Measuring Distance Between Beacon & Device

Suresh SutharApril 24, 2019

2 minutes read

Beacon is BLE(Bluetooth low energy) device which is frequently used in Assets Tracking, Marketing (Sending Push Notification with message and Url), Body Fitness tracking, Door Tracking, etc.

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Example

Let take the example of old age house or mental hospital where abnormal people are leaving and those people are mentor by some other people. It becomes quite a headache when sick people might leave their room or cross the exit door, so avoid this headache a card beacon or wrist beacon is attached to sick people.

When an old person or sick people enter into other room or cross the exit door then the beacon signal will be captured by a gateway device which is mounted in each room or exit door. Gateway after scanning the beacon, it will raise alarm and notify Admin person.

Here Gateway will scan the RSSI value and transmitted power of the beacon and calculate distance between gateway and beacon.

Each Beacon Device emits RSSI (Received Signal Strength Indicator) value which is measured by another device antenna. Higher RSSI value indicates a higher strength of the signal.

As a beacon is low energy device so RSSI value is mostly in Negative number. The RSSI value is always expressed in dBm (decibels per milliwatt).

There are 2 important Parameters for Distance calculation between BLE and Device.

1. RSSI

RSSI stands for Received Signal Strength Indicator which strength of the beacon signal received by the Smartphone or receiving the device.

The RSSI value is used to calculate the approximate distance between BLE and Device. Some time distance is not accurate due to external factors like Low Battery power level of BLE, obstacle between Ble and device or diffraction.

Some RSSI value considerations for signal strength.

RSSI Value	Quality Level
-30dbm	Very Good Signal
-65 dBm	Good Signal
-70 dBm	Average Signal
-80 dBm or less than -80	Poor Signal

2. Tx Power or Measure Power

This is an important parameter for calculating an accurate distance between BLE and device. Tx Power is expected signal level when measured from a one-meter distance.

Tx power is always measured in decibels per milliwatt (dBm)

Distance Calculation Formula

function distanceCalculation(rssiValue) {

var txPowerValue = -62 // Normal ranges between -59 to -65

if ( rssiValue == 0) {

return -1.0;

}

var ratio = rssiValue*1.0/txPowerValue;

if (ratio < 1.0) {

return Math.pow(ratio,10);

}

else {

var distance = (0.89976)*Math.pow(ratio,7.7095) + 0.111;

return distance;

}

}