Filter coefficient control apparatus and method

1. A filter coefficient control apparatus, for controlling a coefficient of a filter of a phase recovering apparatus for an optical coherent receiver, comprising:

a phase offset obtaining means, for obtaining a phase offset between a phase of a symbol received by the optical coherent receiver and a data modulation phase of the symbol;

an autocorrelation calculating means, for calculating an autocorrelation and related statistics of the phase offset; and

a filter coefficient determining means, for determining the coefficient of the filter in accordance with the autocorrelation and related statistics,

wherein the autocorrelation calculating means includes:

a delay sequence obtaining unit, for obtaining a delay sequence by differently delaying the phase offset; and

an autocorrelation sequence obtaining unit, for determining the autocorrelation and related statistics of the phase offset in accordance with the delay sequence.

2. The filter coefficient control apparatus according to claim 1 , wherein the phase offset obtaining means is a subtracter that subtracts a data modulation phase of each symbol in a training sequence input into the phase recovering apparatus from an argument value of each symbol in the training sequence to obtain the phase offset.

3. The filter coefficient control apparatus according to claim 1 , wherein the phase offset obtaining means includes a subtracter and a data modulation phase obtaining unit,

the data modulation phase obtaining unit obtains a data modulation phase of each symbol of an output signal from the phase recovering means, or obtains a data modulation phase of each symbol in a decoded signal obtained by decoding the output signal; and

the subtracter subtracts the data modulation phase from an argument value of each symbol in a signal input into the phase recovering means to obtain the phase offset.

4. The filter coefficient control apparatus according to claim 3 , wherein the data modulation phase obtaining unit obtains a data modulation phase of each symbol in an output signal from the phase recovering apparatus, and comprises:

a 2π/M rounder, for performing 2π/M rounding on the output signal, where M is an order of phase modulation of the symbol;

a multiplier, for multiplying an integer value obtained by the 2π/M rounder with 2π/M; and

an adder, for adding a product obtained by the multiplier with π/M.

5. The filter coefficient control apparatus according to claim 1 , wherein the filter coefficient determining means includes:

an input/output cross-correlation determining unit, for determining cross-correlation between an input and an ideal output of the filter in accordance with the autocorrelation and related statistics of the phase offset;

an input autocorrelation unit, for determining autocorrelation of the input of the filter in accordance with the autocorrelation and related statistics of the phase offset; and

a coefficient calculating unit, for determining the filter coefficient in accordance with the cross-correlation and the autocorrelation.

6. The filter coefficient control apparatus according to claim 5 , wherein the autocorrelation and related statistics is a variance σ n 2 of phase fluctuation caused by channel white noise, a variance σ c 2 of change of the phase offset, and a normalized autocorrelation ρ(k) of the phase offset;

the input/output cross-correlation determining unit determines the cross-correlation R(k) between the input and the ideal output of the filter in accordance with the following equation:

R ( k )=exp{ M 2 σ c 2 [ρ( k )−1]},

where M is an order of phase modulation of a symbol of data input into the phase recovering means; and

the input autocorrelation unit determines the autocorrelation R′(k) of the input of the optimal filter in accordance with the following equation:

R ′( k )=exp{ M 2 σ c 2 [ρ( k )−1 ]}+M 2 σ n 2 δ( k )

where δ(k) is an impulse function, and the value of δ(k) is 1 if the k is 0 while the value of δ(k) is 0 if k is a value other than 0, and where k is an integer and −M<k<M.

7. A filter coefficient control apparatus, for controlling a coefficient of a filter of a phase recovering apparatus for an optical coherent receiver, comprising:

a phase offset obtaining means, for obtaining a phase offset between a phase of a symbol received by the optical coherent receiver and a data modulation phase of the symbol;

an autocorrelation calculating means, for calculating an autocorrelation and related statistics of the phase offset; and

a filter coefficient determining means, for determining the coefficient of the filter in accordance with the autocorrelation and related statistics,

wherein the autocorrelation calculating means includes:

a delay sequence obtaining unit, for obtaining a delay sequence by differently delaying the phase offset;

a noise suppressing unit, for suppressing noise in the delay sequence; and

an autocorrelation sequence obtaining unit, for determining the autocorrelation and related statistics of the phase offset in accordance with the delay sequence having been suppressed of noise.

8. A method for controlling a filter coefficient, for controlling a coefficient of a filter of a phase recovering apparatus for an optical coherent receiver, comprising:

obtaining a phase offset between a phase of a symbol received by the optical coherent receiver and a data modulation phase of the symbol;

calculating an autocorrelation and related statistics of the phase offset; and

determining the coefficient of the filter in accordance with the autocorrelation and related statistics,

wherein the calculating comprises:

obtaining a delay sequence by differently delaying the phase offset; and

determining the autocorrelation and related statistics of the phase offset in accordance with the delay sequence.

9. The method for controlling a filter coefficient according to claim 8 , wherein the obtaining subtracts a data modulation phase of each symbol in a training sequence input into the phase recovering apparatus from an argument value of each symbol in the training sequence to obtain the phase offset.