Lecture 2: Wireless Propagation Channels

Transcription

1 Lecture 2: Wireless Propagation Channels RezaMohammadkhani, UniversityofKurdistan WirelessCommunications,2015 eng.uok.ac.ir/mohammadkhani 1 2 Outline Wireless Propagation Multipath Propagation Large scale fading (Path loss, Shadowing) Small scale fading Path loss models Free-space model Two ray model General ray tracing Simplified model Empirical models mmwave models

2 3 Multipath Propagation 4 Small-scale fading

3 Large-scale fading 5 Path loss Shadowing Propagation Characteristics 6 Path Loss (includes average shadowing) Shadowing (due to obstructions) Multipath Fading

4 7 Path loss models 8 Path Loss Modeling Maxwell s equations Complex and impractical Free space path loss model Too simple Ray tracing models Requires site-specific information Simplified power falloff models Main characteristics: good for high-level analysis Empirical Models Don t always generalize to other environments

5 9 Free Space (LOS) Model Path loss for unobstructed LOS path Power falls off : Proportional to 1/d 2 Proportional to 2 (inversely proportional to ) 10 Ray Tracing Approximation Represent wavefronts as simple particles Geometry determines received signal from each signal component Typically includes reflected rays, can also include scattered and defracted rays. Requires site parameters Geometry Dielectric properties

6 Two Path Model 11 Path loss for one LOS path and 1 ground (or reflected) bounce Ground bounce approximately cancels LOS path above critical distance Power falls off Proportional to d 2 (small d) Proportional to d 4 (d>d c ) Independent of () General Ray Tracing 12 Models all signal components Reflections Scattering Diffraction Reflections generally dominate Requires detailed geometry and dielectric properties of site Similar to Maxwell, but easier math. Computer packages often used

7 13 Simplified Path Loss Model Used when path loss dominated by reflections. Most important parameter is the path loss exponent, determined empirically. P r PK t d d 14 Empirical Channel Models Cellular Models: Okumura model and extensions: Empirically based (site/freq specific), uses graphs Hata model: Analytical approximation to Okumura Cost 231 Model: extends Hata to higher freq. (2 GHz) Multi-slope model Walfish/Bertoni: extends Cost 231 to include diffraction WiFi channel models: TGn Empirical model for n developed within the IEEE standards committee. Free space loss up to a breakpoint, then slope of 3.5. Breakpoint is empirically-based.

8 Okumura s measurements 15 Extensive measurement campaign in Japan in the 1960 s. Parameters varied during measurements: Frequency MHz Distance km Mobile station height 1 10 m Base station height m Environment medium-size city, large city, etc. Propagation loss is given as median values (50% of the time and 50% of the area). Okumura s measurements (2) 16

9 17 Okumura-Hata model How to calculate propagation loss 18 The COST 231-Walfish-Ikegami model How to calculate propagation loss

10 19 Radio Propagation Effects Reflections Diffraction Scattering Radio Propagation effects 20

11 Reflection & Transmission 21 Diffraction 22 Single or multiple edges makes it possible to go behind corners less pronounced when the wavelength is small compared to objects

12 23 Scattering 24 mmwave Propagation

13 25 mmwave: What s the big deal? All existing commercial systems fit into a small fraction of the mmwave band mmwave Propagation (60-100GHz) 26 Channel models immature Based on measurements, few accurate analytical models Path loss proportion to 2 (huge) Also have oxygen and rain absorbtion is on the order of a water molecule require massive MIMO

14 27 mmwave Bands mmwave bands: 57-66GHz 71-76GHz (in the UK and the US) 81-86GHz 92-95GHz (only in the US) 28

15 References 29 A. Goldsmith, Wireless Communications, Cambridge University Press, Andreas Molisch, Wireless communications, Wiley-IEEE Press, 2nd Ed, Wireless Communications course notes by Professor A. Goldsmith, Stanford University. R. Mohammadkhani, Adaptive Impedance Matching to Compensate Mutual Coupling Effects on Compact MIMO Systems, PhD Thesis, 2012, University of Edinburgh, UK