Rate Maximization for RIS-Assisted Downlink Pinching-Antenna Systems

Abstract

This letter investigates a downlink communication system utilizing a single-waveguide pinching-antenna system (PASS) assisted by a reconfigurable intelligent surface (RIS). We focus on the joint optimization of the positions of pinching antennas (PAs) along a waveguide and the phase shifts of the RIS to maximize the achievable rate. A novel three-stage optimization algorithm is developed to address the formulated highly non-convex non-linear programming (NLP) problem. Firstly, the optimal RIS phase shifts are derived in closed-form to constructively combine the reflected signals with the direct link signals. Secondly, the antenna positions are optimized to minimize the large-scale path loss to both the RIS and the user, utilizing a sequential convex approximation (SCA) approach that transforms the problem into a convex form. Finally, a wavelength-scale offset is computed to ensure precise phase coherence at the user. Simulation results demonstrate that the RIS-assisted PASS outperforms conventional fixed-antenna systems and standalone PASS benchmarks. Moreover, the proposed three-stage method achieves near-optimal performance.