Question about identifying active antenna indices and IQ sample logging impact on ranging (nRF54L15 Channel Sounding)

RE: Question about identifying active antenna indices and IQ sample logging impact on ranging (nRF54L15 Channel Sounding)

Edvin — Mon, 10 Nov 2025 15:00:13 GMT

Hello,

This is the reply I got from our Channel sounding team, including some background info:

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First of all, all antenna paths are used in every CS step mode 2, meaning all antennas will be used in every step. If you are using the default T_PM = 10µs and the required T_SW = 10µs, then the antennas may be switched at an interval of 20µs during the mode 2 step. From the application point of view there is not "currently" active antenna, all of this happens in high priority context handled by the controller. If possible you can connect a logic analyzer to the GPIOs which are controlling the antenna switch (https://github.com/nrfconnect/sdk-nrf/blob/df039990cdc5b0187459a83995fa7d61a8afe44a/samples/bluetooth/channel_sounding_ras_initiator/boards/nrf54l15dk_nrf54l15_cpuapp.overlay#L7-L17) to view the exact times of switching.

For the 2:2 configuration there are four antenna paths, N_AP=4, and each antenna path is defined as follows: AP1 is assigned to 1:1, AP2 is assigned to 1:2, AP3 is assigned to 2:1 and AP4 is assigned to 2:2. Where the notation x:y corresponds to antenna x on initiator, antenna y on reflector. (Reference: Core Spec Version 6.2 Vol 6 Part A Section 5.3
Antenna Switching for Channel Sounding).

Furthermore, the antenna path permutation index corresponds to a certain sequence in which the antenna paths will be activated during a CS step according to Table 4.15 in Core Spec Version 6.2 Vol 6 Part H Section 4.7.5 Antenna path permutations. By the way, I think the entire Section 4.7 Phase measurements with antenna switching, might be of interest.

Note that it is this function

https://github.com/nrfconnect/sdk-nrf/blob/df039990cdc5b0187459a83995fa7d61a8afe44a/subsys/bluetooth/controller/cs_antenna_switch.c#L74-L98

that is used by the controller to switch to a given antenna (indexed from 0 unlike the description above/ in spec).

Finally after collecting results from both the initiator and reflector the IQ values measured during a CS procedure for each antenna path is stored here:

https://github.com/nrfconnect/sdk-nrf/blob/a7e36dcbea5d835baea5e96ea80d0b7955e296fd/include/bluetooth/cs_de.h#L78-L79

In the example of a 2:2 configuration

iq_tones[0] contains IQ data from AP1 (antenna 1 on initiator, antenna 1 on reflector)
iq_tones[1] contains IQ data from AP2 (antenna 1 on initiator, antenna 2 on reflector)
iq_tones[2] contains IQ data from AP3 (antenna 2 on initiator, antenna 1 on reflector)
iq_tones[3] contains IQ data from AP4 (antenna 2 on initiator, antenna 2 on reflector)

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Best regards,

Edvin

RE: Question about identifying active antenna indices and IQ sample logging impact on ranging (nRF54L15 Channel Sounding)

Firo_187 — Wed, 05 Nov 2025 19:59:03 GMT

I’m already familiar with the struct and I’m logging the antenna_permutation_index. However, it’s still not entirely clear to me where in the code I can actually see which antenna is being used.

In the file cs_de.c, inside the process_step_data function, I’m working with the struct
bt_hci_le_cs_step_data_mode_2, since my main focus is on Mode 2.
There, I can access the antenna_permutation_index and tone_info.

While I understand the order of the selected antenna paths from the antenna_permutation_index, I still don’t see how to determine which specific physical antenna was active at a given moment.

Could you please clarify how to derive the currently active antenna based on the antenna_permutation_index or any related data?

RE: Question about identifying active antenna indices and IQ sample logging impact on ranging (nRF54L15 Channel Sounding)

Firo_187 — Wed, 05 Nov 2025 13:34:36 GMT

Hi edvin,

Thanks for the reply but is it now possible to log the current active antenna index or not with an multi antenna Configuration?

RE: Question about identifying active antenna indices and IQ sample logging impact on ranging (nRF54L15 Channel Sounding)

Edvin — Wed, 05 Nov 2025 12:50:55 GMT

Hello,

This should be where you can find the IQ data:

https://github.com/nrfconnect/sdk-nrf/blob/a7e36dcbea5d835baea5e96ea80d0b7955e296fd/include/bluetooth/cs_de.h#L78-L79

You should be able to hook onto those using the NCS\nrf\samples\bluetooth\channel_sounding_ras_[initiator/reflector] samples.

Best regards,

Edvin

RE: Question about identifying active antenna indices and IQ sample logging impact on ranging (nRF54L15 Channel Sounding)

Firo_187 — Fri, 31 Oct 2025 21:26:28 GMT

I would also like to mention that I am using the 4×4 antenna array CoreHW CHW1010-ANT 2-1.0 with different polarizations. I am testing various configurations to study the effect of polarization and how it affects the ranging, and therefore I would like to know which antenna is currently active...

RE: Question about identifying active antenna indices and IQ sample logging impact on ranging (nRF54L15 Channel Sounding)

Edvin — Fri, 31 Oct 2025 13:56:28 GMT

Hello,

Sorry for the late reply. I am discussing this with our channel sounding team to see whether it is possible. I will get back to you when I know more. I am sorry for the delay.

Best regards,

Edvin