Researchers at the Massachusetts Institute of Technology (MIT) have developed a wake-up receiver that could help preserve battery life for tiny IoT sensors.

Scientists are striving to develop smaller IoT devices, such as sensors tinier than a fingertip that could make nearly any object trackable. These diminutive sensors have miniscule batteries that are often nearly impossible to replace, so engineers incorporate wake-up receivers that keep devices in low-power sleep mode when not in use, preserving battery life.

Researchers at MIT have developed a wake-up receiver that is less than one-tenth the size of previous devices and consumes only a few microwatts of power. The receiver also incorporates a low-power, built-in authentication system, which protects the device from a certain type of attack that could quickly drain its battery.

Many common types of wake-up receivers are built on the centimetre scale since their antennas must be proportional to the size of the radio waves they use to communicate. Instead, the MIT team built a receiver that uses terahertz waves, which are about one-tenth the length of radio waves. Their chip is barely more than one square millimetre in size.

They used their wake-up receiver to demonstrate effective, wireless communication with a signal source that was several metres away, showing a range that would enable their chip to be used in miniaturised sensors.

For instance, the wake-up receiver could be incorporated into microrobots that monitor environmental changes in areas that are either too small or hazardous for other robots. Also, since the device uses terahertz waves, it could be used in emerging applications, such as field-deployable radio networks that work as swarms to collect local data.

“By using terahertz frequencies, we can make an antenna that is only a few hundred micrometres on each side, which is a very small size,” said Eunseok Lee, a graduate student and lead author of a paper on the wake-up receiver. “This means we can integrate these antennas to the chip, creating a fully integrated solution. Ultimately, this enabled us to build a very small wake-up receiver that could be attached to tiny sensors or radios.”

Lee wrote the paper with his co-advisors and senior authors Anantha Chandrakasan, dean of the MIT School of Engineering, and Ruonan Han, an associate professor who leads the Terahertz Integrated Electronics Group in the Research Laboratory of Electronics; as well as others at MIT, the Indian Institute of Science, and Boston University. The research was presented at last month’s IEEE Custom Integrated Circuits Conference in Texas.

Terahertz waves, found on the electromagnetic spectrum between microwaves and infra-red light, have very high frequencies and travel much faster than radio waves. Sometimes called pencil beams, terahertz waves travel in a more direct path than other signals, which makes them more secure.

However, the waves have such high frequencies that terahertz receivers often multiply the terahertz signal by another signal to alter the frequency, a process known as frequency mixing modulation. Terahertz mixing consumes a great deal of power.

Instead, Lee and his collaborators developed a zero-power-consumption detector that can detect terahertz waves without the need for frequency mixing. The detector uses a pair of tiny transistors as antennas, which consume very little power.

Even with both antennas on the chip, their wake-up receiver was only 1.54 square millimetres in size and consumed less than 3µW of power. This dual-antenna setup increases performance and makes it easier to read signals.

Once received, the chip amplifies a terahertz signal and then converts analogue data into a digital signal for processing. This digital signal carries a token, which is a string of bits. If the token corresponds to the wake-up receiver’s token, it will activate the device.

In most wake-up receivers, the same token is reused multiple times, so an eavesdropping attacker could figure out what it is. Then the hacker could send a signal that would activate the device over and over again, using what is called a denial-of-sleep attack.

“With a wake-up receiver, the lifetime of a device could be improved from one day to one month, for instance, but an attacker could use a denial-of-sleep attack to drain that entire battery life in even less than a day,” said Lee. “That is why we put authentication into our wake-up receiver.”

They added an authentication block that uses an algorithm to randomise the device’s token each time, using a key that is shared with trusted senders. This key acts like a password; if a sender knows the password, they can send a signal with the right token. The researchers do this using a technique known as lightweight cryptography, which ensures the entire authentication process only consumes a few extra nanowatts of power.

They tested their device by sending terahertz signals to the wake-up receiver as they increased the distance between the chip and the terahertz source. In this way, they tested the sensitivity of their receiver, the minimum signal power needed for the device to detect a signal successfully. Signals that travel farther have less power.

“We achieved 5 to 10m longer distance demonstrations than others, using a device with a very small size and microwatt level power consumption,” Lee said.

But to be most effective, terahertz waves need to hit the detector dead-on. If the chip is at an angle, some of the signal will be lost. So, the researchers paired their device with a terahertz beam-steerable array, recently developed by the Han group, to direct the terahertz waves precisely. Using this technique, communication could be sent to multiple chips with little signal loss.

In the future, Lee and his collaborators want to tackle this problem of signal degradation. If they can find a way to maintain signal strength when receiver chips move or tilt slightly, they could increase the performance of these devices. They also want to demonstrate their wake-up receiver in very small sensors and fine-tune the technology for use in real-world devices.

“We have developed a rich technology portfolio for future millimetre-sized sensing, tagging and authentication platforms, including terahertz backscattering, energy harvesting, and electrical beam steering and focusing,” said Han. “Now, this portfolio is more complete with Eunseok’s first-ever terahertz wake-up receiver, which is critical to save the extremely limited energy available on those mini platforms.”

Additional co-authors include Muhammad Ibrahim Wasiq Khan; Xibi Chen, an EECS graduate student; Ustav Banerjee, an assistant professor at the Indian Institute of Science; Nathan Monroe; and Rabia Tugce Yazicigil, an assistant professor of electrical and computer engineering at Boston University.