Joint-Polarization Phase-Noise Estimation and Symbol Detection for Optical Coherent Receivers

The problem of optimal symbol detection in the presence of laser phase noise is studied, for uncoded polarization-multiplexed fiber-optic transmission. To this end, the maximum a posteriori (MAP) symbol detector is presented. Specifically, it is emphasized that obtaining phase-noise point estimates, and treating them as the true values of the phase noise, is in general suboptimal. Furthermore, a pilot-based algorithm that approximates the MAP symbol detector is developed, using approaches adopted from the wireless literature. The algorithm performs joint-polarization phase-noise estimation and symbol detection, for arbitrary modulation formats. Through Monte Carlo simulations, the algorithm is compared to existing solutions from the optical communications literature. It is demonstrated that joint-polarization processing can significantly improve upon the single-polarization case, with respect to linewidth tolerance. Finally, it is shown that with less than 3% pilot overhead the algorithm can be used with lasers having up to 6 times larger linewidths than the most well-performing blind algorithms can tolerate.