The major source limiting cellular system capacity comes from interferences (as oppose to noise). Interference has been recognized as a major bottleneck in increasing capacity and is often responsible for dropped calls.
Major Types of Interference
- Co-Channel Interference
- Adjacent Channel Interference
- Intra-Cell Type
- Inter-Cell Type
Co-channel Cells and Interference
- In a given coverage area, there are several cells that use the same set of frequencies. These cells are called co-channel cells.
- The interference between signals from co-channel cells is called co-channel interference
- Unlike thermal noise which can be overcome by increasing the signal-to-noise ratio (SNR), co-channel interference can’t be overcome by simply increasing the carrier power because an increase in carrier power increases the interference to neighbouring co-channel cells.
- To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance
Co-Channel Reuse Ratio
- The signal to co-channel interference ratio is independent of the transmitted power and becomes a function of the radius of the cell (R) and the distance between centres of the nearest cochannel cells (D).
- The spatial separation between cochannel cells relative to the coverage distance of a cell.(See Figure 5.3)
Figure 5.3 SIR
Co-Channel Reuse Ratio for Various Values of N
Large Q-->Large N -->Small Level of Co-Channel Interference
Small Q-->Small N -->Large Capacity
Co-Channel Signal to Interference Ratio (SIR)
Consider only first tier interference. Assuming all interfering BS’s are equal-distance