Radio waves remember the room
When a WiFi packet crosses a room, it does not travel one path — it reflects off walls, bends around furniture, and passes through people (losing energy on the way). The receiving chip must undo all that distortion to decode the packet, so it first measures it. That measurement is Channel State Information (CSI).
56 numbers, 50 times a second
WiFi (802.11n, 20 MHz) splits its channel into subcarriers — think 56 parallel slivers of spectrum. For each one, the chip records how much the wave was attenuated (amplitude) and shifted (phase). One CSI frame = 56 amplitudes + 56 phases. Our nodes capture ~50 frames per second.
A human body is mostly water; water absorbs and reflects 2.4 GHz radio. A person standing still changes which paths exist; a person moving changes them rhythmically. In the CSI stream you literally watch breathing as a slow oscillation on a handful of subcarriers.
Why CSI beats RSSI
The signal-strength number (RSSI) your phone shows is one coarse average — it barely notices a person. CSI is the full 56-lane picture: it notices a hand wave, and with care, a heartbeat. Same chip, same packets, 56× more information.
Why this matters commercially
Cameras cannot go in bathrooms, data-center cages, hazardous zones, or anywhere privacy is law. Radio sensing sees presence and motion without ever seeing a face — that is why enterprise deployments (data centers, industrial sites) run on exactly the firmware you flashed in the CSI Presence Node solution.