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Radio tomography or Radio tomographic imaging (RTI) is the emerging field which uses radio waves to image the transmission properties of an environment. Radio tomography is similar to computed tomography (CT) but using the medium of radio frequency (RF) sources rather than x-ray sources. Because the wavelength of radio waves is much wider than x-ray waves, radio tomography is used to image large environments at lower resolution. Since radio waves can pass through walls, they can be used to image inside of building structures, in particular, to show the motion of people in an area.

Technology

File:RTI-exp-photo-and-results.png

A prototype radio tomography experiment and estimated image

Radio tomography uses radio frequency sources similar to radar. However, radar is used to image the reflection and scattering properties of a medium. Typical radar devices send an RF pulse and record the back-scattered signal. In contrast, radio tomography images transmission properties, that is, how much each object attenuates the RF signal which passes through it.

Radio tomography, in contrast to other instances of tomography-based imaging methods, can be performed using standard wireless devices . Commercial transceivers (such as those in WiFi devices, cellular phones, or wireless sensors) are typically able to measure received signal strength (RSS). If the transmission power is known, then the path loss can be calculated. Path loss is a measure of the transmission properties of the medium.

To perform radio tomography, many such path loss measurements are needed. When wireless devices can communicate directly with each other (in a mesh networking topology), the number of path loss measurements possible increases as the number of wireless devices squared. Specifically, if there are $n$ wireless devices, there are $n(n-1)$ path loss measurements which can be made. The ability to estimate an image of the transmission properties of the medium increases with the number of nodes and the geometry in which they are deployed.

Radio tomography can either be phase-synchronous or phase-asynchronous. That is, transmission measurements at different points in space can be measured with or without a phase. The phase can provide information about the angle of arriving radio wave, or the phase-shift caused by the transmission medium. Phase measurements typically require highly accurate clocks at wireless devices at different points in space, or a wireless device in motion. Tests of phase-synchronous radio tomography have been called Ultra-narrowband.

Application

Radio tomography has application in both security and smart-home (home automation) systems.

Each year correctional and law enforcement officers are injured because they lack the ability to detect and track offenders through building walls (Hunt 2001). By showing the locations of people and walls within a building, during hostage situations, building fires, or after an earthquake, radio tomography could help law enforcement and emergency responders to know where they should focus their attention and recovery efforts.

Smart-homes promise to reduce energy consumption and improve living by focusing heating and air-conditioning to the part of a home where people are currently located. Smart surround sound systems could direct the 'sweet-spot', and active noise-cancellation could reduce noise exactly where people are located in a room. Interactive video games could use user position and movement within a room or building as inp

Related Research

Ultra-wideband (UWB) radars are being developed to image at what angle and distance the reflectors and scatters are located, even through walls. UWB radar uses one phase-synchronous, high-complexity device. Radars must transmit at high power since scattering losses increase as the distance to the fourth power. In contrast, radio tomography uses simple commercial wireless devices, and measure transmission losses, which increase approximately as the distance squared, and thus can operate at lower transmit powers.

Video surveillance may be insufficient for a variety of security needs. Video is essentially a passive light-wave electromagnetic image, thus it doesn't work in the dark, or through the smoke of a building fire. Video cameras typically have limited view angles and cannot see through walls. Further, video camera surveillance may be limited due to privacy concerns. Radio tomography is too coarse resolution to identify people.

Challenges

The key challenge in radio tomography is the spatial variability of the multipath radio channel. Electromagnetic waves at radio frequencies in cluttered environments do not travel only in straight-line paths, due to diffraction, reflection, and scattering (Rappaport 1996). However, shadowing losses due to interior and exterior walls, floors, and foliage in an environment are very significant and measurable factors in radio links (Durgin 1998). In comparison, x-rays travel in straight paths through a medium, a benefit for CT imaging.

See also

• Tomography

External Links

• Radio tomographic imaging research at the University of Utah
• Wicks, M.C.; Himed, B.; Bracken, J.L.E.; Bascom, H.; Clancy, J., "Ultra narrow band adaptive tomographic radar", 2005 1st IEEE International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, 13-15 Dec. 2005. Pages: 36 - 39