Interference is the sum of all signal contributions that are neither noise not the wanted signal. Lets understand how its effect, its type and what possible source for it.
Effects of Interference
- Interference is a major limiting factor in the performance of cellular systems.
- It causes degradation of signal quality.
- It introduces bit errors in the received signal.
- Bit errors are partly recoverable by means of channel coding and error correction mechanisms.
- The interference situation is not reciprocal in the uplink and downlink direction.
- Mobile stations and base stations are exposed to different interference situation.
Sources of Interference
- Another mobile in the same cell.
- A call in progress in the neighboring cell.
- Other base stations operating on the same frequency.
- Any non-cellular system which leaks energy into the cellular frequency band.
Types of Interference
- There are two types of system generated interference
- Co-channel interference
- Adjacent channel interference
- This type of interference is the due to frequency reuse, i.e. several cells use the same set of frequency.
- These cells are called co-channel cells.
- Co-channel interference cannot be combated by increasing the power of the transmitter. This is because an increase in carrier transmit power increases the interference to neighboring co-channel cells.
- To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance to provide sufficient isolation due to propagation or reduce the footprint of the cell.
- Some factors other than reuse distance that influence co-channel interference are antenna type, directionality, height, site position etc.
- GSM specifies C/I > 9dB.
- In a cellular system, when the size of each cell is approximately the same, co-channel interference is independent of the transmitted power and becomes a function of cell radius(R) and the distance to the center of the nearest co-channel cell (D).
- Q = D / R = Ö3N
- By increasing the ratio of D/R, the spatial separation between the co-channel cells relative to the coverage distance of a cell is increased. In this way interference is reduced from improved isolation of RF energy from the co-channel cell.
- The parameter Q, called the co-channel reuse ratio, is related to the cluster size.
- A small value of Q provides larger capacity since the cluster size N is small whereas a large value of Q improves the transmission quality.
- Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference.
- Adjacent channel interference results from imperfect receiver filters which allow nearby frequencies to leak into the passband.
- Adjacent channel interference can be minimized through careful filtering and channel assignments.
By keeping the frequency separation between each channel in a given cell as large as possible, the adjacent interference may be reduced considerably.