Signal processing apparatus, signal processing method and communication system

A signal processing method comprising:

• separating, for subcarriers, each of a real component and an imaginary component of each polarized wave of a polarization-multiplexed reception signal and, for each subcarrier of each polarized wave, performing imaginary unit multiplication processing of multiplying the imaginary component by an imaginary unit and then performing complex signal processing that is any of (i) processing of branching the imaginary component multiplied by the imaginary unit and adding one of the branched imaginary components to the real component, (ii) processing of branching the real component and adding one of the branched real components to the imaginary component multiplied by the imaginary unit, and (iii) processing of branching each of the real component subjected to convolution and the imaginary component multiplied by the imaginary unit, adding one of the branched imaginary components to one of the branched real components, and subtracting another of the branched imaginary components from another of the branched real components;
• for each subcarrier of each polarized wave, generating, as input signals, the real component and the imaginary component of each polarized wave after the complex signal processing of the subcarrier and phase conjugates of the real component and the imaginary component of each polarized wave after the complex signal processing of a symmetric subcarrier that is another subcarrier in a frequency domain symmetric to a frequency domain of the foregoing subcarrier across a center frequency of the reception signal;
• for each subcarrier of each polarized wave, generating a first addition signal obtained by a process in which each of the real component and the imaginary component of each polarized wave of the subcarrier included in the input signals is multiplied by a complex impulse response, then resulting components are added, and further a phase rotation for frequency offset compensation is applied, and generating a second addition signal obtained by a process in which each of the phase conjugate of the real component and the phase conjugate of the imaginary component of each polarized wave of the symmetric subcarrier included in the input signals is multiplied by a complex impulse response, then resulting components are added, and further a phase rotation opposite to the phase rotation for frequency offset compensation is applied;
• and
• for each subcarrier of each polarized wave, adding or subtracting a transmission data bias correction signal to or from a signal obtained by adding the first addition signal and the second addition signal.

A signal processing apparatus comprising:

• a first compensator that convolves an impulse response for compensating for a frequency characteristic of a receiver and a complex impulse response for wavelength dispersion compensation with each of a real component and an imaginary component of each polarized wave of a polarization-multiplexed reception signal;
• a complex signal processor that, for each polarized wave, performs imaginary unit multiplication processing of multiplying the imaginary component subjected to convolution by an imaginary unit and then performs complex signal processing that is any of (i) processing of branching the imaginary component multiplied by the imaginary unit and adding one of the branched imaginary components to the real component subjected to convolution, (ii) processing of branching the real component subjected to convolution and adding one of the branched real components to the imaginary component multiplied by the imaginary unit, and (iii) processing of branching each of the real component subjected to convolution and the imaginary component multiplied by the imaginary unit, adding one of the branched imaginary components to one of the branched real components, and subtracting another of the branched imaginary components from another of the branched real components;
• an input signal generator that, for each polarized wave, generates, as input signals, the real component and the imaginary component of each polarized wave after the complex signal processing and phase conjugates of the real component and the imaginary component of each polarized wave after the complex signal processing;
• an equalizer that, for each polarized wave, generates a first addition signal obtained by a process in which each of the real component and the imaginary component of each polarized wave included in the input signals is multiplied by a complex impulse response, then resulting components are added, and further a phase rotation for frequency offset compensation is applied, and generates a second addition signal obtained by a process in which each of the phase conjugate of the real component and the phase conjugate of the imaginary component of each polarized wave included in the input signals is multiplied by a complex impulse response, then resulting components are added, and further a phase rotation opposite to the phase rotation for frequency offset compensation is applied;
• and
• a second compensator that, for each polarized wave, adds or subtracts a transmission data bias correction signal to or from a signal obtained by adding the first addition signal and the second addition signal.