Efficient non-binary QC-LDPC coded modulation scheme for long haul optical systems

Abstract Coded modulation which combines channel codes with high dimensional modulation schemes is an attractive technique to improve the spectral efficiency in high speed optical transmission systems. Non-binary low density parity check (NB LDPC) codes can provide larger coding gains with smaller codword lengths compared to its binary counterpart, in addition to reducing the latency of the system. Majority of works based on NB LDPC coded modulation focus on bit interleaved coded modulation (BICM) and the widely used additive white Gaussian noise (AWGN) channel model. However, the Gaussian channel model is not a realistic model for long haul intensity modulated direct detection (IM/DD) optical systems using EDFA amplifiers. In this paper, it is mathematically proved that noise in the received signals after photodetection, can be more accurately represented using chi-square distribution. The efficiency of coded modulation depends up on the mapping between coded symbols and constellation signal points of the modulation scheme. Hence, a symbol interleaved coded modulation with subcarrier modulation for non-binary LDPC codes, based on proposed channel model is presented in this paper to improve the BER performance. The simulation results show that the proposed coded modulation for NB LDPC codes with M-ary quadrature amplitude modulation (QAM) schemes provides better performance than BICM. The analysis of error performance of the proposed system gives a coding gain of 1 dB at BER of 10−7 compared to that of AWGN channel model. The performance improvement of the system is also evaluated in terms of spectral efficiency and link distance.