Evaluation of Spread-Spectrum Sequences in Ultrasonic Sonar for Indoor People Localization

Device-free localization (DFL) systems enable non-intrusive indoor human location without the need to wear a sensor or tag. This can be particularly helpful in healthcare and assisted living applications based on monitoring vulnerable people to improve their quality of life in their own home. For this purpose, this work presents a DFL system based on an ultrasonic asynchronous multistatic sonar, installed on the ceiling, consisting of four transmitters and a receiver that estimates the 3D position of a person's head. The emitted signals propagated through the indoor space are acquired by the receiver, which processes them by means of different proposed methods to estimate the position. Each transmitter emits spread-spectrum signal pulses with good cross-correlation properties, providing an update time of 300 ms and asynchronous operation. Kasami, Zadoff-Chu and orthogonal chirp emission sequences are analyzed by simulation and experimental results. Hence, it can be determined that Zadoff-Chu and chirp stand out due to their performance in terms of Doppler tolerance and accuracy, reaching mean absolute errors of 4.5 cm and 3.5 cm with standard deviations of 0.6 cm and 2 cm, respectively. On the other hand, Zadoff-Chu offers better correlation properties, while chirp provides higher data availability due to its better transducer adaptation.