I. Introduction
In modern coherent polarization division multiplexing (PDM) system in optical fiber, lightning in the rainy days causes transient polarization variations due to Faraday effect in the fibers, resulting in the receiver unlocking [1]. Tracking and equalizing this effect becomes an important issue to ensure the stable operation of communication systems under thunderstorms. In previous studies [2], the rotation of the state of polarization (RSOP) caused by lightning strikes has received focused attention, while the coupling effect between the inherent polarization mode dispersion (PMD) in the channel and the effects of lightning strikes is often neglected. In order to more comprehensively consider the effect of lightning strikes on signal transmission, this paper focuses on the effect of lightning striking on the optical ground wires (OPGWs), namely the transient variations of RSOP and PMD in the channel, emulating the RSOP and PMD in fibers. To this end, a coherent PDM-QPSK system is built in the digital simulation platform. Heidler model [3], retarded field model [4], and a specially built segmented cascaded channel model are used to emulate the polarization variations induced by lightning strikes on signal transmission. Finally, we employ the extended Kalman filtering (EKF) [6] algorithm to track and equalize the RSOP and PMD in the channel and plot the outage probability of the bit error rate (BER) at different baud rates with different DGD. The results are compared with the generalized CMA algorithm, and it is demonstrated that the extended Kalman filtering algorithm is more effective in compensating for the effects of lightning strikes. Meanwhile, the out-of-bounds probability diagram can be used to obtain the maximum communication baud rate allowed by the communication system under certain lightning strike conditions and channel conditions, which provides a reference for the specific settings of modern coherent high-speed information transmission systems.