电子信息专业英语 答案 中英文对应

Unit 1 电子学：模拟和数字

Unit 1-1

第一部分：理想运算放大器和实际限制

The approach to such a situation is to be methodical, and take the necessary time to read and understand each definition in the order that it is listed. Without a real appreciation of what each means, the designer is doomed to failure. The objective is to be able to design a circuit from the basis of the published data, and know that it will function as predicted when the prototype is constructed.In order to discuss the ideal parameters of operational amplifiers, we must first define the terms, and then go on to describe what we regard as the ideal values for those terms. At first sight, the specification sheet for an operational amplifier seems to list a large number of values, some in strange units, some interrelated, and often confusing to those unfamiliar with the subject. It is all too easy with linear circuits, which appear relatively simple when compared with today’s complex logic arrangements, to ignore detailed performance parameters which can drastically reduce the expected performance.2

为了讨论运算放大器的理想参数，我们必须首先定义一些指标项，然后对这些指标项讲述我们所认为的理想值。第一眼看运算放大器的性能指标表，感觉好像列出了大量的数值，有些是陌生的单位，有些是相关的，经常使那些对运放不熟悉的人感到迷惑。对于这种情况我们的方法是花上必要的时间有系统的按照列出的次序阅读并理解每一个定义。如果没有对每一项性能指标有一个真正的评价，设计人员必将失败。目标是能够依据公布的数据设计电路，并确认构建的样机将具有预计的功能。对于线性电路而言，它们与现在的复杂逻辑电路结构相比看起来较为简单，（因而在设计中）太容易忽视具体的性能参数了，而这些参数可极大地削弱预期性能。

Let us take a very simple but striking example. Consider a requirement for an amplifier having a voltage gain of 10 at 50 kHz driving into a 10 kW load.3 A common low-cost, internally frequency-compensated op amp is chosen; it has the required bandwidth at a closed-loop gain of 10, and it would seem to meet the bill.4 The device is connected, and it is found to have the correct gain. But it will only produce a few volts output swing when the data clearly shows that the output should be capable of driving to within two or three volts of the supply rails.5 The designer has forgotten that the maximum output voltage swing is severely limited by frequency, and that the maximum low-frequency output swing becomes limited at about 10 kHz. Of course, the information is in fact on the data sheet, but its relevance has not been appreciated. This sort of problem occurs regularly for the inexperienced designer. So the moral is clear: always take the necessary time to write down the full operating requirements before attempting a design. Attention to the detail of the performance specification will always be beneficial. It is suggested the following list of performance details be considered:

现在让我们来看一个简单但很引人注意的例子。考虑对于一个在50kHz频率上电压增益为10的

放大器驱动10k 负载时的要求。选择一个普通的带有内部频率补偿的低价运放，它在闭环增益为10时具有所要求的带宽，并且看起来满足了价格要求。器件连接后，发现有正确地增益。但是它只能产生几伏的电压变化范围，然而数据却清楚地显示输出应该能驱动达到电源电压范围以内2到3伏。设计人员忽视了最大输出电压变化范围是受频率严格限制的，而且最大低频输出变化范围大约在10 kHz受到限制。当然，事实上这个信息也在数据表上，但是它的实用性并没有受到重视。这种问题经常发生在那些缺乏经验的设计人员身上。所以这个例子的寓意十分明显：在开始设计之前总要花上必要的时间来描写全部的工作要求。关注性能指标的详情总是有益的。建议下面列出的具体的性能指标应该考虑：

1.Closed loop gain accuracy, stability with temperature, time and supply voltage

2.Power supply requirements, source and load impedances, power dissipation

3.Input error voltages and bias currents. Input and output resistance, drift with time

and temperature

4.Frequency response, phase shift, output swing, transient response, slew rate,

frequency stability, capacitive load driving, overload recovery

5.Linearity, distortion and noise

6.Input, output or supply protection required. Input voltage range, common-mode

rejection

7.External offset trimming requirement

Not all of these terms will be relevant, but it is useful to remember that it is better to consider them initially rather than to be forced into retrospective modifications.

1. 在温度，时间和供给电压下的闭环增益的精确性和稳定性

2. 电源要求，电源和负载阻抗，功率消耗

3. 输入误差电压和偏置电流，输入输出电阻，随着时间和温度的漂移

4. 频率响应，相位偏移，输出变化范围，瞬态响应，电压转换速率，频率稳定性，电容性负载驱动，过载恢复

5. 线性，失真和噪声

6. 输入，输出或电源保护要求，输入电压范围，共模抑制

7. 外部补偿调整要求

不是所有的指标项都是有关的，但要记住最初就考虑它们会更好，而不要被迫返工。

Never forget this fact. How many times has a circuit been designed using typical values, only to find that the circuit does not work because the device used is not typical?6 The above statement thus poses a tricky question: when should typical values and when should worst-case values be used in the design? This is where the judgment of the experienced designer must be brought to bear. Clearly, if certain performance requirements are mandatory, then worst-case values must be used. In many cases, however, the desirability of a certain defined performance will be a compromise between ease of implementation, degree of importance, and economic considerations.7

所有参数可以大范围变化

不要忽略这样一个事实。有多少次是在用典型值设计好电路后发现（该电路）只是因为使用的器件不典型而不能工作？这就提出一个棘手的问题：在设计中何时应该使用典型值，何时应该使用

最不利值？这是经验丰富的设计人员也必须进行的判断。显然，如果某些性能要求是强制性的，则一定要用最不利情况下的数值。然而在许多情况下某一规定性能是否可以取得将在易实现性，重要性，经济性之间取得折中。

In the end, we are all controlled by cost, and it is really pointless taking a sledgehammer to crack a nut, Simplicity is of the essence since the low parts count implementation is invariably cheaper and more reliable.8

不超指标设计或超安全标准设计

最后，我们将受制于价格因素，因为杀鸡用牛刀实在是没有意义的。简单极为重要，因为用较少元器件实现（的电路）总是更便宜也更可靠。

As an example of this judgment about worst-case design, consider a low-gain DC transducer amplifier required to amplify 10 mV from a voltage source to produce an output of l V with an accuracy of ±1% over a temperature range of 0~70?C.9 Notice that the specification calls for an accuracy of ±1%. This implies that the output should be 1 V ±10 mV from 0 ~ 70?C. The first step is, of course, to consider our list above, and decide which of the many parameters are relevant. Two of the most important to this (very limited) specification are offset voltage drift and gain stability with temperature. We will assume that all initial errors are negligible (rarely the case in practice). The experienced designer would know that most op amps have a very large open-loop gain, usually very much greater than 10000.

A closed-loop gain change of ±1% implies that the loop gain (as explained later) should change by less than ±100% for a closed-loop gain of 100.10This is clearly so easily fulfilled that the designer knows immediately that he can use typical open-loop gain values in his calculations. However, offset voltage drift is another matter. Many op amp specifications include only typical values for offset voltage drift; this may well be in the order of 5 mV/?C, with an unquoted maximum for any device of 30 mV/?C.11 If by chance we use a device which has this worst-case drift, then the amplifier error could be 30?70 = 2100mV=2.1 mV over temperature, which is a significant proportion of our total allowable error from all sources

作为最不利情况设计的例子，考虑一个低增益直流传感器放大器，要求将电压源输出的10mV信号放大，产生1V的输出，在0~70?C范围内达到±1%的精度。注意，性能要求是±1%的精度。这就是指输出必须在0~70?C温度范围内控制在1 V ±10 mV的限度内。第一步，当然是考虑前面的列表，并决定其中哪些参数是有关的。对这样（非常有限）的参数，两项最重要的指标是电压偏移和对于温度的增益稳定性。我们假设所有的起始误差可以忽略不计（这在实际中是几乎不可能的）。经验丰富的设计人员会知道大多数运放具有极大的开环增益，经常远大于10000。闭环增益±1%的变化意味着环路增益（将在下面说明）的变化在闭环增益为100时应该小于±100%。很明显这将十分容易实现，设计人员会立刻知道计算中他可以使用开环增益的典型值。但是，补偿电压偏移却有所不同。许多运放技术指标仅仅给出补偿电压偏移的典型值，这很可能会在5 V/?C的数量级，而未给出任何器件可以达到的最大值30 V/?C。如果我们碰巧使用的是一个有最不利偏移的器件，那么放大器随温度而产生的误差可为2.1 mV，占所有误差源所产生的总的允许误差的相当大一部分。

Here is a case, then, where one can be confident that the typical value of open-loop gain can be used, but where the maximum value of drift may well cause significant errors. This sort of judgment is essential in careful design, and great care is required in interpreting manufacturers’ data. This consideration must be extended to all the details listed above apart from the fact that worst-case values are often not quoted. It is often found that values given are not 100% tested. Statistical testing is employed which, for example,

guarantees that 90% of all devices fall within the range specified. It could be very inconvenient for the user who relies on the specified performance and then finds that he has several of the ‘other’ 10% actually plugged into his circuit.12

这就是我们可以肯定可使用开环增益典型值的情况，不过最大漂移很可能导致相当大的误差。在仔细的设计中这种判定是必要的，而且理解厂商的数据要更加仔细。这种考虑必须推广到前面列出的所有详细资料，除了最不利值通常是不会注明的。经常发现（技术规格表上）给出的值并非是经过100％测试的。例如，采用统计测试可以保证90％的器件的性能在给定范围之内。对于某些用户可能很不方便，他们依赖于技术指标所给出的性能，而随后发现却有“另外”10% 的器件被用在了他们的电路中。

Unit 2 集成电路

Unit 2-1

第一部分：集成电路

Digital logic and electronic circuits derive their functionality from electronic switches called transistor. Roughly speaking, the transistor can be likened to an electronically controlled valve whereby energy applied to one connection of the valve enables energy to flow between two other connections.1 By combining multiple transistors, digital logic building blocks such as AND gates and flip-flops are formed. Transistors, in turn, are made from semiconductors. Consult a periodic table of elements in a college chemistry textbook, and you will locate semiconductors as a group of elements separating the metals and nonmetals.2 They are called semiconductors because of their ability to behave as both metals and nonmetals. A semiconductor can be made to conduct electricity like a metal or to insulate as a nonmetal does. These differing electrical properties can be accurately controlled by mixing the semiconductor with small amounts of other elements.This mixing is called doping.

A semiconductor can be doped to contain more electrons (N-type) or fewer electrons (P-type). Examples of commonly used semiconductors are silicon and germanium. Phosphorous and boron are two elements that are used to dope N-type and P-type silicon, respectively.3 数字逻辑和电子电路由称为晶体管的电子开关得到它们的（各种）功能。粗略地说，晶体管好似一种电子控制阀，由此加在阀一端的能量可以使能量在另外两个连接端之间流动。通过多个晶体管的组合就可以构成数字逻辑模块，如与门和触发电路等。而晶体管是由半导体构成的。查阅大学化学书中的元素周期表，你会查到半导体是介于金属与非金属之间的一类元素。它们之所以被叫做半导体是由于它们表现出来的性质类似于金属和非金属。可使半导体像金属那样导电，或者像非金属那样绝缘。通过半导体和少量其它元素的混合可以精确地控制这些不同的电特性，这种混合技术称之为“半导体掺杂”。半导体通过掺杂可以包含更多的电子（N型）或更少的电子（P型）。常用的半导体是硅和锗，N型硅半导体掺入磷元素，而P型硅半导体掺入硼元素。

A transistor is constructed by creating a sandwich of differently doped semiconductor layers. The two most common types of transistors, the bipolar-junction transistor (BJT) and the field-effect transistor (FET) are schematically illustrated in Figure 2.1. This figure

shows both the silicon structures of these elements and their graphical symbolic representation as would be seen in a circuit diagram. The BJT shown is an NPN transistor, because it is composed of a sandwich of N-P-N doped silicon. When a small current is injected into the base terminal, a larger current is enabled to flow from the collector to the emitter. The FET shown is an N-channel FET, which is composed of two N-type regions separated by a P-type substrate. When a voltage is applied to the insulated gate terminal, a current is enabled to flow from the drain to the source. It is called N-channel, because the gate voltage induces an N-channel within the substrate, enabling current to flow between the N-regions.

不同掺杂的半导体层形成的三明治状夹层结构可以构成一个晶体管，最常见的两类晶体管是双极型晶体管（BJT）和场效应晶体管（FET），图2.1给出了它们的图示。图中给出了这些晶体管的硅结构，以及它们用于电路图中的符号。BJT是NPN晶体管，因为由N—P—N掺杂硅三层构成。当小电流注入基极时，可使较大的电流从集电极流向发射极。图示的FET是N沟道的场效应型晶体管，它由两块被P型基底分离的N型组成。将电压加在绝缘的栅极上时，可使电流由漏极流向源极。它被叫做N沟道是因为栅极电压诱导基底上的N通道，使电流能在两个N区域之间流动。

Another basic semiconductor structure shown in Figure 2.1 is a diode, which is formed simply by a junction of N-type and P-type silicon. Diodes act like one-way valves by conducting current only from P to N. Special diodes can be created that emit light when a voltage is applied. Appropriately enough, these components are called light emitting diodes, or LEDs. These small lights are manufactured by the millions and are found in diverse applications from telephones to traffic lights.

图2.1所示的另一个基本的半导体结构是二极管，由N型和P型硅连接而成的结组成。二极管的作用就像一个单向阀门，由于电流只能从P流向N。可以构建一些特殊二极管，在加电压时可以发光，这些器件非常合适地被叫做发光二极管或LED。这种小灯泡数以百万计地被制造出来，有各种各样的应用，从电话机到交通灯。

The resulting small chip of semiconductor material on which a transistor or diode is fabricated can be encased in a small plastic package for protection against damage and contamination from the outside world.4 Small wires are connected within this package between the semiconductor sandwich and pins that protrude from the package to make electrical contact with other parts of the intended circuit. Once you have several discrete transistors, digital logic can be built by directly wiring these components together. The circuit will function, but any substantial amount of digital logic will be very bulky, because several transistors are required to implement each of the various types of logic gates.

半导体材料上制作晶体管或二极管所形成的小芯片用塑料封装以防损伤和被外界污染。在这封装里一些短线连接半导体夹层和从封装内伸出的插脚以便与（使用该晶体管的）电路其余部分连接。一旦你有了一些分立的晶体管，直接用电线将这些器件连线在一起就可以构建数字逻辑（电路）。电路会工作，但任何实质性的数字逻辑（电路）都将十分庞大，因为要在各种逻辑门中每实现一种都需要多个晶体管。

At the time of the invention of the transistor in 1947 by John Bardeen, Walter Brattain, and William Shockley, the only way to assemble multiple transistors into a single circuit

was to buy separate discrete transistors and wire them together. In 1959, Jack Kilby and Robert Noyce independently invented a means of fabricating multiple transistors on a single slab of semiconductor material. Their invention would come to be known as the integrated circuit, or IC, which is the foundation of our modern computerized world. An IC is so called because it integrates multiple transistors and diodes onto the same small semiconductor chip. Instead of having to solder individual wires between discrete components, an IC contains many small components that are already wired together in the desired topology to form a circuit.

1947年，John Bardeen、Walter Brattain和and William Shockley发明晶体管的时候。将多个晶体管组装在一个电路上的唯一方法就是购买多个分离的晶体管，将它们连在一起。1959年，Jack Kilby 和 Robert Noyce各自独立地发明了一种将多个晶体管做在同一片半导体材料上的方法。这个发明就是集成电路，或IC，是我们现代电脑化世界的基础。集成电路之所以被这样命名，是因为它将多个晶体管和二极管集成到同一块小的半导体芯片上。IC包含按照形成电路所要求的拓扑结构连在一起的许多小元件，而无需再将分立元件的导线焊接起来。A typical IC, without its plastic or ceramic package, is a square or rectangular silicon die measuring from 2 to 15 mm on an edge. Depending on the level of technology used to manufacture the IC, there may be anywhere from a dozen to tens of millions of individual transistors on this small chip. This amazing density of electronic components indicates that the transistors and the wires that connect them are extremely small in size. Dimensions on an IC are measured in units of micrometers, with one micrometer (1mm) being one millionth of a meter. To serve as a reference point, a human hair is roughly 100mm in diameter. Some modern ICs contain components and wires that are measured in increments as small as 0.1mm! Each year, researchers and engineers have been finding new ways to steadily reduce these feature sizes to pack more transistors into the same silicon area, as indicated in Figure 2.2.

去除了塑料或陶瓷封装后，一个典型的集成电路就是每一边2mm至15mm的方形或矩形硅片。根据制造集成电路的技术水平的不同，在这种小片上可能有几十个到几百万个晶体管，电子器件这种令人惊异的密度表明那些晶体管以及连接它们线是极其微小的。集成电路的尺寸是以微米为单位测量的，1微米是1米的百万分之一。作为参照，一根人的头发其直径大约为100微米。一些现代集成电路包含的元件和连线，是以小到0.1微米的增量来测量的。每年研究人员和工程师都在寻找新的方法来不断减小这些元件的大小，以便在同样面积的硅片上集成更多的晶体管，如图2.2所示。

When an IC is designed and fabricated, it generally follows one of two main transistor technologies: bipolar or metal-oxide semiconductor (MOS). Bipolar processes create BJTs, whereas MOS processes create FETs. Bipolar logic was more common before the 1980s, but MOS technologies have since accounted the great majority of digital logic ICs. N-channel FETs are fabricated in an NMOS process, and P-channel FETs are fabricated in a PMOS process. In the 1980s, complementary-MOS, or CMOS, became the dominant process technology and remains so to this day. CMOS ICs incorporate both NMOS and PMOS transistors.

在集成电路的设计和制造过程中，常用两种主要晶体管技术是：双极和金属氧化物半导体（MOS）。

双极工艺生产出来的是BJT（双极型晶体管），而MOS工艺生产出来的是FET（场效应晶体管）。在20世纪80年代以前更常用的集成电路是双极逻辑，但是此后MOS技术在数字逻辑集成电路中占据了大多数。N沟道FET是采用NMOS工艺生产的，而P沟道FET是采用PMOS工艺生产的。到了20世纪80年代，互补MOS即CMOS成为占主导地位的加工技术，并且延续至今。CMOS集成电路包含了NMOS 和PMOS两种晶体管。

Unit 3 电磁场，天线和微波

Unit 3-1

第一部分：电磁场

The electromagnetic field is a physical field produced by electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction. It is one of the four fundamental forces in the nature (the others are gravitation, the weak interaction, and the strong interaction)

电磁场是由带电物体产生的物理场。它会影响场附近的带电物体的行为。电磁场在空间无限延伸，描述电磁相互作用。它是自然界中四个基本作用力之一（其余为万有引力，弱相互作用，强相互作用）。

The field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. The way in which charges and currents interact with the electromagnetic field is described by Maxwell’s equations and the Lorentz force law. From a classical point of view, the electromagnetic field can be regarded as a smooth, continuous field, propagated in a wavelike manner, whereas from a quantum mechanical point of view, the field can be viewed as being composed of photons.

电磁场可以看成是电场和磁场的结合。电场是由静止电荷产生的，磁场由运动电荷（电流）产生；这两种（电荷）通常被描述为电磁场的源。电荷和电流与电磁场相互作用的方式由麦克斯韦方程组和洛伦兹力定律所描述。从经典的角度，电磁场可认为使平滑、连续的场，以波动方式传播；而从量子力学的角度，场可看作是由光子组成的。

电磁场的结构

The electromagnetic field may be viewed in two distinct ways.

Continuous structure: Classically, electric and magnetic fields are thought of as being

produced by smooth motions of charged objects. For example, oscillating charges produce electric and magnetic fields that may be viewed in a “smooth”, continuous, wavelike manner. In this case, energy is viewed as being transferred continuously through the electromagnetic field between any two locations. For instance, the metal atoms in a radio transmitter appear to transfer energy continuously. This view is useful to a certain extent (radiation of low frequency), but problems are found at high frequencies (see ultraviolet catastrophe). This problem leads to another view.

电磁场可用两种截然不同的观点来看。

连续结构：经典地，电场和磁场被认为是由带电物体的平滑运动产生的。例如，振荡电荷产生电场和磁场，可看成光滑的、连续的波动方式。这时，能量可以看成是在任意两个位置之间通过电磁场连续地传递。例如，在无线电发射机中的金属原子（看来是）连续地传递能量。这个观点在一定程度上（低频辐射）是有用的，但是高频时就有问题（如紫外灾难）。由此产生了另一种观点。

Discrete structure: The electromagnetic field may be thought of in a more “coarse” way. Experiments reveal that electromagnetic energy transfer is better described as being carried away in photons with a fixed frequency. Planck’s relation links the energy E of a photon to its frequency n through the equation: E = h n.where h is Planck’s constant, named in honor of Max Planck, and n is the frequency of the photon. For example, in the photoelectric effect –the emission of electrons from metallic surfaces by electromagnetic radiation – it is found that increasing the intensity of the incident radiation has no effect, and that only the frequency of the radiation is relevant in ejecting electrons.1

离散结构：电磁场可以一种比较“粗略”的方式来考虑。实验表明：电磁场的能量传递可以更好地描述为用固定频率的光子来传递。普朗克的关系式将光子的能量E及其频率 通过下式联系起来E = h ，其中h是为纪念马克斯普朗克而命名的普朗克常量。 是光子的频率。例如在光电效应中，即因电磁辐射而从金属表面发射电子的现象，我们发现增加入射辐射的强度并无影响，只有辐射频率与发射的电子有关。

This quantum picture of the electromagnetic field has proved very successful, giving rise to quantum electrodynamics, a quantum field theory describing the interaction of electromagnetic radiation with charged matter.

已经证明电磁场的量子描述是非常成功的，引出了量子电动力学。量子电动力学是一种描述电磁辐射与带电物体之间相互作用的量子场理论。

电磁场的动力学

In the past, electrically charged objects were thought to produce two types of field associated with their charge property. An electric field is produced when the charge is stationary with respect to an observer measuring the properties of the charge and a magnetic field (as well as an electric field) is produced when the charge moves (creating an electric current) with respect to this observer. Over time, it was realized that the electric and magnetic fields are better thought of as two parts of a greater whole –the electromagnetic

field.2

过去，认为带电物体会产生与它们电荷性质有关的两种场。相对于测量电荷性质的观察者，电荷静止时产生电场，电荷运动（产生电流）时产生磁场（和电场）。随着时间的推移，人们认识到电场和磁场是电磁场这一整体的两个部分。

Once this electromagnetic field has been produced from a given charge distribution, other charged objects in this field will experience a force (in a similar way that planets experience a force in the gravitational field of the Sun). If these other charges and currents are comparable in size to the sources producing the above electromagnetic field, then a new net electromagnetic field will be produced.3 Thus, the electromagnetic field may be viewed as a dynamic entity that causes other charges and currents to move, and which is also affected by them. These interactions are described by Maxwell’s equations and the Lorentz force law.

一旦给定的电荷分布产生了电磁场，那么在电磁场中的其它带电物体就会受到一个力的作用（就像行星受到太阳重力场的力一样）。如果这些电荷和电流的大小与产生上述电磁场的源是可比的，那么将产生一个新的净电磁场。这样，电磁场可以认为是一个动力学实体，它促使其它电荷和电流运动，同时又受它们影响。这些相互作用可由麦克斯韦方程和洛伦兹力定律来描述。

Unit 4 通信和信息论

Unit 4-1

第一部分：远程通信

Today, telecommunication is widespread and devices that assist the process such as television, radio and telephone are common in many parts of the world. There are also many networks that connect these devices, including computer networks, public telephone networks, radio networks and television networks. Computer communication across the Internet is one of many examples of telecommunication.

远程通信是远距离通信的信号传输，在现代，通常这个过程需要电子发射机发射电磁波，但是在早期远程通信包括使用烟火信号，鼓或旗语或日光仪。今天，远程通信很普遍的，助推这一过程的设备如电视，无线电和电话在世界的许多地区都已很普遍。还有连接这些设备的许多网络，包括计算机网络，公共电话网，无线电网和电视网络。互联网上的计算机通信是众多通信的一个例子。

Telecommunication systems are generally designed by telecommunication engineers. Early inventors in the field include Alexander Graham Bell, Guglielmo Marconi and John Logie Baird. Telecommunication is an important part of the world economy with the telecommunication industry’s revenue being placed at just under 3 percent of the gross worl d product.

通信系统通常由通信工程师设计。在这个领域中早期的发明家有Alexander Graham Bell, Guglielmo Marconi 和John Logie Baird。通信在当今的世界经济发展中起着举足轻重的作用，通信产业的税收在世界总产值的比例已接近百分之三。

基本要素

Each telecommunication system consists of three basic elements: a transmitter that takes information and converts it to a signal, a transmission medium over which the signal is transmitted, and a receiver that receives the signal and converts it back into usable information.

每个通信系统包括三个基本要素：采集信息并能将其转换为信号的发射机，传输信号的传输媒介，接收信号并能将其还原为有用信息的接收机。

Consider a radio broadcast for example. The broadcast tower is the transmitter, the radio is the receiver and the transmission medium is free space. Often telecommunication systems are two-way, and a single device acts as both a transmitter and receiver, or transceiver. For example, a mobile phone is a transceiver.

考虑一个无线电广播的例子。广播塔是发射机，收音机是接收机，传输媒介是自由空间。通常通信系统都是双向的，一个设备既做发射机又做接收机，即收发器。例如，移动手机就是一个收发器。

Telecommunication over a phone line is called point-to-point communication because it is between one transmitter and one receiver. Telecommunication through radio broadcasts is called broadcast (or point-to-multipoint) communication because it is between one powerful transmitter and numerous receivers.

电话线上的通信称为点对点通信，因为只在一个发射机和一个接收机之间。通过无线电广播的通信称为广播（一对多）通信，因为通信是在一个大功率的发射机和许多接收机之间。

模拟或数字

Signals can either be analogue or digital. In an analogue signal, the signal is varied continuously with respect to the information. In a digital signal, the information is encoded as a set of discrete values (for example, ones and zeros). During transmission, the information contained in analogue signals will be degraded by noise. Conversely, unless the noise exceeds a certain threshold, the information contained in digital signals will remain intact. This represents a key advantage of digital signals over analogue signals.

信号可以是模拟的，也可以是数字的。在模拟信号中，信号根据信息而连续变化。在数字信号信息被编码为一组离散值（如，1和0）。在传输过程中，模拟信号中的信息会因噪声而退化。相反，只要噪声不超过一定的阈值，数字信号中的信息是不会丢失的。这是数字信号相对于模拟信号一个关键的优点。

网络

A collection of transmitters, receivers or transceivers that communicate with each

other is known as a network. Digital networks may consist of one or more routers that route data to the correct user. An analogue network may consist of one or more switches that establish a connection between two or more users.1 For both types of network, repeaters may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combat attenuation that can render the signal indistinguishable from noise.2

网络是由一个相互通信的发射机、接收机或收发机的集合。数字网络由一个或多个路由器组成，路由器正确地将数据发送给用户。模拟网路由一个或多个交换器组成，交换器在两个或多个用户间建立连接。这两种网络都需要中继器，用于远距离传输时的放大或重建信号。（中继器）用来抗击衰减，再现噪声中难以分辨的信号。

信道A channel is a division in a transmission medium so that it can be used to send multiple streams of information.3For example, a radio station may broadcast at 96 MHz while another radio station may broadcast at 94.5 MHz. In this case, the medium is divided by frequency and each channel receives a separate frequency to broadcast on. Alternatively, one could allocate each channel a recurring segment of time over which to broadcast —this is known as time-division multiplexing and is sometimes used in digital communication.4 信道在传输媒介中分隔区域，以用于使传输媒介传送多个信息流。例如，一个广播站可以工作在96 MHz，而另一个广播站可以工作在94.5 MHz。这时，传输媒介被频率分隔，每个信道接收一个单独的广播频率。另一种方法是给每个信道分配重复的时间段，在这种时间段中进行广播，称为时分多路技术，有时用于数字通信中。

调制

The shaping of a signal to convey information is known as modulation. Modulation can be used to represent a digital message as an analogue waveform. This is known as keying and several keying techniques exist (these include phase-shift keying, frequency-shift keying and amplitude-shift keying). Bluetooth, for example, uses phase-shift keying to exchange information between devices.

传输信息的信号的形成称为调制。调制可将数字信息表示为模拟波形，称为键控法，键控法有几种技术，包括相移键控，频移键控和振幅键控。例如，蓝牙就是通过相移键控在不同设备之间交换信息的。

Modulation can also be used to transmit the information of analogue signals at higher frequencies. This is helpful because low-frequency analogue signals cannot be effectively transmitted over free space. Hence the information from a low-frequency analogue signal must be superimposed on a higher-frequency signal (known as a carrier wave) before transmission. There are several different modulation schemes available to achieve this (two of the most basic being amplitude modulation and frequency modulation). An example of this process in action is a DJ’s voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel

“96 FM”).

调制也可用来以更高的频率传送模拟信号的信息。这一点非常有用，因为低频的模拟信号不能在自由空间中有效地传输。所以，低频模拟信号中的信息在发送之前必须叠加在高频载波上。这种调制也有几种不同的方法，最基本的两种是幅度调制和频率调制。例如，电台主持人的声音就是通过频率调制加载在96 MHz 的载波中发送的（收音机的96 FM台可以收到这个声音）。

Unit 5 多址技术

Unit 5-1

第一部分：多址技术：频分多址、时分多址、码分多址

Multiple access schemes are used to allow many simultaneous users to use the same fixed bandwidth radio spectrum. In any radio system, the allocated bandwidth is always limited. For mobile phone systems the total bandwidth is typically 50MHz, which is split in half to provide the forward and reverse links of the system. Sharing of the spectrum is required in order to increase the user capacity of any wireless network. FDMA, TDMA and CDMA are the three major methods of sharing the available bandwidth to multiple users in wireless system. There are many extensions, and hybrid techniques for these methods, such as OFDM, and hybrid TDMA and FDMA systems. However, an understanding of the three major methods is required for understanding of any extensions to these methods.

多址方案用于使许多用户同时使用同一个固定带宽的无线电频谱。在任何无线电系统中分配的带宽总是有限的。移动电话系统的典型总带宽是50MHz，它被分成两半用以提供系统的前向和反向连接。任何无线网络为了提高用户容量都需要共享频谱。频分多址（FDMA）、时分多址（TDMA）、码分多址（CDMA）是无线系统中由众多用户共享可用带宽的三种主要方法。这些方法又有许多扩展和混合技术，例如正交频分复用（OFDM），以及混合时分和频分多址系统。不过要了解任何扩展技术首先要求对三种主要方法的理解。

频分多址

In Frequency Division Multiple Access (FDMA), the available bandwidth is subdivided into a number of narrower bands. Each user is allocated a unique frequency band in which to transmit and receive. During a call, no other user can use the same frequency band. Each user is allocated a forward link channel (from the base station to the mobile phone) and a reverse channel (back to the base station), each being a single way link. The transmitted signal on each of the channels is continuous allowing analog transmissions. The bandwidths of FDMA channels are generally low (30 kHz) as each channel only supports one user. FDMA is used as the primary breakup of large allocated frequency bands and is used as part of most multi-channel systems. Figures 5.1 and 5.2 show the allocation of the available bandwidth into several channels.

在FDMA中，可用带宽被分为许多个较窄的频带。每一用户被分配一个独特的频带用于发送和接

收。在一次通话中其他用户不能使用同一频带。每个用户分配到一个由基站到移动电话的前向信道以及一个返回基站的反向信道，每个信道都是一个单向连接。在每个信道中传输信号是连续的，以便进行模拟通信。FDMA信道的带宽一般较小（30kHz），每个信道只支持一个用户。FDMA作为大多数多信道系统的一部分用于初步分割分配到的宽频带。将可用带宽分配给几个信道的情况见图5.1和图5.2。

时分多址Time Division Multiple Access (TDMA) divides the available spectrum into multiple time slots, by giving each user a time slot in which they can transmit or receive. Figure 5.3 shows how the time slots are provided to users in a round robin fashion, with each user being allotted one time slot per frame.1

TDMA将可用频谱分成多个时隙，通过分配给每一个用户一个时隙以便在其中发送或接收。图5.3显示如何以一种循环复用的方式把时隙分配给用户，每个用户每帧分得一个时隙。

TDMA systems transmit data in a buffer and burst method, thus the transmission of each channel is non-continuous. The input data to be transmitted is buffered over the previous frame and burst transmitted at a higher rate during the time slot for the channel.2 TDMA cannot send analog signals directly due to the buffering required, thus is only used for transmitting digital data. TDMA can suffer from multipath effects as the transmission rate is generally very high. This leads the multipath signals causing inter-symbol interference.

TDMA以缓冲和爆发方式发送数据。因此每个信道的发射是不连续的。待发送的输入数据在前一帧期间被缓存，在分配给该信道的时隙中以较高速率爆发式发送出去。TDMA不能直接传送模拟信号因为它需要使用缓冲，因而只能用于传输数字形式的数据。由于通常发送速率很高，TDMA会受到多径效应的影响。这导致多径信号引起码间干扰。

TDMA is normally used in conjunction with FDMA to subdivide the total available bandwidth into several channels. This is done to reduce the number of users per channel allowing a lower data rate to be used. This helps reduce the effect of delay spread on the transmission. Figure 5.4 shows the use of TDMA with FDMA. Each channel based on FDMA, is further subdivided using TDMA, so that several users can transmit over one channel. This type of transmission technique is used by most digital second generation mobile phone systems. For GSM, the total allocated bandwidth of 25MHz is divided into 125 channels using FDMA, each having a bandwidth of 200 kHz. These channels are then subdivided further by using TDMA so that each 200 kHz channel allows 8-16 users.

TDMA一般与FDMA结合使用，将可用的全部带宽划分为若干信道。这是为了减少每个信道上的用户数以便使用较低的数据速率。这有助于降低延迟扩展对传输的影响。图5.4显示TDMA结合FDMA的使用。将基于FDMA的各信道进一步用TDMA划分，从而多个用户可以在同一信道上发送信号。这一类传输技术用于大多数第二代移动通信系统。对于GSM系统，分配的全部25MHz带宽被用FDMA分成125个信道，每一个带宽为200kHz。这些信道又用TDMA进一步分割，每一个200kHz的信道可容纳8～16个用户。

码分多址

Code Division Multiple Access (CDMA) is a spread spectrum technique that uses neither frequency channels nor time slots. In CDMA, the narrow band message (typically digitized voice data) is multiplied by a large bandwidth signal which is a pseudo random noise code (PN code).

All users in a CDMA system use the same frequency band and transmit simultaneously. The transmitted signal is recovered by correlating the received signal with the PN code used by the transmitter. Figure 5.5 shows the general use of the spectrum using CDMA.

CDMA是一种扩频技术，既不使用频率信道也不使用时隙。在CDMA中，窄带的消息（典型的是数字话音）被乘以一个宽带的伪随机噪声（PN码）信号。一个CDMA系统中的所有用户使用同一频带而且同时发送。发射的信号通过将接收信号与发送者用的PN码做相关而恢复出来。图5.5显示CDMA系统中频谱的通常使用方式。

CDMA technology was originally developed by the military during World War II. Researches were spurred into looking at ways of communicating that would be secure and work in the presence of jamming.

CDMA技术最初是在第二次世界大战中由军方开发的。当时研究人员受到激励以寻求安全和能够在干扰中正常工作的通信方式。使CDMA有用的一些特性包括：

Some of the properties that have made CDMA useful are:

-S ignal hiding and non-interference with existing systems

-A nti-jam and interference rejection

-I nformation security

-A ccurate ranging

-M ultiple user access

-M ultipath tolerance

—信号隐藏，而且不干扰现有系统

—抗敌方干扰和噪声干扰

—信息安全

—精确测距

—多用户接入

—对多径的适应性

For many years, spread spectrum technology was considered solely for military applications. However, with rapid developments in LSI and VLSI designs, commercial systems are starting to be used.

多年以来，扩频技术一直被认为是只适合于军用。但是随着大规模集成电路（LSI）和超大规模集成电路（VLSI）设计的快速发展，商用系统也开始使用了。

CDMA处理增益

One of the most important concepts required in order to understand spread spectrum techniques is the idea of process gain. The process gain of a system indicates the gain or signal to noise improvement exhibited by a spread spectrum system by the nature of the

spreading and despreading process.3 The process gain of a system is equal to the ratio of the spread spectrum bandwidth used, to the original data bit rate. Thus, the process gain can be written as:

要理解扩频技术最重要的概念之一就是处理增益。系统处理增益是指扩频系统通过扩频和反扩频的性质所表现出来的增益或信噪比的提高。系统处理增益等于使用的扩频带宽与数据原来的比特率之比。因此处理增益可写为：

nfo RF

i p BW BW G =

where BW RF is the transmitted bandwidth after the data is spread, and BW info is the bandwidth of the information data being sent.

其中BW RF 是数据扩展以后的发射带宽，BW info 是所发送信息数据的带宽。

Figure 5.6 shows the process of a CDMA transmission. The data to be transmitted (a) is spread before transmission by modulating the data using a PN code. This broadens the spectrum as shown in (b).

In this example the process gain is 125 as the spread spectrum bandwidth is 125 times greater than the data bandwidth. Part (c) shows the received signal. This consists of the required signal, plus background noise, and any interference from other CDMA users or radio sources.

The received signal is recovered by multiplying the signal by the original spreading code. This process causes the wanted received signal to be despread back to the original transmitted data.

图5.6给出CDMA 传输过程。待发送的数据（a ）在发送前（被）用一个PN 码调制实现扩频。这使频谱扩展，如（b ）所示。在本例中处理增益为125因为扩频带宽是数据带宽的125倍。（c ）是接收信号。它包括要求的信号，附加的背景噪声，以及其它CDMA 用户或无线电信号源的干扰。接收信号通过将信号与原来用于扩频的码进行相乘而恢复出来。However, all other signals uncorrelated to the PN spreading code used become more spread.这一过程使需要的接收信号反扩频恢复成原来的发射数据。然而，所有与所用PN 码不相关的其它信号变得更加扩展。The wanted signal in (d) is then filtered removing the wide spread interference and noise signals.

然后（d ）中的所需信号被滤波出来，而去掉扩频干扰和噪声信号。

CDMA 信号发生

CDMA is achieved by modulating the data signal by a pseudo random noise sequence (PN code), which has a chip rate higher than the bit rate of the data. The PN code sequence is a sequence of ones and zeros (called chips), which alternate in a random fashion. The data is modulated by modular-2 adding the data with the PN code sequence. This can also be done by multiplying the signals, provided the data and PN code are represented by 1 and -1 instead of 1 and 0. Figure 5.7 shows a basic CDMA transmitter

CDMA 通过用伪随机序列（PN 码）调制数据信号来实现，PN 码的码片频率高于数据的比特率。PN 序列是一系列随机交替的1和0（称为码片）。数据通过与PN 码序列做模-2加法被调制。也可以通过信号相乘得到，只要数据和PN 序列都用1和-1表示而不是1和0。图5.7是一个基本的CDMA 发射器。

The PN code used to spread the data can be of two main types. A short PN code (typically 10 ~ 128 chips in length) can be used to modulate each data bit. The short PN code is then repeated for every data bit allowing for quick and simple synchronization of the receiver.

用于数据扩频的PN 码可由两种主要类型。短的PN 码（典型长度10～128码片）可用于调制每一个数据比特。短的PN 码对每一比特数据重复使用，可实现接收机的快速和简单的同步。Figure 5.8 shows the generation of a CDMA signal using a 10-chip length short code. Alternatively a long PN code can be used.

图5.8显示一个使用10个码片的短码CDMA 信号的产生。另外也可以使用长码。Long codes are generally thousands to millions of chips in length, thus are only repeated infrequently. Because of this they are useful for added security as they are more difficult to decode.

长码的程度通常有几千乃至几百万码片，因此不经常重复。因此他们更难以解码，所以有益于增加安全性。

CDMA 前向连接编码

The forward link, from the base station to the mobile, of a CDMA system can use special orthogonal PN codes called Walsh code, for separating the multiple users on the same channel. These are based on a Walsh matrix, which is a square matrix with binary elements, and dimensions which are a power of two.

CDMA 系统中从基站到移动电话的前向连接可以使用称为Walsh 码的特殊正交码来将同一信道的多用户分开。这些码基于Walsh 矩阵，它是由二进制元素构成的方阵，其阶数是2的幂，由一个基Walsh(1)=W 1=0和It is generated from the basis W 1 = 0 and that:

下式生成：

??????=n n n n n W W W W W 2

where Wn is the Walsh matrix of dimension n . For example:

其中W n 是n 阶Walsh 矩阵。例如

??????=10002W

????????????=01101100101000004W

Walsh codes are orthogonal, which means that the dot product of any two rows is zero.

This is due to the fact that for any two rows exactly half the number of bits match and half do not. Each row of a Walsh matrix can be used as the PN code of a user in a CDMA system. By doing this the signals from each user is orthogonal to every other user, resulting in no interference between the signals.4

Walsh码是正交的，就是说任何两行间的点积都是0。这是因为任何两行之间都有一半的比特相同，另一半不同。

Walsh矩阵的每一行都可用作CDMA系统中一个用户的PN码。这一处理过程使每一用户的信号与所有其它用户的信号正交，因而相互之间没有干扰。However, in order for Walsh codes to work the transmitted chips from all users must be synchronized. If the Walsh code used by one user is shifted in time by more than about 1/10 of a chip period with respect to all the other Walsh codes, it loses its orthogonal nature, resulting in inter-user interference.5 不过为了使Walsh码能起作用，所有用户的码片都必须同步。如果一个用户使用的Walsh码在时间上相对于其它所有Walsh码偏移了超过约十分之一的码片周期，就失去了正交性，导致用户间干扰。For the forward link signals for all the users originate from the base station, allowing the signals to be easily synchronized.

对于前向连接所有用户的信号源自基站，因此它们很容易同步。

CDMA反向连接编码

The reverse link is different to the forward link because the signals from each user do not originate from a same source as in the forward link. The transmission from each user will arrive at a different time, due to propagation delay and synchronization errors. Due to the unavoidable timing errors between the users, there is little point in using Walsh codes as they will no longer be orthogonal.6 For this reason simple pseudo random sequence which are uncorrelated, but not orthogonal are used for the PN codes of each user.

反向连接不同于前向连接，因为从各用户发出的信号并不像前向连接那样由同一个源产生。由于传播延迟和同步误差，不同用户发射的信号在不同时刻到达。由于用户之间不可避免的定时偏差，Walsh 码几乎没用，因为它们之间不再正交。由于这一原因，用不相关而又不正交的伪随机序列作为各用户的PN码。

The capacity is different for the forward and the reverse links because of the differences in modulation. The reverse link is not orthogonal, resulting in significant inter-user interference. For this reason the reverse channel sets the capacity of the system.7 由于调制方法的不同，前向和反向连接的容量是不同的。反向连接是非正交的，导致用户间的严重干扰。由于这一原因，反向信道限制了系统的容量。

Unit 6 移动通信

Unit 6-1

第一部分：移动通信

A mobile system is one in which users can physically move while communicating with one another. Examples include pagers, cellular phones, and cordless phones. It is the mobility that has made RF communications powerful and popular. The transceiver carried by the user is called the “mobile unit” (or simply the “mobile”), the “terminal,” or the “hand-held unit.”

The complexity of the wireless infrastructure often demands that the mobiles communicate only through a fixed, relatively expensive unit called the “base station.” Each mobile receives and transmits information from and to the base station via two RF channels called the “forward channel” or “downlink” and the “reverse channel” or “uplink,” respectively.

一个移动通信系统是指用户在这个系统中可以一边和别人互相通信，一边在物理位置上进行移动。例如：传呼机、蜂窝电话和无绳电话。移动性使得射频通信功能强大而且广为流行。用户所持的收发器叫移动单元、终端或手持单元。无线基础设施的复杂性往往要求移动单元只通过一些固定的、较昂贵的称为基站的设备进行通信。每个移动单元通过两个射频信道接收来自基站的信息并向基站发射信息，这两个信道分别称为前向信道或下行链路，以及逆向信道或上行链路。Most of our treatment relates to the mobile unit because, compared to the base station, hand-held units constitute a much larger portion of the market and their design is much more similar to other types of RF systems.

我们大多数讨论的是移动单元，因为和基站相比，手持单元构成市场极大的一块，它们的设计更接近于其他射频系统。

蜂窝系统

With the limited available spectrum (e.g., 25 MHz around 900 MHz), how do hundreds of thousands of people communicate in a crowded metropolitan area? To answer this question, we first consider a simpler case: thousands of FM radio broadcasting stations may operate in a country in the 88-108 MHz band.

对于一个有限的可用频谱（例如：900MHz附近的一个25MHz的频谱），数十万人如何在拥挤的城区里相互通信？为了回答这个问题，首先考虑一种较简单的情况：几千个FM电台可利用88-108MHz 的频带在一个国家里广播。This is possible because stations that are physically far enough from each other can use the same carrier frequency (frequency reuse) with negligible mutual interference, except at some point in the middle where the stations are received with comparable signal levels.1 The minimum distance between two stations that can employ equal carrier frequencies depends on the signal power produced by each.

这是可能的，因为在物理位置上相隔足够远的电台可使用同一载波频率（频率重用），而相互干

扰可以忽略。两个电台的中间位置除外，这里接收到的两个电台信号强度相近。两个可以使用相同载波频率的电台的最小距离是由每个电台发射的信号功率所决定的。

In mobile communications, the concept of frequency reuse is implemented in a “cellular” structure, where each cell is configured as a hexagon and surrounded by six other cells as shown in Figure 6.1(a).

在移动通信系统中，用蜂窝结构来实现频率重用概念，其中每一个蜂窝是六边形的，其周围环绕着6个其它的蜂窝，如图6.1(a)所示。The idea is that, if the center cell uses a frequency f1 for communication, the six neighboring cells cannot utilize this frequency but the cells beyond the immediate neighbors may.2 In practice, more efficient frequency assignment leads to the “7-cell” reuse pattern shown in Figure 6.1(b). Note that in reality each cell utilizes a group of frequencies.

频率重用概念是：如果位于中央的蜂窝使用频率f1进行通信，那么与其相邻的6个蜂窝就不能使用这个频率，但外面不直接相邻的蜂窝可再次使用这个频率。实际上，更有效的频率分配方式是如图6.1(b)所示的“7蜂窝”重用模式。注意：实际上每个蜂窝是使用了一组频率。

The mobile units in each cell of Figure 6.1(b) are served by a base station, and all of the base stations are controlled by a “mobile telephone switching office” (MTSO).

图6.1(b)中的每一个蜂窝中的移动单元都有一个基站提供服务，而所有的基站则有一个移动电话交换机构（MTSO）来控制。

同信道干扰

An important issue in a cellular system is how much two cells that use the same frequency interfere with each other. Called co-channel interference (CCI), this effect depends on the ratio of the distance between two co-channel cells to the cell radius and is independent of the transmitted power.在蜂窝系统中，一个重要的问题是两个使用同一频率的单元之间的干扰有多大。这种干扰叫做同信道干扰，这一效应依赖于两个同信道单元之间的距离与单元半径之比，而与发射功率无关。

Given by the frequency reuse plan, this ratio is approximately equal to 4.6 for the 7-cell pattern of Figure 6.1(b). It can be shown that this value yields a signal-to-co-channel interference ratio of 18 dB.

给定频率重用方案，对于图6.1(b)所示的7蜂窝模式，这个比大约是4.6。可以看出，这个值导致信号－同信道干扰比为18dB

切换

What happens when a mobile unit “roams” from cell A to cell B? Since the power level received from the base station in cell A is insufficient to maintain communication, the mobile must change its server to the base station in cell B. Furthermore, since adjacent cells do not use the same group of frequencies, the channel must also change. Called “handoff,” this process is performed by the MTSO.

当一个移动单元从蜂窝A漫游到蜂窝B时将会发生什么事情？因为从单元A的基站接收到的功率电平不足以维持通信，手机必须将服务器更换为单元B的基站。而且，由于相邻的蜂窝并不使用同一组频率，因此，移动单元还必须更换信道。这一过程叫做切换，是由MTSO来完成的。Once the level received by the base station in cell A drops below a threshold, the MTSO hands off the mobile to the base station in cell B, hoping that the latter is close enough. This strategy fails with relatively high probability, resulting in dropped calls.

一旦基站A接收的电平低于某一阈值，MTSO将手机切换到基站B，希望后者足够近。这种策略失败的可能性比较高，会导致通话的中断。

To improve the handoff process, second-generation cellular systems allow the mobile unit to measure the received signal level from different base stations, thus performing handoff when the path to the second base station has sufficiently low loss.

为了改善切换过程，第二代蜂窝系统使手机能测量接收来自不同基站的信号电平，当到第二个基站的路径损耗足够低时进行切换。

路径损耗和多径衰落

Propagation of signals in a mobile communication environment is quite complex. We briefly describe some of the important concepts here. Signals propagating through free space experience a power loss proportional to the square of the distance, d, from the source.3 在一个移动通信环境里，信号的传播是相当复杂的。这里只简单描述一些重要的概念。在自由空间里传播的信号会有功率损耗，其值正比于离开发射源的距离d的平方。In reality, however, the signal travels through both a direct path and an indirect, reflective path (Figure 6.2).

然而实际上，信号是同时沿着直接路径和间接的反射路径进行传播的，如图6.2所示，在这种情况下，可以看出损耗随距离的四次方增大。It can be shown that in this case, the loss increases with the fourth power of the distance. In crowded areas, the actual loss profile maybe proportional to d2 for some distance and d4 for another.

在一个拥挤的区域里，实际的损耗情况可能对于某些距离是与d2成比例，而对于另一些距离是与d4成比例。

In addition to the overall loss profile depicted in Figure 6.2, another mechanism gives rise to fluctuations in the received signal level as a function of distance. Since the two signals shown in Figure 6.2 generally experience different phase shifts, possibly arriving at the receiver with opposite phases and roughly equal amplitudes, the net received signal may be very small.4

除了图6.2描述的总的损耗情况之外，还有一种机制会引起接收信号的强度随着距离而波动。由于图6.2所示的两个信号通常经受不同的相移，因此有可能到达接收端时相位相反，而幅度却大致相等，这样净接收的信号就可能非常弱。

Called “multipath fading,” this phenomenon introduces enormous variations in the signal level as the receiver moves by a fraction of the wavelength. In reality, since the transmitted signal is reflected by many buildings and moving cars, the fluctuations are quite irregular.