Maybe I'm not framing my question right.Everywhere I read, they mention ho
the head of the symbol(first n samples) is affected due to previous symbol
and so how discarding the CP solves that.(the distortion occurs only i
head).What about the tail of the OFDM symbol,wont that be affected by th
future symbols as much as the head would be by prevoius symbols.Why n
guard interval at the end too?

>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 content
itself(tail
>> of
>> > > the symbol) lends itself to making the channel look like
circular
>> > > convolution.So it really has a dual purpose, and my question is no
the
>> > > contents of the cylic prefix, but how its existence will remov
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 o
a
>> signal they are not taking it over 256 time samples, for example, the
are
>> taking it over an infinitely long signal that is the same 256 samples
>> repeated in each direction. Since the signal is conceptually infinit
it
>> doesn't matter that the echoes from ISI are delayed relative to on
another,
>> as far as the FFT is concerned they are still the same signal and thu
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
>
>