Frequency estimation for optical coherent M-QAM system without removing modulated data phase

For optical coherent M-ary quadrature amplitude modulation (M-QAM) system, the frequency offset can be extracted directly by applying Fast Fourier Transform (FFT) to the signal's argument, without removing the modulated data phase. By categorizing the constellation points and rotating some constellation points by π/4π/4, this algorithm is robust to extract the frequency offset against the noise. Numerical simulations of 16-QAM and 256-QAM coherent systems are presented to demonstrate this algorithm.