BICM in HARQ with Mapping Rearrangement: Capacity and Performance of Practical Schemes

Hybrid ARQ with packet combining for high-order modulations (such as 16-QAM) may be significantly enhanced if the bits-to-symbols mappings are appropriately changed throughout the transmissions. In this paper, we analyze the relationship between such mapping rearrangement (or mapping diversity) and channel coding. We calculate the capacity of the popular bit-interleaved coded modulation (BICM) to draw qualitative and approximate quantitative conclusions that are valid for strong codes approaching the capacity limits. We conclude that the choice/design of the appropriate mapping depends on the targeted spectral efficiency and we demonstrate that certain forms of mapping rearrangement may be counterproductive. We also show that iterative demapping may be successfully applied to significantly reduce the gap between the BICM and coded modulations capacities. The analysis presented is illustrated with the results obtained when the mapping rearrangement is combined with practical turbo codes.