Quality metrics for optical transmission

The quality of optical signals is a very important parameter in optical communications. Several metrics are in common use, like optical signal-to-noise power ratio (OSNR), Qfactor, error vector magnitude (EVM) and bit error ratio (BER). While the BER is the final determinant for a system, a measured raw BER is not necessarily useful to predict the final BER after soft-decision forward error correction (FEC), if the statistics of the noise leading to errors remains unknown. In this respect the EVM is superior, as it allows an estimation of the error statistics. The accuracy of the BER estimate from a measured Q-factor is impaired by the basic requirement that the relevant noise must be Gaussian. Already for plain on-off keying (OOK) signals this assumption is violated if an optical pre-amplifier is employed, and the estimated BER becomes even worse if phase modulation formats are involved. We compare various metrics analytically, by simulation, and through experiments. We employ six quadrature amplitude modulation (QAM) formats at symbol rates of 20 GBd and 25 GBd. The signals were generated by a software-defined transmitter. We conclude that for optical channels with additive Gaussian noise the EVM metric is a reliable quality measure. For nondata-aided QAM reception, BER in the range 10^-6...10^-2 can be reliably estimated from measured EVM.