We propose a joint digital preequalization scheme for a Nyquist wavelength division multiplexing (N-WDM) system to compensate for the intersymbol interference caused by a limited channel bandwidth or aggressive prefiltering. A digital raised-cosine pulse filter is implemented before the reference-based preequalization scheme to enhance the tolerance of the signal to the crosstalk from adjacent channels. The experiment illustrates eight-channel digital–analog-converter generation of the polarization division multiplexing quadrature phase shift keying system at 112 Gb/s per channel on a 25-GHz grid, adopting the reference-based digital preequalization considering narrow-band optical filtering effect at the transmitter. The experimental results show that the proposed preequalization scheme allows for an ultradense channel spacing of 0.89 times of the baud rate in the super N-WDM system with a as large as 5-dB optical signal-to-noise ratio improvement.
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