Dual-Waveguide Pinching Antennas for PLS: Parallel Placement or Orthogonal Placement?

Abstract

Pinching antennas (PAs) represent an emerging flexible-antenna paradigm, where movable PAs arranged along dielectric waveguides enable on-demand customization of wireless channel characteristics. This paper exploits PAs to enhance physical-layer security (PLS) by increasing the channel diversity gap between legitimate users (LUs) and eavesdroppers (Eves). We concentrate on a dual-waveguide PA system, in which each waveguide is equipped with multiple PAs to serve multiple LUs under wiretapping from an Eve. Two waveguide placement schemes are studied: parallel placement and orthogonal placement. To capture practical waveguide propagation effects, we adopt two in-waveguide channel models: in-waveguide phase shifts, and in-waveguide phase shifts and attenuation. We formulate secure sum rate (SSR) and secrecy energy efficiency (SEE) maximization problems, and develop a two-stage optimization framework to tackle these non-convex problems. In the first stage, we design a feasibility-enhanced particle swarm optimization (PSO) algorithm named FeaPSO to search for valid PA positions complying with minimum inter-PA spacing constraints. The second stage leverages successive convex approximation (SCA) to jointly optimize baseband beamforming and artificial noise vectors. Simulation results validate the effectiveness of the proposed two-stage algorithm and confirm that PA adjustment yields substantial gains in both SSR and SEE. Moreover, the simulation outcomes demonstrate the performance benefits brought by orthogonal waveguide placement for PLS enhancement.