OSNR monitoring for QPSK and 16-QAM systems in presence of fiber nonlinearities for digital coherent receivers

OSNR monitoring is indispensable for coherent systems to ensure robustreliable network operation and potentially enable impairment-aware routing for future dynamic optical networks. In a longhaul transmission link with chromatic dispersion (CD) and fiber nonlinearityit is difficult to distinguish between amplifier noise and fiber nonlinearity induced distortions from received signal distributions even after various transmission impairment compensation techniquesthus resulting in grossly inaccurate OSNR estimates. Based on the received signal distributions after carrier phase estimation (CPE)we propose to characterize the nonlinearity-induced amplitude noise correlation across neighboring symbols and incorporate such information into error vector magnitude (EVM) calculation to realize fiber nonlinearity-insensitive OSNR monitoring. For a transmission link up to 1600 km and signal launched power up to 2 dBmexperimental results for 112 Gb/s polarization-multiplexed quadrature phase-shift keying (PM-QPSK) demonstrate an OSNR monitoring range of 10-24 dB with a maximum estimation error below 1 dB. For 224 Gb/s PM-16-quadrature amplitude modulation (PM-16-QAM) systemssimulation results demonstrate an OSNR monitoring range of 18-28 dB with a maximum estimation error below 1 dB. Tolerance of the proposed OSNR monitoring technique to different pulse shapestiming phase offsetspolarization dependent loss (PDL)polarization-mode dispersion (PMD) and WDM effects are also investigated through simulations.