A progressive edge growth algorithm for bit mapping design of LDPC coded BICM schemes

In this paper, we consider the design of the bit mapping in low-density parity-check (LDPC) coded bit-interleaved coded modulation (BICM) schemes. We introduce a two-layer bipartite graph to represent the LDPC coded BICM scheme where a new bit mapping graph linking sub-channels to variable nodes (VNs) is added to the conventional Tanner graph. We propose a progressive edge growth (PEG) algorithm to design the bit mapping for the BICM scheme. The design paradigm is to provide more protections to the VNs that are allocated to the sub-channels with the lowest mutual information. We define a novel concept of unreliable depth profile to classify the reliability of the VNs. By connecting more reliable edges to the least reliable VNs, we can significantly improve the reliable edge distribution for the unreliable VNs, thus improve the extrinsic information in the iterative decoding. The proposed bit mapping algorithm is employed for the design of the LDPC coded BICM with 64-QAM and 256-QAM. Simulation results show that the proposed design can considerably improve the error performance, compared to the conventional consecutive bit mapping strategy.