Multipath in the radio channel creates small-scale fading effects. The three most important effects are:
Rapid changes in signal strength over a small travel distance or time interval
Random frequency modulation due to varying Doppler shifts on different multipath signals
Time dispersion (echoes) caused by multipath propagation delays.
In built-up urban areas, fading occurs because the height of the mobile antennas are well below the height of surrounding structures, so there is no single line-of-sight path to the base station.
Even when a line-of-sight exists, multipath still occurs due to reflections from the ground and surrounding structures.
The incoming radio waves arrive from different directions with different propagation delays. The signal received by the mobile at any point in space may consist of a large number of plane waves having randomly distributed amplitudes, phases, and angles of arrival.
These multipath components combine vectorially at the receiver antenna, and can cause the signal received by the mobile to distort or fade. Even when a mobile receiver is stationary, the received signal may fade due to movement of surrounding objects in the radio channel.
If objects in the radio channel are static, and motion is considered to be only due to that of the mobile, then fading is purely a spatial phenomenon.
The spatial variations of the resulting signal are seen as temporal variations by the receiver as it moves through the multipath field.
Due to the constructive and destructive effects of multipath waves summing at various points in space, a receiver moving at high speed can pass through several fades in a small period of time.