Automatic connection to closest device

RE: Automatic connection to closest device

Amanda Hsieh — Fri, 06 Dec 2024 19:42:36 GMT

This is very similar to our default setting in broadcaster:

As we can see, the broadcast stream occupied more than 40 % of airtime. So Lowering the retransmission time and bitrate could help.

RE: Automatic connection to closest device

RakeLund — Fri, 06 Dec 2024 14:05:24 GMT

I would like to be able to scan for a peripheral to set up an ACL connection while the audio broadcast is running.

I think the current bitrate is 32kbps actually I'll check, and I'm not sure about the number of retransmissions, so I'll have a look at that as well. Is this set by a configuration parameter or should it be part of the application code?

As far as I've understood it earlier, it should be possible to run more than one audio broadcast channel at the same time, but does this mean that this will also be hard to achieve in practise because of how much airtime one stream occupies?

RE: Automatic connection to closest device

Amanda Hsieh — Fri, 06 Dec 2024 13:34:50 GMT

Hi,

Not sure I understand this correctly or not. Would you like to do the scan on the broadcaster while broadcasting?

Since our broadcaster's default setting is using 48KH 96kbps stereo channel output and retransmission time = 2 (which means 120bytes x 3 times per channel per 10ms), this would occupy a lot of airtime. You could lower the bitrate, e.g., using 48kbps or 32kbps, which makes the payload size smaller, so we have more time for scanning, but that will also lower the audio quality.

Could you elaborate in more detail about your use case? It is very likely the radio capacity that is limiting. You can also reduce the number of retransmissions to 1 (a total of 2 transmissions), and also check the MCU load just to be sure using standard Zephyr tools.

-Amanda H.

RE: Automatic connection to closest device

RakeLund — Thu, 05 Dec 2024 13:19:39 GMT

Hi again!

I did have a look at the links you sent when you replied, but forgot to give you a reply, sorry! The first link is broken, not sure what it was supposed to link to.

I looked into the distance measurement toolbox, but I don't really think it's a good match with LE Audio unless it is only used before enabling the broadcast source.

I have experimented with using RSSI values and used the mode of the values as recommended by bluetooth SIG https://www.bluetooth.com/blog/proximity-and-rssi/. This seems to work quite well for connecting automatically when a peripheral is close to the central device (within ~1m).
The downside is that for it to work we have to spend some time collecting samples to have something to filter before initiating the connection. This is not really an issue when our device is only focusing on scanning, but while the broadcast audio is active the broadcasting seems to take most of the bluetooth radio capacity (or MCU capacity?) and I only receive 1 advertisement packet every 1-2 seconds.

Is this expected behavior when combining advertising and scanning? Should it theoretically be possible to get more advertisement samples quicker without stopping the audio broadcast or is this as good as it gets?

RE: Automatic connection to closest device

Amanda Hsieh — Fri, 25 Oct 2024 19:49:31 GMT

Hi,

You're correct that RSSI can be used to determine proximity, but it's not always reliable. I think you can check out my colleague's suggestion in this post and this post.

As for distance measurement, you can take a look at this Bluetooth: nRF Distance Measurement with Bluetooth LE discovery sample which also supports nRF5340. I would double-check with the developer for compatibility with the LE Audio implementation.

Regards,
Amanda H.