The Cellular Concept System Design Fundamentals
The Cellular Concept System Design Fundamentals
Each mobile uses a separate, temporary radio channel to talk to the cell site. The cell site talks to many mobiles at once, using one channel per mobile. Channels use a pair of frequencies for communication—one frequency (the forward link) for transmitting from the cell site and one frequency (the reverse link) for the cell site to receive calls from the users. so mobiles must stay near the base station to maintain communications. The basic structure of mobile networks includes telephone systems and radio services.
Early mobile radio systems
A single high powered transmitter (single cell)
Large coverage area
Low frequency resource utility
Low user capacity
low-power transmitters using 12 conversations (channels)
2. Mobile Telephone System Using the Cellular Concept. Interference problems caused by mobile units using the same channel in adjacent areas proved that all channels could not be reused in every cell.
Frequency Reuse
Because only a small number of radio channel frequencies were available for mobile systems, engineers had to find a way to reuse radio channels to carry more than one conversation at a time. The solution the industry adopted was called frequency planning or frequency reuse.
The coverage area of cells is called the footprint. This footprint is limited by a boundary so that the same group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere
Handoff
The final obstacle in the development of the cellular network involved the problem created when a mobile subscriber traveled from one cell to another during a call. As adjacent areas do not use the same radio channels, a call must either be dropped or transferred from one radio channel to another when a user crosses the line between adjacent cells. Because dropping the call is unacceptable
Example for Frequency Reuse: The concept of Cluster
Consider a cellular system which has a total ofS duplex channels available for use.
The S channels are divided among N cells (cluster).
Each cell is allocated a group of k channels.
The total number of available radio channels can
be expressed as S=kN.