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- With the demand for wireless service increases, the number of channels assigned to a cell 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.
- Techniques discussed below are used in practice to expand the capacity of cellular system.
A. Cell splitting:
- Cell splitting is the process of sub dividing a congested cell into smaller cells, each with its own base station and corresponding reduction in antenna height and transmitted power.
- Cell splitting increases capacity of a cellular system since it increases number of times that channels are reused.
- By defining new cells which have a smaller radius than the original cells and by installing these smaller cells between existing cells, capacity increases due to additional number of channels per unit cell area
- In this D/R ratio is kept constant and entire system is rescaled.
B. Cell sectoring:
- In cell sectoring a single omnidirectional antenna at base station is replaced by several directional antennas, each radiating within a specified sector.
- By using directional antennas power is transmitted in single desired direction decreasing number of interfering co-channel cells and co-channel interference.
- The technique for decreasing co-channel interference and thus increasing system performance by using directional antennas is called sectoring.
- The factor by which the co-channel interference is reduced depends on the amount of sectoring used. A cell is normally partitioned into three 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.
- For cluster size 7, sectoring reduces co-channel cells from 6 to 2 for $120^0$ sectoring and to 1 for $60^0$ sectoring.
- Improvement in S/I ratio.
- Improvement in system capacity.
- Increased number of antennas at base station.
- Decrease in trunking efficiency due to channel sectoring at the base station.
- Increase in number of handoffs, since sectoring reduces the coverage area of the particular group of frequencies.
C. Microcell zone concept:.
- The increased number of hand off, increase load on the switching and control link because of sectoring. A solution to this problem is given by microcell zone concept
- Large control base station is replaced by several lower power transmitters on the age of cell.
- The mobile retains the same channel and the base station simply switches the channel to a different zone site and the mobile moves from zone to zone.
- Since a given channel is active only in a particular zone in which mobile is travelling, base station radiation is localized and interference is reduced.
- The advantage of zone cell technique is that while the cell maintains a particular coverage radius, co-channel interference in the cellular system is reduced. As the large central base station is replaced by several lower power transmitters on ages of cell. Decreased co-channel interference improves signal quality leads to increase in capacity without degradation in trunking efficiency caused by sectoring.
- Decrease co-channel interference which leads to an improvement in signal quality and also leads to an increase in capacity.
- No degradation of trunking efficiency.
- More antennas are required.
- Base station need to be more sophisticated to handle transfer of call from one zone to another zone within the cell.