Selective bit mapping for Chinese DTMB LDPC coded QAM modulation scheme

In this paper, we introduce selective-bit mapping to improve the performance of the LDPC coded QAM modulation scheme for Chinese Digital Terrestrial Television Broadcasting (DTMB) system. The threshold of message-passing decoding can be lowered by matching the binary components of LDPC codeword to the positions in the m-tuples to be mapped into 2mQAM symbols. By our approach, the pattern of mapping is described by bit-mapping polynomials, based on which density evolution using Gaussian approximation can be applied. We optimize the bit mapping polynomials for the LDPC codes of the Chinese DTMB standard. Numerical results illustrate that by the proposed approach the decoding threshold can be lowered by 0.05dB to 0.499dB by different combinations of code rate and order of QAM modulations. Simulation results show that the actual BER improvement varies from 0.09dB to 0.6dB in single-carrier and OFDM modes.