  A training-based channel estimation algorithm is proposed for the
faster-than-Nyquist PDM-mQAM (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 (OSNR) to achieve a certain bit error rate (BER) in the presence of
first-order polarization mode dispersion and phase noise introduced by the
laser linewidth.
