[chbot] Bluetooth tags
cdhmanning at gmail.com
Thu Oct 1 05:39:59 BST 2020
That's an interesting data point. I suppose a monte carlo statistical model
would give consistent results.
Here your scanning is being done by a PC which, I assume, is scanning hard
all the time. On most devices that's going to be chomping the power. eg,
Silabs SOC is around 12mW in RX IIRC which would kill a CR2032 in about 50
hours. so clearly hard scanning is not a winner.
If you're prepared to take a much more relaxed statistical approach and,
say, only worry about people being close for more than a couple of minutes
then you can come up with something a bit more reasonable:
* Set the TX power lower so that devices more than, say, 5 metres away do
not interfere. That makes it unlikely that you will have more than 5 or so
devices close enough. That reduces collisions.
* Reduce beacon interval to 5 seconds. That reduces collisions.
* Goal is now to scan, say, up to 20 devices in, say, 1 minute.
* There is now way less interference which cuts down on the collisions so
more beacons get through.
* Run radio at 2% duty cycle, thus stretching out power to many months.
On Thu, Oct 1, 2020 at 12:53 PM Stephen Irons <stephen at irons.nz> wrote:
> Some weeks ago, we spoke about Covid tracking and using Bluetooth tags. I
> wondered about what would happen if there were many tags in a close
> I had the opportunity to set up a test, and now have some actual data.
> - 104 devices operating as BLE beacons, transmitting an iBeacon-format
> signal every 1.5 s
> - CR2032 battery, plastic housing
> - all bundled together in a plastic bag
> - my PC with a USB BT adaptor acting as a monitor
> - using Python 'beacontools' to monitor beacons
> - with a filter to receive only the BT address prefix that I am
> interested in
> - In 20 scans, the monitor has heard all tags, every time.
> - It takes ~100 ms for the system to report the first tag.
> - It takes 8--10 seconds for the system to report up all 104 tags.
> - In 60 s of scanning, each device is heard an average of 22.8 times.
> - Each device transmit 60/1.5 = 40 times, so we hear just over 50% of
> the transmissions.
> I know that there are other BLE beacons in the area, as well as many WiFi
> networks, BT phones, headphones, TVs, etc in the vicinity.
> The system works better than I imagined it would...
> Stephen Irons
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