Research On Information And Energy Transmission Optimization In UAV Communication System

With the rapid development of the mobile Internet,the demand for wireless data traffic is expanding rapidly,which brings an enormous challenge to the terrestrial communication system.However,it is difficult to deal with this challenge by only deploying terrestrial base stations.In addition,terrestrial base stations are faced with a series of problems,such as high deployment costs and difficulty in steep terrain such as mountainous areas,and vulnerability to natural disasters such as earthquakes.The deployment of unmanned aerial vehicle(UAV)base stations in existing terrestrial communication system can effectively solve the above problems.In the UAV communication system,the UAV base station is in the air with fully controllable mobility,which can shorten the distances to intended users and reduce the path loss to improve the efficiency of information or energy transmission.This paper focuses on improving the efficiency of information or energy transmission in the UAV communication system by optimizing the flying trajectory of the UAV.Two scenarios were considered,namely UAV-enabled wireless information transfer and UAV-enabled wireless power transfer with directional antenna.The first research content considers a UAV-enabled wireless information transfer system,in which a UAV serves as a mobile transmitter to broadcast information to multiple ground users over a finite communication period,and the multi-access method is frequency division multiple access.The goal of this paper is to maximize the minimum achievable rates of all users by jointly optimizing the UAV's trajectory and transmit power allocation over time,subject to UAV's maximum speed constraint and an average transmit power constraint.However,this problem is non-convex and difficult to be solved optimally.As a result,this paper first considers a relaxed problem by ignoring the maximum speed constraint,and obtain its optimal solution via Lagrange dual method.The optimal solution reveals that the UAV should successively hover at a finite number of hovering locations,but it is not suitable for the original problem with the maximum speed constraint.Based on the solution of the relaxed problem,and in order to maximize the hovering time,the UAV should fly at the maximum speed between the hovering locations and access all hovering locations with the shortest distance.In this paper,the trajectory of the UAV is obtained by using the method of solving travelling salesman problem(TSP),which is called the successive hover-and-fly trajectory,and then the corresponding optimal hovering durations and transmit power allocation are obtained.The above is the algorithm proposed for the original problem with maximum speed constraint,and the solution obtained by it is asymptotically optimal.The simulation results validate the superiority of the proposed algorithm compared to several existing algorithms.The second research content considers a UAV-enabled wireless power transfer(WPT)system,in which a UAV equipped with a directional antenna is served as a mobile energy transmitter to charge multiple ground nodes over a finite charging period.The introduction of directional antenna adds a new tradeoff in UAV's altitude and beamwidth to the system,which makes the flying of UAV is no longer limited to two dimensional(2D)space,and thus UAV's mobility is more fully utilized.The goal of this paper is to maximize the minimum energy received by all nodes by jointly optimizing the horizontal trajectory,altitude,and beamwidth of the UAV,subject to UAV's maximum speed constraint,an altitude constraint,and a beamwidth constraint.This paper considers two cases,including the case of two nodes and the case of multiple nodes,and gives an efficient solution to the problem of maximizing the minimum received energy.The simulation results show that the proposed scheme has a larger performance gain than the existing schemes.