Wireless power transfer

RF wireless power

Powering low power devices with RF energy

The number of wireless communication devices is increasing every year, with over 24 billion expected by 2030. These devices are continuously evolving in capability, yet the methods used to power them remain conventional. Currently, devices must either:

  • Be regularly charged with a cable
  • Have their batteries replaced
  • Be placed on a charging pad

All three of these methods require regular maintenance, which can be cumbersome and costly for applications with a large number of devices or with devices that are difficult to locate or access.

A promising solution to this challenge is radio frequency (RF) wireless power transfer. While RF signals have traditionally been used only for data transmission, they can also transfer energy over the air. A device with RF power transfer capabilities can convert RF signals into a direct current (DC) signal to power its circuitry.

RF wireless powering is particularly relevant for applications that involve a large number of low power wireless devices. Examples of such applications include:

  • Sensors in smart buildings and autonomous vehicles
  • Electronic shelf labels
  • Inventory tags for tracking items
  • Low power internet of things (IoT) devices that may be used in production lines or retail settings

With RF wireless power transfer, these devices can be powered while in use and in motion, without the need for manual intervention. This technology significantly reduces the need for energy maintenance, enhancing efficiency and reducing costs.

How does RF wireless power transfer work?

A device can be powered by RF signals through a rectenna, a combination of an antenna and a rectifier. The antenna captures the RF signal, and the rectifier converts the RF signal to a direct current (DC) signal. This DC energy can then be used to directly power the device or stored in a battery or supercapacitor for future use.

A rectifier typically consists of diodes followed by a capacitor based low pass filter (LPF). A rectifier is a non linear circuit element, so its DC power output depends non linearly on the RF input power. The output DC power is influenced not only by the strength of the input RF signal but also by its waveform. For instance, signals with a high peak to average power ratio (PAPR) yield a higher DC power output for a given average incident RF power compared to constant-envelope signals. This is because the peaks of a high PAPR signal are more likely to exceed the diode turn on voltage, compared to a constant envelope signal with the same average power but lower peak power.

AirFuel Alliance RF standard

A standard that defines all procedures between the RF transmitter and receiver ensures interoperability between devices from different vendors, promoting the development of products utilizing RF power transfer technology.

AirFuel Alliance is a global coalition of companies that is developing standards for both near field (magnetic resonance) and far field (using RF wireless powering). The AirFuel Resonant standard, for devices using magnetic resonance, has already been adopted by commercial products. In January 2023, the AirFuel RF baseline system specification was published, and the AirFuel RF interoperability and conformance tests are currently in preparation for release. For a device to be certified by the AirFuel Alliance RF standard, it must satisfy these test cases.

As an active member of the AirFuel Alliance, Rohde & Schwarz plays a pivotal role in advancing the AirFuel RF standard. We leverage our expertise as a test and measurement provider to contribute to the development of industry standards that will shape the future of wireless powering.

3GPP Ambient IoT

The 3rd Generation Partnership Project (3GPP) is a global organization that defines standards for telecommunications. Recognizing the requirements of new IoT applications, such as battery less devices, 3GPP has introduced a new technology category called Ambient IoT.

The number of connections and device density in an Ambient IoT network can be orders of magnitude higher than existing 3GPP IoT technologies. At the same time, the complexity, energy storage capacity, power consumption and network coverage of Ambient IoT devices are orders of magnitude lower than existing 3GPP low power wide area (LPWA) technologies. The low power needs of Ambient IoT devices make RF signals a promising energy source.

"I want to thank Rohde & Schwarz for their leadership in developing a wireless power tester (WPT) for the AirFuel RF standard. The WPT will enable companies to test and validate conformance to the AirFuel RF standard and accelerate the deployment of interoperable RF wireless power solutions."
Dr. Sanjay Gupta, President and Chairman of AirFuel Alliance

RF wireless power at Rohde & Schwarz

Wireless Power Tester (WPT) project

The Rohde & Schwarz Wireless Power Tester (WPT) project is dedicated to developing a comprehensive testing solution for wireless power transmitters and receivers. The project is focused on testing the efficiency of wireless power receivers with different battery statuses. It also works on measuring the RF to DC conversion efficiency for different RF waveforms.

Additionally, the project performs the conformance tests defined by the AirFuel Alliance RF Standard. Since 2022, Rohde & Schwarz has been participating in the yearly IEEE Wireless Power Technology Conference and Expo (WPTCE), which is the largest global event for wireless power research and industry engagement. In May 2024, Rohde & Schwarz presented several AirFuel Alliance conformance tests at WPTCE in Kyoto, Japan.

Rohde & Schwarz experts have delivered a presentation on RF Wireless Power Transfer at the Wireless Congress in 2023. More in-depth information can be found in the submitted paper RF Wireless Power Transfer: A Study on the Power Transfer Efficiency of Different Waveforms and an Overview of the Standardization Efforts.

If you have any questions please contact us.

Press release: Rohde & Schwarz drives AirFuel Alliance RF standardization efforts with first RF wireless power tester prototype

Press release: Rohde & Schwarz drives AirFuel Alliance RF standardization efforts with first RF wireless power tester prototype

Rohde & Schwarz showcased a proof of concept for testing far field wireless power systems at the IEEE Wireless Power Technology Conference and Expo (WPTCE) in Kyoto, Japan. The radio frequency (RF) wireless power tester developed by Rohde & Schwarz utilizes the company’s test and measurement equipment along with wireless charging development kits available on the market. It also includes test automation features and a user friendly web interface. With this setup, Rohde & Schwarz supports the AirFuel Alliance RF Standard.

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