In order to avoid the frequency selective fading Frequency Division Multiplexing (FDM) is better when compared with the single carrier modulation schemes. Thus in FDM, frequency spectrum is divided into sub bands and each sub band carries data with low data rates. Hence when the signal is effected by frequency selective fading, only one sub band will be effected which results in minimal loss of data. But still FDM cannot overcome the multipath propagation. This is a tipical problem in domestic power line.
Orthogonal Frequency Division Multiplexing (OFDM) overcomes the fading due to multipath propagation using ‘cyclic prefix’. In OFDM the frequency spectrum is divided into subcarriers and each subcarrier is orthogonal to each other. This effectively reduces also the Inter Symbol Interferences (ISI).
The orthogonal property of the OFDM carriers is as shown below,
Figure: Orthogonal property of OFDM signals
Thus, when the peak of a carrier occurs, all the other carriers become zero. Hence at the sampling instance there is no interference due to other carriers. This property of OFDM signals allows the overlapping of the sub bands, resulting in increased spectral efficiency.
*To learn how the OFDM is generated please refer to my next post
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