Propagation Models

The use of propagation modeling is a requirement in the RF design process.
The propagation modeling techniques used are meant to determine the
attenuation of the radio wave as it transverses from the transmitter
antenna to that of the receiver’s antenna. The propagation model therefore
is meant to characterize the radio path shown in Figure 9-4.
As with all aspects of radio design, numerous methods are used in the
course of arriving at the desired result, that is, how much attenuation did the
signal experience and does it exhaust the values defined in the link budget.
Some of the most popular propagation models used are Hata, Carey, Elgi,
Longley-Rice, Bullington, Lee, and Cost 231, to mention a few. Each of these models has advantages and disadvantages associated with each of them.
Specifically, some baseline assumptions are used with any propagation
model and need to be understood prior to utilizing them. Most cellular operators
use a version of the Hata model for conducting propagation characterization.
The Carey model, however, is used for submitting information to
the FCC with regards to cell site filing information. Cellular and Personal
Communication Services (PCS) operators utilize either Hata or Cost231 as their primary method for determining path loss. With the introduction of
3G, the use of Cost231 is the model of choice to use that can be applied to
any of the spectrum allocations defined by the ITU.
Regardless of the frequency band of operation, the model used for predicting
coverage needs to factor into it a large amount of variables that
directly impact the actual RF coverage prediction of the site. The positive
attributes affecting coverage are the receiver sensitivity, transmit power,
antenna gain, and the antenna height above average terrain. The negative
factors affecting coverage involve line loss, terrain loss, tree loss, building
loss, electrical noise, natural noise, antenna pattern distortion, and
antenna inefficiency, to mention a few.
With the proliferation of cell sites, the need to theoretically predict the
actual path loss experienced in the communication link is becoming more
and more critical.To date, no overall theoretical model has been established
that explains all the variations encountered in the real world. However, as
the cellular and PCS communication systems continue to grow, a growing
reliance is placed on the propagation prediction tools. The reliance on the
propagation tool is intertwined in the daily operation of the wireless communication
system. The propagation model employed by the cellular and
PCS operator has a direct impact on the capital build program of the company
for determining the budgetary requirements for the next few fiscal
years. Therefore, it is essential that the model utilized for the propagation
prediction tool be understood. The model should be understood in terms of
what it can actually predict and what it cannot predict.
Over the years, numerous articles have been written with respect to
propagation modeling in the cellular communications environment. With
the introduction of PCS, there has been an increased focus on refining the
propagation models to assist in planning out the networks. However, no one
model can predict every variation that will take place in the environment.
To overcome this obstacle, some operators have resorted to utilizing a combination
of models, depending on the environmental conditions relevant to
the situation.
In addition to which model would be the best to utilize, other perturbations
to the model need to be considered. One of the most basic considerations
is determining the morphology that the model will be applied to.
Morphologies are normally defined in four categories: dense urban, urban,
suburban, and rural. The selection of which morphology to utilize at times
is more of an art than a direct science and this often leads to gross assumptions
being made for a geographic area. The morphologies are generally
defined using a rough set of criteria:

■ Dense urban This is normally the dense business district for a
metropolitan area. The buildings for the area generally are 10 to 20
stories or above, consisting of skyscrapers and high-rise apartments.
■ Urban This type of morphology usually consists of building
structures that are from 5 to 10 stories in height.
■ Suburban This morphology is a mix of residential and business with
the buildings ranging from one to five stories, but mainly consisting of
one- to two-story structures.
■ Rural This morphology, as the name applies, generally consists of
open areas with structures not exceeding two stories and that are
sparsely populated.
From these morphologies, it may seem obvious that classifying an area is
rather ambiguous because the geographic size of the area is left to the engineer
to define.
As mentioned before, several propagation models are currently utilized
throughout the industry, and each of the models has pros and cons.
It is through understanding the advantages and disadvantages of each of
the models that a better engineering design can actually take place in a
network.