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1、 现代通信原理课件Chapter OneChapter One1IntroductionChapter OneWhy?Information AgeInformation Superhighway2Application military application;common application;Chapter One3What is a communication system?Communication systems is used to transmit information.1.1 Introduction4Key conceptions or words (1)Informa
2、tion;(2)Message;(3)Signal;(4)Waveforms1.1 Introduction51 Jimes C.Maxwell（1864）2 Heinrich Hertz（1887）3Claude E.Shannon(1948)4.Winner1.1 Introduction6Perspective of communicationWire optic-fiber and networks wireless 1G-2G-3G-B3G WLAN;MIMO and MIMO-OFDM,UWB,optic wireless communication1.1 Introduction
3、7What must be considered when you design a communication system?1.Selection of the information-bearing waveform2.Bandwidth and power of the waveform3.Effect of system noise on the received information4.Cost of the system1.1 Introduction81.2 Digital and Analog source and systemThe generation of commu
4、nication system91)signal processor 2)In analog system it may be an analog low-pass filter;In hybrid one,it may be analog-to-digital converter(ADC)3)2)Function of signal-processing block in hybrid one4)Source coding5)Channel coding(adding parity and others)6)3)Function of Carrier Circuits7)modulation
5、Function of transmitter 101）Classification:wire and wirelessWire channel including twisted-pair telephone line/Coaxial cables,waveguides and fiber-optic cables Wireless channel having air,vacuum and seawater2)Note:The general principles of digital and analog modulation apply to all types of channels
6、,although channel characteristics may impose constraints that favor a particular type of signalingChannel11The receiver takes the corrupted signal at the channel output and converts it to a baseband signal that can be handled by the receiver baseband processor,and then the receiver baseband processo
7、r estimate the source information and output the estimation resultReceiver12The goal is to design communication system that transmit information to the receiver with as little deterioration as possible while satisfying design constrains such as allowable transmitted energy,allowable signal bandwidth
8、 and cost.In digital system measure of deterioration is taken to be bit error rate(BER),while in analog system the performance measure is usually taken to be the signal-to-noise ratio(SNR)at the receiver output.Summary131.2 Digital and Analog source and systemModel of analog communication system141.
9、2 Digital and Analog source and system(model of digital communication system)151）Relatively inexpensive digital circuits may be used2）Privacy is preserved by using data encryption3）Greater dynamic range is possible4）Data from different sources my be transmitted over a common digital transmission sys
10、tem2 the advantage of digital system1.2 Digital and Analog source and system165）Noise does not accumulate in long-distance systems by repeater6）Error in detected data may be small,even in large amount of noise on the received signal7）Error may often be corrected by the use of coding2 the advantage o
11、f digital system1.2 Digital and Analog source and system171)More bandwidth is required generally2)Sychronization is required3.Disadvantages of Digital communication1.2 Digital and Analog source and system181.2 Digital and Analog source and system(classification of communication system)according to p
12、hysical properties of information:Telephone/data/image and so on according to frequency:baseband transmitting/bandpass transmitting system according to transmitting medium:wire/wireless according to multiplexing patterm:Frequency division multiplexing access(FDMA)Time division multiplexing access(TD
13、MS)Code division multiplexing access(CDMA)191.2 Digital and Analog source and systemClassification of communication pattermPeer to peer communication:simplex;half duplex;duplexParallel transmitting communication Series transmitting communication201.Key conception2.Digital information source produces
14、 a finite set of possible messages.Such as typewriter and keyboard 3.Analog information source produces messages that are defined on continuum.4.Digital communication system transfers information from a digital source to the intended receiver(sink)5.Analog communication system transfers information
15、from a analog source to the intended receiver(sink)1.2 Digital and Analog source and system211)Deterministic waveform It can be modeled as a completely specified function of time2)Random waveform(stochastic waveform)3)It can not be completely specified as a function of time and must be modeled proba
16、bilistically4)4)Example:Example:1.3 Deterministic and Random waveforms 221)Property of the book a.from easiness to difficulty b.from concept to performance analysis2)Methods of learning it a.reading it again and again b.grasping the important concept c.understanding the basic communication system by
17、 deriving some key formula d.simulation using Matlab1.4 The property of the book and methods of learning it231.Information ContentIf the probabiltiy of transmitting jth message is Pj，the information content sent from a digital source when it was transmitted is given by:1.9 Information Measure241.9 I
18、nformation MeasureFrom the definition,we have the following results:1)The message that are less likely to occur provide more information2)Information content depends on only the likehood of sending the message and does not depend on possible interpretation of the content as to whether or not it make
19、s sense.252 Unit of information1.9 Information Measure1)Bits,Nats and Hartley262 Unit of information1.9 Information Measure2)Relationship between Bit,Nat and Hartley271)Definition2)If there are m possible different source message in a digital source(m is finite),and Pj is the probabiltiy of sending
20、the jth message,the entropy(also called average information)is:1.9 Information Measure3.Entropy for Digital Source284 Example1-1Example1-1:In a string of 12 symbols,where each symbol consistis of one of four levels,there are 412 different combinations(words),since each levels is equally likely,and a
21、ll the different words equally likely.Thus Pj=(1/4)12,And Ij=log2(1/(1/4)12)=12log2(4)=24bits 1.9 Information Measure295 Source Rate 1.9 Information MeasureSource Rate is defined asExampleExample:A1-2 A telephone touch-tone keypad has the digits 0 to 9 with probability of sending being 0.099 each,pl
22、us“*”and“#”with probability of sending being 0.005 each.If the keys are pressed at a rate of 2keys/pute the data rate for this source.Solution:Using define above,where T=1/2=0.5s/key,then:30For digital systems,the optimum system might be defined as the system that minimizes the BER at the system out
23、put subject to constraints on transmitted energy and channel bandwidth.Thus bit error rate and signal bandwidth are of prime importance in this course.1.10 Channel capacityShannon gives channel capacity as311.10 Channel capacityWhere B is the channel bandwidth in hertz,S/N is the signal-to-noise pow
24、er ratio at the input to the digital receiver.And showed that if the rate of information R(bits/s)was less than C,the BER would approach zero.32In analog system,the optimum system might be defined as the one that achieves the largest signal-to-noise ratio at the receiver output,subject to design con
25、straints such as channel bandwidth and transmitted power.Nyquist showed that if a pulse represents one bit of data,noninterfering pulse could be sent over a channel no faster than 2B pulses/s,where B is the channel bandwidth in hertz.That is now known as the dimensionality theorem.1.10 Channel Capac
26、ity33Example SA1-3Example SA1-3:A computer user plans to buy a higher-speed modem for sending data over his or her analog telephone line.The telephone line has a signal-to-noise ratio(SNR)of 25dB and passes audio frequencies over the range,from 300 to 3200Hz,Calculate the maximum data rate that coul
27、d be sent over the telephone line when there are on errors at the receiving end.Solution:In the term of a power ratio,the SNR=10(25/10)=316.2,the bandwidth is B=3200-300=2900 Hz，we get:1.10 Channel CapacityProblem:Can a 28.8kbit/s modem work on this telephone line?And a 14.4kbits/s modem?34Problems:1.5,1.7,1.9,1.10,1.12,1.13 1.15Reading:page 1-2,1-2,1-3,1-6 1-9,1-10 Homework35