Doppler shift compensation based aerial-RIS communications for emergency search and rescue

To address the severe communication disruptions typically induced by natural disasters and to meet the urgent connectivity demands in their aftermath, Aerial Reconfigurable Intelligent Surface (ARIS) systems have emerged as a promising technological solution. Leveraging flexible deployment and intelligent signal reflection capabilities, ARIS systems can significantly enhance coverage and reliability in challenging environments. However, the Doppler shifts caused by the high mobility of ARIS, along with the lack of dynamic resource allocation, result in degraded communication performance. To tackle these challenges, we propose a Reconfigurable Intelligent Surface (RIS) configuration-based Doppler Shift Compensation (R-DSC) mechanism that dynamically adjusts RIS phase shifts to achieve coherent superposition of reflected signals at the user side. We further formulate a joint R-DSC-based beamforming, resource allocation, and trajectory planning optimization algorithm to maximize the weighted sum rate in emergency search and rescue scenarios. The resulting non-convex problem is addressed using a Block Coordinate Descent (BCD) method. Firstly, subcarrier and power allocation are alternately optimized, where subcarrier variables are relaxed and solved via Branch and Bound algorithm. Secondly, RIS phase shifts are optimized based on spatial geometry and Doppler compensation. Thirdly, the UAV trajectory is initialized by using Bezier curves and refined through gradient projection. Numerical results demonstrate that the proposed algorithm significantly outperforms the traditional Fixed Trajectory Scheme (FTS) and Static Deployment Scheme (SDS) by effectively compensating for Doppler shifts and enhancing communication quality in dynamic rescue tasks, while ensuring baseline connectivity for all users.