Four-dimensional coded modulation with bit-wise decoders for future optical communications

A. Alvarado, E. Agrell

Research output: Contribution to journalArticleAcademicpeer-review

65 Citations (Scopus)

Abstract

Coded modulation (CM) is the combination of forward error correction (FEC) and multilevel constellations. Coherent optical communication systems result in a four-dimensional (4D) signal space, which naturally leads to 4D-CM transceivers. A practically attractive design paradigm is to use a bit-wise decoder, where the detection process is (suboptimally) separated into two steps: soft-decision demapping followed by binary decoding. In this paper, bit-wise decoders are studied from an information-theoretic viewpoint. 4D constellations with up to 4096 constellation points are considered. Metrics to predict the post-FEC bit-error rate (BER) of bit-wise decoders are analyzed. The mutual information is shown to fail at predicting the post-FEC BER of bit-wise decoders and the so-called generalized mutual information is shown to be a much more robust metric. For the suboptimal scheme under consideration, it is also shown that constellations that transmit and receive information in each polarization and quadrature independently (e.g., PM-QPSK, PM-16QAM, and PM-64QAM) outperform the best 4D constellations designed for uncoded transmission. Theoretical gains are as high as 4 dB, which are then validated via numerical simulations of low-density parity check codes.
Original languageEnglish
Pages (from-to)1993-2003
Number of pages11
JournalJournal of Lightwave Technology
Volume33
Issue number10
DOIs
Publication statusPublished - 15 May 2015

Keywords

  • Bit-interleaved coded modulation (BICM)
  • bitwise (BW) decoders
  • channel capacity
  • coded modulation (CM)
  • fiber-optic communications
  • low-density parity-check (LDPC) codes
  • nonlinear distortion

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