Question

Hi,

We performed some tests on EYE Beacons with the purpose of integrating them in our systems: we stumbled upon the transmission power configuration and the impact that this parameter has on the txPower field in the BLE packets, that will later be useful for the computation of RSSI of the beacon and, eventually, its estimated distance from the receiver.

As far as we understand, there are 8 possible values of transmission power: [-14, -11, -8, -5, -2, 2, 4, 8], so we tested all of these values both in iBeacon and Eddystone, knowing that in iBeacon txPower is the reference RSSI measured at 1m distance from the device, while in Eddystone at 0m distance. Please mind that txPower values are the one that everyone can find in the payload of an advertising packet.

These are the results:

Configured Power [dBm]	txPower1m (iBeacon)	txPower0m (Eddystone)	Difference
8	-54	-24	30
4	-60	-24	36
2	-60	-30	30
-2	-60	-32	28
-5	-66	-34	32
-8	-66	-38	28
-11	-74	-43	31
-14	-76	-44	32

So our questions are:

1. Is it possible that the txPower values for different Configured Powers are exactly the same?
   We are asking because at 1 meter, in "somewhat controlled conditions", those are almost always NOT the RSSI values we are getting from the devices, which of course is going to impact even the (already not so reliable) distance calculation.
2. From online documentations, we found out that the typical RSSI variation within 1 meter is about -41 dBm: this means that between Eddystone txPower values and iBeacon ones we should see a similar difference, but as you can see in the table, this is never the case (the average difference between the two values is ~31 dBm); also the difference seems to be "jumpy" and not regular/monotonic as intuitively we can expect (but maybe I'm wrong in this supposition).
   Is there an explanation for this? Maybe something regarding the calibration of the transmission power?

Thanks in advance.

Answer 1

Hello.

Calibrated Power - in the Beacon protocol is a critical parameter that helps estimate the distance between a beacon and a receiving device. For example, this value is sent in Byte 29 of the iBeacon packet and is typically set to a predefined value in the sensor's configuration. It indicates the transmitting power of the beacon at a reference distance of 1 meter and is measured in decibels referenced to 1 milliwatt (dBm).

RFU - Similarly to Calibarated Power, RFU is set for Eddystone protocols. The difference is iBeacon, the calibrated power is reference RSSI value that is the expected RSSI value at 1 meter from the beacon, while for Eddystone, it's the expected RSSI value at 0 meters.

Currently Teltonika is providing beacons with pre-calibrated RSSI at 1m for iBeacon and RSSI at 0m for Eddystone that should be suitable for most use cases, however we plan to introduce user calibration in the future.

https://wiki.teltonika-gps.com/view/EYE_FAQ#What_is_TX_.2F_RFU_.2F_Calibrated_Power.3F