Cellular Concept
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This presentation provides an overview of the cellular concept and key related topics: - Cells are small geographical service areas defined by a base station and radio channels. Multiple cells are grouped into clusters to fully utilize available frequencies through frequency reuse. - Handoff is the process of transferring voice and control signals between cells as a mobile moves between cells during a call. Successful and infrequent handoffs are important. - Interference is reduced through frequency reuse and strategies like cell splitting and sectoring. Cell splitting divides cells into smaller areas served by low-power base stations to increase channel reuse and capacity. Sectoring uses directional antennas to reduce interference from co-channel cells.
Cellular Concept
- 1. PRESENTED BY: DEEPTANU DATTA ROLL NO. 1811EE05 A PRESENTATION on Cellular Concept Guided by : Dr. Sudhan Majhi Associate Professor Department of Electrical Engineering IIT Patna Indian Institute of Technology Patna 1
- 2. Outline 2 Introduction Cells and Clusters Frequency Reuse Channel Assignment Strategies Handoff Handoff Processing Interference Co-channel and Adjacent Channel Signal to Interference Ratio Cell Splitting and Sectoring
- 3. Introduction In older days, large coverage area was achieved by using high power antenna in a tall tower Multiple use of the same frequency was impossible since reuse would lead to interference Cellular Concept was a major breakthrough in solving the problem of limited spectrum and user capacity It works on the principle of frequency reuse Single high-power tall antenna was replaced by multiple low power short antennas 3
- 4. Cells and Clusters Cell is a small geographical area consisting of a base station and a group of radio channels Cluster is a group of cells that collectively use the complete set of available frequencies Number of cells in a cluster : N = i2 + ij + j2 Capacity : C = kNM = sM s = total number of channels available = kN k = number of channels in each cell M = number of clusters in the cellular system 4
- 5. Frequency Reuse• Base stations in adjacent cells are assigned set of channels of different frequencies to reduce interference • Radio channel signal strength reduces exponentially with distance • So, the same set of channels can be used again in a far-away cell so that interference is minimum • Frequency Reuse is this use of the same set of frequency for minimum interference • It leads to increase in the capacity of the system 5
- 6. Channel Assignment Strategies Fixed : Each cell is allocated a fixed number of voice and control channels. Drawback : If all the channels of a cell are fully occupied, then call is terminated even if neighbouring cell has some unoccupied channels Borrowing : Cell can borrow channels from neighbouring unoccupied cells. Chance of call blocking is less Dynamic : Assigned channels for the cells are not fixed. Base station requests a channel from the MSC when call is placed. 6
- 7. T R A N S F E R R I N G O F V O I C E A N D C O N T R O L S I G N A L S F R O M O N E C E L L T O A N O T H E R W H E N M O B I L E M O V E S T O D I F F E R E N T C E L L D U R I N G T H E C O N V E R S A T I O N • T W O T Y P E S : S O F T A N D H A R D Handoff 7
- 8. Handoff Processing Handoff must be performed successfully and as infrequently as possible A particular signal strength level is set as the minimum signal strength level for reliable voice communication Then, threshold at which handoff occurs is set at slightly higher level than reliable communication Difference between these two should be set very carefully 8
- 9. Handoff Processing (Contd….) Δ = Pr,handoff – Pr,minimum usable Pr,minimum usable = minimum signal strength level at which reliable communication occurs Pr,handoff = signal strength level at which handoff is processed If Δ is very high, unnecessary handoffs occur, which over burdens the MSC If Δ is very low, there is a chance of call termination due to insufficient time to process handoff So, an optimum value of Δ should be chosen 9
- 10. Illustration of Handoff Process 10
- 11. Interference in cellular system Mainly due to the following four factors : Another cell in the system A call in progress in a neighbouring cell Other base stations using the same set of frequency Another non-cellular system leaking energy in the cellular frequency band Two types : Co-channel and Adjacent Channel 11
- 12. Co-channel Interference Interference from cells using same set of frequencies Co-channel cells should be placed as far as possible Q = Co-channel Reuse/Interference Ratio D = Distance between centres of nearest co-channel cells R = Radius of the cell Large Q reduces co-channel interference & voice/ call quality is improved Small Q increases co-channel interference, but capacity is increased ‘N’ reduces so more number of channels are available per cell N R D Q 3 12
- 13. Adjacent Channel Interference Interference from adjacent frequency signals Effect of imperfect receiver filtering, which leaks nearby frequencies into the desired passband Minimized by careful filtering and channel assignments Frequency separation between channels of a cell should be as high as possible 13
- 14. Signal to Interference Ratio It is a kind of SNR to measure level of co-channel interference SIR is given by : - S = signal power received from serving base station Ii = Received signal power from ith co-channel cell io = number of co-channel interfering cells 0 1 i i iI S SIR 14
- 15. •CELL SPLITTING •CELL SECTORING Increasing Capacity 15
- 16. Cell Splitting Sub-dividing a congested cell into smaller cells (microcells) each with its own base station and corresponding reduction in antenna height and signal power Increases capacity by increasing the number of times the channels are reused When a cell is splitted into microcells, number of cells are increased Increased number of cells increases number of clusters over the coverage region As cell size is reduced, each cell can now use more number of channels, so capacity increases It increases the number of handoff 16
- 17. Illustration of Cell Splitting 17
- 18. Cell Sectoring 18 Channels in a cell are broken down into sectored groups by directional antenna, reducing co-channel interference SIR is increased by directional antennas in a sector By using directional antennas, a given cell receives interference from only a fraction of the available co- channel cells. This is called sectoring For 120° sectoring, number of co-channel cells reduced from 6 to 2 For 60° sectoring, number of co-channel cells reduced from 6 to 1
- 19. 120° Sectoring 60° Sectoring 19 Illustration of Cell Sectoring
- 20. THANK YOU 20