Advances in Detection and Error correction For coherent Optical Communications: Regular, Irregular, and Spatially Coupled LDPC Code Designs

This chapter focuses on soft-decision decoding of low-density parity-check (LDPC) codes and the evolving spatially coupled (SC) LDPC codes. It first describes the effect of differential coding on coherent optical communication systems and especially on the maximum conveyable information rate (the so-called capacity). The chapter gives a rigorous description of higher-order modulation schemes frequently used in coherent communications and introduces the channel model taking into account phase slips that are due to imperfect phase estimation in the coherent receiver. It also gives an introduction to LDPC codes and shows how irregular LDPC codes can be designed to be well-adapted to differential coding. The chapter considers the case of differential coding and shows different possibilities of degree distribution optimization with the goal to find the best possibility for LDPC-coded differential modulation with the decoder scheduling.