Question

What is the signal power in dB if its value is 0 dBm

Answer 1

To calculate any value in dB we need its basic value, because of dBm determines the absolute value and dB - the relative one.

For this task:

dB = dBm + 10lg[P0m/P0]; where P0m = 1mW, P0 - the basic value (missed).