Path loss & shadowing By eng : mahmoud abdel aziz

OUTLINE Ray Tracing Path Loss Models Free Space Model 2 Ray Model General Ray Tracing Empirical measurement Log Normal Shadowing

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

Path Loss Modeling Maxwell’s equations Free space path loss model -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

Free Space path loss (LOS) Model the signal propagates along a straight line &The channel model called a line-of-sight (LOS) d=vt Path loss for unobstructed LOS path Power falls off : inversely Proportional to d2 Proportional to l2 (inversely proportional to f2)

Ray Tracing Approximation 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

Two Path Model 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 General Ray Tracing 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

Different types of cells : 1- each model is define for a specific environement

Empirical Models 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 distance include the effects of path loss, shadowing, and multipath.

Okumura-Hata model 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 and 1500 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

Okumura-Hata model cont. 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

Okumura-Hata model cont. 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

COST 231- extension to Hata model 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 to small cities 0 dB medium sized cities and suburban areas G = 3 dB metropolitan areas

Indoor Attenuation Factors 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.

Simplified path loss model 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

Log – distance path loss with shadowing 1- as mobile user moves away from its base station , the receiver signal become weaker because of the growing propagation attenuation with the distance. 2- mobile moves in uneven terrain it often travels into a propagation shadow behind a building or a hill or other obstacle much larger than the wavelength of the transmitted signal and the associated received signal level is attenuated significantly. This phenomenon is called shadowing . a log normal distribution is a popular model for characterizing the shadowing process . Log term fading : Overall path loss Log distance path loss a zero mean Gaussian distributed random variable (in dB))

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