ISI Mitigation Capability of MLSE Direct-Detection Receivers

We explain why maximum-likelihood sequence estimation (MLSE) receivers for direct-detection optical communication systems do not achieve the single-pulse performance of threshold receivers, in which there is no intersymbol interference (ISI). For this purpose, we propose a simple and intuitive model to approximate the performance of MLSE receivers under the assumption of moderate ISI. A performance analysis of this model shows that an MLSE receiver does not reach the performance of an ISI-free threshold receiver. By simulations we evaluate the range over which the model accurately approximates the MLSE receiver.