Pixel-Based Fluid Antenna System for Device-Free ISAC: Turning Fluid-State Diversity Into Received-Signal-Strength Sensing

Abstract

Low-cost device-free integrated sensing and communication (ISAC) is constrained by fixed-pattern received-signal-strength (RSSI) measurements with limited spatial contrast. This letter develops a single-RF-chain access point with a 12-bit pixel-based fluid antenna system. Uplink frames support the communication function, while their RSSI supplies sensing observations across aperture states. Dual-difference calibration identifies 22 effective modes; model-based initialization and joint sensing–communication refinement select states under cycle-time and communication-quality constraints. Across 25 positions, selected K22 outperforms monopole, model-based, and full-sweep baselines, achieving 96% exact-position accuracy and 0.02 m mean distance error, with minimum per-device normalized goodput and packet delivery ratio of 0.999 and 97.97%.