Digital signal processing circuit, method, receiver, and communication system

A receiver comprising:

• a detector configured to coherently receive a polarization-multiplexed optical signal transmitted from a transmitter via a transmission line;
• and
• a digital signal processing circuit configured to perform equalization signal processing on the reception signal coherently received,
• wherein the digital signal processing circuit includes
• a chromatic dispersion compensation filter configured to multiply each of a real component and an imaginary component of each of a first polarization and a second polarization of the reception signal by a filter coefficient that compensates for chromatic dispersion,
• an adaptive equalizer configured to receive input of signals indicating the real component and the imaginary component of the first polarization output from the chromatic dispersion compensation filter and signals indicating the real component and the imaginary component of the second polarization output from the chromatic dispersion compensation filter, to multiply each of the input signals indicating the real component and the imaginary component of the first polarization and the input signals indicating the real component and the imaginary component of the second polarization by a complex impulse response, to add the signals each multiplied by the complex impulse response, and to subject the added signals to phase rotation for carrier phase compensation including a frequency offset for each polarization, and further configured to multiply each of signals indicating phase conjugates of the real component and the imaginary component of the first polarization input and signals indicating phase conjugates of the real component and the imaginary component of the second polarization input by a complex impulse response, to add the signals each multiplied by the complex impulse response, to subject the added signals to rotation reverse to the phase rotation for the carrier phase compensation for each polarization, to add, for each polarization, the signals subjected to the phase rotation for the carrier phase compensation and the signals subjected to the rotation reverse to the phase rotation for the carrier phase compensation, and to output the added signals, and
• at least one memory storing instructions;
• and
• at least one processor configured to execute the instructions to update the phase rotation for the carrier phase compensation and the complex impulse response that is multiplied by the adaptive equalizer, by use of an output of the adaptive equalizer.

A digital signal processing method comprising:

• multiplying, by a chromatic dispersion compensation filter, each of a real component and an imaginary component of each of a first polarization and a second polarization of a polarization-multiplexed optical signal transmitted from a transmitter and received by a receiver by a filter coefficient that compensates for chromatic dispersion;
• multiplying, by an adaptive equalizer configured to receive input of signals indicating a real component and an imaginary component of the first polarization output from the chromatic dispersion compensation filter and signals indicating a real component and an imaginary component of the second polarization output from the chromatic dispersion compensation filter, each of the signals indicating the real component and the imaginary component of the first polarization and the signals indicating the real component and the imaginary component of the second polarization by a complex impulse response, adding the signals each multiplied by the complex impulse response, subjecting the added signals to phase rotation for carrier phase compensation including a frequency offset for each polarization, multiplying each of signals indicating phase conjugates of the real component and the imaginary component of the first polarization and signals indicating phase conjugates of the real component and the imaginary component of the second polarization by a complex impulse response, adding the signals each multiplied by the complex impulse response, subjecting the added signals to rotation reverse to the phase rotation for the carrier phase compensation for each polarization, adding, for each polarization, the signals subjected to the phase rotation for the carrier phase compensation and the signals subjected to the rotation reverse to the phase rotation for the carrier phase compensation, and outputting the added signals;
• and
• updating the phase rotation for the carrier phase compensation and the complex impulse response that is multiplied by the adaptive equalizer, by use of an output of the adaptive equalizer.