A method for optimally equalizing distortion of an optical data channel carrying coherent optical signals with a given analog bandwidth B1 [GHz] and a given symbol rate B2 [GSymbols/sec], comprising: a) providing a receiving end with a balanced detector (14a, 14b), for receiving and detecting said optical signals; b) reducing the analog bandwidth B1 of detected optical signals by a factor of N by filtering said optical signals using an Anti-Aliasing Filter with a cutoff frequency which corresponds to N, said Anti-Aliasing Filter having deterministic attributes and introducing known deterministic Inter Symbol Interference; c) sampling said optical signals at the output of said Anti-Aliasing Filter by an Analog to Digital Converter, using a sampling rate of a single sample per symbol; d) post processing the sampled optical signals by a digital processor operating at a processing rate being equal to said sampling rate and to said symbol rate B2, such that said post processing represents compensation of said distortion; and e) reconstructing an input data stream by: e.1) inverting a frequency response of said channel and applying an inverse of the channel to said optical signal received after said post processing, to compensate for channel distortions of Chromatic Dispersion, Polarization Mode Dispersion, timing errors, and amplitude and phase distortions, except for the Inter Symbol Interference introduced by said Anti-Aliasing Filter; and e.2) optimally decoding the output of said digital processor using a Maximum Likelihood Sequence Estimator - MLSE decoder being capable of optimally compensating the known deterministic Inter Symbol Interference, introduced by said Anti-Aliasing Filter. a) providing a receiving end with a balanced detector (14a, 14b), for receiving and detecting said optical signals; b) reducing the analog bandwidth B1 of detected optical signals by a factor of N by filtering said optical signals using an Anti-Aliasing Filter with a cutoff frequency which corresponds to N, said Anti-Aliasing Filter having deterministic attributes and introducing known deterministic Inter Symbol Interference; c) sampling said optical signals at the output of said Anti-Aliasing Filter by an Analog to Digital Converter, using a sampling rate of a single sample per symbol; d) post processing the sampled optical signals by a digital processor operating at a processing rate being equal to said sampling rate and to said symbol rate B2, such that said post processing represents compensation of said distortion; and e) reconstructing an input data stream by: e.1) inverting a frequency response of said channel and applying an inverse of the channel to said optical signal received after said post processing, to compensate for channel distortions of Chromatic Dispersion, Polarization Mode Dispersion, timing errors, and amplitude and phase distortions, except for the Inter Symbol Interference introduced by said Anti-Aliasing Filter; and e.2) optimally decoding the output of said digital processor using a Maximum Likelihood Sequence Estimator - MLSE decoder being capable of optimally compensating the known deterministic Inter Symbol Interference, introduced by said Anti-Aliasing Filter. e.1) inverting a frequency response of said channel and applying an inverse of the channel to said optical signal received after said post processing, to compensate for channel distortions of Chromatic Dispersion, Polarization Mode Dispersion, timing errors, and amplitude and phase distortions, except for the Inter Symbol Interference introduced by said Anti-Aliasing Filter; and e.2) optimally decoding the output of said digital processor using a Maximum Likelihood Sequence Estimator - MLSE decoder being capable of optimally compensating the known deterministic Inter Symbol Interference, introduced by said Anti-Aliasing Filter.