OFDM data matching to ifft size for symbol mapping

Hi all, 

I am trying to simulate a data flow according to the wimax standard. 
Could someone please help to understand how to manage the data
flow dimensions after encoding before mapping to ofdm symbols?
More precisely: 
I generate, for example, a message of 72 words having in mind a 16 QAM
modulation, with an overall coding rate 3/4. This, after rs and cc coding,
interleaving and puncturing, leads to 192 symbols to be transmitted. 
I note that this number is exactly the number of used data subcarriers for
the 256 point ifft. 
The questions are: 
1. can I use, for example, the 1024 points ifft?
2. considering the predetermined allocation of null carriers and pilot
carriers, how do i fill the (720-192)=528 frequencies left? 

Maybe I am missing something

Many thanks

Gian Paolo 

On Tue, 05 Aug 2008 08:14:00 -0500, "gpi" <gp.impo@gmail.com> wrote:

>Hi all, 
>
>I am trying to simulate a data flow according to the wimax standard. 
>Could someone please help to understand how to manage the data
>flow dimensions after encoding before mapping to ofdm symbols?
>More precisely: 
>I generate, for example, a message of 72 words having in mind a 16 QAM
>modulation, with an overall coding rate 3/4. This, after rs and cc coding,
>interleaving and puncturing, leads to 192 symbols to be transmitted. 
>I note that this number is exactly the number of used data subcarriers for
>the 256 point ifft. 
>The questions are: 
>1. can I use, for example, the 1024 points ifft?
>2. considering the predetermined allocation of null carriers and pilot
>carriers, how do i fill the (720-192)=528 frequencies left? 
>
>Maybe I am missing something
>
>Many thanks
>
>Gian Paolo 
>

The WiMAX 'standard' is very broad and has quite a number of possible
configurations, including different signal bandwidths and different
subcarrier quantities.   It also has a MAC which, not surprisingly,
also has a number of possible configurations.   The number of
permutations of possible configurations is quite large, and I think
it's hard to answer your question without knowing which specific
configuration you're trying to simulate.

If you pick a signal bandwidth and number of subcarrier from the menu
that would be a good start, but you'd also have to pick TDD or FDD,
and whether or not you're using OFDMA, etc., etc.   All of that will
help you home in on how the MAC would allocate a data packet and that
will tell you how many subcarriers, where the data goes, etc., etc.

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org

Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php

>If you pick a signal bandwidth and number of subcarrier from the menu
>that would be a good start, but you'd also have to pick TDD or FDD,
>and whether or not you're using OFDMA, etc., etc.   All of that will
>help you home in on how the MAC would allocate a data packet and that
>will tell you how many subcarriers, where the data goes, etc., etc.
>

Thanks for the reply. 

Although MAC governs the transmission, I am wondering if 
the coding prescriptions of the standard (16-2004, at least)
apply to all ifft possibilities or not. 
The MAC should eventually provide transmission burst parameters, but 
I expect to find them specified in the PHY section.
In particular, could it be that what is written in the PHY layer part of
the IEEE Std 802.16-2004 document, for example  Table 215 in Section
8.3.3.2.1, applies to the case of 256 points ifft only.

What do you think?

many thanks

Gian Paolo 

On Wed, 06 Aug 2008 11:46:00 -0500, "gpi" <gp.impo@gmail.com> wrote:

>
>>If you pick a signal bandwidth and number of subcarrier from the menu
>>that would be a good start, but you'd also have to pick TDD or FDD,
>>and whether or not you're using OFDMA, etc., etc.   All of that will
>>help you home in on how the MAC would allocate a data packet and that
>>will tell you how many subcarriers, where the data goes, etc., etc.
>>
>
>Thanks for the reply. 
>
>Although MAC governs the transmission, I am wondering if 
>the coding prescriptions of the standard (16-2004, at least)
>apply to all ifft possibilities or not. 
>The MAC should eventually provide transmission burst parameters, but 
>I expect to find them specified in the PHY section.
>In particular, could it be that what is written in the PHY layer part of
>the IEEE Std 802.16-2004 document, for example  Table 215 in Section
>8.3.3.2.1, applies to the case of 256 points ifft only.
>
>What do you think?
>
>many thanks
>
>Gian Paolo 

That's how the document works, yes.   The codes described in section
8.3.3.2.1 apply only the PHY described as section 8.3.    The codes
for the OFDMA PHY (section 8.4), for example, are described in section
8.4.9.

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org

Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php

>>Thanks for the reply. 
>>
>>Although MAC governs the transmission, I am wondering if 
>>the coding prescriptions of the standard (16-2004, at least)
>>apply to all ifft possibilities or not. 

>>In particular, could it be that what is written in the PHY layer part
of
>>the IEEE Std 802.16-2004 document, for example  Table 215 in Section
>>8.3.3.2.1, applies to the case of 256 points ifft only.

>
>That's how the document works, yes.   The codes described in section
>8.3.3.2.1 apply only the PHY described as section 8.3.    The codes
>for the OFDMA PHY (section 8.4), for example, are described in section
>8.4.9.

Thanks. 
I realized that the number of subcarriers I was referring to was PUSC.
With FUSC the problems seems resolved. 

Gian Paolo