Channel State Information Variability in BLE Positioning Measurements

The Bluetooth Low Energy beacon messages are used in indoor positioning systems (IPS) to estimate the distance between the wireless devices. Recent work shows that channel state information and separate measurements for each of the three channels may increase the accuracy of the IPS, especially using the fingerprinting method. We perform a set of measurements presenting RSSI variability per channel in stationary measurements and with a variation of angle between the beacon and the receiver. The estimated standard deviation of the RSSI (received signal strength index) is minimal (less than one dBm) for stationary transmission. However, our measurements show that if the angle is not constant, the signal level of beacons received on different channels changes by up to 8 dBm, and the channel state is not a reliable source of information for distance estimation. We show that since mobile phones antennas are not perfectly omnidirectional and the multipath fading significantly changes even with a slight turn of the receiver, the channel state information in the BLE positioning system has limited applicability in real-life conditions.