Adaptive incremental redundancy [WCDMA systems]

We develop an information-theoretic model to explain and predict the coding gains of the incremental redundancy over the Chase combining scheme. From this analysis, we identify a new approach to improve the spectral efficiency of the conventional incremental 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 guarantee high probability of successful decoding. Hence, we propose a new scheme that adapts the modulation and coding formats according to simple feedback information to minimize unnecessary channel usage. Our simulation results demonstrate system throughput gains up to 25% in certain conditions can be achieved by the proposed scheme without modifying the existing WCDMA specifications.