Diffraction

How the radio signals propagate around corners…

FIGURE 4.10

Path length difference

Phase difference

Fresnel-Kirchoff diffraction parameter

Fresnel zone: Successive regions with nl/2 constructive/destructive interference.

For n= 1: null

2: peak

3: null …

FIGURE 4.11, 4.12

Radius of Fresnel circle:

Knife-Edge Diffraction

Gain for varying radius

PAGE 132 FIGURE 4.14

This figure shows how much gain (really loss) there is from the diffraction.

For practical wireless transmission, keep 55% or more of the first Fresnel zone open.

Calculate v, look at Figure, read Gain (it applies to power, factor of 20 included in dB)

EXAMPLE 4.7

Rough Surface Scattering

Consider if:

Then

There are many equations for p_s, and measurements can be used, too

FIGURE 4.6

II. Signals vary around a “deterministic” mean

Log-Distance Path Loss

Average path loss

TABLE 3.2

This model even holds true indoors (Eq 4.93). Exponent there depends on Tables 4.3-5.

Log-normal Shadowing

III. Okumura/Hata Models – common, simple, accurate

Simple predictions based on curve-fitting to measured data for urban models. Different curves are developed for different types of regions but are NOT site-specific.

FIGURE 3.23, 3.24

Path Loss using Okumura model:

Amu = Attenuation relative to free space (measurements, Okumura charts)

Gain of transmitting antenna (height only)

Gain of receiving antenna (function of height only)

G_AREA = FIGURE 3.24 path gain average for different types of areas.

SEE EXAMPLE 3.10

Hata Model

See pp. 119-120 for variations on this model

Suitable for cell systems greater than 1km, not PCS systems