cellular system_cell spliting & sectoring.pptx

cell splitting

1. Improving capacity and coverage Power control for interference reduction Cell splitting Sectoring Microcell Zone Use of repeaters

2. Power control to reduce interference In practice systems, the power level of every subscriber is under constant control by the serving BS. Power control not only reduces interference levels but also prolongs battery life. In CDMA spread spectrum systems, power control is a key feature to ensure maximal utilization of the system capacity. Reduced interference leads to higher capacity.

3. Improving capacity As the demand of service increases, system designers have to provide more channels per unit coverage area. Common techniques are: cell splitting, sectoring and microcell zoning. Cell splitting increases the number of BS deployed and allows an orderly growth of the cellular system. Sectoring uses directional antennas to further control interference and frequency reuse. Microcell zoning distributes the coverage of a cell and extends the cell boundary to hard to reach places

4. Cell Splitting Cell splitting is the process of subdividing a congested cell into smaller cells with Their own base stations A corresponding reduction in the antenna height A corresponding reduction in the transmission power Splitting the cells reduces the cell size and thus more number of cells have to be used. more number of cells=more number of clusters=more channels= higher capacity Cell splitting allows a system to grow by replacing large cells by small cells, without upsetting the channel allocation.

5. Cell Splitting Cells are split to add channels with no new spectrum usage. Depending on traffic patterns the smaller cells may be activated or deactivated in order to efficiently use cell resources.

6. Example 1

7. Example 2

8. Advantages of cell splitting : It improves the S/N ratio. It reduces interference which increases capacity. It reduces the cluster size. System capacity gradually expands as demand. Disadvantages of cell splitting : Handoff is more frequent. Channel assignment becomes difficult. All cell are not simultaneously so special care have to be taken for proper allocation of the problem.

9. a. The channel capacity for a cellular telephone are comprised of 7 macrocells with 11 channels per cell b. channel capacity if each Macrocell is split into 4 minicells c. channel capacity if each minicell is further split into 4 microcells Solution: a. To calculate the total system capacity = number of channels per cell x  number of cells.                                                     = 11 x 7 = 77 Hence, the channel capacity of a Macrocell system  = 77 channels/area b. Splitting each Macrocell into 4 minicells increase the total number of cells in the area to 4*7=28 11 channels x 28 cells = 308 channels/area c. further splitting each minicell into 4 microcells increases the total number of cells in the area to 4*28=112 Therefore, 11 channels*112 cells=1232 channels/area

10. Sectoring As opposed to cell splitting , where D/R is kept constant while decreasing R, sectoring keeps R untouched and reduces the D/R. Capacity improvement is achieved by reducing the number of cells per cluster, thus increasing frequency reuse. In order to do this, it is necessary to reduce the relative interference without decreasing the transmitted power.

11. The CCI maybe decreased by replacing the single omnidirectional antenna by several directional antennas, each reading within a specified sector. A directional antenna transmits to and receives from only a fraction of the total number of co-channel cells. Thus CCI is reduced. A cell is normally partitioned into three 120 ◦ sectors, four 90 ◦ sectors or six 60 ◦ sectors

12. Sectori n g A directional antenna transmits to and receives from only a fraction of total of the co-channel cells. Thus CCI is reduced 15

13. Problems with Sectoring 16 Increases the number of antennas at each BS Decrease in trunking efficiency due to sectoring(dividing the bigger pool of channels into smaller groups) Increase number of handoffs(sector-to sector) Many modern BS support sectoring and related handoff without help of MSC

14. Microcell Zone Concept 17 The Problems of sectoring can be addressed by Microcell Zone Concept A cell is conceptually divided into microcells or zones . Each microcell(zone) is connected to the same base station(fiber/microwave link) Doing something in middle of cell splitting and sectoring by extracting good points of both Each zone uses a directional antenna Each zone radiates power into the cell . MS is served by strongest zone As mobile travels from one zone to another, it retains the same channel , i.e. no hand off . The BS simply switches the channel to the next zone site .

15. Micro Zone Cell Concept 18

16. Microcell Zone Concept 19 Reduced Interference (Zone radius is small so small and directional antennas are used). Decrease in CCI improves the signal quality and capacity. No loss in trunking efficiency (all channels are used by all cells). No extra handoffs. Increase in capacity (since smaller cluster size can be used).

17. Repeaters for Range Extension 20 Useful for hard to reach areas Buildings Tunnels Valleys Radio transmitters called Repeaters can be used to provide coverage in these area Repeaters are bi-directional Rx signals from BS Amplify the signals Re-radiate the signals Received noise and interference is also re-radiated