Research On Frequency-domain Adaptive Equalization Algorithm For Mode-division Multiplexed Systems

Space division multiplexing(SDM)is a novel optical fiber communication technology developed in recent years.It can increase the capacity of optical communication system by at least one order of magnitude through spatial channel multiplexing.Space division multiplexing technology can be divided into mode division multiplexing technology with few mode fibers as transmission medium and multi-core multiplexing technology with multi-core fibers as transmission medium.For the long-distance mode division multiplexing system that we focus on,the channel will be affected by various linear damage and nonlinear effects in the few mode fibers,which makes digital signal processing technology become a necessary means to eliminate or suppress these damages.In this paper,we mainly focus on the time-varying effect and mode dependent loss in the mode division multiplexing system.On the one hand,this paper focuses on the modeling of time-varying channel effect in FMFs,the model we proposed can be used to simulate the few mode optical fiber transmission channel with any number of modes while ensuring that the mode coupling strength does not change suddenly.What’s more,two fast tracking algorithms are proposed to deal with the effect.The Frequency-Domain Auto-Step Least Mean Square(FD-AS-LMS)algorithm uses a simple formula related to a priori error to determine the step size of the algorithm,which realizes the fast convergence and tracking of the sudden time-varying channel with low complexity;While the Frequency-Domain Variable-Forgetting-Factor Recursive Least Square(FD-VFF-RLS)algorithm makes use of the fast-tracking characteristics of RLS algorithm itself,and dynamically determines the forgetting cause of the current equalization through the relative relationship between the posterior error and the prior error.When the rotation frequency of the channel transmission matrix reaches 500 krad/s,the Q~2factor of this algorithm is 4.5 d B higher than that of the traditional LMS algorithm.On the other hand,this article focuses on the suppression and equalization of mode-dependent loss effects in few-mode fibers.The USIC algorithm in the frequency domain is proposed.This algorithm further improves the equalization Q~2 value of the channel with lower SNR by first equalizing the channel with higher SNR and serially eliminating the interference to the lower-SNR channel.Compared with the traditional SIC algorithm,it does not require additional high-complexity transmission matrix estimation.The simulation results show that the USIC algorithm can increase the Q~2value of the lower-SNR channel in mode division multiplexing system by about 3.5 d B after being combined with cyclic mode permutation devices,which is beneficial for the overall transmission capacity of the mode division multiplexing system for long distance transmission.