MULTIPATH FADING

Wireless technologies are not free from problems like limitation of the available frequency spectrum, fading and multipath fading. Fading results in sudden drop of signal power in the receiver. Multipath fading results when the transmitted signal bounces off objects like buildings, office cabinets and hills, creating multiple paths for the signal to reach the receiver. The same transmitted signal that follows the different paths reaches the receiver at different times with different phases. Added together, the several incidences of the same signalwith different phases and amplitude may cancel each other, causing signal loss or drop of signal power.







The consequences of multipath fading(fig 3) may be delay spread, short-term fading, long-term fading and Doppler effect. Delay spread results in spreading of the transmitted pulse on the time axis and even in generation of multiple low amplitude pulse trains. It occurs in fixed radio stations.

In mobile environments as the channel condition changes with motion of the receiver, fading causes the short term effect, resulting in fluctuation of the received power over time. The receiver may not adapt to the changes. This degrades the service quality. Short term fading occurs over short term duration. Long term fading results in decreased received power over long time/distance; as time increases, the moving receiver usually goes further away.

The Doppler effect occurs in fast moving mobiles. It results in shift of the frequency randomly. Multipath fading, in effect, either causes low recied signal power or degraded quality of service, both of which are highly unexpected in the future all-wireless and mobile communication. The low received power increases the bit error rate, which, in turn, limits the data rate.