Glossary
The terms that come up when people talk about Wi-Fi sensing
Short definitions, written for people who already know networking and want to learn the vocabulary specific to sensing.
- IEEE 802.11bf
- Amendment to the Wi-Fi standard that defines sensing procedures between 1 and 7.125 GHz and above 45 GHz. Approved in May and published in September 2025.
- Amplitude and phase
- The two quantities CSI reports for each subcarrier. Amplitude is more stable and easier to use; phase is richer but requires correcting offsets between the transmitter and receiver clocks.
- BFI (Beamforming Feedback Information)
- Compressed report of the channel matrix that a client sends to the access point to enable beamforming. It can be used for sensing without firmware access.
- Bistatic and multistatic sensing
- Bistatic: transmitter and receiver are different devices. Multistatic: several pairs take part in the measurement. Monostatic: the same device transmits and receives, as in radar.
- CFO (Carrier Frequency Offset)
- Difference between the carrier frequencies of transmitter and receiver. One of the causes of noise in CSI phase.
- CSI (Channel State Information)
- Channel estimate the receiver computes for each subcarrier from the preamble of each OFDM frame. The main raw material for sensing.
- Device-free
- Sensing that detects a person through the disturbance they cause in the channel, without them carrying anything. The opposite is device-based sensing.
- Doppler
- Frequency shift caused by a moving reflector. The Doppler spectrum of CSI makes it possible to estimate the speed of whoever is moving.
- ESP32
- Espressif's family of Wi-Fi microcontrollers. It exposes CSI in a documented way, which made it the most widely used low-cost platform for research and prototyping.
- MAC randomization
- Use of temporary MAC addresses by mobile operating systems to hinder tracking. It complicates device counting and underlines that this data is personal.
- Monitor mode
- Operating mode in which the network interface receives every frame on the channel, not only those addressed to it. The basis of passive sensing.
- Multipath
- Arrival of the same signal at the receiver along several paths (direct and reflected), with different delays and attenuations. The phenomenon sensing exploits.
- Nexmon
- Firmware patching framework for Broadcom and Cypress Wi-Fi chips that makes it possible to extract CSI from devices such as the Raspberry Pi.
- OFDM
- Orthogonal Frequency-Division Multiplexing. A technique that splits the channel into many narrow subcarriers. Used by Wi-Fi since 802.11a/g.
- Probe request
- Management frame a device sends to discover nearby networks, even when not connected. Makes it possible to estimate device presence.
- RSSI
- Received Signal Strength Indicator: total power of the received frame. Available on any device, but with little resolution for sensing.
- Sensing by proxy (SBP)
- 802.11bf mechanism in which a client asks the access point to run sensing measurements on its behalf and deliver the results.
- Subcarrier
- Each of the narrow frequencies into which OFDM divides the channel. Wider channels mean more subcarriers and more detail in the CSI.
- TR-369 (USP)
- User Services Platform, the Broadband Forum protocol for managing customer premises equipment. Successor to TR-069 and a natural path for distributing sensing agents.