Trajectory Optimization And Resource Allocation For UAV Base Stations With Limited Backhaul Links

With the rapid development of unmanned aerial vehicle(UAV)in recent years,UAVs have been widely used in both military and civil fields.With the advantages such as high mobility,flexible deployment,and line-of-sight channel,UAV communication is particularly suited to assist or replace the traditional terrestrial wireless communication,where UAV base stations(UAV-BS)is one of the main use cases.A UAV as a mobile base station(BS)can be deployed in the air,to provide users with in temporary or emergency on-demand service.The traditional ground BSs can utilize fiber,cable and other wired media to set up a backhaul link to the core network.However,in the UAV-BS communication system,wired media will limit the high mobility of the UAV-BS,thus a UAV-BS usually uses a wireless backhaul link.The wireless spectrum resource is very scarce,so how to use the precious spectrum resource efficiently is essential.Based on this consideration,this thesis specifically studies the UAV-BS communication system with a in-band backhaul link,i.e.,the access link and the backhaul link share the same spectrum,which usually has higher spectrum utilization efficiency than the out-band backhaul link scheme.The main contents of this thesis are given as follows.(1)A UAV-BS communication system with single access point(AP)under the in-band backhaul link constraint is studied.Under the constraints of UAV mobility,collision avoidance,information causality,total bandwidth,and transmit power,an optimization problem that jointly optimizes the UAV’s trajectory,system bandwidth allocation and user power allocation to maximize the user’s minimum communication rate is formulated.The formulated problem is non-convex,and this thesis applies the iterative optimization and successive convex optimization(SCO)methods to propose an efficient algorithm to solve the problem.The simulation results show that the proposed joint trajectory,bandwidth and power optimization algorithm can effectivelyimprove the minimum user rate performance of the UAV-BS system,as compared to several benchmark schemes.(2)A UAV-BS communication system with multiple APs under the in-band backhaul link constraint is studied.In the multiple APs scheme,the UAV-BS can improve the data rate of the backhaul link by selecting the AP with the best channel condition,which avoids the extreme case of the single AP scheme where the backhaul channel is too poor to communicate due to the long distance or the obstruction between the UAV and the AP.Under the UAV mobility and collision avoidance constraints,information causality constraints,total bandwidth constraints,and the UAV transmit power constraints,an optimization problem that jointly optimizes the UAV’s trajectory,the system bandwidth allocation and the user power allocation to maximize the user’s minimum communication rate is considered.The considered problem is non-convex,and this thesis applies the iterative optimization and SCO methods to propose an effective algorithm to solve it.Simulation results show that,as compared to the single AP scheme,the multiple APs scheme can efficiently improve the user’s minimum communication rate performance of the system.