A MAP equalizer for the optical communications channel

A maximum a posteriori equalizer is presented for optical communication systems. Assuming that the span of the intersymbol interference (ISI) does not extend beyond the neighboring bits - as is typically the case for the distortion introduced by polarization mode dispersion (PMD) - we derive the conditional probability distribution function in the electrical domain in the presence of PMD and amplifier spontaneous emission dominated noise. Simulation results with an accurate receiver model and all-order PMD show the success of the MAP equalizer in reducing the ISI due to PMD, and that the analytical conditional PDF we derive provides a good match to the actual distribution.