Delay Analysis of Cooperative Truncated HARQ With Opportunistic Relaying

In this paper, we evaluate the delay experienced by Poisson arriving packets for cooperative truncated hybrid automatic repeat request (HARQ) with opportunistic relaying. We derive the theoretical expressions of the expected waiting time and the packet's sojourn time in the queue of truncated opportunistic cooperative ARQ, HARQ I with and without packet combining (PC), and HARQ II with code combining (CC). The analysis is valid for block Nakagami- $m$ fading channels and any number of relays for both amplify-and-forward (AF) and decode-and-forward (DF) relaying. In opportunistic AF cooperative HARQ, packet retransmission is done by the relay offering the highest instantaneous signal-to-noise ratio (SNR) of the relaying link (source–relay–destination). In opportunistic DF cooperative HARQ, retransmission is done by the relay offering the highest instantaneous SNR of the relay–destination link among the relays that have correctly decoded the transmitted packet by the source. If no relay has correctly decoded, the retransmission is made by the source. Simulation results are also provided to verify the tightness of the derived expressions. The results indicate a significant improvement of packet delivery delays when compared with the absence of cooperation.