Achievable Information Rates for Coded Modulation with Hard Decision Decoding for Coherent Fiber-Optic Systems

摘要

We analyze the achievable information rates (AIRs) for coded modulationschemes with QAM constellations with both bit-wise and symbol-wise decoders,corresponding to the case where a binary code is used in combination with ahigher-order modulation using the bit-interleaved coded modulation (BICM)paradigm and to the case where a nonbinary code over a field matched to theconstellation size is used, respectively. In particular, we consider harddecision decoding, which is the preferable option for fiber-optic communicationsystems where decoding complexity is a concern. Recently, Liga \emph{et al.}analyzed the AIRs for bit-wise and symbol-wise decoders considering what theauthors called \emph{hard decision decoder} which, however, exploits \emph{softinformation} of the transition probabilities of discrete-input discrete-outputchannel resulting from the hard detection. As such, the complexity of thedecoder is essentially the same as the complexity of a soft decision decoder.In this paper, we analyze instead the AIRs for the standard hard decisiondecoder, commonly used in practice, where the decoding is based on the Hammingdistance metric. We show that if standard hard decision decoding is used,bit-wise decoders yield significantly higher AIRs than symbol-wise decoders. Asa result, contrary to the conclusion by Liga \emph{et al.}, binary decoderstogether with the BICM paradigm are preferable for spectrally-efficientfiber-optic systems. We also design binary and nonbinary staircase codes andshow that, in agreement with the AIRs, binary codes yield better performance.