Question

Explain how OFDM system use IFFT to modulate information to multiple sub-carriers.

Answer 1

The Fourier transform is used to convert the signals from time domain to frequency domain and the inverse Fourier transform is used to convert the signal back from the frequency domain to the time domain.The Fourier transform is a powerful tool to analyse the signals and construct them to and from their frequency components. If the signal is discrete in time that is sampled, one uses the discrete Fourier transform to convert them to the discrete frequency form DFT, and vice verse, the inverse discrete transform IDFT is used to back convert the discrete frequency form into the discrete time form.

To reduce the mathematical operations used in the calculation of DFT and IDFT one uses the fast Fourier transform algorithm FFT and IFFT which corresponds to DFT and IDFT, respectively.

In transmitters using OFDM as a multicarrier modulation technology, the OFDM symbol is constructed in the frequency domain by mapping the input bits on the I- and Q- components of the QAM symbols and then ordering them in a sequence with specific length according to the number of subcarriers in the OFDM symbol. That is by the mapping and ordering process, one constructs the frequency components of the OFDM symbol. To transmit them, the signal must be represented in time domain. This is accomplished by the inverse fast Fourier transform IFFT.

So, in summary the signal is easier synthesized in discrete frequency domain in the transmitter and to transmit it must be converted to discrte time domain by IFFT.