Robust Faster-Than-Nyquist PDM-mQAM Systems With Tomlinson–Harashima Precoding

A training-based channel estimation algorithm is proposed for the faster-than-Nyquist polarization division multiplexed m-ary quadrature amplitude modulation (m = 4, 16, 64) systems with Tomlinson-Harashima precoding (THP). This is robust to the convergence failure phenomenon suffered by the existing algorithm, yet remaining format-transparent. Simulation results show that the proposed algorithm requires a reduced optical signal-to-noise ratio to achieve a certain bit error rate in the presence of first-order polarization mode dispersion and phase noise introduced by the laser linewidth.