Training-Aided Joint Frame and Frequency Synchronization for THP FTN Coherent Optical Systems

A joint frame and frequency synchronisation algorithm is proposed for Tomlinson-Harashima precoding-based faster-than-Nyquist (FTN) coherent optical systems. The algorithm is developed by using training sequences based on Golay complementary sequences. Simulation results of a 32-Gbaud FTN system show that the algorithm provides accurate estimates over an optical signal-to-noise ratio (OSNR) range from 10 to 25 dB. When considering the first-order polarisation mode dispersion and using these same training sequences for the equaliser, the algorithm works properly and gives performance similar to when the frequency offset is perfectly known. The required OSNR at a bit error rate of 2×10−2 is about 13 dB and 18.8 dB for 4-QAM and 16-QAM systems, respectively.