Power/Performance Bits: March 24

Backscatter Wi-Fi radio
Engineers at the University of California San Diego developed an ultra-low power Wi-Fi radio they say could enable portable IoT devices. Using 5,000 times less power than standard Wi-Fi radios, the device consumes 28 microwatts while transmitting data at a rate of 2 megabits per second over a range of up to 21 meters.

“You can connect your phone, your smart devices, even small cameras or various sensors to this chip, and it can directly send data from these devices to a Wi-Fi access point near you. You don’t need to buy anything else. And it could last for years on a single coin cell battery,” said Dinesh Bharadia, a professor of electrical and computer engineering at the UC San Diego Jacobs School of Engineering.

This would eliminate the need for frequent recharging or replacement of batteries for connected sensors, according to the researchers.

A set of ultra-low power Wi-Fi radios integrated in small chips, each measuring 1.5 square millimeters in area (grain of rice shown for scale). Photo by David Baillot/UC San Diego Jacobs School of Engineering

“This Wi-Fi radio is low enough power that we can now start thinking about new application spaces where you no longer need to plug IoT devices into the wall. This could unleash smaller, fully wireless IoT setups,” said UC San Diego electrical and computer engineering professor Patrick Mercier. “It could also allow you to connect devices that are not currently connected–things that cannot meet the power demands of current Wi-Fi radios, like a smoke alarm–and not have a huge burden on battery replacement.”

The radio uses backscattering, where it takes incoming Wi-Fi signals from a device or access point, modifies them to encode its own data, then reflects the new signals onto a different Wi-Fi channel to another device or access point.

The device relies on a wake-up receiver to turn on the Wi-Fi radio only when it needs to communicate with Wi-Fi signals, so it can stay in low-power sleep mode the rest of the time, during which it consumes only 3 microwatts of power. It includes a custom IC for backscattering data, and the total chip measures 1.5 square millimeters.

Preventing IoT side-channel attacks
Engineers at Rice University propose a new method to increase security for Internet of Things devices with low power consumption. The team claims the security technique, which takes advantage of pre-existing power management circuitry, is 14,000 times better than current state-of-the-art defenses.

In previous work, the researchers generated paired security keys based on unique chip defects. “This year, the story is similar, but we are not generating keys,” said Kaiyuan Yang, an assistant professor of electrical and computer engineering at Rice. “We are looking at defending against a new type of attack that is specifically for IoT and mobile systems.

“In power and electromagnetic side-channel attacks, the attackers can figure out a secret key when your device is running without opening up the device,” he said. “Once they have your key they can decrypt everything, no matter how good your security software is.

Kaiyuan Yang of Rice University explains side-channel attacks and how the new device mitigates them.

The team’s approach uses on-chip power regulators to obfuscate the information leaked by the power consumption of encryption circuits, Yang said. “Every system-on-a-chip has multiple modules powered by the power management circuits, so the interfaces we need are already there. By replacing existing power management circuitry with our unit, we not only provide a much better way to defend against powerful threats, but also provide a much more energy-efficient solution.”

Yang said the circuit should take no more room on a chip than current power management units, and as a side benefit will provide state-of-the-art power regulation.

The team sees their technique as a way to foil power and electromagnetic side-channel attacks and make them more difficult and expensive for attackers, but notes that a long engineering effort is ahead before it can be deployed in devices.

Bendy supercapacitor
Researchers at University College London and Chinese Academy of Sciences developed a flexible but high-capacity supercapacitor.

The supercapacitor uses a graphene electrode material with pores that can be changed in size to store the charge more efficiently. This tuning maximizes the energy density of the supercapacitor to a record 88.1 Wh/L (Watt-hour per liter), which is the highest ever reported energy density for carbon-based supercapacitors.

“We designed materials which would give our supercapacitor a high power density – that is how fast it can charge or discharge – and a high energy density – which will determine how long it can run for. Normally, you can only have one of these characteristics but our supercapacitor provides both, which is a critical breakthrough,” said Zhuangnan Li of UCL Chemistry.

“Moreover, the supercapacitor can bend to 180 degrees without affecting performance and doesn’t use a liquid electrolyte, which minimizes any risk of explosion and makes it perfect for integrating into bendy phones or wearable electronics.”

Similar fast-charging commercial technology has a relatively poor energy density of 5-8 Wh/L and traditional slow-charging but long-running lead-acid batteries used in electric vehicles typically have 50-90 Wh/L.

The team said that while their supercapacitor has a comparable energy density to state-of-the-art value of lead-acid batteries, its power density is two orders of magnitude higher. “Successfully storing a huge amount of energy safely in a compact system is a significant step towards improved energy storage technology,” said Ivan Parkin, Dean of UCL Mathematical & Physical Sciences and UCL Chemistry Professor. “We have shown it charges quickly, we can control its output and it has excellent durability and flexibility, making it ideal for development for use in miniaturized electronics and electric vehicles. Imagine needing only ten minutes to fully-charge your electric car or a couple of minutes for your phone and it lasting all day.”

The 6cm x 6cm proof-of-concept supercapacitor was made from two identical electrodes layered either side of a gel-like substance which acted as a chemical medium for the transfer of electrical charge. This was used to power dozens of LEDs and was found to be highly robust, flexible and stable. When bent at 180 degrees, it performed almost same as when it was flat, and after 5,000 cycles, it retained 97.8% of its capacity.