Comparison of Advanced Detection Techniques for QPSK Signals in Super-Nyquist WDM Systems

We compare the performance of detecting quadrature-phase-shift-keying (QPSK) in a super-Nyquist wavelength division multiplexing system as a QPSK and quadrature duobinary (QDB) signal using maximum likelihood sequence estimation (MLSE) and maximum a posteriori (MAP) criterion with both simulations and experiments. Three different detection schemes are studied: 1) QPSK detection with a multitap linear equalizer followed by a MLSE or MAP detector; 2) QPSK detection with only a MLSE or MAP detector; and 3) QDB detection with a MLSE or MAP detector, where a multitap linear equalizer and a delay-&-add operation are placed before the MLSE/MAP detector to convert a QPSK signal to a QDB signal. Our simulations and experiments show that the QDB detection with multitap MLSE does not provide better performance than that with two-tap MLSE, and the QDB detection with two-tap MLSE has better filtering tolerance than the QPSK detection with linear equalization followed by a multitap MLSE/MAP detector.