How to Calculate Distance from the RSSI value of the BLE Beacon

I am sure many people are working on the BLE technology and even on the projects like Indoor Location Positioning, distance approximation from the RSSI value of the BLE beacon. This Article is short and simply about the distance calculation from the RSSI value of the BLE beacon.

Important parameters :

RSSI

RSSI stands for Received Signal Strength Indicator. 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. At maximum Broadcasting Power (+4 dBm) the RSSI ranges from -26 (a few inches) to -100 (40-50 m distance).

RSSI is used to approximate distance between the device and the beacon using another value defined by the iBeacon standard: Measured Power (see below).

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 which indicates what’s the expected RSSI at a distance of 1 meter to the beacon. Combined with RSSI, it allows to estimate 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.

Advertising Interval

Beacons do not broadcast constantly. 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 big way.

 

Formula

Distance

Measured Power

RSSI

N (Constant depends on the Environmental factor. Range 2-4)

 

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

 

Measured Power is also known as the 1 Meter RSSI.

 

Let’s check some examples :

Measured Power = -69 (for kontakt BLE beacons)

RSSI = -60, -69, -80

N = 2

1. Distance for RSSI -60 = 10 ^ ((-69 – (-60))/(10 * 2))

= 0.35 meter

2. Distance for RSSI -69 = 10 ^ ((-69 – (-69))/(10 * 2))

= 1 meter

3. Distance for RSSI -80 = 10 ^ ((-69 – (-80))/(10 * 2))

= 3.54 meter

 

Obviously, the Distance calculated is the approximated distance and not the exact distance as for calculating exact distance we have to make the loss factor (from environmental factors) zero.

 

Source – Estimote.com, Kontakt.com, quora.

 

 

 

 

 

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