Approaching Nyquist Limit in WDM Systems by Low-Complexity Receiver-Side Duobinary Shaping

Journal article, 2012

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 pre-filtered 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 ISI induced by the pre-filtering 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 pre-filtering, which permits the mature WDM components to implement pre-filtering. 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-pre-filtered 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-Gbit/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-Gbit/s PM 16-ary quadrature amplitude modulation transmission.