Protocoles de retransmission pour les communications coopératives & schéma de relayage pour des sources corrélées
Haifa Fares Jridi · HAL (Le Centre pour la Communication Scientifique Directe) · 2011
Cooperative communication has recently been introduced as a new method to push forward reliability and capacity of future wireless networks. The basic idea of cooperative communications is that all nodes in a network may help each other to transmit informa- tion to a destination, under constraints on power, complexity or delay. While a single antenna is employed at each node, cooperative communications allow to share several antennas distributed over different nodes in order to benefit from spatial diversity. Nevertheless, it was shown that cooperative communications suffer from throughput degradationdue to the fixed cooperative phase. Therefore, to overcome this drawback, we can use Hybrid automatic repeat request (HARQ), a common technique used to make a wireless link reliable and transmissions more efficient. However, most of these works explore exclusively the case of one source node assisted by one or multiple relay nodes. Only few works extended the idea of cooperative HARQ to the case of user cooperation. In this thesis, we explore cooperative HARQ protocols over the user cooperation system, a configuration that has not been fully explored yet. The considered system consists of two sources which cooperate to communicate statistically independent data to a single destination. Each source can operate in two different modes: transmission mode, by transmitting its own local information, or relaying mode, by helping the partner node to transmit its information. The first part of the thesis is devoted to investigate the impact of employing hybrid automatic repeat request (HARQ) protocols, which send back ACK or NACK to the transmitting nodes, over a turbo coded cooperation scheme. This cooperative scheme is adopted as a starting point since it integrates user cooperation with capacity approaching channel coding, particularly turbo code. Instead of repeating the received information, the source decodes the partner's information and transmits additional parity bits ac- cording to an overall distributed turbo code scheme. We exploit the limited feedback to define three different cooperative HARQ protocols: destination-level HARQ, relay- level HARQ and two-level HARQ. These protocols aim mainly to preserve the overall system throughput by controlling the cooperation phase at the destination node, and to increase diversity by allowing retransmission to the partner node which strengthens the inter-source link.We further analyze the probability of outage of the proposed protocols, characterize their diversity-multiplexing tradeoff and evaluate them according to different parame- ters: channel conditions, transmission rate, and resource allocation. Furthermore, we analyze the improvement of the proposed cooperative HARQ pro- tocols over the original turbo coded cooperation system, as well as over a conventional point-to-point incremental-redundancy HARQ system, in terms of frame error proba- bility and throughput efficiency. Thanks to this analytical study, we define the system retransmission gain and the system cooperation gain, which serve as decision param- eters to determine the geometric conditions under which cooperative HARQ protocols are useful. These proposed protocols fit well for applications such as wireless vehicular networks. In the second part of the thesis, we consider wireless sensor networks, where neigh- boring sensors measure data that is statistically dependent. Source correlation can be exploited at the receiver side. As a third main contribution of the thesis, we derive bounds on the error probability in an uncoded decode-and-forward cooperative system with two correlated information sources and one relay. Furthermore, we demonstrate the performance gain that can be obtained by exploiting the correlation between the sources and we compare these results to a correlation-aware system without relay.