Ultrasonic Device-Free Localization System Using Orthogonal Chirp-Based Multistatic Sonar

Indoor device-free localization (DFL) systems offer an alternative paradigm to traditional location, in which the target to be located does not need to carry any sensor, tag, or electronic device. In contrast to most popular DFL systems, which are based on radio frequency technologies, ultrasonic signals have been used in this work, due to their good accuracy results, to estimate the 3-D position of a person's head. For this purpose, a four-emitter and a single-receiver multistatic sonar installed overhead has been employed. The emission signals follow a time division orthogonal chirp division multiplexing (TD-OCDM) access technique to obtain a 300 ms update time and asynchronous operation. The signals propagated in the indoor space are acquired by the receiver and processed. A sign-data least mean square (SD-LMS) adaptive filter algorithm has been applied in order to remove the background components on the received signal. Then, a specific window from the resulting clean signal is employed to determine the reflected signal at the target time-of-arrival (ToA) by using an adaptive threshold. The ToA obtained, together with the direct path ToA between emitters and the receiver, provide ranges to compute the target location using a localization algorithm. A simulator was used to analyze the proposal, estimating the coverage area of the system at 3.14 m2. Several experiments have been performed with a single user placing the multistatic sonar at heights of 2.60 and 2.80 m above the floor within the coverage area. The results achieved positioning errors lower than 9 cm for the 2.60-m height and 11.6 cm at 2.80 m in 90% of the cases.