  A training symbol-based equalization algorithm is proposed for polarization
de-multiplexing in quadrature duobinary (QDB) modulated polarization division
multiplexedfaster-than-Nyquist (FTN) coherent optical systems. The proposed
algorithm is based on the least mean square algorithm, and multiple location
candidates of a symbol are considered in order to make use of the training
symbols with QDB modulation.Results show that an excellent convergence
performance is obtained using the proposed algorithm under different
polarization alignment scenarios. The optical signal-to-noise ratio required to
attain a bit error rate of 2*10-2 is reduced by 1.7 and 1.8 dB using the
proposed algorithm, compared to systems using the constant modulus algorithm
with differential coding for 4-ary quadrature amplitude modulation(4-QAM) and
16-QAM systems with symbol-by-symbol detection, respectively.Furthermore,
comparisons with the Tomlinson-Harashima precoding-based FTN systems illustrate
that QDB is preferable when 4-QAM is utilized.
