Adaptive Incremental Redundancy

We develop an information-theoretic model to explain and predict the coding gains of the incre- mental redundancy over the Chase combining scheme. From this analysis, we identify a new approach to im- prove the spectral efficiency of the conventional in- cremental redundancy scheme, which uses the same modulation and coding formats and, most notably, the same amounts of channel resources for retransmissions as those for the initial transmissions. Depending on the relative channel conditions between the transmissions, fewer incremental redundant bits than those employed in the first transmission might be enough to guaran- tee high probability of successful decoding. Hence, we propose a new scheme that adapts the modulation and coding formats according to simple feedback informa- tion to minimize unnecessary channel usage. Our sim- ulation results demonstrate system throughput gains up to 25% in certain conditions can be achieved by the proposed scheme without modifying the existing WCDMA specifications.