Key Technologies Research On UAV Relay Based FSO Communication System

More recently,the research on potential use of free space optical(FSO)link as a powerful communication link between unmanned aerial vehicles(UAVs)has created much interest in academia and industry.By integrating the UAV into FSO system,the conventional ground-based FSO system will be revolutionized and upgraded to achieve flexible,reliable and ubiquitous network coverage.Although the huge demand for UAV-based FSO communication,the key technical issues that channel modeling,performance analysis,and network optimization are not solved completely.To address this need,this paper studies the UAV-based FSO communication system,and the main contents are as follows:Firstly,a hovering UAV-based FSO serial multi-hop decode-and-forward relaying system is investigated.In this chapter,considering the joint effect of atmospheric loss,atmospheric turbulence,pointing error,and angle-of-arrival(Ao A)fluctuation,the novel closed-form expressions of the link outage probabilities for ground-to-UAV,UAV-to-UAV,and UAV-to-ground links are derived,and the expression of the end-to-end outage probability for the UAV-based relaying system is also obtained.At the same time,the asymptotic outage performance bounds for different links and system are presented to reveal insights about the impact of AoA fluctuations.The derived conclusions have an important guiding role for the system construction.Secondly,the optimization problem of outage performance of hovering UAV-based FSO serial multi-hop relaying system is studied.Based on the system performance expressions derived above,in this chapter,an optimization strategy is proposed to optimize the end-to-end outage performance by finding the hovering position of UAVs,adjusting beam width,and field of view(Fo V)of receivers in every link.The adjustment of the received beam width is to mitigate the fading of pointing error,the purpose of Fo V optimization is to avoid the bottleneck of Ao A fluctuation,and the position optimization of the UAV is to satisfy the stringent requirement about line of sight(Lo S),and to reduce the loss related to the path distance by shortening the link distance.The simulation results show that the proposed optimization strategy ensures the existence of Lo S link and improves the end-to-end outage performance.At last,a mobile relaying heterogeneous networks assisted by a fixed-wing UAV with a finite size of buffer is investigated,and the energy efficiency of the considered system is optimized by adjusting UAV’s trajectory.In order to make full use of the UAV’s spatial flexibility and solve the imbalance of data rate between radio frequency and FSO links,an iterative algorithm is proposed to optimize the UAV’s trajectory,which can effectively find the locally optimal solution by utilizing the successive optimization method.The simulation results show that the proposed algorithm prevents the relaying buffer overflow and maximize the energy efficiency of the system.