Airborne Acoustic Chirp Spread Spectrum Communication System for User Identification in Indoor Localization

Abstract

In this contribution, we propose a reliable, longrange, communication system based on chirp spread spectrum techniques for aerial ultrasonic channels, to be employed for user identification in ultrasonic indoor localization systems. We adopt binary orthogonal keying, where the binary information, modulates the shape of the chirp instantaneous frequency, mapping zeros onto down-chirps and ones onto up-chirps. In order to reduce the demodulation computational costs, we exploit the band-pass sampling theorem and develop an ad-hoc algorithm to under-sample the received signal. The incoming packet detection and frame synchronization are guaranteed by employing a characteristic chirp signal, typically utilized for localization purposes, as a pilot for the transmitted identification packet. A simple, yet efficient, pilot detection scheme enables live features by constantly listening to the channel, triggering the demodulator only when the pilot is detected. The performance of our system has been evaluated in low power and ultra-low power real-world experiments in a warehouse-like environment. It is demonstrated that our system is capable of error-free communication up to a range of 22 m in the low power setup, where the transmitter had a transmission power of 80 mW, corresponding to a sound pressure level of approximately 37 dBSPL at 1 m distance. In the ultra-low power mode, with a transmission power to 15 mW, corresponding to a sound pressure level of approximately 29 dBSPL at 1 m, the system achieved 99% success rate up to 16 m. Moreover, the capability of the system to be robust against the Doppler effect and in a multi-user environment has been tested via simulations.