RE: Channel Sounding TX power configuration, clarifying the different settings

Amanda Hsieh — Wed, 25 Mar 2026 16:41:15 GMT

Hi,

[quote user=""]What does CONFIG_BT_CTLR_TX_PWR_ANTENNA actually affect? Connection, advertising, CS transmissions, or all of them?[/quote]

The config is used to request a TX power in dBm. The power level set will be equal to or less than the one requested, based on the values supported by the hardware used. Values outside the supported range will be set to the nearest supported value. The value set here represents the actual power level fed to the antenna. When a Front-End Module is used, gain values for the SoC and FEM are calculated automatically to guarantee the closest possible match to the value requested by the user at the RF output. It would affect all radio transmissions.

[quote user=""]What does max_tx_power in bt_le_cs_set_default_settings_param do exactly? Is it a hard cap, or a preference that power control can override?[/quote]

max_tx_power sets the maximum output power (Effective Isotropic Radiated Power) to be used for all CS transmissions. The Value range is between BT_HCI_OP_LE_CS_MIN_MAX_TX_POWER to and BT_HCI_OP_LE_CS_MAX_MAX_TX_POWER. The .max_tx_power shows the maximum power possible; it is the combination of tx_power_delta and phy in the bt_le_cs_set_procedure_parameters_param that decides the tx_power. See this post.

[quote user=""]What does tx_power_delta in bt_le_cs_set_procedure_params mean in practice? What is the "existing power level for the PHY" it refers to?[/quote]

The tx_power_delta parameter of the procedure parameters command affects the remote device's TX power as documented here https://github.com/nrfconnect/sdk-zephyr/blob/d24c26f9852942beb447cd6a352aa95873fb6b7a/include/zephyr/bluetooth/cs.h#L794-L798

[quote user=""]What are the valid discrete TX power levels for CONFIG_BT_CTLR_TX_PWR_ANTENNA on nRF54L15?[/quote]

The power level set will be equal to or less than the one requested, based on the values supported by the hardware used. nRF54L15 supports Configurable TX power with 1 dBm step size from -10 dBm to maximum. See nRF54L15 Datasheet

[quote user=""]Should CONFIG_BT_TRANSMIT_POWER_CONTROL be enabled for a CS application, or does it interfere with CS power settings?[/quote]

The CONFIG_BT_TRANSMIT_POWER_CONTROL is enabled in the channel_sounding_ras_initiato and channel_sounding_ras_reflector to allow CS and ACL to use different PHYs.

[quote user=""]Are there any other TX power related settings I should be aware of for a CS application?[/quote]

This post might help.

[quote user=""]The initiator in our setup is mains powered so we want to maximize its TX power. The reflector is battery powered so we want to find the right balance between range and power consumption. How should we configure TX power on each side, and what is the recommended approach for tuning the reflector's power level?[/quote]

See this post.

Regards,
Amanda H.