Likelihood Ratio Test-Based Blind Carrier Frequency Offset Estimation for Coherent Optical OFDM Systems

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.