Why are two antenna paths required for channel sounding with nRF54L15-DK and nRF Toolbox?

[cwhit]

I'm experimenting with Bluetooth Channel Sounding using the nRF Toolbox Android app and the nRF54L15-DK. I'm curious about why CONFIG_BT_CTLR_SDC_CS_MAX_ANTENNA_PATHS needs to be set to 2 for this (as stated in this video). As far as I can tell, this DK board only has one antenna, so is this based on the number of antennae on the phone? How does this config item relate to the Antenna Configuration Index table in the Core spec (Vol 6, Part A, Table 5.3). Does this result in using either ACI 1 or ACI 4 with the phone being the two-antenna device?

[PaKa]

The requirement for 2 antenna paths is driven from Android 16 requirements. If you only set the 2 antenna path support in the reflector and not state you have 2 antennas then the initiator needs to have 2 antennas for 2 antennas to be used. Not sure if the phone actually does use it though, should be possible to see from the logs.

[Edvin]

Just to clarify, when you set CONFIG_BT_CTLR_SDC_CS_MAX_ANTENNA_PATHS=2 you do not claim that the DK has two antennas. You declare that the SoftDevice Controller can handle the channel sounding algorithm with two antenna paths. So if the DK has one antenna and the phone has two, that makes two antenna paths. DK antenna X and Phone antennas A and B, then you have the paths

DK X <=> Phone A
and
DK X <=> Phone B

If the DK did have two antennas, and you wanted to use that as well, you would have to set the CONFIG_BT_CTLR_SDC_CS_MAX_ANTENNA_PATHS=4, because you would have 4 antenna paths:

DK X <==> Phone A

DK X <==> Phone B

DK Y <==> Phone A

DK Y <==> Phone B

You would also have to set CONFIG_BT_CTLR_SDC_CS_NUM_ANTENNAS=2, to tell the nRF that it has two antennas that it controls.

Best regards,

Edvin