Linear chirp z transform based frequency offset estimation algorithm in M-QAM (M-ary Quadrature Amplitude Modulation) coherent optical communication system

The invention provides a linear chirp z transform based frequency offset estimation algorithm in an M-QAM (M-ary Quadrature Amplitude Modulation) coherent optical communication system, which comprises the following steps that: a smaller amount of M-QAM signal receiving sample values are selected for four-power; the FFT (Fast Fourier Transform) is conducted to obtain a frequency corresponding to the maximum spectral value to be recorded as a roughly estimated value; the linear CET (Chirp z Transform) of four-power signals is conducted near the roughly estimated value to obtain a frequency corresponding to the maximum spectral value; and the frequency is divided by four to obtain a frequency offset estimated value of M-QAM signals. The linear chirp z transform based frequency offset estimation algorithm does not adopt a gradient descent method, a Newton method and other methods depending on a step size and the number of symbols, but adopts a method that the frequency is finely estimated by CZT with a fixed comp