Cellular Telephony
   

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Cellular Telephony is designed to provide communications between two moving device called mobile station.

A service provider must be able to locate and track a caller, assign a channel to the call ,and transfer the channel from base station to base station as the caller moves out of range.

Each cellular service area is divided into small region called cells. each cell contains an antenna and is controlled by a small office, called base station (BS).

Each base station is controlled  by a switching office, called a Mobile Switching Center (MSC). The MSC coordinates communication between all the base station and telephone central office. It is computerized center that is responsible for connecting call, recording call information and billing.
     
Cell size is not fixed and can be increased and decreased depending on the population of the area . the typical radius of a cell is 1 to 12 miles.
     
 
     

 

Frequency Reuse Principal

  
  Neighboring cell cannot use the same set of frequencies for communication because it may create interference for the users located near the call boundaries. The set of  frequencies available is limited and frequencies need to be reused. The two green cells in the figure below can reuse the same set of frequencies:
     
 

     
  How the Mobile Works  
     

 

Transmitting

  
 

To place a call from a mobile station, the caller enter a code of 10 digits (A phone number) and presses the send button. the mobile station then scans the band, seeking a setup channel with a strong signal, and sends the data (phone number) to the closest base station using that channel. The base station relays the data to MSC. The MSC sends the data on to the  telephone central office. If the called party is available, a connection is made and the result is relayed back to the MSC. At this point  the MSC assigns an unused voice channel to the call, and a connection is established.
     

 

Receiving

  

 

When a mobile phone is called, the telephone central office sends the number to the MSC. The MSC searches for the location of mobile station by sending query signal to each cell in a process called paging .Once the mobile station is found, the MSC transmits a ringing signal and ,when the mobile station answers, assign a voice channel to the call, allowing voice communication to begin.
     

 

Handoff

  

 

When the mobiles move from one cell to another. the signal may become weak. To solve this problem ,the MSC monitors the level of the signal every few seconds. If signal is weak then transfers the call to other nearest cell. This is called Handoff.
Hard Handoff
A Mobile station only communicates with one base station. When it moves from one base station to another communication must be broken with the previous base station.
Soft Handoff
A Mobile station communicates with two base station at the same time. This means during handoff, a station may continue with the new base station before breaking from the old one.
     

 

Roaming

  

 

Roaming means, in principal, that a user can have access to communication or can be reached where there is coverage. A Service provider usually has limited coverage. Neighboring service provider can provide extended coverage through a roaming contract.
     
   

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  Contributed by: Sandeep Vishwakarma, Faculty- Aptech Morwa Centre