29-09-2017, 12:42 PM
30-09-2017, 10:58 AM
Orthogonal Frequency Division Multiplexing (OFDM) is a method of encoding digital data at multiple carrier frequencies. OFDM has become a popular scheme for digital broadband communication, used in applications such as digital television and sound broadcasting, DSL Internet access, wireless networks, power line networks and 4G mobile communications.
COFDM means coded orthogonal frequency division multiplexing. It differs from OFDM because in COFDM, forward error correction applies to the signal before transmission. This is done to overcome errors. COFDM and OFDM are sometimes used synonymously.
OFDM is a Frequency Division Multiplexing (FDM) scheme used as a multi-carrier digital modulation method. A large number of closely spaced orthogonal subcarrier signals are used to convey data over several parallel data streams or channels. Each subcarrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift modulation) at a low symbol rate, maintaining overall data rates similar to the conventional single carrier modulation schemes in the same bandwidth.
The main advantage of OFDM over single carrier schemes is its ability to cope with severe channel conditions (eg, high frequency attenuation on a long copper wire, narrow band interference, and selective frequency attenuation due to multipath) without complex EQ filters. Channel equalization is simplified because OFDM can be viewed using many slowly modulated narrowband signals rather than a rapidly modulated broadband signal. The low symbol rate makes the use of a guard interval between symbols affordable, allowing the elimination of interference between symbols (ISI) and using echoes and time propagation (on analogue television these are visible as ghosts and fuzzy respectively ) to achieve diversity, ie an improvement in the signal-to-noise ratio. This mechanism also facilitates the design of single frequency networks (SFNs), where several adjacent transmitters send the same signal simultaneously at the same frequency, since signals from multiple distant transmitters can be combined constructively, rather than interfering as would typically occur in a traditional single-seater system.