Four-Dimensional Coded Modulation with Bit-Wise Decoders for Future Optical Communications

• DOI: 10.1109/jlt.2015.2396118
• Publication Date: 2015-02-03T19:49:02Z
• Authors: Alex Alvarado; Erik Agrell
• Keywords: Coding gain; Forward error correction; Parity bit; Constellation diagram; Modulation (music)

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 use bit-wise decoder, where detection process (suboptimally) separated into two steps: soft-decision demapping followed by binary decoding. In this paper, decoders are studied from an information-theoretic viewpoint. 4D constellations with up 4096 constellation points considered. Metrics predict post-FEC bit-error rate (BER) analyzed. The mutual information shown fail at predicting BER so-called generalized be much more robust metric. For suboptimal scheme under consideration, it also that transmit receive each polarization quadrature independently (e.g., PM-QPSK, PM-16QAM, PM-64QAM) outperform best designed for uncoded transmission. Theoretical gains as high 4 dB, then validated via numerical simulations low-density parity check codes.