Device-Free Localization with Reconfigurable Intelligent Surfaces: A Body Fading Model

Device-free localization (DFL) systems exploit variations in the received strength of signals exchanged between static radio nodes to infer the presence and position of a person. With the deployment of reconfigurable intelligent surfaces (RISs), additional programmable reflection paths can be established between the nodes. To exploit these paths for DFL, this work introduces a deterministic body-fading model based on the scalar theory of diffraction. The human body is represented as a two-dimensional knife-edge object, and its influence on the received electric field is analytically derived. A closed-form expression for the bodyinduced attenuation is obtained as a function of RIS aperture, link geometry, and body position. Simulations are conducted for sub-6 GHz and mmWave frequencies using equal physical apertures, showing qualitatively similar attenuation patterns but stronger spatial confinement at higher frequencies. The Analysis further reveals a diminishing attenuation with increasing RIS aperture, providing a physically grounded basis for RIS-assisted DFL design and other RIS-enabled Radio applications in dynamic environments.