Is it necessary for two devices to be connected to perform Channel Sounding?

[ignatious-p]

Hello,

According to my understanding of Bluetooth Core Specification 6.0, Volume 6, Part H, "Channel Sounding", it is not a requirement that the Initiator and Reflector are in "Peripheral" and "Central" roles during Channel Sounding. There simply must be a way of sending the data between the two devices in a bi-directional manner.

Given that context, here are my questions:

1. I have a couple of nRF54L15 development kits, which I program using NCS V3.0.2. Does the current SDK allow me to modify the Channel Sounding procedure so that I can do it over PAwR or something similar? Do I have granular control like that?

2. According to zephyr/bluetooth/direction.h, there seems to be a way to send connection-less CTE data during direction finding. I believe that the nRF54L15 supports direction finding via AoA and AoD as it is supported in the Direction finding connectionless beacon example. Therefore it seems possible to bodge together a method to do what I propose, however, I am not an expert on this matter. Am I on the right track here?

I greatly appreciate any and all advice/suggestions and corrections. 

[PaKa]

For channel sounding you need to be in a connection and that needs to be encrypted. You can't use broadcast mode.

The nRF54L devices only support AoA transmitter role, no AoD and no receiver mode. For Direction Finding receiver you will need to use the nRF52833 with an external antenna array and use nRF Connect SDK 2.1.0.

CTE is very different from Channel Sounding so they are complementary non-overlapping features.

[ignatious-p]

Thanks for your (rather swift) response, Paka!

PaKa said:

For channel sounding you need to be in a connection and that needs to be encrypted. You can't use broadcast mode.

I could not find this in the specification. Could you please share where it is explicitly mentioned that the devices need to be "Connected"  (in the Bluetooth sense of the word). Regarding encryption, it seems that the Encrypted Advertising - Zephyr example demonstrates that it is possible to transmit encrypted Advertisements. Maybe this can be used to circumvent this limitation. It all depends on the level of granular control that the nRF54L15 provides, which is honestly the meat of my question. It would be ideal if I could build a custom 2.4GHz protocol on top of the radio on it.

PaKa said:

The nRF54L devices only support AoA transmitter role, no AoD and no receiver mode. For Direction Finding receiver you will need to use the nRF52833 with an external antenna array and use nRF Connect SDK 2.1.0.

Once again, I cannot find any data on this. Going purely by the examples provided (as the datasheet doesn't claim otherwise) the Bluetooth: Direction finding connectionless beacon and this part of the datasheet imply that the nRF54L15 supports AoA and AoD. If this is incorrect, I have found no evidence on the contrary so far.

PaKa said:

CTE is very different from Channel Sounding so they are complementary non-overlapping features.

Upon reading further, although AoA, AoD and Channel Sounding require constant tone data to be transferred and then I/Q sampled, Channel Sounding uses packets in the CS_SYNC format which doesn't have CTE fields. Thanks for correcting me here. However, I am aware that there are ways to use CTE data without AoA/AoD, in a method known as Multicarrier Phase Difference (MCPD). This is probably why I mixed up the two. 

I don't mind deviating from the specification. I just want to achieve connection-less phase-based ranging. 

[PaKa]

So what I am talking about is what is allowed with Bluetooth and supported in the Nordic SW. The Bluetooth spec doesn't open up for using Channel Sounding in non-connected mode so that has not been implemented in SW. The device can do this but then it won't be Bluetooth but proprietary.

But instead of you writing a new proprietary solution you may take a look at NDT which is supported for the nRF52832/833/840/5340 which runs ranging in non-connected mode. This takes care of the ranging but you will need to build the application and logic around the ranging on your own.

[ignatious-p]

Thanks again for your advice, Paka.

I now understand that Channel Sounding requires that two devices are paired and the link encrypted before the procedure can occur.

According to section 8.5 of the official Bluetooth LE Primer

To initiate channel sounding, the two devices must first establish an LE-ACL connection (see 7.8.1 LE ACL – LE Asynchronous Connection-Oriented Logical Transport). Encryption must be enabled on the link before the channel sounding control procedures can make use of the connection and therefore the two channel sounding devices need to have been paired.

I was only referring to the Channel Sounding section in the Bluetooth 6.0 Core specification and this is why I didn't see it.

PaKa said:

But instead of you writing a new proprietary solution you may take a look at NDT which is supported for the nRF52832/833/840/5340 which runs ranging in non-connected mode. This takes care of the ranging but you will need to build the application and logic around the ranging on your own.

I am not sure if this is an option for those conducting research as we have to publish our ranging algorithms and as far as I understand, the NDT is a proprietary solution that we link against.

Anyway, thanks for being super helpful, Paka. I think I have all the answers I needed.

[cheng]

if does not need security connection, maybe the zephyr "cs-test" project will be helpful. zephyr/samples/bluetooth/channel_sounding at main · zephyrproject-rtos/zephyr

[PaKa]

This would be a proprietary solution as the security is mandatory for Bluetooth compliance. For research this may not be a problem so you can judge if it will meet your requirements.