FREQUENCY REUSE

The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called " Frequency Reuse" or "frequency planning".

                          IN cellular system we know that large area is divided into small cells . Each cell is provided with set of radio channels , which are different from the neighbour cell's allocated radio channels.
This is done in order to avoid frequency interference .
But the available bandwidth of frequency for cellular communication is less and if we assign as mentioned above then we will be able to cover only a small area.

FREQUENCY REUSE CONCEPT:
                    In this system a "cluster" (Group of cells) is created of particular size of 4, 7,12 etc.
Each cell in a cluster is assigned with different set of radio channels . But a cell in cluster will be assigned with same set of channels as a cell in another cluster so as to increase the capacity of cellular network.

"Frequency reuse pattern"

If each cell is allocated a group of "k" channels and if s channels are divided among N cells then

number of available radio channels is 

S=KN 

capacity is given by:

C=MKN=MS 

Where M is the no of times cluster repeated .

CELLULAR TELEPHONE SYSTEM

CELLULAR SYSTEM:

                                     "Its a type of communication based on "cell" arrangement in geographic area".

           After the development of cordless telephone system , It seemed that we need an another system which can be use in mobile communication.So the cellular system is developed .

This system divides the large geographic area to be served into small chunks of areas shaped in "Hexagon", called the cells.

As we can see in below figure 

"cellular system"

Each cell consists of an base station that an transmitting and receiving antennas mounted on the tower.

It is connected to the "mobile switching center (MSC) " , through microwave link or radio link .

And the MSC is connected to "public switched telephone network"(PSTN).

The MSC can handle 100,000 cellular subscribers in total and 5000 simultaneous subscribers .

In large cities several MSC's are used in single cell to handle all subscribers.

FORWARD VOICE CHANNEL:

                                "Its a link between mobile user and base station where signal or data will be sent from base station to the user".

REVERSE VOICE CHANNEL:

                                                  "Its a link between base station and user , where signal or data will be sent from mobile user to base station".

FORWARD CONTROL CHANNEL:

                                         "This is control channel required to initiate a call , here the signal flows from base station to mobile user".

REVERSE CONTROL CHANNEL:

                                        "This is also an control channel where the signal flows from mobile usejr to base station ".

HOW COMMUNICATION HAPPENS??:

                         Initially the with the help of reverse control channel the mobile checks for the strongest signal and monitors that signal untill it "falls below certain level".

Once it is connected to some base station then it receives confirmation signal through forward control channel. If some one is trying to call a particular user then initial signal is sent to MSC by PSTN indicating a particular user wants to communicate some other user.Then the MSC checks for destination user with the help of "electronic serial number (ESN)" and sends the signal to base station, then base station using its forward control channel wil send signal to user. Now intern the user will send an "Handshaking" signal to base station indicating it is ready to receive the call .then the control is given to "Forward and Reverse voice channels".

   

FREQUENCY REUSE:

                                   "To increase the geographic area with limited frequency band ,a technique called "Frequency Reuse" is used.

In this concept same frequency is assigned to different cells which are separated  by the enough distance so as to avoid "frequency interference ".