Orthogonal frequency-division multiplexing

Orthogonal frequency division modulation (OFDM) is a modulation technique for encoding information into an analog signal. OFDM is typically used to modulate digital information onto a carrier signal.

An OFDM signal may be regarded as the sum of a number of individual sub-carrier signals, each modulated (typically using QAM) by its own modulating signal.

This composite signal is then used to modulate the main carrier.

OFDM modulation and demodulation are typically (as of 2001) implemented using digital filter banks using the Fast Fourier Transform.

When OFDM is used in conjunction with channel coding techniques, it is described as Coded orthogonal frequency division modulation (COFDM). As the overhead of doing this in an already digital system is low, and the gains substantial, practical OFDM systems are all actually COFDM.