1) When radio waves hit a rough surface, the reflected energy scatters in all directions due to scattering. 2) A surface is considered smooth if the minimum to maximum height differences are less than the critical height, and rough if they are greater. 3) The radar cross section is defined as the ratio of scattered power density to incident power density, and is used to model scattering objects in the bistatic radar equation.

1. Wireless Communication
Channels: Large-Scale Pathloss

2. Scattering

3. 3
Scattering
The actual received signal in a mobile radio environment
is often stronger than what is predicted by reflection and
diffraction
Reason:
When a radio wave impinges on a rough surface, the
reflected energy is spread in all directions due to
scattering

4. 4

5. 5
Reflection Vs Scattering
 Reflection: Flat surfaces that have much larger dimension
than wavelength
 Scattering: When the medium consists of objects with
dimensions that are small compared to the wavelength
Testing Surface Roughness using Rayleigh Criterion
hc : Critical Height of Surface Protuberance
Θi : Angle of Incidence
λ : Wavelength
Smooth Surface  Minimum to maximum protuberance h is less than hc
Rough Surface  Minimum to maximum protuberance h is greater than hc

6. 6
Γrough: Reflection Coefficient for Rough Surfaces
Γ : Reflection Coefficient for Smooth Surfaces
ρS : Scattering Loss Factor
σh : Standard deviation of the surface height h about the mean surface height
I0(.) : Bessel Function of the first kind and zero order
Modified Reflection Coefficient for Rough Surfaces

7. Bistatic Radar Equation
7

8. Radar Cross Section Model
8
The radar cross section (RCS) of a scattering object is defined as the ratio of the power
density of the signal scattered in the direction of the receiver to the power density of the
radio wave incident upon the scattering object, and has units of square meters.
Bistatic radar equation
R
T
2
T
T
R
20logd
-
20logd
-
)
30log(4
-
]
RCS[dBm
)
20log(
(dBi)
G
(dBm)
P
(dBm)
P

 


