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

In this contribution, we propose a reliable, long-range, communication system based on chirp spread spectrum (CSS) techniques for aerial ultrasonic channels, to be used 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. To reduce the demodulation computational costs, we exploit the bandpass sampling theorem and develop an ad hoc algorithm to undersample the received signal. The incoming packet detection and frame synchronization are guaranteed using a characteristic chirp signal, typically used 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 ultralow-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 (SPL) of approximately 37 dB textsubscript SPL at 1-m distance. In the ultralow-power mode, with a transmission power to 15 mW, corresponding to a SPL of approximately 29 dB textsubscript SPL 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 multiuser environment has been tested via simulations.