Ultrasonic signal acquisition module for smartphone indoor positioning

Smartphone capabilities can significantly enhance mobile and context-aware applications that depend on location information. Whereas outdoor Location Based Services (LBS) are highly developed thanks to GPS, indoor LBS still demand a positioning technology that provides accurate location data. In this paper, an ultrasonic signal acquisition module for fine-grained indoor positioning of portable devices, such as smartphones or tablets, is presented. It is based on a microcontroller that digitizes the signals coming from a set of ultrasonic beacons placed on the ceiling and transfers the data acquired through Bluetooth Wireless Technology to the smartphone, for their consequent processing. All ultrasonic beacons emit simultaneously, each one a different Kasami code which is BPSK modulated with a carrier frequency of 40kHz. At the reception, the proposed module, that can coexist with an unlimited number of receivers, digitizes the analog signal captured by an electret microphone and sends it to the smartphone. Then, the smartphone carries out the correlation of the received signal with the emitted codes and obtains its absolute position by hyperbolic trilateration. Test results show that the proposed system can achieve similar accuracies to conventional ultrasonic based local positioning systems, with the advantage of services of a smartphone.