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Johns Hopkins EN 600 647 - lecture3 Wireless Communication Fundamentals

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Wireless Communication FundamentalsPhysical Properties of WirelessWireless = WavesPublic Use BandsFree-space Path-lossMulti-path PropagationMulti-Path EffectOther Path-loss ExponentsDigital ModulationExamples of Digital ModulationMulti-transmitter InterferenceWireless Communication FundamentalsDavid [email protected] Properties of WirelessMakes wireless network different from wired networksShould be taken into account by all layersWireless = WavesElectromagnetic radiationSinusoidal wave with a frequency/wavelengthEmitted by sinusoidal current running through a wire (transmitting antenna)Induces current in receiving antennacf Public Use BandsC (speed of light) = 3x108 m/sName 900 Mhz 2.4 Ghz 5 GhzRange 902 - 928 2.4 - 2.48355.15 - 5.35Bandwidth26 Mhz 83.5 Mhz 200 MhzWavelength.33m / 13.1”.125m / 4.9”.06 m / 2.4”Free-space Path-loss Power of wireless transmission reduces with square of distance (due to surface area increase)Reduction also depends on wavelengthLong wave length (low frequency) has less lossShort wave length (high frequency) has more loss24DPLMulti-path PropagationElectromagnetic waves bounce off of conductive (metal) objectsReflected waves received along with direct waveMulti-Path EffectMulti-path components are delayed depending on path length (delay spread)Phase shift causes frequency dependent constructive / destructive interferenceAmplitudeTimeAmplitudeFrequencyOther Path-loss ExponentsPath-Loss Exponent Depends on environment: Free space 2 Urban area cellular 2.7 to 3.5 Shadowed urban cell 3 to 5 In building LOS 1.6 to 1.8 Obstructed in building 4 to 6 Obstructed in factories 2 to 3Digital ModulationModulation allows the wave to carry information by adjusting its properties in a time varying wayDigital modulation using discrete “steps” so that information can be recovered despite noise/interferenceExamples of Digital ModulationBPSKQPSKODFMMulti-transmitter InterferenceSimilar to multi-pathTwo transmitting stations will constructively/destructively interfere with each other at the receiverReceiver will “hear” the sum of the two signals, which usually means


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Johns Hopkins EN 600 647 - lecture3 Wireless Communication Fundamentals

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