無線通信原理與應(yīng)用 第三章 Mobile Radio Propagation Large-Scale Path Loss

1、Mobile Radio Propagation:Large-Scale Path Loss7/20/20221Small-scale and large-scale fading 7/20/20222The three Basic Propagation MechanismReflection: occur from the surface of the earth and from buildings and walls.Diffraction:occurs when the radio path between the transmitter and receiver is obstru

2、cted 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. 7/20/20223SpectrumVLF = Very Low Freque

3、ncy , LF = Low Frequency , MF = Medium Frequency ,HF = High Frequency , VHF = Very High Frequency, UHF = Ultra High Frequency, SHF = Super High Frequency,EHF = Extra High Frequency,UV = Ultraviolet Light, Frequency and wave length: = c/f ,wave length , speed of light c 3x108m/s, frequency f1 Mm300 H

4、z10 km30 kHz100 m3 MHz1 m300 MHz10 mm30 GHz100 m3 THz1 m300 THzvisible lightVLFLFMFHFVHFUHFSHFEHFinfraredUVoptical transmissioncoax cabletwisted pair7/20/20224Frequencies for mobile communicationVHF-/UHF-ranges for mobile radiosimple, small antenna for carsdeterministic propagation characteristics,

5、reliable connectionsSHF and higher for directed radio links, satellite communicationsmall antenna, focusinglarge bandwidth availableWireless LANs use frequencies in UHF to SHF spectrum7/20/20225Free Space Propagation ModelIn free space, the received power is predicted byPr(d): Received power with a

6、distance d between Tx and RxPt: Transmitted powerGt: Transmitting antenna gainGr: Receive antenna gain : The wavelength in meters.d: distance in metersL: The miscellaneous losses L (L=1) are usually due to transmission line attenuation, filter losses, and antenna losses in the communication system.

7、L=1 indicates no loss in the system hardware.7/20/20226EIRP&ERPEIRP: 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

8、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)2.15dB7/20/202279dBi antenna

9、 & 3dBi antenna7/20/20228Path LossThe 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

10、 as theWhen antenna gains are excluded, the antennas are assumed to have unity gain, and path loss is given by(f:MHz,d:km)7/20/20229The far-field region of a transmitting antennaThe Friis free space model is only a valid predictor for Pr for values of d, which are in the far-field of the transmittin

11、g antenna. The far-field of a transmitting antenna is defined as the region beyond the far-field distance df , which is related to the largest linear dimension of the transmitter antenna aperture and the carrier wavelength. The far-field distance is given byTo be in the far-field region, d must sati

12、sfy7/20/202210The Reference DistanceIt 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 dd0, may be related to Pr at d0.If Pr is in units of dBm or dBW, the received po

13、wer is given by 7/20/202211Log-distance path loss modelBoth 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 separati

14、on 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 distanced0 is the close-in reference distance which is determined d is the T-R separation distance7/20/202212If a transmitter produces po

15、wer:Pt=50w, receive sensitivity (minimum usable signal level)is -100dbm.Assume d0=100m, with a 900MHz carrier frequency, n=4,Gt=Gr=1; find the coverage distance d.Transmit Power: Pt=50W=47dBmPr(d0)=-24.5dBmPL(dB)=40log(d/d0)=-24.5-(-100)=75.5dbmIf n=4,log(d/d0)=75.5/40=1.8875,d=7718mExample 1 7/20/2

16、022137/20/202214Log-normal ShadowingThe 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 p

17、redicted by Equation (3.11). 7/20/202215Simulation ResultsDeep shadowingSlight Shadowing7/20/202216Log-normal Shadowing 7/20/202217Determination of Percentage of Coverage Area7/20/202218 as a function of probability of signal above threshold on the cell boundary.7/20/202219Four received power measur

18、ements were taken at distances of 100 m, 200 m, 1 km, and 3 km from a transmitter. These measured values are given in the following table. It is assumed that the path loss for these measurements follows the model in Equation (3.12.a), where d0 = 100 m: (a) find the minimum mean square error (MMSE) e

19、stimate for the path loss exponent, n; (b) calculate the standard deviation about the mean value; (c) estimate the received power at d = 2 km using the resulting model; (d) predict the likelihood that the received signal level at 2 km will be greater than -60 dBm; and (e) predict the percentage of a

20、rea within a 2 km radius cell that receives signals greater than -60 dBm, given the result in (d).Example 27/20/202220The value of n which minimizes the mean square error can be obtained by equating the derivative of J(n) to zero, and then solving for n.(a)Using Equation (3.11), we find = pi(d0)-10n

21、log(di/ 100 m). Recognizing that P(d0) = 0 dBm, we find the following estimates for p, in dBm:The MMSE estimate may be found using the following method. Let pi be the received power at a distance di, and let be the estimate for pi using the path loss model of Equation (3.10). The sum of squared erro

22、rs between the measured and estimated values is given bySetting this equal to zero, the value of n is obtained as n = 4.4.7/20/202221(b)The sample variance 2 = J(n)/4 at n = 4.4 can be obtained as follows.therefore = 6.17 dB, which is a biased estimate. 7/20/202222(c)The estimate of the received pow

23、er at d = 2 km is(d)The probability that the received signal level will be greater than -60 dBm is(e)67.4% of the users on the boundary receive signals greater than-60 dBm, then 92% of the cell area receives coverage above 60dbm 7/20/202223Outdoor Propagation ModelsOkumura Model(150-1920MHz,1km-100k

24、m)Hata Model(150-1500MHz,1km-20km)Egli Model(40-400MHz,0-64km)7/20/202224 Okumura Model not provide any analytical explanation its slow response to rapid changes in terrain 7/20/202225Okumura median attenuation and correction7/20/202226Find the median path loss using Okumuras model for d = 50 km, ht

25、e = 100 m, hre = 10 m in a suburban environment. If the base station transmitter radiates an EIRP of 1 kW at a carrier frequency of 900 MHz, find the power at the receiver (assume a unity gain receiving antenna). Example 37/20/202227HATA model &COST 231 extension7/20/202228Example 4In the suburban o

26、f a large city, d = 10 km, hte = 200 m, hre = 2 m , carrier frequency of 900 MHz, using HATA s model find the path loss. 7/20/202229Indoor propagation models7/20/202230Feature of Indoor Radio ChannelThe distances covered are much smaller, and the variability of the environment is much greater for a

27、much smaller range of T-R separation distances. It has been observed that propagation within buildings is strongly influenced by specific features such as the layout of the building, the construction materials, and the building type.Indoor radio propagation is dominated by the same mechanisms as out

28、door: reflection, diffraction, and scattering. However, conditions are much more variable. 7/20/202231Path attenuation factorsPartition Losses in the same floorPartition Losses between Floors(floor attenuation factors, FAF)7/20/202232Log-distance Path Loss ModelIn door path loss has been shown by ma

29、ny researchers to obey the distance power law Where the value of n depends on the surroundings and building type, and X represents a normal random variable in dB having a standard deviation of sigma. This is identical in form to the log-normal shadowing model of outdoor path attenuation model.7/20/2

30、02233Attenuation Factor ModelWhere nSF represents the exponent value for the “same floor” measurement. The path loss on a different floor can be predicted by adding an appropriate value of FAF7/20/202234Signal Penetration into buildingsRF penetration has been found to be a function of frequency as w

31、ell as height within the buildingMeasurements showed that penetration loss decreases with increasing frequency. Specifically, penetration attenuation values of 16.4dB, 11.6dB,and 7.6dB were measured on the ground floor of a building at frequencies of 441MHz, 896.5MHz, and 1400Mhz, respectly.Results

32、showed that building penetration loss decreased at a rate of 1.9dB per floor from the ground level up to the fifteenth floor and then began increasing above the fifteen floor.7/20/202235Ray Tracing and Site Specific Modeling In recent years, the computational and visualization capabilities of computers have accelerated rapi