Description
Fresnel-zone analysis treats the LOS path between a Wi-Fi TX and RX as a stack of nested ellipsoidal zones. Movement through alternating Fresnel zones produces predictable π-radian phase rotations on the CSI ratio, which converts a generic learning problem into a geometric one: motion direction and zone-crossing rate map directly to displacement and velocity. It is the theoretical underpinning of csi-ratio-based fine-grained sensing.
When it's used
- Sub-cm motion sensing (respiration, gestures)
- Geometric models for
device-free-localization - Theoretical bounds on CSI sensing resolution
- Building-physics intuition for CSI deployments
Limitations
- Assumes a dominant LOS path; breaks under heavy NLOS
- Multipath rich indoor environments smear the ideal zone geometry
- Requires phase-stable CSI (i.e. ratio or sanitised phase)