Robust incremental redundancy hybrid ARQ coding for channels with unknown interference

How to choose an optimal number of parity bits at each transmission instant is one of the main problems in incremental redundancy (IR) coding. When the channel state information (CSI) is unknown, a consequence is that IR code throughput may be degraded considerably compared to the complete CSI case. For a communication subject to the unknown CSI distribution, this paper establishes a theoretical basis for a parity bit selection by means of a risk-sensitive optimal control. A set of optimal coding policies are obtained by the dynamic programming which provide a more stable throughput compared to the non-optimized IR coding. These policies have different levels of robustness which are measured by the so-called risk-sensitive parameter contained in the optimization objective function.