Frequency division multiplexing

What Is Frequency Division Multiplexing?

Frequency division multiplexing (FDM) is a technique for transmitting multiple independent signals over a single shared medium by assigning each signal a distinct portion of the available frequency spectrum. Each signal modulates a separate carrier frequency, and the resulting sub-bands are combined before transmission. At the receiver, filters separate the sub-bands and each signal is demodulated from its carrier. Because the sub-bands occupy different spectral regions simultaneously, FDM enables parallel transmission without the time-sharing overhead that characterizes time-division methods.

The principle dates to the earliest telephone networks of the late nineteenth century and was central to twentieth-century carrier telephony systems, in which a single coaxial cable or microwave link carried hundreds of voice channels, each occupying a 4 kHz slot in a hierarchical frequency plan.

Conventional FDM Architecture

In analog FDM, each input signal modulates a unique subcarrier using amplitude modulation (AM), single-sideband modulation (SSB), or frequency modulation (FM). Guard bands of unused spectrum separate adjacent channels to reduce crosstalk. The modulated channels are then linearly summed to form a composite waveform for transmission. This approach is used in AM and FM broadcast radio, where multiple stations share the broadcast band by occupying non-overlapping carriers, and in cable television, where many video channels coexist on a single coaxial distribution network.

The primary limitation of analog FDM is spectral efficiency: guard bands consume bandwidth that carries no information, and the need for sharp channel filters increases hardware complexity as the number of channels grows.

Orthogonal Frequency Division Multiplexing

Orthogonal frequency division multiplexing (OFDM) is a digital evolution of FDM that eliminates guard bands between subcarriers by choosing carrier spacings that make the subcarriers mathematically orthogonal. Two carriers are orthogonal when the integral of their product over one symbol period equals zero, allowing each subcarrier to be demodulated without interference from its neighbors even though their spectra overlap. This property is achieved by setting the subcarrier spacing equal to the reciprocal of the symbol duration, and the modulation and demodulation are implemented efficiently using the inverse discrete Fourier transform (IDFT) and the discrete Fourier transform (DFT).

OFDM substantially improves spectral efficiency compared to analog FDM and provides inherent robustness against frequency-selective fading, since a deep fade affects only the few subcarriers near the null rather than the entire signal. The standard Keysight OFDM primer for IEEE 802.11 describes how cyclic prefix insertion converts the dispersive multipath channel into a set of parallel flat-fading channels, one per subcarrier.

Layered Division Multiplexing

Layered division multiplexing (LDM) is a variant adopted in the ATSC 3.0 digital television standard. Rather than separating signals in frequency, LDM superimposes two or more signal layers at different power levels on the same bandwidth. A high-power layer carries a robust signal intended for mobile and indoor reception, while a low-power layer carries a higher-data-rate signal for stationary receivers with good antenna conditions. The receiver cancels the high-power layer first, then decodes the lower-power layer. LDM thus increases the total information rate within a fixed channel compared to FDM approaches that would split the bandwidth.

Applications

Frequency division multiplexing has applications in a wide range of fields, including: