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considering an OFDM symbol of x(0)x(1)x(2)x(3)x(4)....x(n-2)x(n-1)x(n). To perform cyclic prefix we prefix some samples from end of this OFDM symbol to the beginning of the OFDM symbol, like

x(n-3)x(n-2)x(n-1)x(n)x(0)x(1)x(2)x(3)........ x(n-4)x(n-3)x(n-2)x(n-1)x(n).

Instead of taking x(n-3)x(n-2)x(n-1)x(n), if we prefix with zeros what will happen?

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There are two major approaches for cyclic extension in OFDM systems - CP (cyclic prefix) and ZP (zero padding, also called Trailing Zeros, TZ). Generally they show the same performance, I mean in AWGN or Fading channel. CP method is the simplest one so it is preferred in the most cases. ZP approach leads to slightly less transmission power level for obvious reason. Doppler performance is the same. The main idea of using ZP as I understood is the existence of equalization techniques for combating spectral nulls that is the major problem of OFDM equalizers. I can't explain this technique because I haven't modeled or implemented it, but you can search some articles in the Web. In one of the article it stands that for CP the simple equalizer exists but symbol recovery isn't guaranteed. But for ZP symbol recovery is guaranteed with complexity of equalizer increasing. I think it is the ckear idea. But I suppose it leads to some matrix subspace algorithms e.g. SVD, so it can be too difficult to implement. I've heard ZP is popular in hydroacoustic OFDM techniques of equalization so you can also refer to this topic.

Hope it helps.

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  • $\begingroup$ As I've written below there is no equalization approach for combating spectral nulls for CP system. But for ZP such approach is proposed in some research topics. It is done by increasing complexity of equalization techniques. You can search for this and find some algorithms. asp.eurasipjournals.com/content/pdf/1687-6180-2004-598709.pdf can be starting point. $\endgroup$ – Serj Jun 22 '15 at 12:06
  • $\begingroup$ ok. Did you mean that way? I understood it wrong. I will delete my comment. $\endgroup$ – Karan Talasila Jun 22 '15 at 12:15
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    $\begingroup$ I mean that way. Actually I haven't deep understanding of ZP equalization performance improvement over CP because I haven't modeled it. But I think the subject is interesting. Because combating spectral nulls is big problem, even MMSE for CP can't do this. $\endgroup$ – Serj Jun 22 '15 at 12:20
  • $\begingroup$ CP provides protection against delay spread. This is becasue of CP' s property wherein windowing a CP added OFDM symbol at different starting location yields the same FFT spectrum which is not the case for ZP added signal. lte-bullets.com/LTE%20in%20Bullets%20-%20Cyclic%20Prefix.pdf $\endgroup$ – Naveen Jun 22 '15 at 18:34

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