[chbot] Bluetooth tags

[Helmut Walle]

All very good points. This raises one further question: how well would 
this work in high-noise environments? I am thinking of the typical 
office, where BT performance between 9am and 5pm is often somewhat less 
than great, but if you work early before everyone else arrives with all 
these wireless devices in their pockets, or late after everybody else 
has gone home, BT performance noticeably picks up... I like the 
suggestion of running in energy-saving mode to achieve both long battery 
life and a reduction of interference. But would that work equally well 
in environments that are fraught with a lot of noise from other 
(non-covid tracing) sources in the same band?

Kind regards,

Helmut.

On 1/10/2020 17:39, Charles Manning wrote:
> 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 
> <mailto: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 proximity.
>
>     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
>
>     *Results*
>
>       * 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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