Handbook of Propagation Effects for Vehicular and ... - Courses

Summary of Recommendations 12-21
probability distributions models are the composite of Rayleigh, Ricean, and lognormal
probability density functions with more or less physical reasoning. Geometrical
analytical models attempt to predict the combined effects of scatterers by applying
electromagnetic theory.
12.11.1 Choosing a Model
The choice as to which model is most appropriate depends very much on the intended
application and what propagation phenomenon one wants to predict. Of the different
types, empirical models do not provide insight into the physics of propagation processes,
but they characterize the sensitivity of the results to important parameters. Statistical
models build upon an understanding of the processes that cause signal variations, but
with simplifying assumptions. Analytical models attempt to describe a particular
propagation scenario deterministically, but then have to use statistics to extend the results
to realistic situations.
12.11.1.1 Empirical Regression Models
The EERS model has already been summarized in this chapter under Section 12.3.1. As
it is based on a large amount of data, it gives reasonably realistic estimates of fading
distributions by roadside trees.
12.11.1.2 Statistical Models
Propagation effects on mobile satellite links are caused when transmissions are reflected
diffusely or specularly, when they penetrate through trees or diffract around buildings.
All these different propagation states can be associated with specific statistical
distributions. Models need to allow for a number of states appropriate to the environment
under consideration. The Loo and Lutz models work well in environments with two
states, the three-state model (Chapter 10), however, covers more outdoor scenarios. The
more states are incorporated into a model, the more specific site and parameter data are
required.
12.11.1.3 Analytical Models
Analytical models are mainly useful for describing canonical propagation phenomena, as
such they can be very useful in studying the effect of varying particular parameters, such
as the bandwidth, or antenna pattern.
12.11.2 Suggestions for Future Work
A deficiency of the models presented in Chapter 11 is that they were derived for
vehicular mobile applications, whereas today’s emphasis of satellite services has shifted
towards hand-held personal user terminals. The environment selections have therefore
also broadened from highways to essentially anywhere, including but not limited to
outdoors, inside buildings, inside cars, aboard trains and airplanes, etc. Although data
characterizing some of these scenarios have been presented in earlier chapters, e.g., fade
measurements into buildings, there has been no concurrent systematic model