Analysis and Research of Multiplexing System

Dec.2001Journal of China University of Mining &. TechnologyVol.11 No. 2Analysis and Research ofMultiplexing SystemGUO Hui -ling(郭惠玲), WANG Tong- jie(王仝杰)， LIU Yue-nan(刘越男)( Department of Mechanical and Electronic Engineering, CUMT, Beiing 100083， P. R. China )Abstract: The development of optical transmission was summarized. The multiplexing system was showin detail. The concepts, characteristic, key technology, expand trend and application prospect of fre-quency - division multiplexing ，time-division multiplexing， code- division multiplexing and wave divisionmultiplexing were ilustrated.Key words: multiplexing; time -division multiplexing ; code. -division multiplexing; wave -division multi-plexingCLC number: TN 914Document code: AArticle ID: 1006- 1266(2001 )02-0183-05With the development of the social economy ，While using broadband symbols, every channelthe requirements for information increase rapidly.with different frequency transmits its own symbolsThe amount of information increases exponentially.simultaneously without interrupting each other ，Communication capacity is required higher andwhich we call frequency- -division multiplexing.higher because of the fast increase of communicationFDM is usually used in broadband networks.service. The potential huge bandwidth capacity ofFDM system has two or more channels. It30THz of optical fiber makes the optical fiber com-performance mainly depends on the bandwidth andmunication become the most important technologyposition of up frequency band and down frequencyto support the increase of communication service.band. Another factor has to be considered is theIn the field of data communication we have re-bandwidth of up data streaming channel. Thesearched the technology of multiplexing. It makeschoice for bandwidth depends on the achieved datamultiple communication channels share the samerate and the ratio of up data rate and down dataline, which can make adequate use of the communi-rate. In order to support a wider range of data ratecation channel. Here, we discuss frequency- divisionand the ratio of up and down data rate while trans-multiplexing ( FDM )，time- division multiplexingmitted bi-directionally, FDM system has to raise the(TDM)，code-division multiplexing (CDM)，andcapacity of alterable- bandwidth channel. In thiswave- division multiplexing (WDM).way, the complexity of analog filter is usually to be1 Frequency-Division Multiplexingincreased[2].In traditional frequency- division method， theA pair of copper wires adopt FDM technologyof single sideband suppressed carrier transmission inparallel data stream is transmitted by the way of di-the time of analog communication, ranging from 3-viding the frequency band into umpty non-orthogo -line and12-line of open wire carrier line, 60-line and中国煤化士revig end the sub-p of filters. The virtue132 line of symmetrical cable to 960- line of smallYHCNMHGimmediate. And thecoaxial cable and 1800-line of middle coaxial ca-shortcoming is the utilization ratio of frequencyble[1.Receivedglaty: 299 - 06 - 18Biography!孔影埔i ling( 1963- ) , female, from Henan Province , associate professor, engaged in the research on computer networks andtelecommunication.184Journal of China University of Mining &. TechnologyVol.11 No. 2spectrum is very low and protect frequency band has64 kb/s by pulse code modulation. TDM is used to .to be left. There are many difficulties for multiplemultiplex it to2, 8，34, 140, 565 m and eventuallyfilters to implement. The common used FDMenter into the PDH optical communication time[".scheme is that data stream is transmitted by the .If we let every communication occupy totalsub- channel composed of a group of orthogonal car-band in the sequence of time separately and repeatriers and the frequency spectrum of sub-channelthe process periodically while using baseband sig-may overlap, which raises the utilization ratio ofnals, the line is divided into many logic channels ac-frequency spectrum.cording to time. We call it time- -division multiplex -On the other hand, algorithm may be used toing (TDM). .decrease the complexity of the system-3l. In tradi- .Unlike FDM，TDM system transmits data intional digital communication systems symbol se-different time slicing within a single band. Sharing .quence is modulated on a carrier to be transmitted inchannel bandwidth with time is coordinated by over-series. The frequency spectrum of every symbolframe. An over-frame includes up and down datamay occupy total available bandwidth of the chan-transmission time slice， silence time slice， up andnel. Orthogonal frequency- division multiplexingdown data transmission time slice and another(OFDM) is a kind of parallel data transmission sys-lence time slice. The time slice of up and down datatem. It is composed of n sub-carriers of upper inter-channel is integral multiple of symbol cycle. Anval. They modulate a single line of data informationover-frame can be shown as A-Q B Q, here A andseparately. After modulation, the symbols of n sub-B means the amount of symbol cycle of up andcarriers are overlapped to send simultaneously.down data transmissions separately; Q means siTherefore，a symbol occupies a portion of totallence time slice, which is used to eliminate the prop-available bandwidth of the channel. These sub car-agation delay of channel and the echo response be-riers keep the orthogonality of frequency spectrumtween receiving time slice. The values of A and Bduring a symbol cycle by choosing the interval ofcan be determined by the ratio of up and down datarate[2].carriers. Signals of sub-carriers are overlapped itOTDM is the prospect for optical communica-frequency spectrum. The receiving end can makeuse of the orthogonality of carriers to recover thetion to develop. It has the following characteristics.1) It raises the transmission rate. Signals cansent signals without distortion, which raise the uti-lization ratio of frequency of spectrum. Comparingbe arranged according to time sequence compactlywith single carrier system with same transmissionand unrepeatablely.2) Sent signals are optical pulse signals. Thrate，symbol cycle of OFDM prolongs n times.optical pulse sequence is only sent in prescriptiveUsually it is longer than the delay extension olchannels，which enhances the ability to eliminatetime slice.3) OTDM utilizes only one optical carrier todisturbance among codes.FDM system is not only suitable for digital TVtransmit multiple light pulse signals, which can im-fields and stereo broadcast, but also traditional highprove the capacity of system greatly.4) OTDM can easily realize to assign the chan-speed telephone connection and wireless LAN. Especially，it is suitable for the integrated service ofnel as needed[5].中国煤化工ate sending timing. Inwireless video， wireless image，audio and data. Itcan realize multimedia communicationl4l.MYHCN MH G of ll-optical communi-cation, an eftective network management and con-2 Time -Division Multiplexing (TDM)trol system has to be researched and developed. TheDuring the transition from analog communica-amplitude of optical signals will attenuate exponen-tion to dig分毁unication, an analog audio sig-tially with the distance varying while transmittingnal of 4 kHz can be converted into digital signal ofthrough optical fiber because of the loss. Thus mag-GUO Hui -ling et al.Analysis and Research of Multiplexing System185nification technology has to be adopted to raise thedition, choosing light source and threshold value arepower of light signals. Optical dispersion exists inalso very effective on the performance of system-8.the optical communication besides loss. Optical dis-CDM technology is based on spread spectrumpersion can lead to disturbance among codes. There-communication. There are two kinds of spread spec-fore，measures must be taken to reproduce opticaltrum schemes. One is direct spread spectrum (DS)signalsEI].technology. The other is frequency hopping spreadNow OTDM is very popular in the networks.spectrum technology (FH)[8].The focus of research is to multiplex in optical field.At present analog cellular mobile communica-In present there are three research directions: thetion has improved a lot. But it cannot satisfy the de-integration of higher speed and time division tech-mands. To increase the capacity of mobile commu-nology; the utility of TDM and high-speed net-nication is becoming the urgent affair. CDM is aworks and all-optical packet networks. With thevery effective approach. Each user is assigned amaturating optical processing technology and opticalpseudocode. So adjacent district can share the frelogic technology all- -optical packet network will be-quency band in common without reusing frequency.come the leading technology of telecommunicationCDM develops very quickly because of its attractivenetworks56.7.characteristic. CDM is one of the spread spectrum3 Code-Division Mul tiplexing (CDM)communication techniques. It has many virtues, forWith the development of multiple access opticalexample，strong anti-jamming， anti- multilevel anetworks, multi-address cutoverbased on all opticaltenuation，high reliability for information transmis-system comes forth. As a rising optical communica-sion，realizing the large capacity of system and CDtion technology, OCDMA combines the virtues ofMA. It also has soft out- of -district switch code.CDMA with that of optical communication. Its ba-CDM is mainly applied in digital cellular com-sic principle is similar to that of ECDM. Opticalmunication system, satellite communication, micro-pulses encode every bit of data stream according to acellular communication and so on. GSM， whichrule on the sending node，which forms a high- speedmakes use of CDM，succeeds in holding most of theoptical sequence. Each pulse is called a code slice.global market quotient.The encoded optical sequence is sent to the receivingThe key technology on CDM has been renode through optical fiber and star coupler. On re-searched in 863 Plan since 1994 in our country.ceiving end if decoder and encoder match well theWe' ve mastered some important technology. We' recoding signal output is self- correlative. The maxi-trying to form our own CDM system device enter-mum self-correlative peak value can be achieved.prise.Otherwise, the output is mutual-correlative, whichcan be received by optical threshold value switch or4 Wave-Division Multiplexing (WDM)high- speed photoelectric receiver. The output elec-In traditional optical transmission by TDM thetric signal is compared with threshold value in com-multiplexing and demultiplexing of the branch sig -parator unit. If the signal value is bigger than thenals, sending and receiving units, clock pick-up cir-threshold value the output is not 0. Otherwise thecuit and signal reproducer work at high multiplexingoutput is 0, which can recover the original data[8].signal rate speed. So the device rate and bandwidth .The key technology of CDM includes code type中国煤化ismission rate to raise.design and encoder and decoder design. The proper-:THCNM H G'_ptical transmission willty of code type can influence the performance of sys-meet wth ditticultes too. In a system with eachtem，such as data transmission rate, signal-to-noisechannel carrying 2. 5 Gbps ( billion bits perratio and number of users. Code type design insecond)，up to 10 Gbps can be delivered a secondcludes corr月秀数搪aracteristic of code, orthogonali-by the optical fiber. WDM is the most effectivety, code size and implementation complexity. In ad-method to raise the capacity of optical fiber channel.186Journal of China University of Mining &. TechnologyVol.11 No. 2WDM is a technology that puts data from dif-3) Access networkferent sources together on an optical fiber， withThere is an important technology in WDM-each signal carried on its own separate light wave-dense wave- -division multiplexing (DWDM)，whichlength of 1 or several hundreds of nanometer. Andhas become the focus of telecommunication networklight carriers with different wavelength will be sepa-development. DWDM applied in Jingjiu Railway is arated at receiving end. In fact the nature of WDM issuccessful example.FDM. Each channel is implemented by dividing fre-5 Concl usionsquency fields. Since each channel is demultiplexedat the receiving end of the transmission back intoThe economic performance of high bit rate sys-the original source， different data formats beingtem rises exponentially. The appearance of high-transmitted at different data rates can be transmit-speed system increases the service transmissionted together-9].pacity. Only small portion of optical fiber band-Using WDM system in optical communicationwidth can be utilized if single high-speed is adoptedtrunk networks has the following virtues.merely. In order to improve the utilization ratio of1) Huge resource of bandwidth can be madeoptical fiber it is necessary to use multiple channeluse of. The transmission capacity of a single opticalsystem.fiber enlarges many times.It's well known that FDM is mainly used in2) Because the wavelength of signal travellinganalog communication time. It's very complicatedin a optical fiber is separate with each other the sig-because it requires high linearity of system and strictnals with different characteristic can be transmittedaccuracy of device. In addition, frequency band re-to integrate and separate many kinds of telecommu-source is very expensive. The user’ s requirementsnication service including digital signals and analogcannot be satisfied within limited frequency band.signals.FDM has been stopped developing in our country.3) Using WDM routing technology to switchTwo factions- TDM and CDM are formed fromand recover networks makes a solid foundation foanalog communication time to digital communicationrealizing transparent and high survival optical net-time. TDM is very popular in today's network andworks in the futurel9].TDM technology is very mature. But its capacity is4) WDM device is passive device. It has greatsmall, which is only twice of analog. Using TDMflexibility in networks design and construction be-to raise transmission capacity is close to the limit ofcause of its simple structure, small size and reliablesilicon, gallium and arsenic, which has almost no la-stability.tent capacity. So the unique solution should turn to5) WDM cannel is transparent to data format.optical multiplexing. There are many kinds of opti-WDM system has three normal structures.cal multiplexing. At present only WDM has beerThey are single fiber unidirectional multiplexingused commercially on a large scale. It's the most ef-transmission, single fiber bi -directional multiplexingfective way to make use of optical fiber band widthtransmission and optical add- drop multiplexingnow. OCDM is a new technology in recent years.transmission.Its virtues are high utilization ratio of frequencyWDM is used widely because of its virtue. It'sband, convenient switching and fine security. But itmainly applied in the following fields.中国煤化工lance and doubtable sta-1) Long distance communication trunk trans-:TYHCNMH Gplication prospect, butmissionsome technology has to be perfected and matured.2) Regional computer networkGUO Hui-ling et al.Analysis and Research of Multiplexing System187Preference[1]张煦.波分多路应用引申至市内网[J].光通讯研究,1998,(3):9-15.[2] 潘亚淘,周 宏,陈 健.下一代的数字用户环路技术[J].电信科学,2000, 16(4):28-33.[3]佟 学俭,乐光新.无线衰落信道中的正交频分复用[J].无线通讯技术,2000,9(4):14-17.[4]潘武.正交频分复用技术在宽带通讯中的应用[J].移动通讯,1997.(3):32-37.[5] 于洋。光时分复用技术[J].通讯世界,2000,6(9):26-27.[6] 于洋.全光通信及其关键技术[J].现代通信,2001,(1):12-14.[7]曹志刚，现代通信原理[M].北京:清华大学出版社,1992.172-190.[8] 张美敦,罗忠生。光码分多址网络技术[J].电信快报,2000,(4):5-9.[9]贺葵。波分复用在光纤网中的应用[J].电信工程技术与标准化,2000,(5):25-27.(上接第177页)References[1] 中国建筑科学研究院.建筑桩基技术规范(JGJ94-94)[M].北京:中国建筑工业出版社，1995.[2]朱国甫 ,李光煜.确定桩侧摩阻力曲线的约束样条拟合方法[J].岩土力学,1994,15(3):1-8.[3] 衡朝阳，吴跃祖,任仲民,等.灌注桩内力测试异常现象分析[A].中国建设科技文库城乡建设卷[C].北京:中国建材工业出版社,1998.[4]费鸿庆,王燕.黄土地基中超长钻孔灌注桩工程性状研究[J].岩土工程学报,2000,22(5):576-580.[5]刘金砺.桩基础设计与计算[M].北京:中国建筑工业出版社,1990.[6]桩基工程手册编写委员会.桩基工程手册[M].北京:中国建筑工业出版社,1995.[7]衡朝阳,王玉海.湿陷性黄土地基处理方案试验研究[J].电力勘测,1998,18<2):9-15..中国煤化工MHCNMHG