Cooperative Communications with Opportunistic Nonorthogonal Amplify-and-Forward Relaying

This paper proposes an opportunistic nonorthogonal amplify-and-forward (ONAF) scheme, assisted by intelligent relay selection. Through analysing the mutual information of the scheme, the novel optimal relay selection criterion is proposed along with its implementation strategy. In order to reduce the system complexity, a sub-optimal selection criterion is then provided. The diversity-multiplexing tradeoff (DMT) analysis shows that the proposed scheme can achieve a diversity gain of the order of the number of candidate relays, and a maximal multiplexing gain of 1. Since the previous work on opportunistic relaying were established under the orthogonal constraint, ONAF is one of the most advanced opportunistic relaying schemes. Our numerical results show the ONAF scheme can outperform the existing nonorthogonal and opportunistic relaying schemes where the relays forward the message using the amplify-and-forward (AF) mode. More importantly, it is a flexible cooperative scheme that can reduce network power consumption, alleviate interference caused among relays´ re-transmissions and avoid the negative impact of the weak source-relay-destination channels.