There are many properties of the air interface which introduce losses. First we need to talk about free space loss , which is the loss purely due to distance. Note that this formula does not include antenna gains . Thus, the higher the frequency, the closer the receiver must be to the transmitter to acheive a good received signal level. Let's do some calculations.
Reflection can occur from buildings or from the earth itself.
Building reflection is a bit more difficult to model, so we will talk about earth reflection. In many wireless communications systems, we may assume the earth is flat, as the path distance d is only a few tens of kilomenters. The angle the reflected signal makes with the ground must be the same on both sides of the reflection point. The distance then comes in as d 4 as opposed to d 2 for free space loss.