Professor Wei Gong and Dr. Longzhi Yuan from the UBIoT Lab have jointly designed and built a third-generation WiFi backscatter communication system, which for the first time have achieved compatibility between backscatter system and native WiFi, allowing the backscatter to utilize ambient WiFi packets as carriers. This enables legitimate data interaction between backscatter nodes and WiFi devices, overcoming the uncertainty of carrier content in the backscatter system. This achievement holds significant importance for driving the widespread deployment and application of backscatter communication technology. The related research findings have been published at the international academic conference ACM MobiSys 2023.
Backscatter communication is a technology that utilizes existing radio frequency signals as carriers for information transmission. It offers advantages such as ultra-low power consumption and minimal system complexity, making it highly advantageous for the Internet of Things (IoT). Its operational process bears certain similarities to how humans perceive the colorful world around them. Just as individuals can see objects due to light propagating to their eyes, objects that are not luminous themselves, such as the moon, the earth, and buildings, can be seen because light reflecting off their surfaces reaches human eyes. In the realm of backscatter communication, radio frequency signals serve as carriers for transmitting information, and backscatter nodes are akin to objects that are visible even though they don't emit light themselves—just as certain objects are visible to the human eye. The receiver, in this analogy, plays the role of the eye. It's important to note that the color of reflected light is influenced not only by the properties of the object but also by the color of the incident light. For instance, when illuminated with red light, a green object appears black to our eyes. Similar phenomena are present in backscatter systems: the demodulated result of the backscattered signal in the receiver is influenced by both the carrier content and tag data, which incurs significant difficulty in the recovering tag information. In fact, this challenge exists in the first- and second-generation WiFi backscatter communication systems. However, in nature, a chameleon actively changes its color to match its environment, effectively aligning the color of reflected light with its surroundings. Inspired by this principle, could the backscatter modulation be adjusted to counteract the impact of carrier content? Following this line of thought, the research team designed a backscatter system that decodes the content of the ambient carrier and adjusts tag modulation accordingly. This innovative approach ultimately reshapes reflected signals to be legitimate WiFi packets containing only tag data—thus, the third-generation WiFi backscatter communication system. Drawing parallels to the color-changing mechanism of chameleons, this system is named "Chameleon," and its functioning is illustrated in the diagram below.
Figure 1: Chameleon working principle
Following the approach, the main technical challenges revolve around how to decode the content of environmental WiFi data packets in the backscatter tag and how to adjust the tag modulation based on the decoding results. To address the first challenge, researchers observed that the baseband processing of 802.11b WiFi signals imparts different envelope waveforms to data '0' and '1'. Based on this insight, they designed a differential demodulation technique. For the second challenge, researchers found through experimental analysis that using the XOR operation between the results of differential demodulation and the target tag data can effectively counteract the influence of carrier content on the reflected signal. Combining these two techniques, backscatter tags can reshape environmental WiFi packets containing arbitrary content into legitimate WiFi packets with only tag message. This system is compatible with WiFi devices such as iPhones, iPads, Android smartphones, laptops, etc. This compatibility holds significant importance for integrating the backscatter system into WiFi networks and contributes to the advancement of the Internet of Things (IoT).
Figure 2: The captured excitation beacon and tag beacon in COTS WiFi devices