What Is OFDM?

OFDM (Orthogonal Frequency-Division Multiplexing) is a multicarrier modulation technique in which a high-rate data stream is divided into many parallel substreams, each modulating a closely spaced narrowband subcarrier. The subcarriers are designed to be mutually orthogonal, meaning the peak response of each subcarrier coincides with the zero crossings of its neighbors, so they can overlap in frequency without interfering with one another at the receiver. Introduced by Robert W. Chang at Bell Laboratories in 1966 and refined over the following decades, OFDM has become the dominant modulation scheme in broadband wireless communications because it transforms a frequency-selective fading channel into a set of parallel flat-fading channels, simplifying equalization.

The modulation and demodulation processes are implemented digitally using the Inverse Fast Fourier Transform (IFFT) at the transmitter and the FFT at the receiver, making OFDM practical at the subcarrier counts needed for modern systems. A cyclic prefix, a copy of the tail of each symbol prepended to its front, absorbs multipath echoes and eliminates intersymbol interference between successive symbols, provided the prefix is longer than the channel's delay spread.

Multicarrier Transmission Principles

The defining characteristic of OFDM is the orthogonal spacing of subcarriers at intervals of 1/T, where T is the useful symbol duration. This spacing allows each subcarrier to be demodulated independently by the FFT without leakage from adjacent channels. Because each subcarrier carries a low symbol rate, its symbol period is long relative to typical multipath delay spreads in wireless channels, which reduces the sensitivity of any individual subcarrier to time-domain echoes. The cyclic prefix converts the linear convolution imposed by the channel into a circular convolution, enabling the channel response on each subcarrier to be modeled as a single complex scalar that can be corrected by a one-tap equalizer per subcarrier. Frequency-domain subcarrier allocation also allows individual subcarriers to be turned off or assigned different modulation orders based on channel quality, a feature known as adaptive bit loading that improves spectral efficiency. NOMA (non-orthogonal multiple access) has been studied as a technique for serving multiple users simultaneously on the same OFDM subcarriers by superposing signals at different power levels, offering capacity gains at the cost of increased receiver complexity.

OFDM in Wireless and Wireline Standards

The combination of multipath resilience, spectral efficiency, and FFT-based implementation has made OFDM the preferred physical-layer waveform in multiple generations of wireless standards. The IEEE 802.11a/g/n/ac/ax family of Wi-Fi standards all use OFDM or its multi-user extension OFDMA. 3GPP Long-Term Evolution (LTE, the core of 4G networks) adopted OFDM for the downlink and a single-carrier variant for the uplink; 5G New Radio (NR) extended OFDM to both directions with flexible subcarrier spacing of 15, 30, 60, 120, or 240 kHz to accommodate diverse use cases from enhanced mobile broadband to ultra-reliable low-latency communications, as detailed in 5G waveform design studies on IEEE Xplore. Digital subscriber line (DSL) systems and power-line communication standards use OFDM to navigate the frequency-selective channels of copper wire and power conductors. The technique has also been applied in acoustic underwater communications and optical fiber systems, where it equalizes chromatic dispersion across the optical band.

Applications

OFDM has applications in a wide range of disciplines, including:

• 4G LTE and 5G NR cellular networks for mobile broadband
• Wi-Fi networks under the IEEE 802.11 family of standards
• Digital television broadcasting (DVB-T, ATSC 3.0)
• ADSL, VDSL, and power-line communication systems
• Acoustic underwater communication links
• Coherent optical fiber transmission systems