无线通信基础(双语)_教学课件_5.ppt

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1、Propagation Over Smooth Planel Two-path modell Critical distanceDiffraction and Fresnel ZonesLog-Distance Path Loss with Shadowingl Log-Distance Path Loss Model l Log-Normal Shadowingl Log-Term FadingA simple model similar to free-space modelAverage received signal power decreases logarith-mically w

2、ith distance d and path loss exponent kPath loss:attenuation of the signall Major advantage of this modelSimple,general applicabilityl Major disadvantageNot accurate,can not describe random effectl With random(arbitrary)shadowing,path losses are different even among MS with same T-R distancel Due to

3、 various clutters on path,shadowing effect is randoml Path loss is also random,and usually normal(in dB)l We have log-normal distribution of path loss(about a mean value)Let be a zero-mean Gaussian distributed random variable(in dB)with standard deviation .The pdf of is given by For example,8 dB is

4、a typical value for in an outdoor cellular system and 5 dB is a value for an indoor environment.As a result,long-term fading is a combination of log-distance path loss and log-normal shadowing.Let denote the overall path loss with shadowing in dB.Then,l Received signal powerNote:both Pr(d)and Lp(d)a

5、re Gaussian random variables with certain mean and variance 2.pdf0Due to the random nature of the log-term fading,the path loss always defined as which is limited to dB with a pre-defined probability.Example 2.7Left measurement table,assume log-normal model,and d0=100mFind path loss exponent k.Find

6、variance 2.Predict received power at d=2 km.d(m)Received power1000 dBm200-20 dBm1000-35 dBm3000-70 dBmSolution to Example 2.7Take expectation E():Solution to Example 2.7With k=4.5,we have samples of Sample varicance:Average received power:-57 dBm Applications of path loss modelsl Used for link-budge

7、t design l Predict large scale coverage for mobile systems2.4.2 Propagation Over Smooth Plane2.4.2 Propagation Over Smooth Plane2.4.4 Log-Distance Path Loss with Shadowing2.4.4 Log-Distance Path Loss with Shadowing2.4.1 Free Space Propagation2.4.1 Free Space Propagation2.4.5 Indoor/Outdoor Path Loss

8、 Model2.4.5 Indoor/Outdoor Path Loss Model2.4.3 Diffraction and 2.4.3 Diffraction and FresnelFresnel Zones Zones2.4.6 Radio Cell Coverage 2.4.6 Radio Cell Coverage lRadio cell coverage is the service area supported by each base station.lThe coverage depends on Service quality requirements The propag

9、ation environmentthe service quality criterion is specified in terms of the propagation path lossthe log-term fading modell Consider the log-term fading model.Determine the cell coverage for the following two situations Without shadowing With shadowingExample 2.8a)Without shadowing,it is required th

10、at,at the cell border,the path loss cannot be dB larger than that at the reference distance d0The path loss in dB at a distance d(dd0)from the base station is The cell coverage is a circle centered at the base station with radius R specified by Example 2.8Showing the above equation for the radius R,

11、we obtain a)Without shadowing,it is required that,at the cell border,the path loss cannot be dB larger than that at the reference distance d0Example 2.8b)With shadowing,the cell coverage is defined as the service area of the base station over which the path loss is limited to dB with a pre-defined p

12、robability.And assume the reference distance d0 is very small compared with the cell radius.Example 2.8The problem of estimating the cell coverage can be approached in two steps:Step 1:Determine the probability a1 that the path loss at location r(d0)is below the threshold ,where the probability is a

13、veraged over the circumference;Step 2:Calculate the probability a2 of the circular area(defined by R)over which the path loss is below the threshold based on a1,where the probability is averaged over the circular area(cell area).Example 2.8The relative path loss at d with respect to the loss at d0 i

14、s given by With shadowing,the path loss in dB at a distance d(d0)from the base station is b)With shadowing,assume the reference distance d0 is very small compared with the cell radius.Example 2.8At the distance d=r(d0),the probability that the relative path loss is limited to dB is b)With shadowing,

15、assume the reference distance d0 is very small compared with the cell radius.Example 2.8b)With shadowing,assume the reference distance d0 is very small compared with the cell radius.Where for and Next,we want to find the probability a2 of the event over a circular area A centered at the base station

16、 with radius R.Example 2.8Thus,we have b)With shadowing,assume the reference distance d0 is very small compared with the cell radius.Example 2.8where If d0R,we haveb)With shadowing,assume the reference distance d0 is very small compared with the cell radius.l Predict the signal strength by consideri

17、ng also the terrain profile.This is more accurate than the statistical approach of log-distance path loss model,but has also more limited applicationsl Most of such models are obtained from systematic interpretation of measurement dataTheir application is limited to similar environmentsl Okumura Mod

18、ell Okumura-Hata Path Loss Modell COST 231 Modell Lees Path Loss Model lOne of the most common models for signal prediction in large urban macrocellslOkumura used extensive measurements of base station-to-mobile signal attenuation throughout Tokyo to develop a set of curves giving median attenuation

19、 relative to free space of signal propagation in irregular terrainlThe Okumura model is restricted to the following range of parameters:The carrier frequency,150,1500MHzAntenna heights of base station and mobile station,hb30,100mThe distance between the base station and mobile station,1,100KmlThe pa

20、th loss in dB is given bywhereis free space path loseis the median attenuation is the antenna height gain factoris the gain due to the type of environmentlThe value of and obtained from Okumuras empirical plots.lOkumura derived empirical formulas forlThe Okumura model has a 1014 dB empirical standar

21、d deviation between the path loss predicted by the model and the path loss associated with the measurements used to develop the model.lEmpirical formulation to match Okumura modellSuitable for large celllThe path loss is represented as a function of The carrier frequency,150,1500MHzAntenna heights o

22、f base station and mobile station,hb30,200m,hm1,10mThe distance between the base station and mobile station,1,20KmlThe path loss in dB is given bywherela(hm)is the correction factor for mobile antenna height,and is given byFor a small and medium cityFor a large cityfc=900MHzhb=50mhm=3mlThe Hata mode

23、l was extended by the European cooperative for scientific and technical research(EURO-COST)to 2GHzWhere a(hr)is the same correction factor as before and CM is 0 dB for medium sized cities and suburbs and 3dB for metropolitan areas.lThe COST 231 model is restricted to the following range of parameter

24、s:The carrier frequency,1.5,2GHzAntenna heights of base station and mobile station,hb30,200m,hm1,10mThe distance between the base station and mobile station,1,20KmlLees model can be used to predict area-to-area path loss.The model consists of two parts:Path loss prediction for a specified set of con

25、ditionsAdjustment factors for a set of conditions different from the specified onelThe model requires two parameters:The power at a 1.6km(1 mile)of interception P0 in dBmThe path-loss exponent k.lThe specified set of conditions is as follows:Carry frequency fc=900MHzBase station antenna height=30.48

26、 m(100 ft)Base station power at the antenna=10 WBase station antenna gain=6 dB above dipole gainMobile station antenna height=3 m(10 ft)Mobile station antenna gain=0 dB above dipole gainro=1mil =1,6 kmrPoPrlThe received signal power in dBm is represented by:Where d0=1.6km,d(d0)is the distance betwee

27、n the mobile station and the base station in km,and n is a constant between 2 and 3 dependent on the geographical locations and the operating frequency ranges.n=2 is recommended for a suburban or open area with f 450MHz.lThe parameter a0(dB)is an adjustment factor for a different set of conditions:W

28、herelThe parameter a0(dB)is an adjustment factor for a different set of conditions:WherelThe value v in a2 is obtained from empirical data and is given by:lA 2dB signal gain is provided by an actual 4dB gain antenna at the mobile unit in a suburban area,and less than 1dB gain received from the same

29、antenna in an urban area for adjusting a5.terrainP0(dBm)kFree space-452.00Open area-494.35Suburban areas-61.73.84Urban area(Philadelphia)-703.68Urban area(Newark)-644.31Urban area(Tokyo)-843.05fc=900MHzhb=50mAntenna gain=6dBTransmitter power=10Whm=3mTransmitterPropagation in Indoor CaseReceiverIndoo

30、r models are less generalizedl Shorter distances are closer to near-fieldl More clutter,scattering,less LOSl Indoor environment differ widely in the materials used for walls and floors,and it is difficult to find generic models that can be accurately applied to determine empirical path loss in a spe

31、cific indoor setting.l Okumura Modell Okumura-Hata Path Loss Modell COST 231 Modell Lees Path Loss Modell Radio Cell Coveragel P2-17l P2-18.2.4.2 Propagation Over Smooth Plane2.4.2 Propagation Over Smooth Plane2.4.4 Log-Distance Path Loss with Shadowing2.4.4 Log-Distance Path Loss with Shadowing2.4.

32、1 Free Space Propagation2.4.1 Free Space Propagation2.4.5 Indoor/Outdoor Path Loss Model2.4.5 Indoor/Outdoor Path Loss Model2.4.3 Diffraction and 2.4.3 Diffraction and FresnelFresnel Zones Zones2.4.6 Radio Cell Coverage 2.4.6 Radio Cell Coverage 2.4.1 Free Space Propagation2.4.1 Free Space Propagati

33、on2.4.2 Propagation Over Smooth Plane2.4.2 Propagation Over Smooth Plane2.4.3 Diffraction and 2.4.3 Diffraction and FresnelFresnel Zones Zones2.4.4 Log-Distance Path Loss with Shadowing2.4.4 Log-Distance Path Loss with Shadowing2.4.5 Indoor/Outdoor Path Loss Model2.4.5 Indoor/Outdoor Path Loss Model

34、Lees Path Loss Model Okumura-Hata Path Loss Model The channel fading can be classified as large-scale fading or small-scale fading.lLong-scale fading arise from the fact that most of the large reflectors and diffracting objects alone the transmission path are distant from the terminallSmall-scale fa

35、ding is the rapid variation of signal levels when the user terminal moves short distances,which is due to reflections of local objects and the motion of the terminal relative to those objects.Shadowing or lognormal fadingTime delay spreadTime-varyingIn section 2.4,we made the assumption that the cha

36、nnel was linear,according to this assumption,all distortions can be characterized by the attenuation or superposition of different signals.In addition,we allow the possibility that the propagation channel may be time varying.As a consequence of these assumptions,the channel can be presented by a dual time time-varying impulse response,defined as Linear time-variant channel.