Dans les systèmes radio mobiles, la diversité représente une technique efficace pour lutter contre l’évanouissement dû aux multi-trajets. La pleine diversité spatiale est atteinte dans les systèmes multiple-input multiple-output (MIMO). Mais, souvent l’intégration d’antennes multiples au niveau de l’émetteur ou du récepteur est coûteuse. Comme alternative, dans les réseaux sans fil multi-hop, la diversité coopérative garantit des gains de diversité spatiale en exploitant les techniques MIMO traditionnelles sans avoir besoin d’antennes multiples. En outre, la diversité coopérative fournit au réseau : un débit important, une énergie réduite et une couverture d’accès améliorée.Dans ce contexte, l’objectif de cette thèse est de concevoir des schémas de codage pour le canal à relais afin de réaliser une meilleure performance en termes de gain de diversité et de gain de codage. D’abord, nous étudions un système de turbo-codage distribué à L-relais en mode soft-decode-and-forward. Ensuite, nous proposons un système de turbocodage coopératif distribué à L-relais en utilisant la concaténation en parallèle des codes convolutifs. Enfin, afin d’améliorer la fiabilité de détection au niveau du noeud relais, nous proposons la technique de sélection d’antenne/relayage-soft. Pour une modulation BPSK, nous dérivons des expressions de la borne supérieure de la probabilité d’erreurbinaire où les différents sous-canaux sont supposés à évanouissement de Rayleigh, indépendants et pleinement entrelacés avec une information instantanée d’état de canal idéal. Une validation des résultats théoriques est également menée par la simulation. / Diversity provides an efficient method for combating multipath fading in mobile radio systems. One of the most common forms of spatial diversity is multiple-input multipleoutput (MIMO), where full diversity is obtained. However, embedding multiple antennas at the transmitter or the receiver can sometimes be expensive. As an alternative to collocated antennas, cooperative diversity in wireless multi-hop networks confirms their ability to achieve spatial diversity gains by exploiting the spatial diversity of the traditional MIMO techniques, without each node necessarily having multiple antennas. In addition, cooperative diversity has been shown to provide the network with importantthroughput, reduced energy requirements and improved access coverage.In light of this, the objective of this thesis is to devise coding schemes suitable for relay channels that aim at showing the best compromise between performance of diversity and coding gains. Firstly, we investigate a distributed turbo coding scheme dedicated to L-relay channels operating in the soft-decode-and-forward mode. Then, we present a proposed distributed turbo coded cooperative (DTCC) scheme, called parallel concatenated convolutional-based distributed coded cooperation. Finally, we investigate antenna/soft-relaying selection for DTCC networks in order to improve their end-to-end performance. Assuming BPSK transmission for fully interleaved channels with ideal channel state information, we define the explicit upper bounds for error probability inRayleigh fading channels with independent fading. Both theoretical limits and simulation results are presented to demonstrate the performances.

Cooperative communications by pooling available resources—for example, power and bandwidth—across the network, is a distributed solution for providing robust wireless transmission. Motivated by contemporary applications in multi-hop transmission and ad hoc networks, the classical three-node relay channel (RC) consisting of a source–destination pair and a relay node has received a renewed attention. One of the crucial aspects of the communication over relay networks (RNs) is the design of proper relaying protocols; that is, how the relay should take part in the transmission to meet a certain quality of service. In this dissertation, we address the design of reliable transmission strategies and quantification of the associated transmission rates over RNs. We consider three canonical examples of RNs: the classical RC, the multiple-access RC (MARC) and the two-way RC.We also investigate the three-node RC and MARC with state. The capacity of the aforementioned RNs is an open problem in general except for some special cases. In the thesis, we derive various capacity bounds, through which we also identify the capacity of some new classes of RNs. In particular, we introduce the class of state-decoupled RNs and prove that the noisy network coding is capacity achieving under certain conditions. In the thesis, we also study the effect of the memory length on the capacity of RNs. The investigated relaying protocols in the thesis can be categorized into two groups: protocols with a finite relay memory and those with infinite relay memory requirement. In particular, we consider the design of instantaneous relaying (also referred to as memoryless relaying) in which the output of the relay depends solely on the presently received signal at the relay. For optimizing the relay function, we present several algorithms constructed based on grid search and variational methods. Among other things, we surprisingly identify some classes of semi-deterministic RNs for which a properly constructed instantaneous relaying strategy achieves the capacity. We also show that the capacity of RNs can be increased by allowing the output of the relay to depend on the past received signals as well the current received signal at the relay. As an example, we propose a hybrid digital–analog scheme that outperforms the cutset upper bound for strictly causal relaying. / <p>QC 20110909</p>

Los canales múltiple-entrada-múltiple-salida (MIMO) han sido ampliamente propuestos para superar los desvanecimientos aleatorios de canal en comunicaciones inalámbricas no selectivas en frecuencia. Basados en equipar tanto transmisores como receptores con múltiple antenas, sus ventajas son dobles. Por un lado, permiten al transmisor: i) concentrar la energía transmitida en una dirección-propia determinada, o ii) codificar entre antenas con el fin de superar desvanecimientos no conocidos de canal. Por otro lado, facilitan al receptor el muestreo de la señal en el dominio espacial. Esta operación, seguida por la combinación coherente de muestras, aumenta la relación señal a ruido de entrada al receptor. De esta forma, el procesado multi-antena es capaz de incrementar la capacidad (y la fiabilidad) de la transmisión en escenarios con alta dispersión.Desafortunadamente, no siempre es posible emplazar múltiples antenas en los dispositivos inalámbricos, debido a limitaciones de espacio y/o coste. Para estos casos, la manera más apropiada de explotar el procesado multi-antena es mediante retransmisión, consistente en disponer un conjunto de repetidores inalámbricos que asistan la comunicación entre un grupo de transmisores y un grupo de receptores, todos con una única antena. Con la ayuda de los repetidores, por tanto, los canales MIMO se pueden imitar de manera distribuida. Sin embargo, la capacidad exacta de las comunicaciones con repetidores (así como la manera en que este esquema funciona con respeto al MIMO equivalente) es todavía un problema no resuelto. A dicho problema dedicamos esta tesis.En particular, la presente disertación tiene como objetivo estudiar la capacidad de canales Gaussianos asistidos por múltiples repetidores paralelos. Dos repetidores se dicen paralelos si no existe conexión directa entre ellos, si bien ambos tienen conexión directa con la fuente y el destino de la comunicación. Nos centramos en el análisis de tres canales ampliamente conocidos: el canal punto-a-punto, el canal de múltiple-acceso y el canal de broadcast, y estudiamos su mejora de funcionamiento con repetidores. A lo largo de la tesis, se tomarán las siguientes hipótesis: i) operación full-duplex en los repetidores, ii) conocimiento de canal tanto en transmisión como en recepción, y iii) desvanecimiento sin memoria, e invariante en el tiempo.En primer lugar, analizamos el canal con múltiples repetidores paralelos, en el cual una única fuente se comunica con un único destino en presencia de N repetidores paralelos. Derivamos límites inferiores de la capacidad del canal por medio de las tasas de transmisión conseguibles con distintos protocolos: decodificar-y-enviar, decodificar-parcialmente-y-enviar, comprimir-y-enviar, y repetición lineal. Asimismo, con un fin comparativo, proveemos un límite superior, obtenido a través del Teorema de max-flow-min-cut. Finalmente, para el número de repetidores tendiendo a infinito, presentamos las leyes de crecimiento de todas las tasas de transmisión, así como la del límite superior.A continuación, la tesis se centra en el canal de múltiple-acceso (MAC) con múltiples repetidores paralelos. El canal consiste en múltiples usuarios comunicándose simultáneamente con un único destino en presencia de N repetidores paralelos. Derivamos una cota superior de la región de capacidad de dicho canal utilizando, de nuevo, el Teorema de max-flow-min-cut, y encontramos regiones de tasas de transmisión conseguibles mediante: decodificar-y-enviar, comprimir-y-enviar, y repetición lineal. Asimismo, se analiza el valor asintótico de dichas tasas de transmisión conseguibles, asumiendo el número de usuarios creciendo sin límite. Dicho estudio nos permite intuir el impacto de la diversidad multiusuario en redes de acceso con repetidores.Finalmente, la disertación considera el canal de broadcast (BC) con múltiples repetidores paralelos. En él, una única fuente se comunica con múltiples destinos en presencia de N repetidores paralelos. Para dicho canal, derivamos tasas de transmisión conseguibles dado: i) codificación de canal tipo dirty paper en la fuente, ii) decodificar-y-enviar, comprimir-y-enviar, y repetición lineal, respectivamente, en los repetidores. Además, para repetición lineal, demostramos que la dualidad MAC-BC se cumple. Es decir, la región de tasas de transmisión conseguibles en el BC es igual a aquélla del MAC con una limitación de potencia suma. Utilizando este resultado, se derivan algoritmos de asignación óptima de recursos basados en teoría de optimización convexa. / Multiple-input-multiple-output (MIMO) channels are extensively proposed as a means to overcome the random channel impairments of frequency-flat wireless communications. Based upon placing multiple antennas at both the transmitter and receiver sides of the communication, their virtues are twofold. On the one hand, they allow the transmitter: i) to concentrate the transmitted power onto a desired eigen-direction, or ii) tocode across antennas to overcome unknown channel fading. On the other hand, they permit the receiver to sample the signal on the space domain. This operation, followed by the coherent combination of samples, increases the signal-to-noise ratio at the input of the detector. In fine, MIMO processing is able to provide large capacity (and reliability) gains within rich-scattered scenarios.Nevertheless, equipping wireless handsets with multiple antennas is not always possible or worthwhile. Mainly, due to size and cost constraints, respectively. For these cases, the most appropriate manner to exploit multi-antenna processing is by means of relaying. This consists of a set of wireless relay nodes assisting the communication between a set of single-antenna sources and a set of single-antenna destinations. With the aid of relays, indeed, MIMO channels can be mimicked in a distributed way. However, the exact channel capacity of single-antenna communications with relays (and how this scheme performs with respect to the equivalent MIMO channel) is a long-standing open problem. To it we have devoted this thesis.In particular, the present dissertation aims at studying the capacity of Gaussian channels when assisted by multiple, parallel, relays. Two relays are said to be parallel if there is no direct link between them, while both have direct link from the source and towards the destination. We focus on three well-known channels: the point-to-point channel, the multi-access channel and the broadcast channel, and study their performance improvement with relays. All over the dissertation, the following assumptions are taken: i) full-duplex operation at the relays, ii) transmit and receive channel state information available at all network nodes, and iii) time-invariant, memory-less fading.Firstly, we analyze the multiple-parallel relay channel, where a single source communicates to a single destination in the presence of N parallel relays. The capacity of the channel is lower bounded by means of the achievable rates with different relaying protocols, i.e. decode-and-forward, partial decode-and-forward, compress-and-forward and linear relaying. Likewise, a capacity upper bound is provided for comparison, derived using the max-flow-min-cut Theorem. Finally, for number of relays growing to infinity, the scaling laws of all achievable rates are presented, as well as the one of the upper bound.Next, the dissertation focusses on the multi-access channel (MAC) with multiple-parallel relays. The channel consists of multiple users simultaneously communicating to a single destination in the presence of N parallel relay nodes. We bound the capacity region of the channel using, again, the max-flow-min-cut Theorem and find achievable rate regions by means of decode-and-forward, linear relaying and compress-and-forward. In addition, we analyze the asymptotic performance of the obtained achievable sum-rates, given the number of users growing without bound. Such a study allows us to grasp the impact of multi-user diversity on access networks with relays.Finally, the dissertation considers the broadcast channel (BC) with multiple parallel relays. This consists of a single source communicating to multiple receivers in the presence of N parallel relays. For the channel, we derive achievable rate regions considering: i) dirty paper encoding at the source, and ii) decode-and-forward, linear relaying and compress-and-forward, respectively, at the relays. Moreover, for linear relaying, we prove that MAC-BC duality holds. That is, the achievable rate region of the BC is equal to that of the MAC with a sum-power constraint. Using this result, the computation of the channel's weighted sum-rate with linear relaying is notably simplified. Likewise, convex resource allocation algorithms can be derived.

Orientador: Prof. Dr. Murilo Bellezoni Loiola / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia da Informação, 2015.

Non-orthogonal multiple access (NOMA), with its exceptional spectrum efficiency, was thought to be a promising technology for upcoming wireless communications. Physical layer security has also been investigated to improve the security performance of the system. Power-domain NOMA has been considered for this paper, where multiple users can share the same spectrum which bases this sharing on distinct power values. Power allocation is used to allocate different power to the users based on their channel condition. Data signals of different users are superimposed on the transmitter's side, and the receiver uses successive interference cancellation (SIC) to remove the unwanted signals before decoding its own signal. There exist an eavesdropper whose motive is to eavesdrop on the confidential information that is being shared with the users. The network model developed in this way consists of two links, one of which considers the relay transmission path from the source to Near User to Far User and the other of which takes into account the direct transmission path from the source to the destination, both of which experience Nakagami-m fading. To degrade the eavesdropper's channel, the jamming technique is used against the eavesdropper where users are assumed to be in a full-duplex mode which aims to improve the security of the physical layer. Secrecy performance metrics such as secrecy outage probability, secrecy capacity, etc. are evaluated and analyzed for the considered system. Mathematical analysis and simulation using MATLAB are done to assess, analyze and visualize the system's performance in the presence of an eavesdropper when the jamming technique is applied. According to simulation results, the active jamming approach enhances the secrecy performance of the entire system and leads to a positive improvement in the secrecy rate.

The cooperative relaying scheme is a promising technique for increasing the capacity and reliability of wireless communication. Even though extensive research has performed in information theoretical aspect, there are still many unresolved practical problems of cooperative relaying system. This dissertation analyzes the performance of cooperative decode-and-forward (DF) relaying systems in the presence of multiple interferers and improve network throughput for these systems. We propose and summarize various systems in the view of network topology, transmission structure, and slot allocation. We present closed-form expressions for the end-to-end outage probability, average symbol-error-probability, average packet-error-probability, and network throughput of the proposed systems. This dissertation shows that the robustness of the destination against interference is more important than robustness of the relay against interference from an interference management perspective, and increasing the number of branches yields better outage and error performance improvements against shadowing than increasing the number of hops. In cellular networks, the cooperative diversity systems can outperform the dual-Rx antenna system, but only when the relay is located in a relatively small portion of the total cell area with respect the the destination mobile terminal. The results also show that since the effective regions of the uplink and the downlink do not overlap, different relays should be utilized for cell sectorization in the uplink and the downlink. Finally, the proposed variable-slot selection DF scheme can reduce the system complexity and make the maximum throughput point in the low and moderate signal-to-interference-plus-noise ratio region.

En la última década, la cooperación entre usuarios ha generado un gran interés por la posibilidad de mejorar la velocidad de transmisión en las redes de comunicaciones inalámbricas. El objetivo es formar un array con las antenas de todos los dispositivos y, de esta forma, aplicar técnicas de procesado espacio-temporal. El esquema de cooperación más sencillo es el canal con relays: todos los terminales que escuchen una comunicación entre dos puntos pueden ayudar a la fuente retransmitiendo lo que hayan recibido.En un sistema realista, los relays no disponen de información sobre el canal en trasmisión. En este escenario, los códigos espacio-temporales (STC, del inglés space-time coding) son la alternativa más eficiente para aprovechar la diversidad introducida por los relays. Sin embargo, los STC clásicos están diseñados para un número limitado y fijo de antenas transmisoras y no se adaptan bien a sistemas cooperativos donde el número de relays puede ser elevado y, sobretodo, puede variar en el tiempo, según los usuarios entren o salgan de la red. El problema principal es la necesidad de usar un código nuevo cada vez que cambie la configuración de la red, generando un importante tráfico de señalización.Esta tesis analiza un código espacio-temporal a bloques de dispersión lineal (LD-STBC, del inglés linear-dispersion space-time block coding), aleatorio y distribuido: a cada relay se le asigna una matriz aleatoria que aplica una transformación lineal al vector que contiene los símbolos de la fuente. Cada matriz se genera de forma independiente y sin ninguna relación con el número de usuarios involucrados. De esta manera, el número de nodos puede variar sin necesidad de modificar los códigos existentes.La forma más intuitiva de construir matrices de dispersión lineal independientes es que sus elementos sean variables aleatorias independientes e idénticamente distribuidas (i.i.d.). Por esta razón, se estudia primero la eficiencia espectral obtenida por este tipo de LD-STBC. Es importante remarcar que la eficiencia espectral es una cantidad aleatoria, ya que es una función de los códigos aleatorios anteriormente descritos. Sin embargo, cuando las dimensiones de las matrices crecen infinitamente pero manteniendo constante la tasa del código (relación entre número de símbolos de la fuente sobre el número de símbolos de los relays), la eficiencia espectral converge rápidamente hacia una cantidad determinista. Este resultado se demuestra usando la teoría de las matrices aleatorias. Por esta razón, el sistema se analiza aproximando la eficiencia espectral con su limite. Por ejemplo, la comparación con el canal directo entre fuente y destino permite definir unas condiciones suficientes en donde el sistema con relays es superior a la comunicación punto a punto.Posteriormente se debe analizar la probabilidad de outage, es decir la probabilidad de que, debido a la baja calidad del canal, la eficiencia espectral sea menor que la velocidad de transmisión solicitada por el sistema. Como ya se ha mencionado anteriormente, los relays se introducen para aumentar la diversidad del canal y, con ella, el número de caminos independientes entre la fuente y el receptor, reduciendo la probabilidad de outage. Para los LD-STBC i.i.d. las prestaciones en términos de outage dependen del tipo de relay (amplify and forward o decode and forward) y son función de la tasa del código, que debe ser cuidadosamente elegida para maximizar el orden de diversidad sin desperdiciar demasiados recursos.Finalmente, en el último capítulo de la tesis se considera otro tipo de LD-STBC, distinto del i.i.d. analizado hasta ahora. En este caso, las matrices de dispersión lineal siguen siendo independientes la una de la otra pero se añade la restricción de que cada una tenga columnas (o filas, según la tasa del código) ortogonales. Así, se consigue que el código siga siendo flexible con respecto a las variaciones en el número de usuarios, pero su estructura permite reducir la interferencia generada por cada relay, como se puede notar comparando su eficiencia espectral con la eficiencia espectral obtenida por el código i.i.d. Cabe destacar que el análisis asintótico de estos códigos (llamados isométricos) se basa en herramientas matemáticas más sofisticadas que las anteriores y, por lo tanto, es necesario un estudio más profundo para poder entender cómo se comporta en términos de outage. / In the last decade, cooperation among multiple terminals has been seen as one of the more promising strategies to improve transmission speed in wireless communications networks. Basically, the idea is to mimic an antenna array and apply distributed versions of well-known space-diversity techniques. In this context, the simplest cooperative scheme is the relay channel: all the terminals (relays) that overhear a point-to-point communication between a source and a destination may decide to aid the source by forwarding (relaying) its message.In a mobile system, it is common to assume that the relays do not have any information about the channel between them and the destination. Under this hypothesis, the best solution to exploit the diversity offered by multiple transmitting antennas is to use space-time coding (STC). However, classical STC's are designed for systems with a fixed and usually low number of antennas. Thus, they are not suitable for relaying in most mobile communications systems where the number of terminals is potentially large and may vary as users join or leave the network. For each new configuration, a new code has to be chosen and notified to the relays, introducing a set-up overhead of signaling traffic.In this dissertation we will propose and analyze a randomized distributed linear-dispersion space-time block code (LD-STBC): each relay is assigned a specific matrix which linearly transforms (left-multiplies) the column vector of source symbols. Each matrix is independently generated and does not depend on the total number of transmitters, which can thus change without interrupting data transmission for a new code--relay assignment.The more intuitive way to build independent linear-dispersion matrices is to fill them with independent and identically distributed (i.i.d.) random variables. Therefore, we will first consider these i.i.d. codes and characterize the resulting spectral efficiency. In order to analyze the performance achieved by the system, we consider a large-system analysis based on random matrix theory. We will show that the random spectral efficiency (function of the random linear-dispersion matrices) converges almost surely to a deterministic quantity when the dimensions of the code grow indefinitely while keeping constant the coding rate. Since convergence is very fast, the random spectral efficiency will be approximated by the deterministic limit in the subsequent analysis. By comparison with the direct link, sufficient conditions are derived for the superiority of relaying.Next, we will analyze the outage probability of the system, that is the probability that the spectral efficiency falls below a given target rate due to channel fading. The main purpose of diversity techniques is to introduce alternative paths from the source to the destination, so that data transmission does not fail when the direct link undergoes deep fading. We will show that the diversity behavior of LD-STBC relaying mainly depends on both the coding rate and the relaying strategy (amplify and forward or decode and forward). It is then important to choose the coding rate that maximizes the diversity order without wasting too many resources.To conclude the dissertation, we will consider a different code based on independent isometric Haar-distributed random linear-dispersion matrices. Thenew code maintains the flexibility of the previous one with respect to variations in the number of relays. However, the more complex structure of the codes allows a noticeable reduction of the interference generated by the relays. Unfortunately, isometric codes also require more sophisticated mathematical tools for their asymptotic analysis. For this reason, we simply introduce the problem by showing that it is possible to have some spectral-efficiency gain with respect to i.i.d. codes. The outage-probability analysis requires a more thorough understanding and will be the subject of future work.

Cooperative wireless communication has received significant attention during recent years due to several reasons. First, since the received power decreases rapidly with distance, the idea of multi-hopping is becoming of particular importance. In multi-hopped communication, the source exploits some intermediate nodes as relays. Then the source sends its message via those relays to the destination. Second, relays can emulate some kind of distributed transmit antennas to form spatial diversity and combat multi-path fading effect of the wireless channel. Parallel Relay Channel is an information theoretical model for a communication system whereby a sender aims to communicate to a receiver with the help of relay nodes. It represents the simplest model for a multi–hop wireless network and a full understanding of the limits of communication over such a channel can potentially shed light on the design of more efficient wireless networks. However, the capacity of the relay channel has been established only for few special cases and little progress has been made toward solving the general case since the early 1980s. In this dissertation, motivated by practical constraints, we study the information theoretical limits of the half-duplex Gaussian Parallel Relay channel , as well as, the transmission strategies for the parallel relay channel with bandwidth mismatch between the first and the second hops. Chapter 2 investigates the problem of communication for a network composed of two half-duplex parallel relays with additive white Gaussian noise (AWGN). There is no direct link between the source and the destination. However, the relays can communicate with each other through the channel between them. Two protocols, i.e., \emph{Simultaneous} and \emph{Successive} relaying, associated with two possible relay scheduling are proposed. The simultaneous relaying protocol is based on \emph{Broadcast-multiaccess with Common Message (BCM)} scheme. For the successive relaying protocol: (i) a \emph{Non-Cooperative} scheme based on the \emph{Dirty Paper Coding (DPC)}, and (ii) a \emph{Cooperative} scheme based on the \emph{Block Markov Encoding (BME)} are considered. The composite scheme of employing BME in \emph{at most} one relay and DPC in \emph{at least} another one is shown to achieve at least the same rate when compared to the \emph{Cooperative} and \emph{Non-Cooperative} schemes. A \emph{``Simultaneous-Successive Relaying based on Dirty paper coding scheme" (SSRD)} is also proposed. The optimum scheduling of the relays and hence the capacity of the half-duplex Gaussian parallel relay channel in the low and high signal-to-noise ratio (SNR) scenarios is derived. In the low SNR scenario, it is revealed that under certain conditions for the channel coefficients, the ratio of the achievable rate of the simultaneous relaying based on BCM to the cut-set bound tends to be 1. On the other hand, as SNR goes to infinity, it is proved that successive relaying, based on the DPC, asymptotically achieves the capacity of the network. Schein and Gallager introduced the Gaussian parallel relay channel in 2000. They proposed the Amplify-and-Forward (AF) and the Decode-and-Forward (DF) strategies for this channel. For a long time, the best known achievable rate for this channel was based on the AF and DF with time sharing (AF-DF). Recently, a Rematch-and-Forward (RF) scheme for the scenario in which different amounts of bandwidth can be assigned to the first and second hops were proposed. In chapter 3, we propose a \emph{Combined Amplify-and-Decode Forward (CADF)} scheme for the Gaussian parallel relay channel. We prove that the CADF scheme always gives a better achievable rate compared to the RF scheme, when there is a bandwidth mismatch between the first hop and the second hop. Furthermore, for the equal bandwidth case (Schein's setup), we show that the time sharing between the CADF and the DF schemes (CADF-DF) leads to a better achievable rate compared to the time sharing between the RF and the DF schemes (RF-DF) as well as the AF-DF.

In this thesis, we investigate cooperation by applying OFDM signals to cooperative relay networks. We consider the single path relay network and the multiple path relay network. Using the amplify-and-forward relay algorithm, we derive the input-output relations and mutual informations of both networks. Using a power constraint at each relay, we consider two relay power allocation schemes. The first is constant gain allocation, where the amplifying gain used in the amplify-and-forward algorithm is constant for all subcarriers. The second is equal power allocation, where each subcarrier transmits the same power. The former scheme does not require CSI (channel state information), while the latter one does. We simulate the mutual informations using the two relay power allocation schemes. Results indicate that equal power allocation gives a slightly higher mutual information for the single path relay network. For the multiple path network, the mutual information is practically the same for both schemes. Using the decode-and-forward relay algorithm, we derive the input-output relations for both networks. The transmitter and each relay are assumed to have uniform power distributions in this case. We simulate the BER (bit error rate) and WER (word error rate) performance for the two networks using both the amplify-and-forward and decode-and-forward relay algorithms. For the single path relay network, amplify-and-forward gives very poor performance, because as we increase the distance between the transmitter and receiver (and thus, add more relays), more noise and channel distortion enter the system. Decode-and-forward gives significantly better performance because noise and channel distortion are eliminated at each relay. For the multiple path relay network, decode-and-forward again gives better performance than amplify-and-forward. However, the performance gains are small compared to the single path relay network case. Therefore, amplify-and-forward may be a more attractive choice due to its lower complexity.

Dans les systèmes radio mobiles, la diversité représente une technique efficace pour lutter contre l'évanouissement dû aux multi-trajets. La pleine diversité spatiale est atteinte dans les systèmes multiple-input multiple-output (MIMO). Mais, souvent l'intégration d'antennes multiples au niveau de l'émetteur ou du récepteur est coûteuse. Comme alternative, dans les réseaux sans fil multi-hop, la diversité coopérative garantit des gains de diversité spatiale en exploitant les techniques MIMO traditionnelles sans avoir besoin d'antennes multiples. En outre, la diversité coopérative fournit au réseau : un débit important, une énergie réduite et une couverture d'accès améliorée.Dans ce contexte, l'objectif de cette thèse est de concevoir des schémas de codage pour le canal à relais afin de réaliser une meilleure performance en termes de gain de diversité et de gain de codage. D'abord, nous étudions un système de turbo-codage distribué à L-relais en mode soft-decode-and-forward. Ensuite, nous proposons un système de turbocodage coopératif distribué à L-relais en utilisant la concaténation en parallèle des codes convolutifs. Enfin, afin d'améliorer la fiabilité de détection au niveau du noeud relais, nous proposons la technique de sélection d'antenne/relayage-soft. Pour une modulation BPSK, nous dérivons des expressions de la borne supérieure de la probabilité d'erreurbinaire où les différents sous-canaux sont supposés à évanouissement de Rayleigh, indépendants et pleinement entrelacés avec une information instantanée d'état de canal idéal. Une validation des résultats théoriques est également menée par la simulation.