In an OFDM scheme, a large number of orthogonal, overlapping, narrow band
sub-channels or subcarriers, transmitted in parallel, divide the available transmission
bandwidth. The separation of the subcarriers is theoretically minimal such that
there is a very compact spectral utilization. The attraction of OFDM is mainly due to
how the system handles the multipath interference at the receiver. Multipath generates
two effects: frequency selective fading and intersymbol interference (ISI).
The "flatness" perceived by a narrow-band channel overcomes the former, and
modulating at a very low symbol rate, which makes the symbols much longer than
the channel impulse response, diminishes the latter. Using powerful error correcting
codes together with time and frequency interleaving yields even more robustness
against frequency selective fading, and the insertion of an extra guard interval
between consecutive OFDM symbols can reduce the effects of ISI even more.
Thus, an equalizer in the receiver is not necessary.
