where d0 means the near-Earth reference distance (which is
determined experimentally) and d is the distance between
the transmitter and receiver. PL (d0) represents the received
strength when the distance is d0 and the same goes for PL (d)
when the distance is d. And the coefficient n indicates the
rate of the loss exponent which varies with the complexity of
environment and is always set to 2⇠4 under indoor circumstances.
X! is to model the shadowing effect, which follows
a Gaussian distribution (in dB) with mean zero and standard
deviation !.