Optimized variable degree matched mapping for protograph LDPC coded modulation with 16QAM

The protograph LDPC (P-LDPC) coded modulation is an efficient scheme using the variable degree matched mapping (VDMM) based on the protograph structure. We propose a VDMM optimization strategy, as a crucial part for the P-LDPC coded modulation, by checking different permutation patterns between the variable nodes in protograph and the modulated bits. Then, we develop a modified EXIT analysis for evaluating the thresholds for P-LDPC coded modulation. For some specified P-LDPC codes and 16QAM constellations, the optimum patterns for VDMM can be obtained by a limited number of searches based on the modified EXIT analysis. We find that for a certain labeling, the edge distributions of the protogragh structure should be taken into consideration, rather than using the water-filling strategy or BICM approach. Decoding simulations, being consistent with the numerical analysis results, show that P-LDPC coded modulation with the optimum pattern noticeably outperforms that with the conventional water-filling pattern.