A Novel LDPC-BICM-ID Scheme with Rotated Mapping QAM in Underwater Acoustic Communication

Low density parity check (LDPC) code has a flexible low decoding complexity and its performance is close to Shannon limit. In the bit-interleaved coded modulation with iterative decoding (BICM-ID), the iterative information exchange between the soft demapper and decoder benefits the bit error rate (BER) performance improvement. For BICM-ID with 16-quadrature amplitude modulation (16-QAM), the bits experience different protection depending on their positions within the symbol, which is known as unequal error protection (UEP). To further improve the LDPC-BICM-ID performance in underwater acoustic communication, a key-bit protection method is developed and the rotating mapping scheme is adopted in this paper. Simulation results show that in comparison with the LDPC-BICM-ID of Gray mapping 16-QAM, our proposed scheme can procure a gain of about 0.8dB at BER of 10
^-5
.