ONE of the problems with using wireless for security applications is that batteries need to be replaced. Sure, these are more affordable and now last longer but in a typical wireless system there might be a dozen that need replacing at varying intervals for the life of the device network. For the electronic security industry, battery-free wireless devices would be a wondrous thing. 
The communication technique devised by researchers at UoW is called ambient backscatter, and anyone who has ever run a wireless background test on a typical city site will understand exactly what this means. Instead of using a powered oscillator to create its own wireless signals, an ambient backscatter device leeches signal power from TV and cellular transmissions that are propagating through the environment and recycles them. 
To prove their concept, researchers built small, battery-free devices with antennas that can detect, harness and reflect a TV signal, which was picked up by other devices. The researchers tested the ambient backscatter technique with credit card-sized prototype devices placed within a metre feet of each other. For each device, the researchers built antennas into ordinary circuit boards that flash an LED light when receiving a communication signal from another device.
Groups of the devices were then tested in a variety of settings in the Seattle area, including inside an apartment building, on a street corner and on the top level of a parking garage. These locations ranged from less than 800m from a TV tower to about 10 kays away.
They found that the devices were able to communicate with each other, even those farthest from a TV tower. The receiving devices picked up a signal from their transmitting counterparts at a rate of 1 kilobit per second. This is enough to send information such as a sensor reading, text messages and contact information.
It’s also feasible to build this technology into devices that do rely on batteries but whose batteries have failed, included security alarm panels. A system could be configured so that when power fails and its backup battery dies, an alarm panel could still send text messages by leveraging power from an ambient TV signal.
“We can now re-purpose wireless signals that are already around us into both a source of power and a communication medium,” said lead researcher Shyam Gollakota, a UW assistant professor of computer science and engineering. 
“It’s hopefully going to have applications in a number of areas including wearable computing, smart homes and self-sustaining sensor networks.”
The researchers published their results at the Association for Computing Machinery’s Special Interest Group on Data Communication 2013 conference in Hong Kong. 
They received the conference’s best-paper award for their research. The applications are endless, the researchers say, and they plan to continue advancing the capacity and range of the ambient backscatter communication network.
“Our devices form a network out of thin air,” said co-author Joshua Smith, a UW associate professor of computer science and engineering and of electrical engineering. 
“You can reflect these signals slightly to create a Morse code of communication between battery-free devices.”

“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”