Free space indicates an ideal, even, and isotropic medium of space. When electromagnetic waves are transmitted in this medium, no reflection, refraction, scattering, or absorption occurs.
Propagation losses are caused only by the energy spread of electromagnetic waves. Satellite communication and microwave line-of-sight (LOS) communication are typical examples of free space propagation. In certain conditions, the antennae of the base station and terminal can be mounted at any height.
In this case, LOS communication between the base station and the terminal is implemented. If a clear line of sight (CLOS) exists between the transmit antenna and receive antenna, then path loss complies with the free space model. The propagation losses in the free space model are as follows:
PL = 32.4 + 20log(d ) + 20log( f )
Where, d indicates the distance between the terminal and the base station. The unit is km.
The preceding formula does not consider the impact of ground reflection, and thus often underestimates propagation loss. This model is applicable to the scenario when the antennas of the base station and terminal are mounted at considerable height and CLOS exists between the base station and the terminal.