Cellular Wireless Networks
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.
