A wirelessly powered 5G relay is in development that could accelerate the development of smart factories.

By adopting a lower operating frequency for wireless power transfer, the proposed relay design solves current limitations, including range and efficiency. In turn, this allows for a more versatile and widespread arrangement of sensors and transceivers in industrial settings, said scientists from Tokyo Institute of Technology.

One of the hallmarks of the Information Age is the greater flow of information. This can be readily seen in high-tech factories and warehouses, where wireless sensors and transceivers end up embedded in robots, production machinery, and automatic vehicles. In many cases, 5G networks orchestrate operations and communications between these devices.

To avoid relying on cumbersome wired power sources, it is possible to energize sensors and transceivers remotely via wireless power transfer (WPT).

Problem with WPT
However, one problem with conventional WPT designs is they operate at 24 GHz. At such high frequencies, transmission beams must be extremely narrow to avoid energy losses. Moreover, power can only transmit if there is a clear line of sight between the WPT system and the target device. Since 5G relays often extend the range of 5G base stations, WPT needs to reach even further, which is yet another challenge for 24 GHz systems.

Schneider Bold

To address the limitations of WPT, a research team from Tokyo Institute of Technology created a solution. In a recent study, they developed a 5G relay they can power wirelessly at a lower frequency of 5.7 GHz.

“By using 5.7 GHz as the WPT frequency, we can get wider coverage than conventional 24 GHz WPT systems, enabling a wider range of devices to operate simultaneously,” said senior author and Associate Professor Atsushi Shirane.

Additionally, the proposed wirelessly powered relay is meant to act as an intermediary receiver and transmitter of 5G signals, which can originate from a 5G base station or wireless devices. The key innovation of this system is the use of a rectifier-type mixer, which performs fourth-order subharmonic mixing while also generating DC power.

Notably, the mixer uses the received 5.7 GHz WPT signal as a local signal. With this local signal, together with multiplying circuits, phase shifters, and a power combiner, the mixer “down-converts” a received 28 GHz signal into a 5.2 GHz signal. Then, this 5.2 GHz signal internally amplifies, up-converts to 28 GHz through the inverse process, and retransmits to its intended destination.

A key innovation is the use of the 5.7 GHz wireless power transfer (WPT) signal as both a means of generating DC power using a rectifier and as an oscillator for the mixing and unmixing circuits. By amplifying the input signal after down conversion to a lower frequency via mixing, this circuit achieves higher efficiency and gain.
Source: 2024 IEEE Symposium on VLSI Technology & Circuits

To drive these internal amplifiers, the proposed system first rectifies the 5.7 GHz WPT signal to produce DC power, which ends up managed by a dedicated power management unit. This ingenious approach offers several advantages.

More Power
“Since the 5.7 GHz WPT signal has less path loss than the 24 GHz signal, more power can be obtained from a rectifier,” Shirane said. “In addition, the 5.7 GHz rectifier has a lower loss than 24 GHz rectifiers and can operate at a higher power conversion efficiency.”

Finally, this proposed circuit design allows for selecting the transistor size, bias voltage, matching, cutoff frequency of the filter, and load to maximize conversion efficiency and conversion gain simultaneously.

Through several experiments, the research team showcased the capabilities of their proposed relay. Occupying only a 1.5 mm by 0.77 mm chip using standard CMOS technology, a single chip can output a high power of 6.45 mW at an input power of 10.7 dBm. Notably, multiple chips could combine to achieve a higher power output. Considering its many advantages, the proposed 5.7 GHz WPT system could contribute to the development of smart factories.

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