Research On Spot Center Location Algorithm In FSO System Under Atmospheric Turbulent Environment

Wireless laser communication is a wireless communication technology that uses laser as the information transmission carrier.When the signal beam is transmitted in the atmospheric channel,atmospheric turbulence will cause light intensity flicker,beam expansion,spot drift,angle-of-arrival undulation and other effects,resulting in distortion of the spot detected at the receiving end,reduced spot positioning accuracy,and increased aiming deviation,which eventually leads to increased BER of the communication system or even communication link interruption.In order to establish and maintain a reliable wireless laser communication link,a block barycenter algorithm is proposed for the problem of poor positioning stability of the laser beam affected by turbulence in the atmospheric environment.The theoretical verification of the algorithm is carried out by simulating the distorted spot and simulating the atmospheric turbulence phase screen;the experimental verification of the algorithm is carried out by using a servo motor system equipped with a CCD camera combined with the positioning algorithm for the positioning and data acquisition of real-time images of the spot.The specific work of this study is as follows:1.The traditional spot localization algorithm is analyzed under the conditions of simulated atmospheric turbulence phase screen.According to the idea of adaptive thresholding method and the characteristics of noise emergence,a block barycenter algorithm is proposed;and for the influence of atmospheric turbulence effect on laser communication link,the calculation process of grayscale center-of-mass method is improved according to the chunking idea,and the block barycenter algorithm is proposed to improve the spot localization accuracy and stability.2.To simulate the Gauss distorted spot,the threshold segmentation algorithm and the block barycenter algorithm,and get the result verified by theory.a.There are four threshold segmentation methods including iterative algorithm,adaptive threshold algorithm,maximum between-class variance algorithm and block threshold segmentation algorithm were used to calculate the deviation of locating point from the standard center,and the traditional gray barycenter method was used to calculate the deviation of locating point from the standard center,the segmentation threshold method is superior to the other three methods,and the positioning accuracy can be improved by 59.5%.b.For the original images with superposition average value of 0 and a variance of5-40,and increasing Additive white Gaussian noise in units of 5,then the block barycenter algorithm,the barycenter algorithm,the centroid algorithm and the weighted barycenter algorithm are used for theoretical comparison,under the noise interference with variance of 40,the positioning accuracy can be improved by 53.1%,64.4% and 48.2% respectively.c.Using an improved subharmonic compensation method,the superimposed power spectrum inversion method is used to simulate the generation of a atmospheric turbulence phase screen,and to simulate the formation of a Gaussian spot by a simulated Gauss beam passing through the phase screen,during the procedure,we use the weak,moderate and strong turbulence intensity respectively for simulation verification.The results show that compared with the other three algorithms,the block barycenter algorithm still maintains a stable positioning accuracy.The median of positioning deviation is 1.29175 in moderate turbulence and 1.7373 in strong turbulence,and the calculation results are better than those of barycenter algorithm,centroid algorithm and weighted barycenter algorithm.3.Experimental verification.The experimental platform is built,and the real-time spot location and data acquisition are carried out by the servo motor system with CCD camera and the positioning algorithm.The results show that compared with the other three localization algorithms,the standard deviation of the block barycenter algorithm in the x-axis direction decreases by 15.45%,48.28% and 53.76%,respectively,the standard deviation of the location offset in the Z-axis direction decreases by 8.93%,54.69% and 76.24% respectively.And under the same conditions,the light power intensity of the spot image acquired by the tracking and positioning system equipped with the block barycenter localization algorithm is the highest.