Battery-Free Cellphones on the Horizon

University of Washington researchers recently demonstrated the world's first battery-free cellphone, created with funding from the U.S. National Science Foundation (NSF) and a Google Faculty Research Award for mobile research.

The battery-free technology harvests energy from the signal received from the cellular base station (for reception) and the voice of the user (for transmission) using a technique called backscattering. Backscattering for battery-free operation is best known for its use in radio frequency identification (RFID) tags, typically utilized for applications such as locating products in a warehouse and keeping track of high-value equipment. An RFID base station (called a reader) "pings" the tag with an RF pulse, which allows the tag to harvest microwatts of energy from it—enough to return a backscattered RF signal modulated with the identity of the item.

"The obvious thing about energy harvesting is you don't need batteries, but it's only in the past few years that anyone has been able to make use of it," said Lee Ratliff, principal analyst of connectivity and Internet of Things (IoT) for IHS Markit in Dallas, TX. "Unfortunately, harvesting generates very little energy; so little, that you really need a new standard. For instance, Wi-Fi signals transmit continuously, but harvesting that energy constantly will only enable transmissions of about 10 feet today. Range will be the big challenge for making this technology successful."

Cellphones today generate their own RF signals, requiring up to 10,000 times as much energy, according to Google Scholar Joshua Smith, a professor of computer science and electrical engineering at the University of Washington. His Battery-Free Cellphone project uses backscatter communications to modulate the returning signal to the base station, but requires two harvesters (compared to RFID's one). One energy harvester on the handset harvests energy coming from the base station; the other harvests energy from the speaker's voice to generate the needed 3.5-microwatts to modulate backscattered speech.

The base station signal (a 900MHz sine wave in the prototype—the same unlicensed frequency used by many consumer cordless telephones, whose cradles serve as their base stations) is received by a software defined radio in the cellphone handset. Passive electronic components there demodulate the incoming signal from 900MHz down to the direct current needed to run the electronics, including a small microcontroller. Likewise, the speaker's voice into the handset uses passive components to harvest enough power to amplitude-modulate (AM) the backscattered 900MHz signal with the speaker's voice. The base station can then extract the speech from the amplitude-modulated backscattered signal and sends the voice along to the connected party.

"The user's speech uses direct transducer modulation of the reflected radio waves to drive a transistor connected to the antenna, which amplitude-modulates the backscattered 900MHz signal that gets picked up by the base station," said Smith.

The key to Smith and colleagues' patented success was moving all the power-consuming parts to the base station, such as the analog-to-digital (A/D) converter required to make the analog voice signal compatible with the digital Internet. Unlike RFID tags which only harvest and return an RF pulse, the challenge for the University of Washington team was to generate the 3.5 microwatts required by their battery-free cellphone continuously while the speaker is making a call.

On the downside, a switch on the handset was required to toggle between reception and transmission modes, much like a walkie-talkie. Also, the range of the battery-free cellphone prototype is limited to about 30 feet today—similar to that of a cordless phone. The addition of a small solar cell harvesting ambient light in the room, like those found on a battery-free calculator, increased the range of the battery-free cellphone to 50 feet.

Greater Range on the Horizon

The reason Smith et. al. call their invention a battery-free cellphone is their roadmap to extend the range  of battery-free sensor networks and machine-to-machine (M2M) applications, until they can meet the requirements of cellphones. By slowing down the bit-rate below that of speech, they have already demonstrated battery-free sensor networks that work over much longer distances.

Deepak Ganesan, a backscatter researcher at the University of Massachusetts at Amherst, agrees. Ganesan is hopeful about using backscatter technology for cellphones eventually, but that will not happen without surmounting many engineering hurtles from the knowledge gained from optimizing battery-free sensor networks and machine-to-machine backscatter communications.

"Backscatter technology is feasible for the long distances required for a battery-free cellphone, but there are a lot of engineering challenges to be achieved over the next five to 10 years to make it a standard," said Ganesan.

Ratliff suggests, "The real potential here is on much simpler applications, such as sensor networks. I don't think there is any chance this technology will be used in real cellphones, now or 10 years from now. It's certainly a misnomer for them to call this a 'cellphone' since it does not use any portion of the cellphone network today."

Ganesan believes backscattering standards eventually will take their place alongside standards like Ethernet, Wi-Fi, and Bluetooth, since it is the first communization technique that requires just micro-watts of power. In addition, he says, we cannot predict what breakthroughs may be made during the development of backscatter sensor networks and machine-to-machine communications that might enable the technology's use for cellphones.

Disney Research is also advancing backscatter communications by boosting the amount of energy it can collect. To achieve longer ranges, Disney Research is demodulating down to DC not just the high-frequency cell tower signal, but also the lower-frequency signals from nearby television and FM radio stations. In the paper Riding the Airways: Ultra-Wideband Ambient Backscatter via Commercial Broadcast Systems, Alanson Sample of Disney Research Pittsburgh wrote, "communication costs dominate the energy consumption, and ultimately limit the utility, of low-power devices and sensor nodes. Backscatter communication based on deliberate and ambient sources has the potential to radically alter this paradigm by offering two to three orders of magnitude better communication efficiency."

Sample and his colleagues created a demonstration system that harvested the energy from 17 different broadcast sources ranging in output from -80 to -39 dBm. Using a multi-band base station, the result was an increase of the backscatter communications distance to 164 feet—still a long way from the goal of miles needed for a cellphone, but suitable for consumer cordless phones.

"Our backscatter nodes simultaneous transmit data by reflecting all incident ambient radio signals from 1MHz up to 2.5GHz, including broadcast protocols such as FM, TV, and cellular," said Sample.

Disney also expects future 5G networks to expand the number of energy sources from which to harvest energy, thus increasing the distance over which backscatter networks can communicate even further. "The role of 5G will be to provide more signal sources for our sensor nodes to backscatter data on top of and thus provides the opportunity to increase range and reliability," said Sample.

NSF and Google Mobile

The inventors of the battery-free cellphone at the University of Washington have spun-off a company called Jeeva Wireless to commercialize the technology. Despite Google's financial support and Smith's status as a Google Scholar, Jeeva Wireless is free to license its battery-free RF technology to other manufacturers.

Smith, also a cofounder of one-year-old Jeeva Wireless, said battery-free sensor networks and machine-to-machine applications would be Jeeva Wireless's first commercial focus, since the cellphone application would require carriers to install modified base stations in all their towers. 

"Details of our business plan are proprietary, but I can tell you we are working with a partner to design a chip, first for long-range unlicensed bands," said Smith. "We have a lot of interest from all kinds of parties, both for battery-free cellphones as an add-on for emergencies when your battery dies, to lengthen the battery life of cellphones, for battery-free sensor networks, and for long-range M2M communications."

In the future, the team hopes to increase the range of the battery-free cellphone, to encrypt conversations for security, and even to stream video by adding a low-power E-ink screen.

R. Colin Johnson is a Kyoto Prize Fellow who ​​has worked as a technology journalist ​for two decades.