通信與電子信息工程專(zhuān)業(yè)英語(yǔ)課文翻譯 第2單元 聶敏

1、Unit 2Modulation TechnologyText AModulation Technology調(diào)制技術(shù)調(diào)制技術(shù)Unit 2v原文：原文：Formula (2.1) introduced the basic function of a sine wave which already indicates the three basic modulation schemes (typically, the cosine function is used for explanation):v譯文：譯文：公式（公式（2.1）引入了一個(gè)基本的正弦函數(shù)，）引入了一個(gè)基本的正弦函數(shù)，它可以表示三

2、種基本調(diào)制方案（通常用余弦函數(shù)來(lái)它可以表示三種基本調(diào)制方案（通常用余弦函數(shù)來(lái)說(shuō)明）：說(shuō)明）：Unit 2vUnit 2v原文：原文：Digital modulation is required if digital data has to be transmitted over a medium that only allows for analog transmission. v譯文：譯文：如果數(shù)字信號(hào)必須在只允許模擬信號(hào)傳輸如果數(shù)字信號(hào)必須在只允許模擬信號(hào)傳輸?shù)拿浇橹袀鬏敚瑒t要求進(jìn)行數(shù)字調(diào)制。的媒介中傳輸，則要求進(jìn)行數(shù)字調(diào)制。Unit 2v原文：原文：One example for wir

3、ed networks is the old analog telephone system to connect a computer to this system a modem is needed .The modem then performs the translation of digital data into analog signals vice versa.v譯文：譯文：模擬電話(huà)系統(tǒng)是有線(xiàn)網(wǎng)絡(luò)的一個(gè)例子，它模擬電話(huà)系統(tǒng)是有線(xiàn)網(wǎng)絡(luò)的一個(gè)例子，它需要用一個(gè)調(diào)制解調(diào)器將計(jì)算機(jī)與系統(tǒng)相連。調(diào)需要用一個(gè)調(diào)制解調(diào)器將計(jì)算機(jī)與系統(tǒng)相連。調(diào)制解調(diào)器完成調(diào)制解調(diào)模數(shù)轉(zhuǎn)換的任務(wù)。數(shù)字傳制解調(diào)器

4、完成調(diào)制解調(diào)模數(shù)轉(zhuǎn)換的任務(wù)。數(shù)字傳輸在有線(xiàn)局域網(wǎng)或計(jì)算機(jī)內(nèi)部是很有用的。輸在有線(xiàn)局域網(wǎng)或計(jì)算機(jī)內(nèi)部是很有用的。Unit 2v原文：原文：Apart from the translation of digital data into analog signals, wireless transmission requires an additional modulation, an analog modulation that shifts the center frequency of the base-band signal generated by the digital modulati

5、on up to the radio carrier. v譯文：譯文：除了數(shù)模轉(zhuǎn)換外，無(wú)線(xiàn)傳輸還需要一個(gè)附除了數(shù)模轉(zhuǎn)換外，無(wú)線(xiàn)傳輸還需要一個(gè)附加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過(guò)加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過(guò)數(shù)字調(diào)制提高形成無(wú)線(xiàn)電載波。數(shù)字調(diào)制提高形成無(wú)線(xiàn)電載波。Unit 2v原文：原文：For example digital modulation translates a 1 Mbps bit-steam into a base-band signal with a bandwidth of 1MHz.There are several reasons why this b

6、ase-band signal cannot be directly transmitted in a wireless system:v譯文：譯文：例如，通過(guò)數(shù)字調(diào)制，將一個(gè)例如，通過(guò)數(shù)字調(diào)制，將一個(gè)1Mbps比特比特流轉(zhuǎn)換成流轉(zhuǎn)換成1MHz帶寬的基帶信號(hào)?；鶐盘?hào)之所帶寬的基帶信號(hào)?；鶐盘?hào)之所有以不能直接在無(wú)線(xiàn)系統(tǒng)中傳輸有如下幾個(gè)原因：有以不能直接在無(wú)線(xiàn)系統(tǒng)中傳輸有如下幾個(gè)原因：Unit 2v原文：原文：Digital transmission is used, for example, in wired local area networks or within a computer.

7、 In wireless networks, however, digital transmission cannot be used. Here, the binary bit-steam has to be translated into an analog signal first. v譯文：譯文：然而，在無(wú)線(xiàn)網(wǎng)絡(luò)中，二進(jìn)制比特流首先然而，在無(wú)線(xiàn)網(wǎng)絡(luò)中，二進(jìn)制比特流首先必須被轉(zhuǎn)換成模擬信號(hào)，因此，不一定必須用到必須被轉(zhuǎn)換成模擬信號(hào)，因此，不一定必須用到數(shù)字傳輸。數(shù)字傳輸。Unit 2v原文：原文：The three basic methods for this translation a

8、re amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). These are discussed in more detail in the following sections.v譯文：譯文：三種基本的數(shù)字調(diào)制是幅移鍵控（三種基本的數(shù)字調(diào)制是幅移鍵控（ASK）、）、頻移鍵控（頻移鍵控（FSK）和相移鍵控（）和相移鍵控（PSK）。下面我）。下面我們要分別詳細(xì)討論。們要分別詳細(xì)討論。Unit 2v原文：原文：Apart from the translation of d

9、igital data into analog signals, wireless transmission requires an additional modulation, an analog modulation that shifts the center frequency of the base-band signal generated by the digital modulation up to the radio carrier. v譯文：譯文：除了數(shù)模轉(zhuǎn)換外，無(wú)線(xiàn)傳輸還需要一個(gè)附除了數(shù)模轉(zhuǎn)換外，無(wú)線(xiàn)傳輸還需要一個(gè)附加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過(guò)加的模擬調(diào)制，

10、將普通基帶信號(hào)的中心頻率通過(guò)數(shù)字調(diào)制提高形成無(wú)線(xiàn)電載波。數(shù)字調(diào)制提高形成無(wú)線(xiàn)電載波。Unit 2v原文：原文：For example digital modulation translates a 1 Mbps bit-steam into a base-band signal with a bandwidth of 1MHz.There are several reasons why this base-band signal cannot be directly transmitted in a wireless system:v譯文：譯文：例如，通過(guò)數(shù)字調(diào)制，將一個(gè)例如，通過(guò)數(shù)字調(diào)制，將

11、一個(gè)1Mbps比特比特流轉(zhuǎn)換成流轉(zhuǎn)換成1MHz帶寬的基帶信號(hào)?；鶐盘?hào)之所帶寬的基帶信號(hào)?；鶐盘?hào)之所有以不能直接在無(wú)線(xiàn)系統(tǒng)中傳輸有如下幾個(gè)原因：有以不能直接在無(wú)線(xiàn)系統(tǒng)中傳輸有如下幾個(gè)原因：Unit 2v原文：原文：Apart from the translation of digital data into analog signals, wireless transmission requires an additional modulation, an analog modulation that shifts the center frequency of the base-band

12、signal generated by the digital modulation up to the radio carrier.v譯文：譯文：除了數(shù)模轉(zhuǎn)換外，無(wú)線(xiàn)傳輸還需要一個(gè)附除了數(shù)模轉(zhuǎn)換外，無(wú)線(xiàn)傳輸還需要一個(gè)附加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過(guò)加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過(guò)數(shù)字調(diào)制提高數(shù)字調(diào)制提高到到無(wú)線(xiàn)電載波無(wú)線(xiàn)電載波頻段頻段。Unit 2v原文：原文：For example digital modulation translates a 1 Mbps bit-steam into a base-band signal with a bandwidth of 1

13、MHz.There are several reasons why this base-band signal cannot be directly transmitted in a wireless system:v譯文：譯文：例如，通過(guò)數(shù)字調(diào)制，將一個(gè)例如，通過(guò)數(shù)字調(diào)制，將一個(gè)1Mbps比特比特流轉(zhuǎn)換成流轉(zhuǎn)換成1MHz帶寬的基帶信號(hào)?；鶐盘?hào)之所帶寬的基帶信號(hào)?；鶐盘?hào)之所有以不能直接在無(wú)線(xiàn)系統(tǒng)中傳輸有如下幾個(gè)原因：有以不能直接在無(wú)線(xiàn)系統(tǒng)中傳輸有如下幾個(gè)原因：Unit 2v原文：原文：Antennas: An antenna must be the order of magnitude o

14、f the wavelength of the signal in size to be effective. For the 1MHz signal in the example this would result in an antenna some hundred meter high, which is obviously not very practical for handheld devices. With 1 GHz, antennas of a few centimeters in length can be used.v譯文：譯文：天線(xiàn)：天線(xiàn)必須由有效信號(hào)的波長(zhǎng)大小來(lái)決定。

15、天線(xiàn)：天線(xiàn)必須由有效信號(hào)的波長(zhǎng)大小來(lái)決定。例如，例如，1MHz的信號(hào)，天線(xiàn)就有數(shù)百米高，這顯然對(duì)的信號(hào)，天線(xiàn)就有數(shù)百米高，這顯然對(duì)便攜設(shè)備來(lái)說(shuō)很不實(shí)際。而便攜設(shè)備來(lái)說(shuō)很不實(shí)際。而1GHz，天線(xiàn)則只有幾米，天線(xiàn)則只有幾米高，這是很有用的。高，這是很有用的。Unit 2v原文：原文：Frequency division multiplexing: Using only base-band transmission, FDM could not be applied. Analog modulation shifts the base-band signals to different carrie

16、r frequencies. The higher the carrier frequency, the more bandwidth that is available for many base-band signals.v譯文：譯文：頻分復(fù)用：頻分復(fù)用不用在頻分復(fù)用：頻分復(fù)用不用在單獨(dú)單獨(dú)的基帶傳的基帶傳輸中。模擬調(diào)制將基帶信號(hào)轉(zhuǎn)換成不同的載波頻輸中。模擬調(diào)制將基帶信號(hào)轉(zhuǎn)換成不同的載波頻率。載波頻率越高，基帶信號(hào)的可用帶寬越寬。率。載波頻率越高，基帶信號(hào)的可用帶寬越寬。Unit 2v原文：原文：Medium characteristics: Path-loss penetration o

17、f obstacles, reflection, scattering, and diffraction depend heavily on the wavelength of the signal. v譯文：譯文：介質(zhì)特性：信道損耗、繞射、反射、散射介質(zhì)特性：信道損耗、繞射、反射、散射和在信號(hào)波長(zhǎng)緊密相關(guān)的衍射。和在信號(hào)波長(zhǎng)緊密相關(guān)的衍射。Unit 2v原文：原文：Depending on the applications, the right carrier frequency with the desired characteristics has to be chosen: long

18、waves for submarines, short waves for handheld devices, very short waves for directed microwave transmission etc.v譯文：譯文：根據(jù)實(shí)際應(yīng)用選擇所需合適根據(jù)實(shí)際應(yīng)用選擇所需合適的的載波頻率特載波頻率特性：潛艇采用長(zhǎng)波，便攜設(shè)備采用短波，定向微性：潛艇采用長(zhǎng)波，便攜設(shè)備采用短波，定向微波傳輸采用超短波等等。波傳輸采用超短波等等。Unit 2v原文：原文：As for digital modulation, three different basic schemes are known

19、for analog modulation: amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). vFigure 2.1 shows a (simplified) block diagram of a radio transmitter for digital data.v譯文：譯文：至于數(shù)字調(diào)制，所知的三種不同的基本模擬調(diào)至于數(shù)字調(diào)制，所知的三種不同的基本模擬調(diào)制類(lèi)型：幅度調(diào)制（制類(lèi)型：幅度調(diào)制（AM）、頻率調(diào)制（）、頻率調(diào)制（FM）和相位）和相位調(diào)制（調(diào)制（PM）。）。v圖（圖（2

20、.1）表示一個(gè)數(shù)字無(wú)線(xiàn)反射區(qū)的簡(jiǎn)化程序框圖。）表示一個(gè)數(shù)字無(wú)線(xiàn)反射區(qū)的簡(jiǎn)化程序框圖。Unit 2v原文：原文：The first step is the digital modulation of data into the analog base-band signal according to one of the schemes presented in the following section. v譯文：譯文：第一步是將數(shù)字調(diào)制數(shù)據(jù)通過(guò)如下部分所第一步是將數(shù)字調(diào)制數(shù)據(jù)通過(guò)如下部分所示的形式轉(zhuǎn)換成模擬基帶信號(hào)，然后將模擬信號(hào)示的形式轉(zhuǎn)換成模擬基帶信號(hào)，然后將模擬信號(hào)的中心頻率提高到無(wú)線(xiàn)

21、電載波頻段，最后信號(hào)通的中心頻率提高到無(wú)線(xiàn)電載波頻段，最后信號(hào)通過(guò)天線(xiàn)傳輸。過(guò)天線(xiàn)傳輸。Unit 2v原文：原文：The receiver(Figure 2.2) receives the analog radio signal via its antenna and demodulates the signal into the analog base-band signal with the help of the known carrier. This would be all that is needed for an analog radio tuned in to a radio

22、station. (The analog base-band signal would constitute the music.)v譯文：譯文：接收機(jī)（圖接收機(jī)（圖2.2）通過(guò)天線(xiàn)接收到模擬無(wú)）通過(guò)天線(xiàn)接收到模擬無(wú)線(xiàn)電信號(hào)，根據(jù)一定的載波對(duì)信號(hào)進(jìn)行解調(diào)轉(zhuǎn)換線(xiàn)電信號(hào)，根據(jù)一定的載波對(duì)信號(hào)進(jìn)行解調(diào)轉(zhuǎn)換成模擬基帶信號(hào)成模擬基帶信號(hào)，全部應(yīng)用在接收廣播電臺(tái)的模全部應(yīng)用在接收廣播電臺(tái)的模擬廣播。（模擬基帶信號(hào)承載音頻。）擬廣播。（模擬基帶信號(hào)承載音頻。）Unit 2v原文：原文：For digital data, another step is needed. Bits or frames have

23、to be detected, i.e., the receiver must synchronize with the sender. How synchronization is achieved, depends on the digital modulation scheme. v譯文：譯文：對(duì)于數(shù)字信號(hào)來(lái)說(shuō)，另一步對(duì)于數(shù)字信號(hào)來(lái)說(shuō)，另一步是是檢測(cè)比特和檢測(cè)比特和幀是至關(guān)重要的。例如接收必須和發(fā)射同步。同幀是至關(guān)重要的。例如接收必須和發(fā)射同步。同步怎樣實(shí)現(xiàn)要根據(jù)數(shù)字調(diào)制模式。步怎樣實(shí)現(xiàn)要根據(jù)數(shù)字調(diào)制模式。Unit 2v原文：原文：After synchronization, the r

24、eceiver has to decide if the signal represents a digital 1 or 0, reconstructing the original data.v譯文：譯文：經(jīng)過(guò)同步，接收機(jī)必須判定是否是由經(jīng)過(guò)同步，接收機(jī)必須判定是否是由0.1代碼表示的信號(hào)重組成原始數(shù)據(jù)。代碼表示的信號(hào)重組成原始數(shù)據(jù)。Unit 2v原文：原文： The digital modulation schemes presented in the following sections differ in many issues, such as spectral efficienc

25、y (I.e., how much power is needed to transfer bits which is very important for portable devices that are battery dependent), and robustness to multi-path propagation. Noise, and interference.v譯文：譯文：數(shù)字調(diào)制模式在下列問(wèn)題中有不同的表示：數(shù)字調(diào)制模式在下列問(wèn)題中有不同的表示：例如光譜效率（比如，傳輸比特所需用多少能量例如光譜效率（比如，傳輸比特所需用多少能量對(duì)電池終端設(shè)備是非常重要的。）、多徑對(duì)電池終

26、端設(shè)備是非常重要的。）、多徑效應(yīng)效應(yīng)、噪聲和干擾。噪聲和干擾。Unit 2v原文：原文：Amplitude shift keyingvFigure 2.3 illustrates amplitude shift keying (ASK), the most simple digital modulation scheme. The two binary values, 1 and 0, are represented by two different amplitudes. In the example, one of the amplitudes is 0 (representing the

27、 binary 0). v譯文：譯文：振幅鍵控振幅鍵控v圖圖2.3所示振幅鍵控（所示振幅鍵控（ASK）是最簡(jiǎn)單的數(shù)字調(diào)）是最簡(jiǎn)單的數(shù)字調(diào)制方式。二進(jìn)制值制方式。二進(jìn)制值0和和1由不同的幅度來(lái)表示。例由不同的幅度來(lái)表示。例中，一個(gè)幅度是中，一個(gè)幅度是0（代表二進(jìn)制（代表二進(jìn)制0）。）。Unit 2v原文原文:This simple scheme only requires low loss heavily influence the amplitude. In a wireless environment, a constant amplitude cannot be guaranteed, s

28、o ASK is typically not used for wireless radio transmission.v譯文：譯文：這種簡(jiǎn)單的方式僅需要低的帶寬，但是容這種簡(jiǎn)單的方式僅需要低的帶寬，但是容易受到干擾的影響。像多徑傳播，噪聲和信道衰易受到干擾的影響。像多徑傳播，噪聲和信道衰減嚴(yán)重影響幅度所產(chǎn)生的效果。在無(wú)線(xiàn)工作環(huán)境減嚴(yán)重影響幅度所產(chǎn)生的效果。在無(wú)線(xiàn)工作環(huán)境下，不能保證穩(wěn)定的幅度，所以通常下，不能保證穩(wěn)定的幅度，所以通常ASK不能用不能用于無(wú)線(xiàn)電傳輸。于無(wú)線(xiàn)電傳輸。Unit 2v原文：原文：However, the wire transmission scheme with th

29、e highest performance, namely optical transmission, uses ASK. Here, a light pulse may represent a 1, while the absence of light represents a 0. v譯文：譯文：然而，有線(xiàn)傳輸方式有很高的傳輸性能，然而，有線(xiàn)傳輸方式有很高的傳輸性能，即光纖傳輸中使用即光纖傳輸中使用ASK，有有光脈沖表示光脈沖表示1，同時(shí)，同時(shí)無(wú)光表示無(wú)光表示0。Unit 2v原文：原文：The carrier frequency in optical systems is some hu

30、ndred T Hz. ASK can also be applied to wireless infra red transmission, using a directed beam or diffuse light.v譯文：譯文：光纖系統(tǒng)中載波的頻率達(dá)到數(shù)百光纖系統(tǒng)中載波的頻率達(dá)到數(shù)百THz。ASK也應(yīng)用于無(wú)線(xiàn)紅外線(xiàn)傳輸通過(guò)直連光束或漫也應(yīng)用于無(wú)線(xiàn)紅外線(xiàn)傳輸通過(guò)直連光束或漫射光。射光。Unit 2v原文：原文：Frequency shift keyingvA modulation scheme often used for wireless transmission is frequen

31、cy shift keying (FSK)(Figure 2.4). The simplest form of FSK, also called binary FSK (BFSK),assigns one frequency f1 to the binary 1 and another frequency f2 to the binary 0. v譯文：譯文：頻移鍵控頻移鍵控vFSK(圖圖2.4)是常用于無(wú)線(xiàn)傳輸?shù)囊环N調(diào)試方式。最是常用于無(wú)線(xiàn)傳輸?shù)囊环N調(diào)試方式。最簡(jiǎn)單的頻移鍵控形式，被稱(chēng)為二進(jìn)制頻移鍵控簡(jiǎn)單的頻移鍵控形式，被稱(chēng)為二進(jìn)制頻移鍵控（BFSK），分配碼元），分配碼元1為頻率為頻率f1

32、，碼元，碼元0對(duì)應(yīng)頻率對(duì)應(yīng)頻率f2。Unit 2v原文：原文：A very simple way to implement FSK is to switch between two oscillators, one with the frequency f1 and the other with f2, depending on the input.v譯文譯文:實(shí)現(xiàn)實(shí)現(xiàn)FSK最簡(jiǎn)單的辦法是最簡(jiǎn)單的辦法是頻率頻率在兩個(gè)振蕩之在兩個(gè)振蕩之間轉(zhuǎn)換，根據(jù)輸入，一個(gè)是頻率間轉(zhuǎn)換，根據(jù)輸入，一個(gè)是頻率為為f1，另一個(gè)為，另一個(gè)為頻率頻率為為f2。Unit 2v原文：原文：To avoid sudden c

33、hanges in phase, special frequency modulators with continuous phase modulation (CPM) can be used. Sudden changes in phase cause high frequencies, which is an undesired side-effect. A simple way to implement demodulation is by using two band pass filters, one for f1 the other for other for f2. v譯文：譯文

34、：為了防止相位跳變，引入了特殊頻率調(diào)制，為了防止相位跳變，引入了特殊頻率調(diào)制，連續(xù)相位調(diào)制。相位隨高頻進(jìn)行跳變，這是難免連續(xù)相位調(diào)制。相位隨高頻進(jìn)行跳變，這是難免的邊效應(yīng)。簡(jiǎn)單方式上的解調(diào)是使用兩個(gè)帶通濾的邊效應(yīng)。簡(jiǎn)單方式上的解調(diào)是使用兩個(gè)帶通濾波，一個(gè)為波，一個(gè)為f1另一個(gè)為另一個(gè)為f2。Unit 2v原文：原文：A computer can then compare the signal levels of the filter outputs to decide which of them is stronger. FSK needs a larger bandwidth compare

35、d to ASK but is much less susceptible to errors.v譯文：譯文：抽樣抽樣判決判決器能檢測(cè)出經(jīng)過(guò)濾波輸出的信號(hào)器能檢測(cè)出經(jīng)過(guò)濾波輸出的信號(hào)電平是否足夠高。電平是否足夠高。FSK較較ASK有有較較高的帶寬，和高的帶寬，和更不易產(chǎn)生錯(cuò)誤。更不易產(chǎn)生錯(cuò)誤。Unit 2vUnit 2v原文：原文：This simple scheme, shifting the phase by 180 each time the value of data changer, is also called binary PSK (BSK).v譯文：譯文：這個(gè)簡(jiǎn)單的方式中，相位

36、每次變化這個(gè)簡(jiǎn)單的方式中，相位每次變化180碼碼元也隨之改變，這就是二進(jìn)制相移鍵控元也隨之改變，這就是二進(jìn)制相移鍵控（BPSK）。）。Unit 2v原文：原文：A simple implementation of a BPSK modulator could multiply a frequency f with +1 if the binary data is 1 and with -1 if the transmitter. v譯文：譯文：BPSK的調(diào)制器實(shí)現(xiàn)是如果是碼元的調(diào)制器實(shí)現(xiàn)是如果是碼元1便乘以便乘以頻率（頻率（f+1）碼元）碼元0則乘以頻率（則乘以頻率（f-1）。）。Unit 2

37、v原文：原文：This can be done using a phase lock loop(PLL). Compared to FSK, PSK is more resistant to interference ,but receiver and transmitter are also more complex.v譯文：譯文：為了數(shù)據(jù)接收的正確性，接收機(jī)在頻率和為了數(shù)據(jù)接收的正確性，接收機(jī)在頻率和相位上同發(fā)射機(jī)實(shí)現(xiàn)同步。這將使用鎖相環(huán)相位上同發(fā)射機(jī)實(shí)現(xiàn)同步。這將使用鎖相環(huán)（PLL）。）。PSK較較FSK有更好的抗干擾能力，但有更好的抗干擾能力，但接收和發(fā)射更復(fù)雜。接收和發(fā)射更復(fù)雜。Un

38、it 2v原文：原文：Advanced frequency shift keyingvA famous FSK scheme used in many wireless is minimum shift keying (MSK). MSK is basically BFSK without abrupt phase changes, i.e., it belongs to CPM schemes, Figure 2.6 shows an example for the implementation of MSK. v譯文：譯文：新型頻移鍵控新型頻移鍵控v在大多數(shù)無(wú)線(xiàn)系統(tǒng)中使用的普遍頻移鍵控頻是

39、最在大多數(shù)無(wú)線(xiàn)系統(tǒng)中使用的普遍頻移鍵控頻是最小頻移鍵控（小頻移鍵控（MSK）。）。MSK是基本沒(méi)有相位突是基本沒(méi)有相位突變的變的BFSK,即屬于即屬于CPM方式。圖方式。圖2.6示出示出MSK實(shí)現(xiàn)的例子。實(shí)現(xiàn)的例子。Unit 2vUnit 2v原文：原文：According to the following scheme, the lower or higher frequency is chosen (either inverted or non-inverted) to generate the MSK signal:vif the even and the odd bit are bot

40、h 0, then the higher frequency f2 is inverted (I.e., f2 is used with a phase shift of 180);v譯文：譯文：通過(guò)如下方式，選擇更低或更高的頻率來(lái)通過(guò)如下方式，選擇更低或更高的頻率來(lái)產(chǎn)生產(chǎn)生MSK信號(hào)：信號(hào)：v如果偶數(shù)為和奇數(shù)位都為如果偶數(shù)為和奇數(shù)位都為0，那么高頻，那么高頻f2部分倒部分倒置（即置（即f2其相位其相位180變化）。變化）。Unit 2v原文：原文：if the even bit is 1, the odd bit 0, then the lower frequency f1 is inver

41、ted. This is the case, e.g., in the fifth to seventh columns of Figure 2.6;v譯文：譯文：如果偶數(shù)位是如果偶數(shù)位是1，奇數(shù)位是，奇數(shù)位是0.那么將低頻那么將低頻f1部分倒置。這就是圖部分倒置。這就是圖2.6例子中第例子中第5到第到第7列的列的圖形。圖形。Unit 2v 原文：原文：if the even bit is 0 and the odd is 1, as in columns 1 to 3, f1 is taken without changing the phase;v if both bits are 1,

42、then the original f2 is taken.v A high frequency is always chosen if even and odd bits are equal. The signal is inverted if the odd bit equals 0. This scheme avoids all phase shifts in the resulting MSK signal.v 譯文：譯文：如果偶數(shù)位是如果偶數(shù)位是0奇數(shù)位是奇數(shù)位是1，那么從，那么從1到到3的波形中的波形中f1并不改變相位。并不改變相位。v 如果兩位都是如果兩位都是1，則選用原，則選用

43、原f2。v 如果奇數(shù)和偶數(shù)位相等通常選用高頻。如果偶數(shù)位等于如果奇數(shù)和偶數(shù)位相等通常選用高頻。如果偶數(shù)位等于0信號(hào)翻轉(zhuǎn)。這種方式避免了信號(hào)翻轉(zhuǎn)。這種方式避免了MSK信號(hào)中的所有相位都翻信號(hào)中的所有相位都翻轉(zhuǎn)。轉(zhuǎn)。Unit 2v原文：原文：Adding a so-called Gaussian low pass filter to the MSK scheme results is Gaussian MSK (GMSK), which is the digital modulation scheme for many European wireless standards. The filter

44、 reduces the large spectrum needed by MSK.v譯文：譯文：另外還有一種經(jīng)過(guò)高斯低通濾波的另外還有一種經(jīng)過(guò)高斯低通濾波的MSK方方式形成所謂的高斯最小頻移鍵控（式形成所謂的高斯最小頻移鍵控（GMSK），這），這許多歐洲無(wú)線(xiàn)標(biāo)準(zhǔn)中的數(shù)字調(diào)制形式。濾波降低許多歐洲無(wú)線(xiàn)標(biāo)準(zhǔn)中的數(shù)字調(diào)制形式。濾波降低了了MSK所需的高頻譜。所需的高頻譜。Unit 2v原文：原文：Advanced phase shift keyingvThe simple PSK scheme can be improved in many ways. The basic BPSK scheme

45、only uses one possible phase shift of 180. The left side of Figure 2.7 shows BPSK in the phase domain (which is typically the better representation compared to the time domain in Figure 2.5).v譯文：譯文：新型相移鍵控新型相移鍵控v簡(jiǎn)單的簡(jiǎn)單的PSK方式在許多方面可以被改進(jìn)。基本的方式在許多方面可以被改進(jìn)?；镜腂PSK方式僅用一次方式僅用一次180度相位轉(zhuǎn)換。圖度相位轉(zhuǎn)換。圖2.7左邊所左邊所示為示為BP

46、SK相位域（這通常比圖相位域（這通常比圖2.5的時(shí)域更具代表的時(shí)域更具代表性）。性）。Unit 2v原文：原文：the right side of Figure 2.7 shows quadrature PSK (QPSK), one of the most common PSK schemes (sometimes also called quaternary PSK).v譯文：譯文：右邊的圖右邊的圖2.7展示了差分相移鍵控展示了差分相移鍵控 （QPSK），是最普通的），是最普通的PSK形式（有時(shí)也稱(chēng)為形式（有時(shí)也稱(chēng)為四進(jìn)制相移鍵控）。四進(jìn)制相移鍵控）。Unit 2v 原文：原文：Here,

47、 higher bit rates can be achieved for the same bandwidth by coding two bits into one phase shift. Alternatively, one can reduce the bandwidth and still achieve the same bit rates as for BPSK.v譯文：譯文：在相同的帶寬中，一個(gè)相位轉(zhuǎn)換編碼兩比在相同的帶寬中，一個(gè)相位轉(zhuǎn)換編碼兩比特實(shí)現(xiàn)了高比特率。另外，減小了帶寬還達(dá)到了特實(shí)現(xiàn)了高比特率。另外，減小了帶寬還達(dá)到了BPSK相同的比特率。相同的比特率。Unit 2

48、v原文：原文：QPSK (and other PSK schemes) can be realized in two variants. The phase shift can always be relative to a reference signal (with the same frequency). If this scheme is used, a phase shift of 0 means that the signal is in phase with the reference signal. v譯文：譯文：QPSK(以及其他以及其他PSK方式方式)能實(shí)現(xiàn)兩種變能實(shí)現(xiàn)兩種

49、變形。相位變換也同參考信號(hào)相關(guān)（同頻）。如果形。相位變換也同參考信號(hào)相關(guān)（同頻）。如果使用這種方法，使用這種方法，0的一個(gè)相位變換意味著參考信號(hào)的一個(gè)相位變換意味著參考信號(hào)的相移。的相移。Unit 2v原文：原文：A QPSK signal will then exhibit a phase shift of 45 for the data 11, 135 for 10, 225 for 00, and 315 for 01 with all phase shifts being relative to the reference signal.v譯文：譯文：QPSK信號(hào)將展寬數(shù)據(jù)信號(hào)將展寬數(shù)

50、據(jù)11的相位變換為的相位變換為45，10的為的為135,00的為的為225和和01的為的為315使得所有相位變換通參考信號(hào)相關(guān)。使得所有相位變換通參考信號(hào)相關(guān)。Unit 2v原文：原文：The transmitter selects parts of the signal as shown in Figure 2.28 and concatenates them. To reconstruct data, the receiver has to compare the incoming signal at the receiver. v譯文：譯文：發(fā)射機(jī)選擇部分如圖發(fā)射機(jī)選擇部分如圖2.8所示

51、的信號(hào)并串所示的信號(hào)并串起來(lái)。為了恢復(fù)信號(hào)，接收機(jī)必須把接收的信號(hào)起來(lái)。為了恢復(fù)信號(hào)，接收機(jī)必須把接收的信號(hào)通參考信號(hào)做比較。其的一個(gè)問(wèn)題涉及接收機(jī)產(chǎn)通參考信號(hào)做比較。其的一個(gè)問(wèn)題涉及接收機(jī)產(chǎn)生參考信號(hào)。生參考信號(hào)。Unit 2v原文：原文：Transmitter and receiver have to be synchronized very often, e.g., by using special synchronization patterns before user data arrives or via a pilot frequency as reference.v譯文：譯文：

52、發(fā)射機(jī)和接收機(jī)必須同步，例如，在用戶(hù)發(fā)射機(jī)和接收機(jī)必須同步，例如，在用戶(hù)信號(hào)到達(dá)前使用特殊的同步模式或通過(guò)參考導(dǎo)頻。信號(hào)到達(dá)前使用特殊的同步模式或通過(guò)參考導(dǎo)頻。Unit 2v 原文：原文：One way to avoid this problem is to use differential QPSK (DQPSK). Here the phase shift is not relative to a reference signal but to the phase of the previous two bits. In this case, the receiver does not n

53、eed the reference signal but only compares two signals to reconstruct data. v DQPSK is used in US wireless technologies IS-136 and PACS and in Japanese PHS.v 譯文：譯文：為避免這種問(wèn)題的一種方法是使用差分四相鍵控為避免這種問(wèn)題的一種方法是使用差分四相鍵控（DQPSK）。相位變換與參考信號(hào)除在前兩個(gè)比特中）。相位變換與參考信號(hào)除在前兩個(gè)比特中都都不相關(guān)，在這種情況下，接收機(jī)不需要參考信號(hào)，只用比不相關(guān)，在這種情況下，接收機(jī)不需要參考信號(hào)，只

54、用比較兩信號(hào)就可以再生數(shù)據(jù)。較兩信號(hào)就可以再生數(shù)據(jù)。v 英國(guó)無(wú)線(xiàn)科技英國(guó)無(wú)線(xiàn)科技IS-136和日本和日本PHS的的PACS都使用都使用DQPSK。Unit 2v原文：原文：One could now think of extending the scheme to more angle are combined coding 4 bits per phase/amplitude change. v譯文：譯文：現(xiàn)在新型相位變換的方法是擴(kuò)展更多的角現(xiàn)在新型相位變換的方法是擴(kuò)展更多的角度。實(shí)例中，每編譯度。實(shí)例中，每編譯3比特相位變換比特相位變換8個(gè)角度。此個(gè)角度。此外，外，PSK結(jié)合結(jié)合ASK的方

55、式就如在的方式就如在9600bps時(shí)延時(shí)延標(biāo)準(zhǔn)的調(diào)制解調(diào)器使用的正交振幅調(diào)制（標(biāo)準(zhǔn)的調(diào)制解調(diào)器使用的正交振幅調(diào)制（QAM）（圖（圖2.8的左邊）。每次相位的左邊）。每次相位/幅度變換，幅度變換，3種不種不同振幅和同振幅和12個(gè)角度編碼個(gè)角度編碼4比特。比特。Unit 2v原文：原文：Problems occur for wireless communication in case of noise or ISI. The more points used in the phase domain, the harder it is to separate them. DQPSK has been

56、 proven as one of the efficient schemes under these considerations.v譯文：譯文：無(wú)線(xiàn)連接中的問(wèn)題有噪聲和無(wú)線(xiàn)連接中的問(wèn)題有噪聲和ISI。相位域。相位域使用的相位點(diǎn)越多越難分離。使用的相位點(diǎn)越多越難分離。DQPSK被證實(shí)是被證實(shí)是在這些考慮因素下最優(yōu)有效的方式之一。在這些考慮因素下最優(yōu)有效的方式之一。Unit 2v原文：原文：A more advanced scheme is a hierarchical modulation as used in the digital TV standard DVB-T. The right

57、 side of Figure 2.8 shows a 64 QAM that contains a QPSK modulation. v譯文：譯文：最新型的方式是使用在最新型的方式是使用在DVB-T標(biāo)準(zhǔn)的數(shù)字標(biāo)準(zhǔn)的數(shù)字電視中使用的分層調(diào)制。圖電視中使用的分層調(diào)制。圖2.8右邊的部分展示右邊的部分展示了用了用QPSK調(diào)制的調(diào)制的64位位QAM。Unit 2v原文：原文：A 64 QAM can code 6 bit per symbol. Here the two most significant bits are used for the QPSK signal embedded in th

58、e QAM signal. If the reception of the signal is good the entire QAM constellation can be resolved.v譯文：譯文：64位位QAM每標(biāo)志編碼每標(biāo)志編碼6比特。把比特。把QPSK信號(hào)中最顯著的兩個(gè)比特嵌入到信號(hào)中最顯著的兩個(gè)比特嵌入到QAM信號(hào)中。如信號(hào)中。如果信號(hào)接收正確，就能完成果信號(hào)接收正確，就能完成QAM的星座圖。的星座圖。Unit 2v 原文：原文：Under poor reception conditions, e.g., with moving receivers, only the QP

59、SK portion can be resolved. A high priority data stream in DVB-T is coded with QPSK using the two most significant bits. The remaining 4 bits represent low priority data. v For TV this could mean that the standard resolution data stream is coded with high priority, the high resolution information wi

60、th low priority. If the signal is distorted, at least the standard TV resolution can be received.v 譯文：譯文：在接收條件差的情況下，例如，在移動(dòng)接收條件下，在接收條件差的情況下，例如，在移動(dòng)接收條件下，只能接收只能接收QPSK信號(hào)部分。高優(yōu)先級(jí)數(shù)據(jù)流信號(hào)部分。高優(yōu)先級(jí)數(shù)據(jù)流DVB-T通過(guò)通過(guò)QPSK編碼由兩個(gè)標(biāo)記比特表示，剩下的編碼由兩個(gè)標(biāo)記比特表示，剩下的4個(gè)比特承載低個(gè)比特承載低優(yōu)先級(jí)數(shù)據(jù)。優(yōu)先級(jí)數(shù)據(jù)。v 如果信號(hào)是扭曲的，最終的電視分辨率也能接收。如果信號(hào)是扭曲的，最終的電視分辨率也能接收