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EELE 5490, Fall, 2009 EELE 5490, Fall, 2009 Wireless Communications Wireless Communications Ali S. Afana Department of Electrical Engineering Class 5 Dec. 4 th , 2009

EELE 5490, Fall, 2009 Wireless Communications

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EELE 5490, Fall, 2009 Wireless Communications. Ali S. Afana Department of Electrical Engineering Class 5 Dec. 4 th , 2009. Outline. Review Free space propagation Received power is a function of transmit power times gains of transmitter and receiver antennas - PowerPoint PPT Presentation

Text of EELE 5490, Fall, 2009 Wireless Communications

  • EELE 5490, Fall, 2009

    Wireless CommunicationsAli S. Afana

    Department of Electrical Engineering

    Class 5

    Dec. 4th, 2009

    EE 552/452 Spring 2007

  • OutlineReviewFree space propagationReceived power is a function of transmit power times gains of transmitter and receiver antennasSignal strength is proportional to distance to the power of -2Reflection: Cause the signal to decay faster. Depends on the height of transmitter and receiver antennasDiffractionScattering

    EE 552/452 Spring 2007

  • DiffractionDiffraction occurs when waves hit the edge of an obstacleSecondary waves propagated into the shadowed regionWater wave exampleDiffraction is caused by the propagation of secondary wavelets into a shadowed region. Excess path length results in a phase shiftThe field strength of a diffracted wave in the shadowed region is the vector sum of the electric field components of all the secondary wavelets in the space around the obstacle.Huygens principle: all points on a wavefront can be considered as point sources for the production of secondary wavelets, and that these wavelets combine to produce a new wavefront in the direction of propagation.

    EE 552/452 Spring 2007

  • Diffraction

    EE 552/452 Spring 2007

  • Diffraction geometryDerive of equation 4.54-4.57

    EE 552/452 Spring 2007

  • Diffraction geometry

    EE 552/452 Spring 2007

  • Diffraction geometryFresnel-Kirchoff distraction parameters, 4.56

    EE 552/452 Spring 2007

  • Diffraction Loss

    EE 552/452 Spring 2007

  • EE 552/452 Spring 2007

  • Fresnel ScreensFresnel zones relate phase shifts to the positions of obstacles

    EE 552/452 Spring 2007

  • Fresnel ZonesBounded by elliptical loci of constant delayAlternate zones differ in phase by 180Line of sight (LOS) corresponds to 1st zoneIf LOS is partially blocked, 2nd zone can destructively interfere (diffraction loss)How much power is propagated this way?1st FZ: 5 to 25 dB below free space prop.

    Obstruction of Fresnel Zones 1st 2nd 0-10-20-30-40-50-600o90180odBTip of ShadowObstructionLOS

    EE 552/452 Spring 2007

  • Fresnel diffraction geometry

    EE 552/452 Spring 2007

  • Knife-edge diffraction

    EE 552/452 Spring 2007

  • Knife-edge diffraction lossGain

    EE 552/452 Spring 2007

  • Multiple knife-edge diffraction

    EE 552/452 Spring 2007

  • ScatteringRough surfacesLamp posts and trees, scatter all directionsCritical height for bumps is f(,incident angle), Smooth if its minimum to maximum protuberance h is less than critical height and rough if greater than that.Scattering loss factor modeled with Gaussian distributionNearby metal objects (street signs, etc.)Usually modeled statistically

    EE 552/452 Spring 2007

  • Propagation ModelsLarge scale models predict behavior averaged over distances >> Function of distance & significant environmental features, roughly frequency independentBreaks down as distance decreasesUseful for modeling the range of a radio system and rough capacity planning, Experimental rather than the theoretical for previous three modelsPath loss models, Outdoor models, Indoor modelsSmall scale (fading) models describe signal variability on a scale of Multipath effects (phase cancellation) dominate, path attenuation considered constantFrequency and bandwidth dependent Focus is on modeling Fading: rapid change in signal over a short distance or length of time.

    EE 552/452 Spring 2007

  • Free Space Path LossPath Loss is a measure of attenuation based only on the distance to the transmitterFree space model only valid in far-field; Path loss models typically define a close-in point d0 and reference other points from there:

    Log-distance generalizes path loss to account for other environmental factorsChoose a d0 in the far field.Measure PL(d0) or calculate Free Space Path Loss.Take measurements and derive empirically.

    EE 552/452 Spring 2007

  • Typical large-scale path loss

    EE 552/452 Spring 2007

  • Log-Normal Shadowing ModelShadowing occurs when objects block LOS between transmitter and receiverA simple statistical model can account for unpredictable shadowing PL(d)(dB)=PL(d)+X0,Add a 0-mean Gaussian RV to Log-Distance PLVariance is usually from 3 to 12.

    EE 552/452 Spring 2007

  • Questions?

    EE 552/452 Spring 2007

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