Wednesday 5 August 2015

Orthogonal Frequency Division Multiplexing

Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wide band digital communications, used in applications such as digital television and audio broadcasting, DSL Internet access, wireless networks, powerline networks, and 4G mobile communications.

OFDM is a frequency division multiplexing (FDM) scheme used as a digital multi-carrier modulation Technique. A large number of closely spaced Orthogonal sub carrier signals are used to carry data on several parallel data streams or channels. Each sub-carrier is modulated with a conventional modulation scheme such as Quadrature amplitude modulation (QAM) or Phase shift Keying(PSK) at a low symbol rate, maintaining total data rates similar to conventional single carrier modulation schemes in 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. Channel equlization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate intersymbol interference (ISI) and utilize echoes and time-spreading (on analogue TV these are visible as ghosting and blurring, respectively) to achieve a diversity gain, i.e. a signal to noise ratio improvement. This mechanism also facilitates the design of single frequency networks (SFNs), where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.

Advantages
High spectral efficiency as compared to other double sideband modulation schemes, spread spectrum, etc.
Can easily adapt to severe channel conditions without complex time-domain equalization.
Robust against narrow-band co-channel interference.
Robust against  ISI and fading caused by multipath propagation.
Efficient implementation using fast fourier transform
Low sensitivity to time synchronization errors.
Tuned sub-channel receiver filters are not required (unlike conventional FDM

Disadvantages
Sensitive to Doppler shift
Sensitive to frequency synchronization problems.
High peak-to-average power ratio requiring linear transmitter circuitry, which suffers from poor power efficiency.
Loss of efficiency caused by cyclic prefix 


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Article By:
J.Jyostna
ECE Department
Asst. Professor 
Sphoorthy Engineering College

Sphoorthy Engineering College


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