Hello, KS.
I don't know if that's what you need, but take a look at these codes that calculate and show the signal strength.
I'm looking at the examples in \ns-3.3 \examples\wireless\power-adaptation-interference.cc and power-adaptation-distance.cc. These have some coding (such as classes and functions) that involve transmission powers.
Hope it helps.
Health and Peace.
Regards,
Edivaldo