Distance Estimation for BLE-based Contact Tracing -- A Measurement Study

Mobile contact tracing apps are - in principle - a perfect aid to condemn the human-to-human spread of an infectious disease such as COVID-19 due to the wide use of smartphones worldwide. Yet, the unknown accuracy of contact estimation by onboard wireless technologies in real use hinders their broader employment. We address this challenge by conducting a measurement study with a custom testbed to show the capabilities and limitations of Bluetooth Low Energy (BLE) in different scenarios. Distance estimation is based on interpreting the signal pathloss with a basic linear and a logarithmic model yielding an accuracy of 2-4 m. Further, we compare our results with accurate ultra-wideband (UWB) distance measurement, which is able to reduce the distance error to 0.9 m. While the BLE distance error may not seem sufficiently accurate, we show that a simple threshold-based contact detector achieves an F1-score accuracy of 64%, which can be improved to 69% when the carrying position of the phone is known. It is further shown that multi-path signal propagation reduces the effect of body shielding and thus increases detection accuracy in indoor scenarios.