Delay Analysis for Adaptive Hybrid ARQ Protocol

This paper analyzes transmission delavs of an adaptive hybrid ARQ communications protocol used with a discrete-time two-state Markovian channel; this channel miodel allows for both time-varying noise levels and memory. The protocol adapts to changes in channel state, i.e., changes in noise level, by selecting one of two block error-correction codes for each packet according to an estimate of the next channel state; the estimate, based on the error pattern in the previous packet, is fed back to the transmitter via the acknowledgment. Our analysis shows that for some channels with moderate or long dwell times, i. e., infrequent changes of channel state, the adaptive hybrid ARQ protocol can dramatically reduce transmission delays; the regime of channel parameters for which this protocol is useful is thus wider than indicated by previous analyses of throughput improvement alone. For some channels evaluated, both the mean and standard deviation of the delay were reduced by factors of more than 3 (the reduction in the standard deviation was sometimes more than 4).