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Estimating Distance to BLE Beacons Using RSSI and TxPower in HarmonyOS

#harmonyos #distanceestimation #blebeacon #rssifiltering

Read the original article：Estimating Distance to BLE Beacons Using RSSI and TxPower in HarmonyOS

Requirement Description

The developer needs to estimate the proximity between the user's device and a nearby BLE beacon to enable location-based behaviors (e.g., showing contextual ads, triggering entry/exit events, or indoor positioning). Since GPS is not suitable for short-range indoor detection, the distance must be calculated based on BLE signal strength using RSSI (Received Signal Strength Indicator) and TxPower (transmission power at 1 meter).

Background Knowledge

BLE beacons (especially iBeacon) include a TxPower field in their advertising payloads, representing the expected RSSI value at 1-meter distance. By comparing the actual measured RSSI with the known TxPower, we can estimate how far away the beacon is from the device.

The general formula used in BLE distance estimation is:

distance = 10 ^ ((txPower - rssi) / (10 * n))
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Where:

  • rssi is the measured signal strength,
  • txPower is the transmission power at 1 meter,
  • n is the path-loss exponent (typically between 2 and 4, depending on the environment).

This method is not highly accurate but is good enough for approximate proximity grouping like Near (<1m), Mid (1–3m), Far (>3m).

Implementation Steps

  1. Extract txPower and rssi from the BLE scan result.
  2. Choose a reasonable n value depending on your environment (e.g., 2 for open space, 3 for indoor).
  3. Apply the distance estimation formula.
  4. Use distance thresholds to trigger behavior (e.g., notify if within 2 meters).
  5. Optionally, smooth results over time to reduce RSSI fluctuation.

Code Snippet / Configuration

Distance Estimation Utility 

export function estimateDistance(rssi: number, txPower: number, n: number = 3): number {
  if (rssi === 0) return -1; // Invalid RSSI
  const ratio = (txPower - rssi) / (10 * n);
  return parseFloat(Math.pow(10, ratio).toFixed(2));
}
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Usage in a BLE Manager

const beacon = {
  uuid: '123e4567-e89b-12d3-a456-426614174000',
  major: 1,
  minor: 1,
  txPower: -59,
  rssi: -70,
};

const distance = estimateDistance(beacon.rssi, beacon.txPower);
console.info(`Beacon estimated at ~${distance} meters.`);
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Interpreting the Result

function getProximityCategory(distance: number): string {
  if (distance < 1) return 'Immediate';
  if (distance < 3) return 'Near';
  if (distance >= 3) return 'Far';
  return 'Unknown';
}Copy codeCopy code
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Test Results

Distance estimation was tested with several beacons placed at known distances (0.5m, 1m, 2m, 3m, 5m). The calculated distance was generally consistent within a ±0.5m range indoors. In real-world conditions (walls, reflections), the results were slightly more variable but still suitable for categorizing proximity into Immediate / Near / Far groups.

Fluctuations in RSSI were observed, so basic filtering (e.g., averaging last 3 readings) was found to improve stability. In low-power scan mode, distance updates arrived every ~1s depending on the beacon interval.

Limitations or Considerations

Factor Notes
Accuracy RSSI is very sensitive to orientation, interference, and obstacles.
Environmental Impact Use a different n value depending on indoor vs outdoor usage.
Multiple Beacons You may receive conflicting distances from overlapping signals.
Smoothing Required Apply filtering (average, median, Kalman) to reduce jitter.
iBeacon Format Only TxPower must be parsed from advertisement data (not always available for non-iBeacon formats).

Written by Arif Emre Ankara

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