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(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,