Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, and where the number of obstacles per unit volume is large.Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the channel.
- The actual received signal in a mobile radio environment is often stronger than what is predicted by reflection and diffraction models alone.
- This is because when a radio wave impinges on a rough surface, the reflected energy is spread out (diffused) in all directions due to scattering. As you can see in figure 10.7
- Flat surfaces that have much larger dimension than a wavelength may be modeled as reflective surfaces.
- Surface roughness is often tested using the Rayleigh criterion which defines a critical height (hc) of surface protuberances for a given angle of incidence θi , given by
- Surface is considered smooth if its minimum to maximum protuberance h is less than hc, and is considered rough if the protuberance is greater than hc.
- For rough surfaces, the flat surface reflection coefficient needs to be multiplied by a scattering loss factor Ps, to account for the diminished reflected field. Assumed that the surface height h. It is a Gaussian distributed random variable with a local mean and found Ps to be given by
whereσh is the standard deviation of the surface height about the mean surface height.
- The scattering loss factor derived by Ament was modified by Boithias to give better agreement with measured results
The reflected E-fields for h>hc can be solved for rough surfaces using a modified reflection coefficient given as
Radar Cross Section Model:
- 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.
- For urban mobile radio systems, models based on the bistatic radar equation may be used to compute the received power due to scattering in the far field. The bistatic radar equation describes the propagation of a wave traveling in free space which impinges on a distant scattering object, and is then reradiated in the direction of the receiver, given by
Where dT and dR are the distance from the scattering object to the transmitter and receiver, respectively