frequency division multiplexing
FDMA
frequency division multiple access

    (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)