Abstract
Estimating the distance between two smartphones plays an important role in a host of applications. For this purpose, smartphones emit and scan for Bluetooth Low Energy (BLE) signals. When a device is detected, the distance is estimated by evaluating the received strength of these signals. The main insight that is exploited for distance estimation is that the attenuation of a signal increases with the distance along which it has traveled. However, besides distance, there are multiple additional factors that impact the attenuation and hence disturb the distance estimation procedure. Among them, frequency-selective hardware and signal propagation belong to the most significant ones. For example, a BLE device transmits packets on three different frequencies (channels), while the transmit power and the receiver sensitivity depend on the frequency. As a result, the received signal strength varies for each channel, even when the distance remains constant. However, the information on which wireless channel a packet has been received is not made available to a smartphone. Hence, this error cannot be compensated, e.g. by calibration. In this paper, we for the first time provide a solution to detect the wireless channel on which a packet has been received by a smartphone application. We experimentally evaluate our proposed technique on multiple different smartphone models. Our results help to make distance estimation on smartphones more robust and accurate.
Highlights
Estimating the distance between two wireless devices has been studied actively by the community, and techniques such as ultra-wideband (UWB), WiFi fine time measurements (FTM) and time-of-flight (TOF) have increased the estimation accuracy considerably throughout the recent years [1]– [5]
Channel-Dependent received signal strength indicator (RSSI): In the Bluetooth Low Energy (BLE) protocol, which is used for distance estimation on smartphones, advertising packets are sent on the 3 different channels 37, 38 and 39, which are spread over almost the entire frequency band used
As a result of this specification, the information on which wireless channel a packet has been received is not made available to a smartphone
Summary
Estimating the distance between two wireless devices has been studied actively by the community, and techniques such as ultra-wideband (UWB), WiFi fine time measurements (FTM) and time-of-flight (TOF) have increased the estimation accuracy considerably throughout the recent years [1]– [5]. Channel-Dependent RSSI: In the BLE protocol, which is used for distance estimation on smartphones, advertising packets are sent on the 3 different channels 37, 38 and 39, which are spread over almost the entire frequency band used. When packets received on different channels are available, the distance estimation error can be reduced by averaging their RSSI values before carrying out the estimation, or by applying more elaborate methods, as proposed e.g., in [18]. Computing the average RSSI over multiple packets can only reduce frequency-dependent errors notably, if their reception channels are known. Contributions: Compared to existing works, we in this paper make the following contributions: 1) We show how exploiting the information on which channel a BLE advertising packet has been received leads to more accurate distance estimations and quantify the achieved accuracy improvements using realworld data.
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