ANALYSIS-METHODS FOR OPTICAL HETERODYNE DPSK RECEIVERS CORRUPTED BY LASER PHASE NOISE

Laser phase noise is a major impairment to the performance of coherent optical communication systems. Differential phase-shift-keying (DPSK) systems are particularly susceptible to its detrimental effects. Due to their nonlinear structure, DPSK receivers are also difficult to analyze accurately. We present two methods for analyzing the effects of phase noise on the performance of an optical heterodyne, binary-DPSK system. The first method utilizes a perturbation solution for filtered phase noise that was derived by Foschini and Vannucci. By comparing the results of this analysis with simulated results, we show that the perturbation solution is accurate for laser linewidths up to at least 10% of the bit rate. Also using this analysis, we verify the accuracy of the widely used approximation whereby the effects of filtering on the magnitude of the phase-noise corrupted signal are neglected. Our second method is based on moments of random variables. Various authors have previously derived expressions for the moments of filtered laser phase noise. However, these formulations suffer from numerical inaccuracies when the amount of phase noise is very small. As the level of phase noise in a practical DPSK system must be small, we derive an improved formulation for the moments of the filtered phase noise that removes the major cause of this numerical instability. The use of these moments to bound the receiver error probability appears at first to be very attractive. However, the very low probabilities involved and the nonlinear nature of the DPSK receiver renders established techniques unsuitable. We therefore apply a maximum-entropy probability density function estimation technique to the problem of analyzing the performance of a DPSK receiver. By comparing results with those obtained using the perturbation analysis, we find that the moment-based method is effectively limited to relatively large error probabilities.