SJTU通信原理概论课件1.ppt

1、Introduction to Communication Principles Course Name:Introduction to Communication Principles Course Code:EI211 Course Instructor:Dianguang Ma Text:Digital and Analog Communication Systems,6th ed.by Leon W.Couch II,ISBN 0-13-081223-4 Contents:Chapters 1 4.About the TextThis book covers the essential

2、s needed for the understanding of wire and wireless communication systems and includes adopted standards.These essentials are How communication systems work:chapters 1 through 5.The effect of noise:chapter 6 and 7.Wire and wireless communication systems:chapter 8.This book is ideal for either a one-

3、semester or a two-semester course.For a one-semester course,the basics of how communication systems work may be taught by using the first five chapters(with selected readings from chapter 8).For a two-semester course,the whole book is used.This book covers practical aspects of communication systems

4、developed from a sound theoretical basis.This book includes over 550 problems.About the AuthorLeon W.Couch,IIProfessor Emeritus and Associate ChairmanElectrical and Computer Engineering DepartmentUniversity of Floridacouchece.ufl.eduCHAPTER 1INTRODUCTIONChapter Objectives How communication systems w

5、ork Frequency allocation and propagation characteristics Computer solutions(MATLAB and MATHCAD)Information measure Coding performanceWhat is a Communication System?Communication systems are designed to transmit information.Communication system designers have four main concerns:Selection of the infor

6、mation-bearing waveform Bandwidth and power of the waveform Effect of system noise on the received information Cost of the system1-2 DIGITAL AND ANALOG SOURCES AND SYSTEMSA digital information source produces a finite set of possible messages.1-2 DIGITAL AND ANALOG SOURCES AND SYSTEMSAn analog infor

7、mation source produces messages that are defined on a continuum.1-2 DIGITAL AND ANALOG SOURCES AND SYSTEMSA digital communication system transfers information from a digital source to the sink.1-2 DIGITAL AND ANALOG SOURCES AND SYSTEMSAn analog communication system transfers information from an anal

8、og source to the sink.Communication Network1-2 DIGITAL AND ANALOG SOURCES AND SYSTEMSA digital waveform is a function of time that can have only a discrete set of values.1-2 DIGITAL AND ANALOG SOURCES AND SYSTEMSAn analog waveform is a function of time that can has a continuous range of values.1-3 D

9、ETERMINISTIC AND RANDOM WAVEFORMSA deterministic waveform can be modeled as a complete specified function of time.1-3 DETERMINISTIC AND RANDOM WAVEFORMSA random waveform(or stochastic waveform)can not be complete specified as a function of time and must be modeled probabilistically.1-6 BLOCK DIAGRAM

10、 Of A COMMUNICATION SYSTEMAll communication systems involves three main subsystems:the transmitter,the channel,and the receiver.The information(message)may be in analog or digital form and it may represent audio,video,or other type of information.They are baseband signals.The signal processing block

11、 at the transmitter conditions the source for more efficient transmission.The signal processor provides source/channel coding.1-6 BLOCK DIAGRAM Of A COMMUNICATION SYSTEMThe transmitter carrier circuit(modulator)converts the processed baseband signal into a frequency band that is appropriate for the

12、transmission medium of the channel.It is needed when the transmission channel is located in a band around fc 0.In this case,s(t)is said to be bandpass.Channels may be classified into two categories:wire and wireless.The channel noise may arise from natural electrical disturbances(e.g.,lighting)or fr

13、om artificial sources(e.g.,switching circuits of a nearby computer).1-6 BLOCK DIAGRAM Of A COMMUNICATION SYSTEMThe channel may contain active amplifying devices(e.g.,transponders).The channel may provide undesirable multipaths between its input and output that have different time delays and attenuat

14、ion characteristics.These characteristics may vary with time,which makes the signal fade.The receiver carrier circuits(demodulator)takes the corrupted signal at the channel output and converts it to a baseband signal.The signal processing block at the receiver“cleans up”this signal and delivers an e

15、stimate of the source information to the information sink.1-7 FREQUENCY ALLOCATIONSWireless communication systems often use the atmosphere for the transmission channel.Here,interference and propagation conditions are strongly dependent on the transmission frequency.To provide some semblance of order

16、 and to minimize interference,government regulations specifies the modulation type,bandwidth,power,and type of information that a user can transmit over designated frequency band.Frequency assignments and technical standards are set internationally by the International Telecommunications Union(ITU).

17、Each member nation of the ITU retains sovereignty over the spectral usage and standards adopted in its territory.However,each nation is expected to abide by the overall frequency plan and standards that are adopted by the ITU.1-7 FREQUENCY ALLOCATIONS1-7 FREQUENCY ALLOCATIONS1-7 FREQUENCY ALLOCATION

18、S1-8 PROPAGATION OF ELECTROMAGNETIC WAVESWireless communication systems often use the atmosphere for the transmission channel.Here,interference and propagation conditions are strongly dependent on the transmission frequency.To provide some semblance of order and to minimize interference,government r

19、egulations specifies the modulation type,bandwidth,power,and type of information that a user can transmit over designated frequency band.Frequency assignments and technical standards are set internationally by the International Telecommunications Union(ITU).Each member nation of the ITU retains sove

20、reignty over the spectral usage and standards adopted in its territory.However,each nation is expected to abide by the overall frequency plan and standards that are adopted by the ITU.1-8 PROPAGATION OF ELECTROMAGNETIC WAVESThe propagation characteristics of electromagnetic waves used in softwire ch

21、annels are highly dependent on the frequency.The electromagnetic spectrum may be divided into three broad bands that have on of three dominant propagation characteristics:ground wave,sky wave,and line of sight(LOS).Ground-wave propagation(below 2MHz):the electromagnetic wave tends to follow the cont

22、our of the Earth,i.e.,diffraction of the wave causes it to propagate along the surface of the Earth.1-8 PROPAGATION OF ELECTROMAGNETIC WAVESThe lowest radio frequency that can be used depends on how long you want to make the antenna.For efficient radiation,the antenna needs to be longer than one ten

23、th of a wavelength.ccfcfc1ExamplemHzsmfchkHzfcc30001010/1031010110481-8 PROPAGATION OF ELECTROMAGNETIC WAVESThe long wave antenna(282 meter high)used to broadcast on 189 kHzThe top of the long wave antennaThe mast of the long wave antennaThe wire to support the long wave antenna1-8 PROPAGATION OF EL

24、ECTROMAGNETIC WAVESSky-wave propagation(in the 2-to-30 range):long-distance coverage is obtained by reflecting the wave at the ionosphere and at the Earths boundaries.1-8 PROPAGATION OF ELECTROMAGNETIC WAVESThe ionization is caused by ultraviolet radiation from the sun,as well as cosmic rays.1-8 PRO

25、PAGATION OF ELECTROMAGNETIC WAVESThe critical frequencies of the ionosphere are the highest frequencies capable of being reflected from the ionized regions.This is related to the maximum electron densities for these regions.1-8 PROPAGATION OF ELECTROMAGNETIC WAVESThe refraction index of the ionosphe

26、re is given by2811fNnwhere N is the free electron density(number of electrons per cubic meter).In an ionized region n is less than 1 because N is greater than 0 and outside the ionized region n is approximately equal to 1 because N is approximately equal to 0.In the ionized region,the wave will be b

27、ent according to Shells law:iirrnnsinsin1-8 PROPAGATION OF ELECTROMAGNETIC WAVESActually,in the ionosphere the waves are refracted(i.e.,bent)gradually in an inverted U shape.1-8 PROPAGATION OF ELECTROMAGNETIC WAVESActually,in the ionosphere the waves are refracted(i.e.,bent)gradually in an inverted

28、U shape.1-8 PROPAGATION OF ELECTROMAGNETIC WAVESLOS propagation(above 30 MHz):the electromagnetic wave propagation in a straight line.There is little refraction by the ionosphere.The signal will propagate through the ionosphere.This mode has the disadvantage that for communication between two terres

29、trial(Earth)stations,the signal path has to be above the horizon.1-8 PROPAGATION OF ELECTROMAGNETIC WAVESThe antenna need to be placed on tall towers so that the receiver antenna can“see”the transmitting antenna.Whats the distance to the radio horizon?kmkmdkmmihdftmh120126208.6320.3926.15364681-8 PR

30、OPAGATION OF ELECTROMAGNETIC WAVESDerivationhhrhdrhhrhdhrhrhrrd52802396022222)(342222222But 1mi.=5280 ft.,so if d and h are measured by mi.and ft.,respectively)(252801)()(52802)()(52802)(feethfeetmilefeethmilesmileshmilesmilesd1-9 INFORMATION MEASUREThe information sent from a digital source when th

31、e jth message is transmitted is given bybitsPIjj)1(log2where Pj is the probability of transmitting the jth message.The average information(entropy)measure of a digital source isbitsPPIPHmjjjmjjj121)1(logwhere m is the number of possible different messages.The source rate is given bysbitsTHR/where T

32、is the time required to send a message.1-10 CHANNEL CAPACITY AND IDEAL COMMUNICATION SYSTEMThe optimum digital system might be defined as the system that minimizes the probability of bit error at the system output subject to constraints on transmitted energy and channel bandwidth.It is possible to i

33、nvent a system with no bit error at the output even we have noise introduced into the channel.The optimum analog system might be defined as the one that achieves the largest signal-to-noise ratio(SNR)at the receiver output subject to constraints on transmitted energy and channel bandwidth.It is impo

34、ssible to design a system with infinite SNR at the output when noise is introduced by the channel.1-10 CHANNEL CAPACITY AND IDEAL COMMUNICATION SYSTEMShannon showed that(for the case of signal plus white Gaussian noise)a channel capacity C(bits/s)could be calculated such that if the rate of informat

35、ion R(bits/s)was less than C,the probability of error would approach zero.The equation for C issbitsNSBC/)1(log2where B is the channel bandwidth in Hz,and S/N is the signal-to-noise power ratio(watts/watts,not dB)at the input to the digital receiver.Systems approach this bound usually incorporate er

36、ror-correction coding.Nyquist showed that if a pulse represents one bit of data,noninterferencing pulses could be sent over a channel no faster than 2B pulses/s.1-11 CODINGIf the data at the output of a digital communication system have errors that are too frequent for the desired use,the errors can

37、 often be reduced by the use of two main techniques:automatic repeat request(ARQ)and forward error correction(FEC).In an ARQ system,when a receiver circuit detects errors in a block of data,it requests that the data block be retransmitted.In an FEC system,the transmitted data are encoded so that the

38、 receiver can correct as well as detect errors.These procedures are also classified as channel coding because they are used to correct errors caused by channel noise.(The purpose of source coding is to extract the essential information from the source and encode it into digital form so that it can b

39、e saved or transmitted using digital techniques.)Coding involves adding extra(redundant)bits to the data stream so that the decoder can reduce or correct errors at the output of the receiver.1-11 CODINGCoding is a mapping of k input binary symbols into n output binary symbols.Codes may be classified

40、 into two broad categories:block codes and convolution codes.Block CodesConvolution Codes(n,k)(n,k,K)MemorylessMemoryR=k/n=1/4 to 7/8R=k/n=to 7/8.k=3 to several hundredk,n=3 to 8,v=(k-1)K=2 to 601-11 CODING1-11 CODINGAll of the codes achieve their coding gains at the expense of bandwidth expansion(Trellis-coded modulation is an exception).Homework 1 1-1,1-4,1-5,1-9,1-15,1-16.