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..
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.