Mumbai university > Comp > SEM 8 > Mobile Communication

Marks: 10M

Year: May 2012

• In cell sectoring a single omnidirectional antenna at base station is replaced by several directional antennas, each radiating within a specified sector.

• By using directional antennas power is transmitted in single desired direction decreasing number of interfering co-channel cells and co-channel interference.

• When sectoring is employed, the channels used in a particular cell are broken down into sectored groups and are used only within a particular sector. For cluster size 7, number of co-channel cells reduced from 6 to 2 for $120^0$ sectoring. This is because only 2 of the 6 co-channel cells receive interference with a particular sectored channel group.

• For omnidirectional antenna there are 6 co-channel cells. For worst condition as shown in figure below there are 2 cells at distance D, 2 cells at distance D + R and 2 cells at distance D - R.

We know,

$\frac{S}{I} = \frac{R^{-n}}{Σ_{i = 1}^{io} D^{-n}}$

$\frac{S}{I} = \frac{R^{-n}}{2D^{-n} + 2(D - R)^{-n} + 2 (D + R)^{-n}}$

For N=12 and n=4 we get S/I = 22.53 db which is greater than 18 db, hence cluster size of 12 can be used. For N=7 S/I is 17.27 db, which is less than 18 db, thus cluster size of 7 cannot be used

• Figure given below shows that mobile at the centre of cell named as A experiences interference only from 2 sectors 1 and 2.

For N=7 and n=4, S/I is found to be as 24.81 db which is less than 18 db, thus cluster size of 7 can be used with sectoring.

• In this way with reduction in interfering co-channel cells using sectoring, on can improve S/I ratio thus increasing the capacity. Increase in capacity is given by,

  $\frac{Old \ \ cluster \ \ size}{New \ \ cluster \ \ size} = \frac{12}{7} = 1.714$