Hybrid Offline-Online Design for Reconfigurable Intelligent Surface Aided UAV Communication

TL;DR

This paper introduces a hybrid offline-online design for RIS-aided UAV communication in urban environments, optimizing system performance by combining statistical and instantaneous channel information.

Contribution

It proposes a novel hybrid design framework that jointly optimizes RIS phase-shifts and UAV trajectory offline, then adapts beamforming online, improving performance over benchmarks.

Findings

Outperforms benchmark schemes in achievable rate.

Balances system performance with CSI overhead.

Uses stochastic successive convex approximation for optimization.

Abstract

This letter considers the reconfigurable intelligent surface (RIS)-aided unmanned aerial vehicle (UAV) communication systems in urban areas under the general Rician fading channel. A hybrid offline-online design is proposed to improve the system performance by leveraging both the statistical channel state information (S-CSI) and instantaneous channel state information (I-CSI). For the offline phase, we aim to maximize the expected average achievable rate based on the S-CSI by jointly optimizing the RIS's phase-shift and UAV trajectory. The formulated stochastic optimization problem is difficult to solve due to its non-convexity. To tackle this problem, we propose an efficient algorithm by leveraging the stochastic successive convex approximation (SSCA) techniques. For the online phase, the UAV adaptively adjusts the transmit beamforming and user scheduling according to the effective I-CSI. Numerical results verify that the proposed hybrid design performs better than various bechmark schemes, and also demonstrate a favorable trade-off between system performance and CSI overhead.