Subcarrier-Joint Pre- and Post-Equalization in Digital Subcarrier-Multiplexing Systems With Optical Filtering Impairments

In this paper, the pre- and post-equalizers of all subcarriers are jointly optimized to compensate for optical filtering impairments caused by cascaded reconfigurable optical add-drop multiplexers (ROADMs) in digital subcarrier-multiplexing (SCM) systems. In the proposed scheme, equalizers of all subcarriers are jointly optimized based on a batch gradient descent algorithm and a novel loss function for a limited bit precision is designed to achieve the optimization. The performance is evaluated by extensive simulations in 4-subcarrier SCM systems with an aggregate symbol rate of 65 GBaud. Compared to current equalization schemes, the proposed scheme obtains generalized mutual information (GMI) gains of 0.1-0.3 bits/symbol after the transmission with 10 ROADMs in a linear channel with filtering penalties and amplified spontaneous emission (ASE) noise or a nonlinear channel where fiber nonlinearity and chromatic dispersion (CD) are introduced. The performance is also evaluated by experiments in 4-subcarrier SCM systems with an overall symbol rate of 32 GBaud. Transmission scenarios with different filtering impairments are emulated by a WaveShaper with different bandwidths. When the bandwidth of the WaveShaper is 26 GHz, the GMI gains of the proposed scheme over the current equalization schemes are 0.2–0.75 bits/symbol.