The basic technology for mobile phones and personal communication systems is a cellular network. A service area is divided into cells. A base station is located in the middle of each cell.
Each cell is allocated a frequency and adjacent cells are allocated a different frequency to avoid interference. The transmitting power is controlled to avoid interference with neighbouring cells. The same frequency is used in non-adjacent cells – this is known as frequency reuse.
When the number of users in a cell increases, the frequency allocated to the cell is no longer enough. There are a number of approaches to overcome this problem.
- Add new channels: Usually, when a system is set up, not all channels are utilised. Growth and expansion is systematically managed by adding new channels when the number of users increases.
- Frequency borrowing: A congested cell borrows frequencies from adjacent cells.
- Cell splitting: Cells in areas with high density traffic are divided into smaller cells. For this purpose, the power level must be reduced to limit signal transmission to within one cell.
- Cell sectoring: A cell is divided into 3 or 6 sectors and each sector is allocated a subset of channels for that cell. Involves replacing a single omni-directional antenna with 3 directional antennas (120o sectoring) or 6 directional antennas (60o sectoring). Each sector uses a directional antenna at the BS and is assigned a set of channels. The number of channels in a sector is the number of channels in a cell divided by the number of sectors. Reduces transmission power, longer battery lifetime.
