【正文】 通過(guò)空氣移動(dòng),他們身后留下低壓區(qū)域。這些振動(dòng)移動(dòng)附近的空氣分子，使分子連接在一起，因此提高空氣壓力略。 is created by vibrations, such as those produced by a guitar string, vocal cords or a speaker cone. These vibrations move the air molecules near them, forcing molecules together, as a result raising the air pressure slightly. The air molecules that are under pressure then push on air molecules surrounding them, which push on the next set of air molecules, and so forth, causing a wave of high pressure to move through the air。新形式的播放,文件格式,壓縮和存儲(chǔ)的數(shù)據(jù)都在一個(gè)看似每天改變，但是這種聯(lián)系的潛在機(jī)制轉(zhuǎn)換為現(xiàn)實(shí)世界的聲音轉(zhuǎn)化為數(shù)字值,操縱這些數(shù)據(jù),最后將他們轉(zhuǎn)化成真實(shí)世界的聲音沒(méi)有變化多少，因?yàn)轳R克斯8. As wavelengths decrease to approach the size of circuit ponents, circuit elements are no longer lumped as such, and leads can act as reflective ponents and/or antennas and lumped elements as electromagnetic resonators.隨著波長(zhǎng)減小到接近于電路元件的尺寸，電路單元就不再是集總的，導(dǎo)線可起到反射元件以及（或）天線的作用，集總單元?jiǎng)t成為電磁諧振器。6. When coupled with light’s short wavelength which allows for miniature sources and detectors and micronsized waveguides, direct detection schemes have allowed for small, lightweight, high bandwidth systems which are petitive in many areas, most notably to the present telemunications transmission, although a myriad of other applications are continually opening up.光的波長(zhǎng)很小，可以使用小型的光源和檢測(cè)器以及微米級(jí)的波導(dǎo)，于是用直接檢測(cè)方案可實(shí)現(xiàn)在許多領(lǐng)域具有競(jìng)爭(zhēng)力的小巧的寬帶系統(tǒng)，這些領(lǐng)域中特別引人注目的是當(dāng)前電信傳輸中的應(yīng)用，盡管無(wú)數(shù)其他應(yīng)用也在不斷涌現(xiàn)出來(lái)。4. The smallness of the optical wavelength, therefore, allows for the miniaturization of transmit and receive modules, which should allow considerable reduction in size, weight, and cost of optical munication systems with respect to microwave/radio wave counterparts.光波波長(zhǎng)之小使發(fā)射和接收模塊得以小型化，這就使光通信系統(tǒng)的尺寸、重量以至價(jià)格與相應(yīng)的微波、無(wú)線電波通信系統(tǒng)相比都大為降低。2. Pulse broadening determines the minimum separation between adjacent pulses, which in turn determines the maximum informationcarrying capacity of the optical fiber.脈沖的展寬決定了相鄰脈沖之間的最小間隔，這一間隔進(jìn)而又決定了光纖的最大信息容量。噪聲和信號(hào)畸變是電子通信的兩個(gè)基本問(wèn)題。信道可以有許多不同的形式，比如自由空間里的微波聯(lián)接，一對(duì)電線，或者一根光纖。4. The availability of such a large collection of system performance curves in a single pilation allows the researcher or system designer to perform tradeoff studies among the various munication type/fading channel/diversity binations so as to determine the optimum choice in the face of his or her available constraints.在一本書(shū)中提供如此大量的系統(tǒng)性能曲線，使研究者或系統(tǒng)設(shè)計(jì)者能在各種通信類型、衰落信道、分集組合之間進(jìn)行比較研究，從而在他或她所面臨的限制之下做出最佳的選擇。至今依然如此。2. It is true of any munication system that the shape and amplitude of the transmitted signal will be continuously degraded by the introduction of noise, and the attenuation along the transmission path.傳輸?shù)男盘?hào)會(huì)不斷地被噪聲的引入和線路上的衰減所損傷，對(duì)任何通信系統(tǒng)都是如此。Unit 6 Exercises (1) Translate the following sentences into Chinese.1. Communication is the transmission of information from one point to another. This transmission requires the ability to vary signals with time in a manner which is unpredictable to the receiver.通信就是信息從一點(diǎn)傳送到另一點(diǎn)。因此基帶子載波頻率為fn = n/T, (n = 0, 1, ... , N1)，第n個(gè)調(diào)制符號(hào)記為X(n)，在一個(gè)符號(hào)周期內(nèi)的OFDM波形可表示： (1)在t = T/N 時(shí)刻采樣這個(gè)波形得到： (2)從上面的表示中可以看到，x(k) 和X(n)構(gòu)成了一對(duì)離散傅里葉變換，因此基帶OFDM波形可根據(jù)N個(gè)調(diào)制符號(hào)的離散傅里葉變換來(lái)生成。即使在子載波間存在頻譜部分重疊，符號(hào)仍能無(wú)損失地恢復(fù)。這樣，不影響正確的解調(diào)，信號(hào)能在接收器中準(zhǔn)確地恢復(fù)。因此，OFDM本質(zhì)上是一個(gè)并行調(diào)制系統(tǒng)，當(dāng)載波數(shù)足夠大時(shí)，這種系統(tǒng)具有抗多徑干擾的能力。每個(gè)子載波可用同樣的方案，如PSK，QAM等，或者用不同的方案數(shù)字調(diào)制。一個(gè)通信系統(tǒng)的成功很大程度上依賴于調(diào)制。它用于通信系統(tǒng)以使信號(hào)特性匹配信道特性，減少噪聲和干擾，在單個(gè)信道上同時(shí)傳輸多個(gè)信號(hào)，并克服某些設(shè)備的限制。所有考慮的相應(yīng)于實(shí)際信道的情況，以及許多情況下得到的BER和SER表達(dá)式均可用手持計(jì)算器來(lái)計(jì)算。通過(guò)使用復(fù)雜的復(fù)用技術(shù)如CDMA或TDMA的擴(kuò)展，并提高服務(wù)的靈活性使得系統(tǒng)容量比原來(lái)的第一代系統(tǒng)提高10倍。通過(guò)在傳輸前將話音波形壓縮使得用戶容量提高了三倍。這些系統(tǒng)用戶容量低，而且因?yàn)槭褂煤?jiǎn)單的無(wú)線電接口而存在安全問(wèn)題。電子通信建立起人和人、計(jì)算機(jī)和計(jì)算機(jī)之間的聯(lián)系。2. Electronic munications is the transfer and movement of data between locations through the use of puters. An electronic munication system includes the equipment needed to support the movement of information, the munication lines and media to carry the information, the puter software and programs to control the flow of information, the personnel to plan, implement, and operate munications, and the management of all these resources. Electronic munications establishes links between people as well as puters.電子通信就是通過(guò)使用計(jì)算機(jī)在不同的地點(diǎn)之間傳輸數(shù)據(jù)。為了實(shí)現(xiàn)這一過(guò)程，已經(jīng)利用工作在射頻、微波以及毫米波頻率上的電磁載波開(kāi)發(fā)了復(fù)雜的技術(shù)。在通信系統(tǒng)內(nèi)部，信息的傳輸經(jīng)常通過(guò)將信息疊加或調(diào)制到作為信號(hào)載體的電磁波上來(lái)實(shí)現(xiàn)。Unit 4 Exercises (1) Translate the following passages into Chinese.1. 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 munication system is usually required. Within a munication 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, munication may also be achieved by using an electromagnetic carrier that is selected from the optical range of frequencies.通信可以廣義地定義為從一處到另一處的信息傳遞。7. When the cutoff frequency is no less than the maximal frequency of the original signal, nor is it greater than the difference between the sampling frequency and the maximal frequency, the original signal may be pletely rebuilt.當(dāng)截止頻率不小于原始信號(hào)的最高頻率，也不大于采樣頻率和最高頻率之差，原始信號(hào)可以完整地重建。后一個(gè)動(dòng)機(jī)并非是最不重要的。4. Operation of the radar is based on the measurement of the time it takes for a pulse transmitted from an antenna to get reflected by the object to be detected and to return at the antenna and the receiver.雷達(dá)的工作是基于對(duì)天線發(fā)出的脈沖被所要探測(cè)的目標(biāo)反射，（然后）回到天線和接收機(jī)所需時(shí)間的測(cè)量。這一結(jié)果驗(yàn)證了所用算法的正確性。 Unit 3 Exercises(1) Translate the following sentences into Chinese.1. A very monly used method of microwave measurements is based on the study of a standing wave pattern formed along the line because of the interference of incident and reflected waves.一個(gè)很常用的微波測(cè)量方法是基于對(duì)駐波圖型的研究，這種駐波是由入射波和反射波之間的干涉而沿著傳輸線形成的。4. The first observation is made on the fundamental relations