Page 179 - Integrated Wireless Propagation Models

M a c r o c e l l P r e d i c t i o n M o d e l s - P a r t 2 : P o i n t - t o - P o i n t M o d e l s 157

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3.2 Fine-T n i n g the Lee Model 22
Both terrain contour and human-made structures strongly affect the received mobile
radio signal strength. But it is difficult to separate the effects of the natural environment
and human-made structures on the received data for two reasons. First, in a natural
environment, the ground is never flat; second, human-made structures in each area are
different. Finding the propagation characteristics, such as the intercept signal level at a
given range (1 km or 1 mile away from the base station) and the slope of path loss along
the radio path, is a challenging task. This section introduces a method that can separate
the effects of human-made structures from those of the terrain contour with a high
degree of confidence. The propagation characteristics differ from different areas, and
the proper ones are used as valuable inputs to the propagation prediction software
program. The theoretical shadow loss and the effective antenna height gain are also
compared with the large amounts of measured data when a mobile is either blocked or
not blocked by terrain from the base station. Finally, a means of fine-tuning the propa­
gation model due to the terrain effect by feeding back the measured data is discussed.
The increasing demand for cellular services offers a great challenge in designing
cellular systems. To be able to design a good cellular system and fine-tune intricate
parameters demands a good design tool. In the meantime, an accurate propagation
prediction tool is a key to successfully designing a cellular system, especially in the early
stages of cellular and PCS cell design. The alternative to the prediction tools is to drive
every route in the interested areas before deciding on the locations of base stations, and
this is both costly and tedious. A more practical and cost-effective solution would be to
collect a sufficient amount of measurement data ahead of the system deployment and
use the statistics gained from these values as an input to fit the prediction tool. It is
necessary to collect the measured data so as to enhance the predicted outputs and at the
same time reveal the characteristics of the region under consideration.
Since every area has its own unique building structures, terrain configuration, and
morphology, it is very difficult to separate the effects caused by the natural obstructions
1
of the area from the human-made obstructions of the measured radio signal data.5• 3 In
general, the parameters can be classified in two categories:
Impact of Human-Made Structures. The effect of human-made constructions can be
translated to a path loss curve of slope and a 1-mile (or 1-km) intercept value, which become
the input parameters for the prediction model. Thus, every region will have its own char­
acteristic slope and intercept values due to the different human-made constructions.2
Impact of the Natural Terrain and Variation. Variations in terrain, valleys, moun­
tains, and so on. result in strong or weak signal reception, depending on the effective
antenna height gain or loss in the nonobstructive case and on the diffraction loss in the
obstructive case due to terrain contours. 1
The methodology of separating the effects of human-made structures from those of
terrain contours using the pre-collected measurement data is discussed here. Then two
deduced parameters are obtained as an input in the prediction tool. These two deduced
parameters are the correct slope and the 1-mile intercept value for the area of interest.
In the real environment, the ground level is never flat, the structures above ground
level are of different types, and the accuracy of the collected measurement data is never
100 percent accurate. We cannot expect the measured data to be totally reliable, but the
accuracy of the terrain database does play a major role in the prediction results.
We will first explain the method of how to separate the factors of human-made
structures from the factors of natural terrain contours. Then examples will be presented