Soft Turbo HARQ combining

In this paper, we consider a hybrid automatic repeat request (HARQ) system with bit-interleaved coded modulation over wireless channels. At higher-order modulations, bit-level combining of log-likelihood ratios (LLRs) is sub-optimal compared to symbol-level maximal ratio combining (MRC). Since symbol level combining is not always feasible, we propose novel bit-level LLR HARQ combining techniques that deploy a modified Turbo principle at the receiver. Our proposed combining methods make use of the information available from previous transmissions to improve the performance of both detection and decoding at the current transmission. We analyze the proposed combining schemes using modified information transfer charts. We show that significant coding and throughput gains can be achieved using our proposed HARQ combining schemes with minimal extra memory requirements.