CELLULAR TELEPHONES
By the late 1960s all the technical elements for cellular were in place, but in the
United States the technology languished through lengthy regulatory debates.
A major issue was frequency spectrum. Plenty of unused spectrum existed in
space reserved for UHF television. In 1974, the FCC designated part of the UHF
spectrum between 800 and 900 MHz for cellular, but they delayed approval of the
service pending hearings on the monopoly aspects of cellular. At the time, the
LECs had a monopoly on landline services, and many interveners urged the FCC
to open cellular to competition. In 1978 an AT&T subsidiary named Advanced
Mobile Phone System (AMPS), installed the first cellular radio demonstration system
in the United States in Chicago. The FCC addressed the monopoly issue by
breaking the spectrum into two segments, allocating half to a non-wireline carrier
and the other to the LEC.
Cellular caught on rapidly and exhausted the initial frequencies. The FCC
expanded the band to 824 to 849 MHz in the mobile to base direction, and 869 to
894 MHz base to mobile. The lower half of each band, called the A band, is designated
for wireline carriers, which are defined roughly as local exchange companies.
The upper half, or B band, is designated for non-wireline carriers, which are non-
LEC common carriers. With the mergers and acquisitions that have occurred since
the original allocations were made, this distinction has all but disappeared. The
FCC grants licenses in both bands to serve a Cellular Geographic Serving Area
(CGSA). A CGSA corresponds to a Metropolitan Statistical Area (MSA), which is a
major metropolitan area defined by the Office of Management and Budget.
Cellular Technology
Cellular overcomes most of the disadvantages of conventional mobile telephone.
Acoverage area is divided into hexagonal cells as shown in Figure 20-2. Frequencies
are not duplicated in adjacent cells, which reducess interference between base stations.
It also allows the carrier to reuse frequencies within the coverage area with a
buffer between cells that are operating on the same band of frequencies. This technique
greatly increases the number of radio channels available compared to a conventional
mobile telephone system, which uses a frequency only once in a coverage
area. The cells are smaller in urban and larger in rural areas.
Figure 20-3 illustrates the general cellular plan. The carrier selects the number
and size of cells to optimize coverage, cost, and total capacity within the serving
area. FCC rules and regulations do not specify these design factors; they are up
to the service provider. The mobile units are frequency agile; that is, they can shift
to any of the voice channels. To operate on either digital or analog channels,
the mobile units also must be dual mode. A dual-mode unit responds to a digital
channel first if one is available. If not, it falls back to analog.
Mobile units are equipped with processor-driven logic units that respond to
incoming calls and shift to radio channels under control of the base station. Each
cell site is equipped with transmitters, receivers, and control apparatus. One or
more frequencies in each cell are designated for calling and control. For incoming
and outgoing calls, the cell-site controller assigns the channel and directs a frequency
synthesizer inside the mobile unit to shift to the appropriate frequency.
An electronic central office serves as a mobile telephone switching office
(MTSO), and controls mobile operation within the cells. The cell-site controllers
connect to the MTSO over datalinks for control signals, and voice channels for
talking. The MTSO switches calls to other mobile units and to the local telephone
system, processes data from the cell-site controllers, and records billing details.
It also controls hand-off so a mobile leaving one cell switches automatically to a
channel in the next cell.
