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Identifying the BLE Advertising Channel for Reliable Distance Estimation on Smartphones

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Document pages: 10 pages

Abstract: As a response to the global COVID-19 surge in 2020, many countries haveimplemented lockdown or stay-at-home policies. If, however, the contact personsof every infected patient could be identified, the number of virustransmissions could be reduced, while the more incisive measures could besoftened. For this purpose, contact tracing using smartphones is beingconsidered as a promising technique. Here, smartphones emit and scan forBluetooth Low Energy (BLE) signals for detecting devices in range. When adevice is detected, its distance is estimated by evaluating its received signalstrength. The main insight that is exploited for distance estimation is thatthe attenuation of a signal increases with the distance along which it hastraveled. However, besides distance, there are multiple additional factors thatimpact the attenuation and hence disturb distance estimation. Among them,frequency-selective hardware and signal propagation belong to the mostsignificant ones. For example, a BLE device transmits beacons on threedifferent frequencies (channels), while the transmit power and the receiversensitivity depend on the frequency. As a result, the received signal strengthvaries for each channel, even when the distance remains constant. However, theinformation on which wireless channel a beacon has been received is not madeavailable to a smartphone. Hence, this error cannot be compensated, e.g., bycalibration. In this paper, we for the first time provide a solution to detectthe wireless channel on which a packet has been received on a smartphone. Weexperimentally evaluate our proposed technique on multiple different smartphonemodels. Our results help to make contact tracing more robust by improving theaccuracy of distance estimation.

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