Cyclic Prefix in OFDM System

Could anyone please explain how Cyclic Prefix in OFDM system converts
linear convolution to the circular convolution?

Thanks in advance!

	 

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On Tue, 27 May 2014 05:54:36 -0500, "runinrainy" <100271@dsprelated>
wrote:

>Could anyone please explain how Cyclic Prefix in OFDM system converts
>linear convolution to the circular convolution?

Remember that the driving impairment is the channel impulse response,
which has some length in time that is a significant portion of the
OFDM symbol period.    This means that if there was no CP, energy from
the previous symbol would bleed into the next proportional to the
channel delay spread.  This would create intersymbol interference,
since any energy from the previous symbol that bleeds into the next
will degrade performance.

Chopping the tail end of the OFDM symbol off and sticking it in the
front means that any energy that bleeds into the FFT window in the
receiver is from the same symbol.    Since basic convolution done with
FFTs is circular, the effect is that this appears as normal
convolution of the transmitted symbol with the channel impulse
response and there is no interference from the previous symbol.

Hope that helps a bit.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com