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Holographic Process Uses WiFi to Generate 3D Images

Photonics.com
May 2017
MUNICH, May 16, 2017 — A holographic imaging process that generates 3D images using the microwave radiation of a Wi-Fi transmitter could be used in automated industrial facilities to track objects as they move through production. Although processes already exist that allow the localization of microwave radiation, even through walls, this process is different in that it allows an entire interior space to be imaged via holographic processing of Wi-Fi or cellphone signals.

The process for acquiring 3D images from the radiation of an unmodified, commercial, narrowband Wi-Fi router does not require any prior knowledge of the emitted radiation and works with any type of signal, including encrypted communication.

Holographic imaging using WiFi, TUM.

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

To develop the process, researchers from Technical University of Munich (TUM) recorded holograms formed by the coherent light emitted from wireless data transmission systems in a phase-coherent fashion, across a meter-sized imaging region. Three-dimesional views of objects and emitters were obtained by feeding the resulting data into digital reconstruction algorithms. Employing a digital implementation of dark-field propagation to suppress multipath reflection, researchers were able to significantly enhance the quality of the resulting images. The hologram of a 10-m-sized building was numerically simulated. The team believes that both localization of emitters and 3D tomography of absorptive objects is feasible using this 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,” said Friedemann Reinhard, director of the Emmy Noether Research Group for Quantum Sensors.

Until now, generating images from microwave radiation has required special-purpose transmitters with large bandwidths. Using holographic data processing, the team was able to generate images using the very small bandwidths of typical household Wi-Fi transmitters operating in the 2.4 and 5 gigahertz (GHz) bands. According to the team, even Bluetooth and cellphone signals could be used. The wavelengths of these devices correspond to a spatial resolution of a few centimeters.

Holographic imaging using WiFi, TUM.
Generating holograms from the microwave radiation of a Wi-Fi transmitter requires merely one fixed and one movable antenna. Using this technology, it is possible to generate a three-dimensional image of the space around the Wi-Fi transmitter. Courtesy of Friedemann Reinhard/Philipp Holl / TUM.

Unlike optical holograms, which require elaborate laser technology, generating holograms with a Wi-Fi router requires just one fixed and one movable antenna.

“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 researcher Philipp Holl. “Future Wi-Fi frequencies, like the proposed 60-gigahertz IEEE 802.11 standard, will allow resolutions down to the millimeter range.”

The team investigated the feasibility of a realistic large-scale reconstruction and tracking application. Its 3D reconstruction of a hologram clearly showed sharp outlines of metallic shelves and bars in their respective reconstructed planes.

Holographic imaging using WiFi, TUM.
From the 'light' of the WLAN router in the basement, the three-dimensional image of a warehouse (right) can be reconstructed with holographic analysis of the microwave radiation. Courtesy of Friedemann Reinhard/Philipp Holl / TUM.

The team is planning several future improvements to their technique. For example, a stationary 2D array of antennas could replace the slow, synthetic aperture approach of mapping a wave front by pointwise scanning. Such a device could provide pictures several orders of magnitude faster.

The capability to treat microwave holograms like optical images would allow the microwave image to be combined with camera images. The additional information extracted from the microwave images could be embedded into the camera image of a smartphone, for example to trace a radio tag attached to a lost item.

Research on the transparency of specific materials would facilitate the development of paint or wallpaper translucent to microwaves, useful for privacy protection, while transparent materials could be deployed in factory halls to allow parts to be tracked.

The researchers also hope that with further advancement, the technology could be used to recover victims buried under an avalanche or a collapsed building. While conventional methods only allow point localization 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 systematically elucidate the easiest way to quickly reach victims.

The research was published in Physical Review Letters (doi: 10.1103/PhysRevLett.118.183901). 

GLOSSARY
hologram
An interference pattern that is recorded on a high-resolution plate, the two interfering beams formed by a coherent beam from a laser and light scattered by an object. If after processing, the plate is viewed correctly by monochromatic light, a three-dimensional image of the object is seen.  
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