1. An apparatus for compensating a nonlinear impairment of a signal traversing an optical communications system, the apparatus comprising: a parallel array of linear convolution filters configured to process a selected set of samples of the signal to calculate an estimate of a nonlinear interference field, the selected set of samples comprising a first sample and a plurality of second samples; and a processor configured to apply the estimated nonlinear interference field to the first sample so as to at least partially compensate the nonlinear impairment, wherein each linear convolutional filter comprises a set of filter taps, each filter tap being configured to compute a product involving a complex coefficient representing an efficiency of nonlinear interaction, and wherein the complex coefficients are quantized to a predetermined number of levels, and the number of complex multiplication operations used to evaluate the output of each linear convolutional filter is less than the number of filter taps.