【正文】 by two different amplitudes. In the example, one of the amplitudes is 0 (representing the binary 0). ?譯文： 振幅鍵控 ?圖 （ ASK）是最簡單的數(shù)字調(diào)制方式。 Unit 2 ?原文： The digital modulation schemes presented in the following sections differ in many issues, such as spectral efficiency (., how much power is needed to transfer bits – which is very important for portable devices that are battery dependent), and robustness to multipath propagation. Noise, and interference. ?譯文： 數(shù)字調(diào)制模式在下列問題中有不同的表示：例如光譜效率（比如，傳輸比特所需用多少能量對(duì)電池終端設(shè)備是非常重要的。同步怎樣實(shí)現(xiàn)要根據(jù)數(shù)字調(diào)制模式。） Unit 2 ?原文： For digital data, another step is needed. Bits or frames have to be detected, ., the receiver must synchronize with the sender. How synchronization is achieved, depends on the digital modulation scheme. ?譯文： 對(duì)于數(shù)字信號(hào)來說，另一步 是 檢測比特和幀是至關(guān)重要的。 Unit 2 ?原文： The receiver(Figure ) receives the analog radio signal via its antenna and demodulates the signal into the analog baseband signal with the help of the known carrier. This would be all that is needed for an analog radio tuned in to a radio station. (The analog baseband signal would constitute the music.) ?譯文： 接收機(jī)（圖 ）通過天線接收到模擬無線電信號(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) ， 全部應(yīng)用在接收廣播電臺(tái)的模擬廣播。 ?圖（ ）表示一個(gè)數(shù)字無線反射區(qū)的簡化程序框圖。 Unit 2 ?原文： Depending on the applications, the right carrier frequency with the desired characteristics has to be chosen: long waves for submarines, short waves for handheld devices, very short waves for directed microwave transmission etc. ?譯文： 根據(jù)實(shí)際應(yīng)用選擇所需合適 的 載波頻率特性：潛艇采用長波，便攜設(shè)備采用短波，定向微波傳輸采用超短波等等。載波頻率越高，基帶信號(hào)的可用帶寬越寬。 Unit 2 ?原文： Frequency division multiplexing: Using only baseband transmission, FDM could not be applied. Analog modulation shifts the baseband signals to different carrier frequencies. The higher the carrier frequency, the more bandwidth that is available for many baseband signals. ?譯文： 頻分復(fù)用：頻分復(fù)用不用在 單獨(dú) 的基帶傳輸中。例如， 1MHz的信號(hào)，天線就有數(shù)百米高，這顯然對(duì)便攜設(shè)備來說很不實(shí)際。 Unit 2 ?原文： For example digital modulation translates a 1 Mbps bitsteam into a baseband signal with a bandwidth of are several reasons why this baseband signal cannot be directly transmitted in a wireless system: ?譯文： 例如，通過數(shù)字調(diào)制，將一個(gè) 1Mbps比特流轉(zhuǎn)換成 1MHz帶寬的基帶信號(hào)。 Unit 2 ?原文： For example digital modulation translates a 1 Mbps bitsteam into a baseband signal with a bandwidth of are several reasons why this baseband signal cannot be directly transmitted in a wireless system: ?譯文： 例如，通過數(shù)字調(diào)制，將一個(gè) 1Mbps比特流轉(zhuǎn)換成 1MHz帶寬的基帶信號(hào)。下面我們要分別詳細(xì)討論?；鶐盘?hào)之所有以不能直接在無線系統(tǒng)中傳輸有如下幾個(gè)原因： Unit 2 ?原文： Digital transmission is used, for example, in wired local area works or within a puter. In wireless works, however, digital transmission cannot be used. Here, the binary bitsteam has to be translated into an analog signal first. ?譯文： 然而，在無線網(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，不一定必須用到數(shù)字傳輸。 Unit 2 ?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 baseband signal generated by the digital modulation up to the radio carrier. ?譯文： 除了數(shù)模轉(zhuǎn)換外，無線傳輸還需要一個(gè)附加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過數(shù)字調(diào)制提高形成無線電載波。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)。Unit 2 Modulation Technology Text A Modulation Technology 調(diào)制技術(shù) Unit 2 ?原文： Formula () introduced the basic function of a sine wave which already indicates the three basic modulation schemes (typically, the cosine function is used for explanation): ?譯文： 公式（ ）引入了一個(gè)基本的正弦函數(shù)，它可以表示三種基本調(diào)制方案（通常用余弦函數(shù)來說明）： Unit 2 ?Unit 2 ?原文： Digital modulation is required if digital data has to be transmitted over a medium that only allows for analog transmission. ?譯文： 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?，則要求進(jìn)行數(shù)字調(diào)制。 Unit 2 ?原文： One example for wired works is the old analog telephone system —to connect a puter to this system a modem is needed .The modem then performs the translation of digital data into analog signals vice versa. ?譯文： 模擬電話系統(tǒng)是有線網(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。數(shù)字傳輸在有線局域網(wǎng)或計(jì)算機(jī)內(nèi)部是很有用的。 Unit 2 ?原文： For example digital modulation translates a 1 Mbps bitsteam into a baseband signal with a bandwidth of are several reasons why this baseband signal cannot be directly transmitted in a wireless system: ?譯文： 例如，通過數(shù)字調(diào)制，將一個(gè) 1Mbps比特流轉(zhuǎn)換成 1MHz帶寬的基帶信號(hào)。 Unit 2 ?原文： The three basic methods for this translation are amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). These are discussed in more detail in the following sections. ?譯文： 三種基本的數(shù)字調(diào)制是幅移鍵控（ ASK）、頻移鍵控（ FSK）和相移鍵控（ PSK）。 Unit 2 ?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 baseband signal generated by the digital modulation up to the radio carrier. ?譯文： 除了數(shù)模轉(zhuǎn)換外，無線傳輸還需要一個(gè)附加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過數(shù)字調(diào)制提高形成無線電載波?；鶐盘?hào)之所有以不能直接在無線系統(tǒng)中傳輸有如下幾個(gè)原因： Unit 2 ?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 baseband si