Wireless Communication

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Wireless Communication: 

Wireless Communication Study Group

Propagation Signal Propagation Characteristics: 

Propagation Signal Propagation Characteristics Path loss: power falloff relative to distance Shadowing: random fluctuations due to obstructions Flat and frequency selective fading: caused by multipath

Propagation Characteristics: 

Propagation Characteristics Path Loss (includes average shadowing) Shadowing (due to obstructions) Multipath Fading P r /P t d= vt P r P t d= vt v Very slow Slow Fast

Path Loss Modeling: 

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

Free Space (LOS) Model: 

Free Space (LOS) Model Path loss for unobstructed LOS path Power falls off : Proportional to 1/d 2 Proportional to l 2 (inversely proportional to f 2 ) d=vt

Ray Tracing Approximation: 

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

Two Path Model: 

Two Path Model 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 l (f)

General Ray Tracing: 

General Ray Tracing Models all signal components Reflections Scattering Diffraction Requires detailed geometry and dielectric properties of site Similar to Maxwell, but easier math. Computer packages often used

Simplified Path Loss Model: 

Simplified Path Loss Model Used when path loss dominated by reflections. Most important parameter is the path loss exponent g , determined empirically.

Empirical Models: 

Empirical Models Okumura model Empirically based (site/freq specific) Awkward (uses graphs) Hata model Analytical approximation to Okumura model Cost 231 Model: Extends Hata model to higher frequency (2 GHz) Walfish/Bertoni: Cost 231 extension to include diffraction from rooftops Commonly used in cellular system simulations

Main Points: 

Main Points Path loss models simplify Maxwell’s equations Models vary in complexity and accuracy Power falloff with distance is proportional to d 2 in free space, d 4 in two path model Main characteristics of path loss captured in simple model P r =P t K[d 0 /d] g Empirical models used in simulations Low accuracy (15-20 dB std) Capture phenomena missing from formulas Awkward to use in analysis