A comparative study of reconfigurable intelligent surfaces with relays in UAV cooperative communications

Abstract

Reconfigurable Intelligent Surface (RIS) is an emerging technique proposed for the 6th Generation (6G) network to manipulate the propagation environment for wireless transmission. Since unmanned aerial vehicle (UAV) offers 3D mobility and high availability of line-of-sight link, the integration of UAV with the RIS (UAV-RIS) have received much attention. This paper aims to study the comparison between the RIS and the conventional active relays for UAV cooperative communications. The numerical results show that the energy efficiencies of the active relays are higher when the UAV is placed close to the middle point between the BS and UE, whereas the RIS performs better when the UAV is close to either the BS or the UE. Besides, decode-and-forward (DF) relay achieves higher energy efficiency than the amplify-and-forward (AF) relays. In addition, the performance of the full-duplex relaying mode is affected by the residual self-interference (SI), while the performance of RIS is significantly affected by the number of elements. There is none of the relaying modes works well in all conditions, and the performance of the relaying modes depends on the system frequency used.