Description
Channel State Information (CSI) is the per-subcarrier complex channel response sampled by an OFDM receiver during normal Wi-Fi packet reception. Each CSI sample is a vector of complex numbers (one per subcarrier per TX/RX antenna pair) describing how the multipath channel attenuates and phase-shifts each frequency bin, which makes it a far richer sensing observable than scalar RSSI. CSI is the foundational primitive of WiFi sensing — every downstream task in this thesis (movement detection, occupancy estimation, crowd dynamics) starts from a CSI time-series.
When it's used
- Every WiFi-sensing pipeline that goes beyond coarse RSSI
- Indoor occupancy / crowd-counting systems on commodity hardware
- Activity, gesture, gait, and fall recognition
- Passive radar and Doppler-based motion sensing
- 802.11bf-style integrated sensing-and-communication research
Limitations
- Available on only a handful of NICs (Intel 5300, Atheros AR9580, Broadcom/Cypress via Nexmon, ESP32 vendor extension); modern chipsets often lock down access
- Phase is not directly usable on most NICs without sanitisation (CFO, SFO, PBD, PDD must be removed or differenced out)
- Highly environment-dependent: small rotations of antennas or furniture changes shift the multipath profile
Source Papers
- ma2020_4782 ↗ — survey of WiFi sensing built directly on CSI
- xie2015_0389 ↗ — precise power-delay-profile extraction from 802.11n CSI
- fallani2026_04be ↗ — recent CSI-based occupancy/health framing
- chen2023_5cbd ↗ — CSI generalisation taxonomy
- halperin2010_94f8 ↗ — original 802.11n CSI tool release on Intel 5300