ABSTRACT
The Friis transmission formula, which was derived in Chapter 3, implies a simple model of propagation loss simply due to a spherical wave spreading out from its source without obstruction. Although this model may have applicability to satellite communications (SATCOM), where the satellite transmitter is radiating into free space, it cannot apply to propagation paths full of obstructions, such as buildings and the earth. Then, a number of physical phenomena will come into play that will influence the propagation loss, including reflection, refraction, absorption, multipath phase interference, and diffraction. The Friis formula for free space propagation predicts a path loss that is proportional to the square of the frequency times the square of the distance between transmitter and receiver (i.e., the range). In contrast to the Friis formula, path loss empirically measured for cell phone communications in urban areas depends on the fourth power of the range and shows the parametric dependence given by the following expression:
L r fempirical ~ 4 2
2h hmb
(5.1)
where r is the distance between transmitter and receiver (i.e., the range), f is the frequency, hb is the height of the base station antenna, and hm is the height of the mobile antenna.
