<p> In this thesis, we consider a multiuser downlink OFDM system for which the channel state information ( CSI) is known to both the transmitter and the receiver. </p> <p> For such a system, we design an optimal precoder that minimizes the total mean square error (MSE) subject to a total power constraint for which a minimum MSE (MMSE) equalizer is employed. We show that, the MMSE precoder can be obtained by optimally allocating the subcarriers and optimally allocating the power. This problem can be solved by a two-stage process, in which we minimize the lower bound of the MSE to obtain the optimal power for each subcarrier, followed by seeking an optimal precoder to achieve this minimized lower bound. Specifically, our subcarrier allocation strategy states that, each subcarrier should be allocated to only one user that has the largest subchannel gain in that subcarrier. </p> <P> Moreover, based on this subcarrier allocation strategy, we perform an optimal power loading and design the corresponding optimal precoder that minimizes the average bit error rate (BER). Here, the MMSE equalizer is also employed. This optimization problem is solved by two stages. In the first stage, we derive the lower bound of the average BER and minimize this lower bound. After we employ the MMSE subcarrier allocation strategy, the optimal power loading problem can be efficiently solved by interior point methods. In order to reduce computation complexity, an alternative, efficient power loading method is proposed here, which is much more efficient when the number of subcarriers is large. In the second stage, to achieve the minimized lower bound, we seek a design of an optimal precoder. Simulation results show that for moderate to high signal-to-noise ratio (SNR), the performance of the minimum BER {MBER) precoder employed with the MMSE equalizer design is superior to several other design methods, including the MMSE precoder design. </p> / Thesis / Master of Applied Science (MASc)

In this thesis, we design advanced interference management techniques for future wireless networks under the availability of perfect and imperfect channel state information (CSI). We do so by considering a generalized imperfect CSI model where the variance of the channel estimation error depends on the signal-to-noise ratio (SNR). First, we analyze the performance of standard linear precoders, namely channel inversion (CI) and regularized CI (RCI), in downlink of cellular networks by deriving the received signal-to-interference-plus-noise ratio (SINR) of each user subject to both perfect and imperfect CSI. In this case, novel bounds on the asymptotic performance of linear precoders are derived, which determine howmuch accurate CSI should be to achieve a certain quality of service (QoS). By relying on the knowledge of error variance in advance, we propose an adaptive RCI technique to further improve the performance of standard RCI subject to CSI mismatch. We further consider transmit-power efficient design of wireless cellular networks. We propose two novel linear precoding techniques which can notably decrease the deployed power at transmit side in order to secure the same average output SINR at each user compared to standard linear precoders like CI and RCI. We also address a more sophisticated interference scenario, i.e., wireless interference networks, wherein each of the K transmitters communicates with its corresponding receiver while causing interference to the others. The most representative interference management technique in this case is interference alignment (IA). Unlike standard techniques like time division multiple access (TDMA) and frequency division multiple access (FDMA) where the achievable degrees of freedom (DoF) is one, with IA, the achievable DoF scales up with the number of users. Therefore, in this thesis, we quantify the asymptotic performance of IA under a generalized CSI mismatch model by deriving novel bounds on asymptotic mean loss in sum rate and the achievable DoF. We also propose novel least squares (LS) and minimum mean square error (MMSE) based IA techniques which are able to outperform standard IA schemes under perfect and imperfect CSI. Furthermore, we consider the implementation of IA in coordinated networks which enable us to decrease the number of deployed antennas in order to secure the same achievable DoF compared to standard IA techniques.

Le système MIMO coopératif est une solution attrayante dans un environnement où les trajets multiples signalent s'avérer être une étape stimulante pour le lien paire communication émetteur-récepteur. En effet, la diversité spatiale fournis par les émetteurs et recievers peut être exploitée pour améliorer la qualité du signal. Cette thèse étudie l'application de la boucle fermée précodeur MIMO pour réduire encore plus l'énergie de transmission dans un tel environnement. La contribution de cette thèse est de proposer des approches globales qui conduisent à l'optimisation globale des transmissions dans le système MIMO coopératif. Tout d'abord, on exploite la diversité spatiale des noeuds, et proposons une technique de sélection de noeud pour réduire l'énergie de transmission. Les noeuds sont sélectionnées en utilisant le linéaire boucle fermée MIMO précodeur max-dmin qui optimise la distance minimale (dmin) de critère pour réduire le BER de la constellation reçue, ce qui abaisse le rapport requis signal sur bruit (SNR). Deuxièmement, nous sommes motivés par une obligation de rendre les paramètres d'évaluation des performances MIMO disponibles aux couches supérieures du protocole. Ainsi, nous proposons une méthode théorique pour obtenir la fonction de distribution de probabilité (pdf) de dmin pour le précodeur max-dmin, nous utilisons le résultat de rapprocher le BER et de la capacité ergodique pour un système MIMO et une valeur de M en utilisant deux sous-canaux dans d'une manière rapide. Nous présentons un scénario qui exige que l'information pertinente soit détectée à partir d'une variété de sources situées à l'intérieur de la haute tension (HT) environnement du poste de réseau intelligent (SG) des applications. Notre contribution comprend le développement d'un outil de simulation basé sur la technique de sélection de noeuds pour le max-dmin distribué MIMO précodage. Pour tenir compte des interactions entre les couches multiples de communication, nous proposons de concevoir un système de communication MIMO coopératif complet basé sur un protocole d'échange de base qui est liée à notre scénario de transmission supposé. On construit en outre toutes les trames de sous-couche MAC, qui sont transmis dans ce système limité par le coût de l'énergie et de la synchronisation. / Cooperative MIMO system worked from the spatial diversity provided by the transmitters and receivers locations to improve the quality of service in the communication exchange. In our work, we explored the application of closed-loop MIMO precoder to further limit the energy of transmission in such environment. Our contribution is to propose approaches that lead to global optimization of transmissions in cooperative MIMO system. Firstly, we exploit spatial diversity of nodes, and then a node selection technique to reduce the energy of transmission. The nodes are selected using the max-dmin linear closed-loop MIMO precoder which, optimizes the minimum distance (dmin) criterion to reduce the Bit Error Rate of the received constellation, thereby lowering the required signal-to-noise ratio. Secondly, we are motivated by a requirement to make the MIMO performance evaluation parameters available to higher protocol layers. Thus, we propose a theoretical method to derive the probability distribution function of dmin for the max-dmin precoder, then we use the result to approximate the Bit Error Rate and ergodic capacity for any MIMO system and any value of modulation size M using 2 subchannels in a rapid manner. To achieve our objective, we present a realistic scenario from an existing application case where data must be collected from a variety of sources located inside a high voltage substation environment (smart grid applications). Our contribution involves the development of a simulation tool based on node selection technique for the max-dmin distributed MIMO precoding. Finally, inside this transmission scenario, we propose a complete communication system based on a basic exchange protocol. We further construct all the MAC sub layer frames that are transmitted in this system constrained by energy and synchronization cost.

Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier Block Transmissions (SCBT) are located at the two opposite edges of block transmission concept. In this thesis a system which lies between OFDM and SCBT is proposed. The new system, namely Block Transmissions on Orthogonal Carriers (BTOC), can be considered as a hybrid form of OFDM and SCBT. BTOC system is investigated under the redundant filterbank precoders and equalizers framework. Peak to average power ratio (PAPR) of BTOC is formulated and compared with the PAPRs of OFDM and SCBT. Effect of frequency offset for BTOC is investigated and comparison between OFDM, SCBT, and BTOC is presented. Simulation results of Zero Padded OFDM (ZP-OFDM), SCBT, and BTOC are included.