How to set transmit power for BLE in nRF Connect SDK?

[bmcdonnell-npm]

How can I adjust the BLE transmit (Tx) power, up to and including +8 dBm? I'm using nRF Connect SDK (NCS) on an nRF52840-DK.

I modified the throughput sample to reverse the roles (GAP central becomes GATT server; GAP peripheral becomes GATT client), added timeslots and radio notifications.

I've tried 2 different approaches so far, neither of which fully works. I'm testing by running the code on one nRF52840-DK, pressing Button #2 to make it the peripheral (advertising), and scanning with an nRF52840-Dongle (with "nRF Connect for Desktop Bluetooth Low Energy Standalone v4.0.0") on my PC to see its Rx signal strength.

I see a possible 3rd approach, but the code doesn't make sense to me, and I'm not sure if it applies.

Approach #1a - mpsl_tx_power_channel_map_set(), NCS v2.0.2

This seems to work up to 0 dBm, i.e. I get higher Rx signal strength at the Dongle when DK is set to 0 dBm compared to -20 dBm. But there is no difference in Rx signal strength comparing Tx power set to 0 and 8 dBm.

Approach #1b - mpsl_tx_power_channel_map_set(), NCS v2.1.0-rc1

Doesn't work. No difference in Rx signal strength, regardless of Tx power setting.

Approach #2 - mpsl_tx_power_radio_supported_power_adjust(), NCS v2.1.0-rc1

Doesn't work. No difference in Rx signal strength, regardless of Tx power setting.

Note, this is the first tag where mpsl_tx_power_radio_supported_power_adjust() is provided.

Possible Approach #3 - Imitate HCI Power Control Sample

I haven't tried this. Does this fit my situation, since I'm not using the HCI in my application?

Also, the set_tx_power() function looks like it's setting values (members of the cp struct) as if to use them to control the tx power, but it never uses them. How does this even work?

throughput_mod__no_tx_pwr_ctrl.zip

throughput_mod__approach_1a.zip

throughput_mod__approach_1b.zip

throughput_mod__approach_2.zip

[bmcdonnell-npm]

I did some further experimentation to simplify things. Attached is code for "Approach 3.2", in which I started over with the throughput example, and only modified it to add a shell command "tx_pwr" which uses your set_tx_power() and get_tx_power() functions. It still has the same problem as my previous attempts, which (more specifically) is that it does not set or get the tx_power on the GAP peripheral device for the connected mode (BT_HCI_VS_LL_HANDLE_TYPE_CONN), regardless of whether the functions are called before or after the connection is established. (It does set it for the central, but only after the connection is established.)

Here is the unmodified code, so you can diff them to see what I added:

throughput_base.zip

With my "Approach 3.2" modification:

throughput_mod__approach_3.2_simplified.zip

Here is the corresponding console output from the peripheral:

How can I set the tx power of the peripheral for the connected state?

[Hung Bui]

Hi Brendan, 

If you call  set_tx_power( BT_HCI_VS_LL_HANDLE_TYPE_CONN  when you are not in a connection you will get error -5. 

Please test with my attached example. 
You can see in the following sniffer trace, the first test is when I  call set_tx_power( BT_HCI_VS_LL_HANDLE_TYPE_CONN .. with value 8 dBm when connected: 

Before and after connection, the RSSI of the advertiser/peripheral are the same (-14 to -17dBm)

And here is when I call  set_tx_power( BT_HCI_VS_LL_HANDLE_TYPE_CONN  with value -8 dbm, notice the different in the RSSI before and after connection.

set_tx_power() is called inside connected() call back. 

peripheral_lbs_txpower (2).zip

[Hung Bui]

Hi Brendan, 

Sorry it was an issue with devzone. I have updated the link. 
Yes you can capture the sniffer trace using a nRF52840 dongle. Please download the sniffer from here and follow the documentation to capture the trace. 

[bmcdonnell-npm]

Hung Bui said:

And here is when I call  set_tx_power( BT_HCI_VS_LL_HANDLE_TYPE_CONN  with value -8 dbm, notice the different in the RSSI before and after connection.

How do I add columns you have in the sniffer? In particular RSSI and Channel?

Here's what mine looks like:

I tried looking for them by adding a column in Column Preferences, but I don't see them there.

Nevermind, I found it. "Apply as Column", as described in Inspecting captured data.

[bmcdonnell-npm]

I'm glad to learn about using the sniffer. It is a useful tool, thanks.

But I'm still stumped on why the last code I provided here ("approach  3.2") isn't working on the peripheral. I tried set_tx_power() only after the connection is established, and it still fails. Even if I put it in the connected() callback like you did in your latest code (peripheral_lbs_txpower (2).zip), it still fails.

In this newly attached "approach 3.3", I'm imitating your latest code more closely, adding it again to the throughput example. It only has set_tx_power() (not get_tx_power()), and only calls it

• once for BT_HCI_VS_LL_HANDLE_TYPE_ADV, and
• once for BT_HCI_VS_LL_HANDLE_TYPE_CONN,

and I call them in the in the same places that you did. Still, as before, it succeeds on all except the peripheral for the CONN state.

throughput_mod__approach_3.3_further_simplified.zip

Can you please look at my code and tell me why this one case isn't working? Do a diff of it against the unmodified throughput example (attached throughput_base.zip) to see how I imitated your latest example.

Console output on the Central device:

Console output on the peripheral:

[bmcdonnell-npm]

I'm making more progress. It seems having both CONFIG_BT_CENTRAL=y and CONFIG_BT_PERIPHERAL=y in prj.conf (as in the throughput sample code) breaks set_tx_power() for the peripheral in the CONN state. Why? Is this expected behavior?

I'll report back later after more testing.

[Hung Bui]

Hi Brendan, 

In the code I provided to you I used this in the connected() callback: 

set_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_CONN,

                     0,

                     -8);

 You can notice that the 2nd argument was 0. It's the connection handle value. By default if you have only one connection , and support only one connection at a time, the value of 0 is correct. But it not the same if you support multiple roles and can have multiple connections. 
In this case you have CONFIG_BT_CENTRAL=y, and the first connection as peripheral doesn't have connection handle equals 0 .

So what you need to do is to add this: 

  uint16_t conn_handle;

  bt_hci_get_conn_handle(conn,&conn_handle);

Then call this: 

set_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_CONN,

                     conn_handle,

                     -8);

After that it should work.

Regarding your question about approach #1 or #2, please pay attention that setting the TXpower with MPSL API only set the "envelop" of the txpower range, the actual TXPower is set by the API of the protocol used in the MPSL timeslot. Please have a look here:  RE: nRF NCS - Set radio TX Power