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 timedivision 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 divisionmultiplexing, code division multiplexing.

(2001-06-28)