Quality Analysis of Uniform and PS QAM Signals Distorted by Arbitrary Filtering Effects of WSS in ROADM-based Optical Network

We propose an analytical scheme to estimate the quality of uniform and probabilistically-shaped (PS)
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-ary QAM signals degraded by spectral narrowing due to the arbitrary filtering effect of wavelength-selective switches (WSSs) in optical nodes equipped with reconfigurable optical add-drop multiplexers (ROADMs). We assume that all of the transmission lines and components used in an optical transport have linear responses and define the total transfer function of the entire path of the signal as the product of all the linear responses. We then obtain the discrete impulse response from the total transfer function to calculate the statistical distribution of the amplitude of the received signal with inter-symbol interference (ISI) after spectral narrowing and finally derive mathematical descriptions to calculate the qualities such as the signal-to-noise ratio (SNR), bit error ratio (BER), and normalized generalized mutual information (NGMI). The proposed scheme can predict accurate values of the qualities of signals with a reasonable calculation cost, even for extremely severe cases of spectral narrowing, where the Gaussian distribution assumption for the signal amplitude is not available. We conduct an experiment to verify the accuracy of the proposed scheme, and show a good agreement between analytical and experimental results on the qualities of 32-Gbaud, dual-polarization QPSK, uniform 16QAM, PS-16QAM, and PS-64QAM signals after propagating through cascaded two WSSs that have symmetric and asymmetric passband shapes with 3-dB bandwidths from 20 to 40 GHz.