Robust and fast coarse frequency offset estimation based on frequency-domain skewness for coherent DSCM systems under severe bandwidth limitations.

A novel, to the best of our knowledge, frequency offset estimation (FOE) scheme is proposed and demonstrated for coherent digital subcarrier multiplexing (DSCM) systems, where frequency offset (FO) leads to severe filtering damage of subcarriers due to bandwidth limitations. The scheme exploits the symmetry of the signal spectrum, which naturally arises from the transmitter's frequency response and introduces frequency skewness as a cost function to search for FO. To achieve fast FOE, the false position (FP) method is employed to iteratively compute frequency shifts. Experimental results show that within 80% of the cumulative distribution function (CDF), the proposed scheme (with an FFT size of 2048) achieves FOE in just three iterations. In the 4-subcarrier system, the maximum error is less than 250 MHz, while in the 6- and 8-subcarrier systems, the maximum error is less than 80 MHz. In a 6-subcarrier system, the proposed scheme exhibits stable performance under different FO values and received optical power (ROP), even under severe filtering conditions where existing schemes tend to degrade or fail. Therefore, the proposed scheme provides a robust FOE solution for coherent digital subcarrier multiplexing systems with strong filtering impairments.