Fundamentals of Empirical Propagation Modeling for Outdoor Scenarios

Main Focus Of The Chapter

Noise is an essential factor in wireless communications due to changing environmental conditions. The surroundings of a wireless network can very different at distinct places greatly affecting communication between nodes.

Noise can be divided into two kinds to analyze the effect of noise in wireless communications: additive noise and multiplicative noise. Additive noise is inherent to the receptor and is usually stable. Multiplicative noise arises from many factors and objects that affect the wireless channel. As a result multiplicative noise can vary greatly from location to location. Multiplicative noise is the main factor of interest when analyzing a wireless channel and propagation models.

The multiplicative noise having no relation with either the transmitter or the receiver can be divided into three main fading processes (see Figure 1): Path Loss, Shadowing (or slow fading) and Fast-Fading (or Multipath fading).

Figure 1.

Fading processes. Adapted from (Saunders & Aragón-Zavala, 2007)

Some of the most important phenomena that generate multiplicative noise are the following:

• Absorption: This phenomenon occurs when objects absorb electromagnetic radiation and convert it into another type of energy such as heat or electricity. All the objects present in the environment generate different levels of absorption, for example, walls, trees, signs -- even the atmosphere.

• Reflection: Reflection occurs when electromagnetic waves hit an object which is much greater than the transmitted wavelength. When this happens, the signal is reflected by the hit surface (see Figure 2). Reflection takes place when the waves hit the ground, walls and furniture. When a wave is reflected, it can be partially refracted (Sarkar, 2003). The degree of reflection depends on the material’s properties, like reflection, absorption and transmission. For example, a metal sheet has a nearly perfect reflection, whereas glass and paper have a nearly perfect transmission. Materials like brick have a certain degree of reflection, absorption and transmission (Webb, 1999).

Figure 2.

Reflection effect

• Diffraction: It occurs when there is an obstacle with sharp borders in the path between the transmitter and the receiver. When the waves reach the obstacle, they bend over the sharp borders (see Figure 3). Diffraction has an advantage: the waves can reach places where there is no line of sight; however, the attenuation is greater (Rappaport, 2002). The most important factor is the diffraction angle. With large angles of diffraction, the signal can surround an obstacle. On the other hand, a narrow angle can form a shadow behind the obstacle, where there will be no signal reception (Webb, 1999).

• Refraction: The refraction phenomenon occurs when electromagnetic waves pass from one material to another with different densities, causing a change of direction (see Figure 3).

Figure 3.

Refraction effect