Doppler shifts in Wi-Fi signals could be used for surveillance, researchers say

Researchers at University College London (UCL) have developed technology that uses Wi-Fi to "see" through walls. It works like radar, detecting Doppler shifts in radio frequency waves, which could potentially be used to detect humans in surveillance and military situations.

In order to find the location of an object or person, the UCL system compares a reference channel, or the system receiving the RF signal, and a surveillance channel, which is monitoring the Doppler shift. The technology has the ability to work from multiple reference channels, including IEEE 802.11 b, g, n and ac standards, as well as GSM and LTE wireless signals.

According to the IEEE, despite similar radar-like detectors based on "household communication bands," UCL's system is unique because it's a one-way system. Where traditional radar surveillance is detectable due to the need to bounce waves off of the target, Wi-Fi surveillance uses only passive radiation, or existing wireless waves, making it undetectable and therefore more secure for military and law enforcement use.

Karl Woodbridge, part of the UCL research team, said part of the technology's appeal is its flexibility and software-defined architecture.

"You keep the same hardware and all you have to do is change the code or recode the system and it will adapt to a completely different sort of signal," Woodbridge said in a UCL document. "That is obviously a lot easier and more adaptable than changing great chunks of hardware every time."

In addition to its ease of deployment, UCL's system could soothe the worries many people have about invasion of privacy where surveillance is involved. Rather than taking a photo or recording other identifying information, the technology's reliance on Doppler shifts simply identifies the presence of humans and is able to do so using existing Wi-Fi infrastructure rather than the installation of additional technology.

In a similar vein, MIT researchers are working on a technology they call "Wi-Vi." This device uses active radars built through walls to detect and capture signals and essentially "see through walls." Further, a separate study using particle filters to interpret small frequency shifts has left researchers with the conclusion that Sequential Importance Resampling (SIR) filters make Doppler shift Wi-Fi technology even more accurate.

As is often the case with military and public safety technology, there is already speculation for commercial uses. This could include monitoring patient activity in the healthcare industry, as well as tracking human activity for retail purposes.

For more:
- see this UCL document
- see this Network World article
- see this TechWorm article

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