An opportunistic protocol for two-way relaying systems with direct link

In this paper, a novel opportunistic protocol is proposed for the two-way relaying systems with direct link. Unlike the existing protocols, the proposed protocol opportunistically exploits analog network coding and the direct link. The proposed protocol switches its strategies to support two traffic flows simultaneously or support only one traffic flow based on instantaneous channel conditions. An extremely tight lower bound of the outage probability is derived, which reveals that full diversity order two is achieved by the proposed protocol. It is demonstrated that the proposed protocol outperforms the existing protocols which also utilize the direct link in the two-way relaying systems in terms of both outage probability and sum-rate. In addition, it is indicated that the proposed protocol has a better outage performance than the analog network network coding (ANC) protocol and larger sum-rate than the opportunistic source selection (OSS) protocol.