Hata

The most prolific path loss model employed in cellular presently is the
empirical model developed by Hata or some variant of it. The Hata model is
an empirical model derived from the technical report made by Okumura, so
the results could be used in a computational model. The Okumura report is
a series of charts that are instrumental in radio communication modeling.
The Hata model is as follows:

It should be noted that some additional conditions are applied when
using the Hata model as compared to the free space equation. The values
utilized are dependent upon the range over which the equation is valid. If
the equation is used with parameters outside the values, the equation is
defined for the results and will be suspect to error.
Therefore, the Hata model should not be employed when trying to predict
a path loss less than 1 km from the cell site or if the site is less than 30
meters in height. This is an interesting point to note since cellular sites are
being placed less than 1 km apart and often below the 30-meter height.
In the Hata model, the value hm is used to correct the mobile antenna
height. The interesting point is that if you assume a height of 1.5 meters for
the mobile, that value nulls out or becomes 0 of the equation.
A critical point to mention here is that the Hata model employs three
correction factors based on the environmental conditions that path loss prediction
is evaluated over. The three environmental conditions are urban,
suburban, and open.