RF-Pen: Practical Real-Time RFID Tracking in the Air

Wireless tracing technologies have seen great potentials in many applications, including drawing and writing, gesture-based commanding, and gaming. Many state-of-the-art systems have recently been proposed along this line. However, none of them can strike a balance among hardware complexity, time delay, and accuracy in real-world scenarios. In this paper, we propose RF-Pen, a practical and complete RFID tracking system that achieves centimeter-level real-time tracing with 4 antennas. To do so, RF-Pen mainly employs two key designs, namely selective hologram and hybrid voting. Our selective hologram places antennas in large separation, which not only expedites the tracking process by producing a handful of good-quality candidate points but also maximizes tracing resolution. Nevertheless, a big challenge is ambiguity. To address this, we introduce hybrid voting that effectively integrates RSSI and phase measurements to evaluate the likelihood of all candidate points. This way, a precise initial position and fine-resolution tracing beams are located. We implement RF-Pen using off-the-shelf readers and tags and compare it against state-of-the-art systems. Results show that with a single reader of 4 antennas, RF-pen achieves a median trajectory error of 2.15 cm and a median position error of 12.8 cm, which are
$3.7\times$
and
$4.1\times$
better than RF-IDraw, respectively.