Revel in the Ambiance
August 30, 2013
When you read a traditional (paper and ink) book, you're making use of an ambient electromagnetic field. The Sun, or a lamp, is providing photons indiscriminately to illuminate your space, and you're making use of the small fraction that reflect from the page and reach your eyes. When you're reading by sunlight, it's about as close to a free lunch as you can get.
When you read, you make use of the ambient radiation in the visible spectrum, but the Sun also provides plenty of invisible infrared and ultraviolet radiation. The Sun's infrared and ultraviolet are the most intense of the invisible ambient radiation, but nature provides other sources.
There are natural processes, such as lightning, that provide ambient radiation at radio wavelengths. The human body, itself, glows brightly at about 100 GHz, and this fact is used as the basis for millimeter wave security scanners. Although the Sun is surprisingly quiet at radio frequencies, other objects in the universe produce radio waves detected by radio astronomers.
As I wrote in a previous article (Harvesting Radio Frequency Energy, November 22, 2011), modern technology has enhanced the glow of the ambient radio spectrum to the point at which it's possible to harvest this energy to power small electronic devices. Useful energy can be harvested from radio and television broadcasting stations, and it's locally generated by Wi-Fi devices in our homes and offices. Just a few months ago, I wrote about research at the University of Washington computer scientists on gesture recognition using ambient Wi-Fi radiation (Gesture Recognition, June 17, 2013).[1-2]
Not exactly ambient field devices, but close cousins, are RFID devices. These are like ambient field devices, but they differ in the fact that the external field is purposefully generated, typically at a fixed frequency. RFID devices are designed to harvest energy at that frequency to energize their electronics, which often include a transmitter.
The idea of using generated and ambient fields to power devices has been around for quite some time. The most interesting example is an invention by electronic music pioneer, Léon Theremin. Theremin's invention, was a covert listening device embedded in a wood carving of the Great Seal of the United States. The seal was a wall decoration in the US ambassador's Moscow home.
Theremin's device incorporated a quarter-wavelength antenna excited by a carrier wave from a remote transmitter operating at about 330 MHz. This carrier wave was phase modulated by a condenser microphone, and speech signals were detected at a remote receiver. More information on this device can be found in my previous article, Cyber Warfare, November 10, 2011.
Some of the same University of Washington engineers who did the Wi-Fi gesture recognition research mentioned above have just published details of an ambient backscatter device at the Association for Computing Machinery's Special Interest Group on Data Communication (SIGCOMM) 2013 conference in Hong Kong. The research team received the best-paper award at that conference.
These ambient backscatter devices operate by harvesting ambient RF energy for power, and then absorbing or reflecting these pre-existing radio signals for communication. Such devices need no battery power and no human attention. Says Shyam Gollakota, an assistant professor of computer science and engineering at the University of Washington and team leader,
|Even I've dabbled in passive RF communications. This is figure four of my January 29, 2013, patent, "Modulated antenna for wireless communications." In this case, I'm modulating an ambient radio frequency field using a Sterba Curtain antenna. (US Patent No. 8,362,961, via Google Patents.)|
"We can repurpose wireless signals that are already around us into both a source of power and a communication medium... It's hopefully going to have applications in a number of areas including wearable computing, smart homes and self-sustaining sensor networks."
A block diagram of an example ambient backscatter device is shown in the figure.
One important application for such devices is for communication with structural health sensors embedded into structures. Such sensors could communicate the presence of a crack. Credit-card sized proof-of-concept devices were constructed and tested in the Seattle area at locations ranging from about a half mile to six and a half miles distant from a television tower (see photograph).
These devices could communicate at a data rate of a kilobit per second over a range of 2.5 feet outdoors, where the ambient signals were stronger, and 1.5 feet indoors, where the ambient signals were weaker. This was more than enough to light a light-emitting diode associated with a particular button press and transfer "funds" from one credit card to another, as shown in a YouTube video.[5-6]
The research was funded by a Google Faculty Research Award and by the National Science Foundation's Research Center for Sensorimotor Neural Engineering at the University of Washington.
|Block diagram of an RF ambient backscatter device. |
(Modified version of fig. 3 of ref. 4, rendered using Inkscape.)
- Michelle Ma, "Wi-Fi signals enable gesture recognition throughout entire home," University of Washington Press Release, June 4, 2013.
- Whole-Home Gesture Recognition Using Wireless Web Site at the University of Washington.
- Devlin M. Gualtieri, "Modulated antenna for wireless communications," US Patent No. 8,362,961, January 29, 2013.
- Vincent Liu, Aaron Parks, Vamsi Talla, Shyamnath Gollakota, David Wetherall and Joshua R. Smith, "Ambient Backscatter: Wireless Communication Out of Thin Air," Paper Prepared for SIGCOMM'13, August 12–16, 2013, Hong Kong, China.
- Michelle Ma, "Wireless devices go battery-free with new communication technique," University of Washington Press Release, August 13, 2013.
- University of Washington Ambient Backscatter Devices, YouTube Video.
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