Multipath, and Fading

• Multipath

  • In a typical wireless environment, there is not a direct line-of-sight (LOS) path between the transmitter and the receiver. In fact, the electromagnetic waves emitted by the transmitter usually take different paths in getting to the receiver, depending upon the obstacles in the environment. As a result, the received signal is actually a sum of the various contributions, each of which differs in both amplitude and phase. In many cases, the signals combine in a destructive manner, thereby severely degrading the signal's strength. The receiver is faced with the difficult task of properly demodulating and decoding the signal into something that resembles the original. Although emerging receiver technologies are combating the multipath problem, it can still seriously inhibit a wireless system's performance.

• Fading

  • Fading describes rapid changes in a radio signal's amplitude over a short period of time or travel distance. Fading is a result of multipath. In fact, it is caused by interference between multiple replicas of the transmitted signal.

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