A cellular system operates by dividing a large geographic service area into cells and assigning the same frequencies to multiple, nonadjacent cells. This is known in the industry as “frequency reuse.” As a subscriber travels across the service area, the call is transferred (handed off) from one cell to another without noticeable interruption.
All the base stations in a cellular system, including radio towers, are connected to a mobile telephone switching office (MTSO) by landline or microwave links. The MTSO controls the switching between the Public Switched Telephone Network (PSTN) and the cell site for all wireline-to-mobile and mobile-to-wireline calls.
There is a huge investment at stake when determining the location of a cell site. The radio tower alone can cost from $250,000 to $1 million. Thus, before a cell site is installed, a number of studies are performed to justify the cost and calculate the return on investment (ROI). Ademographics study, for example, helps forecast the potential subscriber base in the area planned for the cell site.
The study begins with the total population, broken down by sex, age, race, types of households, occupancy rates, and income levels. Much of this information is gleaned from data compiled by the U.S. Census Bureau. By generating a topography map, engineers are able to determine if there will be any obvious interference issues. The goal is to discover problems that would impair the performance of the wireless cell site solution or wireless link.
Sometimes a 50-foot portable crank-up tower is used to create a temporary cell site. Together with vehicles containing both access points and subscriber units, tests are run to find out what may interfere with the signal and demonstrate a realtime cell site coverage area.
After determining any interference issues and the best strategic location for the cell site, a full site survey is done to establish the final plans for cell site deployment. This includes having all the information about the types of mounts needed as well as having potential interference-filtering measures defined. Certified network engineers then determine the best base station configuration and orientation. If no problems are encountered, a tower can be up and running within 6 weeks.
The Telecommunications Act of 1996 specifically leaves in place the authority that local zoning authorities have over the placement of cell towers. It does prohibit the denial of facilities siting based on radio frequency (RF) emissions if the licensee has complied with the Federal Communications Commission’s (FCC’s) regulations concerning RF emissions.
It also requires that denials be based on a reasoned approach and prohibits discrimination and outright bans on construction, placement, and modification of wireless facilities. The FCC mandates that service providers build out their systems so that adequate service is provided to the public. In addition, all antenna structures used for communications must be approved by the FCC, which determines if there is a reasonable possibility that the structure may constitute a menace to air navigation.
The tower height and its proximity to an airport or flight path will be considered when making this determination. If such a determination is made, the FCC will specify appropriate painting and lighting requirements. Thus the FCC does not mandate where towers must be placed, but it may prohibit the placement of a tower in a particular location without adequate lighting and marking.
Low-powered transmitters are an inherent characteristic of cellular radio and broadband personal communication services (PCS). As these systems mature and more subscribers are added, the effective radiated power of the cell site transmitters is reduced so that frequencies can be reused at closer intervals, thereby increasing subscriber capacity.
There are more than 50,000 cell sites operating within the United States and its possessions and territories. Therefore, due to the nature of frequency reuse and the consumer demand for services, cellular and PCS providers must build numerous base sites. The sheer number of towers has caused municipalities to impose new requirements on service providers, such as requiring them to disguise new towers to look like trees, which can add $150,000 to the cost of a tower.