Microwatt-Powered Wake-Up Receiver: A Game-Changer for IoT Device Energy Efficiency

Recently, researchers from MIT demonstrated a new miniaturised wake-up receiver that consumes microwatts of power and can wake up an electronic circuit using terahertz waves. What challenges do wake-up systems have in IoT devices, what did the researchers develop, and how could this help future IoT designs?

What challenges do wake-up systems have in IoT devices?

When it comes to reducing energy consumption in circuits, engineers have several options available to them. One such option is to disable all unneeded peripherals, as each peripheral consumes some amount of power, even if unused. Another option is to reduce the frequency of components, especially CPUs, as higher frequencies in CMOS technologies increase power consumption. In cases where these options don’t save enough power, engineers can turn to deep sleep states whereby most of the system is powered down, with only a tiny amount of power being used to retain the contents of memory so that the system can continue when exiting such sleep modes.

However, trying to wake up from sleep mode comes with some challenges. One such challenge is the need for power in external circuits or peripherals responsible for activating wake up signals. A circuit that enters deep sleep will not be able to use its CPU to execute code, and this means that an external source of energy is needed to trigger a wake-up. In the case of buttons connected to batteries and small solar cells detecting light, this isn’t a problem, but for applications that need to listen to small amounts of sound or wake up on a change in temperature, this can be tricky to solve. 

Another challenge facing engineers is system response time. When a system enters a deep sleep cycle, it can be difficult to quickly respond to signals if those signals are used to wake the system up. For example, a sensor that needs to transmit sensor data upon detecting a spike in readings may likely fail to record a short pulse, as the spike may be long gone before the CPU has had time to boot and execute the routine used to take sensor readings. 

Finally, creating external circuits capable of waking up low-energy devices takes up precious PCB space, which limits how small they can be made. Considering how miniaturisation is driving industries such as IoT, engineers are under increasing pressure to try and reduce the overall size and energy consumption of designs, and wake-up circuits are definitely an area that can provide significant improvements.

Researchers create miniaturised wake-up circuit with microwatt operation

Recognising the challenges faced with wake-up circuits, researchers from MIT recently demonstrated a new wake-up receiver that is ten times smaller than others currently available to engineers. Wake up receivers are used to activate a circuit to wake up when detecting a specific radio transmission, but these circuits can be bulky due to the use of large antenna and filter components. However, the researchers managed to eliminate this by using terahertz waves instead, significantly shrinking the size of the receiving components. 

At the same time, the device also incorporates a low-power authentication system which prevents unauthorised wake-up when detecting similar terahertz signals. However, what makes this device truly impressive is its low power consumption of a few microwatts, meaning that it can stay active for extended periods of time while having minimal effect on battery life. The researchers used a technique known as lightweight cryptography, which ensures the entire authentication process consumes only a few extra nanowatts of power.

With a size of 1 square millimetre, the new chip would be easily integrated into IoT designs, providing devices with an external authenticated wake-up system. Additionally, the use of terahertz waves also improves security, as these radio waves are more directional, requiring a radio source to be pointed directly at the chip to activate. The dual-antenna setup, which measures only 1.54 square millimetres, maximises performance and makes it easier to read signals [source]. 

“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. 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,” - Eunseok Lee, electrical engineering and computer science (EECS) graduate student.

Eunseok Lee is an electrical engineering and computer science (EECS) graduate student at MIT who led the research on the wake-up receiver. The research was presented at the IEEE Custom Integrated Circuits Conference and involved collaboration between researchers from MIT, the Indian Institute of Science, and Boston University. The details of their research can be found in a paper authored by Eunseok Lee, along with co-advisors Anantha Chandrakasan and Ruonan Han and other collaborators from MIT, the Indian Institute of Science, and Boston University [source]. 

How could this new development help IoT designs?

The major advantage of this new wake-up receiver is that it allows IoT devices to stay dormant in sleep mode indefinitely. Devices that enter extended periods of sleep generally use a timer to eventually wake up, check the status of sensors and other systems, and then go back to sleep, but even if this is a fast operation, it still consumes a considerable amount of power. Instead, an IoT sensor connected to the researcher’s wake-up sensor could lie entirely silent until called upon, essentially extending battery life considerably.

This would have profound implications in the field of logistics and asset tracking, whereby such asset trackers would be able to record their environments when at key stages of their journey. For instance, the use of low-power asset trackers in the transportation of medical supplies can be pivotal for determining if a medicine was correctly stored at all times of its journey. Another example would be the use at scanning stations, where a terahertz beam is used to activate asset trackers for the sake of automatic logging. 

Overall, what the researchers have demonstrated is truly exciting, and considering that it was demonstrated to be functional over a distance of several meters, it suggests that such devices could very well soon make it to the consumer market.