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. OFDM is sometimes called Discrete multitone modulation (DMT)
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.
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/DMT systems are all actually COFDM.
Although highly complex, COFDM has high performance under even very challenging channel conditions.
By combining the OFDM technique with error-correcting codes, adaptive equalization and reconfigurable modulation, COFDM has the following properties:
- resistant against link dispersion
- resistant against slowly changing phase distortion and fading
- resistant against frequency response nulls and constant frequency interference
- resistant against burst noise
COFDM also generally has a nearly 'white' spectrum, giving it benign electromagnetic interference properties with respect to other signals.