A Hybrid ARQ scheme employing bit rearrangement based on low-order modulation

For the Hybrid ARQ system in which the transmitter employs bit rearrangement and the receiver employs bit-level combining scheme, a novel optimization criterion of bit rearrangement is proposed based on minimizing the upper bounds on non-coded bit error rate. The optimization criterion maximizes the minimum absolute value of the ratio which is LLR´s (logarithmic likelihood ratios) mean value to standard deviation. Based on the criterion, a novel Hybrid ARQ scheme applicable to high-order QAM modulation is proposed in this paper, in the scheme the transmitter may retransmit part bits of a symbol using low-order modulation and employ bit rearrangement according to the novel optimization criterion. Numerical results show that the proposed scheme can improve the performance significantly.