  With the development of the Internet of Things technology, indoor tracking
has become a popular application nowadays, but most existing solutions can only
work in line-of-sight scenarios, or require regular re-calibration. In this
paper, we propose WiBall, an accurate and calibration-free indoor tracking
system that can work well in non-line-of-sight based on radio signals. WiBall
leverages a stationary and location-independent property of the time-reversal
focusing effect of radio signals for highly accurate moving distance
estimation. Together with the direction estimation based on inertial
measurement unit and location correction using the constraints from the
floorplan, WiBall is shown to be able to track a moving object with
decimeter-level accuracy in different environments. Since WiBall can
accommodate a large number of users with only a single pair of devices, it is
low-cost and easily scalable, and can be a promising candidate for future
indoor tracking applications.
