I. Wireless Channel Modeling

Transcription

1 I. Wireless Channel Modeling April 29, 2008 Qinghai Yang School of Telecom. Engineering Qinghai Yang Wireless Communication Series 1

2 Contents Free space signal propagation Pass-Loss & Fading Shadowing effect Time selective fading Frequency selective fading Spatial selective fading MIMO channel Qinghai Yang Wireless Communication Series 2

3 Free-Space Signal Propagation Propagation in free space always like light (straight line) Isotropic radiator: a single point equal radiation in all directions (three dimensional) Shape of antennas may change the signal strength of a particular direction In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver) P P r t = G r G t λ 4π d P r : received power P t : transmitted power G r, G t : receiver and transmitter antenna gain λ (=c/f): wave length Sometime we write path loss in log scale: Lp = 10 log(pt) 10log(Pr) 2 Qinghai Yang Wireless Communication Series 3

4 Signal Propagation Receiving power additionally influenced by shadowing (e.g. through a wall or a door) refraction depending on the density of a medium reflection at large obstacles scattering at small obstacles diffraction at edges diffraction shadow fading refraction reflection scattering Qinghai Yang Wireless Communication Series 4

5 Signal Propagation: Scenarios Details of signal propagation are very complicated We want to understand the key characteristics that are important to our objective Qinghai Yang Wireless Communication Series 5

6 Shadowing Some sample numbers i.e. reduces to ¼ of signal Refraction, reflection, scattering, and diffraction also reduce signal strength Qinghai Yang Wireless Communication Series 6

7 Path Loss and Fading Slow fading : Caused by shadowing typically log-normal. Fast Fading : Caused by local scatterers near mobile - typically Rayleigh fading if scattering is rich. Qinghai Yang Wireless Communication Series 7

8 Path Loss with Surface Reflection Qinghai Yang Wireless Communication Series 8

9 Fading Qinghai Yang Wireless Communication Series 9

10 Ricean vs. Rayleigh Fading Qinghai Yang Wireless Communication Series 10

11 Microscopic Fading Qinghai Yang Wireless Communication Series 11

12 Doppler Spread Qinghai Yang Wireless Communication Series 12

13 Doppler Spread -Time Selective Fading Qinghai Yang Wireless Communication Series 13

14 Delay Spread Qinghai Yang Wireless Communication Series 14

15 Delay Spread -Frequency Selective Qinghai Yang Wireless Communication Series 15

16 Angle Spread Qinghai Yang Wireless Communication Series 16

17 Angle Spread -Space Selective Fading Qinghai Yang Wireless Communication Series 17

18 Multipath Propagation Qinghai Yang Wireless Communication Series 18

19 Multipath Propagation Qinghai Yang Wireless Communication Series 19

20 Multipath Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Signals from multiple paths may cause interference Qinghai Yang Wireless Communication Series 20

21 Multipath Can Reduce Signal Strength Example: reflection from the ground: received power decreases proportional to 1/d 4 instead of 1/d² due to the destructive interference between the direct signal and the signal reflected from the ground ground Qinghai Yang Wireless Communication Series 21

22 Multipath:Interference Due to constructive and destructive interference of multiple transmitted waves The interference of multipath signals varies with distance on the order of the carrier wavelength Qinghai Yang Wireless Communication Series 22

23 Multipath Can Spread Delay signal at sender Time dispersion: signal is dispersed over time LOS pulse multipath pulses signal at receiver LOS: Line Of Sight Qinghai Yang Wireless Communication Series 23

24 Multipath Can Cause ISI dispersed signal can cause interference between neighbor symbols, Inter Symbol Interference (ISI) Assume 300 meters delay spread, the arrival time difference is 300/3x108 = 1 ms if symbol rate > 1 Ms/sec, we will have serious ISI In practice, fractional ISI can already substantially increase loss rate LOS pulse signal at sender multipath pulses signal at receiver LOS: Line Of Sight Qinghai Yang Wireless Communication Series 24

25 MIMO Channel (Spatial Correlation based ) Qinghai Yang Wireless Communication Series 25

26 MIMO Channel (Spatial Correlation based ) Per-tap spatial correlation matrix : Rn = ΘT ΘR (Kronecker product) Cholesky decomposition MIMO channel matrix Hn: H K H K H R H 1/2 n = R iid T or n = unvec{ n vec( iid )} Qinghai Yang Wireless Communication Series 26

27 MIMO Channel (Ray-Based Method) Each propagation path consists of a superposition of M discrete sub-paths (rays) Path n n,m,aoa Subpath m n,m,aod θ n,m,aoa N Ω MS θ v v δ n,aoa N Ω BS δ n,aod θ n,m,aod θ MS MS array θ BS MS array broadside MS direction of travel BS array BS array broadside Qinghai Yang Wireless Communication Series 27

28 MIMO Channel (Ray-Based Method) Qinghai Yang Wireless Communication Series 28

29 Summary Channel characteristics change over location, time, and frequency Received Signal Power (db) path loss power Large-scale fading log (distance) small-scale fading time frequency Qinghai Yang Wireless Communication Series 29

30 Question Q/A For the fading, is it good or bad? How to evaluate it. Investigate the technologies to exploit the temporal, frequency, and spatial diversities in time, frequency and space selective fading channels, respectively. Qinghai Yang Wireless Communication Series 30