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The tx power control is handled by the radio firmware based upon a country code setting, the nvram file settings (power per rate) and this also includes the antenna gain parameter.
There are a set of commands that can control the tx power levels but these are only used with the mfg test firmware as part of the board testing process, and not allowed to be used for the software image that ships in your product for production.
This means that as long as you are following (very!) closely a hardware design package for chip down, a module solution (with the specified platform file/nvram file) then your design will be ok to submit for regulatory testing. If you change the antenna gain then this will need to be updated in the nvram file to ensure the radio firmware can then calculate the correct limits to meet regulatory limits based on the specified country code setting.
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thank you for the answer and clarification.
In order to optimize the antenna performance and still meet the regulatory limits, we would:
1. compare our antenna path (gain, loss) to the reference antenna used by the manufacturer for the initial certification
2. apply class 1 or 2 change as per FCC regulatory (equivalent antenna type, same or less gain, emission is not worse). See also https://www.lsr.com/white-papers/guidelines-for-fcc-permissive-changes
3. if needed, work with the manufacturer to provide specific nvram settings, which match our specific hardware design.
Software-side, we only need to make sure, to run with the correct country code setting and the radio firmware will take care of the rest.