Beacon is a small Bluetooth Low Energy (BLE) device which keeps broadcasting Bluetooth signal. Beacon signal setting is one of the most important deployment configuration in live environment. It will highly affect the user experience, deployment cost and indoor positioning precision. This article gives a detail introduction of signal performance indicator, the major signal related configuration, the factor to consider for common scenarios and how to measure that Beacon signal performance with Smart Phone APP.
Beacon Signal Introduction
In general, Beacon has omnidirectional antenna so that its signal area is spherical shape. The Beacon is at centre of sphere. Smart Phone could detect the Beacon inside the spherical and in opposite, Smart Phone could not detect the signal when it is out the spherical.
The diameter of the spherical is mainly decided by the Beacon signal transmit power (Tx Power). Higher Tx Power means longer diameter.
Receive Signal Strength Indicator (RSSI): When the Smart Phone is in the range of Beacon signal, the signal strength it receives from Beacon because lower when the distance from Beacon is longer. The signal strength is named as Receive Signal Strength Indicator (RSSI). And it is measured with dBm. For example, 4 dBm, -10 dBm. Higher value means higher strength.
Distance Estimation: Based on the RSSI, Smart Phone APP could calculate the distance from the Beacon. For example, if a Beacon’s broadcast range (the spherical diameter) is 15 meter, based on measure, we know the RSSI is -50 dBm at 1 meter distance and -70 at 3 meter distance. When we get a RSSI is -60 dBm, as -50 > -60 > -70, we could estimate the distance between Smart Phone and the Beacon is between 1 meter and 3 meter. The RSSI based distance calculation is not accurate. First, the Bluetooth signal strength is interfered by environment very much. For example, the Bluetooth signal strength will be absolved by water. So in the humid day, the RSSI will be less than the normal day even the distance between Smart Phone and Beacon is identical.
Besides environment, Beacon’s configuration and Beacon’s antenna performance could also affect the variation of signal’s stableness.
The Advertising Interval and Tx Power are 2 majors configuration which affect the Beacon signal performance.
Beacon keeps broadcast signal with special interval. The interval is Advertising Interval. Smart Phone’s Bluetooth “receiver” will also monitor the Beacon’s signal with special Listening Interval (Note: this is simplified. The Bluetooth protocol’s signal detection is much more complex). When the Beacon is sending signal, if the Smart Phone is not Listening, the Smart Phone “missed” the Beacon. So if the Beacon broadcast more frequently, i.e. less Advertising Interval,
There is less chance that the Smart Phone “missed” the Beacon signal. If it misses many times, the Smart Phone APP might “think” the Beacon doesn’t existing.
And in turn, Smart Phone Bluetooth protocol stack could get more broadcasts from same Beacon. And then Smart Phone Bluetooth could calibrate the signal strength based on the multiple broadcast RSSIs, so the RSSI value will be more accurate.
So less Advertising Interval will lead to more stable Bluetooth Signal. But it is not free, it will also lead to less battery life.
Tx Power (Transmit Power) is the signal strength the Beacon sends the signal. Higher Tx Power means longer signal range.
For example, Picture#1 is the RSSI trend with <100ms interval, 4 dBm Tx Power> and Picture#2 is the RSSI trend with <100ms interval, -30 dBm Tx Power>. The Beacon is 0.5 meter way from the Smart Phone. It is clear that the less Tx power leads to more stable RSSI.
So in order to get stable RSSI and high precision indoor positioning, configure as less as possible Tx Power. And in turn, less Tx power could also lead to less signal interfere.
Picture#1 100ms, +4dBm
Picture#2 100ms, -30dBm
Beacon Antenna Performance and Comparison
Beacon’s signal quality and performance is heavyly affected by its antenna design. Beacon Antenna is the most challenge part of Beacon’s PCB board design. In order to get good Antenna, antenna expert and good test environment & test device are needed.
There are 2 major indicators to measure Beacon antenna design: signal strength and signal stableness.
It is easy to compare 2 Beacons’ signal strength by comparing their max signal range as following 3 steps:
Set the 2 Beacons as same Max Tx Power and Advertising Interval;
Put the 2 Beacons’ at same place
Take the Smart Phone away from the 2 Beacons and record the distance where the first Beacon’s signal missing. The distance is the Beacon’s signal range The Beacon with high signal range means high antenna performance per signal strength.
In general, the Beacon with higher signal strength will also mean it has more stable signal.
Below is on easy test way to compare Beacon signal stableness. It could be tested with Nordic (the chipset provider) company's nRF Master Controller, which could install from Andriod play store. The steps are as below.
Set the 2 Beacons as same Max Tx Power and Advertising Interval;
Put the 2 Beacons’ at same place;
Take the Smart Phone to certain distance away from the 2 Beacons and record the signal strength and trend with nRF Master Controller APP.
The following picture is the comparison of Sensor Yunzi with another 2 Beacon providers Beacons.
All three beacons are broadcasting with 0dbm and 416ms interval. The Smart Phone is SUMSUNG S4. The distance is 1 meter.
From the picture, we can see Sensoro Yunzi's signal strength and stablenss is better than the other 2 Beacons.
Best practice to achieve good signal performance
In common Beacon user scenario, there are 2 kind of usage model.
Single Beacon positioning
Multiple Beacons positioning
For the single Beacon position, whenever the Smart Phone enters the Beacon’s signal range, the Smart Phone could retrieve the Beacon’s ID (UUID/MajorID/MinorID) related content and show that to customer. For this scenario, one or a few (to avoid single point failure) Beacons are deployed and the signal range is important. The Tx Power is needed to set properly to make sure the signal could cover the place but not go over that. Less Tx power could not only save battery life but also avoid interfere the Beacon’s neighborhoods.
For different Beacon chipset, there are different set of Tx Power levels. For example, Ti CC2541 has only 3 level Tx Power levels (0 dBm, -6 dBm and -23 dBm) but Nordic 51822 has 8 Tx Power levels (4 dBm, 0 dBm, -4 dBm, -8 dBm, -12 dBm, -16 dBm, -20 dBm, -30 dBm). So Nordic is better than Ti as it is more flexible to choose proper signal range based on Tx Power.
For this scenario, the Advertising interval should be set at about 200 ms or 400 ms to balance between user experience and battery life.
For the multiple Beacons positioning, one Smart Phone will detect multiple Beacons’ signal and find the position with triangle localization or fingerprint algorithm. In order to get better precision, the Advertising Interval is set to very low for example 100ms or 150ms; and the tx power is low too for example <-20 dBm. As the Advertising interval is low, so choose right battery supply is also important to ensure enough battery life. Please reference http://blog.sensoro.com/choose-right-battery-for-your-beacon/ for more detail.