A Robust Rate Enhancement Scheme for Dual-RIS-Aided Air-Ground Integrated Communication Systems

Unmanned aerial vehicles (UAVs) have been expected to significantly enhance the ground wireless communications by serving as aerial relays. However, the serving beams of the ground transmitters (Txs) are generally downsloping, which limits the signal strength at the UAVs. To tackle this problem, we propose a RIS aided air-ground communication scheme. The reconfigurable intelligent surface (RIS) is deployed on nearby buildings to reflect the downward beam upward to the UAV. Moreover, UAVs are highly susceptible to environmental factors during flight, which can lead to channel state information (CSI) errors. To mitigate the impact of the errors, a distributionally robust optimization problem is formulated, and a conditional value-at-risk (CVaR)-based algorithm is proposed to jointly optimize the active beamforming at the Tx, the phase shifts at the RIS and the trajectory of the UAV. It is found that the proposed scheme can significantly improve the communication rate compared with traditional schemes without RIS.