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NL-Beep: A Ranging System between Multiple Smartphones Using Acoustic Sensing in NLOS Environments

In this paper, we describe NL-Beep, which is a novel ranging method between multiple smartphones using acoustic sensing in non-line-of-sight (NLOS) environments. We need to know the distance between people to maintain social distancing because of the COVID-19 pandemic. Acoustic-based ranging methods that use a smartphone built-in speaker and microphone represent one approach to high-accuracy ranging methods. These methods can be used in line-of-sight (LOS) environments where there are no obstacles between the smartphones. However, in daily life, there are usually many obstacles, including pedestrians, and NLOS environments are the norm rather than the exception. Therefore, we propose the NL-Beep system for NLOS environments. To measure the distance between two smartphones without using a direct signal, NL-Beep efficiently uses a reflected signal from the ceiling in an indoor environment. It can also detect LOS and NLOS environments based on the profile of the received signal, and adapt the method used to estimate the distance between the smartphones. In our experiments, we used two smartphones in a room and estimated the distance between them for several placement configurations. In our results, we obtained 90th-percentile errors of less than 11.97 cm for the distances between the smartphones. In this paper, we also mention the effectiveness and limitations of NL-Beep.