Joint Modulation Format Identification and Frequency Offset Estimation Based on Superimposed LFM Signal and FrFT

We propose a novel technique for joint modulation format identification and frequency offset (FO) estimation. The linear frequency modulation signal which is obtained by a direct current signal and Fractional Fourier Transform (FrFT) is superimposed on the data symbols to mark and identify the modulation formats. Simulation results demonstrate our method can identify the DPBPSK, DPQPSK, DP8PSK, DP16QAM, DP32QAM and DP64QAM modulation formats correctly before chromatic dispersion compensation even the optical signal to noise ratio is extremely low. Additionally, due to the excellent time and frequency analysis properties of FrFT, the FO estimation range is greatly extended compared with the traditional Viterbi-Viterbi method.