Information Rates of Next-Generation Long-Haul Optical Fiber Systems Using Coded Modulation

TL;DR

This paper evaluates the achievable information rates of various coded modulation schemes in long-haul optical fiber systems, comparing hard and soft decision decoding, and analyzes their performance with different equalization strategies.

Contribution

It provides a comprehensive comparison of coded modulation decoding structures and identifies optimal formats and strategies for long-haul optical fiber communications.

Findings

Nonbinary HD codes can achieve rates similar to SD decoders.

Switching from symbol-wise to bit-wise decoding with SD has negligible penalty.

HD binary decoders are unsuitable for spectrally-efficient long-haul systems.

Abstract

A comprehensive study of the coded performance of long-haul spectrally-efficient WDM optical fiber transmission systems with different coded modulation decoding structures is presented. Achievable information rates are derived for three different square QAM formats and the optimal format is identified as a function of distance and specific decoder implementation. The four cases analyzed combine hard-decision (HD) or soft-decision (SD) decoding together with either a bit-wise or a symbol-wise demapper, the last two suitable for binary and nonbinary codes, respectively. The information rates achievable for each scheme are calculated based on the mismatched decoder principle. These quantities represent true indicators of the coded performance of the system for specific decoder implementations and when the modulation format and its input distribution are fixed. In combination with the structure of the decoder, two different receiver-side equalization strategies are also analyzed: electronic dispersion compensation and digital backpropagation. We show that, somewhat unexpectedly, schemes based on nonbinary HD codes can achieve information rates comparable to SD decoders and that, when SD is used, switching from a symbol-wise to a bit-wise decoder results in a negligible penalty. Conversely, from an information-theoretic standpoint, HD binary decoders are shown to be unsuitable for spectrally-efficient, long-haul systems.