Soutenance d'Abdulaziz MOHAMAD - Cooperative Relaying Protocols and Distributed Coding Schemesfor Wireless Multiterminal Networks

2 Mai 2016

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Catégorie : Soutenance de thèse

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In this thesis, we investigate cooperative communication strategies for the slow fading half-duplex Multiple-Access Multiple-Relay Channel (MAMRC), defined as follows: (1) Multiple statistically independent sources communicate with a single destination with the help of multiple relays; (2) Each relay is half-duplex; (3) The links between the different nodes are subject to slow fading and additive white Gaussian noise; (4) Some links may interfere.

The Multiple-Access (MA) part of the channel model, described in time, is generic. MA schemes differ depending on how channel uses are allocated to the senders (sources and relays). In the first part, we assume no channel state information at the transmitters (sources or relays) and no feedback channel between the nodes. We analyze the individual and common outage events of SR/JNCC/JNCD for the slow fading half-duplex MAMRC, the MA part of the channel model, described in time, being generic. The individual and common outage probabilities serve as lower bounds on the Block/packet Error Rate (BLER) for the proposed SR/JNCC schemes. These bounds are tight for finite codewords length (typically a few hundred channel uses). We also examine the behavior of the outage probabilities in the high-SNR regime to determine the diversity order of the cooperative protocol. We present different approaches to implement SR/JNCC at the relays.

In the first approach, the network coding part is based on linear codes over non-binary Galois field (NBNC). We specify a few constraints that the network code must satisfy for the JNCC to achieve the full diversity. For the channel coding part, turbo codes are used to encode the sources' packets, while punctured convolutional codes are used at the relays to generate extra parity bits. Inspired by the earlier work of Jaggi et al., we then come up with a class of simpler very flexible joint network channel binary codes, referred to as Bit-Interleaved XOR (BIXOR) based JNCC. This code construction is not provably full diversity but close to full diversity with high probability. For both classes of JNCC, we apply the concise and elegant factor graph formalism to the decoding problems at the relays and destination, the JNCD algorithms being described as instances of the sum-product message passing algorithm.

In the second part of the thesis, we propose to combine two DF protocols, namely Dynamic Decode-and-Forward (DDF) and Selective Decode-and-Forward (SDF). In Dynamic Selective Decode-and-Forward (D-SDF), the relays decide when they switch from listening to forwarding, which represent an obvious advantage compared to (Static) SDF (S-SDF) to cope with the random nature of wireless environments, and notably with asymmetric error-prone Source-to-Relay (S-R) links. In D-SDF, the condition which determines the switching can vary during the transmission and become less stringent than having successfully decoded all the sources, as in standard DDF. After some time, the relays can adopt an opportunistic behavior and cooperate with any subset of successfully decoded sources. With this additional degree of freedom, sources with poor S-R links will not prevent relays from helping other sources experiencing better link conditions.

Our contribution is twofold: Outage behavior of D-SDF on the one hand, and protocol implementation on the other hand. Regarding the first aspect, we characterize the symmetric individual and common MAMRC outage achievable rates in the case where each (relay) sender employs JNCC and each receiver (relay or destination) implements JNCD. As far as protocol implementation is concerned, we design full-diversity JNCC with optimized coding gain, based on families of rate-compatible multiple distributed turbo codes. We also provide a complete description of JNCD at the receivers (destination), based on the sum-product algorithm. In the third and last part of the thesis, we propose and investigate cooperative Incremental Redundancy Hybrid-ARQ (IR-HARQ) strategies based on Selective Decode and Forward (SDF) relaying for the slow fading Orthogonal Multiple Access Multiple Relay Channel (OMAMRC). In contrast with the system model used in the first two parts, a limited feedback from the destination to the relays and the sources is allowed. The destination uses feedback messages to control the (re)transmission of the different nodes (relays and/or sources) with the aim of improving both the spectral efficiency and the reliability (increasing the possibility of decoding all the packets of the sources). Time slots are used optimally and none of them is wasted.

We show by Monte Carlo simulations based on information outage probabilities that even the simplest feedback strategy relying on common ACK/NACK can improve the throughput of the OMAMRC dramatically compared to the no feedback case. Designing and evaluating practical modulation and coding schemes matched to the described feedback cooperative strategies, i.e., with BLER approaching the information outage probabilities, is a natural future research direction.

La thèse sera suivie d'un pot.