Be aware of the capture effect: a measure of its contribution to BLE advertisements reception

The Bluetooth Low Energy (BLE) networks use three channels to broadcast advertisements. These advertisements and other BLE frames are subject to collisions. The collision probability increases as the number of devices increases. Nonetheless, because of the capture effect, receivers may correctly decode some of the frames involved in a collision. Recent studies proposed models and simulations for the BLE discovery process, showing the impact of the advertisement collisions as well as the importance of an appropriate selection of the parameters related to the discovery process. The impact of the capture effect in these studies is unclear. Here we report the impact of the capture effect on the BLE advertisements. In particular, we show that the capture effect systematically increases the advertisement reception rate. We establish upper and lower bounds to quantify the contribution of the capture effect to the packet delivery ratio in dense scenarios. We make measurements in a controlled environment (a Faraday box) to show the presence of the capture effect in networks with 2 to 14 advertisers. Empirical results show that for 14 devices the capture effect increases the advertisement delivery rate by 2.95 % on average, with a peak of 7.8 %. Using simulations to recreate dense scenarios (up to 1000 advertisers) we show that the capture effect can increase the packet delivery ratio by up to 25 %. Our results have implications in the performance analysis of the discovery process and provide insights for the design of BLE applications and protocols that leverage on BLE advertisements.