Strata: Fine-Grained Acoustic-based Device-Free Tracking.

Next generation devices, such as virtual reality (VR), augmented reality (AR), and smart appliances, demand a simple and intuitive way for users to interact with them. To address such needs, we develop a novel acoustic based device-free tracking system, called Strata, to enable a user to interact with a nearby device by simply moving his finger. In Strata, a mobile (e.g., smartphone) transmits known audio signals at inaudible frequency, and analyzes the received signal reflected by the moving finger to track the finger location. To explicitly take into account multipath propagation, the mobile estimates the channel impulse response (CIR), which characterizes signal traversal paths with different delays. Each channel tap corresponds to the multipath effects within a certain delay range. The mobile selects the channel tap corresponding to the finger movement and extracts the phase change of the selected tap to accurately estimate the distance change of a finger. Moreover, it estimates the absolute distance of the finger based on the change in CIR using a novel optimization framework. We then combine the absolute and relative distance estimates to accurately track the moving target. We implement our tracking system on Samsung Galaxy S4 mobile phone. Through micro-benchmarks and user studies, we show that our system achieves high tracking accuracy and low latency without extra hardware.