[chbot] Bluetooth tags

[Stephen Irons]

This test was done in the EPIC building in Manchester Street. It is a 
fairly noisy 2.4 GHz environment, with 20+ WiFi APs visible, 4 people 
in my office with active BT headphones, cardboard walls and ceiling, 
with plenty of geeky types with BT devices in surrounding offices, 
including upstairs.

Testing was during normal office hours.

On Thu, Oct 1, 2020 at 18:43, Helmut Walle <helmut.walle at gmail.com> 
wrote:
> 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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