The investigation of mapping schemes for LDPC-coded modulated systems

In conventional bit-interleaved coded modulation (BICM) system with convolutional codes, Gray mapping has been proved to be the best candidate over the additive white Gaussian noise (AWGN) channel. However, when it comes to LDPC-coded BICM and BICM with iterative decoding (BICM ID) systems, Gray mapping doesn\´t always promise better performance. A simple class of labeling maps that significantly improves the bit error rates (BER) is presented. Meanwhile, we found out that in these systems the overall performance of modulation depends not merely on the constellation schemes, but also the decoding strategy. Compared with the conventional parameters based Euclidean distance, mutual information theory is proved to be a more accurate tool. The simulation results show that only when combined with the suitable mapping and the "state-of-art" decoding algorithms, the BICM system can give full play to their potential.