Study On Influence Of The Complex Atmosphere On Free Space Optical Communication And Mitigation Techniques

Free space optical communication(FSO)is a kind of wireless communication technology,which uses a laser beam as an information carrier and transmits point-to-point in the space channel with the unobstructed line of sight.FSO communication system combines the advantages of microwave communication and optical fiber communication,such as larger capacity,fast network building speed,license-free operation,increased security,and other characteristics,which is usually used as the "last kilometer" of telecommunications or Local Area Network link between buildings.Although the FSO communication system has so many advantages,its performance is seriously affected by the complex atmospheric environment(various weather such as fog,cloud,rain,snow),beam wandering(pointing errors),and atmospheric turbulence.Therefore,it is of great engineering application value to study the influence mechanism of the atmospheric environment on the performance of free-space optical communication system and the corresponding mitigation techniques.Aiming at the engineering application of free-space optical communication,this paper mainly studies the influence mechanism of the complex atmospheric environment on the communication system and its mitigation techniques by mathematical theory,and the system performance is compared and analyzed by simulation technology.The main contents and innovations of this paper are as follows:(1)The performance of the space-to-ground laser communication system is mainly affected by free space and the complex atmospheric environment.First,the effects of the atmospheric environment(fog,cloud,turbulence),beam spread,pointing errors,and sky background radiation on the communication system are analyzed.And the new mathematical expressions of performance parameters of the intensity modulation/direct detection FSO communication system such as link margin,achievable data rate,and bit error rate obtained by using the received signal optical power and the received sky background optical power.The influence of atmospheric environment on the performance of 1550nm wavelength space-to-ground laser communication system is simulated and calculated,the relationship between altitude and communication system performance is analyzed,and the influence of non-return zero on-off keying modulation scheme and pulse-position modulation scheme on system performance is compared.(2)Considering the influence of weather environment,different turbulence models(Malaga model and Exponential Weibull model),and zero boresight pointing errors on the FSO communication system,we establish different joint channel statistical models and derives the performance parameters of the free-space optical communication systems based on on-off keying modulation,such as average bit error rate,average channel capacity,and outage probability and their new mathematical expressions,using H function and Meijer-G function.Under the conditions of different turbulence intensity,different visibility and different jitter degree,the effects of different beam divergence angle and different receiver aperture size on system performance are simulated and analyzed.(3)Considering the influence of weather environment,Exponential Weibull turbulence model,and nonzero boresight pointing errors on the FSO communication system,we establish a joint channel statistical model for partially coherent beams on the communication system,then derive new mathematical expressions of the performance parameters of the free-space optical communication system based on on-off keying modulation by using Meijer-G function and H function,such as average bit error rate,average channel capacity,and outage probability.For the FSO communication system with a wavelength of 1550 nm and a link length of 1km,the performance parameters of the system are simulated,and the effects of spatial coherence length,the plane spot size of the transmitter,the aperture diameter of the receiver,and line-of-sight offset on the performance of communication system are analyzed.(4)Under the conditions of the Malaga turbulent channel and Nakagami-m fading channel,the performance of hybrid FSO/RF(Free Space Optical Communication/Radio Frequency)communication system based on the selective combination technology is studied.Considering the zero boresight pointing errors,we deduce the average bit error rate and outage probability of the selective combination hybrid FSO/RF system which adopts subcarrier modulation and intensity modulation direct detection scheme and obtains their new closed-form by Meijer-G function and Extended Generalized Bivariate Meijer-G function.The bit error rate and outage probability performance of hybrid FSO/RF system and only FSO system are investigated under different subcarrier modulation schemes,turbulence intensity,pointing errors,and RF channel fading parameter m.(5)The performance of the hybrid FSO/RF communication system(Exponentiated Weibull turbulent channel and Nakagami-m fading channel)with joint aperture averaging,coherent modulation and selective combination techniques is analyzed.Using the mathematical tools Meijer-G function,extended generalized bivariate Meijer-G function and the series expansion of Generalized Gauss-Laguerre integral,the new mathematical expressions of the average bit error rate and outage probability of the system are derived.The BER and outage probability performance of the hybrid FSO/RF parallel system,the mixed FSO/RF dual-hop system and the single FSO system are compared and analyzed under different subcarrier modulation,turbulence intensity,pointing errors and fading parameter m of the RF channel;The influence of aperture averaging effect on the performance of hybrid FSO/RF system is investigated.