A digital signal processing unit (10, 30, 40, 50, 60, 70, 80, 90, 100) for use in a coherent optical receiver for an optical communications network, the digital signal processing unit comprising: an adaptive equaliser (12, 92) comprising a number of input ports for receiving electrical signals, each electrical signal corresponding to a different state of polarization of an optical signal received by the coherent optical receiver and a number of output ports, wherein each of the output ports is connected to a processing branch, wherein a processing branch comprises: at least one symbol sequence estimator; and a carrier phase estimator comprising an input for receiving signal taped from an output of the processing branch, wherein an output of the carrier phase estimator being connected to a phase adjuster interconnecting the respective output port of the adaptive equaliser and the symbol sequence estimator; and a processing block connected to an output of the carrier phase estimator, an output of the processing branch and at least one of the output of the phase adjuster and the outputs of the adaptive equalizer, wherein the processing block is arranged to: perform simultaneous channel response coefficients estimation and equalizer coefficients estimation; provide resulting channel response coefficients to the symbol sequence estimator; and provide resulting equalizer coefficients to the adaptive equalizer; wherein the processing block is arranged to perform a gradient algorithm consisting of a first relationship for estimating the equalizer coefficients and a second relationship for estimating the channel response coefficients, and wherein the first relationship is arranged to estimate an equalizer coefficient, C ( k + 1), by subtracting a first update value from a preceding equalizer coefficient, C ( k), and the second relationship is arranged to estimate a channel response coefficient, h ( k + 1), by adding a second update value to a preceding channel response coefficient, h ( k), wherein both the first update value and the second update value are proportional to an error, e k, between a selected equalized signal sample output from the adaptive equalizer and an estimated channel sample. an adaptive equaliser (12, 92) comprising a number of input ports for receiving electrical signals, each electrical signal corresponding to a different state of polarization of an optical signal received by the coherent optical receiver and a number of output ports, wherein each of the output ports is connected to a processing branch, wherein a processing branch comprises: at least one symbol sequence estimator; and a carrier phase estimator comprising an input for receiving signal taped from an output of the processing branch, wherein an output of the carrier phase estimator being connected to a phase adjuster interconnecting the respective output port of the adaptive equaliser and the symbol sequence estimator; and at least one symbol sequence estimator; and a carrier phase estimator comprising an input for receiving signal taped from an output of the processing branch, wherein an output of the carrier phase estimator being connected to a phase adjuster interconnecting the respective output port of the adaptive equaliser and the symbol sequence estimator; and a processing block connected to an output of the carrier phase estimator, an output of the processing branch and at least one of the output of the phase adjuster and the outputs of the adaptive equalizer, wherein the processing block is arranged to: perform simultaneous channel response coefficients estimation and equalizer coefficients estimation; provide resulting channel response coefficients to the symbol sequence estimator; and provide resulting equalizer coefficients to the adaptive equalizer; perform simultaneous channel response coefficients estimation and equalizer coefficients estimation; provide resulting channel response coefficients to the symbol sequence estimator; and provide resulting equalizer coefficients to the adaptive equalizer; wherein the processing block is arranged to perform a gradient algorithm consisting of a first relationship for estimating the equalizer coefficients and a second relationship for estimating the channel response coefficients, and wherein the first relationship is arranged to estimate an equalizer coefficient, C ( k + 1), by subtracting a first update value from a preceding equalizer coefficient, C ( k), and the second relationship is arranged to estimate a channel response coefficient, h ( k + 1), by adding a second update value to a preceding channel response coefficient, h ( k), wherein both the first update value and the second update value are proportional to an error, e k, between a selected equalized signal sample output from the adaptive equalizer and an estimated channel sample.