Multilevel Coding With Spatially Coupled Repeat-Accumulate Codes for High-Order QAM Optical Transmission

We propose multilevel coding (MLC) with spatially coupled codes to increase the net coding gain (NCG) and reduce the power consumption of forward error correction (FEC) codes in high-order quadratic-amplitude modulation (QAM) optical transmissions. We optimized the degree distribution of the spatially coupled repeat-accumulate (SC-RA) codes by using density evolution to enhance the effect of spatial coupling with a limited code length. We constructed multilevel codes with the SC-RA codes and investigated their performance in numerical simulations. NCGs of 12.5, 13.2, and 13.7 dB were obtained for 16, 64, and 256 QAM. We found that MLC is more efficient in terms of circuit size and computational complexity than the conventional FEC configuration of bit-interleaved coded modulation (BICM). Moreover, its efficiency tends to increase as the modulation order increases. Our estimates suggest that the power consumed in the FEC process could be reduced to about half for 16 QAM, 35% for 64 QAM, and 15% for 256 QAM compared with when BICM is used to obtain similar NCG values.