Improved Pilot-Aided Optical Carrier Phase Recovery for Coherent $M$-QAM

We propose an improved pilot-aided (PA) optical carrier phase recovery (CPR) method, using average operation for noise suppression in PA coherent systems. Performance of this method is investigated and compared with other CPR methods through simulations for 28 Gbaud QPSK, 16-, and 64-QAM systems. Results show that PA-CPR with averaging greatly outperforms PA-CPR without averaging, and achieves better or similar performance compared with other PAor non-PA-CPR methods. In addition, we propose a method using maximum likelihood (ML) phase estimation following PA-CPR with averaging to further improve the laser phase noise tolerance. Specifically, after optimizing the bandwidth of the pilot extraction filter, the maximum linewidth symbol duration products (Δf·T) for 1-dB optical signal to noise ratio penalty at BER of 1 × 10^-3 are improved from 1 × 10^-4, 3.5 × 10^-5, and 1 × 10^-5 (by PA-CPR without averaging) to 6.6 × 10^-4, 2 × 10^-4, and 3.8 × 10^-5 (by PA-CPR with averaging), for 28 Gbaud QPSK, 16-and 64-QAM systems, respectively. When ML is following PA-CPR with averaging, Δf·T values for 1-dB OSNR penalty are further improved to 9 × 10^-4, 4.8 × 10^-4, and 6.5 × 10^-5 for QPSK, 16-and 64-QAM systems, respectively.