In a mobile radio channel, a single direct path between the base station and a mobile is seldom the only physical means for propagation, and hence the free space propagation model is in most cases inaccurate when used alone.

The 2-ray ground reflection model shown in Figure is a useful propagation model that is based on geometric optics, and considers both the direct path and a ground reflected propagation path between transmitter and receiver.

This model has been found to be reasonably accurate for predicting the large-scale signal strength over distances of several kilometers for mobile radio systems that use tall towers (heights which exceed 50 m), as well as for line of-sight, microcell channels in urban environments.

Referring to Figure $h_{t}$ is the height of the transmitter and $h_{r}$ is the height of the receiver. If $E_{0}$ is the free space E-field (in units of V/m ) at a reference distance $d_{0}$ from the transmitter, then for $d \gt d_{0},$ the free space propagating E-field is given by

$$E(d, t)=\frac{E_{0} d_{0}}{d} \cos \left(\omega_{c}\left(t-\frac{d}{c}\right)\right) \quad\left(d \gt d_{0}\right)$$

Two propagating waves arrive at the receiver: the direct wave that travels a distance d' and the reflected wave that travels a distance d”.

The electric field $E_{TOT}( d, t)$ can be expressed as the sum of equations for distances d’ and d” (i.e. direct wave and reflected wave)

$$E_{T O T}(d, t)=\frac{E_{0} d_{0}}{d^{\prime}} \cos \left(\omega_{c}\left(t-\frac{d^{\prime}}{c}\right)\right)+(-1) \frac{E_{0} d_{0}}{d^{\prime \prime}} \cos \left(\omega_{c}\left(t-\frac{d^{\prime \prime}}{c}\right)\right)$$