Anonymous wrote:
> "Oli Filth" <[email protected]> wrote in message
> news:ZbUPf.75351$[email protected]..
> > Zeph80 said the following on 09/03/2006 03:51:
> > > Well, the cylic prefix exists to combat ISI and the contents itself(tail
> of
> > > the symbol) lends itself to making the channel look like a circular
> > > convolution.So it really has a dual purpose, and my question is not the
> > > contents of the cylic prefix, but how its existence will remove ISI
> >
> > To eliminate ISI, you need a time-domain guard interval in-between
> > blocks, which is longer than the length of the channel response.
> >
> > The CP serves this purpose.
>
> Right and the key is the operation of an FFT. When one takes an FFT of a
> signal they are not taking it over 256 time samples, for example, they are
> taking it over an infinitely long signal that is the same 256 samples
> repeated in each direction. Since the signal is conceptually infinite it
> doesn't matter that the echoes from ISI are delayed relative to one another,
> as far as the FFT is concerned they are still the same signal and thus the
> FFT results from each echo add constrcutively in the final result.

Not exactly...

The echoes are from *multipath*, not ISI. This causes two separate
issues in an OFDM system:
* ISI - i.e. one block interfering with the next.
* Frequency-selective fading on a per-block basis.

Use of any time-domain guard interval resolves the ISI problem, even an
all-zeros guard interval.

Using a CP as the guard interval resolves the second problem (well,
makes it easier to deal with) by making the linear convolution of the
channel appear as cyclic convolution of the DFT operation, which is
what you were alluding to above.

Simply put, it allows the fading on each sub-carrier to be
(approximately) corrected post-DFT with a coefficient of the form
r.exp(j*phi).

--
Oli