通信外文翻译外文文献英文文献及译文

通信外文翻译外文文献英文文献及译文Communication System

A 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 suitable 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 inverse of the transmitter. It

changes 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 that noise is added to the signal. This unwanted noise may cause distorions

of sound in a telephone, or errors in a telegraph message or data.

Frequency Diversion Multiplexing

Frequency Diversion Multiplexing(FDM) is a one of analog technologies. A speech signal is 0~3 kHz, single sideband amplitude (SSB) modulation can be used to transfer speech signal to 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 "group" of 4 chEmnels, has been foimed it is

moved around the Lrunk network as a single unit. A hierarchy can be set up with

several channels fonning a "group". several groups a "supergroup" and several

"supergraup" eicher a "nmsrergroup" or "hypergroup".

Groups or supergroups are moved around as single units by the communications

equipment 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 each channel individually involves far more equipment 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 Multiplexing

It is possible, with pulse 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 each

end of the communication links to enable samples to be transmitted

in turn, from 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 signals

between samples can be reconstituted at the receiver.

Pulse Code Modulation

In analog modulation, the signal was used to modulate the amplitude or frequency of a carrier, directly. However, in digital modulation a stream of pulse, representing the original，is created. This stream is then used to modulate a carrier or alternatively is transmitted directly over a cable. Pulse Code Modulation (PCM) is one of the two techniques commonly 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 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 coding.

Optical Fiber Communications

Communication may be broadly defined as the transfer of information from one point to another. When the information is to be conveyed over any distance a

communication system is usually required. Within a communication system the information transfer is frequently achieved by superimposing or modulating the information on to an electromagnetic wave which acts as a carrier for the information

signal. This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation. Sophisticated techniques have been developed for this process by using electromagnetic carrier waves

operating at radio frequencies as well as microwave and millimeter wave frequencies. However，拻 communication?may also be achieved by using an electromagnetic

carrier which is selected from the optical range of frequencies.

In this case the information source provides an electrical signal to a 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 conversion

may be either a semiconductor laser 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,

phototransistor 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 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). Although often simpler to implement, analog modulation with an optical fiber communication system is less

efficient, 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, especially at high modulation frequencies. For these

reasons，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 laser drive circuit directly modulates the intensity of the semiconductor laser with the encoded digital signal. Hence a digital optical signal is launched into the optical fiber cable. The avalanche photodiode detector (APD) is followed by a fronted-end amplifier and equalizer or

filter to provide gain as well as linear signal processing and noise bandwidth reduction. Finally, the signal obtained is

decoded to give the original digital information.

Mobile Communication

Cordless Telephone Systems

Cordless telephone system are full duplex communication systems that use radio to connect a portable handset to a dedicated base station，which is then connected to a

dedicated telephone line with a specific telephone number on the public switched telephone network (PSTN) .In first generation cordless telephone systems

5(manufactured in the 1980s), the portable unit communications only to the dedicated

base unit and only over distances of a few tens of meters.

Early cordless telephones operate solely as extension telephones to a 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 allow

subscribers 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 page

using the cordless telephone. Cordless telephone systems 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 System

A cellular telephone system provides a wireless connection to the PSTN for any user location within the radio range of the system.

Cellular systems accommodate a

large number of users over a large geographic area, within a limited frequency spectrum. Cellular radio systems provide high quality service that is often comparable 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 technique called a handoff enables a call to proceed

uninterrupted 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 mobile

telephone switching office (MTSO), since it is responsible for connecting all mobiles to the PSTN in a cellular system. Each mobile

communicates via radio with one of the base stations and may be handed-off to any number of base stations throughout the duration of a call. The mobile station contains a transceiver, an antenna, and control circuitry，and may be mounted in a vehicle or used as a portable hand-held unit. The

base stations consists of several transmitters and receivers which simultaneously handle

full duplex communications and generally have towers which support several transmitting and receiving antennas. The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone lines

or 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 cellular subscribers and 5000 simultaneous conversations at a time, and

accommodates all billing and system maintenance functions, as well. In large cities, several MSCs are used by a single carrier.

Broadband Communication

As can be inferred from the examples of video phone and HDTV, the evolution of

future communications will be via broadband communication centered around video signals. The associated services make up a diverse set of high-speed and broadband

services ranging from video services such as video phone，video conferencing，video

surveillance, cable television (CATV) distribution, and HDTV distribution to the high-speed data services such as high-resolution image transmission, high-speed data

transmission, and color facsimile. The means of standardizing these various broadband

communication services so that they can be provided in an integrated manner is no other than the broadband integrated services digital network (B-ISDN). Simple put, therefore,

the future communications network can be said to be a broadband telecommunication

system based on the B-ISDN.

For realization of the B-ISDN, the role of several broadband communication

technologies is crucial. Fortunately, the remarkable advances 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 fiber

have advanced to considerable levels. Advances in high-speed device and integrated

circuit 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 standpoint, the B-ISDN has

finally 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 Digital 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 early 1980s with the objective of integrating narrowband services, expanded in scope with the inclusion of broadband services, leading to the standardization of the B-ISDN in late

1980抯 and establishing the concept of asynchronous transfer mode (ATM)

communication in process. In addition, standardization of various video signals is becoming finalized through the cooperation among such organizations as CCITT, the International Radio-communications Consultative Committee (CCIR), and the

International Standards Organization (ISO), and reference protocols for high-speed

packet communication are being standardized 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 communication

technologies will be used in conjunction with broadband standards to realize broadband communication networks. In the broadband communication network, the fiber optic

network will represent the physical medium for implementing broadband communication, while synchronous transmission will make possible the transmission of broadband service signals over the optical medium. Also, the B-ISDN will be essential

as the broadband telecommunication network established on the basis of 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 provided

through the B-ISDN are high-speed data communication services and video

communication services.

Image Acquisition

A TV camera is usually used to take instantaneous images and transform them into electrical signals, which will be further translated into binary numbers for the computer to handle. The 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 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 Processing

Image processing is for improving the quality of the images obtained. 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 Analysis

It 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 identifiable

because of the different brightness levels on either side of them. Using

certain 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 Comprehension

Image Comprehension means understanding what is in a scene. Matching the prestored 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 checked against the binary information representing the scene. Once a match occurs, an object is identified. The template matching process continues until all possible objects in the scene have been identified, otherwise it fails.

通信系统

一般的通信系统由下列部分组成:

信源。它产生信息,可以是写成的或口头的文字,或是某种形式的数据。

发信机。发信机把信息转换为信号,信号形式要适合在通信信道中传输。

通信信道。通信信道用于从发信机传送信号到接收机的媒质。信道可以是无

线电链路或者直接的有线连接。

接收机。接收机可以看作与发信机相反的东西。它把接收到的信号变回成信息,并把信息传给终端,这个终端可以是一台扬声器、电传打字机、或一个计算机数据库。

所有通信信道的一个不好的特征是信号上都叠加了噪声。这种人们不希望存

在的噪声会引起电话中声音失真,或者在电报报文或数据中出现的差错。频分复用_频分复用是一种模拟技术,一个话音信号的频段在〇3千赫,用单边带振幅调

制:SSB)可以把话音信号搬移到新的频段。例如,采用这种调制,搬移4路相类似_

的信号,使之均分520千赫频段。各路之间的间隙成为保护间隔,这些间隔使得在实际系统中允许有点频率误差及滤波不足等缺陷。

一旦这个新的基带信号:由4路组成的“群”:形成后,它就作为一个单一的单元在主干网中传送。可以构成一个分级的体系,即以若干路组成一个“群”, 若干“群”组成一个“超群”,再以若干“超群”组成一个“主群”或“极群”。

通信设备把群或超群作为一些单一的单元来传送。对于无线电设备来说,并不需知道含有多少路。只要有足够的带宽,无线电可以处理一个超群。群的大小采取折中考虑确定:因为处理每一路都需要独立的滤波器、调制器及振荡器,这比每个群所需的这些设备要多得多,但是若有一个部件失效,则将失去与这个群相关的全部话路。

时分复用

采用脉冲编码调制时,有可能利用抽样值之间的时间来传送来自其它电路的信号,这种技术称为时分复用,TDM)。要做到这一点,必须在通信链路的两端采用同步开关,使对来自各路的抽样值能一次轮流传送,于是好几个用户看起来都像在同时使用这条链路,虽然每个用户只有一些周期性的短时隙,抽样值之间的原来的模拟信号却可在接收机中重新构成。

脉冲编码调制

在模拟调制中,用信号来直接调制载波的振幅或频率。但在数字调制中则是产生一个脉冲流来代表原来的信号,然后用这个脉冲流来调制载波,或者直接在电缆中传输。脉冲编码调制是常用的两种数字调制技术之一。

所有的脉冲系统取决于按规定的时间间隔抽样的模拟信号的波形。对模拟话

音输入取样所产生的信号称为脉冲调制信号。在实际中它不是很有用,但可用作进一步形成脉冲编码调制信号的一个中间阶段。数字调制的大多数优点来自发送的

脉冲只有两个电平,这种系统称为二进制系统。在PCM中,把每个抽样值的大小变换成一个二进制数。脉冲编码调制过程有三步:抽样、量化和编码。

光纤通信

广义地说,把信息从一点传送到另一点就称为通信。当信息跨越一段距离被传送时,就需要一个通信系统。在通信系统中,信息传送时通过把信息叠加在电磁波上或对电磁波进行调制来实现,电磁波起着载送信号的作用。这一经过调制的载波随后被传送到要求到达的目的地,在那里被接收,并且通过解调还原成原始信息。在运用电磁载波的领域,高新技术得到进一步的发展,比如射频、微波以及毫米波的频率都被用来作为载波频率。在通信中,也可选择光波的频率作为载波频率。

信源^供电信号给发射机,发射机组成一个电子平台来驱动光源以完成对光载频的调制。光源是由发光二极管或半导体激光管构成的,它完成光一电转换。传输媒介由光纤组成。光接收机包括一个含光检测器的电路驱动平台,用以完成对已调光载波的解调。用于检测光信号和进行光一电转换的器件有:光电二极管 (P-N,P-I-N,或雪崩二极管:、发光三极管以及光敏电阻等。因此,在光系统链路的两端都要求有电接口,并且在现阶段,信号处理通常是通过电路实现的。

模拟或数字的信号均可用来调制光载波。模拟调制是指从光源处发射的连续

光强度的变化,而数字调制则不然,它是通过光强度离散的变化:比如有无光脉冲:来实现的。模拟调制在光系统调制中调制效率较低,而且与数字调制相比, 需要高得多的信噪比。模拟调制所必需的线性不总是来源于半导体光源,尤其是在高频调制中。基于上述原因,与数字光系统相比,模拟通信链路常被限制使用在更短的通信距离和更窄的带宽上。

首先,信源的数字信号被适当地编码以进行光传输。激光器的驱动电路通过这些已编码的数字信号来直接调制激光器的发光强度,然后数字光信号被注入光纤。在接收端,信号通过雪崩发光二极管:APD)后进入前置放大器和均衡器或滤波器,放

大器用来提高增益,滤波器用来对信号进行线性处理和减少噪声带宽, 最后,信号通过解码得到原始信号。

移动通信无绳电话系统

无绳电话系统是全双工通信系统。它通过无线电将手持机与一专用基站连接

起来,而后再接到PSTN上的某条电话线上。在第一代无绳电话系统:制造于20 世

纪80年代:中,手持设备仅能与一个专用的基本设备相连,而且工作距离也只有几十米。

早期的无绳电话只能起到电环延迟的作用,即把无线电收发机与一个PSTN 用户连接起来,且用于室内

最近产生^第二代无绳i话允许用户在都市:如伦敦或香港:中心所在的许多地方使用手持机。现在的无绳电话可以与寻呼机相连,以便使某用户先收到寻呼记录,然后再用无绳电话对此作出响应。当用户外出不在基站服务区域内而无法接收呼叫时,无绳电话系统就可以为这些受距离和移动限制的用户提供服务。典型的第二代:无绳电话的:基站所能覆盖的范围已达到几百米。

蜂窝电话系统

一个蜂窝电话系统可以为在该系统无线电收发范围内的任何用户提供一个到PSTN的无绳电话连接。在一定的频谱内,蜂窝系统可以在一个大区域中拥有相当多的用户。蜂窝无线电系统常可以提供比有线电话系统更高质量的服务。通过限制从每一个基站发射机到一个叫做蜂窝小区的范围来获得高容量,同一条无线电通道可以被其它远距离基站再次利用。当一个用户从某个小区移到另一个小

区时,采用一种复杂的交换技术就可以使摘机状态的手机连续地进行呼叫处理。

一个基本的蜂窝系统由移到站、基站和一个移动交换中心,MSC)组成。由于这个移动交换中心负责一个蜂窝系统所有移动电话到PSTN的连接,因而有时又被称为移动电话交换局,MTSO)。每一次通信都要借助某个基站的无线电波传送信号,并

且在整个呼叫过程中可以摘机,拨打其它任何基站的电话号码。一个移动站包括无线电收发机、天线和控制电路,这种控制电路可以安装在传输系统或手机里。基站包括几台发射机和接收机,能同时进行全双工通信,通常还有用以支持收、发无线电的无线塔。基站就像建立在一个蜂窝小区内所有用户和那些通过电话线或微波链接到MSC的集群呼叫连接之间的一座桥梁。MSC负责负责统一所有基站的操作及整个蜂窝系统到PSTN的连接。一个典型的MSC —次可以处理100000个蜂窝通信用户,同时进行5000个通话,并且完成所有计费和系统维护功能。在大城市中,一个独立的载波可以服务于若干个MSC。

宽带通信

从可视电话及高清电视的出现可以推断出未来通信的变革将是以视频信号为核心的宽带通信。这些相关的业务组成了多种的高速的宽带服务,其范围从视频服务,例如可视电话、会议电视、视频监视、有线电视分配器和高清电视分配器到高速数据服务,例如高分辨率图像传输、高速数据传输及彩色传真。将这些不同的宽带通信业务标准化的最好方法莫过于采用宽带综合业务数字网,这样能够使这些服务以集成的方式提供。简言之,未来的通信网可以比喻为基于宽带综合业务数字网之上的宽带通信系统。

对于宽带综合业务数字网的实现,几项宽带通信技术起着至关重要的作用。幸运的是,电子及光纤领域的显著进展导致了宽带通信技术的成熟。随着宽带综合业务数字网在光纤通信的基础上成为可能,光源、无源器件及光纤的相关制造技术已经有了相当水平的提高。针对宽带通信的高速器件和集成电路技术也有了令人瞩目的进步。取得显著进步的还有软件技术、信号处理技术和视频设备技术等。因此,从技术的角度看,宽带综合业务数字网已经最终达到了可以实现的程度。

另一方面,与宽带通信相关的标准化活动正在不断取得进展。围绕T1委员会的光同步网,SONET)标准逐步成为了 CCITT的同步数字体系标准,为基于光通信的

同步数字传输铺平了道路。随着综合窄带业务的开展,开始于20世纪80 年代早期的综合业务数字网的标准化活动逐步扩展到了宽带业务的范围,并于 80年代末期实现了宽带综合业务数字网的标准化,同时在其进程中建立了异步传输模式,ATM)的概念。此外,视频信号的标准通过几个相关的组织机构的合作,如国际电报电话咨询委员会,CCITT)、国际无线电咨询委员会,CCIR)和国际标准化组织,ISO)最终制定,高速分组通信的参考协议也通过国际标准化组织、国际电报电话咨询委员会及电气电子工程师协会,IEEE)实现了标准化。

种种这些努力使宽带通信得以实现。所以,20世纪90年代是成熟的宽带通信技术结合宽带标准被用于宽带通信网的年代。在宽带通信中,光纤网络代表实现宽带通信的物理媒介,与此同时,同步传输技术使得在光媒介上传送宽带信号成为可能。因此,建立在光媒介上和同步传送技术上的宽带通信网是实现宽带通信的最基本的条件,异步传输模式这一通信方式则使其变成现实。宽带综合业务数字网所提供的最重要的宽带业务就是高速数据通信业务和视频通信业务。图像获取

摄像机被用来获取实时图像并将它们转换成电信号。这些电信号又被进一步

转换成计算机处理的二进制数。摄像机在每一时刻进行逐行扫描,每行被分解成几百个像素,一幅图像被分解成几百行:如625行)。像素的亮度用一定比特的二进制_数表示,例如用8比特表示时,其二进制数的范围是0255,其较大的范围足以包容从实景中获取图像的可能的对比度。这些二进制数被存储在随机存取存储器:必须有很大的容量:中,以备计算机进一步处理。

图像处理

图像处理是用来改善所获取的图像的质量。首先,需要提高信噪比。这里噪声指使图像模糊的任何干扰、缺陷或色差;第二,借助各种计算机手段可以提高对比度,增强图像边缘的锐度。

图像分析

图像分析是为了获取图像中所有物体的轮廓。计算机通过检查存储在其中的

二进制图像信息来识别这些物体的特征。其边缘或边界由于其亮度与周围不同而

可以被识别。采用一定的算法,计算机程序能将图像中所有可能的边缘轮廓提取出来。图像分析也可以寻找线条间的结构和阴影。

图像识别

图像识别即识别图像是什么。方法是用先前存储的二进制视觉信号与二进制

形式表示的代表特殊物体的模式进行比较,这些方法是从人工智能借鉴来的技术,通常称为“模式匹配”。这个模式逐个与代表图像的二进制信息进行检查, 一旦匹配发生,一个物体就被识别。模式匹配过程一直持续到图像中所有的物体都被识别,否则,匹配失败。