Thus far we have examined how to calculate the power radiated from an antenna and, using
the Friis transmission formula, how to calculate the received power at the other end of
a communication link. However, the received signal power is meaningless unless compared
with the power received from unwanted sources over the same bandwidth. Such noise sources
include thermal radiation from the earth and sky, cosmic background radiation, and random
thermal processes in the receiving system. In today’s wireless environment, additional noise
due to nonstationary radio frequency interference from pagers, cellular phones, etc., often
needs to be considered, but in this analysis we will concentrate on natural sources only