Wireless Communication FundamentalsPhysical Properties of WirelessWireless = WavesWave Propagation ExampleFrequency & Public Use BandsFree-space Path-lossOther Path-loss ExponentsMulti-path PropagationMulti-Path EffectModulationMulti-transmitter InterferenceWireless Communication FundamentalsDavid [email protected] Properties of Wirelessz Makes wireless network different from wired networksz Should be taken into account by all layersWireless = Wavesz Electromagnetic radiationz Emitted by sinusoidal current running through a wire (transmitting antenna)z Creates propagating sinusoidal magnetic and electric fields according to Maxwell’s equations:z Fields induce current in receiving antennaWave Propagation Exampleelectricfieldmagneticfieldpropagation directionFrequency & Public Use Bandsλcf =z Propagating sinusoidal wave with some frequency/wavelengthz C (speed of light) = 3x108 m/sWavelengthBandwidthRangeName.06 m / 2.4”.125m / 4.9”.33m / 13.1”200 Mhz83.5 Mhz26 Mhz5.15 - 5.352.4 - 2.4835902 - 9285 Ghz2.4 Ghz900 MhzFree-space Path-loss z Power of wireless transmission reduces with square of distance (due to surface area increase of sphere)z Reduction also depends on wavelengthz Long wave length (low frequency) has less lossz Short wave length (high frequency) has more loss24=λπDPLOther Path-loss Exponentsz 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 3Multi-path Propagationz Electromagnetic waves bounce off of conductive (metal) objectsz Reflected waves received along with direct waveMulti-Path Effectz Multi-path components are delayed depending on path length (delay spread)z Phase shift causes frequency dependent constructive / destructive interferenceAmplitudeFrequencyAmplitudeTimeModulationz Modulation allows the wave to carry information by adjusting its properties in a time varying wayz Amplitudez Frequencyz Phasez Digital modulation using discrete “steps” so that information can be recovered despite noise/interferencez 8VSB - US HDTVz BFSK - Mote Sensor Networksz QPSK - 2 Mbps 802.11 & CMDA(IS-95)Multi-transmitter Interferencez Similar to multi-pathz Two transmitting stations will constructively/destructively interfere with each other at the receiverz Receiver will “hear” the sum of the two signals, which usually means