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Multiplexing
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==Types== Multiple [[variable bit rate]] digital [[bit stream]]s may be transferred efficiently over a single fixed [[Bandwidth (signal processing)|bandwidth]] channel by means of [[statistical multiplexing]]. This is an [[asynchronous serial communication|asynchronous]] mode time-domain multiplexing which is a form of time-division multiplexing. Digital bit streams can be transferred over an analog channel by means of code-division multiplexing techniques such as [[frequency-hopping spread spectrum]] (FHSS) and [[direct-sequence spread spectrum]] (DSSS). In [[wireless communication]]s, multiplexing can also be accomplished through alternating [[polarization (waves)|polarization]] ([[Horizontal plane|horizontal]]/[[vertical direction|vertical]] or [[clockwise]]/[[counterclockwise]]) on each [[adjacent channel]] and satellite, or through [[phased array|phased multi-antenna array]] combined with a [[multiple-input multiple-output communications]] (MIMO) scheme. ===Space-division multiplexing=== {{Main|Space-division multiple access}} In wired communication, [[space-division multiplexing]], also known as space-division multiple access (SDMA) is the use of separate point-to-point electrical conductors for each transmitted channel. Examples include an analog stereo audio cable, with one pair of wires for the left channel and another for the right channel, and a multi-pair [[telephone cable]], a switched [[star network]] such as a telephone access network, a switched Ethernet network, and a [[mesh network]]. In wireless communication, space-division multiplexing is achieved with multiple antenna elements forming a [[phased array antenna]]. Examples are [[multiple-input and multiple-output]] (MIMO), single-input and multiple-output (SIMO) and multiple-input and single-output (MISO) multiplexing. An IEEE 802.11g wireless router with ''k'' antennas makes it in principle possible to communicate with ''k'' multiplexed channels, each with a peak bit rate of 54 Mbit/s, thus increasing the total peak bit rate by the factor ''k''. Different antennas would give different [[multi-path propagation]] (echo) signatures, making it possible for [[digital signal processing]] techniques to separate different signals from each other. These techniques may also be utilized for [[space diversity]] (improved robustness to fading) or [[beamforming]] (improved selectivity) rather than multiplexing. ===Frequency-division multiplexing=== [[File:Frequenzmultiplex001.svg|right|thumb|400px|Frequency-division multiplexing (FDM): The spectrum of each input signal is shifted to a distinct frequency range.]] [[Frequency-division multiplexing]] (FDM) is inherently an analog technology. FDM achieves the combining of several signals into one medium by sending signals in several distinct frequency ranges over a single medium. '''In FDM the signals are electrical signals.''' One of the most common applications for FDM is traditional radio and television broadcasting from terrestrial, mobile or satellite stations, or cable television. Only one cable reaches a customer's residential area, but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers without interference. Receivers must tune to the appropriate frequency (channel) to access the desired signal.<ref name = "Bates">{{citation | title = Voice and Data Communications | first1 = Regis J | first2 = Marcus | last1= Bates| last2 = Bates | year = 2007 | publisher = McGraw-Hill Irwin | isbn = 9780072257328}}</ref> [[Image:WDM operating principle.svg|400px|thumb|One stream, one color, light waves, in WDM]] A variant technology, called [[wavelength-division multiplexing]] (WDM) is used in [[optical communication]]s. ===Time-division multiplexing=== [[File:Telephony multiplexer system.gif|right|thumb|400px|Time-division multiplexing (TDM)]] [[Time-division multiplexing]] (TDM) is a digital (or in rare cases, analog) technology that uses time, instead of space or frequency, to separate the different data streams. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver. If done sufficiently quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path. Consider an application requiring four terminals at an airport to reach a central computer. Each terminal communicated at 2400 [[baud]], so rather than acquire four individual circuits to carry such a low-speed transmission, the airline has installed a pair of multiplexers. A pair of 9600 baud modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed.<ref name = "Bates" /> Some [[Proxy server#Web proxy servers|web proxy servers]] (e.g. [[polipo]]) use TDM in [[HTTP pipelining]] of multiple [[HTTP]] transactions onto the same [[Transmission Control Protocol|TCP/IP connection]].<ref>{{citation |title = rfc2068 - HTTP/1.1 |year = 1997 |doi = 10.17487/RFC2068 |url = http://tools.ietf.org/html/rfc2068 |access-date = 2010-09-23 |last1 = Fielding |first1 = Roy |last2 = Gettys |first2 = Jim |last3 = Mogul |first3 = Jeffrey |last4 = Frystyk |first4 = Henrik |last5 = Berners-Lee |first5 = Tim |url-access = subscription }}</ref> [[Carrier-sense multiple access]] and [[multidrop]] communication methods are similar to time-division multiplexing in that multiple data streams are separated by time on the same medium, but because the signals have separate origins instead of being combined into a single signal, are best viewed as [[channel access method]]s, rather than a form of multiplexing. TD is a legacy multiplexing technology still providing the backbone of most National fixed-line telephony networks in Europe, providing the 2 Mbit/s voice and signaling ports on narrow-band telephone exchanges such as the DMS100. Each E1 or 2 Mbit/s TDM port provides either 30 or 31 speech timeslots in the case of CCITT7 signaling systems and 30 voice channels for customer-connected Q931, DASS2, DPNSS, V5 and CASS signaling systems.<ref>{{Cite book|title=Breakthroughs in telephone technology: from Bell to smartphones|date=2012|publisher=Britannica Educational |editor=Curley, Robert |isbn=978-1-61530-724-1 |location=New York |page=69|oclc=769190186}}</ref> ===Polarization-division multiplexing=== [[Polarization-division multiplexing]] uses the [[polarization (waves)|polarization]] of electromagnetic radiation to separate orthogonal channels. It is in practical use in both radio and optical communications, particularly in 100 Gbit/s per channel [[fiber-optic transmission system]]s. Differential Cross-Polarized Wireless Communications is a novel method for polarized antenna transmission utilizing a differential technique.<ref>{{Citation |title=Differential Cross-Polarized Wireless Communications |author1=Siamack Ghadimi|publisher=Scientific Research|date=2019-04-02}}</ref> ===Orbital angular momentum multiplexing=== {{update section|reason=see [[Talk:Multiplexing#Orbital_angular_momentum_multiplexing_section_needs_updating]]|date=August 2024}} [[Orbital angular momentum multiplexing]] is a relatively new and experimental technique for multiplexing multiple channels of signals carried using electromagnetic radiation over a single path.<ref>{{Cite journal|last1=Tamburini|first1=Fabrizio|last2=Mari|first2=Elettra|last3=Sponselli|first3=Anna|last4=Thidé|first4=Bo|last5=Bianchini|first5=Antonio|last6=Romanato|first6=Filippo|date=2012-01-01|title=Encoding many channels on the same frequency through radio vorticity: first experimental test|url=http://stacks.iop.org/1367-2630/14/i=3/a=033001|journal=New Journal of Physics|language=en|volume=14|issue=3|pages=033001|doi=10.1088/1367-2630/14/3/033001|issn=1367-2630|arxiv=1107.2348|bibcode=2012NJPh...14c3001T|s2cid=3570230}}</ref> It can potentially be used in addition to other physical multiplexing methods to greatly expand the transmission capacity of such systems. {{As of|2012}} it is still in its early research phase, with small-scale laboratory demonstrations of bandwidths of up to 2.5 Tbit/s over a single light path.<ref>{{cite news|url=https://www.bbc.co.uk/news/science-environment-18551284|title='Twisted light' carries 2.5 terabits of data per second|work=BBC News|date=2012-06-25|access-date=2012-06-25}}</ref> This is a controversial subject in the academic community, with many claiming it is not a new method of multiplexing, but rather a special case of space-division multiplexing.<ref>{{cite journal|last1=Tamagnone|first1=Michele|last2=Silva|first2=Joana S.|last3=Capdevila|first3=Santiago|last4=Mosig|first4=Juan R.|last5=Perruisseau-Carrier|first5=Julien|title=The orbital angular momentum (OAM) multiplexing controversy: OAM as a subset of MIMO|journal=2015 9th European Conference on Antennas and Propagation (EuCAP)|date=2015|pages=1–5}}</ref> ===Code-division multiplexing=== [[Code-division multiplexing]] (CDM), [[code-division multiple access]] (CDMA) or [[spread spectrum]] is a class of techniques where several channels simultaneously share the same [[frequency spectrum]], and this spectral bandwidth is much higher than the bit rate or [[symbol rate]]. One form is frequency hopping, another is direct sequence spread spectrum. In the latter case, each channel transmits its bits as a coded channel-specific sequence of pulses called chips. Number of chips per bit, or chips per symbol, is the [[spreading factor]]. This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber or radio channel or other medium, and asynchronously demultiplexed. Advantages over conventional techniques are that variable bandwidth is possible (just as in [[statistical multiplexing]]), that the wide bandwidth allows poor signal-to-noise ratio according to [[Shannon–Hartley theorem]], and that multi-path propagation in wireless communication can be combated by [[rake receiver]]s. A significant application of CDMA is the [[Global Positioning System]] (GPS). [[File:telecommunication-multiplexing.svg|thumb|right|400px|Telecommunication multiplexing]]
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