Microwave radiation

Holograms using wi-fi

Print edition : May 26, 2017

A cross made of aluminum foil between the viewer and the WLAN-router can be reconstructed from the WLAN-hologram as can be seen in the inserted picture. Photo: Friedemann Reinhard/Philipp Holl/TUM

SCIENTISTS at the Technical University of Munich (TUM) have developed a holographic imaging process that enables the radiation of a standard Wi-Fi router to generate three-dimensional images of the surrounding environment, including that behind walls.

While optical holograms require elaborate laser technology, generating holograms with the scattered microwave radiation of a Wi-Fi transmitter requires just one fixed and one movable antenna. Friedemann Reinhard and Philipp Holl of TUM have reported this development in the latest issue of Physical Review Letters.

“Using this technology, we can generate a three-dimensional image of the space around the wi-fi transmitter, as if our eyes could see microwave radiation,” Reinhard said. The researchers envision fields of deployment, especially in automated industrial facilities, where operators could use it to track objects as they move through production halls.

Processes that allow the localisation of microwave radiation, even through walls, or in which changes in a signal pattern signify the presence of an object already exist. The novelty is that an entire space can be imaged via holographic processing of ambient Wi-Fi, cell phone or even Bluetooth signals. So far, generating images from microwave radiation required special-purpose transmitters with large bandwidths. Using holographic data processing, the very small bandwidths of a typical household Wi-Fi router operating between 2.4 GHz and 5 GHz were sufficient, the researchers found; they would correspond to a spatial resolution of a few centimetres. “Instead of using a movable antenna, which measures the image point by point, one can use a larger number of antennas to obtain a video-like image frequency,” said Holl, who executed the experiments. “Future Wi-Fi frequencies, like the proposed 60 GHz standard, will allow resolutions down to the millimetre range.”

Experiments showed the imaging method could recover images of metre-sized objects within a room. The team also suggested that the technique could capture snapshots of a dynamic environment at a rate of about 10 frames a second.

The concept of treating microwave holograms like optical images allows the microwave image to be combined with camera images. The additional information extracted from the microwave images can be embedded into the camera image of a smartphone, for example to trace a radio tag attached to a lost item.

The researchers hope that further advancement of the technology may aid in the recovery of victims buried under an avalanche or a collapsed building. While conventional methods only allow point localisation of victims, holographic signal processing could provide a spatial representation of destroyed structures, allowing first responders to navigate around heavy objects and use cavities in the rubble to identify the easiest approach to quickly reach victims.

Compiled by R. Ramachandran