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.
0 Comments