Multilevel optical systems with MLSD receivers insensitive to GVD and PMD

This paper analyzes optical transmission systems based on high-order modulations such as phase-shift keying signals and quadrature amplitude modulations. When the channel is affected by group velocity dispersion (GVD), polarization mode dispersion (PMD), and phase uncertainties due to the laser phase noise, the optimal receiver processing based on maximum-likelihood sequence detection and its practical implementation through a Viterbi processor is described without a specific constraint on the receiver front end. The implementation issues are then faced, showing that at least a couple of widely known front ends, with proper modifications, can be used to extract the required sufficient statistics from the received signal. The aspects related to the receiver adaptivity, the complexity reduction of the Viterbi processor, and the possibility of employing polarization diversity at the transmitter end are also discussed. It is demonstrated that, as long as a sufficient number of Viterbi processor trellis states is employed, GVD and PMD entail no performance degradation with respect to the case of no channel distortions (the back-to-back case).