OFDM Question

Hello

I have got a simple question about the OFDM transmitter and the spectrum
we obtain after IFFT processing-DAC-IQ modulator. It must be very simple
because I have not found anything to clear my mind. 

I just do not understand how we can obtain a symmetric location of all the
subcarriers after a N-IFFT processing, with N even, if one of the
subcarries is located at 0 frequency: even number of negative frequencies -
DC frequency- odd number of positive frequencies.

For example, in "OFDM for wireless multimedia communications", Richard Van
Nee, the expression for the frequency upconverted (with a carrier frequency
fc) analog OFDM signal (expression 2.1) includes a 0.5*Af (Af is the
spacing frequency between subcarriers) frequency shift, and so the spectrum
is symmetric around the carrier frequency fc, but when is this frequency
shift done in the transmitter?

Thank you

On Jul 17, 7:16&#4294967295;am, "Crisanquito" <crisanc...@hotmail.com> wrote:

> I just do not understand how we can obtain a symmetric location of all the
> subcarriers after a N-IFFT processing, with N even, if one of the
> subcarries is located at 0 frequency: even number of negative frequencies -
> DC frequency- odd number of positive frequencies.
>
> For example, in "OFDM for wireless multimedia communications", Richard Van
> Nee, the expression for the frequency upconverted (with a carrier frequency
> fc) analog OFDM signal (expression 2.1) includes a 0.5*Af (Af is the
> spacing frequency between subcarriers) frequency shift, and so the spectrum
> is symmetric around the carrier frequency fc, but when is this frequency
> shift done in the transmitter?

I don't have that book,so I can't help in that regard.  But keep in
mind that for an FFT (N an even number), you have two unique points -
one at 0 and one at N/2.  They don't have conjugates.

Kevin McGee

kevin  <kevinjmcgee@netscape.net> wrote:

>On Jul 17, 7:16&#4294967295;am, "Crisanquito" <crisanc...@hotmail.com> wrote:

>> I just do not understand how we can obtain a symmetric location of all the
>> subcarriers after a N-IFFT processing, with N even, if one of the
>> subcarries is located at 0 frequency: even number of negative frequencies -
>> DC frequency- odd number of positive frequencies.
>>
>> For example, in "OFDM for wireless multimedia communications", Richard Van
>> Nee, the expression for the frequency upconverted (with a carrier frequency
>> fc) analog OFDM signal (expression 2.1) includes a 0.5*Af (Af is the
>> spacing frequency between subcarriers) frequency shift, and so the spectrum
>> is symmetric around the carrier frequency fc, but when is this frequency
>> shift done in the transmitter?

>I don't have that book,so I can't help in that regard.  But keep in
>mind that for an FFT (N an even number), you have two unique points -
>one at 0 and one at N/2.  They don't have conjugates.

I think it's a little unusal to describe an OFDM signal as having
a main carrier that is not one of the tones, or a frequency
orthogonal to the tones.  

But these descriptions tend to be subtly different each time
a new one is crafted, to keep people on their toes.

Steve