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Communication with unmanned aerial vehicles (UAVs) in current terrestrialnetworks suffers from poor signal strength due to the down-tilt of the accesspoints (APs) that are optimized to serve ground users ends (GUEs). To solvethis, one could tilt the AP antenna upwards or allocate more power to serve theUAV. However, this negatively affects GUE downlink (DL) rates. In this paper,we propose to solve this challenge using a reconfigurable intelligent surface(RIS) to enhance the UAV communication while preserving the 3GPP- prescribeddownwards antenna tilt and potentially improving the DL performance of the GUE.We show that under conjugate beamforming (CB) precoding and proper power splitbetween GUEs and the UAV at the APs, an RIS with phase-shifts configured toreflect radio signals towards the UAV can significantly improve the UAV DLthroughput while simultaneously benefiting the GUEs. The presented numericalresults show that the RIS- aided system can serve a UAV with a required datarate while improving the GUEs DL performance relative to that in a CF- MIMOsystem without a UAV and an RIS. We support this conclusion through simulationsunder a varying numbers of RIS reflecting elements, UAV heights, and powersplit factor.

Improving UAV Communication in Cell Free MIMO Using a Reconfigurable Intelligent Surface