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]

Thank you endode for this answer.

I tested the throughput demo on long range today (throughput_example_connecting_on_long_range provided with the S140 V6) with 5 links (5 nRF52840 peripheral sending to 5 nRF52840 central, softdevice S140v6.alpha), the peripherals were 50m from the centrals. I've run the example one by one following those parameters (PHY 250kbps, conn interval 50ms, no dle, ATT_MTU 23)

And I saw huge perturbations, the data rate was divided by 3 or 4 at each of my tests when the links were exchanging data all together. I was thinking that they ended up on the same channel and they disturbed eachother, but you told me it's not really possible so I'm confused. I'm not sure how to check the channel map but if more than one channel is selected it shouldn't really happen...

Any idea ?

Thank you, -Damien

[endnode]

First of all what is your environment? The fact that you use long range air-speed doesn't mean you will beat interference in difficult environments, 2.4GHz radio is nasty thing which behaves counter intuitive sometime. Then you can debug this on several levels: normally link (HW) capabilities are tested by DTM which now is released (as far as I understand) innRF5 SDK with BT5 features such as different air-speeds. This would test nicely that both devices at that distance have on given channel naturally low packet error rate. If this fails you already have problem in "simple" case meaning either your environment is too difficult for current BLE technology or your HW faulty. If this is OK then you can go to more high-level tests like having throughput demo on single link and again carefully get statistics on tests (not running it once;). An finally do it with multiple links.

(1/2)

[endnode]

(2/2) In case you really see some strange behavior and you have this use-case as core then you might need to invest into radio analyzer (BLE sniffer). And because you use latest BT5 features you would need one of latest commercial analyzers (because free tools like nRF Sniffer are not ready for air-speeds different to 1Mbps) so we are speaking about 10-20k USD investment. Are you sure your problem is so serious and blocking?:)

[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.