Ambient backscatter powers battery-free wireless communication
In what could be a boost to the Internet of things (IoT), University of Washington engineers have created a new battery-free wireless communication system that takes advantage of what the researchers call “ambient backscatter.”
Michelle Ma of the university's news and information office writes, “Two devices communicate with each other by reflecting the existing signals to exchange information. The researchers built small, battery-free devices with antennas that can detect, harness, and reflect a TV signal, which then is picked up by other similar devices.”
Now, harvesting ambient RF energy isn't a new concept. As a child, I recall building an AM receiver that was supposed to employ ambient radio-frequency energy to power one or two stages of transistor audio amplification (it wasn't a crystal set). I don't recall that it was particularly successful, but that may have been because there was little ambient RF energy in the remote location in which I grew up. The inventor who published (in Popular Electronics, perhaps) the circuit I was trying to build undoubtedly lived in a city and had better luck.
In any event, as Harry Ostaffe, VP of marketing and business development at Powercast, puts it in an article hosted by Mouser Electronics, “RF energy is currently broadcasted from billions of radio transmitters around the world, including mobile telephones, handheld radios, mobile base stations, and television/ radio broadcast stations. The ability to harvest RF energy, from ambient or dedicated sources, enables wireless charging of low-power devices and has resulting benefits to product design, usability, and reliability. Battery-based systems can be trickle-charged to eliminate battery replacement or extend the operating life of systems using disposable batteries. Battery-free devices can be designed to operate upon demand or when sufficient charge is accumulated. In both cases, these devices can be free of connectors, cables, and battery access panels, and have freedom of placement and mobility during charging and usage.”
The University of Washington implementation follows the second, battery-free case. But whereas the typical use case is to harvest energy and use it to power a self-contained transceiver (or receiver in the case of my childhood effort), the ambient-backscatter approach “transforms existing wireless signals into both a source of power and a communication medium. It enables two battery-free devices to communicate by backscattering existing wireless signals.” The advantage? “Backscatter communication is orders of magnitude more power-efficient than traditional radio communication.”
Ma quotes lead researcher Shyam Gollakota, a UW assistant professor of computer science and engineering, as saying, “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.”
She further quotes Joshua Smith, a UW associate professor of computer science and engineering and of electrical engineering, as saying, “Our devices form a network out of thin air. You can reflect these signals slightly to create a Morse code of communication between battery-free devices.”
The researchers published their results at the Association for Computing Machinery’s Special Interest Group on Data Communication 2013 conference in Hong Kong. They have received the conference’s best-paper award for their research.