Adaptive post digital filter and inter-symbol interference equalizer for optical communication

An apparatus for recovering information bits from a modulated signal in which information bits are modulated using a coherent modulation technique, the apparatus comprising:

• a radio frequency front end that receives the modulated signal in an analog format;
• an analog-to-digital convertor that converts the modulated signal in the analog format into a digitized signal;
• an estimator that estimates, by processing the digitized signal, a channel impairment in the modulated signal and a modulation carrier used for the coherent modulation technique;
• a filter that filters the processed digitized signal operation using a finite impulse response digital post-filter comprising a number of filter coefficients to generate a filter output;
• and
• an inter-symbol-interference (ISI) equalizer that performs equalization on the filter output to produce estimates of the information bits;
• a performance evaluation module that evaluates a performance of the estimates of the information bits;
• and
• an adaptation module that calculates, using the estimates of the information bits, the filter coefficients by providing a feedback signal to change the filter coefficients to optimize the performance;
• wherein the performance evaluation module includes:
• a performance estimation module that calculates an amount of samples of the filter output that fall within a predetermined window in a constellation map of the coherent modulation technique.

An optical communication system comprising:

• an optical signal transmitter that produces an optical signal in an analog format comprising information bits using a coherent modulation technique;
• an optical transmission network that carries the optical signal;
• and
• an optical signal receiver to:
• receive, via the optical transmission network, the modulated signal in the analog format;
• convert the modulated signal in the analog format into a digitized signal;
• estimate, by processing the digitized signal, a channel impairment in the modulated signal and a modulation carrier used for the coherent modulation technique;
• filter the processed digitized signal operation using a finite impulse response digital post-filter comprising a number of filter coefficients to generate a filter output;
• perform inter-symbol-interference (ISI) equalization on the filter output to produce estimates of the information bits;
• evaluate a performance of the estimates of the information bits;
• and
• adaptively adjust, using the estimates of the information bits, the filter coefficients by providing a feedback signal to change the filter coefficients to optimize the performance;
• wherein the evaluating the performance includes:
• calculating an amount of samples of the filter output that within a predetermined window in a constellation map of the coherent modulation technique.