Changing tx power has no effect under Zephyr

[AndyM]

Hello
I don't see any change in RSSI with different TX power values under Zephyr. 1.9.1

It used to work with Nordic SDK 3.2.0  (see table below)
The HCI  API in Zephyr  that is used to set TX power does not return any errors and query reports the saved tx power, but there is no measurable RSSI difference

API used

bt_hci_cmd_send_sync(BT_HCI_OP_VS_WRITE_TX_POWER_LEVEL,...)

to write and 

bt_hci_cmd_send_sync(BT_HCI_OP_VS_READ_TX_POWER_LEVEL,...) 

to read

The functions return no errors

Under Nordic SK 3.2.0 the  following RSSI was reported at 1m 

TX Power	Measured Power (1 M RSSI -dBM) Max,Average
0 (default)	55,61
8	52,54
7	52,54
6	52,55
5	52,56
4	53,57
3	53,59
2	53,60
-4	60,60
-8	63,62
-12	68,65
-16	73,69
-20	76,72
-40	Not detected,Not detected

In Zephyr it stays the same, around -55 regardless of TX power setting

Any ideas?

Thanks

[tesc]

Hi,

In order for the HCI commands to control the TX power, you also need to set the config:

CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL=y

Please note that from nRF Connect SDK v2.1.0, the SoftDevice Controller will use the CONFIG_BT_CTLR_TX_PWR_* settings, e.g. CONFIG_BT_CTLR_TX_PWR_PLUS_8 for +8 dBm.

Regards,
Terje

[AndyM]

CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL is enabled

Please clarify your last statement about nRF  Connect SDK and a CONFIG_BT_CTLR_TX_PWR_PLUS_8 setting
Does this mean that the TX power cannot be set dynamically? 

I found a function
radio_tx_power_set() in subsys\bluetooth\controller\ll_sw\nordic\hal\nrf5\radio\radio.c

It sets the TX power register directly
Can I use that instead of the HAL API ? Or is this a private function?

[tesc]

Hi,

TX power can be set dynamically using the HCI, yes.

Setting it dynamically is also the workaround prior to nRF Connect SDK 2.1.0, if you want a fixed TX power setting other than the default, since fixed TX power setting does not work for the SoftDevice Controller prior to SDK 2.1.0. Hence the note.

I recommend having a look at the Bluetooth: HCI Power Control example, as it does exactly what you want; it changes TX power dynamically. In main.c you will find set_tx_power() and get_tx_power() functions, that are called from other functions inside main.c. I see that in that example, the CONFIG_BT_CTLR_ADVANCED_FEATURES is also enabled, although I am not sure which exact features used depend on this setting.

Regards,
Terje

[AndyM]

Our code is based on that sample. No errors are returned but the RSSI levels do not change. Verified that in the nRF connect mobile app on Android

What are the alternatives?

I could disable Zephyr BT stack  and use the SDK SoftDevice controller -if I'm not mistaken, CONFIG_BT_LL_SW_SPLIT controls that

Is this a possibility? If so - could you point me to the API to set TX power using SoftDevice controller?

Thanks

[tesc]

Hi,

The HCI Power Control example already uses the SoftDevice Controller. Does the unchanged example as delivered through nRF Connect SDK also not work?

Testing the example here, on an nRF52 DK, I confirm both through power measurement (Power Profiler Kit II) and thorugh nRF Connect on Android, that TX power decreases step-wise (5 seconds on each step) then repeating that pattern indefinitely.

Regards,
Terje