| The vortex beam carrying Orbital Angular Momentum(OAM)can greatly improve the information transmission rate and spectrum utilization of the optical wireless communication system.However,the vortex beam is inevitably affected by atmospheric turbulence in the actual propagation environment,which leads to mode crosstalk and reduces the performance of the communication system.This thesis focuses on the transmission characteristics and crosstalk suppression of vortex beams under atmospheric turbulence,and conducts research on crosstalk suppression methods based on preprocessing and signal detection to improve the performance of vortex optical communications system.The main contents are summarized as follows:1.Analysis of the transmission characteristics of vortex beam in atmospheric turbulence.Based on the atmospheric turbulence and laser propagation theory,the vortex beam propagation model is established,and the transmission characteristics of the vortex beam under different parameters are analyzed,which serves as the basis for the subsequent chapters.2.Research on crosstalk suppression based on feedback preprocessing.Firstly,the vortex beam channel model of the atmospheric turbulence scene is established.Then,a beam"zero forcing"preprocessing scheme based on the crosstalk channel state information is designed,and the performance of the method under imperfect channels is analyzed.Finally,the imperfect channel is compensated based on the least mean square criterion.The results show that the preprocessing scheme in this thesis can suppress mode crosstalk within a certain channel error range and increase the channel capacity of the system.After compensation for imperfect channel,the upper limit of the preprocessing scheme can be further increased.3.Research on crosstalk suppression based on signal detection.Firstly,a vortex beam multiplex communication system based on vertical bell layered space time code is established.Then the crosstalk suppression scheme based on sequential interference cancellation,OAM-based sequential interference cancellation,and markov random field belief propagation algorithm is given.Finally,the performance of the proposed schemes under the aperture mismatch and atmospheric turbulence scene are comprehensively analyzed.The results show that the proposed schemes can effectively suppress the influence of crosstalk on the system.The scheme based on markov random field belief propagation scheme reduces the bit error rate by about 2.6 orders of magnitude,while the scheme based on ordered successive interference cancellation reduces the bit error rate by about 1.7?2 orders of magnitude when the refractive index structure constant is1×10-1 4m-2/3,the aperture mismatch tilt angle is 3×10-5 rad,and the signal-to-noise ratio is 18dB. |
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