The IFFT and FFT are used at every antenna of MIMO system according the MIMO technique used ( Beamforming, Spatial diversity, or Spatial multiplexing). The attached book will benefit you to understand MIMO-OFDM system.

Hi,

Try to look at this ebook, y'll find a source code of MIMO-OFDM system.

good luck :)

http://books.google.fr/books?id=6HwAoeuMr3kC&printsec=frontcover&dq=MIMO-OFDM+WIRELESS+COMMUNICATIONS+WITH+MATLAB&hl=fr&sa=X&ei=BAJqVMjgA8n4yQO6z4DIBA&ved=0CCkQ6AEwAA#v=onepage&q=MIMO-OFDM%20WIRELESS%20COMMUNICATIONS%20WITH%20MATLAB&f=false

In the MIMO system, there is Nt transmitting antennas and therefor  Nt OFDM Channels. Every channel produces its own OFDM symbol. It receives its N QAM  symbols from the MIMO encoder. These N symbols are converted by an IFFT to the required OFDM in time domain. The inverse is made in the receiver path where the OFDM symbols are converted back to the frequency domain by FFT.

The process in every branch of  mimo is similar to that in SISO.

Conceptually,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.

wish you success

I will try to answer the question why IFFT/FFT is used in OFDM. Lets consider the single input single output case.

A single carrier that occupies the entire bandwidth might suffer from frequency selective fading. The technique to get rid of the frequency selectivity is to divide the bandwidth in smaller parts and transmit multiple carriers (we can call these carriers as subcarriers). Lets assume that we have divided the bandwidth in N parts and want to transmit N parallel symbols with N subcarriers.

Now for multicarrier transmission (this is not OFDM yet), we need to modulate N-parallel BPSK/QPSK/QAM symbols to place them on N subcarriers that are operating on different parts of the total transmission bandwidth. We need a bank of N modulators in the transmitter for that purpose. Implementing a bank of N modulator or demodulator is not practical as it would be computationally very complex.

The same job of modulating the N-parallel subcarriers can be done by DFT/IDFT with much less complexity.  OFDM is a multicarrier transmission technique where the subcarriers are orthogonal to one another. The use of DFT/IDFT made the OFDM practical and popular. As mentioned earlier, FFT/IFFT is a less complex form of DFT/IDFT.

Note that, the use FFT/IFFT is just implementation choices and not something mandatory. Nothing forbids the implementation of a bank of N modulators (though it would be impractical).