I am currently working on hobby project where I would like to determine the distance of a Bluetooth enabled mobile device from a wall within a room. The idea is to detect if a mobile phone device (this will avoid the need to give things like IR leds, rfid tags or ultrasound transmitters) is a front of a wall (6m long) and then identify the user based on the unique id (bluetooth address) and enable the user to control the contents on the wall and the contents while be moved more to the position of the mobile phone e.g. roughly following it.
Now I am came up with the idea to hang multiple Bluetooth devices on predefined locations within this room and a front of the wall. Bluetooth devices are coming in three classes one, two and three. Most mobile devices with Bluetooth support are using Class 2 meaning the maximum distance of the phone with other "beacons" can be ~10 meters. Only I am having a problem if it's really feasible. What I am currently now are:
1. Most mobile phones are Class 2 Bluetooth devices (maximum of 10 meters)
2. Maximum connected devices are seven
3. Once the Bluetooth device is connected/"paired" the Received Signal Strength Indicator (RSSI) value can be resolved. Values are in the dBm unit (logarithmic).
4. The indoor environment means less interference of RF (or there should be a lot of WiFi access points microwaves around in the building -- first analysis showed they weren't any)
5. I should be able to calculate the distance of the mobile phone from the "beacon" by using the logarithmic aspect of the RSSI because the value gets lower the farther away the phone is. In combination with mathematics like triangulation or goniometry
6. By using a predefined list of "possible" dBm values we can "fingerprint" the position of the phone within the room -- this might improve the accuracy of the position
My suggested positions for the bluetooth beacons are:
* ------ WALL ------------------- *
| * |
* *
| |
-------------------=------
Now my question is are these assumptions correct and is feasible to create this? Are their any project regarding this available or better solutions to achieve the goals (see above) ? I think it is feasible, but the calculated position won't be super precise but good enough for the earlier mentioned id.
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Thanks. Apparently is quite hard to get it all working. Especially because the RSSI/LQ information differs per mobile phone. I think it might be possible when you have full control of the used bluetooth devices (e.g. modules) and able to get useful RSSI/LQ values. Most phones only can tell if the RSSI value if above or below a given (golden) range.
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by: HenrikBerglundPosted on 2007-10-31 at 23:14:29ID: 20191043
Hi,
Cool project! Here's some comments:
1At least initially, the idea was to let 100m devices have more sesitive receivers also, i.e. they would work at 100 m range even if the other device was a 10m device.I have not tried this.
2. You should not count on a cell phone being capable of 7 simultaneous connections. Perhaps if you write the BT SW on the phone yourself that would be possible.
3. You can get a RSSI even without having a connection by performing an inquiry. Results may not be coming in as often as you want then though. If you actually are connected, there is also another measure called link quality 0-255 that you also may be able to use (although RSSI seems to be the best measure)
Also, note that the Bluetooth Adress is easily changed in a device, so this is not a good idea for a security solution, but as I understand it, that would not be the purpose of this effort.
Good luck! Make sure to let us know how it works out!