关于汽车的外文翻译

关于汽车的外文翻译（共5篇）

关于汽车的外文翻译 篇1

Automobile Brake System

The braking system is the most important system in cars. If the brakes fail, the result can be disastrous. Brakes are actually energy conversion devices, which convert the kinetic energy (momentum) of the vehicle into thermal energy (heat).When stepping on the brakes, the driver commands a stopping force ten times as powerful as the force that puts the car in motion. The braking system can exert thousands of pounds of pressure on each of the four brakes.

Two complete independent braking systems are used on the car. They are the service brake and the parking brake.

The service brake acts to slow, stop, or hold the vehicle during normal driving. They are foot-operated by the driver depressing and releasing the brake pedal. The primary purpose of the brake is to hold the vehicle stationary while it is unattended. The parking brake is mechanically operated by when a separate parking brake foot pedal or hand lever is set.

The brake system is composed of the following basic components: the “master cylinder” which is located under the hood, and is directly connected to the brake pedal, converts driver foot’s mechanical pressure into hydraulic pressure. Steel “brake lines” and flexible “brake hoses” connect the master cylinder to the “slave cylinders” located at each wheel. Brake fluid, specially designed to work in extreme conditions, fills the system. “Shoes” and “pads” are pushed by the slave cylinders to contact the “drums” and “rotors” thus causing drag, which (hopefully) slows the car.

The typical brake system consists of disk brakes in front and either disk or drum brakes in the rear connected by a system of tubes and hoses that link the brake at each wheel to the master cylinder (Figure).

Basically, all car brakes are friction brakes. When the driver applies the brake, the control device forces brake shoes, or pads, against the rotating brake drum or disks at wheel. Friction between the shoes or pads and the drums or disks then slows or stops the wheel so that the car is braked.

In most modern brake systems (see Figure 15.1), there is a fluid-filled cylinder, called master cylinder, which contains two separate sections, there is a piston in each section and both pistons are connected to a brake pedal in the driver’s compartment. When the brake is pushed down, brake fluid is sent from the master cylinder to the wheels.

At the wheels, the fluid pushes shoes, or pads, against revolving drums or disks. The friction between the stationary shoes, or pads, and the revolving drums or disks slows and stops them. This slows or stops the revolving wheels, which, in turn, slow or stop the car.

Automobile Brake System

The braking system is the most important system in cars. If the brakes fail, the result can be disastrous. Brakes are actually energy conversion devices, which convert the kinetic energy (momentum) of the vehicle into thermal energy (heat).When stepping on the brakes, the driver commands a stopping force ten times as powerful as the force that puts the car in motion. The braking system can exert thousands of pounds of pressure on each of the four brakes.

Two complete independent braking systems are used on the car. They are the service brake and the parking brake.

The service brake acts to slow, stop, or hold the vehicle during normal driving. They are foot-operated by the driver depressing and releasing the brake pedal. The primary purpose of the brake is to hold the vehicle stationary while it is unattended. The parking brake is mechanically operated by when a separate parking brake foot pedal or hand lever is set.

The brake system is composed of the following basic components: the “master cylinder” which is located under the hood, and is directly connected to the brake pedal, converts driver foot’s mechanical pressure into hydraulic pressure. Steel “brake lines” and flexible “brake hoses” connect the master cylinder to the “slave cylinders” located at each wheel. Brake fluid, specially designed to work in extreme conditions, fills the system. “Shoes” and “pads” are pushed by the slave cylinders to contact the “drums” and “rotors” thus causing drag, which (hopefully) slows the car.

The typical brake system consists of disk brakes in front and either disk or drum brakes in the rear connected by a system of tubes and hoses that link the brake at each wheel to the master cylinder (Figure).

Basically, all car brakes are friction brakes. When the driver applies the brake, the control device forces brake shoes, or pads, against the rotating brake drum or disks at wheel. Friction between the shoes or pads and the drums or disks then slows or stops the wheel so that the car is braked.

In most modern brake systems (see Figure 15.1), there is a fluid-filled cylinder, called master cylinder, which contains two separate sections, there is a piston in each section and both pistons are connected to a brake pedal in the driver’s compartment. When the brake is pushed down, brake fluid is sent from the master cylinder to the wheels.

At the wheels, the fluid pushes shoes, or pads, against revolving drums or disks. The friction between the stationary shoes, or pads, and the revolving drums or disks slows and stops them. This slows or stops the revolving wheels, which, in turn, slow or stop the car.

The brake fluid reservoir is on top of the master cylinder. Most cars today have a transparent r reservoir so that you can see the level without opening the cover. The brake fluid level will drop slightly as the brake pads wear. This is a normal condition and no cause for concern. If the level drops noticeably over ashort period of time or goes down to about two thirds full, have your brakes checked as soon as possible. Keep the reservoir covered except for the amount of time you need to fill it and never leave a cam of brake fluid uncovered. Brake fluid must maintain a very high boiling point. Exposure to air will cause the fluid to absorb moisture which will lower that boiling point.

The brake fluid travels from the master cylinder to the wheels through a series of steel tubes and reinforced rubber hoses. Rubber hoses are only used in places that require flexibility, such as at the front wheels, which move up and down as well as steer. The rest of the system uses non-corrosive seamless steel tubing with special fittings at all attachment points. If a steel line requires a repair, the best procedure is to replace the compete line. If this is not practical, a line can be repaired using special splice fittings that are made for brake system repair. You must never use copper tubing to repair a brake system. They are dangerous and illegal.

Drum brakes, it consists of the brake drum, an expander, pull back springs, a stationary back plate, two shoes with friction linings, and anchor pins. The stationary back plate is secured to the flange of the axle housing or to the steering knuckle. The brake drum is mounted on the wheel hub. There is a clearance between the inner surface of the drum and the shoe lining. To apply brakes, the driver pushes pedal, the expander expands the shoes and presses them to the drum. Friction between the brake drum and the friction linings brakes the wheels and the vehicle stops. To release brakes, the driver release the pedal, the pull back spring retracts the shoes thus permitting free rotation of the wheels.

Disk brakes, it has a metal disk instead of a drum. A flat shoe, or disk-brake pad, is located on each side of the disk. The shoes squeeze the rotatin

g disk to stop the car. Fluid from the master cylinder forces the pistons to move in, toward the disk. This action pushes the friction pads tightly against the disk. The friction between the shoes and disk slows and stops it. This provides the braking action. Pistons are made of either plastic or metal. There are three general types of disk brakes. They are the floating-caliper type, the fixed-caliper type, and the sliding-caliper type. Floating-caliper and sliding-caliper disk brakes use a single piston. Fixed-caliper disk brakes have either two or four pistons.

The brake system assemblies are actuated by mechanical, hydraulic or pneumatic devices. The mechanical leverage is used in the parking brakes fitted in all automobile. When the brake pedal is depressed, the rod pushes the piston of brake master cylinder which presses the fluid. The fluid flows through the pipelines to the power brake unit and then to the wheel cylinder. The fluid pressure expands the cylinder pistons thus pressing the shoes to the drum or disk. If the pedal is released, the piston returns to the initialposition, the pull back springs retract the shoes, the fluid is forced back to the master cylinder and braking ceases.

The primary purpose of the parking brake is to hold the vehicle stationary while it is unattended. The parking brake is mechanically operated by the driver when a separate parking braking hand lever is set. The hand brake is normally used when the car has already stopped. A lever is pulled and the rear brakes are approached and locked in the “on” position. The car may now be left without fear of its rolling away. When the driver wants to move the car again, he must press a button before the lever can be released. The hand brake must also be able to stop the car in the event of the foot brake failing. For this reason, it is separate from the foot brake uses cable or rods instead of the hydraulic system.

Anti-lock Brake System

Anti-lock brake systems make braking safer and more convenient, Anti-lock brake systems modulate brake system hydraulic pressure to prevent the brakes from locking and the tires from skidding on slippery pavement or during a panic stop.

Anti-lock brake systems have been used on aircraft for years, and some domestic car were offered with an early form. of anti-lock braking in late 1990’s. Recently, several automakers have introduced more sophisticated anti-lock system. Investigations in Europe, where anti-lock brakin

g systems have been available for a decade, have led one manufacture to state that the number of traffic accidents could be reduced by seven and a half percent if all cars had anti-lock brakes. So some sources predict that all cars will offer anti-lock brakes to improve the safety of the car.

Anti-lock systems modulate brake application force several times per second to hold the tires at a controlled amount of slip; all systems accomplish this in basically the same way. One or more speed sensors generate alternating current signal whose frequency increases with the wheel rotational speed. An electronic control unit continuously monitors these signals and if the frequency of a signal drops too rapidly indicating that a wheel is about to lock, the control unit instructs a modulating device to reduce hydraulic pressure to the brake at the affected wheel. When sensor signals indicate the wheel is again rotating normally, the control unit allows increased hydraulic pressure to the brake. This release-apply cycle occurs several time per second to “pump” the brakes like a driver might but at a much faster rate.

In addition to their basic operation, anti-lock systems have two other things in common. First, they do not operate until the brakes are applied with enough force to lock or nearly lock a wheel. At all other times, the system stands ready to function but does not interfere with normal braking. Second, if the anti-lock system fail in any way, the brakes continue to operate without anti-lock capability. A warning light on the instrument panel alerts the driver when a problem exists in the anti-lock system.

The current Bosch component Anti-lock Braking System (ABSⅡ), is a second generation design wildly used by European automakers such as BWM, Mercedes-Benz and Porsche. ABSⅡ system consists of : four wheel speed sensor, electronic control unit and modulator assembly.

A speed sensor is fitted at each wheel sends signals about wheel rotation to control unit. Each speed sensor consists of a sensor unit and a gear wheel. The front sensor mounts to the steering knuckle and its gear wheel is pressed onto the stub axle that rotates with the wheel. The rear sensor mounts the rear suspension member and its gear wheel is pressed onto the axle. The sensor itself is a winding with a magnetic core. The core creates a magnetic field around the winding, and as the teeth of the gear wheel move through this field, an alternating current is induced in the winding. The control unit monitors the rate o change in this frequency to determine impending brake lockup.

The control unit’s function can be divided into three parts: signal processing, logic and safety circuitry. The signal processing section is the converter that receives the alternating current signals form. the speed sensors and converts them into digital form. for the logic section. The logic section then analyzes the digitized signals to calculate any brake pressure changes needed. If impending lockup is sensed, the logic section sends commands to the modulator assembly.

Modulator assembly

The hydraulic modulator assembly regulates pressure to the wheel brakes when it receives commands from the control utuit. The modulator assembly can maintain or reduce pressure over the level it receives from the master cylinder, it also can never apply the brakes by itself. The modulator assembly consists of three high-speed electric solenoid valves, two fluid reservoirs and a turn delivery pump equipped with inlet and outlet check valves. The modulator electrical connector and controlling relays are concealed under a plastic cover of the assembly.

Each front wheel is served by electric solenoid valve modulated independently by the control unit. The rear brakes are served by a single solenoid valve and modulated together using the select-low principle. During anti-braking system operation, the control unit cycles the solenoid valves to either hold or release pressure the brake lines. When pressure is released from the brake lines during anti-braking operation, it is routed to a fluid reservoir. There is one reservoir for the front brake circuit. The reservoirs are low-pressure accumulators that store fluid under slight spring pressure until the return delivery pump can return the fluid through the brake lines to the master cylinder.

汽车制动系统

制动系统是汽车中最重要的系统，

[外文翻译]Automobile Brake System-汽车制动系统

，

如果制动失灵，结果可能是损失惨重的。制动器实际就是能量转换装置，它将汽车的动能（动量）转化成热能（热量）。当驾驶员踩下制动踏板，所产生的制动力是汽车运动时动力的10倍。制动系统能对四个刹车系统中的每个施加数千磅的力。

每辆汽车上使用两个完全独立的制动系统，即行车制动器和驻车制动器。

行车制动器起到减速、停车、或保持车辆正常行驶。制动器是由司机用脚踩、松制动器踏板来控制的。驻车制动器的主要作用就是当车内无人的时候，汽车能够保持静止。当独立的驻车制动器—踏板或手杆，被安装时，驻车制动器就会被机械地操作。

制动系统是由下列基本的成分组成:位于发动机罩下方，而且直接地被连接到制动踏板的“制动主缸”把驾驶员脚的机械力转变为液压力。钢制的“制动管路”和有柔性的“制动软管”把制动主缸连接到每个轮子的“制动轮缸”上。 制动液, 特别地设计为的是工作在极端的情况，填充在系统中。“制动盘”和“衬块”是被制动轮缸推动接触“圆盘”和“回转体”如此引起缓慢的拖拉运动, (希望)使汽车减慢速度。

典型的制动系统布置有前后盘式，前盘后鼓式，各个车轮上的制动器通过一套管路系统连接到制动主缸上。

基本上讲，所有的汽车制动器都是摩擦制动器。当司机刹车时，控制装置会迫使制动蹄，或制动衬片与车轮处的旋转的制动鼓或制动盘接触。接触后产生的摩擦使车轮转动减慢或停止，这就是汽车的制动。

在最基本的制动系统中，有一个制动主缸，这个主缸内部填充制动液，并包含两个部分，每个部分里都有一个活塞，两个活塞都连接驾驶室里的制动踏板。当制动踏板被踩下时，制动液会从制动主缸流入轮缸。在轮缸中，制动液推动制动蹄或制动衬片与旋转的制动鼓或制动盘接触。静止的制动蹄或制动衬片与旋转的制动鼓或制动盘之间产生摩擦力使汽车的运动逐渐减缓或停止。

制动液的装置位于主缸的顶部。目前大多数的车都有一个容易看见的装制动液的装置，为的是不用打开盖子就可以看得见制动液的油面。随着制动踏板的运动制动液就会缓慢的下降，正常情况下是这样的。如果制动液在很短的时间内下降得明显或者下降了三分之二，那么就要尽快的检查你的制动系统了。保持制动液装置充满制动液除非你需要维修它，制动液必须保持很高的沸点。位于在空气中的制动液就会吸收空气中的潮气引起制动液低于沸点。

制动液通过一系列的管路从主缸到达各车轮。橡胶软管只用在需要弹力的地方，比如应用在前轮。在车的行进中上下来回运动。系统的其它部分在所有的连接点上都应用了无腐蚀性的无缝钢管。如果钢线需要修理的话，最好的方法就是代替这条线。如果这不符合实际，那么为了制动系统可以用特殊的装置修理它。你不可以用铜管来修理制动系。它们是危险也是不正确的。

鼓式制动器包括制动鼓，一个轮缸，回拉弹簧，一个制动底版，两个带摩擦层的制动蹄。制动底版固定在轮轴外部的法兰或转向节。制动鼓固定在轮毂上。制动鼓的内部表面与制动蹄的内层之间有空隙。要使用制动器时，司机就要踩下踏板，这时轮缸扩大制动片，对其施加压力，是制动蹄触碰制动鼓。制动鼓与摩擦片之间产生的摩擦制动了车轮，从而使汽车停止。要释放制动器时，司机松开踏板，回拉弹簧拉回制动片，这样车轮会自由转动。

盘式制动器包括制动盘而不是鼓，在它的两面上各有一个薄的制动片或叫盘式制动器的制动片。制动片是靠挤住旋转的制动盘来停住汽车。制动主缸里流出的制动液迫使活塞向里部的金属盘移动，这便使摩擦片紧紧地贴住制动盘。这时制动片与制动盘产生的摩擦使汽车减速、停止，出现了制动行为。活塞分金属或塑料。盘式制动器主要有三种，即：浮动卡钳型、固定卡钳型和滑动卡钳型。浮动卡钳型和滑动卡钳型盘式制动器使用单活塞。固定卡钳型盘式制动器既可以使用两个活塞有可以使用四个活塞。

制动系统是由机械能，液压能或气压能装置驱动的。在机械杠杆适合所有的汽车的驻车制动器中使用。当踩下制动踏板时，杠杆就会推动制动器主缸的活塞给制动液施加压力，制动液通过油管流入轮缸。制动液的压力施加到轮缸活塞以使制动片被压到制动鼓或制动盘上。如果松开踏板，活塞回到原来的位置上，回拉弹簧拉回制动片，制动液返回制动主缸，这样制动停止。

驻动制动器的主要作用是车内无人时，使汽车静止不动。如果车内安装的是独立的驻车制动器，那么驻车制动器是由司机手动的控制。驻车制动器正常是当车已经停止时使用的。向后拉手闸，并把手柄卡在正确的位置上。现在，即使离开汽车也不用害怕它会自己滑走。如果司机要再次启车时，他必须在松开手杆之前按下按钮。在行车制动器失灵的情况下，手闸必须能停住车。正因为这样，手闸与脚闸分开，手闸使用的是绳索或杠杆而不是液力系统。

防抱死制动系统是使汽车制动更安全、更方便的制动装置，它既有调节制动系统的压力来防止车轮被完全抱死的功能，又有防止轮胎在滑的路面上行驶或紧急停车时的滑动。

防抱死制动系统最早应用在航空飞行器上，而且在二十世纪 90年代一些国内的汽车内也安装了这种系统。近来，几个汽车制造商引进了更为复杂的防抱死系统。欧洲使用这种系统已有几十年的时间，通过对其的调查，一位汽车制造商坦言，如果所有的汽车都安装上防抱死制动系统，那么交通事故的发生率会降低7.5%。同时，一些权威人士预测这种系统会提高汽车的安全性。

防抱死制动系统可以在一秒钟内调节几次制动时车轮上的受力，使车轮的滑移受到控制，而且所有的系统基本上都以相同的方式完成。每个车轮都会有一个传感器，电子控制装置能连续检测来自车轮传感器传来的脉冲电信号，并将它们处理转换成和轮速成正比的数值；如果其中一个传感器的信号不断下降，那么这就表明了相应的轮胎趋于抱死，这时电子控制装置向该车轮的制动器发出降低压力的指令。当信号显示车轮转速恢复正常时，电子控制装置会增加制动器的液压。这种循环像司机一样调节制动器，但它的速度更快，达到了每秒循环数次。

防抱死制动系统除了上面基本操作，还有两个特点。首先，当制动系统的压力上升到使轮胎抱死或即将抱死的时候，防抱死制动系统才会启动；当制动系统在正常情况下，防抱死制动系统停止运作。其次，如果防抱死制动系统有问题时，制动器会独立地继续运行。但控制板上的指示灯亮起提醒司机系统出现问题。

目前欧洲汽车生产商，如：宝马、奔驰、宝时捷等广泛使用的是波许（Bosch）防抱死制动系统。这种系统基本组成包括车轮转速传感器，电子控制装置和调节装置。

每个有一个向电子控制装置发出车轮转动情况的信号的传感器，它一般由磁感应传感头和齿圈组成。前面的传感器安在轮毂上，齿圈安在轮网上。后面的传感器安在后部的监测系统上，齿圈安在轮轴上。传感器本身是缠绕电磁核的电线圈，电磁核才线圈的周围产生磁场。当齿圈的齿移动到磁场时，就会改变线圈的电流。电子控制装置会监测这种变化，然后判断车轮是否即将抱死。

电子控制装置有三个作用，即：信号的处理，编辑和安全防护。信号的处理起到转换器的作用，它是将接受的脉冲电信号处理转换成数值，为编辑做准备。编辑就是分析这些数值，计算出需要制动压力。如果检测出车轮即将抱死，电控装置就会计算出数值向调节装置发出指令。

调节装置

当接受到电子控制装置的指令后，液压执行装置会调节制动轮缸的液压的大小。调节装置能保持或减小来自制动主缸的液压，而装置本身是不能启用制动器的。这种装置有三个高速率的电磁阀，两个油液存储器和一个带有内外检测阀的传动泵。调节装置中的电子连接器隐藏在塑料盖下。

每个电磁阀都是其独立控制的，并作用于前轮。后部的制动轮缸受到一个电磁阀控制，并依照------的原理进行调节。当防抱死制动系统运行时，电子控制装置会使电磁阀循环运作，这样既能收回又能释放制动器的压力。当压力释放时，它会释放到液压单元。前部的制动器电路有一个单元。存储器低压存储器，它在低压下存储油液，直到回流泵打开，油液流经制动轮缸进入制动主缸。

不懂外文的翻译家 篇2

林纾读书很刻苦，尤其对古典文学下了不少功夫。二十岁时，他阅读的书已达数千卷之多，这为其奠定了深厚的文学根基与学问基础。1882年，他中了举人后，便放弃科举之学，专心致力于古文研究。

一个偶然的机会，使林纾萌发了翻译外国小说的念头。他有一位朋友叫王寿昌，曾留学法国，并带了不少法国小说回来。两人在一块聊天时，王寿昌介绍了小仲马的《茶花女》的故事情节，林纾听后大为感动，于是，二人协议把其介绍给国人。正好王寿昌懂外文，林纾又有很好的古文基础，于是合作开始了。最先翻译出来的便是《茶花女遗事》，由王寿昌口述，林纾执笔，每译至伤心之处，两人都会抱头痛哭，真是声情并茂。因此，此书译本一炮打响，林纾名声大振，轰动了中国文坛。于是，二人开始了长期合作，先后译出了《黑奴呼天录》《撤克逊劫后英雄略》《迦茵小传》等，均名重一时。

就这样，林纾走上了翻译外国文学之路，其“林译小说”脍炙人口，传诵一时。他先后与几位懂外文的留学生合作，翻译了欧美各国的小说，为外国文学传入中国做出了杰出贡献。

（辽宁*韩蕾）

林纾:不通外文的翻译家 篇3

( -06-15 ) 稿件来源：新华每日电讯 新华视界・文史・阅读

林纾是我国著名翻译家,近代文学翻译的开创者,正式译介西方文学第一人。他虽然是一个精通古汉语却不懂外文的文人,却与魏易、曾钟巩等人合作,以“耳受笔追”的方式翻译了涉及11个国家的107名作家的作品,对西方文献的引进和当时社会文化的进步做出了不可磨灭的贡献,为后世留下了一份宝贵的文化遗产。

通力合作

林纾5岁那年,外祖母开始教他读书识字,启蒙课本是《孝经》。1882年,林纾从一个穷秀才一跃成为举人。然而,后来“七上春官,屡试屡败”的历程,让他在科场失意的同时,看到了太多官场的黑暗。他从此绝意于仕途,专心致志地走上了文学创作道路。

林纾有一位同乡叫王昌寿,1885年曾赴法留学6年,法语精湛。他回国后给林纾讲了许多外国名著,使林大开眼界。于是,两人开始合作翻译小仲马的《茶花女》。由王昌寿口译,林纾用文言文记录下来。那时,闽江的一条小船上常可看到这样一幅景致:王昌寿手捧原著,一边浏览,一边口述;林纾则展纸挥笔。林纾耳聪手疾,文思敏捷,经常是王昌寿刚说完一句,他就已写好一句。一天4个小时下来,记下的文字已有6000多字。18夏天,昌言报馆版本的.《茶花女遗事》公开发售。一时间,洛阳纸贵,很快流传开来。

在之后短暂的27年生命里,林纾与魏易、陈家麟等曾留洋的才子们合作翻译了160余部西洋小说,如英国作家狄更斯著的《大卫・科波菲尔德》,俄国托尔斯泰著的《恨缕情丝》,西班牙塞万提斯的《魔侠传》,英国司各特的《撒克逊劫后英雄略》、笛福的《鲁滨逊漂流记》等。这些西洋小说向中国民众展示了丰富的西方文化,开拓了人们的视野。

多才多艺

除翻译外国名著外,林纾还精通文学写作和绘画。辛亥革命后,他创作了长篇小说《京华碧血录》,书中虽以恋爱故事为主干,但它涉及戊戌变法、义和团起义、八国联军进攻北京等重大历史事件,描写的生活面极其广阔,在当时颇受读者欢迎。后来,他又陆续写了《金陵秋》《劫外昙花》《冤海灵光》《巾帼阳秋》《官场新现形记》等长篇小说和一系列短篇小说,生动地反映了当时的社会现实。

林纾23岁时曾拜师陈文召学画,其名

林纾:不通外文的翻译家气不下于他的文学创作和翻译。他擅长花鸟,得师真传,晚年定居北京后致力于山水创作。作品灵秀略似文征明,浓厚处近戴熙,传世作品有《理安山色图》《仿王椒畦山水图》。军阀吴佩孚在他51岁生日时,愿出巨资请林纾画一幅祝寿图,林纾鄙夷其执政时草菅人命,一口拒绝,在当时的京城传为佳话。对于后学,林纾则竭尽全力予以提携。19,湖南湘潭人齐白石初到京城,想以卖画为生,但当时整个北京城没有几个人知道他。为了摆脱困境,齐白石登门拜访林纾,林纾将其绘画全部收购,并在自己编审的《平报》上发表文章,极力推崇齐白石的画作。由于这些幕后工作,帮助齐白石在北京慢慢有了名气,终成一代国画大师。

外文翻译要求 篇4

根据《普通高等学校本科毕业设计（论文）指导》的内容，特对外文文献翻译提出以下要求：

一、翻译的外文文献的字符要求不少于1.5万（或翻译成中文后至少在3000字以上）。字数达到的文献一篇即可。

二、翻译的外文文献应主要选自学术期刊、学术会议的文章、有关著作及其他相关材料，应与毕业论文（设计）主题相关，并作为外文参考文献列入毕业论文（设计）的参考文献。并在每篇中文译文首页用“脚注”形式注明原文作者及出处，中文译文后应附外文原文。

三、中文译文的基本撰写格式为题目采用小三号黑体字居中打印，正文采用宋体小四号字，行间距一般为1.25倍行距，标准字符间距。页边距为左2.5cm，右2.5cm，上3cm，下2cm,页眉距边界2cm,页脚距边界1cm。页眉为：“杭州电子科技大学本科毕业论文外文翻译”，字体为五号宋体，居中；页面统一采用A4纸。从正文开始编写页码，页码居中。

四、封面格式由学校统一制作（注：封面上的“翻译题目”指中文译文的题目），并按“封面、译文

一、外文原文

一、译文

二、外文原文二”的顺序统一装订。如果只有一篇译文，则可以删除“翻译（2）题目”这一行。

五、封面格式请勿自行改动，学号请写完整。

理学院

2006年9月13日

杭州电子科技大学

毕业设计（论文）外文文献翻译

毕业设计（论文）题目

翻译（1）题目 翻译（2）题目 学院

专业

姓名

班级

学号

外文翻译 篇5

Digital Communications,Fourth Edition

作者：John Proakis 起止页码：1-10

出版日期（期刊号）：2003年1月 出版单位：电子工业出版社

外文翻译译文：

第1章 引 言

在本书中,我们将介绍作为数字通信系统分析和设计基础的基本原理。数字通信的研究主题包括数字形式的信息从产生该信息的信源到一个或多个目的地的传输问题。在通信系统的分析和设计中，特别重要的是信息传输所通过的物理信道的特征。信道的特征-般会影响通信系统基本组成部分的设计。下面阐述一个通信系统的基本组成部分及其功能。

1.1数字通信系统的基本组成部分

图1-1-1 显示了一个数字通信系统的功能性框图和基本组成部分。输出的可以是模拟信号，如音频或视频信号;也可以是数字信号，如电传机的输出，该信号在时间上是离散的，并且只有有限个输出字符。在数字通信系统屮，由信源产生的消息变换成二进制数字序列。理论上，应当用尽可能少的二进制数字表示信源输出（消息）。换句话说.我们要寻求一种信源输出的有效的表示方法,使其很少产生或不产生冗余。将模拟或数宇信源的输出有效地变换成二进制数字序列的处理过程称为信源编码或数据压缩。

由信源编码器输出的二进制数字序列称为信息序列，它被传送到信道编码器。信道编码器的目的是在二进制信息序列中以受控的方式引人一些冗余，以便于在接收机中用来克服信号在信道中传输时所遭受的噪声和干扰的影响。因此，所增加的冗余是用来提高接收数据的可靠性以及改善接收信号的逼真度的。实际上，信息序列中的冗余有助于接收机译出期望的信息序列。例如,二进制信息序列的一种(平凡的）形式的编码就是将每个二进制数字简单重复m次.这里m为一个正整数。更复杂的（不平凡的）编码涉及到一次取k个信息比特,并将毎个k比特序列映射成惟一的n比特序列，该序列称为码字。以这种方式对数据编码所引人的冗余度的大小是由比率n/k作来度擞的。该比率的倒数，即k/n，称为码的速率或简称码率。信道编码器输出的二进制序列送至数宇调制器，它是通信信道的接口。因为在实际中遇到的几乎所有的通信信道都能够传输电信号（波形）,所以数字调制的主要目的是将二进制信息序列映射成信号波形。为了详细说明这一点，假定已编码的信息序列以均匀速率R（b/s）―次一个比特传输,数字调制器可以简单地将二进制数字“0”映射成波形s0(t)而二进制数字“1”映射成波形s1(t)。在这种方式中，信道编码器输出的毎一比特是分别传输的。我们把它称为二进制调制。另一种方式，调制器目一次传输b个已编码的信息比特，其方法是采用M = 2s个不同的波形ST(t)i=1,2，…，M,每一个波形用来传输2s个可能的b比特序列中的一个序列。我们称这种方式为M元调制（M〉2）。注意，每b/R秒就有一个新的b比特序列进入调制器。因此，当信道比特率R固定,与一个b比特序列相应的似个波形之一的传输时间量是二进制调制系统时间周期的b倍。

图1-1-1

数字通信系统的基本模型

通信信道是用来将发送机的信号发送给接收机的物理媒质。在无线传输中，信道可以是大气（自由空间）另一方面，电话信道通常使用各种各样的物理媒质，包括有线线路、光缆和无线(微波)等。无论用什么物理媒质来传输信息，其基本特点是发送信号随机地受到各种可能机理的恶化，例如由电子器件产生的加性热噪声、人为噪声（如汽车点火噪声）及大气噪声（如在雷赛雨时的闪电）。

在数字逋信系统的接收端,数字解调器对受到信道恶化的发送波形进行处理，并将该波形还原成一个数的序列，该序列表示发送数据符号的估计值〔二进制或M元〕。这个数的序列披送至信道译码器，它根据信进编码器所用的关于码的知识及接收数据所含的冗余度重构初始的信息序列。

解调器和译码器工作性能好坏的—个度量是译码序列中发生差错的频度。更准确地说，在译码器输出端的平均比特错误概率是解调器-译码器組合性能的一个度量。一般地，错误概率是下列各种因素的函数:码特征、用来在信道上传输信息的波形的类型、发送功率信道的特征（即噪声的大小、干扰的性质等)以及解调和译码的方法。在后续各章中将详细讨论这些因素及其对性能的影晌。

作为最后一步，当需要模拟输出时，信源译码器从信道译码器接收其输出序列并根据所采用的信源编码方法的有关知识重构由信源发出的原始信号。由于信道译码的差错以及信源编码器可能引入的失真,在信源译码器输出端的信号只是原始信源输出的—个近似。在原始信号与重构信号之间的信号差或信号差的函数是数字通信系统引入失真的一种度量。

1.2通信信道及其特征

正如前面指出的，通信信道在发送机与接收机之间提供了连接。物理信道也许是携带电信号的一对明线；或是在已调光波束上携带信息的光纤；或是水下海洋信道其中信息以声波形式传输；或是自由空间，携带信息的信号通过天线在空间辐射传输。可被表征为通信信道的其他媒质是数据存储媒质如磁带、磁盘和光盘。

在信号通过任何信道传输中的一个共同的问题是加性噪声。一般地，加性噪声是由通信系统内部组成元器件所引起的，例如电阻和固态器件。有时将这种噪声称为热噪声。其他噪声和干扰源也许是系统外面引起的，例如来自信道上其他用户的干扰。当这样的噪声和干扰与期望信号占有同频带时，可通过对发送信号和接收机中解调器的适当设计来使它们的影响最小。信号在信道上传输时可能会遇到的其他类型损伤有信号衰减、幅度和相位失真、多径失真等。

可以通过增加发送信号功率的方法使噪声的影响最小。然而，设备和其他实际因素限制了发送信号的功率电平，另一个基本的限制是可用的信道带宽。带宽的限制通常是由于媒质以及发送机和接牧机中组成器件和部件的物理限制产生的。这两种限制因素限制了在任何通信信道上能可靠传输的数据量,我们将在以后各章中讨论这种情况。下面描述几种通信信道的重要特征。

1.有线信道

电话网络扩大了有线线路的应用，如话音信号传输以及数据和视频传输。双绞线和同轴电缆是基本的导向电磁信道，它能提供比较适度的带宽。通常用来连接用户和中心机房的电话线的带宽为几百千赫(khz)另一方面同轴电缆的可用宽带是几兆赫（Mhz）。信号在这样的信道上传输时，其幅度和相位都会发生失真，还受到加性噪声的恶化。双绞线信道还易受到来自物理邻近信道的串音干扰。因为在全国和全世界有线信道上通信在日常通信中占有相当大的比例，因此,人们对传输特性的表征以及对信号传输时的幅度和相位失真的减缓方法作了大量研究。在第9章中，我们将阐述最佳传输信号及其解调的设什方法。在笫10章和第11章中，我们将研究信道均衡器的设计,它是用来补偿信道的幅度和相位失真的。

2.光纤信道

光纤提供的信道带宽比同轴电缆信道大几个数量级。在过去的20年屮，已经研发出具有较低倌号衰减的光缆，以及用于信号和信号检测的可靠性光子器件。这些技术上的进展导致了光纤信道应用的快速发展，不仅应用在国内通信系统中，也应用于跨大西洋和跨太平洋的通信中。由于光纤信道具有大的可用带宽，因此有可能使电话公司为用户提供宽系列电店业务，包括话音、数据、传真和视频等。

在光纤通信系统中，发送机或调制器是一个光源.或者是发光二极管（LED）或者是激光。通过消息信号改变（调制）光源的强度来发送信息。光像光波一样通过光纤传播，并沿着传输路径被周期性地放大以补偿信号衰减（在数宇传输中，光由中继器检测和再生）。在接收机中，光的强度由光电二极管检测，它的输出电信号的变化直接与照射到光电二极管上的光的功率成正比。光纤信道中的噪声源是光电二极管和电子放大器。

3.无线电磁信道

在无线通信系统中，电磁能是通过作为辐射器的天线耦合到传播媒质的。天线的物理尺寸和配置主要决定于运行的频率。为了获得有效的电磁能量的辐射，天线必须比波长的1/10更长。因此,在调幅（AM）频段发射的无线电台，譬如说在f=1MHz时（相当于波长= C/f=300m）要求天线至少为30m。无线传输天线的其他重要特征和属性将在第5章阐述。

在大气和自由空间中，电磁波传播的模式可以划分为3种类型，即地波传播、天波传播和视线传播。在甚低频（VLF）和音频段，其波长超过10km，地球和电离层对电磁波传播的作用如同波导。在这些频段,通信信号实际上环绕地球传播，由于这个原因，这些频段主要用来在世界范围内提供从海洋到船舶的导航帮助。在此频段中可用的带宽较小（通常是中心频率的1% ~10%）因此通过这些信道传输的信息速率较低，且一般限于数字传输。在这些频率上，最主要的一种噪声是由地球上的雷暴活动产生的，特别是在热带地区。干扰来自这些频段上的用户。

在高频（HF）频段范围内，电磁波经由天波传播时经常发生的问题是信号多径。信号多径发生在发送信号经由多条传播路径以不同的延迟到达接收机的时侯，一般会引起数字通信系统中的符号间干扰。而且经由不同传播路径到达的各信号分量会相互削弱，导致信号衰落的现象.许多人在夜晚收听远地无线电台广播时会对此有体验。在夜晚，天波是主要的传播模式。HF频段的加性噪声是大气噪声和热噪声的组合。

在大约30MHZ之上的频率,即频段的边缘，就不存在天波电离层传播。然而，在30~60MHZ频段有可能进行电离层散射传播，这是由较低电离层的信号散射引起的。也可利用在40~300MHZ频率范围内的对流层散射在几百英里的距离通信。对流层散射是由在10mile或更低高度大气层中的粒子引起的信号散射造成的，一般地，电离层散射和对流层散射具有大的信号传播损耗，要求发射机功率大和天线比较长。

在30MHZ以上频率通过电离层传播具有较小的损耗,这使得卫里和超陆地通信成为可能。因此,在甚高频(VHF)频段和更高的频率，电磁传播的最主要模式是LOS传播。对于陆地通信系统这意味着发送机和接收机的天线必须是直达LOS,没有什么障碍。由于这个原因VHF和特高频(UHF)频段发射的电视台的天线安装在髙塔上,以达到更宽的覆盖区域。

一般地LOS传播所能覆盖的区域受到地球曲度的限制。如果发射天线安装在地表面之上H米的高度,并假定没有物理障碍(如山)那么到无线地平线的距离近似为d=15H KM，例如电视天线安装在300m高的塔上.它的覆盖范围大约67km另一个例子，工作在1GHZ以上频率，用来延伸电话和视频传输的微波中继系统将天线安装在离塔上或高的建筑物顶部。

对工作在VHF和UHF频率范围的通信系统限制性能的最主要噪声是接收机前端所产生的热噪声和天线接收到的宇宙噪声。在10GHZ以上的超髙频（SHF)频段，大气层环境在信号传播中担负主要角色。例如,在10GHZ频率，衰减范围从小雨时的0.003 dB/KM左右到大雨时的0.3dB/KM；在100GHZ,衰减范围从小雨时的0.1dB左右到大雨时的6dB左右。因此，在此频率范围,大雨引起了很大的传播损耗，这会导致业务中断(通信系统完全中断)。

在极高频(EHF)频段以上的频率是电磁频谱的红外区和可见光区，它们可用来提供自由空间的LOS光通信。到目前为止,这些频段已经用于实验通信系统,例如，卫星到卫星的通信链路。

4.水声信道

在过去的几十年中.海洋探险活动不断增多。与这种增多相关的是对传输数据的需求。数据是由位于水下的传感器传送到海洋表面的,从那里可能将数据经由卫星转发给数据采集中心。

除极低频率外，电磁波在水下不能长距离传播。在低频率的信号传输的延伸受到限制，因为它需要大的且功率强的发送机。电磁波在水下的衰减可以用表面深度来表示,它是信号衰减l/e的距离。对于海水,表面深度 250/f，其中f以HZ为单位。例如，在10 khz上,表面深度是2.5m。声信号能在几十甚至几百千米距离上传播。

水声信道可以表征为多径信道,这是由于海洋表面和底部对信号反射的缘故。因为波的运动,信号多径分量的传播延迟是时变的，这就导致了信号的衰落。此外,还存在与频率相关的衰减，它与信号频率的平方近似成正比。声音速度通常大约为1 500m/s，实际值将在正常值上下变化,这取决于信号传播的深度。

海洋背景噪声是由虾、鱼和各种哺乳动物引起的。在靠近港口处，除了海洋背景噪声外也有人为噪声。尽管有这些不利的环境，还是可能设计并实现有效的且高可靠性的水声通信系统,以长距离地传输数字信号。

5.存储信道

信息存储和恢复系统构成了日常数据处理工作的非常重要的部分。磁带（包括数字的声带和录像带）、用来存储大量计箅机数据的磁盘、用作计箅机数据存储器的光盘以及只读光盘都是数据存储系统的例子,它们可以表征为通信信道。在磁带或磁盘或光盘上存储数据的过程，等效于在电话或在无线信道上发送数据。回读过程以及在存储系统中恢复所存储的数据的信号处理等效于在电话和无线通信系统中恢复发送信号。

由电子元器件产生的加性噪声和来自邻近轨道的干扰一般会呈现在存储系统的回读信号中，这正如电话或无线通信系统中的情况。

所能存储的数据量一般受到磁盘或磁带尺寸及密度（每平方英寸存储的比特数）的限制，该密度是由写/读电系统和读写头确定的。例如在磁盘存储系统中，封装密度可达每平方英寸比特(1 in=2.54cm)。磁盘或磁带上的数据的读写速度也受到组成信息存储系统的机械和电子子系统的限制。

信道编码和调制是良好设计的数字磁或存储系统的最重要的组成部分。在回读过程中，信号被解调。由信道编码器引入的附加冗余度用于纠正回读信号中的差错。

1.3 通信信道的数学模型

在通过物理信道传输信息的通信系统设计中，我们发现，建立一个能反映传输媒质最重要特征的数学模型是很方便的。信道的数学模型可以用于发送机中的信道编码器和调制器，以及接收机中的解调器和信道译码器的设计。下面，我们将简要的描述信道的模型，它们常用来表征实际的物理信道。1.加性噪声信道

通信信道最简单的数学模型是加性噪声信道，如图1-3-1所示。在这个模型中，发送信号s（t）被加性随机噪声过程n（t）恶化。在物理上，加性噪声过程由通信系统接收机中的电子元部件和放大器引起，或者由传输中的干扰引起（正如在无线电信号传输中那样）。

如果噪声主要是由接收机中的元部件和放大器引起，那么，它可以表征为热噪声。这种模型的噪声统计地表征为高斯噪声过程。因此，该信道的数学模型通常称为加性高斯噪声信道。因为这个信道模型适用于很广的物理通信信道，并且因为它在数学上易于处理，所以是在通信系统分析和设计中所用的最主要的信道模型。信道的衰减很容易加入到该模型。信号通过信道传输而受到衰减时，接收信号是

r(t)s(t)n(t)式中，是衰减因子。

图1-3-1 加性噪声信道

2.线性滤波器信道

在某些物理信道中，例如有线电话信道，采用滤波器来保证传输信号不超过规定的带宽限制，从而不会引起相互干扰。这样的信道通常在数学上表征为带有加性噪声的线性滤波器，如图1-3-2所示。因此，如果信道输入信号为s（t），那么信道输出信号是

r(t)s(t)c(t)n(t)



c()s(t)dn(t)

式中，c()是信道的冲激响应，表示卷积。

图1-3-2 带有加性噪声的线性滤波器信道 3.线性时变滤波器信道

像水声信道和电离层无线电信道这样的物理信道，它们会导致发送信号的时变多径传播，这类物理信道在教学上可以表征为时变线性滤波器。该线性滤波器可以表征为时变信道冲激响应c（τ；t），这里c（τ；t）是信道在t-τ时刻加入冲激而在τ时刻的响应。因此，τ表示“历时（经历时间）”变量。

上面描述的三种数学模型适当的表征了实际中的绝大多数物理信道。本书将这3 种模型用于通信系统的分析和设计。

1.4 数字通信发展的回顾与展望 值得注意的是，最早的电通信形式，即电报，是一个数字通信系统。电报由S•莫尔斯研制，并在1837年进行了演示试验。莫尔斯设计出一种可变长度的二进制码，其中英文字母用点划线的序列（码字）表示。在这种码中，较频繁发生的字母用短码字表示，不常发生的字母用较长的码字表示。因此，莫尔斯码是第三章所述可变长度信源编码方法的先驱。

差不多在40年之后，1875年，E博多设计出一种电报码，其中每一个字母编成一个固定长度为5的二进制码字。在博多码中，二进制码的元素是等长度的，且指定为传号和空号。

虽然莫尔斯在研制第一个点的数字通信系统（电报）中起了重要的作用，但是现在我们所指的现代数字通信系统起源于奈奎斯特的研究。奈奎斯特研究了再给定带宽的电报信道上，无符号间干扰的最大信号传输速率。他用公式表达了一个电报系统的模型，其中发送信号的一般形式为

s(t)anng(tnT)

式中，g（t）表示基本的脉冲形状，an是以速率1/T bit/s发送的二进制数据序列。奈奎斯特提出了带宽限于W Hz的最佳脉冲形状，并且在脉冲抽样时刻Kt(k=0，1。。)无符号间干扰的条件下的最大比特率。他得出结论：最大脉冲速率是2W脉2，冲/s，该速率称为奈奎斯特速率。

1.INTRODUCTION In this book, we present the basic principles that underlie the analysis and design of digital communication systems.The subject of digital communications involves the transmission of information in digital form from a source that generates the information to one or more destinations.Of particular importance in the analysis and design of communication systems are the characteristics of the physical channels through which the information is transmitted.The characteristics of the channel generally affect the design of the basic building blocks of the communication system.Below, we describe the elements of a communication system and their functions.1-1 ELEMENTS OF A DIGITAL COMMUNICATION SYSTEM Figure 1-1-1 illustrates the functional diagram and the basic elements of a digital communication system.The source output may be either an analog signal, such as audio or video signal, or a digital signal, such as the output of a teletype machine, that is discrete in time and has a finite number of output characters.In a digital communication system, the messages produced by the source are converted into a sequence of binary digits.Ideally, we should like to represent the source output(message)by as few binary digits as possible.In other words, we seek an efficient representation of the source output that results in little or no redundancy.The process of efficiently converting the output of either an analog or digital source into a sequence of binary digits is called source encoding or data compression.The sequence of binary digits from the source encoder, which we call the information sequence, is passed lo the channel encoder.The purpose of the channel encoder is to introduce, in a controlled manner, some redundancy in the binary information sequence that can be used at the receiver to overcome the effects of noise and interference encountered in the transmission of the signal through the channel.Thus, the added redundancy serves to increase the reliability of the received data and improves the fidelity of the received signal.In effect, redundancy in the information sequence aids the receiver in decoding the desired information sequence.For example, a(trivial)form of encoding of the binary information sequence is simply to repeat each binary digit m times,where m is some positive integer.More sophisticated(nontrivial)encoding involves talcing k information bits at a time and mapping each k-bit sequence into a unique n-bit sequence, called a code word.The amount of redundancy introduced by encoding the data in this manner is measured by the ratio n/k.The reciprocal of this ratio, namely k/n, is called the rate of the code or,simply, the code rate.The binary sequence at the output of the channel encoder is passed to the digital modulator, which serves as the interface to the communications channel.Since nearly all of the communication channels encountered in practice are capable of transmitting electrical signals(waveforms), the primary purpose of the digital modulator is to map the binary information sequence into signal waveforms.To elaborate on this point, let us suppose that the coded information sequence is to be transmitted one bit at a time at some uniform rate R bits/s.The digital modulator may simply map the binary digit 0 into a waveform s0(t)and the binary digit 1 into a waveform j,(i).In this manner,each bit from the channel encoder is lransmitted separately.We call this binary modulation.Alternatively, the modulator may transmit b coded information bits at a time by using M = 2s distinct waveforms j.(r), i = 0,1

M1 MHz(corresponding to a wavelength of A = cffr = 300m).requires an antenna of at least 30m.Other important characteristics and attributes of antennas for wireless transmission are described in Chapter 5.Figure 1-2-2 illustrates the various frequency bands of the electromagneticspectrum.The mode of propagation of electromagnetic waves in the atmo-sphere and in free space may be subdivided into three categories, namely,ground-wave propagation, sky-wave propagation, and line-of-sight(LOS)propagation.In the VLF and audio frequency bands, where the wavelengths exceed 10 km, the earth and the ionosphere act as a waveguide for electromagnetic wave propagation.In these frequency ranges, communication signals practically propagate around the globe.For this reason, these frequency bands are primarily used to provide navigational aids from shore to ships around the world.The channel bandwidths available in these frequency bands are relatively small(usually 1-10% of the center frequency), and hence the information that is transmitted through these channels is of relatively slow speed and generally confined to digital transmission.A dominant type of noise at these frequencies is generated from thunderstorm activity around the globe,especially in tropical regions.Interference results from the many users of these frequency bands.Ground-wave propagation, as illustrated in Fig.1-2-3, is the dominant mode of propagation for frequencies in the MF band(0.3-3 MHz).This is the frequency band used for AM broadcasting and maritime radio broadcasting.In AM broadcasting, the range with groundwave propagation of even the more powerful radio stations is limited to about 150 km.Atmospheric noise,man-made noise, and thermal noise from electronic components at the receiver are dominant disturbances for signal transmission in the MF band.Sky-wave propagation, as illustrated in Fig.1-2-4 results from transmitted signals being reflected(bent or refracted)from the ionosphere, which consists of several layers of charged particles ranging in altitude from 50 to 400 km above the surface of the earth.During the daytime hours, the heating of the lower atmosphere by the sun causes the formation of the lower layers at altitudes below 120 km.These lower layers, especially the D-layer, serve to absorb frequencies below 2 MHz, thus severely limiting sky-wave propagation of AM radio broadcast.However, during the night-time hours, the electron density in the lower layers of the ionosphere drops sharply and the frequency absorption that occurs during the daytime is significantly reduced.As a consequence, powerful AM radio broadcast stations can propagate over large distances via sky wave over the F-layer of the ionosphere, which ranges from 140 to 400 km above the surface of the earth.A frequently occurring problem with electromagnetic wave propagation via sky wave in the HF frequency range is signal multipath.Signal multipath occurs when the transmitted signal arrives at the receiver via multiple propagation paths at different delays, tt generally results in intersymbol interference in a digital communication system.Moreover, the signal components arriving via different propagation paths may add destructively, resulting in a phenomenon called signal fading, which most people have experienced when listening to a distant radio station at night when sky wave is the dominant propagation mode.Additive noise at HF is a combination of atmospheric noise and thermal noise.Sky-wave ionospheric propagation ceases to exist at frequencies above approximately 30 MHz, which is the end of the HF band.However, it is possible to have ionospheric scatter propagation at frequencies in the range 30-60 MHz, resulting from signal scattering from the lower ionosphere.It is also possible to communicate over distances of several hundred miles by use of tropospheric scattering at frequencies in the range 40-300 MHz.Troposcatter results from signal scattering due to particles in the atmosphere at altitudes of 10 miles or less.Generally, ionospheric scatter and tropospheric scatter involve large signal propagation losses and require a large amount of transmitter power and relatively large antennas.Frequencies above 30 MHz propagate through the ionosphere with relatively little loss and make satellite and extraterrestrial communications possible.Hence, at frequencies in the VHF band and higher, the dominant mode of electromagnetic propagation is linc-of-sight(LOS)propagation.For terrestrial communication systems, this means that the transmitter and receiver antennas must be in direct LOS with relatively little or no obstruction.For this reason, television stations transmitting in the VHF and UHF frequency bands mount their antennas on high towers to achieve a broad coverage area.In general, the coverage area for LOS propagation is limited by the curvature of the earth.If the transmitting antenna is mounted at a height h m above the surface of the earth, the distance to the radio horizon, assuming no physical obstructions such as mountains, is approximately dr Thus,r represents the “age”(elapsed-time)variable.The three mathematical models described above adequately characterize the great majority of the physical channels encountered in practice.These three channel models are used in this text for the analysis and design of communication systems.1-4 A HISTORICAL PERSPECTIVE IN THE DEVELOPMENT OF DIGITAL COMMUNICATIONS It is remarkable that the earliest form of electrical communication, namely telegraphy, was a digital communication system.The electric telegraph was developed by Samuel Morse and was demonstrated in 1837.Morse devised the variable-length binary code in which letters of the English alphabet are represented by a sequence of dots and dashes(code words).In this code, more frequently occurring letters are represented by short code words, while letters occurring less frequently are represented by longer code words.Thus, the Morse code*was the precursor of the variable-length source coding methods described in Chapter 3.Nearly 40 years later, in 1875, Emile Baudot devised a code for telegraphy in which every letter was encoded into fixed-length binary code words of length 5.In the Baudot code, binary code elements are of equal length and designated as mark and space.Although Morse is responsible for the development of the first electrical digital communication system(telegraphy), the beginnings of what we now regard as modern digital communications stem from the work of Nyquist(1924), who investigated the problem of determining the maximum signaling rate that can be used over a telegraph channel of a given bandwidth without intersymbol interference.He formulated a model of a telegraph system in which a transmitted signal has the general form

s(t)anng(tnT)