Beacon Signal Stability Observations

As previously mentioned, we have been doing some signal strength and stability tests across beacons. While the actual data feeds into our consultancy work, we have some high level observations that we can share here.

The following graph shows the standard deviation of the RSSI @ 1m, for our beacons and a few we don’t sell, measured over a 60 second time period:

beaconsignalstability

Smaller bars are better and represent beacons
whose RSSI varied the least over time.

We found that beacons belonged to one or two groups. Firstly those with very stable RSSI and secondly those with an RSSI that had a standard deviation between about 4 and 6 dBm.

Signal stability is more important when you are using the RSSI to infer distance, either directly from the RSSI itself or indirectly via, for example, the iOS immediate, near and far indicators. RSSI varying without a change of distance might cause more spurious triggering. However, you should keep in mind that environmental factors can often cause variation much larger than the 4 to 6 dBm found in this test. Moving obstacles, for example people, will cause significant variation in RSSI.

We don’t yet know why beacons fall into these two groups. We couldn’t find any correlation between signal stability and manufacturer, antenna type, beacon size or processor chip manufacturer.

However, we found what might be some clues in the research paper An Analysis of the Accuracy of Bluetooth Low Energy for Indoor Positioning Applications by R. Faragher, University of Cambridge, UK; R. Harle, University of Cambridge, UK. The paper explains how Bluetooth LE advertising moves pseudorandomly between radio channels. The channels use different radio frequencies that, in turn, results in fading of the signal at different distances. Different radio frequencies experience constructive and destructive interference at different physical locations. Beacons that move more between channels can cause more rapidly varying received signal strength (RSSI).