Cette thèse est consacrée à l'analyse de l'association de codes LDPC non-binaires (LDPC-NB) à des modulations d’ordre élevé. Cette association vise à améliorer l’efficacité spectrale pour les futurs systèmes de communication sans fil. Notre approche a consisté à tirer au maximum profit de l'association directe des symboles d’un code LDPC-NB sur un corps de Galois avec une constellation de même cardinalité. Notre première contribution concerne la diversité spatiale obtenue dans un canal de Rayleigh avec et sans effacement en faisant subir une rotation à la constellation. Nous proposons d’utiliser l'information mutuelle comme paramètre d’optimisation de l’angle de rotation, et ce pour les modulations de type « BICM » et les modulations codées. Cette étude permet de mettre en évidence les avantages de la modulation codée par rapport à la modulation BICM de l’état de l’art. Par simulation de Monte-Carlo, nous montrons que les gains de codage théoriques se retrouvent dans les systèmes pratiques. Notre deuxième contribution consiste à concevoir conjointement l'étiquetage des points de constellation et le choix des coefficients d'une équation de parité en fonction de la distance euclidienne, et non plus de la distance de Hamming. Une méthode d’optimisation est proposée. Les codes ainsi construits offrent des gains de performance de 0.2 dB et ce, sans ajout de complexité. / This thesis is devoted to the analysis of the association of non-binary LDPC codes (NB-LDPC) with high-order modulations. This association aims to improve the spectral efficiency of future wireless communication systems. Our approach tries to take maximum advantage of the straight association between NB-LDPC codes over a Galois Field with modulation constellations of the same cardinality. We first investigate the optimization of the signal space diversity technique obtained with the Rayleigh channel (with and without erasure) thanks to the rotation of the constellation. To optimize the rotation angle, the mutual information analysis is performed for both coded modulation (CM) and bit-interleaved coded modulation (BICM) schemes. The study shows the advantages of coded modulations over the state-of-the-art BCIM modulations. Using Monte Carlo simulation, we show that the theoretical gains translate into actual gains in practical systems. In the second part of the thesis, we propose to perform a joint optimization of constellation labeling and parity-check coefficient choice, based on the Euclidian distance instead of the Hamming distance. An optimization method is proposed. Using the optimized matrices, a gain of 0.2 dB in performance is obtained with no additional complexity.

Reliability of data transmission is a fundamental problem in wireless communications. Fading in wireless channels causes the signal strength to vary at the receiver and this results in loss of data packets. To improve the reliability, automatic repeat request (ARQ) schemes were introduced. However these ARQ schemes suffer from a reduction in the throughput. To address the throughput reduction, conventional ARQ schemes were combined with forward error correction (FEC) schemes to develop hybrid-ARQ (HARQ) schemes. For improving the reliability of data transmission, HARQ schemes are included in the present wireless standards like LTE, LTE-Advanced and WiMAX. Conventional HARQ systems use the same transmission power and the same number of channel uses in different ARQ rounds. However this is not optimal in terms of minimizing the average transmit power or the average energy spent for successful transmission of a data packet. We address this issue in the first part of the dissertation, where we consider optimal resource allocation in HARQ systems with a limit on the maximum number of allowed transmissions for a data packet. Specifically, we consider the problem of minimizing the packet drop probability (PDP) under an average transmit power constraint or equivalently minimizing the average transmit power under a fixed PDP constraint. We consider both incremental redundancy (IR)-based and Chase combining (CC)-based HARQ systems in our work. For an IR-HARQ system, for the special case of two allowed transmissions for each packet, we provide a solution for the optimal number of channel uses and the optimal power to be used in each ARQ round. For a CC-HARQ system, we solve the problem of optimal power allocation in i.i.d. Rayleigh fading channels as well as correlated Rayleigh fading channels. For the CC-HARQ case, we also provide a low complexity geometric programming (GP) solution using an approximation of the outage probability expression. HARQ systems conventionally use one bit acknowledgement (ACK)/negative ACK (NACK) feedback from the receiver to the transmitter. In the 3GPP-LTE systems, one method for sending these HARQ acknowledgement bits is to jointly code them with the other control signaling information using a specified Reed-Muller code consisting of 20 coded bits. Even though the resources used for sending this control signaling information can inherently provide a diversity gain, the Reed-Muller code with such a short block size is not good at extracting all of the available diversity. To address this issue, in the second part of this dissertation, we propose two new methods: i) based on complex-field coding (CFC), and ii) using repetition across frequency bands, to extract the inherent diversity available in the channel resources and improve the error protection for the HARQ acknowledgement bits along with the other control signaling information. In the second part of the dissertation, we also propose a new signal space diversity (SSD) scheme, which results in transmit signals having constant envelope (CE). The proposed CE-SSD scheme results in a better overall power efficiency due to the reduced back-off requirements on the radio frequency power amplifier. Moreover, the proposed CE-SSD technique can be useful for application scenarios involving transmission of small number of information bits, such as in the case of control signaling information transmission. In conventional HARQ systems, during the retransmission phase, the channel resources are exclusively used for the retransmitted data packet. This is not optimal in terms of efficient resource utilization. For efficient utilization of channel resources during the retransmissions, a superposition coding (SPC) based HARQ scheme was proposed in the literature. In an SPC based HARQ system, an erroneous packet is transmitted together with a new data packet by superposition in the Euclidean space. In the final part of this dissertation, we study performance of different bits-to-symbol mappings for such an SPC based HARQ system.

In 2008, the European Digital Video Broadcasting (DVB) standardization committee issued the second generation of Digital Video Broadcasting-Terrestrial (DVB-T2) standard in order to enable the wide broadcasting of high definition and 3D TV programmes. DVB-T2 has adopted several new technologies to provide more robust reception compared to the first genaration standard. One important technology is the bit interleaved coded modulation (BICM) with doubled signal space diversity plus the usage of low-density parity check (LDPC) codes. Both techniques can be combined at the receiver side through an iterative process between the decoder and demapper in order to further increase the system performance. The object of my study was to design and prototype a DVB-T2 receiver which supports iterative process. The two main contributions to the demapper design are the proposal of a linear approximation of Euclidean distance computation and the derivation of a sub-region detection algorithm for the two-dimensional demapper. Both contributions allows the computational complexity of the demapper to be reduced for its hardware implementation. In order to enable iterative processing between the demapper and the decoder, we investigated the use of vertical shuffled Min-Sum LDPC decoding algorithm. A novel vertical shuffled iterative structure aiming at reducing the latency of iterative processing and the corresponding architecture of the decoder were proposed. The proposed demapper and decoder have been integrated in a real DVB-T2 demodulator and tested in order to validate the efficiency of the proposed architecture. The prototype of a simplified DVB-T2 transceiver has been implemented, in which the receiver supports both non-iterative process and iterative process. We published the first paper related to a DVB-T2 iterative receiver.