Description
The CSI ratio is the per-subcarrier complex quotient of CSI streams from two RX antennas on the same NIC, H_a(f,t) / H_b(f,t). Because the dominant phase errors (CFO, SFO, PBD) are common-mode across antennas of one receiver, the ratio cancels them out, yielding a stable complex feature that retains both amplitude and phase information without any calibration. It is the de-facto preferred CSI feature for fine-grained sensing on commodity hardware.
When it's used
- Robust respiration / heartbeat sensing
- Sub-cm motion tracking and gesture recognition
- Replacing brittle phase-sanitisation pipelines
- Fresnel-zone analyses where phase rotation matters
Limitations
- Requires at least two RX antennas on the same NIC
- Numerator/denominator antenna choice matters; deep-fade antennas destabilise the ratio
- Hardware non-idealities not perfectly common-mode (e.g. per-antenna AGC) leak into the ratio