Holographic tech uses wi-fi routers to see through walls


German scientists claim breakthrough in imaging technology using standard wi-fi signals. Angus Bezzina reports.


A cross made of aluminum foil and the image of it reconstructed from the WLAN-hologram (inset).
Friedemann Reinhard / Philipp Holl / TUM

A standard wi-fi router has more to offer than just high-speed internet access. The device can also be used to visually scan the inside of your home, according to a study by two German scientists.

While wi-fi imaging itself is not a new concept – the principle is akin to radar technology – the narrow bandwidths used by commercial wi-fi routers and the presence of multiple reflections from nearby objects have made it difficult to create an image out of standard wi-fi signals. Until now.

Enter Friedemann Reinhard and Phillip Holl from the Technical University of Munich.

The duo found that the solution to this problem was to use the entire wi-fi signal that reaches a detector, rather than just parts of the data – like the angle of arrival – as previous methods had done.

Reinhard and Holl treated the received wi-fi signal like a hologram – a 2-D encoding of a 3-D image – that had recorded the environment surrounding the router.

To decode the signal, they used standard mathematical principles for holographic imaging and a remote-sensing technique known as synthetic aperture radar.

Using simulated emissions from the wi-fi router in the basement of a warehouse (left), a three-dimensional image of the building (right) can be reconstructed with holographic analysis of the microwave radiation.
Friedemann Reinhard/ Philipp Holl / TUM

Using their wi-fi imaging method, they successfully identified a one-metre-wide metal cross within a two-by-three-metre space.

They were also able to demonstrate a computer simulation of a signal for the interior of a small building.

There are still notable limitations, however, as a large recording area would be required to scan a small building in the real world; and the length of time required to scan an environment is also an issue.

But Holl expresses confidence that both these issues could be resolved using arrays of antennas. “Even using a single row of antennas, carried, for instance, by a drone or truck, could reduce the recording time to a few seconds.”

In the future, he says, the technology could feasibly be used for tasks such as locating products in a multi-storey factory.

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