通信外文翻譯外文文獻(xiàn)英文文獻(xiàn)及譯文

1、通信外文翻譯外文文獻(xiàn)英文文獻(xiàn)及譯文Communication SystemA generalized communication system has the following components:(a) Information Source. This produces a message which may be written or spoken words, or some form of data.(b) Transmitter. The transmitter converts the message into a signal, the form of which is su

2、itable for transmission over the communication channel.(c) Communication Channel. The communication channel is the medium used transmit the signal, from the transmitter to the receiver. The channel may be a radio link or a direct wire connection.(d) Receiver. The receiver can be thought of as the in

3、verse of the transmitter. Itchanges the received signal back into a message and passes the message on to its destination which may be a loudspeaker ， teleprinter or computer data bank.An unfortunate characteristic of all communication channels is thatnoise is added to the signal. This unwanted noise

4、 may cause distorions of sound in a telephone, or errors in a telegraph message or data.Frequency Diversion MultiplexingFrequency Diversion Multiplexing(FDM) is a one of analogtechnologies. A speech signal is 03 kHz, single sideband amplitude (SSB) modulation can be used to transfer speech signal to

5、 new frequency bands,four similar signals, for example, moved by SSB modulation to share the band from 5 to 20 kHz. The gaps between channels are known as guard spaces and these allow for errors in frequency, inadequate filtering, etc in the engineered system.Once this new baseband signal, a "g

6、roup" of 4 chEmnels, has been foimed it ismoved around the Lrunk network as a single unit. A hierarchy can be set up withseveral channels fonning a "group". several groups a "supergroup" and several"supergraup" eicher a "nmsrergroup" or "hypergroup&q

7、uot;.Groups or supergroups are moved around as single units by the communicationsequipment and it is not necessary for the radios to know how many channels are involved. A radio can handle a supergroup provided sufficient bandwidth is available. The size of the groups is a compromise as treating eac

8、h channel individually involves far moreequipment because separate filters, modulators and oscillators are required for every channel rather than for each group. However the failure of one module will lose all of the channels associated with a group.Time Diversion MultiplexingIt is possible, with pu

9、lse modulation systems, to use the between samples to transmit signals from other circuits. The technique is known as time diversion multiplexing (TDM). To do this, it is necessary to employ synchronized switches at eachend of the communication links to enable samples to be transmitted in turn, from

10、 each of several circuits. Thus several subscribers appear to use the link simultaneously. Although each user only has periodic short time slots, the original analog signalsbetween samples can be reconstituted at the receiver.Pulse Code ModulationIn analog modulation, the signal was used to modulate

11、 the amplitude or frequency of a carrier, directly. However, in digital modulation a stream of pulse, representing the original， is created. This stream isthen used to modulate a carrier or alternatively is transmitted directly over a cable. Pulse Code Modulation (PCM) is one of the two techniques c

12、ommonly used.All pulse systems depend on the analog waveform being sampled at regular intervals. The signal created by sampling our analog speech input is known as pulse amplitude modulation. It is not very useful in practice but is used as an intermediate stage towards forming a PCM signal. It will

13、 be seen later that most of the advantages of digital modulation come from the transmitted pulses having two levels only, this being known as a binary system. In PCM the height of each sample is converted into a binary number. There are three step in the process of PCM: sampling, quantizing and codi

14、ng.Optical Fiber CommunicationsCommunication may be broadly defined as the transfer of information from one point to another. When the information is to be conveyed over any distance acommunication system is usually required. Within a communication system the information transfer is frequently achie

15、ved by superimposing or modulating the information on to an electromagnetic wave which acts as a carrier for the informationsignal. This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation. Sophisticat

16、ed techniques have been developed for this process by using electromagnetic carrier wavesoperating at radio frequencies as well as microwave and millimeterwave frequencies. However , 拔 communication?may also be achieved by using an electromagneticcarrier which is selected from the optical range of f

17、requencies.In this case the information source provides an electrical signal toa transmitter comprising an electrical stage which drives an optical source to give modulation of the light-wave carrier. The optical source which provides the electrical-optical conversionmay be either a semiconductor la

18、ser or light emitting diode (LED).The transmission medium consists of an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation optical carrier. Photodiodes (P-N, P-I-N or avalanche) and , in some instances,phototr

19、ansistor and photoconductors are utilized for the detection of the optical signal and the electrical-optical conversion. Thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal processing is usually performed electrically.The optical carrier

20、 may be modulated by using either an analog or digital information signal. Analog modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the light intensity are obtained (i.e. on-off pulses). Althou

21、gh often simpler toimplement, analog modulation with an optical fiber communication system is lessefficient, requiring a far higher signal to noise ratio at the receiver than digital modulation. Also, the linearity needed for analog modulation is not always provided by semiconductor optical source,

22、especially at high modulation frequencies. For thesereasons ， analog optical fiber communications link are generally limited to shorter distances and lower bandwidths than digital links.Initially, the input digital signal from the information source is suitably encoded for optical transmission. The

23、laser drive circuit directly modulates the intensity of the semiconductor laser with the encoded digital signal. Hence a digital optical signal islaunched into the optical fiber cable. The avalanche photodiodedetector (APD) is followed by a fronted-end amplifier and equalizer or filter to provide ga

24、in as well as linear signal processing and noise bandwidth reduction. Finally, the signal obtained isdecoded to give the original digital information.Mobile CommunicationCordless Telephone SystemsCordless telephone system are full duplex communication systems that use radio to connect a portable han

25、dset to a dedicated base station which is then connected to adedicated telephone line with a specific telephone number on the public switched telephone network (PSTN) .In first generation cordless telephone systems5(manufactured in the 1980s), the portable unit communications only to the dedicatedba

26、se unit and only over distances of a few tens of meters.Early cordless telephones operate solely as extension telephones toa transceiver connected to a subscriber line on the PSTN and are primarily for in-home use.Second generation cordless telephones have recently been introduced which allowsubscri

27、bers to use their handsets at many outdoor locations within urban centers such as London or Hong Kong. Modern cordless telephones are sometimes combined with paging receivers so that a subscriber may first be paged and then respond to the pageusing the cordless telephone. Cordless telephone systems

28、provide the user with limited range and mobility, as it is usually not possible to maintain a call if the user travels outside the range of the base station. Typical second generation base stations provide coverage ranges up to a few hundred meters.Cellular Telephone SystemA cellular telephone syste

29、m provides a wireless connection to the PSTN for any user location within the radio range of the system.Cellular systems accommodate alarge number of users over a large geographic area, within a limited frequency spectrum. Cellular radio systems provide high quality service that is often comparable

30、to that of the landline telephone systems. High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic area called a cell so that the same radio channels may be reused by another base station located some distance away. A sophisticated switching techniqu

31、e called a handoff enables a call to proceeduninterrupted when the user moves from one cell to another.A basic cellular system consists of mobile station, base stations and a mobile switching center (MSC). The Mobile Switching Center is sometimes called a mobiletelephone switching office (MTSO), sin

32、ce it is responsible for connecting all mobiles to the PSTN in a cellular system. Each mobilecommunicates via radio with one of the base stations and may be handed- off to any number of base stations throughout the durationof a call. The mobile station contains a transceiver, an antenna, and control

33、 circuitry ， and may be mounted in a vehicle or used as a portable hand-held unit. Thebase stations consists of several transmitters and receivers which simultaneously handlefull duplex communications and generally have towers which support several transmitting and receiving antennas. The base stati

34、on serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone linesor microwave links to the MSC. The MSC coordinates the activities of all the base stations and connects the entire cellular system to the PSTN. A typical MSC handles 100000 cellul

35、ar subscribers and 5000 simultaneous conversations at a time, andaccommodates all billing and system maintenance functions, as well.In large cities, several MSCs are used by a single carrier.Broadband CommunicationAs can be inferred from the examples of video phone and HDTV, the evolution offuture c

36、ommunications will be via broadband communication centered around video signals. The associated services make up a diverse set of high-speed and broadbandvideoservices ranging from video services such as video phone conferencing ， videosurveillance, cable television (CATV) distribution, and HDTV dis

37、tribution to the high-speed data services such as high-resolution image transmission, high-speed datatransmission, and color facsimile. The means of standardizing these various broadbandcommunication services so that they can be provided in an integrated manner is no other than the broadband integra

38、ted services digital network (B-ISDN). Simple put, therefore,the future communications network can be said to be a broadband telecommunicationsystem based on the B-ISDN.For realization of the B-ISDN, the role of several broadband communicationtechnologies is crucial. Fortunately, the remarkable adva

39、nces in the field of electronics and fiber optics have led to the maturation of broadband communication technologies.As the B-ISDN becomes possible on the optical communication foundation, the relevant manufacturing technologies for light-source and passive devices and for optical fiberhave advanced

40、 to considerable levels. Advances in high-speed device and integratedcircuit technologies for broadband signal processing are also worthy of close attention. There has also been notable progress in software, signal processing, and video equipment technologies. Hence, from the technological standpoin

41、t, the B-ISDN hasfinally reached a realizable state.On the other, standardization activities associated with broadband communication have been progressing. The Synchronous Optical Network (SONET) standardization centered around the T1 committee eventually bore fmit in the form of the Synchronous Dig

42、ital Hierarchy (SDH) standards of the International Consultative Committee in Telegraphy and Telephony (CCITT), paving the way for synchronous digital transmission based on optical communication. The standardization activities of the 5integrated services digital network (ISDN), which commenced in ea

43、rly 1980swith the objective of integrating narrowband services, expanded in scope with the inclusion of broadband services, leading to the standardization of the B-ISDN in late1980 擔(dān) and establishing the concept of asynchronous transfer mode (ATM)communication in process. In addition, standardizatio

44、n of various video signals is becoming finalized through the cooperation among such organizations as CCITT, the International Radio-communications Consultative Committee (CCIR), and theInternational Standards Organization (ISO), and reference protocols for high-speedpacket communication are being st

45、andardized through ISO, CCITT, and the Institute of Electrical and Electronics Engineer (IEEE).Various factors such as these have made broadband communication realizable.5Therefore, the 1990s is the decade in which matured broadband communicationtechnologies will be used in conjunction with broadban

46、d standards to realize broadband communication networks. In the broadband communication network, the fiber opticnetwork will represent the physical medium for implementing broadband communication, while synchronous transmission will make possible the transmission of broadband service signals over th

47、e optical medium. Also, the B-ISDN will be essentialas the broadband telecommunication network established on the basisof optical medium and synchronous transmission and ATM is the communication means that enables the realization of the B-ISDN. The most important of the broadband services to be prov

48、idedthrough the B-ISDN are high-speed data communication services and videocommunication services.Image AcquisitionA TV camera is usually used to take instantaneous images and transform them into electrical signals, which will be further translatedinto binary numbers for the computer to handle. The

49、TV camera scans one line at a time. Each line is further divided into hundreds of pixels.The whole frame is divided into hundreds (for example, 625) of lines.The brightness of a pixel can be represented by a binary number with certain bits, for example, 8 bits. The value of the binary number varies

50、from 0 to 255, a range great enough to accommodate all possible contrast levels of images taken from real scene.These binary numbers are sorted in an RAM (it must have a great capacity) ready for processing by the computer.Image ProcessingImage processing is for improving the quality of the images o

51、btained. First, it is necessary to improve the signal-to-noise ratio. Here noise refers to any interference flaw or aberation that obscure the objects on the image. Second, it is possible to improve contrast, enhance sharpness of edges between images through various computational means.Image Analysi

52、sIt is for outlining all possible objects that are included in the scene. A computer program checks through the binary visual information in store for it and identifies specific feature and characteristics of those objects. Edges or boundaries are identifiablebecause of the different brightness leve

53、ls on either side of them.Usingcertain algorithms, the computer program can outline all possible boundaries of the objects in the scene. Image analysis also looks for textures and shadings between lines.Image ComprehensionImage Comprehension means understanding what is in a scene. Matching the prest

54、ored binary visual information with certain templates which represent specific objects in a binary form is technique borrowed from artificial intelligence, commonly referred to as "templeite matching"emplate matching? One by one,the templates are checkedagainst the binary information repre

55、senting the scene. Once a matchoccurs, an object is identified. The template matching process continues until all possible objects in the scene have been identified, otherwise it fails.通信系統(tǒng)一般的通信系統(tǒng)由下列部分組成:信源。它產(chǎn)生信息, 可以是寫成的或口頭的文字, 或是某種形式的數(shù)據(jù)。發(fā)信機(jī)。發(fā)信機(jī)把信息轉(zhuǎn)換為信號(hào), 信號(hào)形式要適合在通信信道中傳輸。通信信道。通信信道用于從發(fā)信機(jī)傳送信號(hào)到接收機(jī)的媒質(zhì)。信道

56、可以是無線電鏈路或者直接的有線連接。接收機(jī)。接收機(jī)可以看作與發(fā)信機(jī)相反的東西。它把接收到的信號(hào)變回成信息 , 并把信息傳給終端, 這個(gè)終端可以是一臺(tái)揚(yáng)聲器、電傳打字機(jī)、或一個(gè)計(jì)算機(jī)數(shù)據(jù)庫。所有通信信道的一個(gè)不好的特征是信號(hào)上都疊加了噪聲。這種人們不希望存在的噪聲會(huì)引起電話中聲音失真, 或者在電報(bào)報(bào)文或數(shù)據(jù)中出現(xiàn)的差錯(cuò)。頻分復(fù)用頻分復(fù)用是一種模擬技術(shù)，一個(gè)話音信號(hào)的頻段在。3千赫，用單邊帶振幅 調(diào)制:SSB）可以把話音信號(hào)搬移到新的頻段。例如，采用這種調(diào)制，搬移4路 相類似_的信號(hào) , 使之均分520 千赫頻段。各路之間的間隙成為保護(hù)間隔, 這些 間隔使得在實(shí)際系統(tǒng)中允許有點(diǎn)頻率誤差及濾波不足等

57、缺陷。一旦這個(gè)新的基帶信號(hào): 由 4 路組成的“群”: 形成后 , 它就作為一個(gè)單一的單元在主干網(wǎng)中傳送?？梢詷?gòu)成一個(gè)分級(jí)的體系, 即以若干路組成一個(gè)“群”, 若干“群”組成一個(gè)“超群”, 再以若干“超群”組成一個(gè)“主群”或“極群”。通信設(shè)備把群或超群作為一些單一的單元來傳送。對(duì)于無線電設(shè)備來說, 并 不需知道含有多少路。只要有足夠的帶寬, 無線電可以處理一個(gè)超群。群的大小采取折中考慮確定: 因?yàn)樘幚砻恳宦范夹枰?dú)立的濾波器、調(diào)制器及振蕩器, 這 比每個(gè)群所需的這些設(shè)備要多得多, 但是若有一個(gè)部件失效, 則將失去與這個(gè)群相關(guān)的全部話路。時(shí)分復(fù)用采用脈沖編碼調(diào)制時(shí), 有可能利用抽樣值之間的時(shí)間來

58、傳送來自其它電路的信號(hào)，這種技術(shù)稱為時(shí)分復(fù)用，TDM）。要做到這一點(diǎn)，必須在通信鏈路的兩端采用同步開關(guān) , 使對(duì)來自各路的抽樣值能一次輪流傳送, 于是好幾個(gè)用戶看起來都像在同時(shí)使用這條鏈路, 雖然每個(gè)用戶只有一些周期性的短時(shí)隙, 抽樣值之間的原來的模擬信號(hào)卻可在接收機(jī)中重新構(gòu)成。脈沖編碼調(diào)制在模擬調(diào)制中, 用信號(hào)來直接調(diào)制載波的振幅或頻率。但在數(shù)字調(diào)制中則是產(chǎn)生一個(gè)脈沖流來代表原來的信號(hào), 然后用這個(gè)脈沖流來調(diào)制載波, 或者直接在電纜中傳輸。脈沖編碼調(diào)制是常用的兩種數(shù)字調(diào)制技術(shù)之一。所有的脈沖系統(tǒng)取決于按規(guī)定的時(shí)間間隔抽樣的模擬信號(hào)的波形。對(duì)模擬話音輸入取樣所產(chǎn)生的信號(hào)稱為脈沖調(diào)制信號(hào)。在實(shí)際中它不是很有用, 但可用作進(jìn)步形成脈沖編碼調(diào)制信號(hào)的一個(gè)中間階段。數(shù)字調(diào)制的大多數(shù)優(yōu)點(diǎn)來自發(fā)送脈沖只有兩個(gè)電平，這種系統(tǒng)稱為二進(jìn)制系統(tǒng)。在 PCMfr，把每個(gè)抽樣值的大 小變換成一個(gè)二進(jìn)制數(shù)。脈沖編碼調(diào)制過程有三步: 抽樣、量化和編碼。光纖通信廣義地說, 把信息從一點(diǎn)傳送到另一點(diǎn)就稱為通信。當(dāng)信息跨越一段距離被傳送時(shí) , 就需要一