Indoor pseudo-ranging of mobile devices using ultrasonic chirps

In this paper, we present an indoor ultrasonic location tracking system that can utilize off-the-shelf audio speakers (potentially already in place) to provide fine-grained indoor position data to modern mobile devices like smartphones and tablets. We design and evaluate a communication primitive based on rate-adaptive wide-band linear frequency modulated chirp pulses that utilizes the audio bandwidth just above the human hearing frequency range where mobile devices are still sensitive. Typically transmitting data, even outside of this range, introduces broadband human audible noises (clicks) due to the non-ideal impulse response of speakers. Unlike existing audio modulation schemes, our scheme is optimized based on psychoacoustic properties. For example, all tones exhibit slowly changing power-levels and gradual frequency changes so as to minimize human perceivable artifacts. Chirps also bring the benefit of Pulse Compression, which greatly improves ranging resolution and makes them resilient to both Doppler shifts as well as multi-path propagation that typically plague indoor environments. The scheme also supports the decoding of multiple unique identifier packets being transmitted simultaneously. By applying a Time-Difference-of-Arrival (TDOA) pseudo-ranging technique the mobile devices can localize themselves without tight out-of-band synchronization with the broadcasting infrastructure. This design is not only scalable with respect to the number of transmitters and tracked devices, but also improves user privacy since the mobile devices compute their positions locally. We show through user studies and experimentation on smartphones that we are able to provide sub-meter (95% < 10cm) accurate indoor positioning in a manner that is imperceptible to humans.