Effective Protocols and Channel Quality Control Mechanisms for Cooperative ARQ with Opportunistic AF Relaying

Incorporating relaying techniques into automatic repeat reQuest (ARQ) in general will provide diversity and throughput enhancements. However, when opportunistic amplify-and-forward (AF) relaying is applied to cooperative ARQ, the system design becomes much more involved. First, our capacity outage analysis shows that the temporal diversities of ARQs with a single AF relay cannot be exploited unless the channel quality to the relay exceeds a threshold. This notion of selective AF relaying is extended to systems with multiple relays in an attempt to jointly explore the temporal and spatial diversities with ARQ. Two types of selective and opportunistic AF relaying schemes are then developed for such kinds of relay-assisted ARQ. And our analysis further shows that the temporal and spatial diversities cannot be fully exploited without the use of overhearing among relays and a proper link quality control mechanism to prescreen the overheard signals. This quality control mechanism is implemented with a set of thresholds designed for each hop of the relaying path. Feasible threshold setting methods are also developed for the proposed ARQ protocols to achieve their potential diversities. In contrast to our designs, the ARQ scheme with the typical opportunistic AF relaying method suffers from severe diversity losses. Simulations also show that the proposed ARQ schemes are more effective in throughput enhancement, and can provide cell-edge users almost three times the throughput gain in comparison with ARQ with no relay-assisted forwarding.