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How OFDM works
When OFDM was first implemented, it was by using banks of sinusoidal generators, e.g. just placing up a whole lot of single carriers in parallel. The use of the discrete Fourier transform (DFT) was originally proposed in 1971 by Weinstein and Ebert, which greatly reduces the implementation complexity of OFDM systems. This was further reduced by the development of the fast Fourier transform (FFT). Shortly after an equalisation algorithm was implemented in order to help suppress both ISI and intersubcarrier interference, which is caused by the channel impulse response and timing and frequency errors.In OFDM the subcarrier pulse which is used for transmission is rectangular. This is why the capability of pulse forming and modulation can be performed by an IDFT, which can be generated very efficiently as an IFFT. Because of this, the receiver only needs a FFT to reverse this process. Taking into account the theories of the Fourier Transform the rectangular pulse shape will end up as a sin(x)/x style of spectrum of the subcarriers. In traditional FDM the sub-channels aren’t orthogonal therefore need to be separated by guard bands which obviously wastes much needed spectrum.
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