雙工無(wú)線(xiàn)語(yǔ)音數(shù)據(jù)傳輸系統(tǒng)外文翻譯

1、<p>　　Triple wireless voice data transmission system design</p><p>　　Student :, </p><p>　　Instructor :, University</p><p>　　Every day, in our work and in our leisure time, we c

2、ome in contact with and use a variety of modern communication media, the most common being the telephone, radio, television, and the Internet. Though these media we are able to communicate (nearly) instantaneously with p

3、eople on diffident continents, transact our daily business, and receive information about various developments and events of note that occur all around the world. Electronic mail and facsimile transmission have made it p

4、ossible </p><p>　　Wireless communications. The development of wireless communications stems from the works of Oersted, Faraday, Gauss, Maxwell, and Hertz. In1820, Oersted demonstrated that an electric curren

5、t produces a magnetic field. On August 29,1831,Michael Faraday showed that an induced current is produced by moving a magnet in the vicinity of a conductor. Thus, he demonstrated that a changing magnetic field produces a

6、n electric field. With this early work as background, James C. Maxwell in 1864 predicted th</p><p>　　In 1894, a sensitive device that could device that could detect radio signals, called the coherer, was use

7、d by its inventor Oliver Lodge to demonstrate wireless communication over a distance of 150 yards at Oxford, England. Guglielmo Marconi is credited with the development of wireless telegraphy. Marconi demonstrated the tr

8、ansmission of radio signals at a distance of approximately 2 kilometers in 1895. Two years later, in 1897 , he patented a radio telegraph system and established the Wireless </p><p>　　The invention of the va

9、cuum tube was especially instrumental in the development of radio communication system .The vacuum diode was invented by Fleming in 1904 and the vacuum triode amplifier was invented by De Forest in 1906, as previously in

10、dicated. The invention of the triode made radio broadcast possible in the early part of the twentieth century. Amplitude modulation (AM) broadcast was initiated in 1920 when radio station KDKA, Pittsburgh, went on the ai

11、r. From that date, AM radio broadcast</p><p>　　The first television system was built in the United States by V. K. Zworykin and demonstrated in 1929. Commercial television broadcasting began in London in 193

12、6 by the British Broadcasting Corporation(BBC) . Five years later the Federal Communications Commission(FCC) authorized television broadcasting in the United States. </p><p>　　ELEMENTS OF AN ELECTRICAL COMMU

13、NICA SYSTEM</p><p>　　Electrical communication systems are designed to send messages or information from a source that generates the message to one more destinations. In general, a communication system can b

14、e represented by the functional block diagram shown . The information generated by the source may be of the form of voice (speech source), a picture (image source), or plain text in some particular language, such as Engl

15、ish , Japanese, German , French, etc. An essential feature of any source that generates inform</p><p>　　A transducer is usually required to convert the output of a source into an electrical signal that is su

16、itable for transmission. For example, a microphone serves as the transducer that converts an acoustic speech signal. At the destination, a similar transducer is required to convert the electrical signals that are receive

17、d into a form that is suitable for the user; e.g., acoustic signals, images, etc.</p><p>　　The heart of the communication system consists of three basic parts, namely, the transmitter, the channel, and the r

18、eceiver. The functions performed by these three elements are described next.</p><p>　　The Transmitter. The Transmitter converts the electrical signal into a form that is suitable for transmission though the

19、physical channel or transmission medium. For example, in radio and TV broadcast, the Federal Communications Commission (FCC) specifies the frequency range for each transmitting station. Hence, the transmitter must transl

20、ate the information signal to be transmitted into the appropriate The Transmitter range that matches the frequency allocation assigned to the transmitter. Thus</p><p>　　In general, the transmitter performs t

21、he matching of the message signal to the channel by a process called modulation. Usually, modulation involves the use of the information signal to systematically vary either the amplitude, frequency, or phase of a sinuso

22、idal carrier. For example, in AM radio broadcast, the information signal that is transmitted is contained in the amplitude variations of the sinusoidal carrier, which is the center frequency in the amplitude modulation.

23、In FM radio broadcast.</p><p>　　In general, carrier modulation such as AM, FM, and PM is performed at the transmitter, as indicated above, to convert the information signal to a form that matches the charact

24、eristics of the channel. Thus, though the process of modulation, the choice of the type of modulated in frequency to match the allocation of the channel. The choice of the type of modulation is based on several factors,

25、such as the amount of bandwidth over the channel, the type of noise and the interference that the signal </p><p>　　In addition to modulation, other functions that are usually performed at the transmitter are

26、 filtering of the information-bearing signal , amplification of the modulated signal, and in case of wireless transmission, radiation of the signal by means of a transmitting antenna.</p><p>　　The channel. T

27、he communications channel is the physical medium that is used to send the signal from the transmitter to the receiver. In wireless transmission, the channel is usually the atmosphere (free space). On the other hand, tele

28、phone channels usually employ a variety of physical media, including wirelines, optical fiber cables, and wireless (microwave radio). Whatever the physical medium for signal transmission, the essential feature is that th

29、e transmitted signal is corrupted in a random </p><p>　　In some radio communication channels, such as the ionospheric channel that is used for long range ,short-wave radio transmission, another form of sign

30、al degradation is multipath propagation. Such signal distortion is characterized as a nonadditive signal disturbance which manifests itself as time variations in the signal amplitude, usually called fading .</p>&

31、lt;p>　　Both additive and nonadditive signal distortions are usually characterized as random phenomena and described in statistical terms. The effect of these signal distortions must be taken into account on the design

32、 of the communication system.</p><p>　　In the design of a communication system, the system, the system designer works with mathematical models that statistically characterize he signal distortion encountere

33、d on physical channels. Often, the statistical description that is used in mathematical model is a result of actual empirical measurements obtained from experiments involving signal transmission over such channels .In su

34、ch cases , there is a physical justification for the mathematical model used in the design of communication syste</p><p>　　The receiver. The function of the receiver is to recover the message signal containe

35、d in the received signal. If the message signal is transmitted by carrier modulation, the receiver performs carrier demodulation in order to extract the message from the sinusoidal carrier. Since the signal demodulation

36、is performed in the presence of additive noise and possibly other signal distortion, the demodulated message signal is generally degraded to some extent by the presence of these distortions in the</p><p>　　B

37、esides performing the primary function of signal demodulation, the receiver also performs a number of peripheral functions, including signal filtering and noise suppression.</p><p>　　Digital Communication Sy

38、stem</p><p>　　An electrical communication system in rather broad terms based on the implicit assumption that message signal is a continuous timevarying waveform. We refer to such continuous-time signal wavef

39、orms as analog sources. Analog signal can be transmitted directly via modulation over the communication channel and demodulated accordingly at the receiver. We call such a i communication system an analog communication s

40、ystem.</p><p>　　Alternatively, an analog source output may be converted into a digital form and the message can be transmitted via digital modulation as a digital signal at the receiver. There are some poten

41、tial advantage to transmitting an analog signal by means of digital modulation. The most important reason is that signal fidelity is better controlled though digital transmission than analog transmission. In particular,

42、digital transmission allows us to regenerate the digital signal in long-distance transmis</p><p>　　In some applications, the information to be transmitted is inherently digital; e.g., in the form of English

43、text, computer data, etc. In such cases, the information source that generates the data is called a discrete (digital)source.</p><p>　　In a digital communication systems , the some applications, the function

44、al operations performed at the transmitter and receiver must be expanded to include message signal discrimination at the transmitter and message signal synthesis or interpolation at the receiver. Additional functions inc

45、lude redundancy removal, and channel coding and decoding. </p><p>　　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 computer whi

46、ch is discrete in time and has a finite number of output characters. In a digital communication system, the message produced by the source are usually converted into a sequence of binary digits as possible. In other word

47、s, we seek inefficient representation of the source output of either an analog or a digital source into a sequence of binary digits is called</p><p>　　The sequence of binary digits from the coerce encoder, w

48、hich we call the information sequence is passed to the channel encoder. The purpose of the channel encoder is to introduce, in a controlled manner, some redundancy in binary information sequence which can be used at the

49、receiver to overcome the effects of noise and interference encountered in the transmission of the signal though the channel. Thus the added redundancy serves to increase the reliability of the received data and improves

50、the </p><p>　　The binary sequence at the output of the channel encoder is passed to the digital modulator, which servers as the interface to the communications channel. Since nearly all of the communication

51、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.</p><p&

52、gt;　　At the receiving end of a digital communication system, the digital demodulator processes the channel-corrupted transmitted waveform and reduces reduce each waveform to a signal number that represents an estimate of

53、 the transmitted data symbol (binary or Mary) . When there is no redundancy in the transmitted information, the demodulator must decide which of the M waveform was transmitted in any given time interval. A measure of how

54、 well the demodulator and encoder perform is the frequency with w</p><p>　　As a final step, when an analog output is desired, the source decoder accepts the output sequence from the channel and , from knowle

55、dge of the source-encoding method used, attempts to reconstruct the original signal from the source.</p><p>　　雙工無(wú)線(xiàn)語(yǔ)音數(shù)據(jù)傳輸系統(tǒng)的設(shè)計(jì)</p><p><b>　　學(xué)生：學(xué)院</b></p><p><b>　　指導(dǎo)老

56、師：漢大學(xué)</b></p><p>　　在日常的工作和生活中，人們每天都要接觸和使用大量的現(xiàn)代通信系統(tǒng)和通信媒介，其中最常見(jiàn)的是電話(huà)，無(wú)線(xiàn)電廣播，電視和因特網(wǎng)。 通過(guò)這些媒介，生活在地球各地的人們可可以隨時(shí)進(jìn)行通信，處理日常事務(wù)，通曉天下大事，了解世界發(fā)展。 而電子郵件和傳真則使得文字信息的遠(yuǎn)距離快速傳遞成為現(xiàn)實(shí)。</p><p>　　無(wú)線(xiàn)通信。無(wú)線(xiàn)通信的出現(xiàn)歸功于噢斯特（Oe

57、rsted ），法拉第（Faraday），高斯（Gauss），麥克斯韋（Maxwell）和 赫茲（Hertz）等人的研究工作。1820年噢斯特證明了電流能夠產(chǎn)生磁場(chǎng)；1831年8月29日，法拉第演示了字一個(gè)導(dǎo)體的附近移動(dòng)磁體產(chǎn)生出感應(yīng)電流，隨后他證明了電磁輻射的存在，他所建立的基本理論，已經(jīng)使用了一個(gè)多世紀(jì)；麥克斯的理論由赫茲在1887年經(jīng)實(shí)驗(yàn)所證明。</p><p>　　1894年，噢利弗-洛奇（Oliver

58、Lodge）發(fā)明了一種稱(chēng)為金屬屑檢測(cè)器的靈敏器件，用于檢測(cè)無(wú)線(xiàn)信號(hào)，他在英國(guó)的牛津使用這種靈敏器件演示了在150碼的距離上進(jìn)行無(wú)線(xiàn)通信；馬克尼（Marconi）的名字則與無(wú)線(xiàn)電報(bào)密切相關(guān)，他于1895年演示了在大約2英里距離上進(jìn)行無(wú)線(xiàn)信號(hào)傳輸，1897年馬可波羅又申請(qǐng)了無(wú)線(xiàn)電報(bào)系統(tǒng)的專(zhuān)利，并建立了無(wú)線(xiàn)電報(bào)和信號(hào)公司。1901年12月12日，馬克尼在紐芬蘭的信號(hào)山收到了發(fā)自英國(guó)康沃爾的無(wú)線(xiàn)信號(hào)，傳輸距離約1700英里。</p>

59、<p>　　真空電子管的發(fā)明，對(duì)無(wú)線(xiàn)通信系統(tǒng)的發(fā)展所起的作用尤其突出。如前所述，弗萊明（Fleming）在1904年發(fā)明了真空二極管，德福雷斯特在1906年發(fā)明了真空三極管放大器。三極管的發(fā)明使得無(wú)線(xiàn)電廣播在20世紀(jì)初期成為現(xiàn)實(shí)，1920年在匹茲堡的KDKA無(wú)線(xiàn)電臺(tái)將調(diào)幅（AM）廣播信號(hào)送進(jìn)了空中，從那時(shí)起，調(diào)幅無(wú)線(xiàn)電廣播在英國(guó)及世界各地迅速發(fā)展。今天我們所熟知的超外差式調(diào)幅無(wú)線(xiàn)接收機(jī)，則是第一次世界大戰(zhàn)期間由埃德溫-阿姆

60、斯壯（Edwin Armstrong）發(fā)明的。無(wú)線(xiàn)通信的另一個(gè)重大進(jìn)展是頻率調(diào)制（FM）的出現(xiàn)，它也是阿姆斯壯最早提出的，1933年阿姆斯壯建立并演示了第一個(gè)FM通信系統(tǒng)。不過(guò)，與AM廣播相比，F(xiàn)M的使用發(fā)展緩慢，直到二戰(zhàn)結(jié)束時(shí)FM廣播開(kāi)始流行并進(jìn)入商用系統(tǒng)。</p><p>　　第一個(gè)電視系統(tǒng)由V.K.Z 于1929年在美國(guó)建立并進(jìn)行演示試驗(yàn)，倫敦的英國(guó)廣播公司（BBC）于1936年開(kāi)始商業(yè)電視廣播，五年后聯(lián)

61、邦通信委員會(huì)（FCC）批準(zhǔn)了美國(guó)的電視廣播。</p><p>　　電通信系統(tǒng)的基本組成</p><p>　　電通信系統(tǒng)的作用，是將產(chǎn)生消息的信源信息發(fā)送到一個(gè)或多個(gè)目的地。一般情況下，通信系統(tǒng)可以用功能框圖進(jìn)行表示。信源所產(chǎn)生的信息可以是聲音（語(yǔ)音圖像），圖像（影像源），或以某些特殊語(yǔ)言如英語(yǔ)，日語(yǔ)，德語(yǔ)，法語(yǔ)等寫(xiě)成的純文本。產(chǎn)生信息的任何一個(gè)信源，都有一個(gè)基本的特征，即它的輸出是以概率參

62、量描述的，也就是說(shuō)一個(gè)信源的輸出是不確定的，否則信息傳輸就失去了意義。</p><p>　　變換器通常用于將信源的輸出變換成適合傳輸?shù)碾娦盘?hào)。例如，用做變換器的話(huà)筒，可以將聲頻的語(yǔ)音信號(hào)變換成電信號(hào)，而攝像機(jī)則將圖像信號(hào)變換成電信號(hào)。在接收端，使用一個(gè)類(lèi)似的變換器將收到的電信號(hào)變換成適合用戶(hù)的形式，如聲響信號(hào)，圖像等。</p><p>　　通信系統(tǒng)的核心由三個(gè)部分構(gòu)成，即發(fā)信機(jī)，信道和接收

63、機(jī)。這三個(gè)基本組成部分完成的功能如下所述。</p><p>　　發(fā)信機(jī)。發(fā)信機(jī)將電信號(hào)變換成適合物理信道或其他傳輸介質(zhì)傳輸?shù)男问?。例如，在無(wú)線(xiàn)電和電視廣播中，聯(lián)邦通信委員會(huì)（FCC）指定的各個(gè)發(fā)射臺(tái)的頻率范圍，因此發(fā)信機(jī)必須將信息信號(hào)轉(zhuǎn)換到合適的頻率范圍來(lái)發(fā)送，以便與分配給此發(fā)信機(jī)的頻率相匹配。這樣，由多個(gè)無(wú)線(xiàn)電臺(tái)發(fā)送的信號(hào)就不會(huì)彼此干擾。電話(huà)通信系統(tǒng)中也需要實(shí)現(xiàn)類(lèi)似的功能，從而讓來(lái)自許多用戶(hù)的電語(yǔ)言信號(hào)在同一條

64、電線(xiàn)上傳輸。</p><p>　　一般而言，發(fā)信機(jī)通過(guò)一個(gè)所謂的調(diào)制過(guò)程來(lái)實(shí)現(xiàn)信號(hào)與信道的匹配。通常，調(diào)制需要使用信息信號(hào)來(lái)系統(tǒng)地改變正弦載波的振幅，頻率或相位分量。例如，在AM無(wú)線(xiàn)電廣播中，發(fā)送的信息包含正弦載波的振幅變換中，此載波是分配給無(wú)線(xiàn)電發(fā)射臺(tái)的頻帶的中心頻率，這是一個(gè)振幅調(diào)制的例子；在FM無(wú)線(xiàn)電廣播中，發(fā)送的信息信號(hào)包含在正弦載波的頻率變化中，而這是一個(gè)頻率調(diào)制的例子；相位調(diào)制則是將信息信號(hào)記載在正弦

65、載波上的第三種調(diào)制方法。</p><p>　　如上所述，諸如AM，F(xiàn)M和PM之類(lèi)的載波調(diào)制，通常是由發(fā)信機(jī)通過(guò)將信息信號(hào)變換成與信道特性相匹配的信號(hào)形式來(lái)完成的。這樣通過(guò)調(diào)制過(guò)程，信息信號(hào)在頻率上發(fā)生了改變（搬移），以便與所分配的信道相匹配。調(diào)制類(lèi)型的選擇主要基于幾個(gè)因素，如所分配的帶寬，信號(hào)在信道傳輸中遇到的噪聲及干擾的類(lèi)型，在發(fā)送前進(jìn)行信號(hào)放大所能采用的電子器件等等?？傊{(diào)制過(guò)程使得來(lái)自許多用戶(hù)的多重信息能

66、夠在同一物理信道上同時(shí)發(fā)送。</p><p>　　除了調(diào)制，其他一些通常由發(fā)信機(jī)完成的功能還包括：信息信號(hào)的濾波，已調(diào)信號(hào)的放大，以及在無(wú)線(xiàn)電通信信號(hào)通過(guò)發(fā)射天線(xiàn)完成的輻射等等。</p><p>　　信道。通信信道是一種物理介質(zhì)，用于將來(lái)自發(fā)信機(jī)的信號(hào)發(fā)送到接收機(jī)。在無(wú)線(xiàn)傳輸中，信道通常是大氣層（自由空間）；電話(huà)信道則可以采用多種物理介質(zhì)，包括電線(xiàn)，光纜和無(wú)線(xiàn)電（微波）信道。不管哪一種用于

67、信號(hào)傳送的物理介質(zhì)，都有一個(gè)基本特征，就是所傳送信號(hào)會(huì)被各種可能的因素以隨機(jī)的方式損傷，引起信號(hào)質(zhì)量惡化的因素是加性噪聲，它產(chǎn)生于接收機(jī)的前端放大器，該噪聲通常稱(chēng)為熱噪聲；在無(wú)線(xiàn)傳輸中，還有接收天線(xiàn)收到的人為噪聲，大氣噪聲等附加的加性噪聲。舉例來(lái)說(shuō)，汽車(chē)點(diǎn)火噪聲是一種人為噪聲，而雷電釋放的電光則是一種大氣噪聲。來(lái)自信道上其他用戶(hù)的干擾則是另一種形式的加性噪聲，這種干擾在無(wú)線(xiàn)和有線(xiàn)通信系統(tǒng)中都會(huì)經(jīng)常出現(xiàn)。</p><p

68、>　　在一些無(wú)線(xiàn)通信信道，例如用于長(zhǎng)距離短波無(wú)線(xiàn)傳送的電離層信道上，引起信號(hào)惡化的另一個(gè)因素是多徑傳播。這種信號(hào)失真的本質(zhì)是一種非加性的信號(hào)干擾，它使得信號(hào)自身的振幅隨著時(shí)間的變化而變化，通常稱(chēng)之為衰落。</p><p>　　加性和非加性信號(hào)失真都有隨機(jī)性的特點(diǎn)，并且通常使用統(tǒng)計(jì)參量加以描述，在設(shè)計(jì)通信系統(tǒng)時(shí)對(duì)這些信號(hào)失真的影響必須認(rèn)真考慮。</p><p>　　在設(shè)計(jì)一個(gè)通信系統(tǒng)時(shí)

69、，系統(tǒng)設(shè)計(jì)者應(yīng)從統(tǒng)計(jì)分析的角度，對(duì)在物理信道上的信號(hào)失真建立數(shù)學(xué)模型。數(shù)學(xué)模型中使用的統(tǒng)計(jì)描述，通常都是實(shí)際測(cè)量的經(jīng)驗(yàn)結(jié)果，來(lái)源于在信道上對(duì)信號(hào)的傳送進(jìn)行實(shí)驗(yàn)。在這種情況下，對(duì)于通信系統(tǒng)設(shè)計(jì)的數(shù)學(xué)模型就要進(jìn)行物理修正。另一方面，在某些通信系統(tǒng)設(shè)計(jì)中，信道的統(tǒng)計(jì)特性都會(huì)隨時(shí)間劇烈變化，在這種情況下，系統(tǒng)設(shè)計(jì)者應(yīng)當(dāng)設(shè)計(jì)一種對(duì)各種信號(hào)都具有魯棒性（rubust）的通信系統(tǒng)，可以通過(guò)讓系統(tǒng)根據(jù)所遇到的信道失真自適應(yīng)地調(diào)整參數(shù)來(lái)實(shí)現(xiàn)。</p

70、><p>　　接收機(jī)。接收機(jī)的功能是恢復(fù)接收信號(hào)中所包含的消息信號(hào)。如果消息信號(hào)是通過(guò)載波調(diào)制發(fā)送的，則接收機(jī)必須完成載波調(diào)制，以便從正弦載波中提取出消息。信號(hào)解調(diào)是在有加性噪聲及其他可能的信號(hào)失真的情況下完成的，由于出現(xiàn)在接收消息信號(hào)的保真度，與調(diào)制類(lèi)型，加性噪聲強(qiáng)度及任何其他加性干擾的類(lèi)型和強(qiáng)度等密切相關(guān)。</p><p>　　除了完成信號(hào)解調(diào)這一主要功能，接收機(jī)還完成其他一系列的外圍功能

71、，包括信號(hào)濾波和噪聲抑制等。</p><p><b>　　數(shù)字通信系統(tǒng)</b></p><p>　　電信系統(tǒng)在很大程度上都是基于這樣一個(gè)簡(jiǎn)單的假設(shè)，即消息信號(hào)是連續(xù)的時(shí)變波形。這種連續(xù)時(shí)間信號(hào)波形稱(chēng)為模擬信號(hào)，產(chǎn)生這種信號(hào)的相應(yīng)的信源稱(chēng)為模擬信源。模擬信號(hào)可以通過(guò)載波調(diào)制直接在通信信道上發(fā)送，然后在接收端進(jìn)行相應(yīng)的解調(diào)，這樣的通信系統(tǒng)稱(chēng)為模擬通信系統(tǒng)。</p&g

72、t;<p>　　除此之外，模擬信源的輸出還可以轉(zhuǎn)換成數(shù)字形式，消息可以通過(guò)數(shù)字調(diào)制后發(fā)送，并在接收端解調(diào)成數(shù)字信號(hào)。使用數(shù)字調(diào)制發(fā)送模擬信號(hào)具有一些潛在的優(yōu)勢(shì)，其中最重要的一點(diǎn)是，比起模擬傳輸，通過(guò)數(shù)字信號(hào)傳送的信號(hào)，其保真度能夠得到更好的控制。尤其重要的是，在長(zhǎng)距離傳送時(shí)，數(shù)字傳輸允許對(duì)數(shù)字信號(hào)進(jìn)行再生處理，這樣就可以在每個(gè)再生點(diǎn)消除噪聲的影響。而與此相反，在長(zhǎng)距離傳送時(shí)，疊加到模擬信號(hào)上的噪聲會(huì)隨著模擬信號(hào)電平的周期性

73、放大而逐次累加。對(duì)于模擬信號(hào)選擇數(shù)字傳輸方式的另一個(gè)原因是，模擬信號(hào)有可能高度冗余，通過(guò)數(shù)字處理，可以在調(diào)制前消除冗余度，從而壓縮帶寬。還有第三個(gè)原因，即數(shù)字系統(tǒng)的實(shí)現(xiàn)成本通常比較低。</p><p>　　在某些應(yīng)用中，發(fā)送的信息本來(lái)就是數(shù)字的，如英語(yǔ)文本形式，計(jì)算機(jī)數(shù)據(jù)等。在這種情況下，產(chǎn)生數(shù)據(jù)的信源就稱(chēng)為離散（數(shù)字）信源。</p><p>　　在數(shù)字通信系統(tǒng)中，發(fā)信機(jī)和接收機(jī)完成的功能

74、還應(yīng)包括：發(fā)射端消息信號(hào)的離散化，以及接收端消息信號(hào)的合成或內(nèi)插。附加的功能還有消除冗余度和譯碼等。</p><p>　　信源輸出可以是模擬信號(hào)，如音頻或視頻信號(hào)；也可以是數(shù)字信號(hào)，如計(jì)算機(jī)的輸出，這種輸出在時(shí)間上是離散的，且輸出字符數(shù)目也是有限的。在數(shù)字通信系統(tǒng)中，信源產(chǎn)生的消息通常變換成一系列的二進(jìn)制數(shù)。實(shí)際上，我們希望使用盡可能少的二進(jìn)制數(shù)來(lái)表示信源輸出（消息）。也就是希望尋找一種信源輸出的有效表示方法，使

75、得冗余度很小甚至沒(méi)有。模擬或數(shù)字信源的輸出，有效地變換成二進(jìn)制數(shù)序列的過(guò)程，稱(chēng)為信源編碼或數(shù)據(jù)壓縮。</p><p>　　來(lái)自信源編碼器的二進(jìn)制數(shù)字序列，稱(chēng)為信息序列，它將被送往信道編碼器。信道編碼的目的是受控方式在二進(jìn)制信息序列中引入一些冗余度，以便接收機(jī)用于提高接收數(shù)據(jù)的可靠性，改善接收信號(hào)的保真度。</p><p>　　信道編碼器輸出的二進(jìn)制序列將送往用做通信信道接口的數(shù)字調(diào)制器。由

76、于在實(shí)際應(yīng)用中，幾乎所有的通信信道都能夠傳送電信號(hào)（波形），因此數(shù)字調(diào)制器的主要作用是將二進(jìn)制信息序列映射成信號(hào)波形。</p><p>　　在數(shù)字通信系統(tǒng)的接收機(jī)末端，數(shù)字解調(diào)器對(duì)信道損傷的發(fā)射波進(jìn)行處理，并將每個(gè)波形還原成數(shù)字，以表示對(duì)所發(fā)送數(shù)據(jù)碼元（二進(jìn)制或二進(jìn)制）的估計(jì)。發(fā)送信息中沒(méi)有冗余度時(shí)，解調(diào)器必須判定在任意給定的時(shí)間間隔內(nèi)，發(fā)送的是M個(gè)波形中的哪一個(gè)。評(píng)價(jià)解調(diào)器和編碼器性能好壞的測(cè)度，是譯碼器輸出端