Performance Research Of UAV-assisted Wireless Optical Communication Relay System

Owing to the merits of high bandwidth,effective cost,high security,etc.,free-space optical(FSO)communication has attracted considerable attention and has been proposed for a crucial role in high data-rate point-to-point transmission.However,there is a strict requirement of line-of-sight(LOS)between the transmitter and the receiver for the FSO communication system.Therefore,establishing a relay node to provide a long-distance transmission,and reliable LOS links is significant for the FSO communication system.Considering the limitations of fixed relay nodes in the case of natural disasters,the establishment of mobile relay is necessary.Thanks to the increasingly mature unmanned aerial vehicle(UAV)technology,UAV-assisted FSO relaying communication systems have been a promising solution for stable communication.With the advantages of high flexibility,easy deployment and low price,the FSO relay communication based on UAV can be widely used in natural disasters and emergency rescue.However,the system performance of the UAV-based FSO relaying communication systems can be seriously affected by atmospheric turbulence,attenuation loss,fluctuation of arrival of angle,pointing error and noises.The intensity and phase of the optical signal fluctuates during this period,in a process referred to as scintillation,since the refractive index of the atmosphere turbulence changes randomly as a result of variations in the atmospheric temperature and atmospheric pressure.The farther the optical signal transmission distance is,the more serious the attenuation loss is.This paper studies the effects of atmospheric turbulence and different noises on the performance of FSO communication system firstly.Considering the influence of atmospheric turbulence,Lognormal and Gamma-Gamma channel models are used to describe the fading process of the optical signal in weak-to-strong atmospheric turbulence.In addition to atmospheric turbulence,noise(such as thermal noise,background noise,quantum noise,etc.)also has a significant influence on FSO communication system.Secondly,the expressions of average bit error rate(BER)and average channel capacity of FSO system under the influence of atmospheric turbulence and noises are derived.Finally,the comparison of system performance under different turbulence conditions and various noises are conducted.The results show that wavelength,turbulence intensity,transmission distance,ambient temperature and symbol rate have a significant influence the performance of FSO communication system.In addition,thermal noise has the most influence on the performance of FSO communication system.The FSO system with OOK modulation has the best performance under low electric signal-to-noise ratio(SNR),while the FSO system with DPSK modulation has the best performance under high electric SNR.Considering the strict requirement of LOS transmission of FSO communication system,this paper uses UAV as the relay node of dual-hop FSO communication system,and adopts direct detection intensity modulation(IM/DD)and amplify and forward(AF)protocols.In the ground-to-UAV link,considering the influence of atmospheric turbulence,the Gamma-Gamma model is used to describe the atmospheric turbulence.This paper further considers the geometric loss caused by pointing error,the natural attenuation loss in the transmission process and the fluctuations of arrival of angle(AOA).In the study,the closed form expressions of probability density function(PDF)and cumulative density function(CDF)of SNR of ground-to-UAV link are derived.In addition,the Málaga model is used to characterize the fading distribution of UAV-to-ground link,and the closed expressions of PDF and CDF of transmitter to receiver SNR of dual hop FSO system are given.Furthermore,the closed form expressions of outage probability and BER under different modulation modes of dual-hop system are further derived.It is worth noting that the performance expression under the proposed Gamma-Gamma/Málaga model has universality,and the model is also suitable for the traditional Gamma-Gamma/Gamma-Gamma dual-hop model.Finally,the accuracy of the formula derivation results is further verified by Monte Carlo simulation.It is found that the field of view angle,UAV position deviation,UAV direction deviation,modulation mode and weather conditions have influences on the system performance.The system transmission performance firstly increases and then decreases with the increase of field angle.Therefore,there is an angle threshold,which makes the system performance the best.In addition,compared with 4FSK,QPSK and BPSK modulation schemes,the system performance of BFSK modulation scheme is the best.