外文翻译--数字通信系统.docx

英文资料及中文翻译 Digital Communication System For modern people, if you do not know “Digital Communication” or “Digital Signal”, it is seem to be outdated, even would be laughed as “a ignorant men” . well then ,what is the “Digital Communication” and what is the “Digital Signal” ? This text is to do an generalize in the way of brief and plain.1、Why Digital?Why are communication system，military and commercial alike，“going digital”？The primary advantage is the ease with which digital signals，compared with analog signals，are regenerated. Figure 1 illustrates an ideal binary digital pulse propagating along a transmission line. The shape of the waveform is affected by two basic mechanisms: (1) as all transmission lines and circuits have some nonideal frequence transfer function，there is a distorting effect on the ideal and pulse; and (2) unwanted electrical noise or other interference further distorts the pulse waveform. Both of these mechanisms cause the pulse shape to degrade as a function of line lenge，as shown in Figure 1. During the time that the transmitted pulse can still be reliably identified before it is degraded to an ambiguous state，the pulse is amplified by a digital amplifier that recovers its original ideal shape. The pulse is thus “rebor” or regenerated. Circuits that perform this function at regular intervals along a transmission system are called regenerative repeaters.Distance 1 Distance 2 Dance 3 stance 4 Distance 5Original Some signal Degraded Signal is badly Amplification dstortiuon dstortiuon dstortiuon degraded regenerate pulse Propagation distance Figure 1 Pulse degradation and regenerationDigital circuit are less subject to distortion and interfernce than analog circuit . Becanse binary circuits opoerate in one of two states fully on fully of to be meaningful，a disturbance must be large enough to change the circuit ooperating point from one state to the other. Such two-state operation facilitates signal regeneration and thus prevents noise and other disturbances from accumulating in transmission. Analog signals，however，are not two-state signals; they can take an infinite variety of shapes. With analog circuits，even a small disturbance can render the reproduced waveform unacceptably distorted. Once the analog signal is distorted，the distortion cannot be removed by amplification. Because accumulated noise is irrevocably bound to analog signals，they cannot be perfectly regenerated. With digital techniques，extremely low error rates produres high signal fidelity are possible through error detection and correction but similar procedures are not available with analog.There are other important advantages to digital communications. Digital circuit are more reliable and can be produced at a lower cost than analog circuit . Also, digital hardwave lends itself to more flexble implementation than analog hardwave (e.g., microprocessor, digital switching, and large-scale integrated (LSI) circuit). The combing of digital signals using time-division multiplexing (TDM) is simpler than the combing of analog signals using frequcency-division multiplexing (FDM). Different types of digital signals (data, telegraph, telephone, television) can be treated as identical signals in transmission and switching -a bit is a bit. Also, for convenient switching, digital messages can be handled in autonomous groups called packets. Digital techniques lend themselves naturally to signal processing functions that project against interferene and jamming,or that provide encryption and privacy. Also much data communication is from computer to computer, or from digital instruments or terminal to computer. Such digital terminations are naturally best served by digital communication links.What are the costs associated with the beneficial attributes of digital cimmunication system? Digital system tend to be very signal-processing intensive compared with analog. Also, digital system need to allocate a significant share of their resources to the task of synchroniztion at various levels . With analog system , on the other hand , synchroniztion often is accomplished more easily. One disadvantage of a digital communication system is non-graceful degradation. When the signal-to-noise ratio drops below a certain threshold, the quality of service can change suddenly from very poor. In cintrast, most communication aystem degrade more gracefully.2、Typical Blook Diagram and Transformations The function block diagram shown in Figure 2 illustrates the signal flow and the signal-processing steps through a typical digital communication system (DCS). This figure can serve as a kind of road map, guiding the reader through the chapter. The upper blocks - format, source encode, encrypt, channel encode, multiplex ,pulse modulate, bandpass modulate, frequency spread, and multiple access-denote signal transformations from the source to the transmitter. The lower block denote signal transformations from the receiver to the sink, essentially reversing the signal processing steps performed by the upper blocks. The modulate and demodulate/detect blocks together are called a modern. The term “modern” often encompasses several of the signal processing steps shown in Figure 2 ; When this is the case, the modern can be though of as the “brain” of the system. The transmitter and receiver can be though of as the “muscles” of the system. For wireless applications, the transmitter and consists of a frequency up-conversion stage to a radio frequency (RF), a high-power amplifier(LNA). Frequency down-conversion is performed in the fornt end of the receiver and/or the demodulator.Figure 2 illustrates a kind of reciprocity between the block in the upper transmitter part of the figure and those in the lower receiver part. The signal processing steps that take place in the transmitter are, for the most part, reversed in the reveiver. In Figure 2 , the input information source is converted to binary digits (bis); the bits are then grouped to from digital message or message. Each such symbol (mi，where i=1,2,3，M) can be regarded as a memerber of finite alphabet set containing M members. Thus, for M=2, the message symbol mi is binary (meaning that it constitutes just a signal bit). Even though binary symbol fall within the general definition of M-ary, nevertheless the name M-ary is usually applied to those cases where M2 ; hence, such symbol are each made up of a sequence of two or more bits. (Compare such a finite alphabet in a DCS with an analog system, where the message waveform is typically a member of an infinite set of possible waveform ). For system that use channel coding (error correction coding), a sequence of message symbol is denoted ui. Because a message symbol or a channel symbol can consist of a single bit or a group of bits, a sequence of such symbol is also described as a bit stream, as shown in Figure 2.Consider the key signal processing block shown in Figure 2 , Only formatting, modulation, demodulation/detection, and synchronization are essential for a DCS. Formatting, transform the source information into bits, thus assuring compatibility between the information and the signal processing within the DCS. From this point in the figure up to the pulse-modulation block, the information remains in the form of a bit stream. Modulation is the process by which message symbols or channel symbols (when channel coding is used) are converted to waveforms that are compatible with the requirements imposed by the transmission channel . Pulse modulation is an essential step because each symbol to be transmitted must first be transformed from a binary representation (voltage levels representing binary ones and zeros) to a baseband waveform. The term baseband refers to a signal whose spectrum extends from (or near) dc up to some finite value, usually less than a few megahertz. The pulse-modulation block usually includes filteringfor minimizing the transmission bandwidth. When pulse modulation is applied to binary sym-MultipleacessFrequencyspreadDigitalBasepasswaveform si(t)XMTInformationsource From other sourcesMessagesymbols PulseModulateMulti-plexChannelencodeEncryptSourceencodeFormatBandpassmodulate InformationsinkDigital input mi ui gi(t)DigitalBasebandwaveformSynchro-nization m(t) u(t) DetectZ(t)Multi-plexChanneldecodeDecryptSourcedecodeFormatMessagesymbolsDemodu-late&Sa- mpleRCV To other destination r(t)DigitalBasepasswaveformMultipleacessFrequen-cy des- readFrequen-cy des- read Figure 2 A typical digital communication systembols, the resulting binary waveform is called a pulse-code modulation (PCM) waveform. There are several types of PCM waveform. In telephone applications, these waveforms are often called line codes. When pulse modulation is applied to nonbinary symbols, the resulting waveform is called an M- ary pulse modulation wave form. There are several types of such waveforms,where the one called pulse-amplitude modulation (PAM) is emphasized. After pulse modulation , each message symbol or channel symbol takes the form of a baseband waveform gi(t) , where i=1,2,3，M. In any electronic implementation, the bit stream, prior to pulse-modulation, is represented with voltage levels. One might wonder why there is a separate block for pulse modulation when in face different voltage levels for binary ones and zeros can be viewed as impulse or as ideal rectangular pulses, each pulse occupying one bit time. There are two important differences between such voltage levels and the baseband waveforms used for modulation. First, the pulse-modulation block allows for a variety of binary and M-ary pulse-waveform types. Second, the filtering within the pulse-modulation block yields pulses that occupy more than just time. Filtering yields pulses that are spread in time, thus the pulses are “smeared” into neighboring bit-times. This filtering is sometimes referred to as pulse shaping; it is used to contain the transmission bandwidth within some desired spectral region.For an application involving RF transmission, the text important step is bandpass modulation; it is required whenever the transmission medium will not support the propagation of pulse-like waveforms. For such cases, the medium requires a bandpass waveform si(t), where i=1,2,M. The term bandpass is used to indicate that the baseband waveform gi(t) is frequency translate by a carrier wave to a frequency that is much larger than the spectral content of gi(t). As si(t) propagates over the hannel, it is impated by the channel characteristics, which can be described in term of the channels impulse response hc(t). Also, at various point along the signal route, additive random noise distorts the received signal r(t), so that its reception must be termed a corrupted version of the signal si(t) that was launched at the transmitter. The received signal r(t) can be expressed as: r(t)= si(t)* hc(t)+n(t) i=1,2,M.where “*” respresents a convolution operation, and n(t) represents a noise.In the reverse direction, the receiver front end and/or the demodulator provides frequency down-conversion for each bandpass waveform r(t). The demodulator restores r(t) to an optimally shaped baseband pulse z(t) in preparation for detection. Typically, there can be several filters associated with the receiver and demodulator filtering to remove unwanted high frequency terms (in the frequency down-conversion of bandpass waveforms), and filtering for pulse shaping. Equalization can be described as a filtering option that is used in or after the demodulator to reverse any degrading effects on the signal that were caused by the channel. Equalization becomes essential whenever the impulse reponse of the channel hc(t), is so poor that the received signal is badly distortion caused. An equalizer is implement to compensate for (i.e., remove or diminish) any signal distortion caused by a nonideal hc(t). Finally, the sampling step transforms the shaped pulse z(t) to a sample z(t), and the detection step transforms z(t) to an estimate of the channel symbol ui or an estimate of the message symbol mi (if there is no channel coding). As we known,demodulation is degrading the digital meaning of that waveform.The other signal processing steps within the modern are design option for specific system needs. Source coding produces analog-to-digital (A/D) conversion (for analog sources) and removes redundant (unneeded) information. Note that a typical DCS would either use the source coding option (for both digitizing and compressing the source information ), or it wpuld either use the simpler formatting transformation(for digitizing alone). A system would not use both source coding and formatting, because the former already includes the essential step of digitizing the information. Encryption, which is used to provide communication privacy, prevents unauthorized users from understanding message and from injecting false message into the system. Channel coding, for a given data rate, can reduce the probability of error, PE, or reduce the required signal-to-noise ratio to achieve a desired PE at the expense of transmission bandwidth or decoder complexity. Multiplexing and multiple-access procedures combine signals that might have different characteristics or might originate from different sources, so that they can share a portion of the communications resource(e.g., spectrum, time). Frequency spreading can produce a signal that is relatively invulnerable to interference (both natural and intentional) and can be used to enhance the privacy of the communications. It is also a valuable technique used for multiple access.数字通信系统对于现代人来说，如果不知道“数字通信”或“数字信号”，那就显得很落伍了，甚至会被人讥笑“孤陋寡闻”的。那么何为“数字信号”，何为“数字通信”呢？本文就来做一个简单浅显易懂的概括说明。一、为什么要进行数字化在现今，无论是军用还是商用，为什么通信系统都在进行“数字化”呢？这有许多原因，其中最重要的原因是，数字信号与模拟信号相比，更易于再生。如图1所示，是沿传输线传输的一种理想二进制数字脉冲。其波形的形状要受到两个基本因素的影响：第一，所有传输线和电路都具有非理想的频率传递函数，因而使理想脉冲产生了失真；第二，电子噪声或其他干扰进一步使脉冲波形产生失真。这两种失真机理实际线路长度的函数，都会引起脉冲形状发生畸变。还来看图1，在传输脉冲恶化到模糊状态之前，传输脉冲仍可被可靠的识别，由数字放大器将脉冲放大并恢复最初的理想的形状，脉冲就这样“再生”了。在传输系统中，在规定时间间隔内执行这种功能的电路成为“再生中继器”。图1 脉冲编码再生与模拟电路相比，数字电路不易产生失真和干扰。因为二进制数字电路工作在全通或全断的开关状态下才有意义，所以干扰必须足够大才能使电路从一种状态变到另一种状态。这两种工作状态有助于信号的再生，因而能在传输中有效地抑制噪声和其他累积干扰。然而模拟信号不是双态信号，它的波形有无限多个。在模拟电路中，即使很小的干扰也能导致信号产生难以接受的失真。失真一旦产生，就无法通过放大器去驱除。由于模拟信号和累积噪声密不可分，所以不能完全再生。若采用数字技术，通过检错和纠错可获得极低的差错概率，从而高保真信号，而模拟系统没有这样类似的技术。数字通信系统还有其他的优点：数字电路比模拟电路更可靠，且其生产成本比模拟电路低；数字硬件比模拟硬件更具灵活性，比如微处理器、数字开关、大规模集成电路等；时分复用（TDM）信号比频分复用（FDM）信号的模拟信号更简单；不同类型的数字信号（数据、电报、电话、电视等）在传输和交换中都被看成是相同的信号比特信号；为方便交换，还可将数字信号以数据包（packet）的形式进行处理。数字技术本身借助与信号出处理，具有抗人为干扰和自然干扰的功能，还能够提供加密和隐私处理。计算机与计算机之间、数字设备或终端与计算机之间的数据通信需求越来越多，这些数字终端可以通过数字通信链路获得最好的服务。数字通信系统获得这些优点的代价是什么呢？与模拟系统相比，数字系统侧重于信号处理技术，并在通信的各个阶段，都需要分配一部分共享的资源用于实现同步。而在模拟系统中，同步相对比较容易。数字通信系统的另一个缺点是具有门限效应。即当信噪比下降到一定限度时，服务质量就会急剧恶化，而大部分模拟通信系统服务质量的下降则比较缓慢。二、典型的通信系统框图如图2所示，其功能框图描述了典型数字通信系统（DCS）的信号流程和信号处理过程，该图作为一种地图引导读者浏览本文的内容。方框图的上部表示从信源到发送端的信号传输过程，包括格式化、信源编码、加密、信道编码、多路复用、脉冲调制、带通调制、频率扩展和多址接入；下部表示从接收端到信宿的信号传输过程，基本上是方框图上部信号处理的反过程。调制和解调/检测方框合称为调制解调器。术语“调制解调器”通常是由图2所示的信号处理过程中的一部分构成，这时调制解调器相当于系统的“大脑”，而发送端和接收端则相当于系统的“肌肉”。在无线电应用中，发送端将频率上变频到射频频段（RF），经过高功率放大器馈送到天线。接收端部分由天线和低噪声放大器（LNA）组成下变频由接受器或调制器的前级末端完成。图2显示出上部的发送方框图和下部的接受方框图存在可逆性，发送方框图中大部分信号处理步骤与接受部分方框图中的步骤相反。在图2中，输入信息源先转换成二进制数字（比特），然后将其组合为数字信息或信息符号。每个码元（Mi，i=1,2,3，M）都可看成是长度为M个码元集合元素中的一个组成部分，因而当M =2是，消息码元Mi就是二进制数的(这意味着它仅包含1比特的信息)。尽管二进制数符号也包含在M进制数的定义中，通常M进制数用于M 2的情形，所以每个M符号都由两个或两个以上比特构成（与DCS中这种有限码元集不同，模拟系统的信号波形集是无限的）。对于采用信道编码（纠错编码）的系统而言，信息码元标识为ui。由于每个信息码元或信道由一个或一组比特构成，这样的码元序列也成为比特流，如图2所示。考察图2所示的框图，对于DCS，关键信号处理过程仅包括的格式化、调制、解调/检测和同步。格式化把源信息转换为数字比特，以保证在DCS内信息与信号处理的一直性。在图2中，脉冲调治2方框之前的信息仍是比特流的形式。调制过程将信息码牙或信道码元（采用信道码元）转换成与传输信道特性匹配的波形。脉冲调制是必不可少的步骤，因为要传输每个符号必须先将从二进制代码（表示二进制1或0的电压电平）转换成基波形式。术语基带是直流延伸到某个有限值的信号频谱，这个值通常是小于几MHZ的有限值。脉冲调制方框通常包含使传输基带最小化的滤波器。当对二进制数符号应用脉冲调治时，产生的二进制数波形就称为脉冲编码调制波形。PCM（脉冲编码调制）波形有几种类型，在电话通信中，这些波形通常称为线路码。当脉冲调制用于非二进制符号时，产生的波统称为M进制调制波形，这样的波形也有好几种类型，这里我们重点介绍脉冲幅度调制（PAM）。经过脉冲调制后每个信息码元或信道码元都转变为基带波