(天線與傳播)

1、Antennas and Propagation(天線與傳播)Chapter 5IntroductionnAn antenna is an electrical conductor（電子導線） or system of conductors（導線系統(tǒng)）n發(fā)送 - radiates electromagnetic energy into spacen接收 - collects electromagnetic energy from spacenIn two-way communication, the same antenna can be used for transmission and r

2、eceptionn假定：兩個方向上使用同一頻率Radiation Patterns（輻射模式）nRadiation patternnGraphical representation of radiation properties of an antennanDepicted as two-dimensional cross sectionnBeam width (or half-power beam width) （光束寬度或稱半功率光束寬度）nMeasure of directivity of antenna （計算天線方向性的計量值）半功率光束寬度表示一個角度，在這個角度內(nèi)，天線輻射功率至

3、少是該天線在最佳方向上輻射功率的一半nReception patternnReceiving antennas equivalent to radiation patternTypes of AntennasnIsotropic antenna (idealized) （各向同性天線）nRadiates power equally in all directionsn輻射模式：以天線為中心的一個球體n通常表示為三維模式的一個二維剖面nDipole antennas （偶極天線）nHalf-wave dipole antenna (or Hertz antenna) （半波偶極天線或稱赫茲天線）

4、nQuarter-wave vertical antenna (or Marconi antenna) （1/4波垂直天線或稱馬可尼天線）nParabolic Reflective Antenna （拋物反射天線）半波偶極波垂直天線Antenna Gain （天線增益）nAntenna gainnPower output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)n天線增益不是為了似的

5、輸出功率大于輸入功率，而主要是為了方向性nEffective areanRelated to physical size and shape of antennan針對不同天線的經(jīng)驗計算公式見表5.2, 如：拋物線，拋物面面積為A，則其有效面積為0.56A.Antenna GainnRelationship between antenna gain and effective areanG = antenna gainnAe = effective areanf = carrier frequencync = speed of light ( 3 108 m/s)n = carrier wav

6、elength22244cAfAGeePropagation ModesnGround-wave propagationnSky-wave propagationnLine-of-sight propagationGround Wave PropagationGround Wave PropagationnFollows contour of the earth (沿地表傳播)nCan Propagate considerable distancesnFrequencies up to 2 MHz1kmnExamplenAM radio (調(diào)幅無線電)Sky Wave PropagationS

7、ky Wave PropagationnSignal reflected from ionized layer of atmosphere back down to earthnSignal can travel a number of hops, back and forth between ionosphere and earths surfacenReflection effect caused by refraction (折射)。盡管實際效果看起來象反射nExamplesnAmateur radionCB radio （民用波段無線電）n國際廣播 ( BBC, VOA)Line-of

8、-Sight PropagationLine-of-Sight PropagationnTransmitting and receiving antennas must be within line of sightnSatellite communication signal above 30 MHz not reflected by ionospherenGround communication antennas within effective line of sight (有效的直線范圍) due to refraction。因為微波會被大氣彎曲或折射，大多數(shù)微波彎曲程度與地球曲率相同

9、，因而比直線廣播傳輸?shù)倪h些nRefraction bending of microwaves by the atmospherenVelocity of electromagnetic wave is a function of the density of the mediumnWhen wave changes medium, speed changesnWave bends at the boundary between mediums光波地平線和無線電地平線Line-of-Sight EquationsnOptical line of sightnEffective, or radio

10、, line of sightnd = distance between antenna and horizon (km)nh = antenna height (m)nK = adjustment factor to account for refraction, rule of thumb (經(jīng)驗法) K = 4/3hd57. 3hd57. 3Line-of-Sight EquationsnMaximum distance between two antennas for LOS propagation:nh1 = height of antenna onenh2 = height of

11、antenna two2157. 3hh天線高度計算n例5.2:假定天線高度100m, 另一天線在地平線上，則兩天線直線傳播的最大距離是n假設接收天線10 m，試問為獲得同樣傳播距離，發(fā)送天線高度幾何？n h1=7.842/1.33=46.2 mkmKhd4113357. 357. 3)3 .13(57. 3411Kh84. 73 .1357. 3411KhLOS Wireless Transmission Impairments (傳輸損傷)nAttenuation and attenuation distortionnFree space lossnNoisenAtmospheric ab

12、sorptionnMultipathnRefractionnThermal noiseAttenuationnStrength of signal falls off with distance over transmission mediumnAttenuation factors for unguided media:nReceived signal must have sufficient strength so that circuitry in the receiver can interpret the signalnSignal must maintain a level suf

13、ficiently higher than noise to be received without errornAttenuation is greater at higher frequencies, causing distortionn跨頻帶的衰減均等化Free Space LossnFree space loss, ideal isotropic antenna nPt = signal power at transmitting antennanPr = signal power at receiving antennan = carrier wavelengthnd = prop

14、agation distance between antennasnc = speed of light (3 10 8 m/s)where d, , and c are in the same units (e.g., meters)222244cfddPPrtFree Space LossnFree space loss equation can be recast:dPPLrtdB4log20log10 dB 98.21log20log20d dB 56.147log20log204log20dfcfdFree Space LossnFree space loss accounting

15、for gain of other antennasnGt = gain of transmitting antennanGr = gain of receiving antennanAt = effective area of transmitting antennanAr = effective area of receiving antenna trtrtrrtAAfcdAAdGGdPP2222224Free Space LossnFree space loss accounting for gain of other antennas can be recast as rtdBAAdL

16、log10log20log20 dB54.169log10log20log20rtAAdf表5.2: 天線增益與等效面積Categories of NoisenThermal NoisenIntermodulation noise (互調(diào)噪聲)nCrosstalk (串擾)nImpulse Noise Thermal NoisenThermal noise due to agitation of electrons （電子的熱攪動）nPresent in all electronic devices and transmission medianCannot be eliminatednFun

17、ction of temperaturenParticularly significant for satellite communication n因為衛(wèi)星地面站收到的衛(wèi)星信號較弱Thermal NoisenAmount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:nN0 = noise power density in watts per 1 Hz of bandwidthnk = Boltzmanns constant = 1.3803 10-23 J/KnT = tem

18、perature, in kelvins (absolute temperature)W/Hz k0TN Thermal NoisenNoise is assumed to be independent of frequencynThermal noise present in a bandwidth of B Hertz (in watts):or, in decibel-wattsTBNkBTNlog10 log 10k log10BTlog10 log 10dBW 6 .228Noise TerminologynIntermodulation noise occurs if signal

19、s with different frequencies share the same mediumnInterference caused by a signal produced at a frequency that is the sum or difference of original frequenciesnCrosstalk unwanted coupling between signal paths （不希望的信號路徑之間的偶合）nImpulse noise irregular pulses or noise spikesnShort duration and of relat

20、ively high amplitudenCaused by external electromagnetic disturbances, or faults and flaws in the communications system （外部電磁干擾或通信系統(tǒng)的故障或缺陷）脈沖噪音是數(shù)字數(shù)據(jù)誤碼的一個主要來源如：0.01秒的脈沖在56kbps的傳輸中，會破壞560bits數(shù)據(jù)Expression Eb/N0nRatio of signal energy per bit to noise power density per Hertz R: 數(shù)據(jù)比特率 ; S 信號功率nThe bit err

21、or rate for digital data is a (decreasing) function of Eb/N0nGiven a value for Eb/N0 to achieve a desired error rate, parameters of this formula can be selectednAs bit rate R increases, transmitted signal power must increase to maintain required Eb/N0TRSNRSNEbk/00Other ImpairmentsnAtmospheric absorp

22、tion water vapor （水蒸氣） and oxygen contribute to attenuationnMultipath obstacles reflect signals so that multiple copies with varying delays are receivednRefraction bending of radio waves as they propagate through the atmosphereMultipath Propagation反射(R), 散射(S), 衍射(D)Multipath PropagationnReflection

23、(反射) - occurs when signal encounters a surface that is large relative to the wavelength of the signalnDiffraction(衍射) - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wavenScattering occurs when incoming signal hits an object whose size in the order of the w

24、avelength of the signal or lessThe Effects of Multipath PropagationnMultiple copies of a signal may arrive at different phasesnIf phases add destructively, the signal level relative to noise declines, making detection more difficultnIntersymbol interference (ISI)nOne or more delayed copies of a puls

25、e may arrive at the same time as the primary pulse for a subsequent bitTypes of FadingnFast fadingnSlow fadingnFlat fading (平面衰落)nSelective fadingnRayleigh fading (瑞利衰落)nRician fading (瑞森衰落)Error Compensation Mechanisms(差錯補償機制)nForward error correctionnAdaptive equalizationnDiversity techniquesForwa

26、rd Error CorrectionnTransmitter adds error-correcting code to data blocknCode is a function of the data bitsnReceiver calculates error-correcting code from incoming data bitsnIf calculated code matches incoming code, no error occurrednIf error-correcting codes dont match, receiver attempts to determine bits in error and correctAdaptive Equalization（自適應均衡）nCan be applied to transmissions that carry analog or digital informationnAnalog voice or videonDigital data, digitized voice or videonUsed to