Content of the preamble for OFDM channel estimation

Hello,

i'm currently working on a channel estimation algorithm for OFDM. I
already programmed the main part according to the implementation in DVB-T.
But my system is rather abstract. First a symbol will be sent only with
pilots to make the channel estimation. All the other symbols after that
are full of purely real data (without pilots). With the help of the
channel freq. resp. (which calculated in the CE procedure) the channel
equalization will be realized. My question is: is the content of the
preamble important for the CE? Which kind of values can i use in this
preamble?

thanks in advance!

It depends on what kind of channel are you assuming, how long will it remain 
the same.

Also since ofdm is wideband one technique would be to cycle your pilot 
through
all the tones.
and then the equalizer can make a decision based on (1) when was
the last pilot on that tones spot. (2) what were the pilots value in nearby 
tones.

Since you would not want to wait till the pilot is running through all the 
tones,
you would want the pilot to be sent along with the data.

There are a lot of sequences with a strong auto correlation that you
can use for the pilot hopping sequence so receiver can first lock on
to the pilot, then data

This method is typicaly used with a very short(often nonexistant) preamble,
In this technique you will have a constant pilot overhead in your data tones 
though....

On Sat, 24 Jun 2006 08:16:56 -0500, "skywalker601" <kataculy@yahoo.de>
wrote:

>Hello,
>
>i'm currently working on a channel estimation algorithm for OFDM. I
>already programmed the main part according to the implementation in DVB-T.
>But my system is rather abstract. First a symbol will be sent only with
>pilots to make the channel estimation. All the other symbols after that
>are full of purely real data (without pilots). With the help of the
>channel freq. resp. (which calculated in the CE procedure) the channel
>equalization will be realized. My question is: is the content of the
>preamble important for the CE? Which kind of values can i use in this
>preamble?
>
>thanks in advance!

That diverges from DVB-T a bit, but having symbols dedicated entirely
for pilots is used in most bursty OFDM systems like 802.11 or 802.16.
In those cases the symbols used for channel estimation can also be
used for synchronization, so autocorrelation properties are important
as well.

Other than that certain pilot arrangements will have better PAPR than
others.   In other words, if you did something nutty like put all
pilots the same magnitude with the same phase, you're going to have a
serious PAPR problem asssociated with that symbol.   So usually a
pattern with a fair amount of entropy (i.e., lower PAPR) and a
reasonable autocorrelation sequence (if you want the synchronization
properties) is used.
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org