An opportunistic-based protocol for bidirectional cooperative networks

In this paper, a new opportunistic source selection (OSS) protocol is studied in bidirectional cooperative networks. Unlike existing protocols, this protocol exploits multiuser nature of the bidirectional cooperative networks and it opportunistically supports two traffic flows based on instantaneous channel conditions. This makes the OSS protocol much more reliable than existing protocols. In order to show the performance improvement, we first derive a lower bound of the outage probability of the OSS protocol. Numerical results demonstrate that this lower bound is extremely tight and it indicates that the OSS protocol achieves full diversity order two in a bidirectional cooperative network with two sources and one relay. Then exact and approximate lower bounds of average bit error rates (BERs) at both sources in the OSS protocol are derived. Those lower bounds are very close to the exact average BERs as shown by numerical results. Lastly, an optimum power allocation scheme is developed for the OSS protocol. This scheme can optimize the outage probability, average BER, and data-rate of the OSS protocol at the same time.