Convert RSSI Value of the BLE (Bluetooth Low Energy) Beacons to Meters

Due to the pandemic situation because of coronavirus (Covid-19) many developers are working on Bluetooth, NFC, Geolocation, GPS/GPRS technologies to implement a social distance approach or showing the affected area, etc.

How to manage social distancing using Bluetooth technology?

You should have basic knowledge about the below parameters, which are in use with Bluetooth technology.

RSSI (Received Signal Strength Indicator)

For Bluetooth, it is the strength of the beacon’s signal as seen on the receiving device, e.g. a smartphone. The signal strength depends on distance and Broadcasting Power value. Bluetooth works with broadcasting signals and that broadcasting power value is around 2–4 dBm — and due to that, the signal RSSI strength will be around -26 (a few inches) to -100 (40–50 m distance).

To calculate Bluetooth Proximity between two unpaired/paired devices and the beacon using another value defined by the iBeacon standard; i.e. Measured Power

Due to external factors influencing radio waves — such as absorption, interference, or diffraction — RSSI tends to fluctuate. The further away the device is from the beacon, the more unstable the RSSI becomes.

Measured Power

Measured Power is a factory-calibrated, read-only constant that indicates what’s the expected RSSI at a distance of 1 meter to the beacon. Combined with RSSI, it allows estimating the distance between the device and the beacon.

However, you can change it in the raw chips using Arduino, etc if you have access to the AT-commands of the BLE chip. There is another factor that also affects the continuous signal, i.e. Advertising Interval (Have a look at it below)

Advertising Interval

Beacons do not broadcast constantly like WiFi. They ‘blink’ 😉 instead. Advertising Interval describes the time between each blink. The value ranges between 100 ms and 2000 ms.

The shorter the interval, the more stable the signal. Keep in mind that adjusting Advertising Interval will impact battery life in a long way.

This was the basic about you should know before implementing anything with BLE.

Now Let’s talk about the formula and conversion.

Formula

Keep four parameters in your mind

• Distance
• Measured Power
• RSSI
• N (Constant depends on the Environmental factor. Range 2–4, low to-high strength as explained above)

Distance = 10 ^ ((Measured Power -RSSI)/(10 * N))

BLE uses Measured Power is also known as the 1 Meter RSSI. So consider the value of Measured Power = -69

RSSI = Obtained RSSI value from the devices (For example, you obtained -80)

N = 2 (Consider Low strength)

Put the values into the above formula to calculate the distance:

Distance for RSSI -80 = 10 ^ ((-69 -(-80))/(10 * 2)) = 3.54 meters

Similarly, Let’s say your RSSI values are…

1. Distance for RSSI -75 = 10 ^ ((-69 -(-75))/(10 * 2)) = 1.995 meters ≅ 2 meters
2. Distance for RSSI -69 = 10 ^ ((-69 -(-69))/(10 * 2)) = 1 meter

If you want to calculate your distance, just copy the below expression, change your RSSI value (Highlighted as bold fonts), and paste it to the google search engine.

10 ^ ((-69 -(<RSSI_VALUE>))/(10 * 2))

Note: Please note that this calculated distance is approximated.

Wrapping Up…

If you are looking for an example to alert people when some device comes nearby you, or in another language, If you want to implement a social distancing concept using BLE You can use this forked project on GITHUB. You can also contribute to make it work with high accuracy.

If you find a way, about how to calculate the exact distance with BLE technology then please comment below, Your help will be appreciated.

You can check the code of 1point5 application for a social distancing app.

If you find joy and value in what I do, please consider supporting my work with a donation — however much you can afford, it means and helps more than you can imagine.

References: Bluetooth Wikipedia, BLE Formula, Bluetooth electrical engineering book

Get my stories in your feeds by subscribing to me, or become a vocal+ member to read all stories of thousands of other writers, participate in all challenges and get a payout with low fees and less payout threshold on Vocal Media.

© Originally Published at BLE to Meter Conversion, also published on Medium by Rakshit Shah (Me).