Battery-free wireless sensing and beyond

Pradhan, Swadhin
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The future of Internet-of-Things (IoT) demands seamless interaction between users and devices. The vision is also of one, where sensing and actuation interfaces blend into everyday objects. If all these interactions and sensing interfaces are realized without the need of an integrated power source and built upon hardware so cheap and simple that it can be installed or discarded easily; then, any physical space can become truly context-aware. Firstly, to achieve this vision of IoT-enabled smart space, we first design and develop RIO, a novel battery-free touch-sensing user interface (UI) primitive. With RIO, any surface can be turned into a touch-aware surface by directly attaching RFID tags to them. RIO is built using the technique of impedance tracking: when a human finger touches the surface of an RFID tag, the impedance of the antenna changes. This change manifests as a variation in the phase of the RFID backscattered signal and is used by RIO to track fine-grained touch movement over both o↵-the-shelf and custom-built tags. Secondly, we build a system that analyzes ball motion using a single RFID reader antenna and RFID tags. Despite significant work on wireless sensing, most existing works focus on sensing translation movement or absolute localization. However, rotational motion is also essential, especially in sports analytics (e.g., tracking ball movement), yet has been under-explored. Motivated by the need, we use RFID tags to sense a ball’s speed, direction, spin, and rotation axis. In particular, we exploit the polarization in RFID to enable motion sensing. We develop a model to capture the impact of polarization on the received signal and an optimization framework to incorporate the model to estimate the ball movement. We implement our system, Tag-based Inertial Measurement Unit (TIMU), and demonstrate its effectiveness through an extensive evaluation. To the best of our knowledge, this is the first RFID system that can sense general motion using a single RFID antenna. Finally, we further develop a system called RTSense, which enables RFID tags to sense room temperature. Our key insight is that the impedance of the RFID tag changes with the temperature, and such a change can be reflected in the tag reading. Thus, we can piggy-back communication channels with sensing information. However, it is challenging to achieve high accuracy and robustness against environmental changes. To address these challenges, we first develop a detailed analytical model that captures the impact of temperature change on the tag impedance. We then build a system that leverages a pair of tags that respond differently to the temperature change to cancel out the environmental changes