Blind Polarisation Demultiplexing and Carrier Recovery Using FIR-based Variational AutoEncoder Equaliser for Probabilistic Constellation Shaping in Optical Fibre Communications

We investigate through simulations the potential of Finite Impulse Response (FIR)-based Variational AutoEncoder-inspired (VAE-FIR) equaliser for polarisation demultiplexing, Carrier Phase Recovery (CPR), and Carrier Frequency Offset (CFO) estimation in the context of Probabilistic Constellation Shaped (PCS) transmissions in coherent optical fibre communication systems. Additionally, we compare the performance of this novel estimator with the conventional Constant Modulus Algorithm (CMA) and Pilot-Aided Carrier Phase Recovery (PA-CPR). Our study shows that the VAE-FIR clearly outperforms the conventional approach in terms of polarisation demultiplexing, even with PCS where the CMA fails. We also show the ability of the VAE-FIR to track the phase evolution. Its ability to compensate for the carrier's phase effects is however limited to linewidths of a few dozen kHz and a hundred kHz for CFO, showing that the VAE-FIR may be used to compensate for the small residual phase noise or residual frequency mismatch.