With the IoT poised to spread throughout more of the environment, not just linking up ‘smart’ homes but also tracking movements and monitoring vital infrastructure with low-energy devices, engineers are focused on new battery technology and energy-harvesting methods. Another way to reduce or eliminate the need to replace batteries could be using the new wake-up receiver from researchers at the University of California, San Diego.
The chip wakes up IoT devices only when they need to communicate and perform their function, letting them stay dormant the rest of the time and reducing power use. The technology could be useful for applications that do not always need to be transmitting data, like wearable health monitors that take readings several times a day.
“The problem now is that these devices do not know exactly when to synchronise with the network, so they periodically wake up to do this even when there's nothing to communicate. This ends up costing a lot of power,” said electrical and computer engineer Professor Patrick Mercier. “By adding a wake-up receiver, we could improve the battery life of small IoT devices from months to years.”
The wake-up receiver is an ultra-low power chip that continuously looks out for a specific radio signal, called a wake-up signature, that tells it when to wake up the main device. It needs only a very small amount of power to stay on and do this – 22.3nW, about half a millionth of the power it takes to run an LED night light.
The team, led by Mercier and Professors Drew Hall and Gabriel Rebeiz, said a key part of the receiver’s design is that it targets higher-frequency radio signals than other wake-up receivers. The signals are in the frequency of 9 gigahertz, in the realm of satellite communication, air traffic control and vehicle speed detection. Targeting a higher frequency allowed researchers to shrink everything, including the antenna, transformer and other off-chip components, down into a much smaller package – at least 20 times smaller than previous nanowatt-level work. The team used ultra-low power electronics developed by the Mercier and Hall laboratories, and advanced antenna and radio technologies developed by the Rebeiz laboratory.
Unlike other nanowatt-powered receivers, the new device can perform well over a wide temperature range. The engineers said performance is consistent from -10ºC up to 40ºC. Typically, performance in low-power wake-up receivers drops if the temperature changes by even just a few degrees. “For indoor use, this is not a big deal, but for outdoors use it needs to work over a wide temperature window. We specifically addressed that in this work,” Mercier said.
The researchers claimed the receiver's combination of nanowatt-level power consumption, 4.55cm2 size, -69.5 dBm sensitivity and temperature performance is the best published to date. “This will enable all sorts of new IoT applications,” Mercier said.
The team admitted a “small trade-off” in latency. There is a 0.54s delay between the receiver detecting the wake-up signature and waking up the device. A delay of this length is “not a problem” for the intended applications, however, they added.
“You don't need high-throughput, high-bandwidth communication when sending commands to your smart home or wearables devices, for example, so the trade-off of waiting for a mere half a second to get a 100,000-times improvement in power is worth it,” Mercier said.
The work was published in IEEE Journal of Solid-State Circuits.
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