Answer:

In any coverage area, the source of income increases for the service providers as increases the number of subscriber.  Therefore to increase the number of subscriber,service provider used capacity expansion technique which is used for expands the cellular network capacity. There are basically four methods to expand the capacity of cellular telephone networks that are by obtain additional spectrum for new subscribers, change the cellular architecture including cell splitting and sectoring, change the frequency allocation methodology and to change the modem and access technology.

For cellular systems, it is not always possible to counter the increasing demand by increasing the coverage area because of the limitations in obtaining new land with suitable requirements. So there are three methods for dealing with an increasing subscriber capacity that are:

• Cell Splitting and
• Cell Sectoring
• Micro-cells zone

Cell splitting: This is a technique which has the capability to add new smaller cells in specific areas of the system. It is based on the cell radius reduction and minimizes the need to modify the existing cell parameters. It involves the process of sub-dividing a congested cell into smaller cells where each cell with its own base station and a corresponding reduction in antenna size and transmitting power. This increases the capacity of a cellular system since it increases the number of times that channels are reused. There are few challenges in increasing the capacity by reducing the cell radius where the first challenge is to add the new base stations without moving the existing base station units and the other challenge is to meet the increasing demand that may vary between coverage areas of the system. In cellular system, when traffic density starts to grow and frequency channels in each cell cannot provide enough calls then original cells split into smaller cells where the new radius is the half of the original. Then each new cell carries equal and same traffic load. Thus the new traffic load becomes 4 times the old traffic load of the station.There are two types of cell splitting techniques that are permanent splitting and dynamic splitting. These splitting techniques allow orderly growth of the cellular system.

Permanent splitting : This technique is planned ahead of time. All the designing aspects like channel allocation, transmitting power, traffic load has to be fully completed in advance. So we can say that this is the fixed one techique which doesnot entertain any changes.

Dynamic splitting: All the designing and planning aspects are done in practical field in this technique. There will be no ideal or real channel in cell site.

Cell sectoring: It is a technique which can increase the SIR without necessitating an increase in the cluster size. In any coverage area, let us assumed that the base station is located in the center of a cell and radiates uniformly in all the directions behaving as an omni - directional antenna. However, it has been found that 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 where each antenna radiating within a specified sector. This technique for reducing co-channel interference wherein by using suitable directional antennas, a given cell would receive interference and transmit with a fraction of available co-channel cells is called ’sectoring’.The factor by which the co-channel interference is reduced depends on the amount of sectoring used. A cell is normally portioned into three 120º sectors as shown in fig. (a) or six 60º sectors as shown in fig.(b)

Frequency reuse in cellular system implies that in a given coverage area there are several cells that use the same set of frequencies. These calls are called co-channel cells and the interference between signals from these cells is called co-channel interference.The main concept of frequency reuse means more users can be served in a same cell.

Here, S = No. of duplex channels in the cellular system

               K = No. of channels per cell

               N = No. of cells (4,7 or 12)

               M = No. of times a cluster is repeatedly used

Available channels are grouped in blocks and assigned to cells

   $\large K= \dfrac {S} {N}$

The system capacity (total no. of channels) in the system is

  $\large {C= MS =\ MNK}$

If the cluster size, N, is reduced while the cell size is constant, more clusters, M, are required and thus the greater the capacity. But, the smaller, N, the smaller the frequency re-use distance and the higher the interference.The frequecny reuse factor of a cellular system is given by:

$\large\dfrac{1}{N}$

means each cell within a cluster is only assigned 1/N of the total available channels in the system.

Microcell zone: In cellular system if sectoring is employed then the requirement of  number of handoff  are increased which also increased the load on switching and control link elements. This problem is overcome by using the concept new microcell zone. All microcells operates within the cell at the same frequency which reduces the interference.The new microcell knows where to locate the mobile unit in a specified zone of the cell and then deliver the power to that zone which increase the system capacity .The microzone site equipments are always being small that can be mounted on the side of buildings.

Cellular capacity and coverage improvement can be done through four major techniques they are

1. Cell Splitting: It is the technique of splitting the congested cells (macrocells) into microcells to increase the capacity and coverage. The umbrella technique is used. It is used to increase the frequency of use.
2. Cell Sectoring: It uses multiple directional antennas instead of using a single Omni-directional antenna. The reuse ratio has an impact on the capacity of the cells. S/I ratio varies proportionally to the Reuse ratio.
3. Repeaters: Repeater is also called a Re-transmitter. It is used for the extension of coverage.
4. Micro Cell Zones: This is based on using the sectored cells and uses Lee's MicroCell Zone concept.

Improving capacity is important because it helps in increasing the number of channels (or a number of users).

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