Opportunistic nonorthogonal amplify-and-forward cooperative communications

Introduction: Cooperative communications [1] create diversity gains for network users with a single antenna. The classical cooperative scheme consists of two orthogonal time slots (TS): a broadcasting TS for a source (S) to transmit its signal to the intended destination (D), and a relaying TS for a relay (R) to retransmit S’s signal to D. There are amplify-and-forward (AF) and decode-and-forward (DF) cooperative strategies. To enhance system performance, nonorthogonal cooperative transmission that allows S to continue to transmit during the relaying TS [2], and distributed cooperation that introduces multiple relays for signal retransmissions [3] were introduced. Integrating the two approaches, the distributed nonorthogonal AF (DNAF) scheme [4] was proposed. It has a better diversity-multiplexing trade-off (DMT) [5] performance than the orthogonal distributed cooperation schemes [3]. However, distributed cooperation brings technical challenges such as user interference and power consumption owing to the retransmissions of multiple relays. Opportunistic relaying cooperation [6] alleviates those challenges by always selecting the best relay for signal retransmission. It was shown that provided the same number of relays are available, opportunistic relaying can achieve the sameDMT performance as distributed cooperation. However, most of the current research on opportunistic relaying [6, 7] is restricted to orthogonal transmission, limiting the ultimate potential of system performance. Results concerning the theoretical and simulation performance of nonorthogonal opportunistic relaying are unknown. Addressing this issue, this Letter proposes an opportunistic nonorthogonal AF (ONAF) scheme. The DMT analysis and simulation results of the scheme are presented.