A Fast Coarse Frequency Offset Estimation Using Skewness-Based Root Search for Baud-Rate Sampling Coherent Optical Systems

Baud-rate sampling is recognized as an effective technique for reducing power consumption in coherent communication system. However, the absence of an out-of-band spectrum means that any frequency offset (FO) between the local oscillator (LO) and the signal light will lead to a portion of the spectrum being filtered out after the signal passes through an anti-aliasing filter (AAF). This presents a significant challenge for the practical application of baud-rate sampling coherent systems. To address this issue, a novel coarse FO estimation (FOE) scheme that utilizes a skewness-based root search in the frequency domain is proposed, utilizing the naturally existing frequency response at the transmitter side, which results in symmetry of the signal spectrum around the zero frequency. Specifically, the scheme employs spectral skewness as an indicator, identifying the FO at the point where the skewness equals zero. To achieve fast coarse FOE, the false position method is used to iteratively refine the search interval, requiring only two to three iterations to determine the FO within 50 MHz of the FOE error. This approach significantly reduces the number of root searches by a factor of 120 compared to brute-force search algorithms with a 50 MHz step size. The effectiveness of the proposed FOE method is validated through experimental verification on a 118 Gbaud polarization-division multiplexing 16-QAM system. The experimental results indicate that the coarse FOE error is less than 150 MHz with an FFT size of 512. The results demonstrate the potential of the FOE scheme for practical deployment in baud-rate sampling coherent systems.