Channel Modelling ETIM10. Propagation mechanisms

Transcription

1 Channel Modelling ETIM10 Lecture no: 2 Propagation mechanisms Ghassan Dahman \ Fredrik Tufvesson Department of Electrical and Information Technology Lund University, Sweden Fredrik Tufvesson - ETIM10 1

2 Contents Free space loss Propagation mechanisms Reflection Transmission Diffraction Scattering Waveguiding Examples from propagation scenarios Fredrik Tufvesson - ETIM10 2

3 Free-space loss If we assume RX antenna to be isotropic: P RX 4 d 2 P TX d A RX Attenuation between two isotropic antennas in free space is (free-space loss): L free d 4d Fredrik Tufvesson - ETIM10 3

4 Free-space loss Friis law Received power, with antenna gains G TX and G RX : GRXGTX P d P P G G L d 4d RX TX TX RX TX free 2 P d P G L d G RX db TX db TX db free db RX db 4d P G 10log G TX db TX db 10 RX db In free space, the received power decays with distance at a rate = 20 db/decade 2 Valid in the far field only Fredrik Tufvesson - ETIM10 4

5 Free-space loss What is far field? The free-space loss calculations are only valid in the far field of the antennas. Far-field conditions are assumed far beyond the Rayleigh distance (also known as Fraunhofer distance): d R 2 2 a L where L a is the largest dimesion of the antenna. Another rule of thumb is: At least 10 wavelengths / 2-dipole / Parabolic 2 2r L a d R / 2 / 2 L a 2r 2 8r d R Fredrik Tufvesson - ETIM10 5

6 Example Cellular phone, height 10 cm, f c =900 MHz Rayleigh distance d R =2*0.1 2 /0.333=6 cm For this device the limit is the 10-lambda rule of thumb. Microwave link, antenna diameter 1.2 m, f c =26 GHz Rayleigh distance d R =2*1.2 2 /0.011=250 m For this device the limit is the Rayleigh distance Fredrik Tufvesson - ETIM10 6

7 Fresnel Zones Fresnel zones: successive regions where secondary waves have Tx- Rx path length which are greater than the LOS path length (i.e., phase diff=). The radius of the nth zone: = The propagation follows a space loss as long as the first Fresnel zone is clear Fredrik Tufvesson - ETIM10 7

8 The reference distance d 0 For path-loss propagation models, a close-in distance d 0 is selected such that it lies in the far-field region. = 10 ( ). 20log( ) d For practical systems in the 1-2 GHz region, d 0 is typically chosen to be 1 m in indoor environments, and 100 m or 1 km for outdoor environments. For distances d>d break, the above equation doesn t hold anymore Fredrik Tufvesson - ETIM10 8

9 Propagation mechanisms i r 1 2 t Reflection and transmission Diffraction Scattering Waveguiding Fredrik Tufvesson - ETIM10 9

10 Reflection and transmission (Snell s law) e r conductivity permittivity 1 2 reflected angle sin t sin e 1 2. transmitted angle t Fredrik Tufvesson - ETIM10 10

11 TM and TE waves behave differently Reflection coefficient Transmission coefficient Fredrik Tufvesson - ETIM10 11

12 Ground reflection (2-ray model) For the following scenario the power goes like for distances greater than Fredrik Tufvesson - ETIM10 12

13 The d -4 law = ( ). ( ) if d = ( ). if d > However.. n=4 is not a universal decay exponent Theoretical model is not fulfilled in practice Breakpoint is rarely where theoretically predicted Second breakpoint at the radio horizon (the curvature of the Earth) Fredrik Tufvesson - ETIM10 13

14 Transmission through walls layered structures Total transmission coefficient d T T 1T 2 e j 1R 1 R 2 e 2j total reflection coefficient 1 2 e j e 2j T 1 T 2 with the electrical length in the wall 2 1 d layer cos t Fredrik Tufvesson - ETIM10 14

15 Example: The COST-Hata-Model The COST-Hata-Model is formulated as, For suburban or rural environments: where, L = Median path loss (db) f = Frequency of Transmissionn (MHz) h B = Base station height (m) d = Link distance (km) h R = Mobile station height (m) a(h R ) = Mobile station height correction factor Fredrik Tufvesson - ETIM10 15

16 Diffraction Single or multiple edges makes it possible to go behind corners less pronounced when the wavelength is small compared to objects Fredrik Tufvesson - ETIM10 16

17 Diffraction, Huygen s principle Each point of a wavefront can be considered as a source of a spherical wave Bending around corners and edges Fredrik Tufvesson - ETIM10 17

18 Diffraction coefficient by a screen expjk 0 x The diffraction angle + Total field The Fresnel integral is defined F F F exp j t 2 dt. 2 0 with the Fresnel parameter Fresnel integral ) Fredrik Tufvesson - ETIM10 18

19 Diffraction in real environments For real environments we can represent buildings and objects as multiple screens Fredrik Tufvesson - ETIM10 19

20 Diffraction by multiple screens: Bullington s method tangent Replace all screens with one equivalent screen Height determined by the steepest angle Simple but a bit optimistic equivalent screen E total expjk 0 x 1 2 expj /4 2 F F Fredrik Tufvesson - ETIM10 20

21 Diffraction by multiple screens: Epstein-Petersen Method L 1 L 2 L 3 Compute diffraction loss for each screen separately and add the losses L tot =L 1 +L 2 +L 3 Diffraction The same approach is used also for the ITU model, but with an empirical correction factor Fredrik Tufvesson - ETIM10 21

22 Diffraction by multiple screens: empirical models w The ITU multi-knife-edges model The ITU proposed an extreme simple model for diffraction losses. = + 20 where is the diffraction loss from each separate screen (in db), and is a correcting factor depends Diffraction the distances () among the different screens. The values of :,, are selected from certain assumed distributions that depends on the environment. Q: Use the abovementioned equation to explain the reason behind using the log-normal distribution to model the large-scale fading (shadowing) Fredrik Tufvesson - ETIM10 22

23 Scattering Specular reflection Specular reflection Scattering Smooth surface Rough surface In all cases the law of reflection is obeyed. Flat surfaces that have much larger dimension than are modeled as reflective services. However, the roughness of such surfaces induces non-specular components (scattering) Fredrik Tufvesson - ETIM10 23

24 Scattering by rough surfaces: Kirchhoff theory Requires only the probability density function of the surface amplitude assume no shadowing from surface (no influence among different points) calculate an effective reflection coefficient for the surface for Gaussian surface distribution angle of incidence rough smooth exp 2 k 0 h sin 2 standard deviation of height Fredrik Tufvesson - ETIM10 24

25 Scattering by rough surfaces: Perturbation theory Spatial correlation function h 2 W E r h r h r 2-D location vectors h r h r More accurate than Kirchhoff theory, especially for large angles of incidence and rougher surfaces Generalizes Kirchhoff theory Include shadowing effects by the surface includes spatial correlation of surface how fast are the changes in height calculate an effective reflection coefficient for the surface Fredrik Tufvesson - ETIM10 25

26 Modeling received power due to scattering in the far field (urban mobile systems) For urban mobile systems, the following model is used to compute the received power due to scattering in the far field. = [ ] 30 log where and are the distances from the scattering object to Tx and Rx. RCS (Radar Cross Section) is the surface area of the scattering object. In several European cities, for medium and large buildings located 5 10 km away, RCS values were found to be in the range 14.1dB to 55.7dB Fredrik Tufvesson - ETIM10 26

27 Waveguiding This process models propagation in street canyons, corridors, and tunnels. In wireless channels, this process deviates from the ideal assumptions (e.g., street crossings, rough surfaces, cars, pedestrians, ) Propagation measurements fitted a law, where n varies between 1.5 and Fredrik Tufvesson - ETIM10 27

28 How does the signal reach the receiver Outdoor-to-indoor transmitter receiver Fredrik Tufvesson - ETIM10 28

29 2337 How does the signal reach the receiver In the office Fredrik Tufvesson - ETIM10 29

30 How does the signal leave the transmitter at the roof Fredrik Tufvesson - ETIM10 30

31 In all offices Fredrik Tufvesson - ETIM10 31

32 How does the signal reach the receiver outdoor urban h=23 m h=29 m h=29 m 0 N W E S Cathedr al RX h=30 m Yar d 80 Street 1 TX m 0 h=30 m h=28 m h=28 m BOF h=25 m h=24 m h60 m Transmitter Mottagare Fredrik Tufvesson - ETIM10 32

33 Signal arrives from some specific areas Cathedral Through yard Over BOF Street 1 Street Fredrik Tufvesson - ETIM10 33

34 Diffraction, reflection, scattering, transmission Fredrik Tufvesson - ETIM10 34

35 Outdoor 300 MHz peer-to-peer scenario Center frequency: 285 MHz, 20MHz bandwidth Peer-to-peer measurement: TX, 1.8m (BS) RX, 2.1m (MS) Four routes: 322, 320, 80, 110 m semi-rural scenario sub-urban scenario

36 Digital 3D map of the environment

37 Visualized paths for a particular Tx/Rx position

38 Interaction points, 20 strongest MPCs LOS NLOS Radius of circles reflects power, color reflects delay

39 Interaction points, 20 strongest MPCs NLOS NLOS Radius of circles reflects power, color reflects delay

40 Multipath components tend to appear in clusters, moving Rx Dead cluster As Rx is moving (10 wavelengths, approximately 10 meters), clusters disappear and new clusters appear. Active clusters during the Rx movement New cluster