Orthogonal Frequency Division Multiplexing (OFDM)

by Jean-Paul Linnartz

Orthogonal Frequency Division Multiplexing (OFDM) is special form of multi-carrier modulation, patented in 1970. It is particularly suited for transmission over a dispersive channel. In a multipath channel, most conventional modulation techniques are sensitive to intersymbol interference unless the channel symbol rate is small compared to the delay spread of the channel. OFDM is significantly less sensitive to intersymbol interference, because a special set of signals is used to build the composite transmitted signal. The basic idea is that each bit occupies a frequency-time window which ensures little or no distortion of the waveform. In practice, it means that bits are transmitted in parallel over a number of frequency-nonselective channels.

Applications of OFDM are found in

 

 

OFDM Transmitter based on I-FFT

 
 
 
 

In an OFDM transmitter, blocks of k incoming bits are encoded into n channel symbols. Before transmission, an n-point Inverse-FFT operation is performed. When the signals at the I-FFT output are transmitted sequentially, each of the n channel symbols appears at a different (subcarrier) frequency. Such coding across subcarriers is necessary. If one subcarrier experiences deep fading, this leads to erasure of the data symbol on this subcarrier.

 

 

 

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