A novel low-complexity coherent receiver solution is presented to improve spectral efficiency in wavelength-division multiplexing (WDM) systems. It is based on the receiver-side partial-response equalization and maximum-likelihood sequence detection (MLSD) in prefiltered WDM systems. The partial-response equalization shapes the channel into an intermediate state with a known partial response which is finally recovered by MLSD without the need of channel estimation. In this scheme, the severe intersymbol interference induced by the prefiltering can be shared between the partial-response equalization and MLSD. Therefore, a tradeoff can be made between complexity and performance. The feasibility of receiver-side partial-response shaping relaxes the efforts and requirements on the transmitter-side prefiltering, which permits the mature WDM components to implement prefiltering. In addition, the partial-response equalization or shaping structure is also improved based on our prior art, which further simplifies the overall scheme. For near-baudrate-spacing optically prefiltered WDM systems, duobinary response is experimentally proved as a good intermediate response to shape. Due to the short memory of the duobinary response, the complexity of the receiver based on duobinary shaping has been reduced to a low level. As a whole, the proposed scheme provides a good alternative to Nyquist-WDM at comparable spectral efficiencies. With the proposed receiver-side duobinary shaping technique, three sets of experiments have been carried out to verify the improved duobinary shaping scheme and meanwhile demonstrate the main features, including 5 × 112-Gb/s polarization-multiplexed quadrature phase-shift keying (PM-QPSK) WDM transmission over a 25-GHz grid, single-channel 40-Gbaud PM-QPSK experiment, and 30-GHz-spaced 3 × 224-Gb/s PM 16-ary quadrature amplitude modulation transmission.
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