Spring 2019 Gavel Spring Gavel 2019 - Page 20

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 20 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.