Rate-adaptive HARQ in relay-based cooperative transmission

In this paper we analyze the maximum achievable throughput of Hybrid ARQ (HARQ) protocol for transmission over cooperative block-fading channels. It is assumed that the instantaneous channel state information (CSI) is not available at the encoders and only its outdated version is available via the error-free feedback channel. The outdated CSI is then used to adapt the transmission rate of all cooperating nodes. Considering source-relay-destination scenario, we use Dynamic Programming (DP) to find the optimal rate adaptation policy for both the Source node and the Relay node. Such an approach yields a significant increase in throughput compared to non-adaptive fixed-rate HARQ. Moreover, the maximum achievable throughput is shown for the cooperative HARQ channel which is applicable to both adaptive and non-adaptive models.