Maximum likelihood sequence detection for mitigating nonlinear effects

Coherent detection allows for a more effective compensation of transmission impairments in the electrical domain. However, in order to be effective, a detection strategy should be based on an accurate channel model capable of providing sufficiently accurate signal statistics. While in the linear regime such a model is available and linear impairments such as chromatic dispersion and polarization-mode dispersion can be almost fully compensated by adaptive equalizers, this is not the case for nonlinear impairments, whose mitigation is essentially based on heuristic strategies. One of the most considered strategies is the back-propagation (BP) technique, based on channel inversion. It is shown that BP is most effective only in dispersion-unmanaged links, while a low-complexity Viterbi detector with proper metrics can achieve better results in the case of dispersion-managed links. It is also shown that, in the cases where it is effective, BP is far from approaching optimal performance. Indeed, proper processing after BP can significantly increase performance.