EITN85, FREDRIK TUFVESSON ELECTRICAL AND INFORMATION TECHNOLOGY

Transcription

1 Wireless Communication Channels Lecture 2: Propagation mechanisms EITN85, FREDRIK TUFVESSON ELECTRICAL AND INFORMATION TECHNOLOGY Contents Free space loss Propagation mechanisms Transmission Reflection Diffraction Scattering Waveguiding Examples from propagation scenarios VT 2018 Wireless Communication Channels 2 1

2 Free-space loss If we assume RX antenna to be isotropic: 2 P RX l = Ł 4p d ł P TX d A RX Attenuation between two isotropic antennas in free space is (free-space loss): 2 4pd L free ( d) = Ł l ł VT 2018 Wireless Communication Channels 3 Free-space loss Friis law Received power, with antenna gains G TX and G RX : GRX GTX l P ( d) = P = P G G L d Ł4pd ł ( ) RX TX TX RX TX free 2 Valid in the far field only! ( ) ( ) P d = P + G - L d + G RXdB TXdB TXdB freedb RXdB 4pd = P + G - 10log + G Ł l ł TX db TX db 10 RX db 2 VT 2018 Wireless Communication Channels 4 2

3 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: d = R 2 2 a L l where L a is the largest dimesion of the antenna. Another rule of thumb is: At least 10 wavelengths l / 2-dipole l / 2 Parabolic 2r L a = l / 2 d R = l / 2 L a = 2r 2 8r d R = l VT 2018 Wireless Communication Channels 5 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 VT 2018 Wireless Communication Channels 6 3

4 The d -4 law (1) For the following scenario the power goes like for distances greater than VT 2018 Wireless Communication Channels 7 The d -4 law (2) 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 VT 2018 Wireless Communication Channels 8 4

5 Propagation mechanisms Qi Qr e 1 e 2 Q t Reflection and transmission Diffraction Scattering Waveguiding VT 2018 Wireless Communication Channels 9 Complex dielectric constant conductivity dielectric constant, permittivity Describes the dielectric material in one single parameter Examples Rel. permittivity conductivity Concrete Gypsum Wood Glass Air 1 VT 2018 Wireless Communication Channels 10 5

6 Reflection and transmission Qe Qr reflected angle e 1 e 2 transmitted angle sinb t sinb e = P 1 P 2. Q t VT 2018 Wireless Communication Channels 11 TM and TE waves behave differently Reflection coefficient Transmission coefficient T = 1-r 2 VT 2018 Wireless Communication Channels 12 6

7 Transmission through walls layered structures d T 1 T 2 Total transmission coefficient T= T 1T 2 e?jj 1+R 1 R 2 e?2jj total reflection coefficient _= _ 1+_ 2 e?j2j 1+_ 1 _ 2 e?2jj with the electrical length in the wall J= 2^V P 1 d layer cosb t VT 2018 Wireless Communication Channels 13 Diffraction Single or multiple edges makes it possible to go behind corners less pronounced when the wavelength is small compared to objects VT 2018 Wireless Communication Channels 14 7

8 Diffraction, Huygen s principle Each point of a wavefront can be considered as a source of a spherical wave Bending around corners and edges VT 2018 Wireless Communication Channels 15 Diffraction coefficient exp?jk 0x The Fresnel integral is defined X F F X F =Xexp?j^t 2 dt. 2 0 Total field with the Fresnel parameter X F =J k 2d 1 d 2 V d 1 +d 2 Fresnel integral VT 2018 Wireless Communication Channels 16 8

9 Diffraction in real environments For real environments we can represent buildings and objects as multiple screens VT 2018 Wireless Communication Channels 17 Diffraction Bullington s method tangent Replace all screens with one equivalent screen Height determined by the steepest angle Simple but a bit optimistic equivalent screen VT 2018 Wireless Communication Channels 18 9

10 Diffraction 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 VT 2018 Wireless Communication Channels 19 Scattering Specular reflection Specular reflection Scattering Smooth surface Rough surface VT 2018 Wireless Communication Channels 20 10

11 Kirchhoff theory scattering by rough surfaces calculate distribution of the surface amplitude assume no shadowing from surface calculate a new reflection coefficient for Gaussian surface distribution angle of incidence _ rough =_ smooth exp?2 k 0a h sinf 2 standard deviation of height VT 2018 Wireless Communication Channels 21 Pertubation theory scattering by rough surfaces a 2 h W _ =E r h r h r+_ h r+_ h r _ Include shadowing effects by the surface includes spatial correlation of surface how fast are the changes in height based on calculation of an effective dielectric constant More accurate than Krichhoff theory, especially for large angles of incidence and rougher surfaces VT 2018 Wireless Communication Channels 22 11

12 Waveguiding Waveguiding effects often result in lower propagation exponents n =1.5-5 This means lower path loss along certain street corridors VT 2018 Wireless Communication Channels 23 How does the signal reach the receiver Outdoor-to-indoor transmitter receiver VT 2018 Wireless Communication Channels 24 12

13 How does the signal reach the receiver In the office VT 2018 Wireless Communication Channels 25 How does the signal leave the transmitter at the roof VT 2018 Wireless Communication Channels 26 13

14 In all offices VT 2018 Wireless Communication Channels 27 How does the signal reach the receiver outdoor urban Transmitter h=23 m h=29 m h=29 m 0 20 N W E S Cathedr al RX3 53 Yar h=30 m d 80 Street 1 TX m 0 h=30 m h=28 m h=28 m BOF h=25 m h=24 m h»60 m Receiver VT 2018 Wireless Communication Channels 28 14

15 Signal arrives from some specific areas Cathedral Through yard Over BOF Street 1 Street 2 VT 2018 Wireless Communication Channels 29 Diffraction, reflection, scattering, transmission VT 2018 Wireless Communication Channels 30 15

16 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 VT 2018 Wireless Communication Channels 31 Digital 3D map of the environment VT 2018 Wireless Communication Channels 32 16

17 Visualized paths for a particular Tx/Rx position VT 2018 Wireless Communication Channels 33 Interaction points, 20 strongest MPCs LOS NLOS Radius of circles reflects power, color reflects delay VT 2018 Wireless Communication Channels 34 17

18 Interaction points, 20 strongest MPCs NLOS NLOS Radius of circles reflects power, color reflects delay VT 2018 Wireless Communication Channels 35 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 VT 2018 Wireless Communication Channels 36 18

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