Radiowave Propagation: Physics and Applications

8

EMPIRICAL PATH LOSS AND FADING MODELS

8.1 INTRODUCTION

While the physical models studied in Chapter 7 provide insight into reflection, refraction, and diffraction effects that can occur in propagation between two locations near the Earth's surface, the simplified descriptions of the Earth surface (often modeled as a smooth sphere) and diffraction from obstacles that were used result in only moderate accuracy. The site-specific methods considered in Chapter 7 also require knowledge of terrain along the propagation path; such knowledge sometimes may not be available for fixed transmitters and receivers, or for all possible paths in communication systems involving mobile users or broadcast applications. The methods of Chapter 7 are also oriented primarily to situations involving reflection and/or diffraction from a small number of locations along the propagation path. In many propagation environments, particularly those in urban and suburban areas, there are many possible reflection and diffraction points, resulting in a highly complex propagation environment that is not easily treated using purely physical models.

In complex terrestrial point-to-point propagation problems that exceed the limitations of the methods given in Chapter 7, it is common to apply—in lieu of a physical approach—empirical models for predicting propagation losses. This is also true for point-to-area propagation problems as encountered, for example, in predicting cellular communication coverage. As often occurs ...