In-advance Joint Timing/Frequency Synchronization Using FrFT Pilot for Digital Subcarrier Multiplexing Systems

In the digital subcarrier multiplexing (DSM) system, the absence of guard bands between the adjacent low-baud-rate subcarriers exacerbates the impact of frequency offset (FO) on system performance. Hence, FO must be removed before de-multiplexing with root raised cosine (RRC) filters, as well as before chromatic dispersion compensation (CDC). However, traditional frequency offset estimation (FOE) algorithms for the single carrier (SC) system are not suitable. In this paper, we propose an in-advance FOE and frequency offset compensation (FOC) method that uses a specially designed fractional Fourier Transform (FrFT) pilot sequence. Concurrently, this scheme allows for timing synchronization. It has been demonstrated to be robust against amplified spontaneous emission (ASE) noise, small roll-off factor, fiber nonlinear interference, and strong filtering effects. Furthermore, the computational complexity of the coarse timing synchronization is significantly reduced by up to 50%. Numerical and experimental results show that the proposed scheme works effectively in both the back-to-back (B2B) and the long-haul transmission scenarios.