Efficient Cooperative HARQ for Multi-Source Multi-Relay Wireless Networks

In this paper, we compare the performance of three different cooperative Hybrid Automatic Repeat reQuest (HARQ) protocols for slow-fading half-duplex orthogonal multiple access multiple relay channel. Channel State Information (CSI) is available at the receiving side of each link only. Time Division Multiplexing is assumed, where each orthogonal transmission occurs during a time-slot. Sources transmit in turns in consecutive time slots during the first transmission phase. During the second phase, the destination schedules in each time-slot one node (source or relay) to transmit redundancies based on its correctly decoded source messages (its decoding set) with the goal to maximize the average spectral efficiency. Bidirectional limited control channels are available from sources and relays towards the destination to implement the necessary control signaling of the HARQ protocols. Among the three proposed HARQ, two follow the Incremental Redundancy (IR) approach. One consists in sending incremental redundancies on all the messages from the scheduled node decoding set (Multi-User encoding) while the other one helps a single source (Single User encoding) chosen randomly. The third one is of the Chase Combining (CC) type, where the selected node repeats the transmission (including modulation and coding scheme) of one source chosen randomly from its decoding set. Monte-Carlo simulations confirm that the IR-type of HARQ with Multi-User encoding offers the best performance, followed by IR-type of HARQ with Single User encoding and CC-type of HARQ. We conclude that IR-type of HARQ with Single User encoding offers the best trade-off between performance and complexity for a small number of sources in our setting.