Passive Human Sensing Enhanced by Reconfigurable Intelligent Surface: Opportunities and Challenges

Reconfigurable intelligent surfaces (RISs) have flexible and exceptional performance in manipulating electromagnetic waves and customizing wireless channels. These capabilities enable them to provide a plethora of valuable activity-related information for promoting wireless human sensing. In this article, we present a comprehensive overview of passive human sensing using radio-frequency signals with the assistance of RISs. Specifically, based on the specific applications, we categorize the state-of-the-art human sensing techniques into three types, including human imaging, localization, and activity recognition. Meanwhile, we would also summarize the strengths and weaknesses of the existing RIS-empowered human sensing systems. Furthermore, we explore the application of RISs in human micro-motion sensing, and propose a vital-sign monitoring system enhanced by RISs. Some preliminary experiments are presented to demonstrate the potential of RISs in contactless multiperson vital-sign sensing. Finally, we discuss the technical challenges and opportunities in this field.