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Frequency-division multiple access (FDMA) is achannel access method used in some multiple-access protocols. FDMA allows multiple users to send data through a singlecommunication channel, such as acoaxial cable ormicrowave beam, by dividing thebandwidth of the channel into separate non-overlappingfrequency sub-channels and allocating each sub-channel to a separate user. Users can send data through a subchannel by modulating it on acarrier wave at the subchannel's frequency. It is used insatellite communication systems and telephone trunklines.
FDMA splits the total bandwidth into multiple channels. Each ground station on the earth is allocated a particular frequency group (or a range of frequencies). Within each group, the ground station can allocate different frequencies to individual channels, which are used by different stations connected to that ground station. Before the transmission begins, the transmitting ground station looks for an empty channel within the frequency range that is allocated to it and once it finds an empty channel, it allocates it to the particular transmitting station.
Alternatives includetime-division multiple access (TDMA),code-division multiple access (CDMA), orspace-division multiple access (SDMA). These protocols are utilized differently, at different levels of the theoreticalOSI model.
Disadvantage:Crosstalk may cause interference among frequencies and disrupt the transmission.
FDMA is distinct fromfrequency division duplexing (FDD). While FDMA allows multiple users simultaneous access to a transmission system, FDD refers to how the radio channel is shared between theuplink anddownlink (for instance, the traffic going back and forth between a mobile-phone and amobile phone base station).Frequency-division multiplexing (FDM) is also distinct from FDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel, like in acar radio. FDMA, on the other hand, is an access method in thedata link layer.
FDMA also supportsdemand assignment in addition to fixed assignment.Demand assignment allows all users apparently continuous access of theradio spectrum by assigning carrier frequencies on a temporary basis using a statistical assignment process. The first FDMAdemand-assignment system for satellite was developed byCOMSAT for use on theIntelsat seriesIVA andV satellites.
There are two main techniques:
