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Holographic analysis of wi-fi data generates 3D images of surroundings

Researchers from the Technical University of Munich (TUM) have developed a way of generating holograms using the microwave signals of a wi-fi transmitter, enabling them to produce three-dimensional images of surrounding environments.

The researchers envision the process being used for tracking objects in industry 4.0 scenarios, or for locating trapped victims buried under avalanches or collapsed buildings.

Credit: Technical University of Munich

Until now, generating images from microwave radiation has required special-purpose transmitters or elaborate laser technology. In the latest issue of Physical Review Letters, however, the researchers describe how just two antennas, one fixed and one moveable, can be used to produce holograms with the technique.

‘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,’ explained Friedemann Reinhard, director of the Emmy Noether Research Group for Quantum Sensors at TUM.

The technique, known as wi-fi holography, can be used to process the very small bandwidths of wi-fi, Bluetooth and cell phone signals – operating in the 2.4 and 5 gigahertz bands. The wavelengths of these devices correspond to a spatial resolution of a few centimetres.

Reinhard remarked: ‘Of course, this raises privacy questions. After all, to a certain degree even encrypted signals transmit an image of their surroundings to the outside world. However, it is rather unlikely that this process will be used for the view into foreign bedrooms in the near future. For that, you would need to go around the building with a large antenna, which would hardly go unnoticed.’

The use of multiple antennas to detect signals enables the researchers to take an alternate approach in producing their holograms, and potentially improve the quality of them with future wi-fi developments. ‘Instead of a 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,’ explained Philipp Holl, who executed the experiments of the research. ‘Future wi-fi frequencies, like the proposed 60 gigahertz IEEE 802.11 standard will allow resolutions down to the millimetre range.’

According to the researchers, already established image processing methods can be used to enhance wi-fi holography, such as the dark-field methodology used in microscopy to improve the recognition of weakly diffracting structures, or white-light holography, where the remaining small bandwidth of the wi-fi transmitter can be used to eliminate noise from scattered radiation.

Treating microwave holograms like optical images will allow the additional information of a microwave image to be combined and embedded into a camera image, such as that of a smart phone, enabling functions such as the tracing of a radio tag attached to a lost item.

The researchers are still at the beginning of the development; further research into the transparency of specific materials could lead to the development of paint or wall paper translucent to microwaves, enabling the protection of privacy. Transparent materials could also be deployed in factory halls to allow certain objects to be tracked as part of Industry 4.0 automation.

The researchers hope that further advancement of the technology may aid in the recovery of victims buried under avalanches or collapsed buildings. While conventional detection methods only allow the 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 work out the easiest way to approach victims.

The research was funded by the Emmy Noether Programme of the German Research Foundation (DFB) and the TUM Junior Fellow Fund.

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