The ferquency used by the one transmitter can not be reused again until there is enough geographical seperation so that one station does not interfere with another station assigned to that frequency.
the key to the cellular system is to make efficient use of availaible channels by using LOW POWER TRANSMITTER to allow frequency reuse at much SMALLER DISTANCES.
NOW CELL is an elementary area .
the formula for distance between cells using same frequency is..

D= SQUAREROOT(3K)*R
where D=distance between the cells
K=frequency reuse factor
R= radius of each cell(assuming cells of equal radius)
ABOVE FORMULA IS DERIVED CONSIDERING geometry of cell. here i have not derived the formula.
but D is a function of S/I ratio.

dear faris you asked me to help you to know how is the connection between PLMN and the MSC of GSM can be done?usually there are 3 ways of connection :
1-microwave link.
2-optical fiber link
3-twisted 2 wire
e-mail me for more details…

hi there
drop me your adress to answer you about the planning,the distance may differ as the output power of TRX of the neighbouring cells and the geografical situation is playing important action too,therefore the frequency reuse is important in GSM cell planning…