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1、Unit 2Modulation Technology,Text A,Modulation Technology调制技术,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说明):译文：公式（2.1）引入了一个基本的正弦函数，它可以表示三种基本调制方案（通常用余弦函数来说明）：,Un
2、it 2,Unit 2,原文：Digital 数字modulation调制 is required if digital data has to be transmitted over a medium that only allows for analog模拟 transmission传输.译文：如果数字信号必须在只允许模拟信号传输的媒介中传输，则要求进行数字调制。,Unit 2,原文：One example for wired networks有线网络 is the old analog telephone system to connect a computer to this syst
3、em a modem调制解调器 is needed.The modem then performs the translation of digital data into analog signals vice versa.译文：模拟电话系统是有线网络的一个例子，它需要用一个调制解调器将计算机与系统相连。调制解调器完成调制解调模数转换的任务。数字传输在有线局域网或计算机内部是很有用的。,Unit 2,原文：Apart from the translation of digital data into analog signals,wireless transmission无线传输 requi
4、res an additional modulation,an analog modulation that shifts转变 the center frequency of the base-band signal generated产生 by the digital modulation up to the radio carrier无线电载波.译文：除了数模转换外，无线传输还需要一个附加的模拟调制，将普通基带信号的中心频率通过数字调制提高形成无线电载波。,Unit 2,原文：For example digital modulation translates a 1 Mbps bit-st
5、eam 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无线系统:译文：例如，通过数字调制，将一个1Mbps比特流转换成1MHz带宽的基带信号。基带信号之所有以不能直接在无线系统中传输有如下几个原因：,Unit 2,原文：Digital transmission数字传输 is used,for example,in wired local a
6、rea 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.译文：然而，在无线网络中，二进制比特流首先必须被转换成模拟信号，因此，不一定必须用到数字传输。,Unit 2,原文：The three basic methods for this translation are amplitude shift key
7、ing 幅移键控(ASK),frequency shift keying 频移键控(FSK),and phase shift keying 相移键控(PSK).These are discussed in more detail in the following sections部分.译文：三种基本的数字调制是幅移键控（ASK）、频移键控（FSK）和相移键控（PSK）。下面我们要分别详细讨论。,Unit 2,原文：Apart from the translation of digital data into analog signals,wireless transmission requir
8、es 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.译文：除了数模转换外，无线传输还需要一个附加的模拟调制，将普通基带信号的中心频率通过数字调制提高形成无线电载波。,Unit 2,原文：For example digital modulation translates a 1 Mbps bit-steam into a
9、 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:译文：例如，通过数字调制，将一个1Mbps比特流转换成1MHz带宽的基带信号。基带信号之所有以不能直接在无线系统中传输有如下几个原因：,Unit 2,原文：Apart from the translation转换 of digital data into analog signals,wireless t
10、ransmission requires an additional modulation调制,an analog modulation that shifts the center frequency of the base-band signal generated 产生by the digital modulation up to the radio carrier无线电载波频段.译文：除了数模转换外，无线传输还需要一个附加的模拟调制，将普通基带信号的中心频率通过数字调制提高到无线电载波频段。,Unit 2,原文：For example digital modulation transl
11、ates 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传输 in a wireless system:译文：例如，通过数字调制，将一个1Mbps比特流转换成1MHz带宽的基带信号。基带信号之所有以不能直接在无线系统中传输有如下几个原因：,Unit 2,原文：Antennas天线:An antenna must be the order of
12、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.译文：天线：天线必须由有效信号的波长大小来
13、决定。例如，1MHz的信号，天线就有数百米高，这显然对便携设备来说很不实际。而1GHz，天线则只有几米高，这是很有用的。,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 i
14、s available for many base-band signals.译文：频分复用：频分复用不用在单独的基带传输中。模拟调制将基带信号转换成不同的载波频率。载波频率越高，基带信号的可用带宽越宽。,Unit 2,原文：Medium characteristics:Path-loss penetration of obstacles,reflection,scattering,and diffraction depend heavily on the wavelength of the signal.译文：介质特性：信道损耗、绕射、反射、散射和在信号波长紧密相关的衍射。,Unit 2,原
15、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.译文：根据实际应用选择所需合适的载波频率特性：潜艇采用长波，便携设备采用短波，定向微波传输采用超短波等等。,Unit 2,原文：As for dig
16、ital 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.译文：至于数字调制，所知的三种不同的基本模拟调制类型：幅度调制（AM）、频率调制（FM）和相位调制（PM）。图（2.1）表示一
17、个数字无线反射区的简化程序框图。,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.译文：第一步是将数字调制数据通过如下部分所示的形式转换成模拟基带信号，然后将模拟信号的中心频率提高到无线电载波频段，最后信号通过天线传输。,Unit 2,原文：The receiver(Figure 2.2)receives the analog
18、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.)译文：接收机（图2.2）通过天线接收到模拟无线电信号，根据一定的载波对信号进
19、行解调转换成模拟基带信号，全部应用在接收广播电台的模拟广播。（模拟基带信号承载音频。）,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.译文：对于数字信号来说，另一步是检测比特和帧是至关重要的。例如接收必须和发射同步。同步怎样实现
20、要根据数字调制模式。,Unit 2,原文：After synchronization,the receiver has to decide if the signal represents a digital 1 or 0,reconstructing the original data.译文：经过同步，接收机必须判定是否是由0.1代码表示的信号重组成原始数据。,Unit 2,原文：The digital modulation schemes presented in the following sections differ in many issues,such as spectral e
21、fficiency(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.译文：数字调制模式在下列问题中有不同的表示：例如光谱效率（比如，传输比特所需用多少能量对电池终端设备是非常重要的。）、多径效应、噪声和干扰。,Unit 2,原文：Amplitude shift keyingFig
22、ure 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).译文：振幅键控图2.3所示振幅键控（ASK）是最简单的数字调制方式。二进制值0和1由不同的幅度来表示。例中，一个幅度是0（代表二进制0
23、）。,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.译文：这种简单的方式仅需要低的带宽，但是容易受到干扰的影响。像多径传播，噪声和信道衰减严重影响幅度所产生的效果。在无线工作环境下，不能保证稳定的幅度，所以通常ASK不能
24、用于无线电传输。,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.译文：然而，有线传输方式有很高的传输性能，即光纤传输中使用ASK，有光脉冲表示1，同时无光表示0。,Unit 2,原文：The carrier frequency in optical system
25、s is some hundred T Hz.ASK can also be applied to wireless infra red transmission,using a directed beam or diffuse light.译文：光纤系统中载波的频率达到数百THz。ASK也应用于无线红外线传输通过直连光束或漫射光。,Unit 2,原文：Frequency shift keyingA modulation scheme often used for wireless transmission is frequency shift keying(FSK)(Figure 2.4).
26、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.译文：频移键控FSK(图2.4)是常用于无线传输的一种调试方式。最简单的频移键控形式，被称为二进制频移键控（BFSK），分配码元1为频率f1，码元0对应频率f2。,Unit 2,原文：A very simple way to implement FSK is to switch between two oscillators,
27、one with the frequency f1 and the other with f2,depending on the input.译文:实现FSK最简单的办法是频率在两个振荡之间转换，根据输入，一个是频率为f1，另一个为频率为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
28、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.译文：为了防止相位跳变，引入了特殊频率调制，连续相位调制。相位随高频进行跳变，这是难免的边效应。简单方式上的解调是使用两个带通滤波，一个为f1另一个为f2。,Unit 2,原文：A computer can then compare the signal levels of the filter outputs to deci
29、de which of them is stronger.FSK needs a larger bandwidth compared to ASK but is much less susceptible to errors.译文：抽样判决器能检测出经过滤波输出的信号电平是否足够高。FSK较ASK有较高的带宽，和更不易产生错误。,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).译文：这个
30、简单的方式中，相位每次变化180码元也随之改变，这就是二进制相移键控（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.译文：BPSK的调制器实现是如果是码元1便乘以频率（f+1）码元0则乘以频率（f-1）。,Unit 2,原文：This can be done using a phase lock loop(PLL).Compared to F
31、SK,PSK is more resistant to interference,but receiver and transmitter are also more complex.译文：为了数据接收的正确性，接收机在频率和相位上同发射机实现同步。这将使用锁相环（PLL）。PSK较FSK有更好的抗干扰能力，但接收和发射更复杂。,Unit 2,原文：Advanced frequency shift keyingA famous FSK scheme used in many wireless is minimum shift keying(MSK).MSK is basically BFSK
32、without abrupt phase changes,i.e.,it belongs to CPM schemes,Figure 2.6 shows an example for the implementation of MSK.译文：新型频移键控在大多数无线系统中使用的普遍频移键控频是最小频移键控（MSK）。MSK是基本没有相位突变的BFSK,即属于CPM方式。图2.6示出MSK实现的例子。,Unit 2,Unit 2,原文：According to the following scheme,the lower or higher frequency is chosen(either
33、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);译文：通过如下方式，选择更低或更高的频率来产生MSK信号：如果偶数为和奇数位都为0，那么高频f2部分倒置（即f2其相位180变化）。,Unit 2,原文：if the even bit is 1,the odd bit 0,then the lowe
34、r frequency f1 is inverted.This is the case,e.g.,in the fifth to seventh columns of Figure 2.6;译文：如果偶数位是1，奇数位是0.那么将低频f1部分倒置。这就是图2.6例子中第5到第7列的图形。,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 take
35、n.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.译文：如果偶数位是0奇数位是1，那么从1到3的波形中f1并不改变相位。如果两位都是1，则选用原f2。如果奇数和偶数位相等通常选用高频。如果偶数位等于0信号翻转。这种方式避免了MSK信号中的所有相位都翻转。,Unit 2,原文：Adding a
36、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.译文：另外还有一种经过高斯低通滤波的MSK方式形成所谓的高斯最小频移键控（GMSK），这许多欧洲无线标准中的数字调制形式。滤波降低了MSK所需的高频谱。,Unit 2,原文：Ad
37、vanced phase shift keyingThe 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).译文：新型相移键控简单的P
38、SK方式在许多方面可以被改进。基本的BPSK方式仅用一次180度相位转换。图2.7左边所示为BPSK相位域（这通常比图2.5的时域更具代表性）。,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).译文：右边的图2.7展示了差分相移键控（QPSK），是最普通的PSK形式（有时也称为四进制相移键控）。,Unit 2,原文：Here,higher bit rates can
39、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.译文：在相同的带宽中，一个相位转换编码两比特实现了高比特率。另外，减小了带宽还达到了BPSK相同的比特率。,Unit 2,原文：QPSK(and other PSK schemes)can be realized in two variants.The phase
40、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.译文：QPSK(以及其他PSK方式)能实现两种变形。相位变换也同参考信号相关（同频）。如果使用这种方法，0的一个相位变换意味着参考信号的相移。,Unit 2,原文：A QPSK signal will then exhibit a phase
41、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.译文：QPSK信号将展宽数据11的相位变换为45，10的为135,00的为225和01的为315使得所有相位变换通参考信号相关。,Unit 2,原文：The transmitter selects parts of the signal as shown in Figure 2.28 and concatenates them.To recons
42、truct data,the receiver has to compare the incoming signal at the receiver.译文：发射机选择部分如图2.8所示的信号并串起来。为了恢复信号，接收机必须把接收的信号通参考信号做比较。其的一个问题涉及接收机产生参考信号。,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 pil
43、ot frequency as reference.译文：发射机和接收机必须同步，例如，在用户信号到达前使用特殊的同步模式或通过参考导频。,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
44、signal but only compares two signals to reconstruct data.DQPSK is used in US wireless technologies IS-136 and PACS and in Japanese PHS.译文：为避免这种问题的一种方法是使用差分四相键控（DQPSK）。相位变换与参考信号除在前两个比特中都不相关，在这种情况下，接收机不需要参考信号，只用比较两信号就可以再生数据。英国无线科技IS-136和日本PHS的PACS都使用DQPSK。,Unit 2,原文：One could now think of extending th
45、e scheme to more angle are combined coding 4 bits per phase/amplitude change.译文：现在新型相位变换的方法是扩展更多的角度。实例中，每编译3比特相位变换8个角度。此外，PSK结合ASK的方式就如在9600bps时延标准的调制解调器使用的正交振幅调制（QAM）（图2.8的左边）。每次相位/幅度变换，3种不同振幅和12个角度编码4比特。,Unit 2,原文：Problems occur for wireless communication in case of noise or ISI.The more points us
46、ed in the phase domain,the harder it is to separate them.DQPSK has been proven as one of the efficient schemes under these considerations.译文：无线连接中的问题有噪声和ISI。相位域使用的相位点越多越难分离。DQPSK被证实是在这些考虑因素下最优有效的方式之一。,Unit 2,原文：A more advanced scheme is a hierarchical modulation as used in the digital TV standard DV
47、B-T.The right side of Figure 2.8 shows a 64 QAM that contains a QPSK modulation.译文：最新型的方式是使用在DVB-T标准的数字电视中使用的分层调制。图2.8右边的部分展示了用QPSK调制的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 th
48、e signal is good the entire QAM constellation can be resolved.译文：64位QAM每标志编码6比特。把QPSK信号中最显著的两个比特嵌入到QAM信号中。如果信号接收正确，就能完成QAM的星座图。,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
49、 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.译文：
50、在接收条件差的情况下，例如，在移动接收条件下，只能接收QPSK信号部分。高优先级数据流DVB-T通过QPSK编码由两个标记比特表示，剩下的4个比特承载低优先级数据。如果信号是扭曲的，最终的电视分辨率也能接收。,Unit 2,原文：Multi-carrier modulationSpecial modulation schemes that stand somewhat apart from the others are multi-carrier modulation(MCM),orthogonal frequency division multiplexing(OFDM)or coded O
