Orthogonal frequency-division multiplexing

In <a href="/facts/Telecommunications/vxoj1cxR">telecommunications</a>, orthogonal frequency-division multiplexing (OFDM) is a type of <a href="/facts/Digital_transmission/1yAhaANd">digital transmission</a> used in digital <a href="/facts/Modulation/aAXPjINv">modulation</a> for encoding digital (binary) data on multiple <a href="/facts/Carrier_wave/ae98pBlY">carrier</a> frequencies. OFDM has developed into a popular scheme for <a href="/facts/Wideband/wv1yPnkF">wideband</a> <a href="/facts/Digital_communication/1yAhaANd">digital communication</a>, used in applications such as digital television and audio broadcasting, <a href="/facts/Digital_subscriber_line/3pY5osov">DSL</a> <a href="/facts/Internet_access/EMbwSrnK">internet access</a>, <a href="/facts/Wireless_network/yr4SWTdl">wireless networks</a>, <a href="/facts/Power-line_communication/y0QpHy0V">power line networks</a>, and <a href="/facts/4G/Lfq6mSxl">4G</a>/<a href="/facts/5G/yVb460m4">5G</a> mobile communications.
OFDM is a <a href="/facts/Frequency-division_multiplexing/lgj6TMU1">frequency-division multiplexing</a> (FDM) scheme that was introduced by Robert W. Chang of <a href="/facts/Bell_Labs/fKP5Gk0H">Bell Labs</a> in 1966. In OFDM, the incoming <a href="/facts/Bitstream/Pf9oGYyY">bitstream</a> representing the data to be sent is divided into multiple streams. Multiple closely spaced <a href="/facts/Orthogonal_functions/2MmN826Q">orthogonal</a> subcarrier signals with overlapping spectra are transmitted, with each carrier <a href="/facts/Modulation/aAXPjINv">modulated</a> with <a href="/facts/Binary_digit/S972NSaD">bits</a> from the incoming stream so multiple bits are being transmitted in parallel. Demodulation is based on <a href="/facts/Fast_Fourier_transform/caT7UUCh">fast Fourier transform</a> algorithms. OFDM was improved by Weinstein and Ebert in 1971 with the introduction of a <a href="/facts/Guard_interval/1vCJVQtV">guard interval</a>, providing better orthogonality in transmission channels affected by multipath propagation. Each subcarrier (signal) is modulated with a conventional modulation scheme (such as <a href="/facts/Quadrature_amplitude_modulation/02j52SWG">quadrature amplitude modulation</a> or <a href="/facts/Phase-shift_keying/TFnpYe64">phase-shift keying</a>) at a low <a href="/facts/Symbol_rate/YlQ3JHVr">symbol rate</a>. This maintains total data rates similar to 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 <a href="/facts/Communication_channel/MtlxEShV">channel</a> conditions (for example, <a href="/facts/Attenuation_distortion/PFvmcjqd">attenuation</a> of high frequencies in a long copper wire, narrowband <a href="/facts/Interference_(communication)/BVce5g8d">interference</a> and frequency-selective <a href="/facts/Fading/RsDwBbdz">fading</a> due to <a href="/facts/Multipath_propagation/dJFXvAbW">multipath</a>) without the need for complex equalization filters. Channel <a href="/facts/Equalization_(communications)/44Mc0WXw">equalization</a> is simplified because OFDM may be viewed as using many slowly modulated <a href="/facts/Narrowband/76nBsRml">narrowband</a> signals rather than one rapidly modulated <a href="/facts/Wideband/wv1yPnkF">wideband</a> signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate <a href="/facts/Intersymbol_interference/6oGRSogt">intersymbol interference</a> (ISI) and use echoes and time-spreading (in <a href="/facts/Analog_television/4NJJew1c">analog television</a> visible as <a href="/facts/Ghosting_(television)/oybe05LJ">ghosting</a> and blurring, respectively) to achieve a <a href="/facts/Diversity_gain/ga5PFjW9">diversity gain</a>, i.e. a <a href="/facts/Signal-to-noise_ratio/qohClhyG">signal-to-noise ratio</a> improvement. This mechanism also facilitates the design of <a href="/facts/Single_frequency_network/1QPcPIpB">single frequency networks</a> (SFNs) where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be re-combined constructively, sparing interference of a traditional single-carrier system.
In coded orthogonal frequency-division multiplexing (COFDM), <a href="/facts/Forward_error_correction/cLUTUJrA">forward error correction</a> (convolutional coding) and time/frequency interleaving are applied to the signal being transmitted. This is done to overcome errors in mobile communication channels affected by multipath propagation and <a href="/facts/Doppler_effect/bzrK1BKf">Doppler effects</a>. COFDM was introduced by Alard in 1986 for <a href="/facts/Digital_Audio_Broadcasting/9GNkj4kL">Digital Audio Broadcasting</a> for <a href="/facts/Eureka_(organisation)/eU32YTw6">Eureka</a> Project 147. In practice, OFDM has become used in combination with such coding and interleaving, so that the terms COFDM and OFDM co-apply to common applications.

Orthogonal frequency-division multiplexing open-in-new

Orthogonal frequency-division multiplexing