The Atmospheric Turbulence Mitigation Scheme In Orbital-Angular-Momentum Multiplexed System

Angular momentum can be divided into Spin angular momentum(SAM)and Orbital angular momentum(OAM),where SAM is related to the polarization of light,while OAM is identified by the spiral angular sturcture.In nature,OAM is the basic physical quantity of classical and quantum mechanics.For a single phone,the value of OAM topological charge can be infinite,and the mode functions for different OAM topological charges are orthogonal.It has shown that both channel capacity and spectrum efficiency can be greatly improved by using OAM modes.However,atmospheric turbulence(AT)causes time-varying crosstalk among different OAM modes.The OAM-multiplexed free space optical(FSO)communication link is inevitably interfereced by AT,which significantly impair the bit error rate(BER)performance of the communication link.In the dissertation,four AT mitigation schemes in the OAM-multiplexed FSO communication link have been proposed.Theoretical analysis and numerical simulation are simutaneously presented.The main works and results are the following:(1)A mitigation scheme based on Gerchberg-Saxton phase correction algorithm is first proposed,where the wave front phase distortion is first recoverd by using Gerchberg-Saxton phase retrieval algorithm.By the recovery,the energy dispersion of OAM modes is reduced and the crosstalk caused by AT is mitigated.The theoretical analysis and numerical simulations are presented to verify the proposed scheme.It is shown that the proposed scheme can mitigate the crosstalk,especially for the interferece caused by moderate and weak AT.The BER performance of the system is improved two or three orders of magnitude when the refractive index structure constant of AT Cn2 is 2×10-15m-2/3 and singal to noise ratio(SNR)is 20dB.(2)Later,a mitiagation scheme based on multiuser detection(MUD)equivalent model for the OAM-multiplexed FSO communication link is presented.An equivalent MUD model for the OAM-multiplexed FSO communication link disturbed by AT is first presented,where the crosstalk of each OAM mode is regarded as the user's modulation informaiton.These modulation informaiton are useful for retrieving each user's information at receiver.Then a turbulence mitigation scheme based on decorrelation detection method is present.The numerical results show that the proposed scheme can mitigate the crosstalk caused by moderate or weak AT effectively.When the turbulence strength Cn2 is 1×10-15m-2/3 and SNR is 26dB,the BER performance of the OAM-multiplexed communication link is 2-3 fold improved in comparison with those results without the proposed scheme.In addition,the proposed scheme is more effective for an OAM-multiplexed FSO communication link with larger OAM mode topological charge intervals.(3)A mitigation scheme based on Multiple-input Multiple-output(MIMO)equalization is then proposed for the OAM-multiplexed FSO communication link,where the MIMO equalization algorithm is adopted to compensate the energy losss caused by AT for the system.The feasibility of the scheme is obtained by theoretical analysis,and the effectiveness is verified by numerical simulations.The results show that the proposed scheme is effective for the crosstalk caused by moderate to weak AT.It is not good for the crosstalk caused by strong AT.When the turbulence strenhgth is 1×10-15m-2/3 and SNR is 20dB,the BER performance of OAM-multiplexed system is a 2 fold increase in comparison with those results without the proposed scheme.(4)At last,a mitigation scheme base on spatial diversity(SD)is proposed for the OAM-multiplexed FSO communication link,where the crosstalk caused by AT is equivalent to the fading effects in wireless channels.Equal gain diversity combining method and selective diversity combining method are discussed.The numerical results show that the performance of OAM-multiplexed communication is improved by exploiting the space diversity gain.The BER performance of the mitigation scheme can improved greately.When the turbulence strength Cn2 is S×10-15 m-2/3 and SNR is 22dB,the BER performance of OAM-multiplexed system is a 3 fold increase in comparison with those results without the proposed scheme.