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.

# Use of FFT and IFFT in OFDM

Started by ●October 15, 2006

Reply by ●October 15, 20062006-10-15

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