Description
The Doppler effect is the frequency shift induced on a propagating wave by relative motion between source and observer (or, in radar geometry, by motion of a reflector along the bistatic baseline). In WiFi/mmWave sensing it appears as a time-varying phase offset on each CSI sample whose rate is proportional to radial velocity. Doppler features — frequency-time spectrograms, micro-Doppler signatures, and Doppler-velocity bins — are the bridge between a passive radio observation and a kinematic interpretation (speed, direction, gait cycle).
When it's used
- Doppler-frequency spectrograms for HAR, gait, and gesture recognition
- Crowd-flow velocity estimation from CSI
- Passive WiFi radar range-Doppler maps
- Vital-sign sensing through chest-wall micro-Doppler
Limitations
- Frequency resolution bounded by observation window length (Heisenberg-style trade-off with time resolution)
- Multiple movers project onto the same Doppler bin without spatial separation
- CFO drift on commodity NICs can masquerade as Doppler