Mobile signals propagate through Electromagnetic waves. The carrier frequencies can vary in 800 MHz band or 2000 MHz band depending upon the technology used. The transmission may vary from simple LOS to the one that has lots of obstruction, such as hills, buildings, lamp posts etc. These obstructions can cause reflection, diffraction or scattering of the desired signal due to which the signal gets faded by the time it reaches the receiver.
Phenomenon of Reflection , Diffraction and Scattering
It occurs when the EM wave strikes an object that has large dimensions as compared to the wavelength of the propagating wave. Example: walls of building, clear ground.
It occurs by obstructions with sharp irregularities. Secondary waves resulting from the obstructing surfaces are present all over the space, behind the obstacle also.
Example: Edges of walls, pyramids, etc.
3) Scattering It occurs when the dimension of the obstruction are smaller than the wave length of the signal.example: Lamp posts, small stones, street lights, etc. Phenomenon of reflection, diffraction and scattering are illustrated in Figure 1.
Thus, the radio channels are extremely random and do not offer easy and early analysis. Hence, we model them. Just as a civil engineer creates a model of the construction to predict the look and many other parameters, similarly, many researchers have tried to predict behaviour of the mobile radio channels.
These models predict the power received at the receiver (MS or BS) under certain given conditions. The models are classified as
- Large signal fading models
- Small signal fading model
Fading refers to the fluctuations in the amplitude and phase of the radio signal while it passes through the channel( medium).
In a purely reflective environment, Two Ray Ground Reflection model is used. For a purely diffractive environment, Knife Edge diffraction model is used. Models used for scattering environment are Raman scattering model. The next section deals with these derivable model in detail.