Leakage-Constrained Secure Transmission for Pinching Antenna Systems With Joint User Selection, Precoding, and Antenna Positioning

Abstract

Pinching antenna systems (PASSs) provide spatially reconfigurable transmission by activating movable pinching antennas along dielectric waveguides. This study investigates secure multi-user downlink transmission in PASS-enabled networks with colluding eavesdroppers. Instead of relying only on secrecy-rate maximization, an explicit leakage constraint is imposed to limit the aggregated eavesdropping signal-to-noise ratio of each scheduled confidential stream. To exploit both digital and spatial degrees of freedom, user selection, secure baseband precoding, and continuous pinching-antenna positioning are jointly optimized. The resulting mixed-integer non-convex problem is addressed using an alternating optimization framework. For fixed antenna positions, fractional programming, successive convex approximation, and a binary-promoting quadratic penalty are combined to obtain tractable convex quadratically constrained quadratic subproblems. For fixed baseband variables, the antenna positions are updated using penalty-based projected gradient ascent with Armijo backtracking. Simulation results demonstrate that the proposed design improves the legitimate sum-rate and provides effective leakage control compared with fixed-position, non-intelligent scheduling, greedy scheduling, secrecy-rate-maximization, and conventional fixed-position antenna benchmarks, while further confirming its behavior under imperfect eavesdropper CSI, finite PA positioning resolution, binary recovery, and reduced-scale exhaustive-search evaluation.