## Multipath fading

#### If either the transmitter, receiver or reflectors are moving within a multipath environment, the path lengths will vary with time and so the relative phases between signals will also vary with the position of the users. The result is that the receiver experiences a combined signal with fluctuating amplitude and phase as a function of time.
#### The depth of the resulting fading of the signal is very dependent on whether a strong line-of-sight path exists between transmitter and receiver, in which case the fading signal envelope usually conforms to a Ricean statistical distribution, whereas the line-of-sight path is obscured, and energy arrives at the receiver by a large number of reflections, then a Rayleigh distribution of the envelope level is more likely to occur. The characteristics of the fading signal envelope make a significant impact on the bit error rate performance for a digital communications link. An excellent book on multipath fading is Jakes (1993), and a very thorough text on the performance of data modems in fading is given in Proakis (1989).