無(wú)線通信網(wǎng)絡(luò)第5章天線與傳播課件

1、Antennas and PropagationChapter 5IntroductionAn antenna is an electrical conductor or system of conductorsTransmission - radiates electromagnetic energy into spaceReception - collects electromagnetic energy from spaceIn two-way communication, the same antenna can be used for transmission and recepti

2、onRadiation PatternsRadiation patternGraphical representation of radiation properties of an antennaDepicted as two-dimensional cross sectionBeam width (or half-power beam width) Measure of directivity of antennaReception patternReceiving antennas equivalent to radiation patternTypes of AntennasIsotr

3、opic antenna (idealized)Radiates power equally in all directionsDipole antennasHalf-wave dipole antenna (or Hertz antenna)Quarter-wave vertical antenna (or Marconi antenna)Parabolic Reflective Antenna Antenna GainAntenna gainPower output, in a particular direction, compared to that produced in any d

4、irection by a perfect omnidirectional antenna (isotropic antenna)Effective areaRelated to physical size and shape of antennaAntenna GainRelationship between antenna gain and effective areaG = antenna gainAe = effective areaf = carrier frequencyc = speed of light ( 3 108 m/s) = carrier wavelengthProp

5、agation ModesGround-wave propagationSky-wave propagationLine-of-sight propagationGround Wave PropagationGround Wave PropagationFollows contour of the earthCan Propagate considerable distancesFrequencies up to 2 MHzExampleAM radioSky Wave PropagationSky Wave PropagationSignal reflected from ionized l

6、ayer of atmosphere back down to earthSignal can travel a number of hops, back and forth between ionosphere and earths surfaceReflection effect caused by refractionExamplesAmateur radioCB radioLine-of-Sight PropagationLine-of-Sight PropagationTransmitting and receiving antennas must be within line of

7、 sightSatellite communication signal above 30 MHz not reflected by ionosphereGround communication antennas within effective line of site due to refractionRefraction bending of microwaves by the atmosphereVelocity of electromagnetic wave is a function of the density of the mediumWhen wave changes med

8、ium, speed changesWave bends at the boundary between mediumsLine-of-Sight EquationsOptical line of sightEffective, or radio, line of sightd = distance between antenna and horizon (km)h = antenna height (m)K = adjustment factor to account for refraction, rule of thumb K = 4/3Line-of-Sight EquationsMa

9、ximum distance between two antennas for LOS propagation:h1 = height of antenna oneh2 = height of antenna twoLOS Wireless Transmission ImpairmentsAttenuation and attenuation distortionFree space lossNoiseAtmospheric absorptionMultipathRefractionThermal noiseAttenuationStrength of signal falls off wit

10、h distance over transmission mediumAttenuation factors for unguided media:Received signal must have sufficient strength so that circuitry in the receiver can interpret the signalSignal must maintain a level sufficiently higher than noise to be received without errorAttenuation is greater at higher f

11、requencies, causing distortionFree Space LossFree space loss, ideal isotropic antenna Pt = signal power at transmitting antennaPr = signal power at receiving antenna = carrier wavelengthd = propagation distance between antennasc = speed of light ( 3 10 8 m/s)where d and are in the same units (e.g.,

12、meters)Free Space LossFree space loss equation can be recast:Free Space LossFree space loss accounting for gain of other antennasGt = gain of transmitting antennaGr = gain of receiving antennaAt = effective area of transmitting antennaAr = effective area of receiving antennaFree Space LossFree space

13、 loss accounting for gain of other antennas can be recast asCategories of NoiseThermal NoiseIntermodulation noiseCrosstalkImpulse Noise Thermal NoiseThermal noise due to agitation of electronsPresent in all electronic devices and transmission mediaCannot be eliminatedFunction of temperatureParticula

14、rly significant for satellite communicationThermal NoiseAmount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:N0 = noise power density in watts per 1 Hz of bandwidthk = Boltzmanns constant = 1.3803 10-23 J/KT = temperature, in kelvins (absolute temperature)Thermal N

15、oiseNoise is assumed to be independent of frequencyThermal noise present in a bandwidth of B Hertz (in watts):or, in decibel-wattsNoise TerminologyIntermodulation noise occurs if signals with different frequencies share the same mediumInterference caused by a signal produced at a frequency that is t

16、he sum or difference of original frequenciesCrosstalk unwanted coupling between signal pathsImpulse noise irregular pulses or noise spikesShort duration and of relatively high amplitudeCaused by external electromagnetic disturbances, or faults and flaws in the communications systemExpression Eb/N0Ra

17、tio of signal energy per bit to noise power density per HertzThe bit error rate for digital data is a function of Eb/N0Given a value for Eb/N0 to achieve a desired error rate, parameters of this formula can be selectedAs bit rate R increases, transmitted signal power must increase to maintain requir

18、ed Eb/N0Other ImpairmentsAtmospheric absorption water vapor and oxygen contribute to attenuationMultipath obstacles reflect signals so that multiple copies with varying delays are receivedRefraction bending of radio waves as they propagate through the atmosphereMultipath PropagationMultipath Propaga

19、tionReflection - occurs when signal encounters a surface that is large relative to the wavelength of the signalDiffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio waveScattering occurs when incoming signal hits an object whose size in the order of t

20、he wavelength of the signal or lessThe Effects of Multipath PropagationMultiple copies of a signal may arrive at different phasesIf phases add destructively, the signal level relative to noise declines, making detection more difficultIntersymbol interference (ISI)One or more delayed copies of a puls

21、e may arrive at the same time as the primary pulse for a subsequent bitTypes of FadingFast fadingSlow fadingFlat fadingSelective fadingRayleigh fadingRician fadingError Compensation MechanismsForward error correctionAdaptive equalizationDiversity techniquesForward Error CorrectionTransmitter adds error-correcting code to data blockCode is a function of the data bitsReceiver calculates error-correcting code from incoming data bitsIf calculated code matches incoming code, no error occurredIf error-correcting codes dont match, receiver attempts to determine bits in error and correct