I have read wikipedia on fading but I still dont understand what the difference is between slow and fast fading. I understand that slow fading (or shadowing) can be caused by being shadow by building. But what causes fast fading?

Also how does the Doppler effect play a part in this?

Any help would be really appreciated.

Thanks,

Bobby

P.S. I am specifically looking at fading in cell phones.

Fast fading would be multipath fading. Multipath fading is caused by multiple reflections coinciding on the receiving antenna and cancelling (or increasing, but that's not the fading part).

The statisitics of fading are characterized by rician and rayleigh distributions. Rician distributions assume a direct path from the transmitter to the receiver as well as infinite reflected paths. It is primarily used for characterizing satellite communications. Rayleigh distributions are characterized by infinite reflections of all possible attenuations, and no direct path. It is used to characterize worst case urban or indoor communications.

Multipath essentially causes standing waves, and when you walk through the nulls you get dropouts. The nulls appear (for example) when two different equal amplitude reflections took a path that puts them 180 degrees out of phase at the antenna, so they cancel.

Doppler is a different effect, caused by moving receivers/transmitters. Doppler cause the frequency to appear changed, which must be compensated for in some systems.

Multipath gets more complex the more you look into it. The delayed signals are actually the equivilent of an FIR (finite impulse response) filter. For broadband signals, the effective frequency response of a fast fading channel is determined by the mix of reflections.

The wikipedia treatment takes a slightly different approach, focusing more on the impulse response.