RF-Keypad: A Battery-Free Keypad Based on COTS RFID Tag Array

With the explosive increase in the Internet of Things (IoT) devices, there is a rising demand for seamless and intuitive interactions between users and smart devices. Existing solutions require either dedicated sensors with microcontroller and battery power or modification of the hardware. This article presents RF-Keypad to realize a battery-free and wireless touch input interaction solution via commercial off-the-shelf (COTS) radio frequency identification (RFID) devices. RF-Keypad can easily turn an ordinary rigid object into an interaction touch keypad by deploying a tag array on the surface of the object. RF-Keypad extracts the received signal strength (RSS) variation feature of the touch action and builds an RSS-based model to detect the touch events, which is robust to the tag position and orientation. To acquire better performance, best touch area that poses distinct RSS variation feature is studied and optimal placement of the tag array is also investigated to eliminate mutual coupling effect between adjacent tags. Two touch detection algorithms based on RSS variance (RV) and the variance of the RSS variance (VoRV) are proposed, respectively, where the RV-based algorithm suits the scenario of fixed antenna-tag distance and the VoRV-based algorithm is designed for the scenario of variable antenna-tag distance. We implement a prototype of RF-Keypad with commodity RFID devices. Extensive experiments show that RF-Keypad achieves a touch detection accuracy of higher than 96% under fixed antenna-tag scenario and higher than 91% under variable antenna-tag distance scenario. An intelligent door access application is developed to highlight the practicality and scalability of our solution.