Critical to the quality of communication between a base-station transmitter and a mobile receiver is the signal's propagation channel. As it is transmitted over the air, the signal is subject to fading. The signal can be absorbed or reflected by obstacles such as buildings, mountains, and trees, and the resulting fading can influence its amplitude and phase significantly.rnReflection, diffraction, and local scattering can cause a signal to take multiple paths from base station to receiver (Figure 1). In a so-called multipath scenario, several copies of the same signal arrive at the receiver with different path lengths at different times and with varying amplitudes and phases. With a moving receiver, additional challenges occur such as maxima and minima of signal strength and Doppler shift.
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