FPGA Central

Let's say my transmittid frame is like this:

|| Preamble || GI1 | OFDM symbol 1 || GI2 | OFDM symbol 2 ||

Removing GI1 would remove any interference effects caused by the
preamble.

But what if the last few samples of OFDM symbol 1 is interfered by GI2?

Wouldn't a structure like this:

|| Preamble || GI1 | OFDM symbol 1 | GI1' || GI2 | OFDM symbol 2 | GI2'
||

With GI1' and GI2' postfix able to prevent such problems? GI2' would be
also useful if there is windowing before transmission right?

I've read many papers but not many of them talk about postfixs.

Anyone can explain this?

Thx

"Ant_Magma" <[email protected]> wrote in message
news:[email protected] oups.com...
> Let's say my transmittid frame is like this:
>
> || Preamble || GI1 | OFDM symbol 1 || GI2 | OFDM symbol 2 ||
>
> Removing GI1 would remove any interference effects caused by the
> preamble.
>
> But what if the last few samples of OFDM symbol 1 is interfered by GI2?
>
> Wouldn't a structure like this:
>
> || Preamble || GI1 | OFDM symbol 1 | GI1' || GI2 | OFDM symbol 2 | GI2'
> ||
>
> With GI1' and GI2' postfix able to prevent such problems? GI2' would be
> also useful if there is windowing before transmission right?
>
> I've read many papers but not many of them talk about postfixs.
>
> Anyone can explain this?

Two reasons.

1. The guard interval is used primarily to guard against multi-path
interference. Generally, this interference is a delayed version of the
signal. So you'd typically have the Preamble leaking into GI1, and
OFDMSymbol1 leaking into GI2, but not the reverse.

2. It doesn't really matter. If you're concerned about GI2 interfering with
OFDMSymbol1, then when demodulating OFDMSymbol1, you can just back up and
use part of GI1 instead of the last part of OFDMSymbol1. By backing up,
you've cyclically shifted the data you're demodulating, so you need to
either undo the cyclic shift before doing the FFT, or remove a phase ramp
after doing the FFT.

--
Eric Backus
R&D Design Engineer
Agilent Technologies, Inc.
425-356-6010 Tel

Regarding backing up and using GI1, assuming my guard interval is
greater than the delay spread and good frame synchronization. I suppose
i need not worry about that right?

"Ant_Magma" <[email protected]> wrote in message
news:[email protected] oups.com...
> Regarding backing up and using GI1, assuming my guard interval is
> greater than the delay spread and good frame synchronization. I suppose
> i need not worry about that right?

Yes, the guard interval must be greater than the delay spread, and the
difference between the two limits how much you can back up. I think a
typical case would be to back up by only a small amount, perhaps to avoid
any windowing between the OFDM symbols, and assume that most of the delay
spread really is "delay". Even if your assumption is wrong, it probably
won't be wrong by much and you'll probably still get reasonable performance
from the system.

--
Eric Backus
R&D Design Engineer
Agilent Technologies, Inc.

Thx eric!