Frequency-division multiple access

Frequency Division Multiple Access or FDMA is an channel access method that is used by radio systems to share a certain radio spectrum between multiple users. The users are individually allocated one or several frequency bands, allowing them to access the radio system without interfering with each other.

Features

* FDMA requires high-performing filters in the radio hardware, in contrast to TDMA and CDMA.
* FDMA is not vulnerable to timing problems as TDMA. Since a predetermined frequency band is available for the entire period of communication, stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA.
* Due to the frequency filtering, FDMA is not sensitive to near-far problem which is pronounced for CDMA.

It is important to distinguish between FDMA and frequency-division duplexing (FDD). While FDMA allows multiple users simultaneous access a certain system, FDD refers to how the radio channel is shared between the uplink and downlink (for instance, the traffic going back and forth between a mobile-phone and a base-station). Furthermore, frequency-division multiplexing (FDM) should not be confused with FDMA. The former is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel. FDMA, on the other hand, is an access method in the data link layer.

FDMA also supports demand assignment in addition to fixed assignment. "Demand assignment" allows all users apparently continuous access of the radio spectrum 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.

See also

TDMA, CDMA, SDMA

References