Description
CSI phase is the angle component of the complex channel response per subcarrier. Raw phase from commodity NICs is contaminated by carrier-frequency offset (CFO), sampling-frequency offset (SFO), and packet-boundary detection delay (PBD), so it is rarely used directly. Two practical workarounds dominate: linear-fit phase sanitisation across subcarriers, and inter-antenna phase difference, which cancels common-mode RX errors. Sanitised phase carries sub-wavelength path-length information that amplitude cannot match — critical for fine-grained motion (respiration, micro-gestures) and angle-of-arrival estimation.
When it's used
- Inter-antenna phase difference for device-free localisation and AoA
- Respiration / vital-sign sensing from phase periodicity
- High-resolution gesture recognition
- DFS reports / 802.11bf phase-based sensing primitives
Limitations
- Hardware-specific: phase calibration tables are NIC-dependent
- Phase wrap and reference ambiguity make absolute phase non-portable
- Multipath fading produces deep nulls that destabilise phase locally
Source Papers
- fallani2026_04be ↗ — sanitised phase for in-home health sensing
- hou2023_bf83 ↗ — phase-aware few-shot CSI features
- xie2015_0389 ↗ — phase calibration for power-delay-profile extraction
- wang2015_48cf ↗ — phase-difference features for activity sensing
- ma2020_4782 ↗ — phase-handling chapter of the WiFi sensing survey