A battery-free RFID-based indoor acoustic localization platform

The ability to precisely localize RFID tags with low latency and low cost is desirable for applications such as inventory, asset tracking and robotic-manipulation of tagged items. However, because of multipath propagation effects, traditional RFID positioning methods based on RF Received Signal Strength Indication (RSSI) have limited accuracy (meter-scale) or require large numbers of reference tags and readers, which increases both latency and cost. Acoustic localization systems can be more precise, but typically require a battery-powered sensor platform, which both increases tag size, weight and limits tag lifetime. In addition, conventional acoustic localization systems are not integrated with existing RFID infrastructure. This paper presents a working prototype of a RFID-based system that localizes a custom battery-free, EPC Gen2-compatible UHF tag. The system uses the RFID communication channel for synchronization and inventory, and acoustic propagation delays for distance measurement. The system consists of a custom passive tag (WISP) with acoustic tone-detector that receives and times ultrasound signals, an off-the-shelf EPC Gen2 UHF RFID reader, and an array of ultrasonic beacons. By measuring the Time of Arrival (ToA) of the ultrasound, the passive WISP tag can determine its location relative to the ultrasonic beacons. Time synchronization between the tag being tracked and the ultrasonic beacons is accomplished by using a "spy WISP." The spy WISP listens to the RFID communication traffic between the reader and the tracked tag and triggers the ultrasonic beacons in a fashion that is synchronous with the RFID traffic. The localization performance of the prototype is characterized, and the tradeoffs among power consumption, precision, latency and range are discussed.