A Review of Key Wireless Physical Layer ConceptsOverviewElectromagnetic SpectrumFrequency, Period, and PhaseWavelengthTime and Frequency DomainsDecibelsCoding TerminologyPhase-Shift Keying (PSK)QAMAntennaReflection, Diffraction, ScatteringReflection, Diffraction and ScatteringMultipath PropagationDoppler ShiftChannel CapacityShannon's TheoremHamming DistanceError Correction ExampleTurbo CodesFrequency Hopping Spread SpectrumSpectrumDirect-Sequence Spread SpectrumDS SpectrumSummaryHomework 3Reading Assignment 3Reading Assignment 3 (Cont)Reading Assignment 3 (Cont)Optional Listening MaterialListening (Cont)3-1©2010 Raj JainCSE574sWashington University in St. LouisA Review of Key Wireless A Review of Key Wireless Physical Layer ConceptsPhysical Layer ConceptsRaj JainProfessor of Computer Science and Engineering Washington University in Saint LouisSaint Louis, MO [email protected]/Video recordings of this lecture are available at:http://www.cse.wustl.edu/~jain/cse574-10/3-2©2010 Raj JainCSE574sWashington University in St. LouisOverviewOverview Basic Concepts:¾ Coding, Phase-Shift Keying (PSK), QAM, Decibels¾ Channel Capacity, Nyquist Theorem, Shannon's Theorem, Hamming Distance, Error Correction¾ Antenna, Reflection, Diffraction and Scattering, Multipath Propagation Recent Development:¾ Spread Spectrum, Code Division Multiple Access¾ OFDM ¾ Turbo Codes3-3©2010 Raj JainCSE574sWashington University in St. LouisElectromagnetic SpectrumElectromagnetic Spectrum Wireless communication uses 100 kHz to 60 GHzWireless3-4©2010 Raj JainCSE574sWashington University in St. LouisFrequency, Period, and PhaseFrequency, Period, and Phase A Sin(2πft + θ), A = Amplitude, f=Frequency, θ = PhasePeriod T = 1/f, Frequency is measured in Cycles/sec or Hertz3-5©2010 Raj JainCSE574sWashington University in St. LouisWavelengthWavelength Distance occupied by one cycle Distance between two points of corresponding phase in two consecutive cycles Wavelength = λ Assuming signal velocity v¾λ= vT¾λf = v¾ c = 3*108 m/s (speed of light in free space) = 300 m/μsDistanceAmplitude3-6©2010 Raj JainCSE574sWashington University in St. LouisTime and Frequency DomainsTime and Frequency DomainsFrequencyAmplitudeFrequencyAmplitudeFrequencyAmplitudef3fAAf3fA/3A/33-7©2010 Raj JainCSE574sWashington University in St. LouisDecibelsDecibels Attenuation = Log10 PinPout Example 1: Pin = 10 mW, Pout=5 mWAttenuation = 10 log 10(10/5) = 10 log 102 = 3 dB Example 2: Pin = 100mW, Pout=1 mWAttenuation = 10 log 10(100/1) = 10 log 10100 = 20 dBBelPinPoutdecibel Attenuation = 10 Log10 VinVoutdecibel Attenuation = 20 Log103-8©2010 Raj JainCSE574sWashington University in St. LouisCoding TerminologyCoding Terminology Signal element: Pulse (of constant amplitude, frequency, phase) Modulation Rate: 1/Duration of the smallest element =Baud rate Data Rate: Bits per second Data Rate = Fn(Bandwidth, signal/noise ratio, encoding)PulseBit+5V0-5V+5V0-5V3-9©2010 Raj JainCSE574sWashington University in St. LouisPhasePhase--Shift Keying (PSK)Shift Keying (PSK) Differential PSK: 0 = Same phase, 1=Opposite phaseA cos(2πft), A cos(2πft+π) Quadrature PSK (QPSK): Two bits11=A cos(2πft+45°), 10=A cos(2πft+135°), 00=A cos(2πft+225°), 01=A cos(2πft+315°)Sum of two signals 90° apart in phase (In-phase I , Quadrature Q), Up to 180° phase difference between successive intervals111000 01013-10©2010 Raj JainCSE574sWashington University in St. LouisQAMQAM Quadrature Amplitude and Phase Modulation QAM-4, QAM-16, QAM-64, QAM-256 Used in DSL and wireless networksBinary QAM-401100001 11QAM-16IQIQIQ3-11©2010 Raj JainCSE574sWashington University in St. LouisAntennaAntenna Transmitter converts electrical energy to electromagnetic waves Receiver converts electromagnetic waves to electrical energy Same antenna is used for transmission and reception Omni-Directional: Power radiated in all directions Directional: Most power in the desired direction Isotropic antenna: Radiates in all directions equally Antenna Gain = Power at particular point/Power with IsotropicExpressed in dBiPr= PtGtGr(λ/4πd)2Omni-Directional Directional Isotropic3-12©2010 Raj JainCSE574sWashington University in St. LouisReflection, Diffraction, ScatteringReflection, Diffraction, ScatteringEflection ⇒ Phase shiftiffractioncattering3-13©2010 Raj JainCSE574sWashington University in St. LouisReflection, DiffractionReflection, Diffractionand Scatteringand Scattering Reflection: Surface large relative to wavelength of signal¾ May have phase shift from original¾ May cancel out original or increase it Diffraction: Edge of impenetrable body that is large relative to λ¾ May receive signal even if no line of sight (LOS) to transmitter Scattering¾ Obstacle size on order of wavelength. Lamp posts etc. If LOS, diffracted and scattered signals not significant¾ Reflected signals may be If no LOS, diffraction and scattering are primary means of reception3-14©2010 Raj JainCSE574sWashington University in St. LouisMultipath PropagationMultipath PropagationInter-symbol Interference Delay Spread = Time between first and last versions of signal Fading: Fluctuation in amplitude, phase or delay spread Multipath may add constructively or destructively ⇒ Fast fading3-15©2010 Raj JainCSE574sWashington University in St. LouisDoppler ShiftDoppler Shift If the transmitter or receiver or both are mobile the frequency of received signal changes Moving towards each other => Frequency increases Moving away from each other => Frequency decreasesFrequency difference = velocity/WavelengthExample: 2.4 GHz => l= 3x108/2.4x109= .125m120km/hr = 120x1000/3600 = 33.3 m/s Freq diff = 33.3/.125 = 267 Hz3-16©2010 Raj JainCSE574sWashington University in St. LouisChannel CapacityChannel Capacity Capacity = Maximum data rate for a channel Nyquist Theorem: Bandwidth = BData rate < 2 B Bi-level Encoding: Data rate = 2 × Bandwidth05V Multilevel: Data rate = 2 × Bandwidth × log 2MExample: M=4, Capacity = 4 × Bandwidth3-17©2010 Raj JainCSE574sWashington University in St. LouisShannon's TheoremShannon's Theorem Bandwidth = B HzSignal-to-noise ratio = S/N Maximum number of bits/sec = B log2(1+S/N) Example: Phone wire bandwidth = 3100 HzS/N = 30 dB 10 Log 10S/N = 30Log 10S/N = 3S/N = 103= 1000Capacity = 3100 log 2(1+1000)= 30,894 bps3-18©2010 Raj
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