How to set iBeacon TX power byte

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I am working on the ESP32 microcontroller and I would like to implement iBeacon advertising feature. I have been reading about the iBeacon. I have learnt about the specific format that iBeacon packet uses: https://os.mbed.com/blog/entry/BLE-Beacons-URIBeacon-AltBeacons-iBeacon/

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From what I understand, iBeacon preset is set and not meant to be modified. I must set a custom UUID, major and minor numbers such as:

uint8_t Beacon_UUID[16] = {0x00,0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x88,0x99,0xAA,0xBB,0xCC,0xDD,0xEE,0xFF};

uint8_t Beacon_MAJOR[2] = {0x12,0x34};

uint8_t Beacon_MINOR[2] = {0x56,0x78};

The only thing that I am confused about is the TX Power byte. What should I set it to?

According to the website that I have referred above:

Blockquote A scanning application reads the UUID, major number and minor number and references them against a database to get information about the beacon; the beacon itself carries no descriptive information - it requires this external database to be useful. The TX power field is used with the measured signal strength to determine how far away the beacon is from the smart phone. Please note that TxPower must be calibrated on a beacon-by-beacon basis by the user to be accurate. Blockquote

It mentions what is TxPower and how it should be determined but I still cannot make any sense out of it. Why would I need to measure how far away the beacon is from the smart phone if? That should be done by the iBeacon scanner not the advertiser(me).

ibeacon bluetooth-lowenergy c
1 Answers

When you are making a hardware device transmit iBeacon, it is your responsibility to measure the output power of the transmitter and put the corresponding value into the TxPower byte of the iBeacon transmission.

Why? Because receiving applications that detect your beacon need to know how strong your transmitter is to estimate distance. Otherwise there would be no way for the receiving application to tell if a medium signal level like -75 dB is from a nearby weak transmitter or a far away strong transmitter.

The basic procedure is to put a receiver exactly one meter away from your transmitter and measure the RSSI at that distance. The one meter RSSI is what you put into TxPower byte of the iBeacon advertisement.

The specifics of how to measure this properly can be a bit tricky, because every receiver has a different "specificity" meaning they will read a higher or lower RSSI depending on their antenna gain. When Apple came out with iBeacon several years ago, they declared the reference receiver an iPhone 4S -- this was the newest phone available at that time. You would run beacon detection app like AirLocate (not available in the App Store) or my Beacon Locate (available in the App Store). The basic procedure is to aim the back of the phone at the beacon when it is exactly one meter away and use the app to measure the RSSI. Many detector apps have a "calibrate" feature which averages RSSI measurements over 30 seconds or so. For best results when calibrating, do this with both transmitter and receiver at least 3 feet above the ground and minimize metal or dense walls nearby. Ideally, you would do this outdoors using two plastic tripods (or do the same inside an antenna chamber.)

It is hard to find a reference iPhone 4S these days, and other iPhone models can measure surprisingly different RSSI values. My tests show that an iPhone SE 2nd edition measures signals very similarly to an iPhone 4S. But even these models are not made anymore. If you cannot get one of these, use the oldest iPhone you can get without a case and take the best measurement you can as described above. Obviously a ideal measurement requires more effort -- you have to decide how much effort you are willing to put into this. An ideal measurement is only important if you expect receiving applications to want to get the best distance measurements possible.

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