"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.
>
> i.e.
>
> TX SIGNAL:
>
> Block 1 Block 2
> +---------+ +---------+
> | | | |
> | | | |
> ---+ +-----------+ +---------- -> t
>
>
> RX SIGNAL: (Convolved with channel response)
>
> Block 1 Block 2
> +---------+ +---------+
> | \__ | \__
> | \_____ | \_____
> ---+ \-+ \ -> t
> ^
> ^
> Blocks do not overlap.
>
>
>
> The CP serves this purpose.
>
>
> --
> Oli

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. I guess
this is analagous to a perfect rake receiver. And all it costs is 20% (or
so) of your bandwidth because you're not transmitting information when the
CP is sent and a significant chunk of hardware to implement the FFT
operation.

-Clark