Wireless technology is exploding as the hardware becomes cheaper and uses less power. Chances are good that the upward trajectory will continue over the next few years, as companies are betting that smart devices and remote controls that use the short-range IEEE 802.15.4 protocol will find a place on the market. Now, researchers have used one of the features that provides that protocol with robustness against noise in order to track the movements of people around a room they couldn't otherwise see into.
IEEE 802.15.4 is a short-range wireless standard that allows simple, low bandwidth transmission. Because it's extremely low power and resistant to interference, it has been adopted by Zigbee-compatible smart grid devices, and there are companies hoping it will replace the infrared transmitters in remote controls. Even though it operates in the same unlicensed frequency band as Bluetooth and some forms of WiFi, 802.15.4 avoids interference and crowding in part by sending its transmissions along multiple paths, ensuring that data can find its way around potential sources of interference.
This feature of 802.15.4 is what some researchers latched on to in order to probe a room that acted as a black box. Their work hasn't been through peer review, but it has been placed in the arXiv preprint system, where it was spotted by Technology Review.
The basic idea behind the work is the assumption that transmissions between any two IEEE 802.15.4 devices travel within an ellipse centered on a straight line between those points. It's not possible to know which path a given transmission took, as different paths will be subject to different interference conditions. But it is possible to figure out when something along the paths changes position, because the signal strength of a subset of the paths will change as a result of the altered interference.
On its own, this sort of signal would have atrocious resolution. But the authors suggest that positional information could be provided by a mesh of wireless nodes: each transmitter/receiver pair would provide a distinct slice across the room, and allow the position of the motion to be reconstructed in much the same way a CAT scan reconstructs three-dimensional information from a series of image slices through a sample.
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