Adaptive coded modulation for nonlinear fiber-optical channels

A low complexity hybrid polar low-density parity check (LDPC) coded modulation (CM) scheme is introduced for a single-wavelength fiber-optical channel. In the proposed scheme, we exploit a low complexity probabilistic shaping together with a four-dimensional (4D) mapping to reduce the complexity of a 4D non-binary LDPC CM scheme for fiber-optic channels. The proposed scheme has a flexible structure and it can be used as an adaptive-rate CM scheme. The numerical results show that the forward error correction (FEC) threshold of the introduced CM scheme can be significantly improved by probabilistic shaping with a negligible increase in the system complexity. In particular, the FEC threshold for the uncoded symbol error rate of the introduced CM scheme with 4D 16-ary quadrature amplitude modulation can be improved from 0.058 to 0.072 by exploiting a shaping overhead (redundancy) of 0.016 for an information bit error rate of 10^-5.