges the antennas into an optimal configuration so that the spatial correlation between all antenna elements is minimum. With this design, the precoder is fixed for fixed antenna placement and the transmitter does not require any feedback of channel state information (partial or full) from the receiver. We also derive precoding schemes to exploit non-isotropic scattering distribution parameters of the scattering channel to improve the performance of space-time codes applied on MIMO systems in non-isotropic scattering environments. However, these schemes require the receiver to estimate the non-isotropic parameters and feed them back to the transmitter. ¶ The idea of precoding based on fixed parameters of MIMO channels is extended to maximize the capacity of spatially constrained dense antenna arrays. It is shown that the theoretical maximum capacity available from a fixed region of space can be achieved by power loading based on previously unutilized channel state information contained in the antenna locations. We analyzed the correlation between different modal orders generated at the transmitter region due to spatially constrained antenna arrays in non-isotropic scattering environments, and showed that adjacent modes contribute to higher correlation at the transmitter region. Based on this result, a power loading scheme is proposed which reduces the effects of correlation between adjacent modes at the transmitter region by nulling power onto adjacent transmit modes. ¶ Furthermore, in this thesis a general space-time channel model for down-link transmission in a mobile multiple antenna communication system is developed. The model incorporates deterministic quantities such as physical antenna positions and the motion of the mobile unit (velocity and the direction), and random quantities to capture random scattering environment modeled using a bi-angular power distribution and, in the simplest case, the covariance between transmit and receive angles which captures statistical interdependency. The Kronecker model is shown to be a special case when the power distribution is separable and is shown to overestimate MIMO system performance whenever there is more than one scattering cluster. Expressions for space-time cross correlations and space-frequency cross spectra are given for a number of scattering distributions using Gaussian and Morgenstern's family of multivariate distributions. These new expressions extend the classical Jake's and Clarke's correlation models to general non-isotropic scattering environments.

Future wireless communications expect to experience a spectrum shortage problem. One practical solution is spectrum sharing. This thesis studies precoding strategies to allocate communication resources for spectrum sharing in multi-user wireless systems from a game-theoretic perspective. The approaches for the precoding games are developed under different constraints. It is shown that the precoding game with spectral mask constraints can be formulated as a convex optimization problem and a dual decomposition based algorithm can be exploited to solve it. However, the problem is non-convex if users also have total power constraints. This study shows that an efficient sub-optimal solution can be derived by allocating the bottleneck resource in the system. The sub-optimal solution is proved to be efficient and can even achieve an identical performance to that of the optimal solution in certain cases, but with significantly reduced complexity. / Communications