OFDM Spectral Overlap
Conventional Frequency Division Multiplex (FDM) Multi-Carrier Modulation Technique
In conventional FDM, each carrier frequency is separated by a guard band to prevent interference. The frequencies in the guard band area cannot be used to carry information. The Transmitter contains an IFFT block and the receiver contains an FFT block. FFT and IFFT are a linear transform pair, defined by the following equations. Orthogonal Frequency Division Multiplex (OFDM) Multi-Carrier Modulation Technique
OFDM sub-carriers have a sinc (sin(x)/x) frequency response resulting in overlapin the frequency domain. This overlap does not cause interference due to the orthogonality of the sub-carriers.
- The OFDM receiver uses a time and frequency synchronized FFT to convert the OFDM time waveform back into the frequency domain. In this process the FFT picks up discrete frequency samples, corresponding to the peaks of the carriers. At these frequencies, all other carriers pass through zero eliminating interference between the sub-carriers.
- The FFT requires strict adherence to:
- An integer number of cycles during a symbol period
- An integer number of cycles separating the sub-carriers
- No phase or amplitude changes during symbol period
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