Multistage Bit-Wise Receivers for 4D Modulation Formats in Optical Communications

Over the last years, coded modulation (CM) schemes for coherent fiber-optic communications have gained huge interest in research motivated by the application of four-dimensional (4D) higher-order modulation formats. One possible candidate is the bit-interleaved coded modulation (BICM) paradigm, which however will cause a rate loss compared to the optimum multilevel coding (MLC) approach if no Gray labeling is possible. In this paper, a multistage bit-wise CM scheme to close the rate gap between MLC and BICM is presented. Numerical results for relevant 4D signal sets are provided.