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 inter-symbol interference in a wavelength-divisionmultiplexed (WDM) system. In this paper, we investigate the complexity and performance of transmit side pulse shaping using digital finite-impulse-response (FIR) or frequency-domainequalization (FDE) filters to increase the SE in a polarizationdivision-multiplexed quadrature-phase-shift-keyed (PDM-QPSK) 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 (RRC) spectrum shape has better performances than the raised-cosine (RC) spectrum shape with that channel spacing. A FDE filter with a fast Fourier transform (FFT) size of 64 and an overlap length of 8 points performs slightly better than a 17-tap FIR filter. Index Terms--Coherent communication, pulse shaping, Nyquist filter, digital finite-impulse-response (FIR) filter, digital frequency-domain-equalization (FDE) filter, overlap-and-save FDE