the limiting operation the signal level does not change but the pulse width of the signal changes according to the rising edge timing and the falling edge timing of the signal therefore as long as the rising and falling timings are stable in the signal the decision threshold is kept substantially constant in the limiting operation as shown in fig2 the controller 18 calculates an optimal decision threshold based on the above operations of the limiting amplifier 14 the dc feedback amplifier 15 controls the dc level of the feedback signal to the limiting amplifier 14 based on the decision threshold set by the controller 18 to control the dc level of the positive signal and the negative signal output from the limiting amplifier 14 fig7 is a flowchart of a decision threshold setting process performed by the controller 18 the controller 18 receives the monitor signal indicating the receiving power of an optical signal from the power monitor 11 and sets an initial value of the decision threshold step s 1 then the controller 18 calculates an initial value of the error rate based on the initial value of the decision threshold and the threshold control signal from the fec 17 step s 2 the controller 18 determines whether the error rate satisfies a predetermined condition step s 3 when the.