无线通信基础(双语)-教学课件-(6).ppt

1、Review of Last Class,2、Time dispersion,3、Impairments of the Wireless Channel,1、Four Basic Propagation Mechanism,Fading,Large-scale path loss and shadowing,Small-scale fading,Review of Last Class,Radio Propagation Model“.characterization of radio wave propagation as a function of frequency,distance a

2、nd other conditions.”,Review of Last Class,General ray-tracing method(simulation)Multiple ray-tracing with propagation effects(reflection,diffraction,LOS path,scattering,etc)Might consider building material(steel,concrete,brick,etc)Empirical methodOn-site measurementCurve-fitting Statistical method,

3、Review of Last Class,Chapter 2-Lecture 2,Large-scale path loss and shadowing(1),2.4 Large-scale path loss and shadowing,The path loss is dependent on the distance between the transmitter and receiver It also depends on the propagation environmentTerrain characteristicsWavelengthTransmitting and rece

4、iving antenna heights A path model is important for determination of the base station(cell)coverage area,Large-Scale Fading,2.4 Large-scale path loss and shadowing,2.4.2 Propagation Over Smooth Plane,2.4.4 Log-Distance Path Loss with Shadowing,2.4.1 Free Space Propagation,2.4.5 Indoor/Outdoor Path L

5、oss Model,2.4.3 Diffraction and Fresnel Zones,2.4.6 Radio Cell Coverage,The Friis Equation Free-Space Path Loss,1、Free Space propagation,2.4 Large-scale path loss and shadowing,No interference,no obstructions There is no loss of energy in free space,but there is attenuation due to the spreading of t

6、he waves,1、Free Space propagation,The Friis Equation,Where:Pt=total power radiated by an isotropic source Gt=transmitting antenna gain Gr=receiving antenna gain d=distance between transmitting and receiving antennas=wavelength of the carrier signal=c/fc fc=carrier frequency,1、Free Space propagation,

7、The Friis Equation,What troubles some people when they see this question is that the path loss also increases as the square of the frequency.While it is true that absorption of RF by various materials(buildings,trees,water vapor,etc.)tends to increase with frequency,remember we are talking about“fre

8、e space”here.,1、Free Space propagation,The Friis Equation,Effective area of an isotropic antenna,However,from electromagnetic theory,we note that the effective area of an isotropic antenna in any direction is given by:A=2/(4),2.4 Large-scale path loss and shadowing,1.Free space power flux density(W/

9、m2)power radiated over surface area of sphereS Pt/(4 d2)By covering some of this area,receivers antenna“catches”some of this flux.,1、Free Space propagation,The Friis Equation,2.Effective area of an isotropic antenna The power received by an antenna of effective area A is given by:PR=SAHowever,from e

10、lectromagnetic theory,we note that the effective area of an isotropic antenna in any direction is given by:A=2/(4)ThusPR=SA=Pt 2/(4 d)2,1、Free Space propagation,The Friis Equation,3.Directional Radiation While the isotropic antenna is a useful illustrative device,most antennas are not isotropic.Inst

11、ead,they have gain that is a function of the azimuth angle and the elevation angle.When nonisotropic antennas are used,we get,1、Free Space propagation,The Friis Equation,d D and d,where d is the distance between the transmitting antenna and receiving antenna D is the largest linear dimension of ante

12、nna is the carrier wavelength ddf,Assumes far-field,1、Free Space propagation,1、Free Space propagation,The Friis Equation,The term(4d/)2 is known as the free-space path loss denoted by Lp(d),which is,1、Free Space propagation,Free-Space Path Loss,What does“dB”mean?,dB stands for deciBel or 1/10 of a B

13、el The Bel is a dimensionless unit for expressing ratios and gains on a log scaleGains add rather than multiplyEasier to handle large dynamic ranges,1、Free Space propagation,1、Free Space propagation,1、Free Space propagation,What does“dB”mean?,Ex:Attenuation from transmitter to receiverPT=100,PR=10at

14、tenuation is ratio of PT to PRPT/PRdB=10 log(PT/PR)=10 log(10)=10 dBUseful numbers:1/2dB-3 dB1/1000dB=-30 dB,1、Free Space propagation,What does“dB”mean?,dB can express ratios,but what about absolute quantities?Similar units reference an absolute quantity against a defined reference.n mWdBm=n/mWdB n

15、WdBW=n/WdB Ex:1 mWdBW=？dBW,1、Free Space propagation,Example 2.2Transmitter power:50W.Antenna:unit gain.Carrier frequency:900 MHz.Distance:d=100m.Find:1)dBm value of the transmitter,2)dBm value of the received power,3)Pr(d=10 Km)=?,Relative distance,1、Free Space propagation,Example 2.3In door WLAN wi

16、th fc=900 MHz.find the required transmit power in order to guarantee minimum received power of 10 uW with distance up to 10 m.What if fc=5 GHz?Assume unit antenna gains.,1、Free Space propagation,Free space propagation does not apply in a mobile environment and the propagation path loss depends not o

17、nly on the distance and wavelength,but also on the antenna heights of the mobile station and the base station,and the local terrain characteristics such as buildings and hills.A simple two-path model can be used to illustrate the effect of transmitting and receiving antenna heights.,2.4 Large-scale

18、path loss and shadowing,2、Propagation Over Smooth Plane,Free Space Propagation,Friis EquationFree-Space Path LossWhat does“dB”mean?,Review of Last Class,Propagation Over Smooth Plane,Two-path model,Review of Last Class,2、Propagation Over Smooth Plane,Two-Path Model,The propagation distance of the di

19、rect path is：,The propagation distance of the reflected path is：,Two-Path Model,2、Propagation Over Smooth Plane,Given that dhthr,and,It can be derived that,Two-Path Model,2、Propagation Over Smooth Plane,Given that dhthr,we have d1 d and d2d.However,since the carrier wavelength is very small compared

20、 with d,a slight change in the propagation distance can cause a significant change in the received signal carrier phase.,Two-Path Model,2、Propagation Over Smooth Plane,How is two-ray model obtained?,Taken into account the phase difference,the received signal power is,Where f and f are the amplitude

21、attenuation and carrier phase shift introduced by the reflection.If f 1 and f=for 1,then,It had been derived that,Two-Path Model,2、Propagation Over Smooth Plane,How is two-ray model obtained?,And the corresponding path loss is:,Two-Path Model,2、Propagation Over Smooth Plane,If the product d is much

22、greater than the product ht and hr,we can approximate sin by,thus,And the corresponding path loss is:,Two-Path Model,2、Propagation Over Smooth Plane,As a consequence of the assumption that dht,hr,the angel is small,and cancels out of the equation,leaving it to be essentially frequency independentIt

23、shows an inverse fourth-power law,rather than the inverse-square law of free-space propagation.This points to a far more rapid attenuation of the power received.It shows the effect of the transmit and receive antenna heights on propagation losses.The dependence on antenna height makes intuitive sens

24、e.,And the corresponding path loss is:,Two-Path Model,2、Propagation Over Smooth Plane,Critical distance,Define Critical distanceIf ddc,then signal power falls off with d-4.If ddc,then signal power falls off with d-2.,2、Propagation Over Smooth Plane,An approximation for dc can be obtained by setting,

25、Critical distance,2、Propagation Over Smooth Plane,The critical distance can be used for system design.For example,if propagation in a cellular system obeys the two-ray model then the critical distance would be a natural size for the cell radius,since the path loss associated with interference outsid

26、e the cell would be much larger than path loss for desired signals inside the cell.,Critical distance,2、Propagation Over Smooth Plane,f=900MHz ht=50m hr=2m Gt=1 Gr=1 transmit power normalized ht hr,Two-Path Model,2、Propagation Over Smooth Plane,This plot can be separated into three segments:For smal

27、l distance(ddc the signal components only combine destructively.,Two-Path Model,2、Propagation Over Smooth Plane,Example 2.4 If two-ray model is suitable for urban micro-cell with ht=10 m,hr=3 m,and fc=2 GHz.Find dc.What if for indoor microcell with ht=3 m,hr=2 m?Find the power loss for dc.,Compared

28、with the free space model?,-96dB-83dB,How about d=3Km?,2、Propagation Over Smooth Plane,This model is a simple but very good ray-tracing modelAccurate in predicting large-scale signal strength over large distance with tall towersUsed widely in prediction cellular propagation lossEffective when a single ground reflection dominates the multipath propagation effect,Two-Path Model,2、Propagation Over Smooth Plane,Free-Space Path Loss Two-Path Model,2.4 Large-scale path loss and shadowing,Conclusions,Compare the free space path loss model with the two path model.,Problems,