Improving Capacity and Coverage in Cellular Systems
Book Section 3.7
When a cells capacity is no longer able to provide an acceptable GOS, several methods are available to increase system capacity. Also, in areas where radio reception is poor, repeaters can be used to improve radio coverage
1. Cell Splitting (Microcells) 1. Repeaters
2. Sectoring
3. Microcell Zones
How is it implemented:
The Cell Splitting method takes a large cell that can no longer provide acceptable service and divides it into smaller cells. These smaller cells are laid out to use the frequency reuse pattern already in the system.
Advantages:
Because cells are smaller, system capacity increases. Also, less power is used by mobiles and base stations.
Drawbacks:
Handoffs become more common. To prevent handoffs and dropped calls, umbrella cells are needed (p67) for high speed traffic.
Cost of Implementation:
Many new base stations are needed, increasing system complexity and load of MSC.
How is it implemented:
The cell is divided into sectors by using directional antennas. Common sectoring methods divide cells into 3 or six sectors.
See figure 3.10,11
Advantages:
The S/I ratio increases because interference is received from only 1 direction rather than all directions. This makes it possible for cluster size to be reduced, allowing more channels to be allocated to each cell.
Drawbacks:
Decreased trunking efficiency due to fewer channels per sector. Doesn’t work well in high density urban areas due to reflections.
Cost of Implementation:
Increased complexity at base station due to additional antennas.
How is it implemented:
Consists of a single base station with a distributed antenna system.
Advantages:
Only one base station needed. Because the system is distributed, less power is needed per antenna, and cells can be closer together without increasing SIR. The cluster size can be reduced, making it possible to allocate more channels to each cell. Handoff between microcells within the cell is handled by the base station, reducing the load on the MSC.
Drawbacks:
Base stations are more complex as they must coordinate multiple antennas.
Cost of Implementation:
Antennas are connected to the base station via coaxial cable, fiber optics, or microwave link. Many antenna sights are needed.
How is it implemented:
A repeater is placed in a location that amplifies weak radio signals. The repeater can be placed at the entrance to a tunnel for tunnel coverage, or at a mountaintop, to service a valley on the other side.
Advantages:
Allows cell phone use in otherwise inaccessible areas without the additional cost of a new base station.
Drawbacks:
Doesn’t increase system capacity.
Cost of Implementation:
A repeater has to be placed at the desired location.