Perturbation based hard decision nonlinearity compensation

A method, comprising, in each of one or more stages in a digital signal processor for a coherent optical receiver:

• obtaining an input stream of soft data symbols;
• generating a stream of perturbation terms that are representative of optical nonlinearity of an optical transmission channel;
• and
• using the perturbation terms to compensate respective ones of the soft data symbols in the input stream of soft data symbols for the optical nonlinearity, wherein:
• the generating of the stream of perturbation terms comprises converting the input stream of soft data symbols to an input stream of hard data symbols and operating on the input stream of hard data symbols to produce the perturbation terms;
• the operating on the input stream of hard data symbols comprises forming weight coefficients and forming, for each of the perturbation terms, a weighted sum of the hard data symbols using the weight coefficients;
• and
• the digital signal processor is configured to produce the input stream of soft data symbols of a first of the one or more stages from a stream of measurements of an optical signal received by the coherent optical receiver,
• wherein the using of the perturbation terms to compensate respective ones of the soft data symbols comprises subtracting the perturbation terms from respective ones of the said soft data symbols to generate the compensated ones of the soft data symbols, and wherein the method is performed in a series of the stages, wherein each stage evaluates a different nonlinear contribution and applies the evaluated different nonlinear contribution to the input stream of soft data symbols, and wherein in each stage after the first of the series, the obtaining comprises obtaining the compensated soft data symbols generated by the preceding one of the stages of the series.