Title:research On Performance Of Mimo-fso Adaptive Modulation System Under Pointing Errors

Free-space optical communication(FSO)will be affected by atmospheric turbulence and pointing errors in applications.The fading characteristics of atmospheric channels will cause random fluctuations in the intensity and phase of the transmitted optical signal,thereby affecting the stability and reliability of the communication system.For wireless optical communication systems,the scintillation and pointing errors are important factors limiting link performance.Adaptive subcarrier modulation technology and space diversity technology can effectively suppress the fading caused by atmospheric turbulence and pointing errors,and improve the communication performance of the system.The main research contents of this paper include:1.The basic principle of FSO system,Malaga atmospheric turbulence channel model and pointing error probability distribution model are described.Based on Meijer G function,the expression of Malaga atmospheric turbulence joint probability density function with pointing errors is derived.According to the joint probability density function and Meijer G function,the asymptotic expressions of spectral efficiency,average bit error rate(BER)and outage probability of adaptive SISO system and average BER of non-adaptive SISO(single input single output)system are derived.The performances of non-adaptive SISO system with pointing error and adaptive SISO system are compared and analyzed.Compared with nonadaptive modulation,adaptive modulation can significantly improve the average BER performance of SISO system.2.Based on the joint probability density function of pointing error and Malaga atmospheric turbulence,the asymptotic expression of system BER under three spatial diversity combination modes of maximum ratio combining(MRC),equal gain combining(EGC)and selective combining(SC)at the receiver of MIMO-FSO system is derived by using Meijer G function.Under the conditions of different beam waist radius,jitter deviation,transmitting and receiving aperture and turbulence intensity,The effects of three merging methods on the average BER of the system are compared and analyzed.When average BER is 10-4 and the number of receiving apertures is 4,compared with EGC,the MRC has a diversity gain of about 6dB,so the MRC can better improve the performance of the communication system.3.The joint probability density function expression of pointing error and atmospheric turbulence channel based on generalized power series is derived.Based on this expression,the asymptotic expressions of MIMO channel probability density function,system spectral efficiency and average bit error rate(BER)of adaptive MIMO-FSO spatial diversity system using EGC and MRC are derived respectively.The performances of adaptive spectral efficiency,spectral efficiency and outage probability under the two merging modes are simulated,and the average BER performance of non-adaptive and adaptive MIMO-FSO systems is compared and analyzed.Compared with σs/a=1.5,when the receiving aperture of σs/a=2 is 5,the MRC diversity gain is improved by 12dB.