Filtered Carrier Phase Estimator for High-Order QAM Optical Systems

We investigate, using Monte Carlo simulations, the performance characteristics and limits of a low-complexity filtered carrier phase estimator (F-CPE) in terms of cycle slip occurrences and SNR penalties. In this work, the F-CPE algorithm has been extended to include modulation formats whose outer-ring symbols have a QPSK symmetry, and which are applicable to metro and long-haul optical networks. The proposed joint-polarization approach, where the number of non-null symbols in a simplified QPSK partition is increased, shows a further improvement in robustness against cycle slips, resulting in cycle-slip-free operation at symbol rate 32 GBd and laser linewidths up to 900 kHz, for the range of investigated signal to noise ratios. In addition, it reduces SNR penalties for only a small incremental complexity. We also propose a method for constellation alignment that exploits F-CPE computational blocks to minimize the electronic footprint, in order to compensate for an arbitrary rotation, introduced by F-CPE.