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Path loss & shadowing

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Path loss & shadowing

By eng : mahmoud abdel aziz

Ray Tracing Path Loss Models

Free Space Model

2 Ray Model

General Ray Tracing

Empirical measurement

Log Normal Shadowing

Slow

Fast

Very slow

Path Loss (includes average shadowing)

Shadowing (due to obstructions)

Multipath Fading

Pr/Pt

Pt

Pt

Pr

v

d=vt

d=vt

Maxwell’s equations

-Complex and impractical

Free space path loss model

- Too simple

Ray tracing models

- Requires site-specific information

Empirical Models

Don’t always generalize to other environments

Simplified power falloff models

Main characteristics: good for high-level analysis

d=vt

the signal propagates along a straight line &The channel model called a line-of-sight (LOS)

Path loss for unobstructed LOS path

Power falls off :

inversely Proportional to d2

Proportional to l2 (inversely proportional to f2)

1- In ray tracing we assume a finite number of reflectors with known location and dielectric properties.

2- Represent wavefronts as simple particles

3- Typically includes reflected rays, can also include scattered and defracted rays.

4-Requires site parameters

* Geometry

* Dielectric properties

1-Path loss for one LOS path and 1 ground (or reflected) bounce

2- path loss has alternate min & max as d is increase

3- Power falls off

Proportional to d2 (small d)

Proportional to d4 (d>dc=4hrht\ l)

1-Models all signal components

Reflections

Scattering

Diffraction

1-Requires detailed geometry and dielectric properties of site

2- GRT method uses geometrical optis

3- Computer packages often used

1- each model is define for a specific environement

1-A number of path loss models have been developed over the years to predict path loss in typical wireless environments such as large urban macrocells, urban microcells, and, more recently, inside buildings These models are mainly based on empirical measurements over a given distance in a given frequency range and a particular geographical area or building.

2- Analytical models characterize Pr/Pt as a function of distance, so path loss is well defined. In contrast, empirical measurements of Pr/Pt as a function of distanceinclude the effects of path loss, shadowing, and multipath.

Most popular model

Based on measurements made in and around Tokyo in 1968.

between 150 MHz and 1500 MHz

1- Output parameter : mean path loss (median path loss) LdB

2- Validity range of the model :

• Frequency f between 150 MHz and1500 Mhz

• TX height hb between 30 and 200 m

• RX height hm between 1 and 10 m

• TX - RX distance r between 1 and 10 km

3 types of prediction area :

•Open area : open space, no tall trees or building in path

•Suburban area : Village Highway scattered with trees and house Some obstacles near the mobile but not very congested

• Urban area : Built up city or large town with large building and houses

Village with close houses and tall

Definition of parameters :

hm : mobile station antenna height above local terrain height [m]

dm: distance between the mobile and the building

h0 : typically height of a building above local terrain height [m]

hb:base station antenna height above local terrain height [m]

r :great circle distance between base station and mobile [m]

R=r x 10 great circle distance between base station and mobile [km]

f: carrier frequency [Hz]

fc=f x 10 carrier frequency [MHz]

λ: free space wavelength [m]

3

6

• Okumura takes urban areas as a reference and applies correction factors

Okumura-Hata model for medium to small cities has been extended

to cover 1500 MHz to 2000 MHz (1999)

LdB = F + B log10 R – E + G

F = 46.3 + 33.9 log10 fc – 13.82 log10 hb

E designed for medium tosmall cities

0 dB medium sized cities and suburban areas

G =

3 dB metropolitan areas

1- Indoor environments differ widely in the materials used.

2- Thus, it is difficult to find generic models that can be accurately applied to determine empirical path loss.

3- Indoor path loss models must accurately capture the effects of attenuation across floors

4- the attenuation per floor is greatest for the first floor that is passed through and decreases with each

subsequent floor passed through.

when tight system specifications must be met or the best locations for base stations

or access point layouts must be determined.

- the following simplified model for path loss as a function of distance is commonly used for system design:
K :depends on the antenna characteristics and the average channel attenuation,

do: is a reference distance for the antenna far-field

assumed to be 1-10 m indoors and 10-100 m outdoors

γ: is the path loss exponent