How does cylic prefix work in OFDM?

[Sur]

Everywhere I read they talk about the head of a symbol being affected by previous symbol, and so how cyclic prefix takes care of that.How bout the tails of the symbols-won't they be affected equally due to future symbols.How is that taken care of?

 

[eda_wiz]

cylic prefix

Cyclic prefix is like duplicate[redundant] copy of the symbol.. so even if that part of the symbol is damaged we can reconstruct the symbol effectively. thus they prevent ISI at the cost of extran bandwidth..

 

[shedeed]

Re: cylic prefix in ofdm

If you have multiple versions of the same message signal arriving at the receiver, the rediver won't know where is the start of the frame?? which affects when the reciver will start carrying out the fourier transform.
Adding a portion of the signal that bears no information and is used-in a complex manner- to determine the start of the frame using autocorretion property

 

[udit]

Re: cylic prefix in ofdm

Hi sur,
there are a lot of issues for using cyclic prefix in ofdm
1. It prevents contamination of a block by intersymbol interference from the previous block.
2. it makes the received block appear to be periodic with period M. This produces the appearance of circular convolution, which is essential to the proper functioning of the FFT operation.
3. cyclic prefix is used to maintain orthogonality between sub carriers. One of the
condition for orthogonality is to have integer number of cycles in the FFT sampling
window.
4.The length of the cyclic prefix is the maximum expected length of the channel impulse response. It may be larger tan that too.
5. Overlap-save or overlap-add signal processing techniques could also be used to avoid the extra overhead of the cyclic prefix.


saurav Dutt

 

[mimomod]

cylic prefix in ofdm

Hi all,

In my simple mind, I understand it that the cyclic prefix in OFDM is more or less used to avoid the efect of frequency selective channel response by transforming the linear convolution between the input signal and the channel into a circular convolution one.

By doing that, the freq. selective channel response is transformed into several paralel flat fading channel which makes the equalization procedure is much less complicated.

By the way, udit mentioned that overlap-save or overlap-add techniques could be used to avoid the overhead due to the cyclic prefix. I think overlap-save or overlap-add is used in other techniques, e.g. convolution of very long discrete-time signal with a relatively short channel response. I still don't see any connection between overlap-save or overlap-add with OFDM. Furthermore, cyclic prefix is a must in OFDM which means that we should allow this "overhead" to make the OFDM systems work.

 

[zamir]

Re: cylic prefix in ofdm

Hi all,

In my simple mind, I understand it that the cyclic prefix in OFDM is more or less used to avoid the efect of frequency selective channel response by transforming the linear convolution between the input signal and the channel into a circular convolution one.

By doing that, the freq. selective channel response is transformed into several paralel flat fading channel which makes the equalization procedure is much less complicated.

By the way, udit mentioned that overlap-save or overlap-add techniques could be used to avoid the overhead due to the cyclic prefix. I think overlap-save or overlap-add is used in other techniques, e.g. convolution of very long discrete-time signal with a relatively short channel response. I still don't see any connection between overlap-save or overlap-add with OFDM. Furthermore, cyclic prefix is a must in OFDM which means that we should allow this "overhead" to make the OFDM systems work.

zamir
hi,
lpz read the following paper for cyclic extension of the ttransmitted symbol which helps in synchronization of tinmingb and frequency synchronization
[/list][/list]"ML Estimation of Timing and
Frequency OÆset in Multicarrier
Systems"[/u][/i]\]