COFDM

Coded Orthogonal Frequency-Division Multiplexing (COFDM) — is a digital multi-carrier modulation scheme, which uses a large number of closely-spaced orthogonal sub-carriers. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation) at a low symbol rate, maintaining data rates similar to conventional single-carrier modulation schemes in the same bandwidth. In practice, OFDM signals are generated using the Fast Fourier transform algorithm.

The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions — for example, multipath and narrowband interference — without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal.

The orthogonality of the sub-carriers results in zero cross-talk, even though they are so close that their spectra overlap. Low symbol rate helps manage time-domain spreading of the signal (such as multipath propagation) by allowing the use of a guard interval between symbols. The guard interval also eliminates the need for a pulse-shaping filter. [1]