【正文】 附　錄附錄A　外文資料1. INTRODUCTIONIn this book, we present the basic principles that underlie the analysis and design of digital munication subject of digital munications involves the transmission of information in digital form from a source that generates the information to one or more destinations. Of particular importance in the analysis and design of munication systems are the characteristics of the physical channels through which the information is transmitted. The characteristics of the channel generally affect the design of the basic building blocks of the munication system. Below, we describe the elements of a munication system and their functions.11 ELEMENTS OF A DIGITAL COMMUNICATION SYSTEMFigure 111 illustrates the functional diagram and the basic elements of a digital munication system. The source output may be either an analog signal, such as audio or video signal, or a digital signal, such as the output of a teletype machine, that is discrete in time and has a finite number of output characters. In a digital munication system, the messages produced by the source are converted into a sequence of binary digits. Ideally, we should like to represent the source output (message) by as few binary digits as possible. In other words, we seek an efficient representation of the source output that results in little or no redundancy. The process of efficiently converting the output of either an analog or digital source into a sequence of binary digits is called source encoding or data pression.The sequence of binary digits from the source encoder, which we call the information sequence, is passed lo the channel encoder. The purpose of the channel encoder is to introduce, in a controlled manner, some redundancy in the binary information sequence that can be used at the receiver to overe the effects of noise and interference encountered in the transmission of the signal through the channel. Thus, the added redundancy serves to increase the reliability of the received data and improves the fidelity of the received effect, redundancy in the information sequence aids the receiver in decoding the desired information sequence. For example, a (trivial) form of encoding of the binary information sequence is simply to repeat each binary digit m times,where m is some positive integer. More sophisticated (nontrivial) encoding involves talcing k information bits at a time and mapping each kbit sequence into a unique nbit sequence, called a code word. The amount of redundancy introduced by encoding the data in this manner is measured by the ratio n/ reciprocal of this ratio, namely k/n, is called the rate of the code or,simply, the code rate.The binary sequence at the output of the channel encoder is passed to the digital modulator, which serves as the interface to the munications nearly all of the munication channels encountered in practice are capable of transmitting electrical signals (waveforms), the primary purpose of the digital modulator is to map the binary information sequence into signal waveforms. To elaborate on this point, let us suppose that the coded information sequence is to be transmitted one bit at a time at some uniform rate R bits/s. The digital modulator may simply map the binary digit 0 into a waveform s0(t) and the binary digit 1 into a waveform j,(i). In this manner,each bit from the channel encoder is lransmitted separately. We call this binary modulation. Alternatively, the modulator may transmit b coded informationbits at a time by using M = 2s distinct waveforms j.(r), i = 0,1M 1, one waveform for each of the 2 possible 6bit sequences. We call this Mary modulation (M2). Note that a new 6bit sequence enters the modulator every b/R seconds. Hence, when the channel bit rate R is fixed, the amount of time available to transmit one of the M waveforms corresponding to a 6bit sequence is b times the time period in a system that uses binary modulation.The munication channel is the physical medium that is used to send the signal from the transmitter to the receiver. In wireless transmission, the channel may be the atmosphere (free space). On the other hand, telephone channels usually employ a variety of physical media, including wire lines,optical fiber cables, and wireless (microwave radio). Whatever the physical medium used for transmission of the information, the essential feature is that the transmitted signal is corrupted in a random manner by a variety of possible mechanisms, such as additive thermal noise generated by electronic devices,manmade noise, ., automobile ignition noise,and atmospheric noise,..electrical lightning discharges during thunderstorms.At the receiving end of a digital munications system, the digital demodulator processes the channelcorrupted transmitted waveform and reduces the waveforms to a sequence of numbers that represent estimates of the transmitted data symbols (binary or Mary). This sequence of numbers is passed to the channel decoder, which attempts to reconstruct the original information sequence from knowledge of the code used by the channel encoder and the redundancy contained in the received data.A measure of how well the demodulator and decoder perform is the frequency with which errors occur in the decoded sequence. More precisely,the average probability of a biterror at the output of the decoder is a measure of the performance of the demodulatordecoder bination. In general, the probability of error is a function of the codc characteristics, the types of waveforms used to transmit the information over the channci, the transmitter power, the characteristics of the channel, ., the amount of noise, the nature of the interference, etc., and the method of demodulation and decoding. These items and their effect on performance will be discussed in detail in subsequent chapters.As a final step, when an analog output is desired, the source decoder accepts the output sequence from the channel decoder and,