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Orthogonal frequency-division multiplexing (OFDM)
It is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme in wideband digital communication, where wireless or over copper wires, use in applications such as digital television and audio broadcasting,
DSL broadband internet access, wireless networks, and 4G mobile communications.
It is essentially identical to coded OFDM (COFDM) and discrete multi-tone modulation (DMT), and is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method.
A large number of nearly spaced orthogonal sub-carrier signals are used to carry data on different parallel data streams or channels.
Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation and phase-shift keying) at a low symbol rate, maintaining all data rates similar to conventional single-carrier modulation schemes at the same bandwidth.
The primary advantage of OFDM over single-carrier schemes is-
-its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without complex equalization filters.
The Channel equalization is simplified cause OFDM may be viewed as using many slowly modulated narrowband signals rather than a rapidly modulated wideband signal.
The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to selecte intersymbol interference (ISI) and utilize echoes and time-spreading to find a diversity gain,
so a signal-to-noise ratio improvement.
This mechanism also facilitates the design of single frequency networks (SFNs), this diffrente adjacent transmitters send same signal simultaneously in the same frequency, as the signals from multiple distant transmitters may be combined constructively, otherwise the interfering as would typically occur at a traditional single-carrier system.
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