High throughput Two-Stage Soft/Hard Codecs for Optical Communications

We consider the problem of designing two-stage codecs with one hard decoding stage to reduce the decoder complexity for optical fiber applications. We will show that these schemes offer performance close to the information-theoretical limits while keeping low the decoder complexity. We focus the attention on the design of schemes for the binary input AWGN channel (BAWGN) in the positive SNR regime correspondent to high rates. The encoder includes two-stage encoders and one inner and simple "channel polarizing code". The polarizing code aims at creating a worse channel and a better one from a given BAWGN channel. The rationale of this asymmetry is that "very good channels" admit simpler hard-input decoders instead of more complex soft-input decoders. We observe that for a code-coder working in the positive SNR regime the first stage is a capacity-achieving encoder, coupled with a soft-input iterative decoder, and the second stage is a very high-rate encoder coupled with a simple hard input decoder (BCH or RS-like). The rate splitting between the two stages allows to construct flexible and low complexity encoding schemes for a high SNR range.