Centralized Scheduling Strategies for Cooperative HARQ Retransmissions in Multi-Source Multi-Relay Wireless Networks

In this paper, we investigate centralized scheduling strategies for cooperative incremental redundancy retransmissions in the slow-fading half-duplex multiple access multiple relay channel. Time Division Multiple Access is assumed for the sources and the relays. Sources transmit successively in time slots for the first phase. The second phase consists of a limited number of time slots for retransmissions. In each time slot, the destination schedules a node (being a relay or a source) to retransmit, conditional on the knowledge of the correctly decoded source sets of each node (which is itself for a source). A scheduled relay retransmission uses Joint Network and Channel Coding on its correctly decoded source messages (cooperative retransmission). Several node selection strategies are proposed based on the maximization of the long-term aggregate throughput under a given constraint of fairness. Monte-Carlo simulations show that these strategies outperform the state of the art one based on the minimization of the probability of the common outage event after each time-slot. Moreover, the long-term aggregate throughput reached with these strategies is close to the upper-bound, calculated by the exhaustive search approach. The same conclusion remains valid for both symmetric and asymmetric source rate scenarios.