In telecommunications, multiplexing refers to a set of techniques that enable the sharing of the usable electromagnetic spectrum of a telecommunications channel (the channel passband) among multiple users for the transfer of individual information streams. It is assumed that the user information streams join at a common access point to the channel. The term “multiple access” is usually applied to multiplexing schemes by which multiple users who are geographically dispersed gain access to the shared telecommunications facility or channel. Various methods of multiplexing and multiple access are in common use.
In frequency-division multiplexing (FDM) and frequency-division multiple access (FDMA), the passband of a channel is shared among multiple users by assigning distinct and nonoverlapping sections of the electromagnetic spectrum within the passband to individual users. The information stream from a particular user is encoded into a signal whose energy is confined to the part of the passband assigned to that user. See also Radio spectrum allocations.
Time-division multiplexing (TDM) and time-division multiple access (TDMA) permit a user access to the full passband of the channel, but only for a limited time, after which the access right is assigned to another user. Normally the access rights are assigned in a cyclical order to the competing users. However, statistical time-division multiplexing assigns time on the channel on a demand basis, which typically increases the number of users who may be accommodated on the same channel, but may result in delays in accessing the channel during periods when the demand exceeds the supply.
In code-division multiple access (CDMA), all users are assigned the entire passband of the channel and are permitted to transmit their information streams simultaneously. To maintain the ability to recover the individual signals at the receiver, at the transmitter each signal has impressed on it a characteristic signature.
Space-division multiple access (SDMA) refers to the use of the same portion of the electromagnetic spectrum over two or more spatially distinct transmission paths. In most applications of space-division multiple access, the paths are formed by multibeam antennas, in which each beam is directed toward a different geographic area. See also Antenna (electromagnetism).
In wavelength-division multiplexing (WDM) schemes, transmission systems that employ the optical portion of the electromagnetic spectrum, such as those using fiber-optic cables as the transmission medium, share the total available passband of the medium by assigning individual information streams to signals of different wavelengths or “colors.” See also Optical communications; Optical fibers.
Polarization refers to the direction or geometric orientation of the electric field vector of an electromagnetic field. Polarization-division multiplexing and polarization-division multiple access assign electric fields of different polarization to individual channels or users. See also Electrical communications; Polarization of waves; Polarized light.
