On the Uplink Optimization of RIS-Assisted Multi-User HAPS Communications

Optimizing wireless communications is crucial to meet the growing connectivity demands. High Altitude Platform Stations (HAPS) and Reconfigurable Intelligent Surfaces (RIS) offer promising solutions to achieve this goal. Indeed, HAPS, with its strong communication links to the ground, and RIS, with its passive control capability of signals, can collaborate to improve communications quality. In this paper, we examine the RIS-assisted non-orthogonal HAPS uplink, aiming to enhance the ground users' sum data rate under power and phase shifting constraints. This problem involves joint power control at the users and phase shift configuration at the RIS, which is inherently non-convex. To solve it, we propose to use several optimization methods, including random optimization, particle swarm optimization, Whale optimization algorithm, and alternating optimization (AO). Numerical results demonstrate the efficiency of the investigated methods, with a preference for AO for its trade-off between performance and complexity.