OFDM system

Hi,

It is clear that OFDM systems may have different SNR values for each
subcarrier due to frequency selectivity of the channel. If an OFDM packet
consists of n number of FEC blocks and in case of adaptive modulation and
coding systems, a single FEC block may have different modulation scheme.
However, is it possible for OFDM (e.g.IEEE 802.11a/n) systems that the
block error probability of FEC blocks differ within the same packet? Also,
is that possible to have different subcarrier allocation for different FEC
blocks in a packet? Say for example first FEC blocks contains subcarriers
1,2,3; second FEC block contains 3,4 and so on.

Thanks!

On Thu, 28 Aug 2008 13:03:34 -0500, "patrick_dbg"
<sanindland@gmail.com> wrote:

>Hi,
>
>It is clear that OFDM systems may have different SNR values for each
>subcarrier due to frequency selectivity of the channel. If an OFDM packet
>consists of n number of FEC blocks and in case of adaptive modulation and
>coding systems, a single FEC block may have different modulation scheme.
>However, is it possible for OFDM (e.g.IEEE 802.11a/n) systems that the
>block error probability of FEC blocks differ within the same packet? Also,
>is that possible to have different subcarrier allocation for different FEC
>blocks in a packet? Say for example first FEC blocks contains subcarriers
>1,2,3; second FEC block contains 3,4 and so on.
>
>Thanks!

I think I understand the gist of your question, but if I don't, please
feel free to clarify.

First, 802.11 does not use subcarrier adaptation like you are
describing.   Nevertheless, just from an academice perspective, it is
certainly possible to do what you describe and systems have been
proposed that operate that way.

If a block-oriented FEC is used and there is dynamic fading in the
channel, then even if a constant modulation and code rate is used it
is possible for the fading to affect each block differently.   So even
with constant modulation and coding the probability of block error can
differ between FEC blocks.

And if the modulation and coding is adapted differently between FEC
blocks then the probability of error will very likely be different
between blocks.   The adaptation criterion and algorithm used would
have a big effect on how the modulation and code was selected, so the
performance would depend a lot on the adaptation algorithm.

I hope that provides some insight.


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

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

>I think I understand the gist of your question, but if I don't, please
>feel free to clarify.
>
>First, 802.11 does not use subcarrier adaptation like you are
>describing.   Nevertheless, just from an academice perspective, it is
>certainly possible to do what you describe and systems have been
>proposed that operate that way.
>
>If a block-oriented FEC is used and there is dynamic fading in the
>channel, then even if a constant modulation and code rate is used it
>is possible for the fading to affect each block differently.   So even
>with constant modulation and coding the probability of block error can
>differ between FEC blocks.
>
>And if the modulation and coding is adapted differently between FEC
>blocks then the probability of error will very likely be different
>between blocks.   The adaptation criterion and algorithm used would
>have a big effect on how the modulation and code was selected, so the
>performance would depend a lot on the adaptation algorithm.
>
>I hope that provides some insight.
>
>
>Eric Jacobsen
>Minister of Algorithms
>Abineau Communications
>http://www.ericjacobsen.org
>
>Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
>

I once again thank you for your response. Is IEEE 802.11a/n transmits
encoded bits blockwise (several blocks in a packet for a user)? Then, what
will the real meaning for block error probability, if I get acknowledge for
packets that I have sent?

Thanks a lot!