Polarization-induced distortion effects on the information rate in single-mode fibers.

In this paper, we examine information theoretical properties of single-mode fibers in the presence of polarization-induced distortion effects. We derive some capacity results and further obtain several nonergodic achievable rates. In this work, however, mostly linear distortions are considered. Since polarization-dependent loss (PDL) is a nonunitary phenomenon, information rate loss caused by PDL is fundamentally inevitable. Interestingly, it is shown that in the presence of channel state information at the transmitter, PDL can increase the capacity in some scenarios. We analytically found also that the highest average capacity improvement from the knowledge of PDL at the transmitter is equal to the mean PDL of the link, and this benefit vanishes at high signal-to-noise ratio. In order to achieve the ergodic capacity, it is established that sending uncorrelated Gaussian signals with equal power via both polarizations is the optimum transmit strategy. As it turns out from the results, perhaps counterintuitively, in the presence of PDL, polarization mode dispersion (PMD) always improves the maximum outage rate; however, the PMD impact on the maximum throughput and the maximum two-layer expected rate is trivial. Finally, an extension to the simple Gaussian noise model of fiber nonlinearity is explored. All theoretical results are illustrated by numerical simulations.