On Large Deviations of HARQ with Incremental Redundancy over Fading Channels

The hybrid automatic repeat request (HARQ) protocol with incremental redundancy (IR) or HARQ-IR protocol is an effective means to transmit signals over unreliable channels. It can achieve ergodic capacity with limited feedback of channel state information (CSI) from a receiver. In this paper, using large deviations, we derive the quality of service (QoS) exponent of the HARQ-IR protocol over fading channels. This QoS exponent allows us to compare the throughput of HARQ-IR and the effective capacity of an adaptive transmission system with a buffer that requires instantaneous CSI. It is shown that the HARQ-IR protocol can be more effective when a higher QoS exponent is desired.