Network-Coded Cooperation for Multi-Unicast with Non-Ideal Source-Relay Channels

Network coding is considered as a promising technique to improve diversity gain and network throughput in multi-source relay systems. This paper presents an opportunistic network coded cooperation scheme in wireless multi-unicast system with non-ideal source-relay channels. In the conventional network-coding-based cooperation schemes, the relay merges the messages received from multiple sources and always forwards them to the destinations without checking their reception status. The destinations recover the sources´ messages either from the direct transmissions or from the relay forwarding. Such mode is easy to implement, but may lead to error propagation in the event of decoding failures at relay. In contrast to these works, the proposed scheme is opportunistic, where the relay forwarding is determined by the quality of S → R channels, that is, the relay does not assist transmission unless it has correctly decoded the received sources´ messages. Systematic performance analysis in the form of outage probability and spectral efficiency is performed in this paper. Comparisons with the conventional network coded multi-unicast and Incremental Relaying protocol are made under a fixed system energy constraint. The outage results show that the proposed scheme performs better than the conventional network-coded schemes when source-relay links has poor quality. Comparing with Incremental Relaying protocol, the opportunistic scheme achieves a reduced system outage probability as well as a higher spectral efficiency. This scenario can be extended to general multi-user environment without much cost.