1、1 现代通信原理课件Chapter OneChapter One2IntroductionChapter OneWhy?Information AgeInformation Superhighway3Application military application;common application;Chapter One4What is a communication system?Communication systems is used to transmit information.1.1 Introduction5Key conceptions or words (1)Inform
2、ation;(2)Message;(3)Signal;(4)Waveforms1.1 Introduction61 Jimes C.Maxwell（1864）2 Heinrich Hertz（1887）Claude E.Shannon(1948)4.Winner1.1 Introduction7Perspective of communicationWire optic-fiber and networks wireless 1G-2G-3G-B3G WLAN;MIMO and MIMO-OFDM,UWB,optic wireless communication1.1 Introduction
3、8What 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 Introduction91.2 Digital and Analog source and system The generation of comm
4、unication system Transmitter Information input m(t)Signal processing channel Signal processing Carrier circuits Receiver noise Carrier circuits Information output 10signal processor In analog system it may be an analog low-pass filter;In hybrid one,it may be analog-to-digital converter(ADC)2)Functio
5、n of signal-processing block in hybrid one Source coding Channel coding(adding parity and others)3)Function of Carrier Circuits modulationFunction of transmitter 111）Classification:wire and wirelessWire channel including twisted-pair telephone line/Coaxial cables,waveguides and fiber-optic cables Wi
6、reless channel having air,vacuum and seawater2)Note:The general principles of digital and analog modulation apply to all types of channels,although channel characteristics may impose constraints that favor a particular type of signalingChannel12The receiver takes the corrupted signal at the channel
7、output and converts it to a baseband signal that can be handled by the receiver baseband processor,and then the receiver baseband processor estimate the source information and output the estimation resultReceiver13The goal is to design communication system that transmit information to the receiver w
8、ith as little deterioration as possible while satisfying design constrains such as allowable transmitted energy,allowable signal bandwidth 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
9、signal-to-noise ratio(SNR)at the receiver output.Summary141.2 Digital and Analog source and system Model of analog communication system Transmitting end Information source modulator channel Demodu-lator Sink Or(acceptant)Receiving end noise 151.2 Digital and Analog source and system(model of digital
10、 communication system)161）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 system2 the advantage of digital system1.2 Digital and Ana
11、log source and system175）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 of digital system1.2 Digital and Analog source and syste
12、m181)More bandwidth is required generally2)Sychronization is required3.Disadvantages of Digital communication1.2 Digital and Analog source and system191.2 Digital and Analog source and system(classification of communication system)according to physical properties of information:Telephone/data/image
13、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(TDMS)Code division multiplexing access(CDMA)201.2 Digital
14、 and Analog source and system Classification of communication patterm Peer to peer communication:simplex;half duplex;duplex Parallel transmitting communication Series transmitting communication21Key conception Digital information source produces a finite set of possible messages.Such as typewriter a
15、nd keyboard Analog information source produces messages that are defined on continuum.Digital communication system transfers information from a digital source to the intended receiver(sink)Analog communication system transfers information from a analog source to the intended receiver(sink)1.2 Digita
16、l and Analog source and system22Deterministic waveform It can be modeled as a completely specified function of timeRandom waveform(stochastic waveform)It can not be completely specified as a function of time and must be modeled probabilisticallyExample:Example:1.3 Deterministic and Random waveforms)
17、cos()(00tAt231)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 deriving some key formula d.simulation using Matlab1.4 The
18、property of the book and methods of learning it241.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 MeasurebitPPIjjj)(log)1(log22251.9 Information MeasureFrom the definition,
19、we have the following results:The message that are less likely to occur provide more information1)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 makes sense.262 Unit of information1.9 Infor
20、mation MeasurebitsPIjbitsj)(log2,natsPIjnatsj)ln(,HartleyPIjhartleyj)(log10,1)Bits,Nats and Hartley272 Unit of information1.9 Information Measure2)Relationship between Bit,Nat and Hartley)2/(log)2/(ln10,hartleyjnatsjbitjIII28Definition If there are m possible different source message in a digital so
21、urce(m is finite),and Pj is the probabiltiy of sending the jth message,the entropy(also called average information)is:1.9 Information MeasurebitsPPIPHmjjjmjjj121)1(log3.Entropy for Digital Source294 Example1-1Example1-1:In a string of 12 symbols,where each symbol consistis of one of four levels,ther
22、e are 412 different combinations(words),since each levels is equally likely,and all the different words equally likely.Thus Pj=(1/4)12,And Ij=log2(1/(1/4)12)=12log2(4)=24bits 1.9 Information Measure305 Source Rate 1.9 Information MeasureSource Rate is defined asbitsTHR ExampleExample:A1-2 A telephon
23、e touch-tone keypad has the digits 0 to 9 with probability of sending being 0.099 each,plus“*”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:bitsPPIPHmjjjmjjj121)1(logkeybits/38.3005.01log)005.0(2099.01log)099
24、.0(102log1101010Using define above,where T=1/2=0.5s/key,then:sbitsTHR/76.65.0/38.331For digital systems,the optimum system might be defined as the system that minimizes the BER at the system output subject to constraints on transmitted energy and channel bandwidth.Thus bit error rate and signal band
25、width are of prime importance in this course.1.10 Channel capacityShannon gives channel capacity asNSBC1log2321.10 Channel capacityWhere B is the channel bandwidth in hertz,S/N is the signal-to-noise power ratio at the input to the digital receiver.And showed that if the rate of information R(bits/s
26、)was less than C,the BER would approach zero.33In 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 constraints such as channel bandwidth and transmitted power.Nyquist showed that if a pulse represent
27、s 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 Capacity34Example SA1-3Example SA1-3:A computer user plans to buy a higher-speed modem for sending dat
28、a 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 could be sent over the telephone line when there are on errors at the receiving end.Solution:In the t
29、erm of a power ratio,the SNR=10(25/10)=316.2,the bandwidth is B=3200-300=2900 Hz，we get:1.10 Channel CapacitysbitsNSBC/24097)2.3161(log29001log22Problem:Can a 28.8kbit/s modem work on this telephone line?And a 14.4kbits/s modem?35Problems: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 Homework
