Security sensing needs to work when lights are off, cameras are blind, and the intruder carries nothing. WiFi CSI detects bodies by how they disturb the radio field — device-free, darkness-invariant, and effective through many interior walls.
NLOS motion through building materials
WiFi at 2.4 and 5 GHz penetrates drywall, plaster, and wooden doors. An intruder moving in an adjacent room perturbs multipath paths that traverse the wall — amplitude shifts propagate to the receiver even without line of sight.
This is zone-level detection: “motion occurred in this area,” not “person at coordinates (x, y).” Concrete, brick, and metal-shielded walls attenuate heavily; interior drywall partitions are the practical case.
The detection ladder places through-wall localization at rung 10 — partial motion detection yes, precise reconstruction no.
Darkness and obstacle invariance
CSI does not depend on ambient light. WiVi (CVPR Workshop 2019) demonstrated this quantitatively: a vision classifier dropped from 95% to 46.67% accuracy with lights off, while a WiFi CSI classifier held at 95.83%. Their system disables the vision module when brightness falls below threshold and falls back to WiFi-only — decision-level fusion, not feature fusion, so one modality failing does not corrupt the other.
A PIR sensor in a dark corner may miss an intruder crawling below its cone. A camera needs illumination or infrared supplementation. WiFi sensing reads the same signal at midnight and noon.
Coverage is whole-room from a node pair, not a single cone. Obstacles that block line of sight to a camera may still allow radio paths to reach the receiver.
Device-free vs MAC tracking
MAC-address security tracks phones and laptops — useless when the intruder carries nothing, or when you want to detect any body regardless of device ownership. CSI sensing detects the physical perturbation of a human body on the channel itself.
This also means CSI cannot be defeated by leaving a phone behind. The body is the signal.
What you learn and what you do not
| Signal | Available | Not available |
|---|---|---|
| Motion in a zone | Yes | — |
| Approximate timing | Yes | — |
| Identity | — | No (research: 91% gait ID among 20 enrolled subjects; not Wavey scope) |
| Appearance | — | No |
| Precise position | — | No (without dense node grid) |
| Through concrete walls | — | No |
Wavey baselines the protected space during known-empty periods and flags motion and presence that violate the baseline. Pair with conventional alarms for verification — CSI tells you something moved, not who.
Further reading
- Motion & activity detection — motion event semantics
- Detection ladder — through-wall feasibility
- Getting started