Re: [Discuss-gnuradio] Re: Discuss-gnuradio Digest, Vol 65, Issue 45

[Tom Rondeau]

CHIN-YA HUANG wrote:
> > Message: 6
> > The OFDM system uses fft_length number of subcarriers and 
> > occupied_tones 
> > number of used subcarriers. The used subcarriers are mapped to the 
> > middle of the fft_length subcarriers leaving the same number of 
> > guardband subcarriers on either side. The middle two subcarriers are 
> > also nulled to avoid DC affects.
> >
> > The mapping is done in gr_ofdm_mapper_bcv.cc. If you want to play with 
> >
> > this, I'd make a copy of this one as a different version of the mapping.
> >
> > Tom
> >
> >     
>
> Thanks Tom.
>
> Do you mean mapping to the fft_length?
>   

No, the fft_length is the total number of subcarriers. Only 
occupied_tones number of subcarriers carry data.

> But, my questions is when received a packet, I saw the code in 
> gr_ofdm_mapper_bcv.cc that
> the bits in the packet is sequentially to the subcarrier in order, and then do 
> modulation according to desired constellation. Do I misunderstand anything?
>   

Seems like you got it.

Tom