Orthogonal frequency-division multiplexing
A digital modulation scheme using multiple orthogonal subcarriers.
Orthogonal frequency-division multiplexing (OFDM) is a digital transmission technique that encodes binary data across multiple carrier frequencies. It has become a widely adopted method for wideband digital communication, appearing in digital television and audio broadcasting, DSL internet, wireless networks, power line communications, and 4G/5G mobile systems.
OFDM is a form of frequency-division multiplexing first introduced by Robert W. Chang at Bell Labs in 1966. In this scheme, the incoming bitstream is split into several parallel streams. These streams are transmitted over closely spaced orthogonal subcarriers whose spectra overlap, with each subcarrier modulated by bits from the original stream, allowing multiple bits to be sent simultaneously. Demodulation relies on fast Fourier transform algorithms. In 1971, Weinstein and Ebert improved OFDM by adding a guard interval, which helps maintain orthogonality in channels affected by multipath propagation. Each subcarrier is modulated using a conventional scheme like quadrature amplitude modulation or phase-shift keying at a low symbol rate, keeping total data rates comparable to single-carrier systems using the same bandwidth.
The main advantage of OFDM over single-carrier methods is its ability to handle severe channel conditions—such as high-frequency attenuation in long copper wires, narrowband interference, and frequency-selective fading from multipath—without requiring complex equalization filters. Channel equalization is simpler because OFDM uses many slowly modulated narrowband signals instead of one rapidly modulated wideband signal. The low symbol rate makes a guard interval affordable, which eliminates intersymbol interference and allows echoes and time-spreading to improve signal-to-noise ratio through diversity gain. This also enables single frequency networks, where multiple transmitters send the same signal at the same frequency, and signals from distant transmitters can combine constructively rather than interfering.
In coded orthogonal frequency-division multiplexing (COFDM), forward error correction using convolutional coding and time/frequency interleaving is applied to the signal. This helps overcome errors in mobile channels affected by multipath and Doppler effects. COFDM was introduced by Alard in 1986 for Digital Audio Broadcasting under the Eureka Project 147.
- Introduced by
- Robert W. Chang of Bell Labs
- Year introduced
- 1966
- Improved by
- Weinstein and Ebert
- Year improved
- 1971
- Improvement
- introduction of a guard interval
- Cofdm introduced by
- Alard
- Cofdm year
- 1986
Lore & Background
OFDM is a frequency-division multiplexing (FDM) scheme introduced by Robert W. Chang of Bell Labs in 1966. In OFDM, the incoming bitstream is divided into multiple streams, and closely spaced orthogonal subcarrier signals with overlapping spectra are transmitted. Each subcarrier is modulated with a conventional scheme such as quadrature amplitude modulation or phase-shift keying at a low symbol rate, maintaining total data rates similar to single-carrier schemes in the same bandwidth. Demodulation is based on fast Fourier transform algorithms. In 1971, Weinstein and Ebert improved OFDM by introducing a guard interval, providing better orthogonality in transmission channels affected by multipath propagation.
Coded orthogonal frequency-division multiplexing (COFDM) applies forward error correction (convolutional coding) and time/frequency interleaving to the signal. This is done to overcome errors in mobile communication channels affected by multipath propagation and Doppler effects. COFDM was introduced by Alard in 1986 for Digital Audio Broadcasting for Eureka 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.
The orthogonality of subcarriers requires that the subcarrier spacing is Δf = k/TU Hertz, where TU is the useful symbol duration and k is a positive integer. This eliminates crosstalk between sub-channels and removes the need for inter-carrier guard bands. OFDM generally has a nearly 'white' spectrum, giving it benign electromagnetic interference properties with respect to other co-channel users.
Reader's Guide
OFDM's main advantage over single-carrier schemes is its ability to cope with severe channel conditions—such as attenuation of high frequencies in a long copper wire, narrowband interference, and frequency-selective fading due to multipath—without the need for complex equalization filters. Channel equalization is simplified because OFDM uses 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, eliminating intersymbol interference and allowing echoes and time-spreading to achieve a diversity gain. This mechanism also facilitates the design of single frequency networks, where several adjacent transmitters send the same signal simultaneously at the same frequency, as signals from multiple distant transmitters may be re-combined constructively.
OFDM is sensitive to Doppler shift and frequency synchronization problems, and has a high peak-to-average-power ratio requiring linear transmitter circuitry with poor power efficiency. It also suffers loss of efficiency caused by the cyclic prefix/guard interval. Despite these disadvantages, OFDM has been adopted in numerous wired and wireless standards, including ADSL, VDSL, DVB-T, DAB, IEEE 802.11 wireless LAN standards, LTE, and DOCSIS 3.1. The multiple access technology OFDMA is used in several 4G and pre-4G cellular networks and mobile broadband standards.
Did You Know?
- Weinstein and Ebert improved OFDM in 1971 by introducing a guard interval.
- OFDM is used in applications such as digital television, DSL internet access, wireless networks, and 4G/5G mobile communications.
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