Use of FFT and IFFT in OFDM

Looking at the discrete equation for DFT and its inverse(algorithm wise FFT
and IFFT), it is evident that the results are the same, especially if you
consider the magnitude.
After the base band symbol mapping, the result looks to be in the
frequency domain already. Why go through a conversion to time domain
through IFFT and again mapping this onto a frequency up conversion.

Amby said the following on 15/10/2006 14:21:
> Looking at the discrete equation for DFT and its inverse(algorithm wise FFT
> and IFFT), it is evident that the results are the same, especially if you
> consider the magnitude.

Which results are the same?  If you mean that processing some stuff with 
an IDFT followed by a DFT results in the original values, then yes.


> After the base band symbol mapping, the result looks to be in the
> frequency domain already. Why go through a conversion to time domain
> through IFFT and again mapping this onto a frequency up conversion.

What do you mean by a "frequency up conversion"?

In OFDM, your data is treated as a frequency-domain representation of 
the signal.  To get a signal that you can transmit, you need to be in 
the time domain, so you have to use an IDFT (IFFT).  (The alternative is 
to use each data point to modulate a separate carrier, as in FDM, but 
this is inefficient.)

At the receiver, to get back to the frequency domain, yuo have to use a 
DFT (FFT).


-- 
Oli