Cellular network is an underlying technology for mobile phones,
personal communication systems, wireless networking etc. The technology is
developed for mobile radio telephone to replace high power transmitter/receiver
systems. Cellular networks use lower power, shorter range and more transmitters
for data transmission.
Features of Cellular Systems
Wireless Cellular
Systems solves the problem of spectral congestion and increases user capacity.
The features of cellular systems are as follows −
·
Offer very high capacity in a limited spectrum.
·
Reuse of radio channel in different cells.
·
Enable a fixed number of channels to serve an arbitrarily large number
of users by reusing the channel throughout the coverage region.
·
Communication is always between mobile and base station (not directly
between mobiles).
·
Each cellular base station is allocated a group of radio channels within
a small geographic area called a cell.
·
Neighboring cells are assigned different channel groups.
·
By limiting the coverage area to within the boundary of the cell, the
channel groups may be reused to cover different cells.
·
Keep interference levels within tolerable limits.
·
Frequency reuse or frequency planning.
·
Organization of Wireless Cellular Network.
Cellular network is
organized into multiple low power transmitters each 100w or less.
Shape of Cells
The coverage area of
cellular networks are divided into cells, each cell having its own
antenna for transmitting the signals. Each cell has its own frequencies. Data
communication in cellular networks is served by its base station transmitter,
receiver and its control unit.
The shape of cells
can be either square or hexagon −
Square
A square cell has
four neighbors at distance d and four at distance Root 2
d
- Better if all adjacent
antennas equidistant
- Simplifies choosing and
switching to new antenna
Hexagon
A hexagon cell shape
is highly recommended for its easy coverage and calculations. It offers the
following advantages −
- Provides equidistant
antennas
- Distance from center to vertex equals length of side
Frequency Reuse
Frequency reusing is
the concept of using the same radio frequencies within a given area, that are
separated by considerable distance, with minimal interference, to establish
communication.
Frequency reuse
offers the following benefits −
- Allows communications within
cell on a given frequency
- Limits escaping power to
adjacent cells
- Allows re-use of frequencies
in nearby cells
- Uses same frequency for
multiple conversations
- 10 to 50 frequencies per
cell
For example,
when N cells are using the same number of frequencies
and K be the total number of frequencies used in systems. Then
each cell frequency is calculated by using the formulae K/N.
In Advanced Mobile
Phone Services (AMPS) when K = 395 and N = 7, then frequencies per cell on an
average will be 395/7 = 56. Here, cell frequency is 56.
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