Distortion compensating apparatus, optical receiving apparatus, and optical transmitting and receiving system

1. A distortion compensating apparatus for compensating for distortion in a waveform of a received light signal through digital signal processing, the distortion compensating apparatus comprising:

a plurality of fixed amount compensators which compensate for the distortion in the waveform at respective given compensating amounts, wherein a combination of operating states of the plurality of fixed amount compensators is changed by on/off switching of each of the plurality of fixed amount compensators; and

a controller which controls the on/off switching and thereby changes the combination, based on the distortion in the waveform caused by an optical transmission path through which the received light signal propagates,

wherein the plurality of fixed amount compensators are cascaded,

wherein each of the plurality of fixed amount compensators has coefficient patterns including an off coefficient pattern that makes the each of the plurality of fixed amount compensators switched off, and

wherein the controller switches the operating state of each of the plurality of fixed amount compensators by selecting a coefficient pattern except the off coefficient pattern when each of the plurality of fixed amount compensators is switched on, and selecting the off coefficient pattern when each of the plurality of fixed amount compensators is switched off.

2. The distortion compensating apparatus according to claim 1 , further comprising a variable compensator which compensates for the distortion in the waveform at a variable compensating amount, wherein

the plurality of fixed amount compensators and the variable compensator are cascaded.

3. The distortion compensating apparatus according to claim 1 , wherein the distortion in the waveform is caused by wavelength dispersion of the received light signal.

4. The distortion compensating apparatus according to claim 1 , further comprising non-linear compensators which are alternately cascaded with the plurality of fixed amount compensators.

5. The distortion compensating apparatus according to claim 2 , wherein the controller variably controls the variable compensating amount of the variable compensator based on the distortion in the waveform and a total compensating amount of operating fixed amount compensators.

6. The distortion compensating apparatus according to claim 2 , wherein the plurality of fixed amount compensators and the variable compensator each include a digital filter which multiplexes an input signal and a coefficient pattern.

7. The distortion compensating apparatus according to claim 6 , wherein the digital filter is a finite impulse response filter or an infinite impulse response filter.

8. A distortion compensating apparatus for compensating for distortion in a waveform of a received light signal through digital signal processing, the distortion compensating apparatus comprising:

a plurality of fixed amount compensators which compensate for the distortion in the waveform at respective given compensating amounts, wherein a combination of operating states of the plurality of fixed amount compensators is changed by on/off switching of each of the plurality of fixed amount compensators;

a variable compensator which compensates for the distortion in the waveform at a variable compensating amount; and

a controller which controls the on/off switching and thereby changes the combination, based on the distortion in the waveform caused by an optical transmission path through which the received light signal propagates,

wherein the plurality of fixed amount compensators are cascaded,

wherein the plurality of fixed amount compensators and the variable compensator are cascaded, and wherein each of the plurality of fixed amount compensators comprises:

an input-end switch which selectively switches between a first output and a second output of an input signal;

a digital filter which inputs therein the first output of the input-end switch and which compensates for the distortion in the waveform of the first output at the given compensating amount; and

a delay processor which inputs therein the second output of the input-end switch and which performs delay processing on the second output; and

an output-end switch which selectively outputs either the first output compensated by the digital filter or the second output performed the delay processing by the delay processor, wherein

the controller switches between the input-end switch and the output-end switch so that the first output compensated by the digital filter is outputted when each of the plurality of fixed amount compensators is switched on, and so that the second output performed the delay processing by the delay processor is outputted when each of the plurality of fixed amount compensators is switched off.

9. The distortion compensating apparatus according to claim 1 , wherein each of the plurality of fixed amount compensators comprises a sign controller which inverts a plus or minus of the given compensating amount of each of the plurality of fixed amount compensators.

10. A distortion compensating apparatus for compensating for distortion in a waveform of a received light signal through digital signal processing, the distortion compensating apparatus comprising:

a plurality of fixed amount compensators which compensate for the distortion in the waveform at respective given compensating amounts,

wherein a combination of operating states of the plurality of fixed amount compensators is changed by on/off switching of each of the plurality of fixed amount compensators,

wherein the plurality of fixed amount compensators are cascaded,

wherein each of the plurality of fixed amount compensators comprises a sign controller which inverts a plus or minus of the given compensating amount of each of the plurality of fixed amount compensators, and wherein the sign controller keeps the sign of an imaginary component of a complex signal inputted into and outputted from each of the plurality of fixed amount compensators when the given compensating amount of each of the plurality of fixed amount compensators is a plus value, and inverts the sign of the imaginary component of the complex signal when the given compensating amount of each of the plurality of fixed amount compensators is a minus value.

11. An optical receiving apparatus comprising a distortion compensating apparatus for compensating for distortion in a waveform of a received light signal through digital signal processing, the distortion compensating apparatus comprising a plurality of fixed amount compensators which compensate for the distortion in the waveform at respective given compensating amounts, wherein a combination of operating states of the plurality of fixed amount compensators is changed by on/off switching of each of the plurality of fixed amount compensators; and

a controller which controls the on/off switching and thereby changes the combination, based on the distortion in the waveform caused by an optical transmission path through which the received light signal propagates,

wherein the plurality of fixed amount compensators are cascaded,

wherein each of the plurality of fixed amount compensators has coefficient patterns including an off coefficient pattern that makes the each of the plurality of fixed amount compensators switched off, and

wherein the controller switches the operating state of each of the plurality of fixed amount compensators by selecting a coefficient pattern except the off coefficient pattern when each of the plurality of fixed amount compensators is switched on, and selecting the off coefficient pattern when each of the plurality of fixed amount compensators is switched off.

12. An optical transmitting and receiving system comprising a distortion compensating apparatus for compensating for distortion in a waveform of a received light signal through digital signal processing, the distortion compensating apparatus comprising a plurality of fixed amount compensators which compensate for the distortion in the waveform at respective given compensating amounts, wherein a combination of operating states of the plurality of fixed amount compensators is changed by on/off switching of each of the plurality of fixed amount compensators; and

a controller which controls the on/off switching and thereby changes the combination, based on the distortion in the waveform caused by an optical transmission path through which the received light signal propagates,

wherein the plurality of fixed amount compensators are cascaded,

wherein each of the plurality of fixed amount compensators has coefficient patterns including an off coefficient pattern that makes the each of the plurality of fixed amount compensators switched off, and

wherein the controller switches the operating state of each of the plurality of fixed amount compensators by selecting a coefficient pattern except the off coefficient pattern when each of the plurality of fixed amount compensators is switched on, and selecting the off coefficient pattern when each of the plurality of fixed amount compensators is switched off.