Re: [Discuss-gnuradio] question about ofdm subcarrier-mapping

[bin zan]

Thanks Tom, your information completely answered my question. In my case, the  subcarriers start from 1, I guess that's why there will be 1 number difference.

Thanks for your time,
Bin

On Thu, Feb 18, 2010 at 11:11 AM, Tom Rondeau <address@hidden> wrote:

On Wed, Feb 17, 2010 at 4:48 PM, bin zan <address@hidden> wrote:
> Hi,
>     For a fft-length 64, occupied-tones 32 ofdm mapping will actually use
> subcarrier (16-30) and (33-47) according to the code. However, when I plot
> the results from ofdm_receiver-fft_out_c.dat, I find it will have a peak at
> subcarrier 33 always, and the occupied tone seems start from 16 and end at
> 49. Can any one give me some explanation on that?
>
> Thank you,
> Bin

DC offset. There's a large amount of energy that gets built up in the
DC carrier, so we drop it. Otherwise, it completely messes up the
symbol in that bin. Since we often use even numbers of bins, we don't
really have a "DC subcarrier," and to make it easier for the examples,
I just dropped the center two, knowing that at least one of them would
be hit by this effect. In fact, Matt and I in our work last week ended
up dropping the center 4 so that this large amount of energy didn't
bleed into the next subcarrier, too.

Even though the center two subcarriers are dropped, we still use all
occupied_tones number of carriers, so this is going to take an extra
one on either side. So it should use carriers (15 - 30) and (33 - 48).
Also, the center two subcarriers that are dropped should be 31 and 32
and I mostly see the DC peak happen in the right-most bin (in this
case, 32). Are your numbers coming from starting the counting of
subcarriers at 0?

Tom