Hello fayiqa,
As we know the purpose of power budget is to ensure that enough power reaches the receiver to maintain reliable performance during the entire system lifetime. The minimum average power required by the receiver is the receiver sensitivity. Both receiver sensitivity and transmitted power play an important role in calculating the power power and in addition to that “system margin” as well. I am posting a link where it has been discussed in detail about different parameters to be taken into consideration. I hope it helps you.
Regards