Interference in mobile/cellular communication

Interference in mobile communication:

Pic: Interference in between two water waves

The limiting of available electromagnetic spectrum results in the time congestion which results in interference. The interference can occur from clash with another mobile in the same cell or because of a cell in the adjacent cell. There can be interference between the base stations operating at same frequency band or any other non-cellular energy leaking inadvertently into the frequency band of the cellular system. If there is an interference in the voice channels, cross talk is heard and will appear as noise between the users. Moreover the interference also depends upon the area and population. The interference can be divided into 2 parts: co-channel interference and adjacent channel interference.

1. Co-channel interference(CCI): For the efficient use of available spectrum, it is necessary to reuse frequency bandwidth over relatively small geographical areas. However, increasing frequency reuse also increases interference which decreases system capacity and services quality. The cell where the same set of frequencies is used are called co-channel cells. Co-channel interference is the cross talk between two different radio transmitters using the same radio frequency as is the case with the co-channel cells. 

     The reasons of the CCI can be because of either adverse weather climate or poor radio frequency planning  or over crowded radio spectrum. If the cell size and the power transmitter at the base stations are same then CCI will become independent of the transmitted power and will depend on radius of the cell(R) and the distance between the interfering co-channel cells (D). if D/R ratio is  increased, then the effective distance between the co-channel cells will increase and interference will decrease. The parameter Q is called the frequency reuse ratio is related to the cluster size. For hexagonal geometry:

                              Q=D/R=

The effect of  co-channel interference can be minimized by optimizing the frequency assignments of the base stations and their transmit powers. Tilting the base station antenna to limit the spread of the signals in the system can also be done.

2. Adjacent channel interference (ACI):  This type of interference is caused by adjacent channels i.e. channels in adjacent cell. It is the signal impairment which occurs to one frequency due to presence of another signal on a nearby frequency. This occurs when imperfect receiver filters allow nearby frequencies to leak into the passband. His problem is enhanced if the adjacent channel user is transmitting in a close range compared to the subscribers receiver while the receiver attempts to receive a base station on a channel. This is called near-far effect. 

The more adjascent channels are packed into the channel block, the higher the spectral efficiency, provided that the performance degradation can be tolerated in the system link budget. This effect can also occur of a mobile close to a base station transmits on a channel close to one being used by a weak mobile. This problem might occur if the base station has problem in discriminating the mobile user from the “bleed over” caused by the  close adjacent channel mobile. 

Adjacent channel interference occurs more frequently in small cell clusters and heavily used cells. If the frequency separation between channel is kept large this interference can be reduced to some extent. This assignment of channels is given such that they do not form a contiguous band of frequencies within a particular cell and frequency separation is maximized. Efficient assignment strategies are very much important in making the interference as less as possible.