Frequency-division multiple access

FDMA allocates each user a dedicated frequency band, or sub-channel, for the entire duration of a call or data session. The total available spectrum is divided into contiguous slices, each with a guard band to prevent adjacent-channel interference. In a typical FDMA system, a transmitter filters its signal to fit within its assigned band, and the receiver uses a tunable filter to select the desired channel while rejecting others. This separation in the frequency domain allows simultaneous transmissions without collisions, but it requires precise frequency synchronization and careful filtering. FDMA is often used in conjunction with frequency-division duplexing (FDD), where uplink and downlink use separate frequency bands, as in the original Advanced Mobile Phone System (AMPS).¹

FDMA has been employed in a wide range of systems, from early analog radio and television broadcasting to satellite communications and cellular networks. In the first-generation (1G) analog cellular systems, such as AMPS and Nordic Mobile Telephone (NMT), FDMA was the sole multiple access method, with each call occupying a single 30 kHz or 25 kHz channel. Later, second-generation (2G) systems like GSM combined FDMA with TDMA, where each frequency channel is further divided into time slots. In satellite communications, FDMA is used in both fixed and mobile services, often with frequency reuse across different beams. Modern systems, including LTE and 5G, use orthogonal frequency-division multiple access (OFDMA), a variant that employs many closely spaced subcarriers, but the fundamental principle of frequency separation remains central.²

FDMA offers several advantages, including simplicity, low latency, and constant bit rate, which are beneficial for real-time voice communication. It also avoids the need for complex synchronization between users, as each operates on a distinct frequency. However, FDMA is inefficient when users are idle, because the allocated band remains unused. It also suffers from spectral waste due to guard bands and from the difficulty of implementing narrow filters for very small channel spacings. In addition, FDMA is susceptible to intermodulation distortion in nonlinear amplifiers, which can cause interference between adjacent channels. These limitations have driven the shift toward more flexible methods like OFDMA, which can allocate subcarriers dynamically based on demand.³

Beyond its mainstream use, FDMA has niche applications and historical footnotes. For example, the first commercial mobile telephone service, launched by Bell System in 1946, used FDMA with 120 kHz channels. In military communications, FDMA is used in frequency-hopping spread spectrum systems, where the hopping pattern is coordinated to avoid collisions. In amateur radio, FDMA is used in some digital modes like D-STAR, which divides a 6.25 kHz channel into sub-bands for voice and data. Another edge case is the use of FDMA in power-line communication, where different frequency bands are assigned to different services, such as home networking and smart grid metering. Additionally, the concept of FDMA has been extended to optical communications, where wavelength-division multiplexing (WDM) is essentially FDMA in the optical domain.⁴

Guard band

A small unused frequency interval between adjacent channels to prevent interference.

Frequency-division duplexing (FDD)

A technique where uplink and downlink use separate frequency bands.

Orthogonal frequency-division multiple access (OFDMA)

A multi-user version of OFDM that assigns subsets of subcarriers to different users.