Reconfigurable Intelligent Surfaces Assist UAV Communication

In recent years,due to the high data demand and the large number of users,energy consumption is the bottleneck problem that the future wireless network has to face.The signals sent by the base station(BS)can be reflected to users in a passive manner by means of a reconfigurable intelligent surface(RIS),which freely controls the phase shift,unmanned aerial vehicles(UAV)carries RIS to constantly adjust the reflector position according to the changing environment to maintain a continuous visual link between the transmitter and receiver.Through joint optimization of active beamforming emitted by BS and reflected passive beam by RIS,energy efficiency analysis of the system is carried out.The main contents are as follows:First,the energy efficiency optimization of a single RIS assisted UAV communication system is considered.The high energy efficiency and low cost of RIS are utilized to establish a reliable communication link in combination with the flexible mobility of UAV.The power consumption and energy consumption model of the whole system are calculated by the changes of UAV horizontal position.In order to study the gain of this deployment scheme,BS uses UAV equipped with RIS to provide service to users,this section assumes the maximum transmission ratio and proposes an efficient iterative algorithm that optimizes both BS emits beamforming and RIS phase shift matrix to maximize the overall energy efficiency.Secondly,energy efficiency optimization of multi-UAVs communication system assisted by deployment of multiple RISs is considered.Multiple RISs are used to reflect more channels to improve network capacity and energy efficiency.Discuss RIS ideal phase shift and non-ideal phase shift respectively,the active beam emitted by BS and RIS reflection phase shift framework is optimized jointly,establish a number of reliable communication links,use linear conic relaxation method(LCR)to solve the matching problem between users and RISs,and propose a multi-objective function problem based on energy efficiency maximization.Due to the non-convexity and high complexity of this problem,a joint alternate optimization framework was proposed to solve this challenging problem.Fractional programming and Lagrange decoupling method were used to transform it into convex function to obtain the overall optimal energy efficiency.Finally,simulation experiments are conducted on the RIS-assisted UAV communication energy efficiency problem proposed in this paper to verify the effectiveness and feasibility of the combination of reconfigurable intelligent surface and UAV to improve the system energy efficiency.Numerical results show that the aid of RIS improves the energy efficiency of the overall communication system.