[link|http://www.spectrum.ieee.org/WEBONLY/publicfeature/jun02/wire.html|Eell, I think it's mifty]

Excerpt:

The most important technology in a point-to-multipoint (non-mesh) NLOS system is its smart base-station antenna. Instead of a single omnidirectional or sectorized antenna, these systems use an array of radiating elements. Each element is fed a version of the signal to be transmitted that differs from the others only in its amplitude and phase (time delay). The signals radiated by the array elements combine with each other in space to form one or more beams of carefully calibrated strength propagating in specific directions. The directions are so chosen that the beams--after bouncing off assorted objects in the environment, like mountains, buildings, motor vehicles, and even aircraft--all reach the location of the intended subscriber at the same time and in phase with one another [see figure]. When the beams combine constructively, the result is a strong signal at the receiver, which can therefore use an indoor antenna.

Sounds good, but how do they do it? The answer is by first monitoring signals received from the subscriber unit to determine the characteristics of the environment and then by generating a complementary signal. For example, if the subscriber unit has a simple omnidirectional whip antenna, the signal it transmits will, in general, undergo multipath distortion--that is, it will take multiple paths to the base station, bouncing off various objects, being attenuated to various degrees, and undergoing various delays, depending on the different path lengths.