Relay Selection for Two-Way Relay Channels With MABC DF: A Diversity Perspective

We propose relay-selection techniques for a two-way relay channel, where two terminals simultaneously transmit to each other with the help of a relay. In particular, we focus on a multiple-access broadcast two-way relay channel with a decode-and-forward (DF) relaying policy, which is a main technique for bidirectional relay networks. Two main relay selection metrics, which depend on the instantaneous channel conditions and allow a single best relay to assist the bidirectional communication between the two end sources, are investigated. The first one is the conventional max-min technique, and the second one is the max-sum technique that maximizes the instantaneous sum rate. We prove that the max-min relay-selection policy extracts the maximum diversity gain and is a suitable selection scheme for the high signal-to-noise ratio (SNR) regime, whereas the max-sum scheme is appropriate for low SNRs. An intelligent switching between max-min and max-sum is also proposed. The proposed schemes are analyzed in terms of outage probability and diversity gain and are validated via explicit theoretical and numerical results.