Adaptive equalizer, equalization method, and optical communication system

1. An adaptive equalizer comprising:

an adaptive filter including a first digital filter compensating for a distortion of an input signal, and a first tap coefficient updater adaptively updating a tap coefficient of the first digital filter according to a polarization state of the input signal by a convergence operation;

a second digital filter compensating for the distortion of the input signal;

a second tap coefficient updater adaptively updating a tap coefficient of the second digital filter according to the polarization state of the input signal by a convergence operation; and

a tap-coefficient control circuit setting the tap coefficient converged by the second tap coefficient updater as an initial value of the tap coefficient in the first digital filter which is to be updated by the first tap coefficient updater, arranging the tap coefficients converged by the second tap coefficient updater in descending order of contribution degree to the convergence operation of tap coefficient update in the first tap coefficient updater, judging the tap coefficient not less than upper specified number to be valid and the tap coefficient less than the specified number to be invalid, and setting the tap coefficient of the first digital filter corresponding to the tap coefficient judged to be invalid to zero not to be used in a calculation of the first tap coefficient updater until a next judgment result is made.

2. The adaptive equalizer according to claim 1 , wherein the tap-coefficient control circuit, as a first judgment, collectively arrange I and Q values on an IQ plane in descending order of absolute value for a plurality of tap coefficients updated by the second tap coefficient updater, and judges the tap coefficient not less than the upper specified number to be valid and the tap coefficient less than the specified number to be invalid, and

the larger the absolute value of the tap coefficient, the greater the degree of contribution to the convergence operation of the tap coefficient update in the first tap coefficient updater.

3. The adaptive equalizer according to claim 2 , wherein the tap-coefficient control circuit, as a second judgment for judging the tap coefficient to be valid A, valid B or invalid after the first judgment, judges the tap coefficient to be valid A if the tap coefficient judged to be valid at the first judgment was judged to be valid A or valid B in a previous second judgment, judges the tap coefficient to be valid B if the tap coefficient judged to be valid at the first judgment is a first threshold value or more and was judged to be invalid in the previous second judgment, judges the tap coefficient to be invalid if the tap coefficient judged to be valid at the first judgment is less than the first threshold value and was judged to be invalid in the previous second judgment, judges the tap coefficient to be invalid if the tap coefficient judged to be invalid at the first judgment was judged to be invalid in the previous second judgment, judges the tap coefficient to be valid A if the tap coefficient judged to be invalid at the first judgment is a second threshold value or more and was judged to be valid A or valid B in the previous second judgment, and judges the tap coefficient to be invalid if the tap coefficient judged to be invalid at the first judgment is less than the second threshold value and was judged to be valid A or valid B in the previous second judgment.

4. The adaptive equalizer according to claim 3 , wherein the tap-coefficient control circuit does not use the tap coefficient judged to be valid A but instead sets a previous update result of the first tap coefficient updater as an initial value of the tap coefficient of the first digital filter that is to be updated by the first tap coefficient updater, and sets the tap coefficient judged to be valid B as the initial value of the tap coefficient of the first digital filter which is to be updated by the first tap coefficient updater.

5. The adaptive equalizer according to claim 1 , wherein the first tap coefficient updater updates the tap coefficient of the first digital filter by a sequential update algorithm, and

the second tap coefficient updater updates the tap coefficient of the second digital filter by a sequential update algorithm.

6. The adaptive equalizer according to claim 5 , wherein the adaptive filter includes a third digital filter compensating for the distortion of the input signal using the tap coefficient obtained by converging the update operation of the first tap coefficient updater.

7. The adaptive equalizer according to claim 5 , wherein a step size of the sequential update algorithm of the first tap coefficient updater is changed according to a ratio of the number of tap coefficients judged to be invalid to the overall number of tap coefficients updated by the second tap coefficient updater.

8. The adaptive equalizer according to claim 7 , wherein as the ratio increases, the step size is increased.

9. An equalization method comprising:

compensating for a distortion of an input signal by a first digital filter;

adaptively updating a tap coefficient of the first digital filter according to a polarization state of the input signal by a convergence operation of a first tap coefficient updater;

compensating for the distortion of the input signal by a second digital filter;

adaptively updating a tap coefficient of the second digital filter according to the polarization state of the input signal by a convergence operation of a second tap coefficient updater; and

setting, by a tap-coefficient control circuit, the tap coefficient converged by the second tap coefficient updater as an initial value of the tap coefficient in the first digital filter which is to be updated by the first tap coefficient updater,

wherein the tap-coefficient control circuit arranges the tap coefficients converged by the second tap coefficient updater in descending order of contribution degree to the convergence operation of tap coefficient update in the first tap coefficient updater, judges the tap coefficient not less than upper specified number to be valid and the tap coefficient less than the specified number to be invalid, and sets the tap coefficient of the first digital filter corresponding to the tap coefficient judged to be invalid to zero not to be used in a calculation of the first tap coefficient updater until a next judgment result is made.

10. An optical communication system comprising:

an optical receiver receiving an optical signal and converting the received optical signal into an electrical signal;

an AD converter converting a signal output from the optical receiver into a digital signal; and

an adaptive equalizer comprising:

an adaptive filter including a first digital filter compensating for a distortion of an input signal, and a first tap coefficient updater adaptively updating a tap coefficient of the first digital filter according to a polarization state of the input signal by a convergence operation;

a second digital filter compensating for the distortion of the input signal;

a second tap coefficient updater adaptively updating a tap coefficient of the second digital filter according to the polarization state of the input signal by a convergence operation; and

a tap-coefficient control circuit setting the tap coefficient converged by the second tap coefficient updater as an initial value of the tap coefficient in the first digital filter which is to be updated by the first tap coefficient updater, arranging the tap coefficients converged by the second tap coefficient updater in descending order of contribution degree to the convergence operation of tap coefficient update in the first tap coefficient updater, judging the tap coefficient not less than upper specified number to be valid and the tap coefficient less than the specified number to be invalid, and setting the tap coefficient of the first digital filter corresponding to the tap coefficient judged to be invalid to zero not to be used in a calculation of the first tap coefficient updater until a next judgment result is made,

the adaptive equalizer performing equalization processing that compensates for distortion of a signal output from the AD converter due to polarization fluctuation.