Rasearch On Interference Suppression Methods In Orbital Angular Momentum Mode Division Multiplexed Optical Communication System

With the rapid development of society,the demand for the channel capacity of communication system is gradually increasing.However,the traditional communication technology can provide limited channel capacity.Therefore,the communication technology with large capacity,high confidentiality and high speed has become the development trend of the future communication technology.In order to solve the channel capacity crisis,in addition to the existing methods of wavelength division multiplexing and polarization multiplexing,Orbital Angular Momentum(OAM)multiplexing technology,as a new form of multiplexing,provides a new method for increasing network bandwidth and expanding channel capacity.And it has become a research hotspot in the field of optical communication.However,in practical application,the transmission capacity and transmission distance of OAM multiplexed space optical communication system or OAM multiplexed optical fiber communication system are limited due to certain interference factors.Therefore,interference suppression technology is a very important research direction in OAM mode multiplexed optical communication system.In this paper,the interference suppression technology in space optical communication system and optical fiber communication system using OAM mode reuse is taken as the research direction.On the basis of studying the principle of interference factors,the phase correction technology in space optical communication and nonlinear equalization technology in optical fiber communication are studied respectively.The main research work and innovation points are as follows:1.Study on phase correction based on W WF algorithm in OAM tree space optical communicationA phase correction method based on Wirtinger Flow(WF)algorithm was proposed to solve the problem of wavefront phase distortion caused by atmospheric turbulence in OAM mode multiplexing free space optical communication system.The correction accuracy was improved and the structure of wavefront sensing in adaptive optical system was simplified.Simulation results show that compared with the traditional GS(Gerchberg-Saxton)algorithm,the performance parameters of the system,such as Root Mean Square Error(RMSE),mode purity,crosstalks and bit Error rate,are significantly improved.At the same time,it avoids the problem that the correction method based on traditional GS algorithm is easy to fall into the local optimal solution.2.Nonlinear Decision Method Based on KNN in OAM Optical Fiber Communication SystemIn order to reduce the complexity of Digital Signal Processing(DSP)module at the receiving end in OAM multiplexing optical fiber communication system,a nonlinear decision method based on KNN is proposed.Experimental results on 8-OAM multiplexed optical fiber communication system show that the complexity of the proposed method is reduced by at least one order of magnitude and the bit error rate performance is reduced compared with the traditional Volterra equalizer.On this basis,considering to further reduce the system error rate and the length of training data,a nonlinear decision method based on probability KNN is proposed.According to the probability density and distribution near the test point,the method calculates the appropriate parameters through heuristic learning method,which not only retains the simplicity of KNN,but also optimizes the performance.Experimental results show that compared with the method of nonlinear decision based on KNN,the performance significantly lower bit error rate,save a part of the result of the choice of k value of cross validation of computational overhead,and to some extent the nonlinear decision based on KNN method for data dependencies,thereby reducing the length of the training set,the length of the training data was reduced by one third.3.Nonlinear Decision Method Based on Naive Bayes in OAM Optical Fiber Communication SystemAiming at the stochastic characteristics of nonlinear models in OAM multiplexing optical fiber communication systems,a nonlinear decision method based on Naive Gaussian Bayes is proposed.The method uses the statistical characteristics of the signals to obtain the posterior probability of the signals to judge the signals,so as to solve the problem of randomness of nonlinear models in the system.Experimental results on 8-OAM multiplexed optical fiber communication system show that the proposed method can significantly reduce the bit error rate and complexity compared with the traditional Volterra equalizer.On this basis,a nonlinear decision method based on data-defined Naive Bayes(DNB)algorithm is proposed to further consider the need to reduce the bit error rate of the system.This method improves the Gaussian model in the nonlinear decision method based on Naive Gaussian Bayes.Training data were used to fit the probability distribution model.Experimental results show that compared with the traditional Volterra equalizer,the nonlinear decision method based on KNN,the nonlinear decision method based on probabilistic KNN and the nonlinear decision method based on Naive Gaussian Bayes,the bit error rate of the proposed method is significantly reduced.When l=-4 OSNR=23dB,the bit Error rate reaches the limit of Forward Error Correction(FEC).