ABSTRACT
The performance of a wireless communication system is heavily dependent on the channel over which the transmitted signal propagates. Typically in a wireless communication system the channel consists of multiple paths between the transmitter and receiver with different attenuation and delay. The paths have different attenuation and delays because of the different distances between transmitter and receiver along different paths. For certain transmitted signals the entire transmitted signal may experience a deep fade (large attenuation) due to destructive multipath cancellation. The multipath signals may also add constructively giving a larger amplitude. In addition to the multipath effect of the channel on the transmitted signal, there are other effects on the transmitted signal due to the channel. One of these is distance related and is called the propagation loss. The larger the distance between the transmitter and receiver the smaller the received power. Another effect is known as shadowing. Shadowing occurs due to
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building and other obstacles obstructing the line of sight path between the transmitter and receiver. This causes the received signal amplitude to vary as the receiver moves out from behind buildings or moves behind buildings. In this chapter we examine models of fading channels and methods of mitigating the degradation in performance due to fading. We first discuss in detail models for the multipath fading effects of wireless channels. Then briefly we discuss the models for propagation loss as a function of distance and shadowing. Next we show an example how diversity in receiving over multiple, independent faded paths can significantly improve performance. We conclude by discussing the fundamental limits on reliable communication in the presence of fading.
