Superimposed LDPC coded modulation: Construction and optimization

A superimposed Low-density parity-check (LDPC) coded modulation scheme is proposed in this paper, including the mapping rule, the node-by-node demapping algorithm. A bit-level power allocation method is used to achieve the shaping gain on the criterion that the mutual information between channel inputs and outputs is maximized. An optimization method based on Gaussian approximation is introduced to search the degree distributions of the employed LDPC codes. Simulatio results show that a low bit-error rate is achieved within only 1.5 dB from the Shannon capacity of 4PAM with a code length 4000 at spectral efficiency 1bit/s/Hz.