Carrier-Phase Estimation for 16-QAM Optical Coherent Systems Using QPSK Partitioning With Barycenter Approximation

This paper presents a hardware-efficient carrier phase estimator with high-linewidth tolerance for 16-QAM optical coherent systems. The laser phase noise is estimated using quaternary phase-shift keying (QPSK) partitioning complemented with a low complexity angle-based barycenter approximation as opposed to the classical Viterbi and Viterbi algorithm. The various stages necessary for partitioning and removing the modulation on the received symbols for carrier phase recovery are presented. We show that the phase offset in the middle ring for a 16-QAM constellation can be removed through a simple comparison with the symbols lying on the inner and outer rings of the constellation thus enabling all the symbols to be efficiently utilized for carrier phase recovery. We assess the performance of different filter structures for 16-QAM with filter half width 8 and 16. Simulation results demonstrate that combined linewidth symbol duration product Δν · T_s of 10^-4 is tolerable at the target BER of 10^-2 and 10^-3 when using the barycenter algorithm. Finally, carrier phase recovery in a 16-QAM experiment is investigated to validate the performance of the proposed algorithm.