Using so-called ambient backscatter, these devices can reportedly interact with users and communicate with each other without using batteries because they exchange information by reflecting or absorbing pre-existing radio signals from TV and cellular transmissions.
The researchers have 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.
‘We can repurpose 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 currently taking place in Hong Kong.
‘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.’
According to the university, smart sensors could be built and placed permanently inside nearly any structure, then set to communicate with each other. It is claimed sensors placed in a bridge could monitor the health of the concrete and steel, then send an alert if one of the sensors picks up a hairline crack. The technology can also be used for communication – text messages or emails – in wearable devices.
The researchers tested the ambient backscatter technique with credit card-sized prototype devices placed within several 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 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 half a mile away from a TV tower to about 6.5 miles away.
Researchers demonstrated how one payment card can transfer funds to another card by leveraging the existing wireless signals around them; ambient RF signals are the power source and the communication medium.
They found that the devices were able to communicate with each other, including the ones farthest from a TV tower. The receiving devices picked up a signal from their transmitting counterparts at a rate of 1 kilobit per second when up to 2.5 feet apart outdoors and 1.5 feet apart indoors. This is enough to send information such as a sensor reading, text messages and contact information.
It is further claimed that the new system could be built into devices that do rely on batteries, such as smartphones. It could be configured so that when the battery dies, the phone could still send text messages by leveraging power from an ambient TV signal.
The researchers plan to continue advancing the capacity and range of the ambient backscatter communication network.