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

TL;DR

The paper tackles the challenge of quantifying achievable information rates for next-generation coded modulation in long-haul optical fiber systems using practical decoders. It adopts the mismatched decoder framework to derive AIRs for soft- and hard-decision decoders with bit-wise or symbol-wise demapping, across PM-$M$QAM formats up to $M=256$ and two receiver equalization schemes (EDC and DBP). Key findings show that nonbinary hard-decision codes can approach soft-decision AIRs and that bit-wise soft-decoding incurs negligible penalties relative to symbol-wise decoding; however, hard-decision binary decoders are unsuitable for high spectral efficiency. With full-field DBP, AIRs can reach up to about $12$ bits/sym for distances up to $6000$ km for $PM$-$16$QAM, and even higher performance is achievable with higher-order formats and SD decoding, guiding practical transceiver design and complexity trade-offs for long-haul CM optical 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.

Figures (9)

• Figure 1: General schematic of the optical communication system analyzed in this work.
• Figure 2: Two different implementation alternatives for the CM encoder in Fig. \ref{['fig:SystemSchematic']}.
• Figure 3: The four CM decoder implementations analyzed in this work.
• Figure 4: Graph showing relationships between the information-theoretic quantities presented in this paper. Lines between nodes indicate an inequality, where the arrows point towards the upper bound. Dotted arrows indicate inequalities which become equalities for the AWGN channel.
• Figure 5: AIRs vs. SNR for different modulation formats for the AWGN channel.