Frequency offset drift monitoring: Enabling simultaneously optimum performance and minimum cost of frequency offset estimation

Frequency offset (FO) is an important impairment in coherent transmission systems induced by wavelength mismatch between a free-running local oscillator (LO) and the transmit laser. A fast-Fourier-transform-based FO estimation (FFT-FOE) scheme is commonly employed. The obtained FOE value can be used to compensate FO of a relatively long duration of the signal. On the other hand, the drift of the laser frequency leads to time-varying FO, and FOE should operate in intervals to track the FO drift (FOD) for the best system performance. However, the complexity of FFT is high, and continuous operation of FFT-FOE results in huge energy consumption since it is unclear when FO drifts to an intolerable value. This Letter aims to solve such a problem: How often should FFT-FOE be operated to guarantee system performance with the lowest complexity? Here, we propose a FOD monitoring scheme based on sparse fast Fourier transform, by which optimum performance and minimum cost of FOE are achieved simultaneously. A downsampling process is further proposed to reduce the complexity of subsequent FFT-FOE. The performance is experimentally verified with 28 Gbaud coherent systems.