Mumbai University > Electronics and Telecommunication > Sem 7 > Mobile Communication

Marks: 10 M

Year: May 2012

1. In cell sectoring a single omnidirectional antenna at base station is replaced by several directional antennas, each radiating within a specified sector.
2. By using directional antennas power is transmitted in single desired direction decreasing number of interfering co-channel cells and co-channel interference.
3. 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.
4. 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,

$\frac{s}{I} = \frac{R^{-n}}{\sum_{i=1}^{io}D^{-n}}$

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

$\frac{s}{I} = \frac{R^{-n}}{2R^{-n} \left[\left(\frac{D}{R}\right)^{-n} + (1-\frac{D}{R})^{-n} + (1+\frac{D}{R})^{-n}\right]}$

$\frac{s}{1} = \frac{1}{2 \left[(\sqrt{3N})^{-n} + (1 - \sqrt{3n})^{-n} + (1+ \sqrt{3N})^{-n}\right]} ........(\frac{D}{R}=\sqrt{3N})$

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.

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

As after sectoring there are only 2 co-channel cells, assume distance of these 2 cells be D and D+R.

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

$\frac{s}{I} = \frac{R^{-n}}{R^{-n}(\frac{D}{R})^{-n}+R^{-n}(1+ \frac{D}{R})^{-n}}$

$\frac{s}{I} = \frac{1}{(\sqrt{3N})^{-n} + (1+\sqrt{3N})^{-n}}$

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.

1. 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,

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