Laser Linewidth Tolerance of Decision-Aided Maximum Likelihood Phase Estimation in Coherent Optical $M$-ary PSK and QAM Systems

This letter extends the decision-aided (DA) maximum likelihood (ML) phase estimation in an $M$ -ary phase-shift keying (PSK) system to quadrature amplitude modulation (QAM) signals. The block length effect is simulated for both $M$-ary PSK and QAM systems, and also investigated in $M$ -ary PSK systems through analysis. Our results show that the tolerance of DA ML to laser linewidth for square 16-QAM can be increased by $>$10 times compared to the block $M$th power scheme.