Orthogonal frequency division modulation
Orthogonal Frequency Division Modulation (OFDM) is a modulation technique for modulating digital information into an analog carrier signal. Discrete MultiTone modulation (DMT) is another term for OFDM.
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.
The benefit of using OFDM is that it is easy to filter out noise or interfering frequency ranges suffers from interference, some carriers within that range can be disabled or made to run slower.
When OFDM is used in conjunction with channel coding, 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. COFDM is especially used for DVB-T ans DAB (digital audio broadcasting).