Study Of MIMO-OAM Based Free Space Optical Communication Channels

With the development of the times,the limits of capacity,transmission rate,flexibility and security of wireless communications network become the bottleneck of the development of information technology.Therefore,the researchers put forward the Free Space Optical(FSO)communications system,as one of the response.In addition,FSO become the research focus in the field rapidly.As the data carrier in Orbital Angular Momentum(OAM)multiplexing technique,vortex beam(also called "OAM beam")has the ability of resist atmosphere turbulence(AT)and the OAM multiplexing.Therefore,the vortex beam has significant potential in FSO system.However,vortex beam is easily be distorted while propagating through AT condition.Result in the transmission performance and the reliability of FSO system will be constrained.To reduce the AT induced distortion and improve the performance of FSO system,we study the FSO channel of MIMO-OAM system.We simulate Von-Karman turbulence spectrum model by multi random phase screen method,and we study the propagation of laser beam propagate in FSO channel.We use scintillation index(SI)measure the signal intensity fluctuation(as the quality of AT channel),and utilize the aperture equalization to resist the FSO channel induced time domain distortion of signal.Then,we analyze the SI mitigation of aperture equalization under different AT strength,transmission distance and topological charges.Moreover,different modes of laser beam have different AT resistance.Vector vortex beam has best AT resistance.To reduce SI of vortex beam further,we utilize spatial diversity technique at transmitter and receiver to mitigate the AT induced signal intensity fluctuation.Then demonstrate that spatial diversity has the great potential of SI mitigation by theory and simulation.The performance of different spatial diversity configuration is also been analyzed.In addition,the spatial diversity configuration induced crosstalk distribution distortion is considered.Then,we analyze the phase pattern of tilted transmitted beam and compensate the distortion by the conjugate phase pattern,then improve the stability of OAM FSO channel.Not only the resist of AT induced distortion is considered,we also consider the OAM modulation and multiplexing in FSO system.To study the OAM modulation and multiplexing techniques,the crosstalk of OAM channels is considered.In this paper,we propose a new crosstalk definition base on space spectrum theory to describe the degree of crosstalk of OAM channels better.After the propagation in AT,the crosstalk of OAM channel become significant,which make the detection of the transmitted signal in OAM channel very difficult.To realize the accurate detection of vortex beam signal,we propose a crosstalk distribution statistical feature extraction based topological charges(TC)detection scheme.Determine the transmitted OAM stated by extracting valid information of TC carried by crosstalk distribution as much as possible.This scheme shows better detection performance,and has the ability of error rate estimation that tradition scheme do not.