Rate Allocation and Adaptation for Incremental Redundancy Truncated HARQ

This paper considers incremental redundancy hybrid ARQ (HARQ) transmission over independent block-fading channels. The transmitter, having no knowledge of the instantaneous channel state information (CSI) can or - allocate the transmission rate knowing the statistics of the channel, or - adapt the transmissions rates using the outdated CSI, i.e., the one experienced by the receiver in the past transmissions that resulted in a packet decoding failure. Aiming at throughput maximization problems under constraint on the outage probability, we show how to optimize the rate-adaptation and rate-allocation policies using dynamic programming framework. Numerical examples obtained in a Rayleigh-fading channel show that rate adaptation provides notable gains over a rate allocation and non-adaptive HARQ, and, for high SNR, only a few transmissions are necessary to approach closely the ergodic capacity.