The terahertz region of the wireless spectrum is a thoroughly maddening part of the electromagnetic spectrum. It’s sometimes referred to as a “dead zone” between the frequencies used for radio signals and optical signals. The spectrum band (300 gigahertz to 30 terahertz) has its appealing properties, but its daunting physics has historically made it difficult to harness for practical applications.
Yet the region remains attractive to researchers, resulting in a slew of new applications in recent years. One of the latest is an ultrasmall terahertz wake-up receiver chip designed by MIT engineers. The receiver requires only a few microwatts of power and includes a low-power authentication system that shields it from denial-of-sleep attacks.
With the proliferation of smaller Internet of Things (IoT) devices, wake-up receivers are becoming increasingly relevant today, says Eunseok Lee, a graduate student in MIT’s electrical engineering and computer science department. Lee and other MIT researchers presented a paper on their research at the 2023 IEEE Custom Integrated Circuits Conference, held in April in San Antonio. Lee works in the Terahertz Integrated Electronics Group, which is currently working on ultrasmall platforms using terahertz frequencies. “We want to find [different] applications for these frequencies…and thought that they’d be a good candidate for an ultrasmall sensing platform as well,” Lee says.
A wake-up receiver keeps an electronic device dormant until it is needed. “Think about your mobile phone,” Lee says. “If it is turned on constantly, it will consume a whole lot of power, right? So what a wake-up receiver does is keep an electronic device at a very low power mode [until] we send a signal to the receiver so that it can activate the entire system.” In other words, the tiny receiver can preserve the battery life of very small IoT devices, which already have limited battery capacity.
Like all antennas, those for wake-up receivers need to be proportional to the size of the wavelengths they receive. Most conventional wake-up receivers use Wi-Fi or Bluetooth, with frequencies around 2.4 GHz and wavelengths around 12.5 centimeters. This requires existing receivers to be built on the centimeter scale as well. “However, if we increase our carrier frequency to a few tens or a few hundreds of gigahertz, its wavelength is also reduced, and this can help to reduce the antenna size so that it can fit on very small, millimeter-size sensing platforms,” Lee says.
The MIT researchers built their receiver to communicate on terahertz frequencies, which correspond to wavelengths between 1 and 0.03 millimeters. Their receiver chip was 1.54 mm 2 and used less than 3 microwatts of power. “We demonstrated that our receiver can support communication over a distance of 5 to 10 meters, with microwatt-level power consumption,” Lee says.
Terahertz waves are also occasionally called pencil beams because they disperse less over a given distance than radio waves do.This makes terahertz waves more secure, but at their high frequencies, they also can’t travel very far. Usually, the terahertz signal is multiplied by another signal to change its frequency and help it propagate, but this mixing uses a lot of power. “As we have a limited power budget because of the very small battery on our sensor, what we did was use terahertz self-mixing,” says Lee. The team used a pair of tiny transistors as antennas for their detector and mixed two terahertz frequencies at a negligible power cost.
“We also deployed a wake-up authentication circuit inside our terahertz receiver,” Lee states. While there are many security protocols for wake-up receivers, in the field of integrated circuits, no one had tried to develop a wake-up authentication system before, he says. This system prevents denial-of-sleep attacks, in which an attacker attempts to activate the device repeatedly to drain its battery. The MIT team used an algorithm to randomize the device’s token using a key that is shared with trusted senders. They adopted a technique called lightweight cryptography to incorporate the token randomization, which consumed only a few nanowatts of power.
Potential applications of the miniature terahertz wake-up receiver include installing it inside microbots that monitor areas either too small or too unsafe for humans, indoor security applications where the sensors can be deployed unobtrusively, and in robot swarms to collect localized data. “Currently, we are also working on making a harvesting platform using terahertz waves,” Lee adds, “and also on the angular sensitivity of the wake-up receiver.” They also want to be able to fine-tune terahertz technologies for real-world uses.