Frequency-division multiple access
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Frequency Division Multiple Access or FDMA is an access technology that is used by radio systems to share the radio spectrum. The terminology “multiple access” implies the sharing of the resource amongst users, and the “frequency division” describes how the sharing is done: by allocating users with different carrier frequencies of the radio spectrum.
In FDMA the given Radio Frequency (RF) bandwidth is divided into smaller frequency bands called subdivisions. Each subdivision has its own carrier frequency. A control mechanism is used to ensure that two or more earth stations do not transmit in the same subdivision at the same time. Essentially, the control mechanism designates a receive station for each of the subdivisions.
In demand assignment systems, the control mechanism is also used to establish or terminate the voice-band links between the source and destination stations. Consequently, any of the subdivisions is used by any of the participating earth stations at any given time. If each subdivision carries only one 4-KHz voice-band channel, this is known as a single-channel per carrier (SCPC) system.
When several voice-band channels are frequency-division multiplexed to form a composite baseband signal comprised of groups, supergroups, or even mastergroups, a wider subdivision is assigned. This is referred to as multiple-channel per carrier (MCPC).
FDMA also supports demand assignment besides fixed assignment. Demand assignment allows all users continuous and equal access of the entire transponder bandwidth by assigning carrier frequencies on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellite was developed by COMSAT for use on the Intelsat series IVA and V satellites.
In contrast to this methodology with Time Division Multiple Access (TDMA), where users are separated in time, Code Division Multiple Access CDMA where users are separated by codes also exists as well as other multiple access schemes: SDMA – Space division multiple access, CSMA – Carrier sense multiple access, and MF-TDMA - Multi-Frequency TDMA
