Researchers at the Massachusetts Institute of Technology (MIT) are creating a type of sensor smaller than a fingertip that could pave the way for ever-smaller Internet of Things (IoT) devices. These sensors use activation receivers that keep devices in a low-power "suspend" mode when not in use, preserving battery life. This activation receiver is less than a tenth the size of previous models and consumes only a few microwatts – its chip is just over 1 square millimetre in size. It also incorporates an authentication system capable of protecting against a certain type of attack that can quickly drain the battery. Other common types of activation receivers are centimetre-sized, as their antennas must be proportional to the size of the radio waves they use to communicate. Instead, the MIT team developed a model that uses waves on the terahertz scale, about one-tenth the length of radio waves. Waves in the terahertz range are found in the electromagnetic spectrum between microwaves and infrared light, have very high frequencies and travel much faster than radio waves. Sometimes called “pencil beams”, terahertz waves travel a more direct path than other signals, which makes them safer. The researchers used this activation receiver to demonstrate wireless communication several meters away, showing that this range could allow it to be used in miniaturized sensors. For example, the receiver could be incorporated into micro-robots that monitor environmental changes in areas too small for other robots to reach. In addition, because the device uses waves in the terahertz range, it could be used in emerging applications, such as field-deployable radio networks that work as swarms to collect localised data. "By using frequencies in the terahertz range, we can make an antenna with only a few hundred micrometers on each side and integrate them into the chip. In short, this allows us to build a very small receiver to be connected to small sensors or radios," explains Eunseok Lee, a graduate student in electrical engineering and computer science and lead author of a paper on the activation receiver. The research was presented at the IEEE Custom Integrated Circuits Conference. Energy consumption Often receivers multiply the terahertz signal by another signal to change its high frequency, a process known as frequency mix modulation. However, mixing in terahertz consumes a lot of power. The MIT researchers then developed a detector with zero power consumption that can detect terahertz waves without the need for frequency mixing. The solution uses a pair of tiny transistors as antennas that consume very little power. Even with two antennas on the chip, the new activation receiver is only 1.54 square millimeters in size and consumes less than 3 microwatts of power. This dual antenna configuration maximizes performance and makes it easier to read signals. Once the signals are received, the chip amplifies them and converts analog data into a digital signal for processing. This digital signal carries a token (sequence of bits) which, if it matches the receiver's token, will activate the IoT device. What about security? In most activation receivers, the same token is reused multiple times, so an attacker could discover it and send a matching signal to activate the device repeatedly, using what is called a "sleep denial attack". "With an activation receiver, the life of devices can be extended from a day to a month, for example, but an attacker could use a 'sleep denial attack' to deplete the entire battery life in less than a day. That's why we put authentication on our activation receiver," Lee explains. The new receiver authentication block uses an algorithm to randomise tokens on the device, using a key that is shared with trusted senders. This key works like a password - if a sender knows the password, they can send signals with the correct token. The researchers do this using a technique known as lightweight cryptography, which ensures that the authentication process consumes only a few extra nanowatts of power. A device was tested sending signals in the terahertz range to the activation receiver and the researchers increased the distance between the chip and the source. In this way, they were able to identify the receiver's sensitivity, i.e. the minimum signal strength required for the device to successfully detect signals. Signals that travel further have less energy. "We were able to reach distances of 5 to 10 metres more than others, using a device with a very small size and power consumption at the microwatt level," details Lee. To be most effective, terahertz waves need to hit the detector directly. If the chip is at an angle, some of the signal will be lost. So the researchers paired the device with a newly developed steerable array of terahertz beams to precisely direct the waves. Using this technique, communication can be sent to multiple chips with minimal signal loss. In the future, the researchers want to solve this problem of signal degradation. If they can find a way to maintain signal strength when the receiver chips move or tilt slightly, they could increase the performance of the devices. They also want to demonstrate their wake-up receiver on very small sensors and tweak the technology for use in real-world devices. They have already developed a rich technology portfolio for future millimetre-sized detection, marking and authentication platforms, including terahertz back scattering, energy harvesting and electrical beam steering and focusing. Now, that portfolio will be even more complete with the first terahertz activation receiver, which is essential for saving the extremely limited power valuable in miniplatforms.