Generation of Spectrally Efficient Nyquist-WDM QPSK Signals Using Digital FIR or FDE Filters at Transmitters

Nyquist pulse shaping can be used to increase the spectral efficiency (SE) by narrowing the signal spectrum without introducing intersymbol interference in a wavelength-division-multiplexed (WDM) system. In this paper, we investigate the complexity and performance of pulse shaping at the transmitter side using digital finite-impulse-response (FIR) or frequency-domain-equalization (FDE) filters to increase the SE in a polarization-division-multiplexed quadrature-phase-shift-keyed WDM system. Simulation results show that FIR filters with 17 taps allow for a reduction in channel spacing to 1.1 times the symbol rate within a 1 dB penalty. The root-raised-cosine spectrum shape has better performances than the raised-cosine spectrum shape with that channel spacing. An FDE filter with a fast Fourier transform size of 64 and an overlap length of 8 points performs slightly better than a 17-tap FIR filter.