>> 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
Reply by kevin●July 18, 20092009-07-18
On Jul 17, 7:16�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
Reply by Crisanquito●July 17, 20092009-07-17
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