Zeph80 said the following on 09/03/2006 18:53:
> Maybe I'm not framing my question right.Everywhere I read, they mention how
> the head of the symbol(first n samples) is affected due to previous symbols
> and so how discarding the CP solves that.(the distortion occurs only in
> head).What about the tail of the OFDM symbol,wont that be affected by the
> future symbols as much as the head would be by prevoius symbols.

Technically, no. By definition, any realisable channel response must be
causal - something at time t = 1 can't possibly affect something that
happened at t = 0.

However, the question of whether so-called precursor rays are an issue
depends on your frame of reference. If your timing algorithm is
designed with t = 0 defined as the strongest incoming ray, rather than
the first incoming ray, then it will appear that precursors are
affecting previous symbols. But in actual fact, its the fact that
you're windowing blocks too late that's causing the interference.

If you design your timing recovery algorithm to sync to the first
incoming ray, then there will be no "precursors".


> Why no guard interval at the end too?

There is nothing to stop you utilise a cyclic postfix as well, the maths
is identical.

Be aware though, a lot of standard OFDM synchronisation algorithms in
the literature rely on the correlation statistics of the standard cyclic
prefix structure. Adding a postfix will probably require quite a bit of
re-working of these algorithms.



--
Oli