22 March 2018

What is interference in communication

The performance of a cellular system is highly affected by interference. Interference is the main problem in increasing capacity. basically, there are two types of interference in the cellular system.
  1. Co-channel interference
  2. Adjacent channel interference
Co-channel interference :

In the frequency reuse method, several cells use the same set of frequencies in the given service area. The cell using the same set of the channel are called co-channel cells. The interference between signals from these cells is called as co-channel interference.

While the order to reduce co-channel interference, the co-channel interference cell are actually separated by the smallest distance to provide isolation.
The minimum distance that allows the same frequency to be reused depends on factor like a number of co-channel cells near the center cell.

Now let us discuss how to find frequency reuse distance D  in co-channel interference  
D= √3N  R 
N =  i + i j +  j2
Here we have to calculate different values of D  for given values of N.

D=3 , N=3 (i=1, j=1)

D=4.58R, N=7(i=1, j=2)

D= 6R, N=12(i=2, j=2)

If all the cell transmits the same power, then N increases and the frequency reuse distance D increases. This increases D reduces the probability interference can occur.

The ratio D/R is called the co-channel reuse ratio. It is expressed as,

                                Q = D/R = √3N

A small value of Q provides large capacity as the cluster size N is small. A large Q, i.e a large value of D/R indicates that the spatial separation between the co-channel cell relative to the coverage distance of a cell is increased.
Adjacent channel interference :       

While the interference resulting from the channel that is adjacent in frequency to the desired channel is called adjacent channel interference.

If the user that is using an adjacent channel and is transmitting in close range to subscriber receiver then while the receiver tries to receive a base station on the desired channel the problem can be critical. It is called the near-far effect.

Be proper filter and channel assignment the adjacent channel interference can be minimized. As each cell is given only part of the available channels a cell needs to assign channels that are adjacent in frequency.

In adjacent channel interference is frequency reuse when the frequency reuse is high there are more chances for interference because of neighboring channels and because of separation concept. 

Let us now talk about the basic source of interference :
  • Any other mobile in the same cell.
  • A call in always progress in the neighboring of a cell.
  • In most of the other base station operating on the same frequency.
  • Any non-cellular system which leaks energy into the cellular frequency band.