Determining RF Coverage
in Criminal Cases
By Richard Miletic
In criminal cases, certain techniques are used
to determine the location of a mobile phone
at the time of a crime. This article explains
new methods of figuring out radio frequency
(RF) coverage with a high degree of accuracy
in order to establish the location of the
phone in relation to the serving cell site at
the time of the incident.
In order to determine the location of the
mobile phone, a subpoena is served upon
the cellular network service operator. The
service operator provides Call Detail Records
(CDR) and Cell Site Location Information
(CSLI) for a certain time period for a
specific mobile phone number and a list of
the cell sites in the general area. Serving cell
sites are the sites that the cell phone was
connected to during the calls. Among other
things, the CDR and CSLI include cell site
ID information that indicates which cell was
handling the call. This is how the location of
the phone is matched to the location of the
cell site. The problem with this is that the
phone can be anywhere within the coverage
area of the cell and the exact location is
unknown. It is important to determine the
coverage areas of the serving cells so that one
knows the general location of the mobile.
Due to the highly technical nature of the
information and its importance in many
cases, it is highly beneficial to hire an expert
witness who has experience in cellular
network design, optimization, and mobile
phone location.
Background
They are easily distinguishable because
of the triangular shape. Some towers will
have multiple network operators using
the same tower and/or have multiple
technologies on the same tower. The
antennas are designed to transmit over
a 120-degree beam width such that by
combining all three sectors the result
is full coverage around the cell. This is
done to increase coverage and capacity
as opposed to having a single antenna
transmitting a circular pattern.
In a typical cellphone network, a cell site is
comprised of three sectors with one transmit
and two receive antennas per sector. (See
Figure 1.)
With that said, some cells are cells that are omnidirectional (circular)
and some have three, four, and six sectors, but the three-sector cell is by
far the most dominant in the industry. Each sector on a four-sector cell
covers 90 degrees. Each sector on a six-sector cell covers 60 degrees.
Whatever the case, the cell site information is provided and thus the
Figure 2: Cell site sectors
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THE GAVEL
RF coverage analysis can be done accordingly. Figure 2 is a simple
representation of the difference between an omnidirectional,
three-sector, and six-sector cell.
Cellular networks consist of a number of cell sites that connect
to each other and to processing servers and telephone switches
via high speed copper, fiber optic, or microwave links. In a large
metropolitan area such as Chicago, there may be over 1,000
outdoor or macro cell sites serving the mobile phone users. These
outdoor or macro cell sites consist of a small building containing
the processing equipment and cabling and antennas that are
mounted on a building or tower.
For large indoor venues such as sports stadiums, hospitals, and
airports, Distributed Antenna Systems (DAS) are installed.
These consist of a number of antennas mounted inside the venue
connected with coax and fiber to a central point. The wireless
operators connect the cellular network to the DAS at this point.
The purpose of these antenna systems is to increase the capacity
(number of served users) and the coverage in order to support
high density of users and provide better coverage indoors.
When a cellphone initiates a call, the cell site sector with the
best coverage and quality of service is assigned based on real time
signal strength and quality measurements by the
phone and the cell site. A series of communication
messages between the phone and the network
occur in order to assign a frequency and channel
number for communication. After the call is
established, the phone continuously measures the
signal strength from the serving and surrounding
cells in case it needs to hand off to another cell
site. Details of the call are kept in the network
processing equipment for billing, engineering, and
call processing.