Adaptive equalizer, adaptive equalization method, and optical communication system

1. An adaptive equalizer comprising:

an adaptive equalization filter configured to adaptively compensate, by updating a tap coefficient, for a waveform distortion caused by a polarization fluctuation of a received signal;

a first tap coefficient updater configured to calculate a tap coefficient according to the polarization fluctuation of the received signal using a variable step size and update the tap coefficient of the adaptive equalization filter with the calculated tap coefficient by itself;

a second tap coefficient updater configured to receive the received signal as an input and calculate a tap coefficient according to the polarization fluctuation of the received signal using a fixed step size by itself without receiving the an output of the adaptive equalization filter;

a polarization state estimator configured to estimate a polarization state of the received signal using the tap coefficient calculated by the second tap coefficient updater; and

a step size updater configured to obtain a step size corresponding to the polarization state estimated by the polarization state estimator and update the variable step size with the obtained step size.

2. The adaptive equalizer according to claim 1 , wherein each of the first tap coefficient updater and the second tap coefficient updater calculates the tap coefficient using a sequential updating algorithm for obtaining the tap coefficient by updating the tap coefficient for each step size so as to minimize a comparison error between a transmitted known signal and a true value of the known signal.

3. The adaptive equalizer according to claim 1 , wherein the step size updater includes a memory that stores, in advance, step sizes at which optimal error rate characteristics can be obtained for respective predetermined ranges of at least one of a polarization state fluctuation amount and a polarization state fluctuation speed.

4. The adaptive equalizer according to claim 1 , wherein the polarization state estimator includes at least one of

a moving average circuit configured to obtain a moving average of an estimated value of the polarization state of the received signal and supply the obtained average value to the step size updater, and

a peak detection circuit configured to detect a peak from the estimated polarization state fluctuation amount using a hysteresis characteristic.

5. An adaptive equalization method comprising:

a step of executing adaptive equalization filtering of adaptively compensating, by updating a tap coefficient, for a waveform distortion caused by a polarization fluctuation of a received signal;

a first tap coefficient updating step of calculating a tap coefficient according to the polarization fluctuation of the received signal using a variable step size and updating the tap coefficient of the adaptive equalization filtering with the calculated tap coefficient by itself;

a second tap coefficient updating step of receiving the received signal as an input and calculating a tap coefficient according to the polarization fluctuation of the received signal using a fixed step size by itself without receiving the an output of the adaptive equalization filtering;

a polarization state estimation step of estimating a polarization state of the received signal using the tap coefficient calculated in the second tap coefficient updating step; and

a step size updating step of obtaining the step size corresponding to the polarization state estimated in the polarization state estimation step and updating the variable step size with the obtained step size.

6. The adaptive equalization method according to claim 5 , wherein each of the first tap coefficient updating step and the second tap coefficient updating step includes a step of calculating the tap coefficient using a sequential updating algorithm for obtaining the tap coefficient by updating the tap coefficient for each step size so as to minimize a comparison error between a transmitted known signal and a true value of the known signal.

7. The adaptive equalization method according to claim 5 , wherein the step size updating step includes a step of selecting the step size using a memory that stores, in advance, step sizes at which optimal error rate characteristics can be obtained for respective predetermined ranges of at least one of a polarization state fluctuation amount and a polarization state fluctuation speed.

8. The adaptive equalization method according to claim 5 , wherein the polarization state estimation step includes at least one of

a step of obtaining a moving average of an estimated value of the polarization state of the received signal and supplying the obtained average value to the step size updating step, and

a step of detecting a peak from the estimated polarization state fluctuation amount using a hysteresis characteristic.

9. An optical communication system comprising:

a chromatic dispersion compensator configured to compensate for a distortion caused by chromatic dispersion of a received signal;

an adaptive equalizer, according to claim 1 , configured to compensate for a polarization fluctuation of an output signal of the chromatic dispersion compensator; and

a decoder configured to decode an output signal of the adaptive equalizer.

10. The optical communication system according to claim 9 , further comprising an optical receiver configured to receive an optical signal and convert the received optical signal into the received signal.