LAUSR

This letter proposes two carrier frequency offset (CFO) estimators by utilizing the likelihood ratio test (LRT) for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems. The zero-information in real and imaginary parts of the effective data subcarriers can be detected by LRT for flexible modulation formats. An LRT-based blind estimator (LRT-BE) algorithm is then proposed by using power fluctuations of the zero-information. An LRT-based simplified fine estimator (LRT-SFE) is subsequently developed to reduce complexity by approximating the cost function as a cosine function. The closed-form expression of CFO estimate is derived by using three arbitrary test values, which is computationally efficient. The blind zero-information detection procedure transfers the LRT-based methods to a sort of semi-blind estimator which improves the estimation accuracy, and the optimal threshold derived by LRT makes the proposed methods be robust to $M$ -QAM modulations. Simulation results verify the accuracy, flexibility and efficiency of the proposed algorithms. At the MSE of 10−4, the SNR gain of LRT-SFE is upto 6.5 dB compared to other blind estimators.