Orthogonal frequency-division multiplexing

Orthogonal frequency-division multiplexing (OFDM) sends data simultaneously over many closely spaced subcarriers whose waveforms are mathematically orthogonal, allowing their spectra to overlap without mutual interference.

Orthogonality: A mathematical relation in which functions or vectors have zero inner product under a specified measure. OFDM spaces subcarrier frequencies so each is orthogonal to the others over a symbol interval.
Orthogonal frequency-division multiple access: A multiple-access method that assigns groups of OFDM subcarriers to different users. It extends OFDM by scheduling subcarriers across users in cellular systems.
Single-carrier transmission: A transmission method that conveys data using one modulated carrier rather than many simultaneous subcarriers. It can have lower peak power variation, while OFDM often simplifies equalization in dispersive channels.
Multipath propagation: The arrival of a transmitted signal at a receiver along multiple paths with different delays. OFDM’s long symbol intervals and cyclic prefixes help manage delayed copies of signals.
Discrete Fourier transform: A transform that represents a finite sequence as a sum of discrete-frequency complex sinusoids. It efficiently maps OFDM symbols between frequency-domain subcarriers and time-domain samples.
Wi-Fi: A family of wireless local-area networking standards based on IEEE 802.11. Many Wi-Fi generations use OFDM to transmit robust high-rate signals indoors.
Frequency-division multiplexing: A method that carries multiple signals simultaneously in distinct frequency bands. OFDM is a tightly spaced, orthogonal form of frequency-division multiplexing.
Frequency-selective fading: A channel condition in which signal attenuation varies across frequency because of multipath effects. OFDM divides a wideband channel into narrow subcarriers that can be equalized individually.
Cyclic prefix: A copy of a signal’s ending appended to its beginning before transmission. It turns channel delay spread into a guard interval that helps preserve subcarrier orthogonality.
LTE (telecommunication): A 4G mobile telecommunications standard developed by the 3rd Generation Partnership Project. LTE uses OFDM on its downlink and a related transform-precoded scheme on its uplink.

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