Still-Person Presence Detection via WiFi Micro-Doppler

The hardest sensing case for conventional sensors is a person who is not moving. PIR reports empty. Cameras work but raise privacy concerns. Wavey can detect micro-presence — a still body whose chest rises and falls with respiration — through micro-Doppler analysis of WiFi CSI phase.

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Micro-Doppler physics

Breathing displaces the chest wall by a few millimeters at 0.1–0.5 Hz (6–30 breaths per minute). This creates a micro-Doppler modulation on the CSI phase: a narrowband periodic signal far weaker than footstep energy but structurally distinct from an empty room.

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Detection requirements

Bandpass filtering in the respiration band (0.1–0.5 Hz) followed by periodicity detection (autocorrelation or peak finding in the spectrum) confirms presence even when macro-motion variance is near zero. A complementary approach treats the breathing spectrogram as an image: in-car child-presence detection uses CSI power autocorrelation for macro motion and spectrogram enhancement plus CNN for breathing classification — 99% detection at 5 GHz with 58 subcarriers sampled at 30 Hz. The image-classification path on spectrograms is increasingly common when packet rate and SNR are sufficient; ESP32 deployments need careful placement and consistent traffic to reach comparable confidence.

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Still person vs empty room

The decision is SNR-dependent. An empty room has no periodic component in the respiration band. A still person adds one. The signal-to-noise ratio depends on distance, body orientation relative to the link, and how much multipath the torso perturbs. This is why presence is rung 5 on the detection ladder — achievable with good setup, not guaranteed everywhere. Wavey treats it as awareness, not a medical vitals measurement.

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Why PIR fails here

PIR detects changes in infrared radiation within a narrow cone. A motionless person emits steady IR — no change, no trigger. The sensor reports empty after its timeout. CSI does not measure heat. It measures how the radio channel changes. Periodic chest displacement modulates phase even when the person is otherwise still. This is a fundamental sensing mechanism difference, not a software improvement on the same hardware.

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Where presence matters

• Occupancy that does not time out on a still reader, sleeper, or worker
• Elder-care monitoring — confirming someone is in the room
• Smart-home automation — lights and climate that hold while you are present

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Further Reading