Patent ID: 8660438

Claim:
A digital coherent receiver comprising: a front end that converts a light signal into an electric signal by using a signal light and a local oscillator light; an A/D convertor that converts the electric signal of the front end into a digital signal; and a processor that calculates a frequency for separating a spectrum of the digital signal converted by the A/D convertor into two halves as a spectrum gravity center, estimates a frequency offset of the digital signal based on the calculated spectrum gravity center, and reduces the frequency offset of the digital signal based on the estimated frequency offset, wherein the spectrum gravity center is calculated according to the following equation spectrum ⁢ ⁢ gravity ⁢ ⁢ center = ∑ i = - N 2 + 1 N 2 ⁢ ⁢ i * P ⁡ ( i ) ∑ i = - N 2 + 1 N 2 ⁢ P ⁡ ( i ) * Δ * 1 fb .