无线通信基础(双语)_教学课件_17.ppt

5.2 5.2 Random AccessRandom Access5.1 5.1 Multiple access in a radio cellMultiple access in a radio cell5.35.3 Conflict-free multiple access technologies Conflict-free multiple access technologies Techniques commonly used to enhance the spectrum utilization in a mobile communication system include:l Data compressionl Bandwidth reductionl Channel assignmentl Choice of multiple access methodThe overall spectral efficiency of a mobile communication system can be estimated based on a knowledge of:l Channel spacing in kHzl Cell area in km2l Frequency reuse factorl Multiple access scheme usedThe spectral efficiency of a mobile communication system can be represented as a combination of two independent components:lOne component that depends on the system parameterslThe other component that depends on the multiple access method usedl FDMA Systemsl TDMA Systemsl CDMA SystemsThe overall system spectral efficiency for a mobile communications system may be defined in the following ways:Definition 5.1Definition 5.2Spectral Efficiency of FDMA(FDMA)AssumingBS-the total frequency spectrum bandwidth for transmissions in one directionBg-the guard band of which is used in each of the edgeBc-the bandwidth of a single channelSpectral Efficiency of FDMA(FDMA)Thus,the number of channels that can be simult-aneously supported isLet Nctl be the number of allocated control channels and Ndata be the number of data channels,thenNs=Ndata+NctlSpectral Efficiency of FDMA(FDMA)Since each user in service is assigned a data channel,Ndata is also the maximum number of simultaneous users in each cell cluster,and we getFrom which we have the inequalitySpectral Efficiency of FDMA(FDMA)Consider one cluster as the system,the spectral efficiency of FDMA is defined asIn the AMPS system,the system bandwidth is 12.5MHz,the channel spacing is 30KHZ,and the edge guard spacing is 10KHz.The number of channels allocated for control signaling is 21.FindlThe number of channels available for message transmissionlThe spectral efficiency of FDMASystem Spectral EfficiencyFrequency reuse is employed to increase the capacity of the entire cellular system.Therefore,the total number of channels available for data traffic per cell is given by Therefore,from the definition of the overall system spectral efficiency,we getSystem Spectral EfficiencySystem Spectral EfficiencySuppose a cellular system in which the one-way bandwidth of the system is 12.5MHz,the channel spacing is 30kHz,and the guard band at each boundary of the spectrum is 10kHz,if(1)the cell area is 6 km2,(2)the frequency reuse factor is 7,and(3)21 of the available channels are used to handle control signaling,calculatelThe total number of available channels per clusterlThe number of available data channels per clusterlThe number of available data channels per celllThe system spectral efficiency in units of channels/MHz/km2Spectral Efficiency of Wideband TDMA(W-TDMA)Assumingp -the time duration for the preambleTf -the frame durationLd -the number of information data symbols in each slott -the time duration for the trailerLs -the total number of symbols in each slotSpectral Efficiency of Wideband TDMA(W-TDMA)Then,we haveSpectral Efficiency of Narrowband TDMA(N-TDMA)LetBc -the bandwidth of an individual userBg -the guard spacingNu -the number of subbandsThen,the number of subbands isSpectral Efficiency of Narrowband TDMA(N-TDMA)Spectral Efficiency of Narrowband TDMA(N-TDMA)The spectral efficiency of narrowband TDMA is proportional to that of wideband TDMA,and the proportionality constant isThe spectral efficiency of narrowband TDMA is then given byCell Capacity of TDMA SystemsThe cell capacity is defined as the maximum number of mobile users that can be supported simultaneously in each cell.With TDMA,the maximum number of simultaneous users that can be accommodated during one use of the available frequency spectrum isandNs is the total number of TDMA channels available for the entire cellular system without frequency reuse.With frequency reuse,the cell capacity is Cell Capacity of TDMA SystemsWhere N is the frequency reuse factor.Systems Spectral EfficiencyThe maximum number of radio channels in an N-TDMA system is given by the system bandwidth divided by the bit rate of each userThus,the cell capacity isSystems Spectral EfficiencyThis value is adjusted because of efficiency in bandwidth usage due to the modulation schemeIf we consider the overhead necessary for TDMA,the effective number should be Systems Spectral EfficiencyThe overall spectral efficiency of the system in bits/unit bandwidth/cell can be expressed asDefinition:the cell capacity of a DS-CDMA system is defined as the maximum number of mobile stations that can be supported during one use of the wireless channel in a single cell,under the constraint that quality of service requirements are met.Consider the intercell interferenceConsider the background noiselPerfect automatic power controllUnity source activity factor(i.e.,persisitent transmissions)lThe cell-cite antenna has only one sectorlEvery cell handles the same type of trafficSource activity factorImperfect power controlFrequency reuse efficiencyNumber of cell sector5.45.4 Spectral efficiency Spectral efficiencyl P 6-7.Overview of Wireless Communications3 11.1 Definition of Wireless communication1.2 Historical overview of wireless communication1.3 Types of wireless communications1.4 The development of wireless communicationCharacterization of the Wireless Channel3 22.1 Radio propagation environment2.2 Linear time-variant channel model2.3 Channel correlation functions2.4 Large-scale path loss and shadowing2.5 Small-scale multipath fadingl Four Basic Propagation Mechanisml Factors Contributing to Power Loss l FadingLarge-scale path loss and shadowingSmall-scale fadingpath loss and lognormal shadowingpredicting the coverage and availability of a particular servicelocal environment and the movement of the radio terminal for general transmitter and receiver design2.4 Large-scale path loss and shadowing2.4.1 Free Space Propagation2.4.1 Free Space Propagation2.4 Large-scale path loss and shadowing2.4.2 Propagation Over Smooth Plane2.4.2 Propagation Over Smooth Plane2.4 Large-scale path loss and shadowing2.4.3 Diffraction and 2.4.3 Diffraction and FresnelFresnel Zones Zones2.4 Large-scale path loss and shadowing2.4.4 Log-Distance Path Loss with Shadowing2.4.4 Log-Distance Path Loss with Shadowing2.4 Large-scale path loss and shadowing2.4.5 Indoor/Outdoor Path Loss Model2.4.5 Indoor/Outdoor Path Loss ModelLees Path Loss Model Okumura-Hata Path Loss Model 2.4 Large-scale path loss and shadowing2.4.6 Radio Cell Coverage 2.4.6 Radio Cell Coverage l Without shadowingl With shadowing2.2 Linear time-variant channel modelImpulse responseDoppler spreadTransfer functionDelay-Doppler spread2.3 Channel correlation functionsFourier Trans.Fourier Trans.2.3 Channel correlation functionsFlat slow fadingFlat fast fadingFrequency selective slow fadingFrequency selective fast fadingFrequency selective fast fadingFrequency Selective slow fadingFlat fast fadingFlat slow fadingSymbol interval TsSymbol interval TsTcTsTsSymbol interval TsTsBaseband signal bandwidth BsBdBcBaseband signal bandwidth BsBsBs2.5 Small-scale multipath fading2.5.2 Second-order statistics2.5.2 Second-order statistics 2.5.1 First-order statistics 2.5.1 First-order statistics l NLOS-Rayleigh distributionl LOS-Rician distributionl Level Crossing Ratel Average Fade DurationBandpass transmission Techniques for Mobile Radio3 33.1 Introduction3.2 Digital Modulation3.3 Probability of Transmission Error3.4 Spread SpectrumFor wireless communications,the criteria commonly used to evaluate the suitability of a modulation scheme include:lCompact power spectral densitylGood transmission performance lSmall(or no)envelope fluctuations after band pass filtering IQThe phase change between adjacent QPSK symbols is limited to the set/4-QPSK/4-DQPSKTo combat the effect of the phase distortion on signal detection,/4-DQPSK is/4 shift QPSK combined with differential encoding.The MSK signal is a binary frequency shift keying signal withlContinuous phase lMinimum frequency spacingMSKGMSKl Coherent Reception in an AWGN Channel l Coherent Reception in a Flat Slow Rayleigh Fading Channel3.3 Probability of transmission error3.4.2 Spreading Codes3.4.2 Spreading Codes3.4.1 Spread-Spectrum Principles3.4.1 Spread-Spectrum Principles3.4.3 Direct-Sequence Spread Spectrum3.4.3 Direct-Sequence Spread Spectrum3.4.4 Frequency-Hopped Spread Spectrum 3.4.4 Frequency-Hopped Spread Spectrum 3.4.5 Code Division Multiple Access3.4.5 Code Division Multiple Access Fundamentals of Cellular Communications3 44.1 Introduction4.2 Frequency reuse and mobility management4.3 Cell cluster concept4.4 Cochannel and adjacent channel interference4.5 Call blocking and delay at the cell-site4.6 Other mechanisms for capacity increase4.7 Channel assignment strategieslCellular Communications and Frequency ReuselMobility ManagementlCapacity Expansion by Frequency ReuselCellular Layout for Frequency ReuselGeometry of Hexagonal CellslFrequency Reuse RatioMultiple Access Techniques3 55.1 Multiple access in a radio cell5.3 Conflict-free multiple access technologies5.4 Spectral efficiency5.2 Random Access