‘Wasted’ WiFi signals can power small devices

Researchers have developed a way to harvest energy from WiFi signals to power small devices.

The amount of WiFi sources to transmit information wirelessly between devices has grown exponentially. This results in the widespread use of the 2.4GHz radio frequency that WiFi uses, with excess signals available to be tapped for alternative uses.

To harness this under-utilized source of energy, researchers developed a technology that uses tiny smart devices known as spin-torque oscillators (STOs) to harvest and convert wireless radio frequencies into energy to power small electronics.

In their new study, the researchers successfully harvested energy using WiFi-band signals to power a light-emitting diode (LED) wirelessly, without using any battery.

“We are surrounded by WiFi signals, but when we are not using them to access the Internet, they are inactive, and this is a huge waste,” says Yang Hyunsoo, a professor from the computer electrical and computer engineering department at the National University of Singapore, who spearheaded the project.

“Our latest result is a step towards turning readily-available 2.4GHz radio waves into a green source of energy, hence reducing the need for batteries to power electronics that we use regularly. In this way, small electric gadgets and sensors can be powered wirelessly by using radio frequency waves as part of the Internet of Things. With the advent of smart homes and cities, our work could give rise to energy-efficient applications in communication, computing, and neuromorphic systems.”

Spin-torque oscillators are a class of emerging devices that generate microwaves, and have applications in wireless communication systems. However, the application of STOs is hindered due to a low output power and broad linewidth.

While mutual synchronization of multiple STOs is a way to overcome this problem, current schemes, such as short-range magnetic coupling between multiple STOs, have spatial restrictions. On the other hand, long-range electrical synchronization using vortex oscillators is limited in frequency responses of only a few hundred MHz. It also requires dedicated current sources for the individual STOs, which can complicate the overall on-chip implementation.

To overcome the spatial and low frequency limitations, the research team came up with an array in which eight STOs are connected in series. Using this array, the 2.4 GHz electromagnetic radio waves that WiFi uses was converted into a direct voltage signal, which was then transmitted to a capacitor to light up a 1.6-volt LED. When the capacitor was charged for five seconds, it was able to light up the same LED for one minute after the wireless power was switched off.

In their study, the researchers also highlighted the importance of electrical topology for designing on-chip STO systems, and compared the series design with the parallel one. They found that the parallel configuration is more useful for wireless transmission due to better time-domain stability, spectral noise behavior, and control over impedance mismatch. On the other hand, series connections have an advantage for energy harvesting due to the additive effect of the diode-voltage from STOs.

“Aside from coming up with an STO array for wireless transmission and energy harvesting, our work also demonstrated control over the synchronizing state of coupled STOs using injection locking from an external radio-frequency source,” says Raghav Sharma, the first author of the paper. “These results are important for prospective applications of synchronized STOs, such as fast-speed neuromorphic computing.”

To enhance the energy harvesting ability of their technology, the researchers are looking to increase the number of STOs in the array they had designed. In addition, they’re planning to test their energy harvesters for wirelessly charging other useful electronic devices and sensors.

The research team also hopes to work with industry partners to explore the development of on-chip STOs for self-sustained smart systems, which can open up possibilities for wireless charging and wireless signal detection systems.

The results appear in Nature Communications. Additional researchers are from NUS and Japan’s Tohoku University.

Source: National University of Singapore

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