FreqTune-PASS: Frequency-Tuned Beamforming for Multi-User Pinching-Antenna Systems

Abstract

Pinching-antenna systems (PASS) reconfigure large-scale propagation environments by activating low-cost radiation points along a dielectric waveguide. However, existing multi-user PASS designs rely on pinching-antenna (PA) position optimization or activation, whose mechanical reconfiguration is much slower than user switching in time-division multiple access systems. To overcome this mismatch, a frequency-tuned beamforming framework (FreqTune-PASS) is proposed, where all PA positions remain fixed and only a user-specific carrier-frequency offset is adjusted in each slot. Since each PA-radiated signal experiences a distinct combination of in-waveguide and free-space delays, a common frequency offset induces path-specific phase rotations and thus reshapes the coherent superposition at the scheduled user. Based on a delay-domain system model, the fast-timescale frequency-offset optimization and the slow-timescale PA-deployment problems are formulated. A closed-form globally optimal offset is derived for the two-PA case, while a low-complexity one-dimensional search and an approximate closed-form offset under equal-spacing deployment are developed for the multi-PA case. The deployment analysis further shows that full-aperture equal spacing maximizes the guaranteed phase-tuning span and yields near-arithmetic relative delays for uniformly distributed users. Numerical results demonstrate that the proposed framework substantially improves both the sum rate and the worst-user rate over fixed-frequency transmission and approaches the coherent-combining upper bound.