Mobile Radiotelephone in the U.S.
As the demand for wireless service increases, the number of channels assigned to a cell eventually becomes insufficient to support the required number of users. At this point, cellular design techniques are needed to provide more channels per unit coverage area
Methods of Improving the coverage and capacity:
- Cell Splitting
- Microcell Zone Concept
Figure 5.5 Cell Splitting
Cell splitting is the process of subdividing a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitter power.(see figure 5.5)
- Cell splitting increases the capacity of a cellular system since it increases the number of times that channels are reused
- New cells which have a smaller radius than the original cells and by installing
- These smaller cells (called microcells) between the existing cells
- Cell splitting allows a system to grow by replacing large cells with smaller cells, while not upsetting the channel allocation scheme required to maintain the minimum co-channel reuse ratio Q between co-channel cells.
- The technique for decreasing co-channel interference and thus increasing system capacity by using directional antennas is called sectoring. (see figure 5.6)
- The factor by which the co-channel interference is reduced depends on the amount of sectoring used.
- A cell is normally partitioned into three 1200 sectors or six 60° sectors.
- When sectoring is employed, the channels used in a particular cell are broken down into sectored groups and are used only within a particular sector.
Microcell Zone Concept:
Figure 5.6 Sectoring (a)120 0(b)600
- The increased number of handoffs required when sectoring is employed results in an increased load on the switching and control link elements of the mobile system.
- In this scheme, each of the three (or possibly more) zone sites is connected to a single base station and share the same radio equipment.
- The zones are connected by coaxial cable, fiber optic cable, or microwave link to the base station. Multiple zones and a single base station make up a cell
- This approach is superior to sectoring since antennas are placed at the outer edges of the cell, and any base station channel may be assigned to any zone by the base station.
Advantage of the zone cell
- The co-channel interference in the cellular system is reduced
- Improves the signal quality
- Leads to an increase in capacity, without the degradation in trunking efficiency caused by sectoring