The Nyquist wavelength-division-multiplexing technique (N-WDM) enables a solution to achieve high spectral efficiency (SE) in long-haul transmission systems. Compared to polarization-division-multiplexing (PDM) quadrature-phase-shift-keying modulation, multilevel modulation scheme such as PDM 64-ary quadrature amplitude modulation (64QAM) is much more sensitive to intrachannel noise and interchannel crosstalk caused by N-WDM. We experimentally demonstrated the generation and transmission of the 6 × 144-Gb/s N-WDM PDM-64QAM signal over 80-km single-mode fiber SMF-28 with erbium-doped-fiber-amplifier-only amplification. It achieved a net SE of 10 bits/s/Hz assuming 20% forward error correction overhead. This experiment was successfully enabled by digital-signal-processing (DSP) pre-equalization of transmitter-side impairments and DSP post-equalization of the channel and receiver-side impairments. Nyquist-band is considered in the pre-equalization to enhance the tolerance of the PDM-64QAM to the aggressive spectral shaping. The benefit from the proposed Nyquist-band pre-equalization scheme was also experimentally demonstrated.
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