On the Performance of Hybrid-ARQ with Incremental Redundancy and with Code Combining over Relay Channels

In this paper, we consider a relay network consisting of a source, a relay, and a destination. The source transmits a message to the destination using hybrid automatic repeat request (HARQ). The relay overhears the transmitted messages over the different HARQ rounds and tries to decode the data packet. In case of successful decoding at the relay, both the relay and the source cooperate to transmit the message to the destination. The channel realizations are independent for different HARQ rounds. We assume that the transmitter has no channel state information (CSI). Under such conditions, power and rate adaptation are not possible. To overcome this problem, HARQ allows the implicit adaptation of the transmission rate to the channel conditions by the use of feedback. There are two major HARQ techniques, namely HARQ with incremental redundancy (IR) and HARQ with code combining (CC). We investigate the performance of HARQ-IR and HARQ-CC over a relay channel from an information theoretic perspective. Analytical expressions are derived for the information outage probability, the average number of transmissions, and the average transmission rate. We illustrate through our investigation the benefit of relaying. We also compare the performance of HARQ-IR and HARQ-CC and show that HARQ-IR outperforms HARQ-CC.