无线通信系统第4章课件.ppt

1、12/1/20221Mobile Radio Propagation:Small-Scale Fading and Multipath 12/1/2022212/1/20223Fading(small-scale)is a more rapid fluctuation of signals caused by constructive and destructive interference between two or more versions of the same signal(multipath)Doppler effect,due to moving terminals or su

2、rroundings signals wider than coherence bandwidth of radio channelSmall-Scale fading12/1/20224Time-variant multipath channel12/1/20225 Doppler ShiftExample-Carrier frequency fc=1850 MHz (i.e.=16.2 cm)-Vehicle speed v=60 mph=26.82 m/s-If the vehicle is moving directly towards the transmitter-If the v

3、ehicle is moving perpendicular to the angle of arrival of the transmitted signalHzf165162.082.260f12/1/20226Coherence Bandwidth&Coherence Time12/1/20227Characteristics of Radio Channel(Large path loss)Multipath(reflection,scattering)Time-variant(time-varying,a consequence of the constantly changing

4、physical characteristics of the media,e.g.moving of objects)It is reasonable to characterize the time-variant multipath channels statistically.Fadingpath loss,slow fading,fast fading(multipath and time-variant)Time spread(multipath)Frequency spread(time-variant)12/1/20228Impulse response of wireless

5、 channelsThe mobile channel can be modeled as a linear,time varying filter with impulse response h(t,t),where t is the channel multipath delay for a fixed t.The impulse response h(t,t)completely characterizes the channel and is a function of both t and tThe received signal can be expressed as a conv

6、olution of the channel impulse response h(t,t)with the transmitted signal x(t)ttttdtthxthtxtyor),()(),()()(ttttdtxthtxthty)(),()(),()(12/1/20229(,)complex passband channel(,)complex baseband equivalent channel :time variation due to motion/Doppler shift :multipath delay(time-dispersion)bh th ttttt12

7、/1/202210Random Characteristics of wireless channelsThe impulse response of the time-variant multipath channel is a random processThe signal passed through a time-variant multipath channel is a random processdescribe multipath delay as the excess delay,relative to the first arriving multipath compon

8、ent with excess delay t0=0Discretize excess delay in N equally spaced“bins”,such that all multipath wave components inside bin no.i are represented by one component with delay .iitt 2Maximum channel bandwidth that this model can represent:t1/2t?12/1/202211the time varying discrete-time impulse respo

9、nse model for a multipath radio channel12/1/20221210(,)(,)exp(,)()Nbiiiih ta tjttttt ttSo far,we have channel modelGood:it gives every detail about the channelBad:it is hard to see any essential characteristics of channels,such as what signal can pass,what signal can not passWe need a few major para

10、meters for easyCompare different channels(delay,bandwidth,spectrum,etc)Develop design guide lines for wireless signals12/1/202213Such parameters can be derived from channel modelSpecifically,from“power delay profile”of the channelPower delay profile is the spatial/time average over a local areaIt is

11、 a function:power delay,i.e.,the average received power with some delay.2120|()|()iNjbiiihEa et tt12/1/202214qAre measured by channel sounding techniquesqPlots of relative received power as a function of excess delay qThey are found by averaging intantenous power delay measurements over a local area

12、qLocal area:no greater than 6m outdoorqLocal area:no greater than 2m indoorPower Delay Profiles12/1/202215Continuous Wave(CW)SignalHighbandwidth(Wideband)Signal-Received power for CW signals undergoes rapid fades over small distances-Received power for wideband signals changes very little of small d

13、istances.1.However,the local area average of both signals are nearly identical.Different Baseband Signals12/1/202216Parameters of Mobile Multipath Channels Time Dispersion Parameters Coherent bandwidth Doppler Spread Coherence Time12/1/202217Time Dispersion Parameters Mean excess delaykkkkkkkkkkPPaa

14、)()(22ttttt RMS delay spreadkkkkkkkkkkPPaa)()()(2222222ttttttttwhere12/1/202218Power delay ProfileReceived Signal Level(dBm)-105-100-95-90050100150200250300350400450RMS Delay Spread(t)=46.4 nsMean Excess delay(t)=45 nsMaximum Excess delay 10 dB=110 ns Noise threshold12/1/202219-30 dB-20 dB-10 dB 0 d

15、B0125(s)t st38.4 11.01.001.0)0)(01.0()2)(1.0()1)(1.0()5)(1(_ 22222_207.21 11.01.001.0)0)(01.0()2)(1.0()1)(1.0()5)(1(st st37.1)38.4(07.2121.37 sExample12/1/202220Coherence BandwidthCoherence bandwidth is a statistical measure of the range of frequencies over which the channel can be consider“flat”(i.

16、e.,a channel which passes all spectral components with approximately equal gain and linear phase)Two frequencies separated greater than Bc have different channel responseIf the coherence bandwidth is defined as the bandwidth over which the frequency correlation function is above 0.5,then the coheren

17、ce bandwidth is approximatelyt51cBWhere t RMS delay spreadRMS delay spread(time dispersion parameters)and coherence bandwidth are parameters which describe multipath nature of the radio channel12/1/202221Since Bc is greater than 30 kHz,AMPS will work without an equalizer.However,GSM requires 200 kHz

18、 bandwidth which exceeds Bc,thus an equalizer would be needed for this channel.12/1/202222Doppler Spread and Coherence TimeCoherence time is actually a statistical measure of the time duration over which the channel impulse response is essentially invariant,and quantifies the similarity of the chann

19、el response at different times.A popular definition of coherence time for digital communications isvffTmmc423.0Doppler Spread and Coherence Time are parameters which describe the time varying nature of the channel in a small-scale region.12/1/202223ExampleThe spatial sampling interval required to ma

20、ke small-scale propagation measurements is Tc/2.How many samples will be required over 10m travel distance if fc=1900 MHz and v=50 m/s.How long would it take to make these measurements,assuming they could be made in real time from a moving vehicle?Solution12/1/202224 Fading Effects Due to Multipath

21、Time Delay Spread12/1/20222512/1/202226nDue to Doppler SpreadnRate of change of the channel characteristics is larger than theRate of change of the transmitted signalnThe channel changes during a symbol period.nThe channel changes because of receiver motion.nCoherence time of the channel is smaller

22、than the symbol period of the transmitter signal BS:Bandwidth of the signalBD:Doppler SpreadTS:Symbol PeriodTC:Coherence BandwidthOccurs when:BS TCFast Fading12/1/202227 Due to Doppler Spread Rate of change of the channel characteristics is much smaller than theRate of change of the transmitted sign

23、alOccurs when:BS BDandTS TCBS:Bandwidth of the signalBD:Doppler SpreadTS:Symbol PeriodTC:Coherence BandwidthSlow Fading12/1/202228Fast fading&Slow fading12/1/202229Types of Small-Scale Fading 12/1/20223012/1/202231Small scale fading and multipathImpulse response model of channelDiscrete channel mode

24、lParameters of channelsrms delay spreadCoherence bandwidthDoppler Spread Coherence TimeRelationship to signal bandwidth and symbol rate12/1/202232Equalization Some techniques to mitigate fading12/1/202233The intersymbol interference of received symbols(bits)must be removed before decision making(the

25、 case is illustrated below for a binary signal,where symbol=bit):Decision circuitAdaptiveequalizerSymbols with ISISymbols with ISI removed“Clean”symbolsDecision time instantDecision threshold12/1/202234d(k)y(k)y(k 1)y(k2)Z-1Z-1Z-1y(k N)d(k)w0kw1kw2kwNkAdaptive Algorithm+-e(k)Time Domain Equalization

26、12/1/202235 Diversity 12/1/202236Space diversity12/1/202237xxsc(t)sc(t-iTc)xsc(t-NTc)DemodDemodDemody(t)DiversityCombinerdkCDMA RAKE Receiver12/1/202238Channel coding12/1/202239OFDM12/1/202240Fading DistributionsDescribes how the received signal amplitude changes with time.Remember that the received

27、 signal is combination of multiple signals arriving from different directions,phases and amplitudes.With the received signal we mean the baseband signal,namely the envelope of the received signal(i.e.r(t).Its is a statistical characterization of the multipath fading.Two distributions Rayleigh Fading

28、 Ricean Fading12/1/202241Rayleigh fading12/1/20224212/1/2022430.000010.00010.0010.010.110102030GAUSSIAN CHANNELRAYLEIGH FADINGsignal-to-noise ratio(dB)bit error probability,binary antipodal signals12/1/202244Ricean fadingWhen there is a dominant stationary(nonfading)signal component present,such as

29、a line-of sight propagation path,the small-scale fading envelope distribution is Ricean.12/1/202245Statistical Models for Flat Fading ChannelThe Ricean distribution degenerates to a Rayleigh distribution when the dominant component fades away.12/1/202246 1.For the power delay profiles in Figure,esti

30、mate the 90%correlation and 50%correlation coherence bandwidths.12/1/2022472.If a baseband binary message with a bit rate Rb=100 kbps is modulated by an RF carrier using BPSK,answer the following:(a)Find the range of values required for the rms delay spread of the channel such that the received sign

31、al is a flat-fading signal.(b)If the modulation carrier frequency is 5.8 GHz,what is the coherence time of the channel,assuming a vehicle speed of 50 kilometers per hour?(use 4.40b)(c)For your answer in(b),is the channel fast or slow fading?(d)Given your answer in(b),how many bits are sent while the

32、 channel appears static?(e)A CDMA Rake receiver is able to exploit multipath when the channel is(a)flat;(b)slow;(c)fast;(d)frequency selective12/1/2022483.Try to explain the meaning of the following:(i)the channel is frequency-nonselective;(ii)the channel is slowly fading;(iii)the channel is frequency-selective.