Selective and Opportunistic AF Relaying for Cooperative ARQ: An MLSD Perspective

Selective and opportunistic amplified-and-forward (SOAF) relaying has been shown more effective than the typical opportunistic AF relaying for cooperative automatic repeat request (ARQ). The key to exploit the diversity potential of SOAF ARQ lies in a proper quality control mechanism for the relayed signals. Although quality control methods have been proposed from the viewpoint of capacity outage probability, the methods are shown herein not enough for SOAF ARQ to exploit the full diversity in packet error rates (PERs). Reinvestigating the quality control problem from the perspective of maximum likelihood sequence detection, our analysis shows that the PERs can attain the full spatial diversity offered by relays in every ARQ round only if the thresholds to control the received signal qualities of the forwarding relays increase with the signal-to-noise ratio (SNR). Sufficient conditions on the thresholds settings are proposed, which explicitly characterize the relationship among the threshold requirements, the SNR, and the minimum codeword distance of the employed channel code. An effective threshold searching algorithm is further developed for SOAF ARQ to exploit both the diversity and the SNR gains in PER. Simulations with trellis codes also verify our analysis and the effectiveness of the proposed quality control method.