Orthogonal frequency-division multiplexing (OFDM) — essentially identical to Coded OFDM (COFDM) and Discrete multi-tone modulation (DMT) — is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.

OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, wireless networking and broadband internet access.

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 equalization 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 handle time-spreading and eliminate intersymbol interference (ISI). This mechanism also facilitates the design of single-frequency networks, 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.

Example of applications

The following list is a summary of existing OFDM based standards and products. For further details, see the Usage]] section in the end of the article.

Cable

• ADSL and [[VDSL broadband access via POTS copper wiring.
• Power line communication (PLC).
• Multimedia over Coax Alliance (MoCA) home networking.
• ITU-T G.hn, a standard which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables)

Wireless

• The wireless LAN radio interfaces IEEE 802.11a, g, n and HIPERLAN/2.
• The digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB.
• The terrestrial digital TV system DVB-T.
• The terrestrial mobile TV systems DVB-H, T-DMB, ISDB-T and MediaFLO forward link.
• The cellular communication systems Flash-OFDM]]
• The mobile broadband 3GPP Long Term Evolution air interface named High Speed OFDM Packet Access (HSOPA)
• The Wireless MAN / Fixed broadband wireless access (BWA) standard IEEE 802.16 (or WiMAX).
• The Mobile Broadband Wireless Access (MBWA) standards IEEE 802.20, IEEE 802.16e (Mobile WiMAX) and WiBro.
• The wireless Personal Area Network (PAN) Ultra wideband (UWB) IEEE 802.15.3a implementation suggested by WiMedia Alliance.