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Unit 2 Modulation Technology,Text A,Modulation Technology 調(diào)制技術(shù),Unit 2,原文：Formula (2.1) introduced the basic function of a sine wave which already indicates the three basic modulation schemes (typically通常, the cosine 余弦function is used for explanation說明): 譯文：公式（2.1）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 數(shù)字modulation調(diào)制 is required if digital data has to be transmitted over a medium that only allows for analog模擬 transmission傳輸. 譯文：如果數(shù)字信號必須在只允許模擬信號傳輸?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 networks有線網(wǎng)絡(luò) is the old analog telephone system —to connect a computer to this system a modem調(diào)制解調(diào)器 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。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：Apart from the translation of digital data into analog signals, wireless transmission無線傳輸 requires an additional modulation, an analog modulation that shifts轉(zhuǎn)變 the center frequency of the base-band signal generated產(chǎn)生 by the digital modulation up to the radio carrier無線電載波. 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，將普通基帶信號的中心頻率通過數(shù)字調(diào)制提高形成無線電載波。,Unit 2,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無線系統(tǒng): 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?；鶐盘栔幸圆荒苤苯釉跓o線系統(tǒng)中傳輸有如下幾個(gè)原因：,Unit 2,原文：Digital transmission數(shù)字傳輸 is used, for example, in wired local area networks or within a computer. In wireless networks, however, digital transmission cannot be used. Here, the binary bit-steam 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，因此，不一定必須用到數(shù)字傳輸。,Unit 2,原文：The three basic methods for this translation are amplitude shift keying 幅移鍵控(ASK), frequency shift keying 頻移鍵控(FSK), and phase shift keying 相移鍵控(PSK). These are discussed in more detail in the following sections部分. 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）。下面我們要分別詳細(xì)討論。,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 base-band 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，將普通基帶信號的中心頻率通過數(shù)字調(diào)制提高形成無線電載波。,Unit 2,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: 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。基帶信號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轉(zhuǎn)換 of digital data into analog signals, wireless transmission requires an additional modulation調(diào)制, an analog modulation that shifts the center frequency of the base-band signal generated 產(chǎn)生by the digital modulation up to the radio carrier無線電載波頻段. 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，將普通基帶信號的中心頻率通過數(shù)字調(diào)制提高到無線電載波頻段。,Unit 2,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傳輸 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?；鶐盘栔幸圆荒苤苯釉跓o線系統(tǒng)中傳輸有如下幾個(gè)原因：,Unit 2,原文：Antennas天線: An antenna must be the order of magnitude of 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. 譯文：天線：天線必須由有效信號的波長大小來決定。例如，1MHz的信號，天線就有數(shù)百米高，這顯然對便攜設(shè)備來說很不實(shí)際。而1GHz，天線則只有幾米高，這是很有用的。,Unit 2,原文：Frequency division multiplexing: Using only base-band transmission, FDM could not be applied. Analog modulation shifts the base-band signals to different carrier frequencies. The higher the carrier frequency, the more bandwidth that is available for many base-band 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。模擬調(diào)制將基帶信號轉(zhuǎn)換成不同的載波頻率。載波頻率越高，基帶信號的可用帶寬越寬。,Unit 2,原文：Medium characteristics: Path-loss penetration of obstacles, reflection, scattering, and diffraction depend heavily on the wavelength of the signal. 譯文：介質(zhì)特性：信道損耗、繞射、反射、散射和在信號波長緊密相關(guān)的衍射。,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。,Unit 2,原文：As for digital modulation, three different basic schemes are known for analog modulation: amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). Figure 2.1 shows a (simplified) block diagram of a radio transmitter for digital data. 譯文：至于數(shù)字調(diào)制，所知的三種不同的基本模擬調(diào)制類型：幅度調(diào)制（AM）、頻率調(diào)制（FM）和相位調(diào)制（PM）。 圖（2.1）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：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. 譯文：第一步是將數(shù)字調(diào)制數(shù)據(jù)通過如下部分所示的形式轉(zhuǎn)換成模擬基帶信號，然后將模擬信號的中心頻率提高到無線電載波頻段，最后信號通過天線傳輸。,Unit 2,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 station. (The analog base-band signal would constitute the music.) 譯文：接收機(jī)（圖2.2）通過天線接收到模擬無線電信號，根據(jù)一定的載波對信號進(jìn)行解調(diào)轉(zhuǎn)換成模擬基帶信號，全部應(yīng)用在接收廣播電臺的模擬廣播。（模擬基帶信號承載音頻。）,Unit 2,原文：For digital data, another step is needed. Bits or frames have to be detected, i.e., the receiver must synchronize with the sender. How synchronization is achieved, depends on the digital modulation scheme. 譯文：對于數(shù)字信號來說，另一步是檢測比特和幀是至關(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。,Unit 2,原文：After synchronization, the receiver has to decide if the signal represents a digital 1 or 0, reconstructing the original data. 譯文：經(jīng)過同步，接收機(jī)必須判定是否是由0.1代碼表示的信號重組成原始數(shù)據(jù)。,Unit 2,原文： The digital modulation schemes presented in the following sections differ in many issues, such as spectral efficiency (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. 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電池終端設(shè)備是非常重要的。）、多徑效應(yīng)、噪聲和干擾。,Unit 2,原文：Amplitude shift keying Figure 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 binary 0). 譯文：振幅鍵控 圖2.3所示振幅鍵控（ASK）是最簡單的數(shù)字調(diào)制方式。二進(jìn)制值0和1由不同的幅度來表示。例中，一個(gè)幅度是0（代表二進(jìn)制0）。,Unit 2,原文:This simple scheme only requires low loss heavily influence the amplitude. In a wireless environment, a constant amplitude cannot be guaranteed, so ASK is typically not used for wireless radio transmission. 譯文：這種簡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，噪聲和信道衰減嚴(yán)重影響幅度所產(chǎn)生的效果。在無線工作環(huán)境下，不能保證穩(wěn)定的幅度，所以通常ASK不能用于無線電傳輸。,Unit 2,原文：However, the wire transmission scheme with the highest performance, namely optical transmission, uses ASK. Here, a light pulse may represent a 1, while the absence of light represents a 0. 譯文：然而，有線傳輸方式有很高的傳輸性能，即光纖傳輸中使用ASK，有光脈沖表示1，同時(shí)無光表示0。,Unit 2,原文：The carrier frequency in optical systems is some hundred T Hz. ASK can also be applied to wireless infra red transmission, using a directed beam or diffuse light. 譯文：光纖系統(tǒng)中載波的頻率達(dá)到數(shù)百THz。ASK也應(yīng)用于無線紅外線傳輸通過直連光束或漫射光。,Unit 2,原文：Frequency shift keying A modulation scheme often used for wireless transmission is frequency 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. 譯文：頻移鍵控 FSK(圖2.4)是常用于無線傳輸?shù)囊环N調(diào)試方式。最簡單的頻移鍵控形式，被稱為二進(jìn)制頻移鍵控（BFSK），分配碼元1為頻率f1，碼元0對應(yīng)頻率f2。,Unit 2,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. 譯文:實(shí)現(xiàn)FSK最簡單的辦法是頻率在兩個(gè)振蕩之間轉(zhuǎn)換，根據(jù)輸入，一個(gè)是頻率為f1，另一個(gè)為頻率為f2。,Unit 2,原文：To avoid sudden changes 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. 譯文：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，這是難免的邊效應(yīng)。簡單方式上的解調(diào)是使用兩個(gè)帶通濾波，一個(gè)為f1另一個(gè)為f2。,Unit 2,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 compared to ASK but is much less susceptible to errors. 譯文：抽樣判決器能檢測出經(jīng)過濾波輸出的信號電平是否足夠高。FSK較ASK有較高的帶寬，和更不易產(chǎn)生錯(cuò)誤。,Unit 2,,Unit 2,原文：This simple scheme, shifting the phase by 180 each time the value of data changer, is also called binary PSK (BSK). 譯文：這個(gè)簡單的方式中，相位每次變化180碼元也隨之改變，這就是二進(jìn)制相移鍵控（BPSK）。,Unit 2,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. 譯文：BPSK的調(diào)制器實(shí)現(xiàn)是如果是碼元1便乘以頻率（f+1）碼元0則乘以頻率（f-1）。,Unit 2,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. 譯文：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)（PLL）。PSK較FSK有更好的抗干擾能力，但接收和發(fā)射更復(fù)雜。,Unit 2,原文：Advanced frequency shift keying A 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. 譯文：新型頻移鍵控 在大多數(shù)無線系統(tǒng)中使用的普遍頻移鍵控頻是最小頻移鍵控（MSK）。MSK是基本沒有相位突變的BFSK,即屬于CPM方式。圖2.6示出MSK實(shí)現(xiàn)的例子。,Unit 2,,Unit 2,原文：According to the following scheme, the lower or higher frequency is chosen (either inverted or non-inverted) to generate the MSK signal: if the even and the odd bit are both 0, then the higher frequency f2 is inverted (I.e., f2 is used with a phase shift of 180); 譯文：通過如下方式，選擇更低或更高的頻率來產(chǎn)生MSK信號： 如果偶數(shù)為和奇數(shù)位都為0，那么高頻f2部分倒置（即f2其相位180變化）。,Unit 2,原文：if the even bit is 1, the odd bit 0, then the lower frequency f1 is inverted. This is the case, e.g., in the fifth to seventh columns of Figure 2.6; 譯文：如果偶數(shù)位是1，奇數(shù)位是0.那么將低頻f1部分倒置。這就是圖2.6例子中第5到第7列的圖形。,Unit 2,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; if both bits are 1, then the original f2 is taken. 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. 譯文：如果偶數(shù)位是0奇數(shù)位是1，那么從1到3的波形中f1并不改變相位。 如果兩位都是1，則選用原f2。 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信號翻轉(zhuǎn)。這種方式避免了MSK信號中的所有相位都翻轉(zhuǎn)。,Unit 2,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 reduces the large spectrum needed by MSK. 譯文：另外還有一種經(jīng)過高斯低通濾波的MSK方式形成所謂的高斯最小頻移鍵控（GMSK），這許多歐洲無線標(biāo)準(zhǔn)中的數(shù)字調(diào)制形式。濾波降低了MSK所需的高頻譜。,Unit 2,原文：Advanced phase shift keying The simple PSK scheme can be improved in many ways. The basic BPSK scheme 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). 譯文：新型相移鍵控 簡單的PSK方式在許多方面可以被改進(jìn)?；镜腂PSK方式僅用一次180度相位轉(zhuǎn)換。圖2.7左邊所示為BPSK相位域（這通常比圖2.5的時(shí)域更具代表性）。,Unit 2,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). 譯文：右邊的圖2.7展示了差分相移鍵控 （QPSK），是最普通的PSK形式（有時(shí)也稱為四進(jìn)制相移鍵控）。,Unit 2,原文：Here, 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. 譯文：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BPSK相同的比特率。,Unit 2,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. 譯文：QPSK(以及其他PSK方式)能實(shí)現(xiàn)兩種變形。相位變換也同參考信號相關(guān)（同頻）。如果使用這種方法，0的一個(gè)相位變換意味著參考信號的相移。,Unit 2,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. 譯文：QPSK信號將展寬數(shù)據(jù)11的相位變換為45，10的為135,00的為225和01的為315—使得所有相位變換通參考信號相關(guān)。,Unit 2,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. 譯文：發(fā)射機(jī)選擇部分如圖2.8所示的信號并串起來。為了恢復(fù)信號，接收機(jī)必須把接收的信號通參考信號做比較。其的一個(gè)問題涉及接收機(jī)產(chǎn)生參考信號。,Unit 2,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. 譯文：發(fā)射機(jī)和接收機(jī)必須同步，例如，在用戶信號到達(dá)前使用特殊的同步模式或通過參考導(dǎo)頻。,Unit 2,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 need the reference signal but only compares two signals to reconstruct data. DQPSK is used in US wireless technologies IS-136 and PACS and in Japanese PHS. 譯文：為避免這種問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除在前兩個(gè)比特中都不相關(guān)，在這種情況下，接收機(jī)不需要參考信號，只用比較兩信號就可以再生數(shù)據(jù)。 英國無線科技IS-136和日本PHS的PACS都使用DQPSK。,Unit 2,原文：One could now think of extending the scheme to more angle are combined coding 4 bits per phase/amplitude change. 譯文：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3比特相位變換8個(gè)角度。此外，PSK結(jié)合ASK的方式就如在9600bps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（QAM）（圖2.8的左邊）。每次相位/幅度變換，3種不同振幅和12個(gè)角度編碼4比特。,Unit 2,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 proven as one of the efficient schemes under these considerations. 譯文：無線連接中的問題有噪聲和ISI。相位域使用的相位點(diǎn)越多越難分離。DQPSK被證實(shí)是在這些考慮因素下最優(yōu)有效的方式之一。,Unit 2,原文：A more advanced scheme is a hierarchical modulation as used in the digital TV standard DVB-T. The right side of Figure 2.8 shows a 64 QAM that contains a QPSK modulation. 譯文：最新型的方式是使用在DVB-T標(biāo)準(zhǔn)的數(shù)字電視中使用的分層調(diào)制。圖2.8右邊的部分展示了用QPSK調(diào)制的64位QAM。,Unit 2,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 the QAM signal. If the reception of the signal is good the entire QAM constellation can be resolved. 譯文：64位QAM每標(biāo)志編碼6比特。把QPSK信號中最顯著的兩個(gè)比特嵌入到QAM信號中。如果信號接收正確，就能完成QAM的星座圖。,Unit 2,原文：Under poor reception conditions, e.g., with moving receivers, only the QPSK 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. For TV this could mean that the standard resolution data stream is coded with high priority, the high resolution information with low priority. If the signal is distorted, at least the standard TV resolution can be received. 譯文：在接收條件差的情況下，例如，在移動接收條件下，只能接收QPSK信號部分。高優(yōu)先級數(shù)據(jù)流DVB-T通過QPSK編碼由兩個(gè)標(biāo)記比特表示，剩下的4個(gè)比特承載低優(yōu)先級數(shù)據(jù)。 如果信號是扭曲的，最終的電視分辨率也能接收。,Unit 2,原文：Multi-carrier modulation Special modulation schemes that stand somewhat apart from the others are multi-carrier modulation (MCM), orthogonal frequency division multiplexing (OFDM) or coded OFDM standards IEEE 802.11a and HiperLAN2. 譯文：多載波調(diào)制 特殊的調(diào)制方式與其形式不同的代表有多載波調(diào)制（MCM）、正交頻分復(fù)用、和編碼的正交頻分復(fù)用，都應(yīng)用在歐洲數(shù)字無線電系統(tǒng)DAB和WLAN標(biāo)準(zhǔn)的IEEE 802.11a和HiperLAN2中的內(nèi)容。,Unit 2,The main attraction of MCM is its good ISI mitigation property. MCM splits the high bit rate stream into many lower bit rate streams (see Figure 2.9), each stream being sent using an independent carrier frequency. MCM的主要影響力是其好的ISI延遲屬性，MCM把高速比特流分割成許多低速比特流（見圖2.9），每個(gè)數(shù)據(jù)流用獨(dú)自的載波頻率傳輸。,Unit 2,原文：If, for example , n symbols/s have to be transmitted, each subcarrier transmits n/c symbols/s with c being the number of subcarriers. One symbol could, for example represent 2 bits as in QPSK. DAB, for example, uses between 192 and 1 536 of these subcarrier. The physical layer of HiperLAN2 and IEEE 802.11a uses 48 subcarriers for data. 譯文：如果n表示傳輸間隔，每個(gè)子載波傳輸n/c個(gè)間隔，其中c表示子載波的數(shù)目。一個(gè)間隔能承載如例QPSK的2比特。DAB用192到1536之間的子載波。HiperLAN2和IEEE 802.11a的物理層和使用48的數(shù)據(jù)子載波。,Unit 2,原文：Figure 2.10 shows the superposition of orthogonal frequencies. The maximum of one subcarrier frequency appears exactly at a frequency where all other subcarriers equal zero. 譯文：圖2.10示出正交頻率的疊加。副載波頻率的最大值剛好出現(xiàn)在所有的子載波都為0的頻率處。,Unit 2,原文：Using this scheme, frequency selective fading only influences some subcarriers, and not the whole signal – an additional benefit of MCM. Typically, MCM transmits symbols. This helps the receiver to handle multi-path propagation. 譯文：使用這種方式，頻率選擇性衰減并不會影響所有的信號只是一些子載波，這也是MCM的一個(gè)額外的好處。通常MCM傳輸間隔的防衛(wèi)間隔是在單間隔和群間隔之間。這有助于接收機(jī)處理多信道傳輸。,Unit 2,原文：OFDM is a special method of implementing MCM using orthogonal carriers. Computationally, this is a very efficient algorithm based on fast Fourier transform (FFT) for modulation/demodulation. If additional error-control coding across the symbols in different subcarriers is applied, the system is referred to as COFDM. 譯文：OFDM實(shí)現(xiàn)使用正交載波的MCM的一種特殊方法。計(jì)算上，調(diào)制解調(diào)的基于快速傅里葉變換是一種非常有效是算法。如果貫穿應(yīng)用在不同子載波間隙附加的錯(cuò)誤控制編碼，系統(tǒng)應(yīng)當(dāng)使用COFDM。,Unit 2,Unit 2 Modulation Technology,Text B,PCM 脈沖編碼調(diào)制,Unit 2,原文：Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals, which was invented by Alec Reeves in 1937. It is the standard from digital audio in computers and various Blue-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems. 譯文：1937年，Alec Reeves發(fā)明了一種用數(shù)字信號表示被抽樣的模擬信號的方式，即脈沖編碼調(diào)制（PCM）。對于數(shù)字電話、DVD格式以及計(jì)算機(jī)的各種藍(lán)光壓縮光盤盒數(shù)字音頻而言，PCM是一種標(biāo)準(zhǔn)形式。,Unit 2,原文：A PCM stream is a digital representation of an analog signal, in which the magnitude of the analogue signal is sampled regularly at uniform intervals, with each sample being quantized to the nearest value within a range of digital steps. 譯文：PCM數(shù)據(jù)流是模擬信號的數(shù)字表示，模擬信號的幅度被間隔規(guī)則抽樣，每個(gè)樣值被量化為數(shù)字量階內(nèi)最近的值。,Unit 2,原文：PCM is dependent on three separate operations, sampling, quantizing, and coding. Many different schemes for performing these three functions have evolved during recent years, and we shall describe the main ones. In these descriptions we shall see how a speech channel of telephone quality may be conveyed as a series of amplitude values, each value being represented, that is, coded a sequence of 8 binary digits. 譯文：PCM依賴于抽樣，量化和編碼三種單獨(dú)的操作。 在二十年內(nèi)演變出了許多不同的實(shí)現(xiàn)能這三種功能的方法，我們將描述最主要的方法。我們將會了解到語音信道的電話質(zhì)量可以由一系列每個(gè)的幅度值來表示，如一串編碼的8位二進(jìn)制。,Unit 2,原文：Furthermore, we shall prove that a minimum theoretical sampling frequency of order 6.8 kilohertz (kHz) is required to convey a voice channel occupying the range 300 Hz to 3.4 kHz. 譯文：進(jìn)一步說，我可以證明6.8kHz序列的最小理論抽樣頻率能表示語音信道需占用300Hz到3.4kHz。,Unit 2,原文：Practical equipment, however, normally use a sampling rate of 8 kHz, and if 8-digit per sample value are used, the voice channel becomes represented by a stream of pulse with a repetition rate of 64 kHz. 譯文：然而，實(shí)用設(shè)備通常使用的抽樣頻率是8kHz，如果每次抽樣值是8位，語音信道將變成由速率重復(fù)的64kHz的脈沖流表示。,Unit 2,原文：Reexamination of our simple example shows us that the speech signal of maximum frequency 3.4 kHz has been represented by a signal of frequency of 64 kHz. However, if only 4-digits per sample value had been used, the quality of frequency of transmission would drop, and the repetition rate of the pulses would be reduced to 32 kHz. 譯文：重新檢查抽樣值得出語音信號的最大頻率3.4kHz已經(jīng)由頻率為64kHz的信號表示。然而，如果抽樣值僅為4位，傳輸質(zhì)量會降低，并且根據(jù)脈沖的速率將降低到32kHz。,Unit 2,原文：Thus the quality of transmission is dependent on the pulse repetition rate, and for digital communication system these two variables may be interchanged most efficiently. 譯文：因此脈沖重復(fù)率決定傳輸速率，對數(shù)字通信系統(tǒng)來說這兩個(gè)數(shù)值可以有效的互換。,Unit 2,原文：Digital transmission provides a powerful method for overcoming noisy environments. Noise can be introduced into transmission path in many different ways: perhaps via a nearby lighting strike, the sparking of a car ignition 譯文：數(shù)字傳輸提供了一種克服自然噪聲的有效方法。傳輸信道中引入噪聲的途徑有很多：也許一個(gè)近的雷擊，汽車點(diǎn)火系統(tǒng)的叫聲，或者通信設(shè)備自身的熱低噪聲水平。,Unit 2,原文：system, or the thermal low-level noise with the communication equipment