Chapter 4. Mobile Radio Propagation Large-Scale Path Loss

Transcription

1 Chapter 4 Mobile Radio Propagation Large-Scale Path Loss

2 The mobile radio channel places fundamental limitations on the performance. The transmission path between the T-R maybe very complexity. Radio channels are extremely random and do not offer easy analysis. Modeling the radio ch. has been one of the most difficult parts of mobile system design.

3 4.1 introduction to radio wave propagation electromagnetic wave propagation can generally be attributed to reflection, diffraction, and scattering. Propagation models predicting the average received signal strength at a given distance from the transmitter (large-scale propagation models) the instantaneous received signal strength may fluctuate rapidly (small-scale fading).

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6 4.2 Free Space Propagation Model to predict received signal strength when the T and R have a clear, unobstructed LOS path. Satellite systems and microwave LOS radio links typically undergo free space propagation. PG G λ Pr ( d ) ( W), d L 2 t t r 0 = 2 2 ( 4π ) 0 d 是接收功率的参考点, d = 1 m( 室内 ),100m或 1km( 室外 ) d0 2D r( ) = r( 0) 0 f =, f P d P d, d d d d 是远场距离 d λ G, G为发射和接收天线增益, L是与传播无关系统损耗因子, λ是波长, t r d( d > d) 是发射和接收天线之间的距离, D为天线的最大物理尺寸 0

7 An isotropic is an ideal antenna, and is often used to reference antenna gains in wireless systems. The effective isotropic radiated power (EIRP) is defined as: EIRP=PtGt Effective radiated power (ERP) is used instead of EIRP to denote the maximum radiated power as compared to a half-wave dipole antenna. A dipole antenna has a gain of 1.64 (2.15dB) above an isotrope. In practice, antenna gains given in unit of dbi (isotropic antenna) or dbd (dipole antennas).

8 The path loss represents signal attenuation as a positive quantity measured in db. 2 P t GG t rλ PL( db) = 10log = 10log ( ) 2 2 Pr 4π d L

9 4.3 Relating Power to Electric Field It can be proven that any radiator produces electric and magnetic fields. i 0 P z θ d θ r L y x ϕ

10 E E 0 = πε0c d jω cd 2 3 ilcosθ 1 ilsinθ jω c c il 0 cosθ jωc c Hϕ = + 4π c d d Eϕ = H = H = r r d表示辐射场成分, d d c 2 e e jω jω 2 0 c θ = πε0c d d jω cd θ 表示感应场成分, 表示静电场成分 c c ( t dc) e ( t dc) jω c ( t dc)

11 The relationship between the received power and the electric field is as follows. 2 the power flux density d / is given by P d R E fs ( P W m) 2 2 EIRP PG t t E E E = = = = = 2 2 4πd 4πd R η η fs ( 2 W / m ) 是固有阻抗, 自由空间中为 η=120πω=377 Ω, 表示远场电场辐射部分大小 Pd may be thought of as the EIRP divided by the surface area of a sphere with radius d. 2

12 The power received at distance d, is given as 2 E PG G Pr( d) = PA = λ d e Ae W 120π = 4 d L ( ) 2 t t r 2 2 π 0 ( ) It is useful to relate the received power level to a receiver input voltage 2 PtGt 1m 1m Vant open circuit P( d) r = Rant V V ant 4R ant To match receiver

13 4.4 The Three Basic Propagation Mechanisms Reflection occurs when a propagation electromagnetic wave impinges upon a very large dimensions object when compared to the wavelength of propagating wave. Reflections occur from the surface of the earth and from buildings and walls. Reflection from dielectrics η sinθ η sinθ η sinθ η sinθ Γ =, Γ = 2 t 1 i 2 i 1 t η2sinθt + η1sinθ η2sinθi + η1sinθt Brewster angle (reflection coefficient equals 0) sin ( θ ) ( ) B = ε1 ε1+ ε2 Reflection from perfect conductors Γ = 1, Γ = 1

14 Ground Reflection (Two-Ray) Model: It is found to be reasonably accurate for predicting the large-scale signal strength over distances of several Km for mobile radio systems that use tall towers (heights which exceed 50 m), as well as for LOS microcell channels in urban environments

15 T( 发射机 ) E LOS ETOT = ELOS + Eg R( 接收机 ) h t E i E r = E g h r θ i θ 0 d

16 ' ( ' ) Ed 0 0 d ELOS d, t = cos ω ' c t d d c ', 为直达路径 '' ( '' ) Ed 0 0 d Eg d, t =Γ cos '' ωc t d d c Γ为地面发射系数, 地面全发射时 Γ=-1 '', 为反射波路径 ' '' Ed 0 0 d Ed 0 0 d TOT (, ) = cos ω 1 cos ' c +(-) ω '' c E dt t t d c d c 当收发天线之间距离 d远远大于 h + h 时, d处接收功率为 : hh 2 2 t r Pr = PG t tgr, d 4 双线模型的路径损耗为 : 比自由空间损耗大得多 t ( ) PL( db) = 40 logd 10 logg + 10 logg + 20 logh + 20 logh f t r t r

17 Diffraction occurs when the radio path between T-R is obstructed by a surface that has sharp irregularities (edges). It allows radio signals to propagate around the curved surface of the earth, beyond the horizon, and to propagate behind the obstacles. Received field strength decreases rapidly as receiver moves deeper into the obstructed region, the diffraction still exists and of the has sufficient strength to produce a useful signal.

18 Fresnel Zone Grometry T β α h t -h r h obs -h r γ R d 1 d 2

19 直射和绕射路径差和相应的相位差分别为 : ( d + d ) 2π 2π h ( d + d ) h, ϕ = = 2 dd λ λ dd Fresnel Kirchoff 绕射参数 ν为 : ν = h Knife-edge Diffraction model 0 ( ν) ( 1+ j) ( ν ) d (( 2 π ) ) 2 ( d + d ) 1 2 λdd the electric field strength,e,of a knife-edge diffracted wave is E E d = F = exp j t 2 dt 2 ν the diffraction gain is G ( db) = 20log F d 1 2

20 5 0 刃形绕射增益 (G d db) 费涅尔绕射参数 υ

21 The actual received signal is often stronger than what is predicted by reflection and diffraction models alone. Scattering occurs when the wave travels through objects with dimensions that are small compared to the λ, and where the number of obstacles per unit volume is large. Scattered waves are produced by rough surfaces, small objects, or by other irregularities. In practice, foliage, street signs, and lamp posts induce Scattering.

22 A surface is considered smooth if its minmax protuberance h is less than h c, and is considered rough if the protuberance is greater than h c. λ hc =, θi为入射角, 8sinθ ρ σ s h i ρ 散射损耗系数为 : 2 πσ hsinθi = exp 8 λ 为表面高度的标准偏差 s

23 4.5 practical link budget design using path loss Models Log-distance path loss model The average large-scale path loss for an arbitrary T-R separation is expressed as a function of distance by using a path loss exponent,n, PL( d) d d 0 PL( db) PL( d ) 10n log n d = 0 + d0

24 表 4.2 不同环境下路径损耗指数 环境 路径损耗指数,n 自由空间 2 市区蜂窝 2.6~3.5 市区蜂窝阴影 3~5 建筑物内视距传播 1.6~1.8 被建筑物阻挡 4~6 被工厂阻挡 2~3

25 Log-normal Shadowing The surrounding environmental clutter may be vastly different at two different locations having the same T-R. At any value of d,the PL(d) at a particular location is random and distributed lognormally about the mean value. d PL( d) [ db] = PL( d) + Xσ = PL( d0) + 10n log + Xσ, d0 X 是均值为 0, 标准偏差为 σ的高斯随机变量, 单位 db σ

26 The log-normal distribution describes the random shadowing effects which occur over a large number of measurement locations which have the same T-R separation, but have different levels of clutter on the propagation path.

27 4.6 Outdoor Propagation Model Radio transmission in a mobile communications system often takes place over irregular terrain. Some of the commonly used outdoor propagation models are now discussed. Longley-Rice Model Durkin Model Okumura Model Hata Model PCS Extension to Hata Model Walfish and Bertoni Model Wideband PCS Microcell Model

28 4.7 Indoor Propagation Models characterizing radio propagation inside buildings. The indoor radio channel differs from the traditional mobile radio channel in two aspects--the distances covered are much smaller, and the variability of the environment is much greater for a much smaller range of T-R separation distances. Indoor channels may be classified either as LOS or obstructed (OBS),with varying degrees of clutter.

29 Partition losses (same floor) Partition losses between Floors Log-distance Path Loss Model Ericsson Multiple Breakpoint Model Attenuation Factor Mode

30 4.7 Signal Penetration into Buildings RF penetration is a function of frequency as well as height within the building. The loss decreases with increasing frequency. The loss decreased at a rate of 2dB per floor from the ground level up to the ninth floor and then increased above the ninth floor. The antenna pattern in the elevation plane also plays an important role in how much signal penetrates a building from the outside.

31 The percentage of widows, when compared with the building face surface area, impacts the level of RF penetration loss, as does the presence of tinted metal in the windows. The angle of the transmitted wave upon the face of the building also has a strong impact on the penetration loss.