Frequency-division multiplexing


Frequency-division multiplexing

Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.

Non telephone

FDM can also be used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal. A television channel is divided into subcarrier frequencies for video, color, and audio. DSL uses different frequencies for voice and for upstream and downstream data transmission on the same conductors, which is also an example of frequency duplex.

Where frequency division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA) [ cite book
last = White
first = Curt
year = 2007
title = Data Communications and Computer Networks
publisher = Thomson Course Technology
location = Boston, MA
pages = 140-143
isbn = 1-4188-3610-9

] .

FDMA is the traditional way of separating radio signals from different transmitters.

In the 1860 and 70s, several inventors attempted FDM under the names of Acoustic telegraphy and Harmonic telegraphy. Practical FDM was only achieved in the electronic age. Meanwhile their efforts led to an elementary understanding of electroacoustic technology, resulting in the invention of the telephone.

Telephone

For long distance telephone connections, 20th century telephone companies used L-carrier and similar co-axial cable systems carrying thousands of voice circuits multiplexed in multiple stages.

For short distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier. Those cables didn't allow such large bandwidths, so only 12 voice channels (Double Sideband) and later 24 (Single Sideband) were multiplexed into four wires, one pair for each direction with repeaters every several miles, approximately 10 km. See 12-channel carrier system. By the end of the 20th Century, FDM voice circuits had become rare. Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM.

Since the late twentieth century Digital Subscriber Lines have used a Discrete multitone (DMT) system to divide their spectrum into frequency channels.

The concept corresponding to frequency division multiplexing in the optical domain is known as wavelength division multiplexing.

ee also

* Duplex (telecommunications)
* Single-sideband modulation
* Orthogonal frequency-division multiplexing (OFDM),
* Time-division multiplexing (TDM)
* Time-division multiple access
* Code division multiple access
* Bias tee

References


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Frequency Division Multiplexing — Frequency Division Multiplexing,   Frequenzmultiplex Verfahren …   Universal-Lexikon

  • frequency-division multiplexing — dažninis tankinimas statusas T sritis informatika apibrėžtis Telekomunikacijose – ↑kanalų tankinimas, kai ↑pakanaliams skiriamos atskiros dažnių juostos. atitikmenys: angl. frequency division multiplexing ryšiai: dar žiūrėk – kanalas dar žiūrėk – …   Enciklopedinis kompiuterijos žodynas

  • frequency-division multiplexing —    Abbreviated FDM. A method of sharing a transmission channel by dividing the bandwidth into several parallel paths, defined and separated by guard bands of different frequencies designed to minimize interference. All signals are carried… …   Dictionary of networking

  • Frequency Division Multiplexing — FDM (Frequency Division Multiplex) ist ein nachrichtentechnisches Multiplexverfahren, mit dem gleichzeitig mehrere Signale auf mehrere Träger verteilt übertragen werden können. Die Träger sind mehreren unterschiedlichen Frequenzen zugeordnet,… …   Deutsch Wikipedia

  • Frequency Division Multiplexing —   Refer instead to Multiplexing …   International financial encyclopaedia

  • frequency-division multiplexing —    (FDM)    A system in which the available transmission frequency range is divided into narrow subchannels for use by a number of low speed devices …   IT glossary of terms, acronyms and abbreviations

  • frequency division multiplexing — See FDM …   Dictionary of telecommunications

  • frequency division multiplexing — …   Useful english dictionary

  • frequency-division multiplexing access —    (FDMA)    Technique for sharing a single transmission channel among two or more users by allocating each an exclusive frequency band within the channel …   IT glossary of terms, acronyms and abbreviations

  • Orthogonal frequency-division multiplexing — Passband modulation v · d · e Analog modulation AM · …   Wikipedia