CELLULAR CONCEPT

Cellular Concept

Traditional mobile service was structured similar to television broadcasting: One very
powerful transmitter located at the highest spot in an area would broadcast in a radius of
up to fifty kilometers. The Cellular concept structured the mobile telephone network in a
different way. Instead of using one powerful transmitter many low-powered transmitter
were placed through out a coverage area. For example, by dividing metropolitan region
into one hundred different areas (cells) with low power transmitters using twelve
conversation (channels) each, the system capacity could theoretically be increased from
twelve conversations using one hundred low power transmitters.
The cellular concept employs variable low power levels, which allows cells to be
sized according to subscriber density and demand of a given area. As the populations
grows, cells can be added to accommodate that growth. Frequencies used in one cell
cluster can be reused in other cells. Conversations can be handed over from cell to cell to
maintain constant phone service as the user moves between cells.
The cellular system design was pioneered by during’70s by Bell Laboratories in
the United States, and the initial realization was known as AMPS (Advanced Mobile
Phone Service). The AMPS cellular service was available in United States in 1983.
AMPS is essentially generation 1 analog cellular system in contrast to generation 2
digital cellular systems of GSM and CDMA (1S-95).
Cells :
A cell is the basic geographic unit of cellular system. The term cellular
comes from the honeycomb areas into which a coverage region is divided. Cells are base
stations transmitting over small geographic areas that are represented as hexagons. Each
cell size varies depending upon landscape. Because of constraint imposed by natural
terrain and man-made structures, the true shape of cell is not a perfect hexagon.
A group of cells is called a cluster. No frequencies are reused in a cluster.
Features of Digital Cellular Systems:

                                                            n          Small cells

                                                            n              Frequency reuse
                                                            n              Small, battery-powered
handsets
                                                            n                  Performance of
handovers

Cellular System Characteristics

General

characteristics of digital cellular systems Cellular radio systems allow the subscriber to place and receive
telephone calls over the wire-line telephone network where ever cellular coverage is provided. Roaming capabilities extend service to users traveling outside their “outside” home service areas.
The distinguishing features of digital cellular systems compared to other
mobile radio systems are:

characteristics of digital 

• Small cells

cellular systems
A cellular system uses many base stations with relatively small
coverage radii (on the order of a 100 m to 30 km).

• Frequency reuse

The spectrum allocated for a cellular network is limited. As a result
there is a limit to the number of channels or frequencies that can be
used. For this reason each frequency is used simultaneously by
multiple base-mobile pairs. This frequency reuse allows a much
higher subscriber density per MHz of spectrum than other systems.
System capacity can be further
increased by reducing the cell size (the coverage area of a single base
station), down to radii as small as 200 m.

• Small, battery-powered handsets In addition to supporting much

higher densities than previous systems, this approach enables the use
of small, battery-powered handsets with a radio frequency that is
lower than the large mobile units used in earlier systems.

• Performance of handovers

In cellular systems, continuous coverage is achieved by executing a “handover”
(the seamless transfer of the call from one base station to another) as the mobile
unit crosses cell boundaries. This requires the mobile to change frequencies
under control of the cellular network.