Mobile Communications Fundamentals

Even though the term “cellular” is often used in North America to denote
analog AMPS systems, most, though not all, mobile communications systems
are cellular in nature. Cellular simply means that the network is
divided into a number of cells, or geographical coverage areas, as shown in
Figure 1-4. Within each cell is a base station, which contains the radio
transmission and reception equipment. It is the base station that provides
the radio communication for those mobile phones that happen to be within
the cell. The coverage area of a given cell is dependent upon a number of
factors such as the transmit power of the base station, the transmit power
of mobile, the height of the base station antennas, and the topology of the
landscape. The coverage of a cell can range from as little as about 100 yards
to tens of miles.
Specific radio frequencies are allocated within each cell in a manner that
depends on the technology in question. In most systems, a number of individual
frequencies are allocated to a given cell and those same frequencies
are reused in other cells that are sufficiently far away to avoid interference.
With CDMA, however, the same frequency can be reused in every cell.
Although the scheme shown in Figure 1-4 is certainly feasible and is sometimes
implemented, it is common to sectorize the cells, as shown in Figure
1-5. In this approach, the base station equipment for a number of cells
is co-located at the edge of those cells, and directional antennas are used to
provide coverage over the area of each cell (as opposed to omnidirectional
antennas in the case where the base station is located at the center of a cell). Sectorized arrangements with up to six sectors are known, but the
most common configuration is three sectors per base station in urban areas,
with two sectors per base station along highways.
Of course, it is necessary that the base stations be connected to a switching
network and for that network to be connected to other networks, such
as the Public Switched Telephone Network (PSTN) in order for calls to be
made to and from mobile subscribers. Furthermore, it is necessary for information
about the mobile subscribers to be stored in a particular place on
the network. Given that different subscribers may have different services
and features, the network must know which services and features apply to
each subscriber in order to handle calls appropriately. For example, a given
subscriber may be prohibited from making international calls. Should the
subscriber attempt to make an international call, the network must disallow
that call based upon the subscriber’s service profile.