On the Secrecy Rate of Multi-User Pinching Antenna Systems

Abstract

Pinching-antenna systems (PASS) control wireless propagation by activating or repositioning radiating elements along dielectric waveguides. This geometric flexibility is attractive for sixth-generation indoor networks, but its physical-layer security benefit may be overestimated if in-waveguide attenuation is neglected or a passive eavesdropper is assumed to occupy a known location. This thesis investigates secrecy sum-rate maximization in a multi-user PASS downlink while jointly accounting for waveguide attenuation and uncertainty in passive-eavesdropper locations. The proposed system employs spatial-division multiple access, user-specific beamforming, movable pinching antennas, and artificial noise transmitted in the null space of the legitimate-user channel. A passive eavesdropper is represented by an unknown position within a circular threat zone, without requiring a nominal location estimate. Sample-average approximation maps this continuous spatial uncertainty to a finite collection of representative eavesdropper channels. The joint optimization of the beamformers, reduced null-space artificial-noise covariance, and pinching-antenna positions is non-convex. A two-block alternating-optimization algorithm is developed. Successive convex approximation updates the beamformers and artificial-noise covariance, while projected gradient ascent with Armijo backtracking updates the antenna positions subject to minimum-spacing constraints. Simulation results demonstrate the superiority of the proposed attenuation-aware and uncertainty-aware design as compared to fixed-position baseline by achieving a considerably higher mean secrecy sum-rate.