Description
CSI amplitude is the magnitude component of the complex channel response, |H(f, t)|, derived per subcarrier from raw CSI. It is the most stable CSI feature on commodity NICs because it is unaffected by the carrier frequency offset and packet-detection delay that corrupt phase. Amplitude time-series capture path-length changes from human bodies as smooth low-frequency modulations and from limb motion as higher-frequency Doppler ripples — the workhorse feature for occupancy, motion, and activity tasks.
When it's used
- Subcarrier-level occupancy and crowd-counting features
- Motion detection through amplitude variance / standard deviation
- Spectrogram inputs for HAR and gesture recognition
- Fingerprinting baselines where phase is unavailable
Limitations
- Sensitive to Automatic Gain Control jumps; needs AGC normalisation
- Loses sub-wavelength path-length resolution that phase-based methods capture
- Subject to bursty hardware noise on consumer NICs (Intel 5300 in particular)