Push the limit of device-free acoustic sensing on commercial mobile devices

Device-free acoustic sensing has obsessed with renovating human-computer interaction techniques for all-sized mobile devices in various applications. Recent advances have explored sound signals in different methods to achieve highly accurate and efficient tracking and recognition. However, accuracies of most approaches remain bottlenecked by the limited sampling rate and narrow bandwidth, leading to restrictions and inconvenience in applications. To bridge over the aforementioned daunting barriers, we propose PDF, a novel ultrasound-based device-free tracking scheme that can distinctly improve the resolution of fine-grained sensing to submillimetre level. In its heart lies an original Phase Difference based approach to derive time delay of the reflected Frequency-Modulated Continuous Wave (FMCW), thus precisely inferring absolute distance, catering to interaction needs of tinier perception with lower delay. The distance resolution of PDF is only related to the speed of actions and chirp duration. We implement a prototype with effective denoising methods all in the time domain on smartphones. The evaluation results show that PDF achieves accuracies of 2.5 mm, 3.6 mm, and 2.1 mm in distance change, absolute distance change, and trajectory tracking error respectively. PDF is also valid in recognizing 2 mm or even tinier micro-movements, which paves the way for more delicate sensing work.