Study On The Turbulence Mitigation Scheme In Free-space Optical Communications Using Orbital Angular Momentum Multiplexing

Free-space optical (FSO) communication employing orbital angular momentum (OAM)multiplexing is a novel FSO communication scheme, who can tremendously improve transmissionrate of the system without extra spectrum bandwidth. However, it is well known that terrestrial FSOlinks usually are subject to channel fading induced by atmospheric turbulence. The OAM states ofbeams are inevitably affected by atmospheric turbulence (AT) while transmitting in the free space.It has important theoretical significance and reference value to study the method of mitigatinginterference of atmospheric turbulence on OAM-multiplexed FSO communication.In this paper, the study was performed on restraining the effect of atmospheric turbulence onboth a single OAM state and crosstalk of different OAM states in the multiplexing communication.First of all, the effect of atmospheric turbulence on transmission of OAM beams in free space wassimulated by using the model of atmospheric turbulence. On this basis, a scheme of error correctioncoded OAM multiplexing FSO communication through atmospheric turbulence by using channelerror correction code was studies in this paper. The numerical simulation results show that the errorcorrection method can effectively restrain the effect of turbulence noises on reliability ofOAM-multiplexed FSO communication system and reduce the system BER. The error correctionmethod can improve the system performance by making BER decline1to2orders of magnitude inthe condition of weak and medium atmospheric turbulence (C2n<10-14).Then, the OAM multiplexing FSO communication scheme based on wavefront correction wasstudied by using Shark-Hartmann wavefront correction algorithm in adaptive optics which cancorrect the distortions of OAM superposition state caused by atmospheric turbulence by means ofphase compensation. The research results show that Shark-Hartmann wavefront correctionalgorithm can effectively reconstruct the phase space of transmission beams that is damaged byatmospheric turbulence and then obviously decrease atmospheric turbulence interference. Theperformance of the system is improved in all the condition of weak, medium and strongatmospheric turbulence. The BER of the system can decline0.5to1orders of magnitude with thesub-channels of l=-2,-1,+1,+2.Lastly, in this paper, the OAM multiplexing FSO communication scheme based on lineardecorrelation multi-user detection (MUD) method was proposed. Atmosphere turbulence candestroy the phase structure of the OAM beams, make the orthogonality among different OAM states no longer maintain. Meanwhile, it still gives rise to the power spread of different OAM states,which can make the communication systems produce crosstalk. However, the method of OAMstates multi-user detection can effectively utilize crosstalk information among OAM states, thenmitigate the crosstalk caused by atmospheric turbulence. Numerical calculation results show thatthis linear decorrelation OAM states MUD method can weaken the correlation of different OAMchannels and remove the inter-channel interference to some extent, then the system performancewill be improved. The BER of the system can decline1to2orders of magnitude in the condition ofweak and medium atmospheric turbulence (C2n<10-14) with the sub-channels of l=+1,+2,+3,+4.