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I'm currently studying some OFDM techniques, and have found that in order to avoid Inter-Symbol Interference (ISI), there needs to be some space in between symbols. Lets assume this length to be $\mu$. This can either be done by adding a cyclic prefix to the beginning of the OFDM symbol, or by adding a simple guard interval (no transmission).

It seems to me like a cyclic prefix is typically used, but I'm not sure why this is used over a guard interval. using a guard interval would save power, and since the first $\mu$ points of the received signal will be ignored anyways, wouldn't it be better to save power? I'm not sure what the advantages of using a cyclic prefix are. Thanks for any insight!

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A silent guard interval would likely increase PAPR (peak to average power), which requires a sender capable of more dynamic range (without clipping or generating power supply noise, etc.)

A long enough cyclic prefix prevents multipath echoes from one symbol interfering with the portion of the next symbol after the prefix. And a symbol interfering with itself from only its own multipath echo can be modeled as a flat fading channel, which may be easier to equalize or to error correct over a far wider range of multipath channel responses.

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  • $\begingroup$ would the sender need to be capable of a more dynamic range? I understand the the PAR would be increased, but the sender would send the same information during the normal symbol right? Would it be the receiver that would need to handle a higher ranger? Thanks for the explanation thought! this helps! $\endgroup$ – gerrgheiser May 9 '16 at 4:18
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if there is cyclic prefix, the receiver can also use it as a reference for oscillator synchronization, which is a very important point.

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It repeats the end of the symbol so the linear convolution of a frequency-selective multipath channel can be modeled as circular convolution, which in turn may transform to the frequency domain via a discrete Fourier transform. This approach accommodates simple frequency domain processing, such as channel estimation and equalization.

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