Scientists are trying to develop ever smaller IoT devices, such as sensors smaller than a fingertip, that can make virtually any object trackable. These tiny sensors often have tiny batteries that are nearly impossible to replace, so engineers include wake-up receivers that keep devices in low-power sleep mode when not in use, preserving battery life.

Researchers at the Massachusetts Institute of Technology have developed a new wake-up receiver that is one-tenth smaller than previous devices and consumes only a few microwatts of power. It also includes a built-in, low-power authentication system that protects the device from certain types of attacks that can drain the battery quickly.

Many common alarm clock receivers are built on the centimeter scale, as 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 waves in the terahertz range, about one-tenth the length of radio waves. The size of its microcircuits barely exceeds 1 square millimeter.

They used wake-up receivers to demonstrate efficient wireless communication with a signal source several meters away, demonstrating the range that would allow their chips to be used in miniature sensors.

For example, a wake-up receiver could be placed on microrobots that monitor environmental changes in areas that are too small or dangerous for other robots. Also, because the device uses terahertz-band waves, it can be used in new applications such as field-deployable radio networks that work in swarms to collect localized data.

“Using terahertz frequencies, we can make an antenna of only a few hundred micrometers on each side, which is very small. This means that by integrating these antennas on the chip, we can create a fully integrated solution. Electrical engineering and computer science (EECS) graduate student and lead of the wake up report.” “Ultimately, this allowed us to create a very small wake-up receiver that could be hooked up to tiny sensors or radios,” says co-author Yunseok Lee.

Li co-authored the paper with co-advisers and senior authors, Ananta Chandrakasan, dean of the MIT School of Engineering, and Ruonan Khan, Vannevar Bush Professor of Electrical Engineering and Computer Science, who leads the Energy Efficient Circuits and Systems Group. EECS, which manages Terahertz Integrated electronics in the electronics research laboratory; as well as others at MIT, the Indian Institute of Science and Boston University. The work was presented at the IEEE Special Integrated Circuits Conference.

Reducing the size of the recipient

Terahertz waves in the electromagnetic spectrum between microwaves and infrared light have very high frequencies and travel much faster than radio waves. Lee explains that terahertz waves, sometimes called “pencil beams,” act more directly than other signals, making them safer.

However, the waves have such high frequencies that terahertz receivers often multiply the terahertz signal with another signal to change the frequency; this is a process known as mixed frequency modulation. Terahertz mixing consumes a lot of energy.

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

Even with both antennas on the chip, the wake-up receivers were only 1.54 square millimeters in size and consumed less than 3 microwatts of power. This dual antenna setup maximizes performance and makes signals easier to read.

After the chip receives them, it amplifies the terahertz signal and then converts the analog data into a digital signal for processing. This digital signal contains an token, which is a string of bits (0’s and 1’s). If the token matches the wake-up receiver token, it will wake the device.

Strengthening security

Most wake-up receivers use the same token multiple times so an eavesdropping attacker can figure out what it is. A hacker can then send a signal that will repeatedly activate the device using a so-called “sleep denial” attack.

“With the wake-up receiver, for example, a device’s lifespan could be extended from one day to one month, but an attacker could use a sleep-denial attack to consume all battery life in less than a day. That’s why we authenticate on our wake-up receiver,” he explains.

They added an authentication block that uses an algorithm to randomize the device token each time using a key provided to trusted senders. This key acts as a password; If the sender knows the password, he can send a signal with the correct token. The researchers do this using a technique known as lightweight cryptography, which ensures that the entire authentication process consumes only a few extra nanowatts of energy.

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

“Using a device that is very small in size and consumes microwatts of power, we showed a distance of 5-10 meters longer than the others,” says Li.

But to be most effective, terahertz waves must hit the detector. If the chip is at an angle, some of the signal will be lost. So the researchers combined their device with a terahertz steerable grating recently developed by Hahn’s group to precisely steer terahertz waves. Using this method, communication can be sent to multiple chips with minimal signal loss.

In the future, Li and colleagues want to address this signal degradation problem. If they can find a way to preserve signal strength while their receiver chips are moving or slightly tilted, they could improve the performance of these devices. They also want to display wake-up receivers in very small sensors and fine-tune the technology for use in real devices.