%0 Journal Article
%@ 0733-8724
%A Alvarado, A
%A Agrell, E
%D 2015
%F discovery:1461283
%J Journal of Lightwave Technology
%K Bit-interleaved coded modulation,  bit-wise decoders,  channel capacity,  coded modulation,  fiber-optic communications,  nonlinear distortion,  low-density parity-check codes
%N 10
%P 1993-2003
%T Four-Dimensional Coded Modulation with Bit-wise Decoders for Future Optical Communications
%U https://discovery.ucl.ac.uk/id/eprint/1461283/
%V 33
%X 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.
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