Cyclic prefix insertion is an important function during OFDM signal generation.
A cyclic prefix is necessary to prevent interference from previously transmitted OFDM symbols.
The intersymbol interference can be viewed as a direct result of multipath propagation.
At first glance, cyclic prefix insertion may be regarded as a useless operation since it is merely repeats a copy of the existing data in the OFDM symbol and does not add any new information.
However, it is instrumental for multiple reasons.
First, it helps maintain orthogonality between subcarriers in the receiver, which is one of the foundations of an orthogonal frequency division transmission.
It also provides a periodic extension to the OFDM signal through which the “linear convolution” operation performed on the transmitted signal by the channel can be approximated by a “circular convolution” operation.
Mimicking a circular convolution with a cyclic prefix is quite important if you want OFDM to represent the modulated signal in the frequency domain.
The validity of the frequency-domain equalization performed in the receiver is only ensured if channel response can be viewed as circular convolution, something that cyclic prefix insertion can ensure.
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