无线通信原理与应用-第二章传输技术基础课件.ppt

1、Transmission FundamentalsChapter 2Electromagnetic SignalnFunction of timenCan also be expressed as a function of frequencynSignal consists of components of different frequenciesTime-Domain ConceptsnAnalog signal-signal intensity varies in a smooth fashion over timenNo breaks or discontinuities in th

2、e signalnDigital signal-signal intensity maintains a constant level for some period of time and then changes to another constant levelnPeriodic signal-analog or digital signal pattern that repeats over timens(t+T)=s(t)-t +nwhere T is the period of the signalTime-Domain ConceptsnAperiodic signal-anal

3、og or digital signal pattern that doesnt repeat over timenPeak amplitude(A)-maximum value or strength of the signal over time;typically measured in voltsnFrequency(f)nRate,in cycles per second,or Hertz(Hz)at which the signal repeatsTime-Domain ConceptsnPeriod(T)-amount of time it takes for one repet

4、ition of the signalnT=1/fnPhase()-measure of the relative position in time within a single period of a signalnWavelength()-distance occupied by a single cycle of the signalnOr,the distance between two points of corresponding phase of two consecutive cyclesSine Wave ParametersnGeneral sine wavens(t)=

5、A sin(2ft+)nFigure 2.3 shows the effect of varying each of the three parametersn(a)A=1,f=1 Hz,=0;thus T=1sn(b)Reduced peak amplitude;A=0.5n(c)Increased frequency;f=2,thus T=n(d)Phase shift;=/4 radians(45 degrees)nnote:2 radians=360=1 periodSine Wave ParametersTime vs.DistancenWhen the horizontal axi

6、s is time,as in Figure 2.3,graphs display the value of a signal at a given point in space as a function of timenWith the horizontal axis in space,graphs display the value of a signal at a given point in time as a function of distancenAt a particular instant of time,the intensity of the signal varies

7、 as a function of distance from the source Frequency-Domain ConceptsnFundamental frequency-when all frequency components of a signal are integer multiples of one frequency,its referred to as the fundamental frequencynSpectrum-range of frequencies that a signal containsnAbsolute bandwidth-width of th

8、e spectrum of a signalnEffective bandwidth(or just bandwidth)-narrow band of frequencies that most of the signals energy is contained inFrequency-Domain ConceptsnAny electromagnetic signal can be shown to consist of a collection of periodic analog signals(sine waves)at different amplitudes,frequenci

9、es,and phasesnThe period of the total signal is equal to the period of the fundamental frequency Relationship between Data Rate and BandwidthnThe greater the bandwidth,the higher the information-carrying capacitynConclusionsnAny digital waveform will have infinite bandwidthnBUT the transmission syst

10、em will limit the bandwidth that can be transmittednAND,for any given medium,the greater the bandwidth transmitted,the greater the costnHOWEVER,limiting the bandwidth creates distortions Data Communication TermsnData-entities that convey meaning,or informationnSignals-electric or electromagnetic rep

11、resentations of datanTransmission-communication of data by the propagation and processing of signals Examples of Analog and Digital Data nAnalognVideonAudionDigitalnTextnIntegers Analog SignalsnA continuously varying electromagnetic wave that may be propagated over a variety of media,depending on fr

12、equencynExamples of media:nCopper wire media(twisted pair and coaxial cable)nFiber optic cablenAtmosphere or space propagationnAnalog signals can propagate analog and digital data Digital SignalsnA sequence of voltage pulses that may be transmitted over a copper wire mediumnGenerally cheaper than an

13、alog signalingnLess susceptible to noise interferencenSuffer more from attenuationnDigital signals can propagate analog and digital dataAnalog SignalingDigital SignalingReasons for Choosing Data and Signal CombinationsnDigital data,digital signalnEquipment for encoding is less expensive than digital

14、-to-analog equipmentnAnalog data,digital signalnConversion permits use of modern digital transmission and switching equipmentnDigital data,analog signalnSome transmission media will only propagate analog signalsnExamples include optical fiber and satellitenAnalog data,analog signalnAnalog data easil

15、y converted to analog signalAnalog TransmissionnTransmit analog signals without regard to content nAttenuation limits length of transmission link nCascaded amplifiers boost signals energy for longer distances but cause distortionnAnalog data can tolerate distortionnIntroduces errors in digital dataD

16、igital TransmissionnConcerned with the content of the signalnAttenuation endangers integrity of datanDigital SignalnRepeaters achieve greater distancenRepeaters recover the signal and retransmitnAnalog signal carrying digital datanRetransmission device recovers the digital data from analog signalnGe

17、nerates new,clean analog signalAbout Channel CapacitynImpairments,such as noise,limit data rate that can be achievednFor digital data,to what extent do impairments limit data rate?nChannel Capacity the maximum rate at which data can be transmitted over a given communication path,or channel,under giv

18、en conditions Concepts Related to Channel CapacitynData rate-rate at which data can be communicated(bps)nBandwidth-the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium(Hertz)nNoise-average level of noise over the communications pathnErro

19、r rate-rate at which errors occurnError=transmit 1 and receive 0;transmit 0 and receive 1Nyquist BandwidthnFor binary signals(two voltage levels)nC=2BnWith multilevel signalingnC=2B log2 MnM=number of discrete signal or voltage levelsSignal-to-Noise RationRatio of the power in a signal to the power

20、contained in the noise thats present at a particular point in the transmissionnTypically measured at a receivernSignal-to-noise ratio(SNR,or S/N)nA high SNR means a high-quality signal,low number of required intermediate repeatersnSNR sets upper bound on achievable data rate power noisepower signall

21、og10)(10dBSNRShannon Capacity FormulanEquation:nRepresents theoretical maximum that can be achievednIn practice,only much lower rates achievednFormula assumes white noise(thermal noise)nImpulse noise is not accounted fornAttenuation distortion or delay distortion not accounted forSNR1log2 BCExample

22、of Nyquist and Shannon FormulationsnSpectrum of a channel between 3 MHz and 4 MHz;SNRdB=24 dBnUsing Shannons formula251SNRSNRlog10dB 24SNRMHz 1MHz 3MHz 410dBBMbps88102511log10626CExample of Nyquist and Shannon FormulationsnHow many signaling levels are required?16log4log102108log222662MMMMBCClassifi

23、cations of Transmission MedianTransmission MediumnPhysical path between transmitter and receivernGuided MedianWaves are guided along a solid mediumnE.g.,copper twisted pair,copper coaxial cable,optical fibernUnguided MedianProvides means of transmission but does not guide electromagnetic signalsnUsu

24、ally referred to as wireless transmissionnE.g.,atmosphere,outer spaceUnguided MedianTransmission and reception are achieved by means of an antennanConfigurations for wireless transmissionnDirectional nOmnidirectional General Frequency RangesnMicrowave frequency rangen1 GHz to 40 GHznDirectional beam

25、s possiblenSuitable for point-to-point transmissionnUsed for satellite communicationsnRadio frequency rangen30 MHz to 1 GHz nSuitable for omnidirectional applicationsnInfrared frequency rangenRoughly,3x1011 to 2x1014 HznUseful in local point-to-point multipoint applications within confined areas Ter

26、restrial MicrowavenDescription of common microwave antennanParabolic dish,3 m in diameternFixed rigidly and focuses a narrow beamnAchieves line-of-sight transmission to receiving antennanLocated at substantial heights above ground level nApplicationsnLong haul telecommunications servicenShort point-

27、to-point links between buildingsSatellite MicrowavenDescription of communication satellitenMicrowave relay stationnUsed to link two or more ground-based microwave transmitter/receiversnReceives transmissions on one frequency band(uplink),amplifies or repeats the signal,and transmits it on another fr

28、equency(downlink)nApplicationsnTelevision distributionnLong-distance telephone transmissionnPrivate business networksBroadcast RadionDescription of broadcast radio antennasnOmnidirectionalnAntennas not required to be dish-shapednAntennas need not be rigidly mounted to a precise alignmentnApplication

29、snBroadcast radionVHF and part of the UHF band;30 MHZ to 1GHznCovers FM radio and UHF and VHF televisionMultiplexingnCapacity of transmission medium usually exceeds capacity required for transmission of a single signalnMultiplexing-carrying multiple signals on a single mediumnMore efficient use of t

30、ransmission mediumMultiplexingReasons for Widespread Use of MultiplexingnCost per kbps of transmission facility declines with an increase in the data ratenCost of transmission and receiving equipment declines with increased data ratenMost individual data communicating devices require relatively mode

31、st data rate supportMultiplexing TechniquesnFrequency-division multiplexing(FDM)nTakes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signalnTime-division multiplexing(TDM)nTakes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signalFrequency-division MultiplexingTime-division Multiplexing