1. In a coherent receiver of an optical communications system, a method of data symbol recovery comprising: a phase noise estimator computing a respective probabilistic phase error for each of a plurality of data symbol estimates; a phase rotator computing a phase rotation based on the probabilistic phase error estimates, using a filter function; the phase rotator applying the phase rotation to at least one data symbol estimate to generate a corresponding rotated symbol estimate; and a decision circuit processing each rotated symbol estimate to generate corresponding decision values of each data symbol; wherein computing the respective probabilistic phase error comprises calculating an expected value of phase noise based on a conditional probability density function, the probability density function being conditioned on at least one symbol estimate.