On Wed, Feb 17, 2010 at 4:48 PM, bin zan [email protected] 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