Long range coexistence

[damien5606]

Hello,

We'd like to connect 50 bluetooth 5 devices (nRF52840) to a Receiver (made of multiple centrals nRF52840) with a long range (PHY 250kbps, conn interval 50ms, no dle, ATT_MTU 23)

As I understood there're 37 bluetooth channels + 3 for advertising. If I assume that a nRF52840 central can be connected at 5 nRF52840 peripheral at the same time (I guess on the same bluetooth channel), I would need 10 nrf52840 centrals in my receiver to connect all my devices. So I'd use 10 channels.

Do you think they will interfere with eachother (knowing we'll work with long distance signal so very weak ones) or the space between the channel is big enough ?

Thank you a lot,

-Damien

[endnode]

Hi Damien,

The problem of shared channels is only on 3 advertising channels but once the connection link is established all remaining 37 channels (unless devices restrict some of them through channel map) are used through frequency hopping mechanism. Since BLE is based on timed events and most of the time there is silence the capacity of whole 37-channel spectrum is enormous and even if some collisions happen here and there the link is ready to handle it transparently (so applications on top don't even notice except small drops in actual data throughput). Normally you can have hundreds or even thousands of active links with very little interference in the spectrum.

On advertising channels again devices use different timing (and there is mandatory jitter added so even if they meet on few events and interfere for few packets they will drift quickly to different timings and problem is solved) but in certain situations you might experience more crowded situations and interference. But that's when you have hundreds or rather thousands of active broadcasters or dozens of very active scanners vs. few low-frequency advertisers. So far you should be fine (and maybe some refresh about how BT LE PHY and Link Layer work would help;)

Cheers Jan

[damien5606]

I was in open air environment. I had no problem with one link, I've tested it many times and the result were as good as expected by Nordic and the repeatability was great. The problem appear when I had the 4 other links, the data rate drop by 3 or 4, and a weird thing is that some links are way faster than the others (while the receivers are all at the same place and the emitter are all at an other place, 50m further, the antennas are facing the same direction, at the same distance from the ground)

Link 1) 15.78 Kbps Link 2) 11.31 Kbps Link 3) 10.30 Kbps Link 4) 28.22 Kbps Link 5) 13.22 Kbps

I've done many tests, and the results were similar to this one, and changing a lot from one test to an other. Thank you for your help.