无线通信原理与.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,*,Chapter,3,:,Mobile Radio Propagation:Large-Scale Path Loss,10/8/2024,1,3.1 Introduction to Radio wave Propagation,Small-scale and large-scale fading,10/8/2024,2,3.2 Free Space Propagation Model,In free space,the received power is predicted by Firiis Equ.,P,r,(d):Received power with a distance d between Tx and Rx,P,t,:Transmitted power,G,t,:Transmitting antenna gain,G,r,:Receive antenna gain,:The wavelength in meters.,d:distance in meters,L:The miscellaneous losses L(L=1)are usually due to transmission line attenuation,filter losses,and antenna losses in the communication system.L=1 indicates no loss in the system hardware.,10/8/2024,3,Reflection,:occur from the surface of the earth and from buildings and walls.,Diffraction,:occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities(edges).,Scattering,:occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength,and where the number of obstacles per unit volume is large.,3.3 The three Basic Propagation Mechanisms,10/8/2024,4,reflection（反射）at large obstacles,Scattering(散射)at small obstacles,diffraction(衍射)at edges,10/8/2024,5,EIRP&ERP,EIRP:Effective Isotropic Radiated Power,Represents the maximum radiated power available from a transmitter in the direction of maximum antenna gain,as compared to an isotropic radiator.,ERP:Effective Radiated Power,ERP is used instead of EIRP to denote the maximum radiated power as compared to a half-wave dipole antenna(instead of an isotropic antenna).,In practice,antenna gains are given in units of dBi(dB gain with respect to an isotropic sourse)or dBd(dB gain with respect to a half-wave dipole),2.15dB,10/8/2024,6,9dBi antenna&3dBi antenna,10/8/2024,7,Path Loss,The path loss,which represents signal attenuation as a positive difference(in dB)between the effective transmitted power and the received power.,The path loss for the free space model when antenna gains are included is given by quantity measured in dB,is defined as the,When antenna gains are excluded,the antennas are assumed to have unity gain,and path loss is given by,(f,:MHz,d:km),10/8/2024,8,The,Friis,free space model is only a valid predictor for Pr for values of d,which are in the far-field of the transmitting antenna.,The far-field of a transmitting antenna is defined as the region beyond the far-field distance,d,f,which is related to the largest linear dimension of the transmitter antenna aperture and the carrier wavelength.The far-field distance is given by,To be in the far-field region,d must satisfy,The far-field region of a transmitting antenna,10/8/2024,9,The Reference Distance,It is clear that equation does not hold for d=0.For this reason,large-scale propagation models use a known received power reference point.The received power,Pr(d),at any distance dd,0,may be related to Pr at d,0,.,If P,r,is in units of,dBm,or,dBW,the received power is given by,10/8/2024,10,3.4 Link budge design using path loss model,Log-distance path loss model,Both theoretical and measurement-based propagation models indicate that average received signal power decreases logarithmically with distance,whether in outdoor or indoor channels.The average large-scale path loss for an arbitrary T-R separation is expressed as a function of distance by using path loss exponent n.,n is the path loss exponent which indicates the rate at which the path loss increases with distance,d,0,is the close-in reference distance which is determined,d is the T-R separation distance,10/8/2024,11,Path-loss exponents,10/8/2024,12,If a transmitter produces power:Pt=50w,receive sensitivity(minimum usable signal level)is-100dbm.Assume d,0,=100m,with a 900MHz carrier frequency,n=4,G,t,=G,r,=1;find the coverage distance d.,Transmit Power:Pt=50W=47dBm,Pr(d,0,)=-24.5dBm,PL(dB)=40log(d/d,0,)=-24.5-(-100)=75.5db,If n=4,log(d/d,0,)=75.5/40=1.8875,d=7718m,Example 1,10/8/2024,13,The model in Equation(3.11)does not consider the fact that the surrounding environmental clutter may be vastly different at two different locations having the same T-R separation.This leads to measured signals which are vastly different than the,average,value predicted by Equation(3.11).,Log-normal Shadowing,10/8/2024,14,Log-normal Shadowing,10/8/2024,15,Determination of Percentage of Coverage Area,10/8/2024,16,U(r)as a function of probability of signal above threshold on the cell boundary.,10/8/2024,17,Example 2,A local average signal strength field measurements,the measured data fit a distant-dependent mean power law model having a log-normal distribution about the mean.Assume the mean power law was found to be .If a signal of 1mW was received at d0=1m from the transmitter,and at a distance of 10m,10%of the measurements were stronger than-25dBm,define the standard deviation,for the path loss model at d=10m.,10/8/2024,18,Four received power measurements were taken at d。