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OFDM otherwise called orthogonal frequency division multiplexing is a modulation and a multiple access technique that can be applied to mobile communications. OFDM or Multitone modulation as it is sometimes called is the basis for several commercial wireless applications. In OFDM the segments are according to frequency there by dividing the spectrum into a number of equally spaced tones, which are orthogonal with each other and carries a portion of user information on each tone. As the mobile cellular wireless system operates under harsh and challenging channel conditions, the wireless channel is distinct and much more unpredictable than the wired because of the factors such as multipath fading, shadow fading, Doppler spread and time dispersion or delay spread. OFDM over comes the ISI (intersymbol interference) in a multipath environment.
In order to combat these effects the modern wireless systems employ a variety of signal processing techniques, which include the factors such as equalization, error correction coding, spread spectrum, interleaving and diversity. The sinusoidal waveforms making up the OFDM tones have the very special property of being the only eigen functions of a linear channel. With this property and the incorporation of small amount of guard time, called the cyclic prefix to each symbol enables the orthogonality between tones to be preserved in the presence of multipath. The cyclic prefix allows the tones to be realigned at the receiver thus regaining orthogonality and is used to absorb transients
from previous bursts caused by multipath. Thus OFDM eliminates the effect of multipath, ISI (intersymbol interference), ICI (intercarrier interference) in Mobile channels.
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Last edited by rajrocks; 1st October 2011 at 05:59 PM.