There is a performance criterion of cellular mobile systems like:
a) Voice quality.
b) Service Quality like coverage and quality of service.
c) Number of Dropped calls.
d) Special features like call forwarding, call diverting, call barring.
As the demand for wireless service increases, the number of channels assigned to cell becomes insufficient to support required number of users.
At this point, cellular design techniques are needed to provide more channels per unit coverage area.
There are 3 techniques for improving cell capacity in cellular system, namely:
- Cell Splitting.
- Coverage Zone Approach.
A) CELL SPLITTING:
- It is 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.
- Cell splitting increases capacity of cellular system since it increases number of times that channels are reused, it preserves frequency reuse plan.
- It defines new cells which have smaller radius than original cells and by installing these smaller cells called microcells between existing cells, that is radius will be half of the original cell.
- Thus capacity increases due to additional number of channels per unit area, but does not disturb the channel allocation scheme required to maintain the minimum co-channel reuse ratio Q between co-channel cells.
- This is another method to increase cellular capacity and coverage by keeping cell radius unchanged and decreasing D/R ratio.
- In this approach, capacity improvement is achieved by reducing the number of cells in a cluster and thus increasing the frequency reuse.
- The co-channel interference in a cellular system may be decreased by replacing a single Omni-directional antenna at the base station by several directional antennas, each radiating within a specified sector.
The factor by which the co-channel interference is reduced depends on the amount of sectoring used.
a) 1200 sectoring b) 600 sectoring
- Improvement in Signal capacity.
- Improvement in signal to interference ratio.
- Increases frequency reuse.
- Increase in number of handoffs.
- Increase in number of antenna at each base station.
C) COVERAGE ZONE/ MICROCELL ZONE CONCEPT:
- This approach was presented by Lee to solve the problem of an increased load on the switching and control link elements of the mobile system due to sectoring.
- It is based on a microcell concept for 7 cell reuse.
- In this scheme, each of the three zone sites are connected to a single base station and share the same radio equipment.
- Multiple zones and a single base station make up a cell. As a mobile travels within the cell, it is served by the zone with the strongest signal.
- 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.