Network controller and signal quality estimating method

A receiver employing a digital coherent signal processing technique, the receiver comprising:

• a local oscillator configured to generate local oscillated light;
• an optical hybrid circuit configured to mix an input optical signal and the local oscillated light generated by the local oscillator;
• a photoelectric converter configured to convert an optical signal mixed by the optical hybrid circuit into an electric signal;
• an analogue to digital converter configured to convert the electric signal converted by the photoelectric converter into a digital signal;
• a frame synchronizer configured to synchronize a frame in a signal converted by the analogue to digital converter;
• an error corrector configured to measure a signal quality of a frame-synchronized signal generated by the frame synchronizer and correct the frame-synchronized signal with an error code;
• a noise generator configured to generate a noise to add on the frame-synchronized signal, based on a control signal;
• and
• a noise controller configured to generate the control signal to adjust an amount of the noise to be generated by the noise generator, based on the signal quality measured by the error corrector.