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.
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