Clock time independent distance measurement between smartphones using audible PRBS

This paper presents an approach for distance mea-surement between two smartphones. The approach uses the smartphone's microphone(s) and speaker(s) to simultaneously emit and record audio to determine the propagation delay, and hence distance. Each device in succession emits a unique maximum length sequence (MLS) audio signal. Both devices record emitted signals in a single continuous recording. An automated comparison of the recordings from both devices - and specifically the temporal position of the emitted signals within each recording - allows the audio propagation delay between the devices to be estimated. A custom web application was developed that performs signal emission and recording on each device and transmits the resulting recordings to a master device for automated analysis. This facilitates vendor and operating system independent deployment. Initial tests, carried out using Samsung Galaxy A10 smartphones running Android 10, yielded a mean error of 32.29 mm. Because the distance is determined by the propagation delays in each locally acquired audio, there is no need for clock time synchronization. Possible future refinements of the system are discussed, including representation of the positions of microphone and loudspeaker within each phone, that may significantly improve precision. Accurately determining the distance may also aid time mismatch correction.