Leveraging UAVs with Intelligent Reflecting Surfaces for Energy-Efficient Communications with Cell-Edge Users

In this paper, we consider the downlink communication of a multi-antenna base station with its associated single-antenna user equipments (UEs) assisted by an intermediate unmanned aerial vehicle (UAV) to avoid service starvation of cell-edge UEs and meet the required quality-of-service. Specifically, we investigate the advantages of using intelligent reflecting surfaces (IRS) at the UAV to achieve the end-to-end communication with high energy efficiency. The objective is to maximize the total energy efficiency of the system, by jointly optimizing the beamforming vector at the base station and the phase shifts matrix of the passive reflecting elements at the UAV. We propose an iterative algorithm based on maximum-ratio transmission, alternating optimization techniques and difference of convex (DC) programming, by assuming channel state information availability. Numerical results are provided, and show that significant performance gains can be achieved via the assistance of the IRS equipped UAV as compared to conventional relaying. In particular, the gain in EE is more than 45% when optimal power allocation is implemented in the system, and can even attain 50% when a large number of reflecting elements are mounted on the UAV.