【文章內(nèi)容簡(jiǎn)介】 munication system and their ELEMENTS OF A DIGITAL COMMUNICATION SYSTEM Figure illustrates the functional diagram and the basic elements of a digital munication 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 a digital munication system, the messages produced by the source are converted into a sequence of binary , we should like to represent the source output(message)by as few binary digits as other words, we seek an efficient representation of the source output that results in little or no 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 sequence of binary digits from the source encoder, which we call the information sequence, is passed lo the channel 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 , 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 example, a(trivial)form of encoding of the binary information sequence is 黃河科技學(xué)院畢業(yè)論文（文獻(xiàn)翻譯）simply to repeat each binary digit m times,where m is some positive sophisticated(nontrivial)encoding involves talcing k information bits at a time and mapping each kbit sequence into a unique nbit sequence, called a code 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 Basic elements of a digital munication system 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 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/ 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 call this binary , the modulator may transmit b coded information bits at a time by using M = 2s distinct waveforms j.(r), i = 0,1M1 MHz(corresponding to a wavelength of A = cffr = 300m).requires an antenna of at least important characteristics and attributes of antennas for wireless transmission are described in Chapter illustrates the various frequency bands of the mode of propagation of electromagnetic waves in the atmosphere and in free space may be subdivided into three categories, namely,groundwave propagation, skywave propagation, and lineofsight(LOS) the VLF and audio frequency bands, where the wavelengths exceed 10 km, the earth and the ionosphere act as a waveguide for electromagnetic wave these frequency ranges, munication signals practically propagate around the this reason, these frequency bands are primarily used to provide navigational aids from shore to ships around the channel bandwidths available in these frequency bands are relatively small(usually 110% of the center frequency), and hence the information that is transmitted through these channels is of relatively slow speed and generally confined to digital dominant type of noise at these frequencies is generated from thunderstorm activity around the globe,especially in tropical results from the many users of these frequency （文獻(xiàn)翻譯）Groundwave propagation, as illustrated in , is the dominant mode of propagation for frequencies in the MF band( MHz).This is the frequency band used for AM broadcasting and maritime radio AM broadcasting, the range with groundwave propagation of even the more powerful radio stations is limited to about 150 noise,manmade noise, and thermal noise from electronic ponents at the receiver are dominant disturbances for signal transmission in the MF propagation, as illustrated in results from transmitted signals being reflected(bent or refracted)from the ionosphere, which consists of several layers of charged particles ranging in altitude from 50 to 400 km above the surface of the the daytime hours, the heating of the lower atmosphere by the sun causes the formation of the lower layers at altitudes below 120 lower layers, especially the Dlayer, serve to absorb frequencies below 2 MHz, thus severely limiting skywave propagation of AM radio , during the nighttime hours, the electron density in the lower layers of the ionosphere drops sharply and the frequency absorption that occurs during the daytime is significantly a consequence, powerful AM radio broadcast stations can propagate over large distances via sky wave over the Flayer of the ionosphere, which ranges from 140 to 400 km above the surface of the frequently occurring problem with electromagnetic wave propagation via sky wave in the HF frequency range is signal multipath occurs when the transmitted signal arrives at the receiver via multiple propagation paths at different delays, tt generally results in intersymbol interference in a digital munication , the signal ponents arriving via different propagation paths may add destructively, resulting in a phenomenon called signal fading, which most people have experienced when listening to a distant radio station at night when sky wave is the dominant propagation noise at HF is a bination of