frequency division multiple access
frequency division multiplexing
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frequency division multiplexing
<communications> (FDM) The simultaneous transmission of multiple separate
signals through a shared medium (such as a wire, optical fibre, or light beam)
by modulating, at the transmitter, the separate signals into separable frequency
bands, and adding those results linearly either before transmission or within
the medium. While thus combined, all the signals may be amplified, conducted,
translated in frequency and routed toward a destination as a single signal,
resulting in economies which are the motivation for multiplexing. Apparatus at
the receiver separates the multiplexed signals by means of frequency passing or
rejecting filters, and demodulates the results individually, each in the manner
appropriate for the modulation scheme used for that band or group.
Bands are joined to form groups, and groups may then be joined into larger
groups; this process may be considered recursively, but such technique is common
only in large and sophisticated systems and is not a necessary part of FDM.
Neither the transmitters nor the receivers need be close to each other; ordinary
radio, television, and cable service are examples of FDM. It was once the
mainstay of the long distance telephone system. The more recently developed time
division multiplexing in its several forms lends itself to the handling of
digital data, but the low cost and high quality of available FDM equipment,
especially that intended for television signals, make it a reasonable choice for
many purposes.
Compare wavelength division multiplexing, time division multiplexing, code
division multiplexing.
(2001-06-28)
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frequently asked question
Frequency Modulation
<communications> (FM) A method of encoding data by varying the frequency
of a constant amplitude carrier signal.
Contrast Amplitude Modulation.
(2001-04-02)
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Frequency Shift Keying
<communications> (FSK) The use of frequency modulation to transmit
digital data, i.e. two different carrier frequencies are used to represent zero
and one.
FSK was originally used to transmit teleprinter messages by radio (RTTY) but can
be used for most other types of radio and land-line digital telegraphy. More
than two frequencies can be used to increase transmission rates.
(1997-07-14)
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