Performance Analysis Of UAV OWC System Based On Relay Collaboration

With the development of unmanned aerial vehicles(UAV)technology gradually mature,combining UAV with mobile communication technology cannot only make up for the problem of insufficient network coverage in remote areas,but also enable rapid network deployment in areas where emergency communication is required(for example,earthquake disasters,etc.)or areas where temporary needs for bigger capacity(such as sporting events),to alleviate the pressure on the ground communication facilities and enhance robustness of mobile communication networks.The emergence of free-space optical communication(FSO)is mainly to solve the spectrum congestion problem of the existing radio frequency band.The existing 4G frequency band uses a frequency lower than 6 GHz,and the 5G network being deployed has a frequency lower than 100 GHz.While the spectrum of optical signal is located in the near-infrared band,and has powerful features such as high data rates,low cost,unregulated spectrum and immunity to interference,etc.The combination of UAV and FSO cannot only show the advantages of the UAV which is easy to deploy,flexible and convenient,but also achieve the overall improvement of the communication network performance by means of the high data rates and superior security.This thesis mainly studies the channel characteristics of UAV optical wireless communication system,including Ground-UAV link,UAV-UAV link,Two-hop FSO communication system and multi-hop hybrid RF/FSO communication system with UAV as relays.Different from the RF channel,the channel of the optical signal is mainly affected by atmospheric attenuation,atmospheric turbulence and pointing errors.Since atmospheric turbulence and atmospheric attenuation are the inherent characteristics of optical channels,we only introduce it in general.Pointing error characteristics are dependent on the precision of the mechanical design of the communication system.Therefore,we focus on the impact of pointing errors on the UAV optical wireless communication system.We first established a system model of the UAV relaying optical communication system after adopting the acquisition,tracking and pointing mechanisms.On the basis of this,with the help of mathematical methods such as probability analysis,we have established a mathematical analysis model.Then,by considering various random parameters comprehensively(position offset of the ground,position offset and direction offset of the UAV,etc.),we derive the probability density function(pdf)of the pointing errors in Ground-UAV and UAV-UAV links.Based on this,we studied the performance of a two-hop UAV relaying optical communication system,by considering the characteristics of atmospheric turbulence,path loss and pointing errors,we derive the joint probability density function of the communication system.We also derive a closed-form expressions of performance metrics such as the outage probability and symbol error rate.To the end,with computer simulation,we use the performance indicators to assess the impact of various parameter changes on system performance.We also investigated a multi-hop hybrid RF/FSO UAV relaying communication system,due to the variability of the flight position,a high altitude can result in a large coverage,but it,in turn,causes a larger path loss.Therefore,there is a tradeoff between coverage and link quality.Take into account this characteristic,we derive the optimal altitude of the UAVs when the outage performance is minimum.And we also analyzed the influence of pointing errors on the system performance by using of outage probability,finally,theoretical analysis results are simulated and verified by computer simulation.