This thesis focuses on the design of linear precoding schemes for downlink multiple-input multiple-output (MIMO) networks. These schemes are designed to be amenable to implementation in wireless networks that allow rate-limited feedback of channel state information (CSI). In the first half of this thesis, memoryless quantization codebooks are designed and incremental vector quantization techniques are developed for the representation of CSI in MIMO point-to-point links and isolated (single-cell) downlink networks. The second half of the thesis seeks to design linear precoding schemes for the multi-cell downlink networks that can achieve improved performance without requiring significantly more communication resources for CSI feedback than those required in the case of an isolated single-cell. For the quantization problem, smooth optimization algorithms are developed for the design of codebooks that possess attractive features that facilitate their implementation in practice in the addition to having good quantization properties. As one example, the proposed approach is used to design rank-2 codebooks that have a nested structure and elements from a phase-shift keying (PSK) alphabet. The designed codebooks have larger minimum distances than some existing codebooks, and provide tangible performance gains. To take advantage of temporal correlation that may exist in the wireless channel, an incremental approach to the Grassmannian quantization problem is proposed. This approach leverages existing codebooks for memoryless quantization schemes and employs a quantized form of geodesic interpolation. Two schemes that implement the principles of the proposed approach are presented. A distinguishing feature of the proposed approach is that the direction of the geodesic interpolation is specified implicitly using a point in a conventional codebook. As a result, the approach has an inherent ability to recover autonomously from errors in the feedback path. In addition to the development of the Grassmannian quantization techniques and codebooks, this thesis studies linear precoder design for the downlink MIMO networks in the cases of small networks of arbitrary topology and unbounded networks that have typical architectures. In particular, a linear precoding scheme for the isolated 2-cell network that achieves the optimal spatial degrees of freedom of the network is proposed. The implementation of a limited feedback model for the proposed linear precoding scheme is developed as well. Based on insight from that model, other linear precoding schemes that can be implemented in larger networks, but with finite size, are developed. For unbounded networks of typical architecture, such as the hexagonal arrangement of cells, linear precoding schemes that exploit the partial connectivity of the network are presented under a class of precoding schemes that is referred to as spatial reuse precoding. These precoding schemes provide substantial gains in the achievable rates of users in the network, and require only local feedback. / Thesis / Doctor of Philosophy (PhD)

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This thesis examines various signal processing techniques that are required for establishing efficient (near optimal) communications in multiuser multiple-input multiple-output (MIMO) environments. The central part of this thesis is dedicated to acquisition of information about the MIMO channel state - at both the receiver and the transmitter. This information is required to organize a communication set up which utilizes all the available channel resources. Realistic channel model, i.e., the spatial channel model (SCM), has been used in this study, together with modern long-term evolution (LTE) standard. The work consists of three major themes: (a) estimation of the channel at the receiver, also known as tracking; (b) quantization of the channel information and its feedback from receiver to the transmitter (feedback quantization); and (c) reconstruction of the channel knowledge at the transmitter, and its use for data precoding during communication transmission. / Communications

Future cellular networks are expected to support extremely high data rates and user capacities. This thesis investigates the downlink of a wireless cellular system that takes advantage of multiple antennas at base stations and mobile stations, frequency reuse across all cells, and cooperation among base stations. We identify asynchronous interference resulting from multi-cell communication as a key challenge, prove the existence of a downlink/uplink duality in that case, and present a linear precoding scheme that exploits this duality. Since this result is not directly extendable to orthogonal frequency division multiplexing (OFDM), we propose a `hybrid' algorithm for two cooperating base stations, which combines linear and nonlinear precoding. This algorithm minimizes the sum mean squared error of the system and is extendable to OFDM. Finally, we consider the problem of user selection for multiuser precoding in OFDM-based systems. We extend an available single-cell user selection scheme to multiple cooperating cells.

Future cellular networks are expected to support extremely high data rates and user capacities. This thesis investigates the downlink of a wireless cellular system that takes advantage of multiple antennas at base stations and mobile stations, frequency reuse across all cells, and cooperation among base stations. We identify asynchronous interference resulting from multi-cell communication as a key challenge, prove the existence of a downlink/uplink duality in that case, and present a linear precoding scheme that exploits this duality. Since this result is not directly extendable to orthogonal frequency division multiplexing (OFDM), we propose a `hybrid' algorithm for two cooperating base stations, which combines linear and nonlinear precoding. This algorithm minimizes the sum mean squared error of the system and is extendable to OFDM. Finally, we consider the problem of user selection for multiuser precoding in OFDM-based systems. We extend an available single-cell user selection scheme to multiple cooperating cells.

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Cette thèse à pour but d'étudier des alternatives à l’augmentation des faisceaux dans des systèmes satellite THD deprochaine génération, évaluant des stratégies pour augmenter de manière significative la capacité totale du système. Dansce contexte, les schémas de réutilisation de fréquence (FR) agressives viennent naturellement dans l'esprit afin d'augmenterles ressources globales en bande passante et, par conséquent, augmenter la capacité globale du système. Cependant, cesschémas mènent à une augmentation des interférences co-canal, rendant l'utilisation du spectre supplémentaire pastellement efficace. Visant à trouver une solution à ce défi, des techniques basées sur des interférences ont été évaluées dans un contexte satellitaire réaliste, correspondant au précodage linéaire et la réutilisation fréquentielle fractionnelle (FFR). Le précodage linéaire est une technique MIMO qui permet l’application des schémas de réutilisation plus agressifs en traitantconjointement les signaux transmis afin de pré-compenser les interférences co-canal. Cette technique a été étudiée dans lecadre des systèmes THD et sa performance dérivé en considérant une caractérisation réaliste antenne, prouvantl'importante amélioration dans la capacité totale du système. Des stratégies de scheduling ont été également étudiées etdes algorithmes évalués, prouvant que d'autres améliorations peuvent être réalisées considérant mécanismes intelligentsde scheduling. Une autre voie d'augmenter les ressources spectrales par faisceau a été étudiée considérant des schémasFFR, utilisés en grande partie dans les réseaux mobiles terrestres (c.-à-d. WiMax, LTE…). / The purpose of this Ph.D. thesis has been to investigate alternatives to the beam scaling in NG-HTS systems, assessingadvanced strategies to significantly increase total system capacity, without further exploding the number of beams. In thiscontext, aggressive frequency reutilization (FR) strategies come naturally into mind as a potential mean to increase overallbandwidth resources and therefore, boost system capacity. However, it leads to an increase on co-channel interferences,rendering the usage of additional spectrum not as efficient. Aiming to find a solution to this challenge, advancedinterference-based system techniques have been assessed in a realistic NG-HTS context, corresponding to Linear Precodingand Fractional FR (FFR) schemes. Linear Precoding is a MIMO-based technique which allows considering more aggressiveFR schemes by jointly processing the transmitted signals in order to pre-compensate co-channel interferences. Thistechnique has been studied in the frame of NG-HTS systems and their performances derived considering realistic antennacharacterization, proving significant improvement in total system capacity. Scheduling strategies have been alsoinvestigated and schedule algorithms assessed, showing further improvements can be achieved considering smartscheduling mechanisms. Another way to increase spectral resources per beam has been then investigated considering FFR schemes, used mostly in mobile terrestrial networks (i.e. WiMAX, LTE…). FFR scheme application has been characterized and adapted to the particularities of a realistic HTS satellite context and its gains in total capacity have been derived. A natural synergy between Linear Precoding and FFR has been then studied, leading to further improvements on total system capacity.