(b) the testimony is based on sufficient facts or data;
(c) the testimony is the product of reliable principles
and methods; and
(d) the expert has reliably applied the principles and
methods to the facts of the case.
The prosecution wants to prove the phone could have
been at the incident, and the defense wants to prove
that the phone could not have been there or was
somewhere else. This all comes down to determining
the actual coverage of the cell site at the time of the
crime. Determining whether the phone and incident
were within or not within the coverage area at the
same time may be key to determining the outcome
of the case. It is critical to get this right because an
innocent person may go to prison.
Figure 6: Terrain and clutter
Determining Actual Coverage
In order to determine the actual coverage of a cell, a combination
of sophisticated RF modeling and drive testing must be performed.
RF modeling is done with software that imports antenna patterns,
network characteristics, terrain, morphology, traffic data, and building
databases. Through proven algorithms, it creates a coverage plot or
heat map. The accuracy of the coverage plot can be further improved
by drive testing samples of the area and fine tuning the modeling
parameters with actual measured data. Cellular network providers
have used this method for many years to design and optimize cellular
phone networks.
The cell site antenna transmit pattern is key to understanding its
coverage. Many different types of antennas are available, depending
on the need for the network design. The antenna manufacturers
test their antenna models in an enclosed RF chamber in order
to determine the profile pattern. The RF chamber blocks out any
external radio signals so it does not disrupt the test. The output
of this test is a profile that may look something like the photo on
the left in Figure 5. The image on the right in Figure 5 is a 3D
representation because in reality antennas transmit in 3D space. The
blue color indicates a stronger signal because this is closest to the RF
source.
The antenna patterns in Figure 5 look similar to a circle or an arc,
but not exactly. Antennas also have side or back lobes, as can be seen
Figure 5: 2D antenna test pattern (left), 3D antenna pattern (right)
22
THE GAVEL
in the left side of the figure to the left of the main lobe. These are
functions of the antenna design.
Many variables can affect the actual coverage of an antenna in the
real world, including the following:
• Antenna height.
• Transmit power (output power at the antenna).
• Antenna down tilt (angle of antenna pointing down from
horizontal).
• Terrain (hills, mountains, valleys, etc.).
• Clutter (trees, grasses, foliage, buildings, etc.).
• Traffic patterns (busy times usually decrease the coverage).
• Interference (signals from other cell sites, external transmissions,
or generated from poor cell site cable connections).
Another major factor affecting coverage area is the terrain and
clutter. The world is not a flat surface. Mountains, hills, and crevices
will block radio signals. In addition to terrain, there is morphology
or clutter. This consists of everything on top of the terrain, including
trees, grasses, water, and rocks. These either block or soak up RF,
essentially decreasing the coverage area. Buildings have a significant
impact on RF coverage. Steel, concrete, and tempered glass are all
materials that will block or reflect RF.
Finally, the amount of traffic on the cellular network has a significant
impact on the coverage of the cell. This is dependent on the
technology, but in general the more traffic on a cell, the smaller the
cell’s coverage area. There is a maximum number of calls the cell site
can handle. As the number of users increase, the noise they create
also increases. Therefore, in order to maintain
good call quality, the network prioritizes the
callers. The ones that have a poor audio channel
may be delayed or blocked from calling. These
are phones that are in a high interference or poor
coverage area, which is typically at the boundary
of the coverage area. If the network blocks these
users from the cell, the coverage area essentially
decreases. It is therefore important to look at the
time of day and peak and nonpeak times when
determining coverage.
Figure 7 shows a coverage plot of an area with
several cell sites. The sites are shown in black,