Mini devices can help save small sensor battery life

By Adam Zewe | MIT News Agency

Scientists are trying to develop ever smaller internet-of-things devices, such as sensors smaller than a fingertip that can make almost any object traceable. These tiny sensors house minuscule batteries that are often nearly impossible to replace, so engineers incorporated a wake-up receiver that keeps the device in a low-power “sleep” mode when not in use, conserving battery life.

Researchers at MIT have developed a new wake-up receiver that is less than a tenth the size of previous devices and consumes only a few microwatts of power. Their receivers also incorporate a built-in, low-power authentication system, which protects the device against certain types of attacks that can quickly drain the battery.

Many types of wake receivers are built on the centimeter scale because their antennas must be proportional to the size of the radio waves they use to communicate. Instead, the MIT team built a receiver that utilizes terahertz waves, which are about one-tenth the length of radio waves. Their chips are barely more than 1 square millimeter in size.

They used their wake-up receiver to demonstrate effective wireless communication with signal sources several meters away, displaying the range that their chip allows to use in miniature sensors.

For example, a wake receiver could be incorporated into a microrobot that monitors environmental changes in areas that are too small or dangerous for other robots to reach. Also, because the device uses terahertz waves, it could be used in new applications, such as field-deployable radio networks that act as swarms to collect local data.

“Using terahertz frequencies, we can make antennas that are only a few hundred micrometers on each side, which is very small. This means we can integrate these antennas onto the chip, creating a fully integrated solution. Ultimately, this will allow us to build very small wake-up receivers that can be attached to small sensors or radios,” said Eunseok Lee, an electrical engineering and computer science (EECS) graduate student and lead author of the paper on wake-up receivers.

Lee wrote paper with co-advisor and senior author Anantha Chandrakasan, dean of the MIT School of Engineering and Vannevar Bush Professor of Electrical Engineering and Computer Science, who led Energy Efficient Circuits and Systems Groupand Ruonan Han, a professor at EECS, who led Integrated Terahertz Electronics Group in Electronics Research Laboratory; as well as others at MIT, the Indian Institute of Science, and Boston University. This research was presented at the IEEE Custom Integrated Circuits Conference.

Minimize recipients

Terahertz waves, found on the electromagnetic spectrum between microwaves and infrared light, have a very high frequency and travel much faster than radio waves. Sometimes called “pencil beams,” terahertz waves travel a more direct path than other signals, which makes them more secure, explains Lee.

However, these waves have such a high frequency that terahertz receivers often multiply the terahertz signal by other signals to change the frequency, a process known as frequency mixing modulation. Terahertz mixing consumes a lot 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 an antenna, which consumes very little power.

Even with both antennas on the chip, their wake-up receiver measures just 1.54 square millimeters and consumes less than 3 microwatts of power. This dual antenna setup maximizes performance and makes signal reading easier.

Once received, their chip amplifies the terahertz signal and then converts the analog data into a digital signal for processing. This digital signal carries a token, which is a series of bits (0s and 1s). If the token matches the recipient’s wake token, it will activate the device.

Increase security

In most receiver wakes, the same token is reused multiple times, so an eavesdropping attacker can figure it out. Then the hacker can send a signal that will activate the device repeatedly, using what’s called a denial-of-sleep attack.

“With the receiver awake, the device’s lifetime can be increased from one day to one month, for example, but an attacker can use a denial-of-sleep attack to consume its entire battery life in even less than a day. . That’s why we put authentication into our build recipients,” he explains.

They added an authentication block that uses an algorithm to randomize the device token over time, using keys shared with trusted senders. This key works like a password — if the sender knows the password, they can send a signal with the proper token. The researchers did 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 a terahertz signal to wake receivers 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 required for the device to successfully detect the signal. Signals that travel farther have less power.

“We achieved demonstration distances of 5 to 10 meters farther than others, using devices with very small sizes and microwatt-level power consumption,” said Lee.

But to be most effective, the terahertz waves have to hit the detector hard. If the chip is tilted, some of the signal will be lost. So, the researchers paired their device with a terahertz beam-steerable array, recently developed by Han’s group, to precisely orient terahertz waves. Using this technique, communications can be sent across multiple chips with minimal signal loss.

In the future, Lee and his colleagues want to address this problem of signal degradation. If they could find a way to maintain signal strength when the receiver chip moved or tilted slightly, they could improve the performance of this device. They also wanted to demonstrate their wake receiver in a very small sensor and refine the technology for use in real-world devices.

“We have developed a rich portfolio of technologies for future millimeter-sized sensing, tagging, and authentication platforms, including terahertz backscattering, energy harvesting, and electric beam steering and focusing. Now, the portfolio is further complete with Eunseok’s first terahertz wake-up receiver, which is critical to conserving the very limited energy available on the mini-platform,” said Han.

Additional co-authors include Muhammad Ibrahim Wasiq Khan PhD ’22; Xibi Chen, an EECS graduate student; Ustav Banerjee PhD ’21, assistant professor at the Indian Institute of Science; Nathan Monroe PhD ’22; and Rabia Tugce Yazicigil, assistant professor of electrical and computer engineering at Boston University.