如何使用蓝牙信号强度计算android设备之间的距离？

Question

double getDistance(int rssi, int txPower) {
    return Math.pow(10d, ((double) txPower - rssi) / (10 * 2));
}

I tried calculating distance between the Bluetooth devices using the above equation. But it returns wrong results. The results are in the format "3.4565216E9". But the results varies from "3.4565216E9" to "9.9645352E9" randomly. Can someone suggest a different method

Answer 1

A contour plot of radio signal strength indication (RSSI) even under ideal conditions is roughly maple leaf shaped. The variation you are seeing is almost certainly a consequence of whether you are in or between the high signal "fingers" of the leaf.

To get a useful result you must either use a rotating antenna and compute the RMS signal strength (think WW2 battleship radar) or a multi-pole antenna and phase trickery to get the same result.

After you get that to work you will have to do signal processing to reject reflections and when you get that to work stably you will have to conduct a full scale field test to work up a fall-off profile at 15 degree intervals from the sensor perpendicular so you can transform RSSI to range.

Since I don't think commercial BT hardware will allow you to entertain these options I suggest you not try to use RSSI for ranging.

If this is a university project then for the purposes of the exercise you need to fix the position and orientation of the transponder and antenna in a space you can mark up with polar coordinates, and map the contours. Tedious and time consuming? Yes.

Answer 2

There is no easy way to measure the distance using RSSI. One can take measurements of distance vs. RSSI and then construct a mathematical rule for it. However, that will be different for every different bluetooth chip inside phones.

There is a library called Android Beacon Library: https://altbeacon.github.io/android-beacon-library/

These guys are trying to have a community where people experiment and find these readings, but still not many models have been registered. Nonetheless, you can use it to get a general sense of distance. By default it uses Nexus 5 measurements unless if you specify other measurements.

The distance can be output as in this sample code:

        beaconManager.addRangeNotifier(new RangeNotifier() {
            @Override 
            public void didRangeBeaconsInRegion(Collection<Beacon> beacons, Region region) {
                if (beacons.size() > 0) {
                    Log.i(TAG, "The first beacon I see is about "+beacons.iterator().next().getDistance()+" meters away.");        
                }
            }
        });