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1、1,Principles of Communications,2,Chapter 1 Introduction1.1 Historical Review of CommunicationOrigin of ancient communicationTwo modes of communicationDevelopment of modern communication,3,1.2 Message,information&signalMessage：speech,letters,figures,imagesInformation：effective content of message.Diff
2、erent types of messages may contain the same informationSignal：the carrier of messageWhat transmitted in a communication system is signal.,4,Measurement of information：#“quantity of message”information content#Ex:“Rainfall will be 1 mm tomorrow”information content small“Rainfall will be 1 m tomorrow
3、”information content large“The sun will rise in the east tomorrow morning”information content equals zero#Information content I=I P(x)，P(x)Occurrence probability#Definition：I=loga 1/P(x)=-logaP(x)#Usually,set a=2,the unit of the information content will be called a bit.#For an equal probability bina
4、ry symbol:I=log2 1/P(x)=log2 1/(1/2)=1 bit,5,#For an equal probability M-ary symbol:I=log2 1/P(x)=log2 1/(1/M)=log2 Mbit If M=2k，then I=k bit,6,1.3 Digital Communication1.3.1 Basic conceptTwo categories of signals Analog signal：Its voltage or current can be expressed by a continuous function of time
5、.For example,speech signal.Digital signal：Its voltage or current can only take finite number of discrete values.For example,digital computer data signal.,7,Analog Signal&Digital Signal,Analog signals,Digital signals,8,Two kinds of communication systems Analog communication system Requirement High fi
6、delity Criterion Signal to noise ratio Basic issue parameter estimation Digital communication system Requirement correct decision Criterion Error probability Basic issur statistical decision theory,9,1.3.2 Advantages of Digital CommunicationFinite number of possible values of signalsCorrect decision
7、 may be achieved,Fig.1.3.2 Distortion and restoration of digital signal waveforms,(a)Waveforms of distored digital signal,10,Error correcting techniques can be used.Digital encryption can be used.Different kinds of analog&digital message can be integrated to transmitDigital communication equipment:D
8、esign and manufacture are easierWeight&volume are smallerDigital signal can be compressed by source coding to reduce redundency.Out put S/N increases with bandwidth according to exponential law.,11,1.3.3 Digital Communication System Model,12,13,Specifications of Digital Communication Systems,Relatio
9、nship between efficiency&reliability(rate accuracy)Transmission rate：Symbol rate:RB BaudInformation rate:Rb bit/secondFor M-ary system：Rb=RB log2 MMessage rate:RM Error probability：Symbol error probability Pe=number of received symbols in error/total number of transmitted symbols,14,Bit error probab
10、ility Pb=number of received bits in error/total number of transmitted bitsWord error probability Pw=number of received words in error/total number of transmitted wordsRelationship between symbol error probability and bit error probabilityPb=Pe x M/2(M-1)Pe/2,15,Relationship between word error probab
11、ility and bit error probabilityFor binary system,If a word is consisted of k bits,thenPw1(1 Pe)k Utilization factor of frequency bandUtilizaition factor of energy,16,1.4 Channel Wireless channelOrigin of wireless communicationRequirement of electromagnetic wave emission on wavelengthDivision of freq
12、uency band(wavelength),17,Division of frequency band,Frequency Name Typical application band(kHz)3 30 Very low frequency Long-distance navigation,(VLF)Underwater comm.Sonar30 300 Low frequency Navigation,underwater comm.(LF)radio beaconing300 3000 Medium frequency Broadcasting,maritime comm.(MF)dire
13、ction-finding,distress calling,coast guard,18,Division of frequency band,Frequency Name Typical application band(MHz)3 30 High frequency Long-distance broadcasting,telegraph,(HF)telephone,fax,search and lifesaving,comm.between aircrafts&ships,and between ship&coast,amateur radio 30 300 Very high fre
14、quency TV,FM broadcasting,land traffic,air(VHF)traffic,control,taxi,police,navigation,aircraft communication300 3000 Ultra high frequency TV,cellular phone network,microwave(UHF)link,radio sounding,navigation,satellite communication,GPS,surveillance radar,radio altimeter,19,Division of frequency ban
15、d,Frequency Name Typical application band(GHz)3 30 Super high frequency Satellite comm.,radio altimeter,(SHF)microwave link,aircraft radar,meteorological radar,public land vehicle communication 30 300 Extremely high Radar landing system,satellite frequency(EHF)comm.,vehicle comm.,railway traffic 300
16、 3000 Submillimeter wave Experiment,not designated（0.1 1 mm）,20,Division of frequency band,Frequency Name Typical application band(THz)43 430 Infrared Optical communication(7 0.7 m)430 750 Visible light Optical communication(0.7 0.4 m)750 3000Ultraviolet Optical communication(0.4 0.1 m)Note:kHz=103
17、Hz,MHz=106 Hz,GHz=109 Hz,THz=1012 Hz,mm=10-3 m,m=10-6 m,21,Ground wave,Frequency：below 2MHzDiffraction：Propagation distance：hundreds to thousands of km,Ground surface,22,D layer:60 80 kmE layer:100 120 kmF layer:150 400 kmF1 layer:140 200 kmF2 layer:250 400 km At night:D layer:disappearsF1 layer:dis
18、appears(Or,F1 and F2 are combined as F layer),Ionosphere Structure,23,Sky-waveIonosphere Height：60 400 kmOne hop max.propagation distance：4000 kmPropagation distance by multi-hops:10000 kmFrequency：2 30 MHz,24,Frequency：30 MHzPropagation distance:d 2+r 2=(h+r)2,orh D 2/50(m)where D km,Line-of-sight
19、propagation,Figure 1.4.3 Line-of-sight propagation,25,Radio relay,26,Geostationary satellite,27,Stratosphere communication,HAPS(High Altitude Platform Station),28,Figure 1.4.5 Atmosphere attenuation,Atmosphere attenuation,29,Scatter communication,Ionosphere scatteringFrequency:30 60 MHzTroposphere s
20、catteringFrequency:100 4000 MHzMeteor-tail scatteringFrequency:30 100 MHz,30,Wired channelOpen wiresSymmetrical cablesCoaxial cables,Fig.1.4.8,31,Table 1.4.3 General electrical characteristics of wired channels,32,Optical fiberStructure,33,Transmission loss,34,35,Usually,assume f ei(t)can be express
21、ed as k(t)ei(t)So,eo(t)=k(t)ei(t)+n(t)where k(t)is called multiplicative interference,and is a complicated function which reflects the characteristics of the channel.In the simplest condition:k(t)=const.,expressing attenuation.When k(t)=const.,it is a constant parameter channel.For example,coaxial c
22、able.When k(t)const.,it is called a random parameter channel.For example,vehical cellular network communication channel.,36,Coding channel model:Binary coding channel model where,P(0/0),P(1/1)corrrect transfer probabilities P(0/1),P(1/0)error transfer probabilities P(0/0)=1-P(1/0)P(1/1)=1-P(0/1),37,
23、4-ary coding channel model,Transmitting end,Receiving end,Figure 1.4.12 4-ary coding channel model,38,Influence of channel characteristics on signal transmission,Constant parameter channel time-invariant linear networkLink：a segment of physical line where no exchange existsAmplitude frequency charac
24、teristics:,39,Compensation of frequency distortion,40,Phase frequency characteristics:Ideal characteristic:phase-()=k;group delay-()=d()/d=kInfluence of distortion:waveform distortion,inter-symbol interferenceLinear distortion including frequency distortion&phase distortion can be corrected by linea
25、r compensation network.Nonlinear distortion:nonlinear amplitude characteristic,frequency deviation,phase jittering,41,Random parameter channelCommon characteristics attenuation:varying with time transmission delay:varying with time multi-path propagation:fast fadingCharacteristics of received signal
26、:Let transmitting signal be A cos 0t，after transmission through n paths,the received signal R(t)can be expressed as:where ri(t)amplitude of received signal passing over i-th path i(t)delay of the received signal passing over i-th path i(t)=-0 i(t)X c(t)X s(t),42,where V(t)envelope of the received si
27、gnal R(t)(t)phase of the received signal R(t)i.e.,Because ri(t)and i(t)are slowly varied,ri(t)and i(t)are also slowly varied.Hence,Xc(t),Xs(t)and V(t),(t)are also slowly varied.Hence,R(t)can be considered a narrow band signal(random process).,43,It can be seen from the following equationAfter transm
28、ission,the transmitting signal A cos 0t：*amplitude A becomes slowly varied amplitude V(t);*phase 0 becomes slowly varied phase(t)；*spectrum becomes narrow band spectrum from single frequency.,44,Frequency selective fadingAssume:there are only two paths with identical attenuation and different delays
29、,Transmitting signal is f(t)，received signals are af(t-0)and af(t-0-)；spectrum of transmitting signal is F()。then f(t)F()af(t-0)a F()e-j0 af(t-0-)a F()e-j(0+)af(t-0)+af(t-0-)a F()e-j0(1+e-j)H()=a F()e-j0(1+e-j)/F()=ae-j0(1+e-j)|1+e-j|=|1+cos-jsin|=|(1+cos)2+sin21/2|=2|cos(/2)|3 categories of signal：
30、*deterministic signal*random phase signal*fluctuation signal,45,1.5 Noise in Channel,Classified according to origins：Man-made noise：electric sparks,Natural noise：lightning,atmosphere noies,thermal noise,.Classified according to characteristics:impulse noise narrow band noise fluctuation noiseMain noise involved in the following discussion on communication systems is:white noise thermal noise is a kind of typical white noise1.6 Brief Summary,
