A Novel Hybrid ARQ Scheme Based on LDPC Code Extension and Feedback

The design of an efficient hybrid automatic repeat request (ARQ) scheme based on rate compatible low density parity check (LDPC) codes is considered. It has been shown that extending as well as puncturing of LDPC codes can produce good rate compatible LDPC codes for the additive white Gaussian noise channel. One issue with the traditional LDPC-based hybrid ARQ methods is that the throughput drops off significantly at low signal-to-noise ratios (SNRs). In this paper we introduce a coding scheme which is capable of using puncturing, extending and feedback at the same time to address this issue. Appropriate choice of feedback functions along with optimum combining of received signals for the belief propagation decoder mitigates the throughput drop- off issue at low SNRs, while having a small feedback overhead from the receiver. A powerful mother code is generated via the progressive edge growth algorithm and is used for puncturing and extending in the proposed scheme. Clustering the codewords of the longest codebook is used to decrease the overhead of the feedback connection. Simulation analysis of the throughput shows that our scheme could get as close as 0.5 dB to the Shannon limit while having up to 2 dB gain compared to previous works at low SNRs.