Recently, the time synchronization via optical fiber, (e.g., two-way fiber time transfer technique, and round-trip time transfer technique) is more and more attractive, due to its higher precision and longer distance. However, because this method is based on the assumption that the forward and backward propagation delays of one optical fiber are nearly the same, it may fail to achieve time synchronization due to link asymmetry. To resolve this problem, we propose a time filtering method to detect link asymmetry and mitigate time asynchronization caused by link asymmetry. Specifically, we create an array to record four time parameters, i.e., (1) round-trip time, (2) time offset between Server and Client, (3) the difference of round-trip time, and (4) the difference of time offset. The four time parameters are calculated for each synchronization period. The third and fourth parameters are used to detect link asymmetry, and the second parameter is used to compensate the symmetry of the transmission delay, and minimize the time asynchronization. We simulate our method on a two-way fiber time synchronization system with OptiSystem. Simulation results show that our method can quickly detect link asymmetry within 1 second, and achieves nanoseconds time synchronization in the presence of link asymmetry.
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