It describes the basic aspects of 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
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2. Outline
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 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
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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
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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
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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.
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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
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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
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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
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10. Illustration of Handoff Process
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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
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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
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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
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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
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15. •CELL SPLITTING
•CELL SECTORING
Increasing Capacity
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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
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17. Illustration of Cell Splitting
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18. Cell Sectoring
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 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
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Illustration of Cell Sectoring

20. THANK YOU
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