Flex-PAM modulation formats for future optical transmission system

We propose four strategies to establish the operation of the transmitter of flexible modulation formats based on different PAM constellations on the in-phase and quadrature axes of the two polarizations exploited for optical fiber transmission (Flex-PAM). We verify that the strategy based on forcing all the four PAMs to operate at the same bit error rate (BER) - the “same-BER” strategy - besides being the easiest to be implemented enables back-to-back performances only slightly sub-optimal with respect to the strategy that minimizes the overall Flex-PAM signal-to-noise ratio, given the target BER. We also show how nonlinear propagation performances of a WDM comb of Flex-PAM channels can be optimized operating on the power ratio between the four PAM constellations.