Low-Complexity Design for Pinching-Antenna-Enhanced Secure Communications

Abstract

Pinching antenna systems (PASS) enable dynamic antenna reconfiguration by sliding pinching antennas (PAs) along dielectric waveguides, offering new spatial degrees of freedom for physical layer security. This paper considers a PASS-enhanced secure downlink and maximizes the achievable secrecy rate by jointly optimizing the transmit beamformer and PA positions. For fixed PA positions, we show that the optimal beamformer is obtained in closed form as the dominant generalized eigenvector of a generalized Rayleigh quotient. To efficiently optimize PA locations, we reformulate PA positions as inter-PA spacing variables, which simplifies the constraints and yields a projected gradient ascent algorithm with a closedform Euclidean projection. In particular, the projection admits a thresholding structure and a closed-form expression for the associated Lagrange multiplier, thereby avoiding the need for generic convex solvers. We further extend the design to a two-timescale setting to reduce reconfiguration overhead and incorporate finiteresolution PA positioning via an efficient discrete projection. Simulations demonstrate fast convergence and significant secrecyrate gains over fixed-antenna and random-placement baselines.