Cellular wireless networks operate by dividing geographic areas into smaller sections called cells. Each cell contains a base station that can communicate with mobile devices within its cell. To increase network capacity, cells reuse the same set of radio frequencies by ensuring cells using the same frequencies are sufficiently distant from each other. This allows many frequencies to serve many cells simultaneously. The document discusses key concepts like frequency reuse, cell size, handoffs between cells, and increasing capacity through smaller cell sizes and cluster sizes.

1. CELLULAR WIRELESS
NETWORKS
Dr. Loai Bani Melhim 1

2. Introduction (1)
• In the early years of mobile radio systems, a large
coverage was achieved by using a single high-
powered transmitter with the antenna mounted on
tall tower.
• Although a large coverage could be attained by
this approach, it does not allow the reuse of the
same radio frequencies due to interference.
Dr. Loai Bani Melhim 2

3. Introduction (1)
Dr. Loai Bani Melhim 3

4. Introduction (2)
Cellular technology
• is the foundation of mobile wireless communications
• supports users in locations that are not easily served
by wired networks.
• is the underlying technology for
• mobile telephones,
• personal communications systems,
• wireless Internet and
• Wireless Web applications.
Dr. Loai Bani Melhim 4

5. Introduction (3)
Cellular technology is a technique
• that was developed to increase the capacity available for
mobile radio telephone service.
The way to increase the capacity of the system is
• to use lower power systems with shorter radius and
• to use numerous transmitters/ receivers.
Dr. Loai Bani Melhim 5

6. Principles of Cellular Networks
• Cellular Network Organization
• Frequency Reuse
• Increasing Capacity
• Operation of Cellular Systems
• Mobile Radio Propagation Effects
• Handoff
• Power Control
• Traffic Engineering
Dr. Loai Bani Melhim 6

7. Principles of Cellular Networks
• Cellular Network Organization
• Frequency Reuse
• Increasing Capacity
• Operation of Cellular Systems
• Mobile Radio Propagation Effects
• Handoff
• Power Control
• Traffic Engineering
Dr. Loai Bani Melhim 7

8. Cellular Network Organization (1)
• Use multiple low-power transmitters (100 W or
less)
• Areas divided into cells
• Each served by its own antenna
• A Band of frequencies allocated to each cell.
• Each cell is served by a Base Station (BS), consisting of
transmitter, receiver and control unit.
• Adjacent cells are assigned different frequencies to
avoid interference or crosstalk.
• Cells sufficiently distant from each other can use the
same frequency band.
Dr. Loai Bani Melhim 8

9. Cellular Network Organization (2)
• To accommodate a
large number of users
over a large geographic
area, the cellular
telephone system uses
a large number of low-
power wireless
transmitters to create
cells.
Dr. Loai Bani Melhim 9

10. Cellular Network Organization (3)
Dr. Loai Bani Melhim
• Channels
(frequencies)
used in one cell
can be reused in
another cell some
distance away,
which allows
communication by
a large number
stations using a
limited number of
radio frequencies.
Cellular telephone system with a number of
low-power wireless transmitters to create
cells
10

11. Cellular Network Organization (2)
Dr. Loai Bani Melhim 11

12. Cellular Network Organization (3)
Dr. Loai Bani Melhim 12

13. Cell shape
Criteria/ recommendations
• Antennas in all adjacent cells must be equidistant
(hexagonal pattern)
• This simplifies the task of
• Determining when to switch the user to another antenna
• Choosing another antenna
• Hexagonal pattern
• Provides for equidistant antennas
• Radius of hexagon=length of side of hexagon= R
• Distance b/w cells, d = (3/2)R + (3/2)R = 3R
Dr. Loai Bani Melhim
NOTE: A precise hexagonal pattern is not used
13

14. Cell
Each cellular base station is allocated a group of radio
channels to be used within a small geographic area called
a cell.
Dr. Loai Bani Melhim 14

15. Principles of Cellular Networks
• Cellular Network Organization
• Frequency Reuse
• Increasing Capacity
• Operation of Cellular Systems
• Mobile Radio Propagation Effects
• Handoff
• Power Control
• Traffic Engineering
Dr. Loai Bani Melhim 15

16. Frequency Reuse OR Frequency
Planning
Frequency Reuse:
The same frequency band or channel used in a cell can be
reused in another cell as long as they are far a part.
Object:
to increase coverage and capacity
Dr. Loai Bani Melhim 16

17. FREQUENCY REUSE CONCEPT
• Cellular telephone
systems rely on an
intelligent allocation and
reuse of channels.
• Each base station is
given a group of radio
channels to be used
within a cell. Base
stations in the
neighboring cells are
assigned completely
different set of channel
frequencies.
Dr. Loai Bani Melhim 17

18. FREQUENCY REUSE CONCEPT
• Cells with the same
number use the same
set of frequencies,
called reusing cells or
N cells.
Dr. Loai Bani Melhim
• N cells which collectively use the available frequencies
are known as cluster S (S = k.N). If a cluster is replicated
M times within a system, then the total number of duplex
channels or (capacity, C)
C = M.k.N= M.S.
18

19. FREQUENCY REUSE CONCEPT
Dr. Loai Bani Melhim
• N cells which
collectively use the
available
frequencies are
known as cluster S
(S = k.N).
• If a cluster is
replicated M times
within a system,
then the total
number of duplex
channels or
(capacity, C)
C = M.k.N= M.S.
19

20. FREQUENCY REUSE CONCEPT
Dr. Loai Bani Melhim 20

21. Frequency Reuse
• Adjacent cells assigned different frequencies to
avoid interference or crosstalk
• It is not practical to attempt to use same
frequency band in two adjacent cells except
CDMA, (Code Division Multiple Access)
• Objective is to reuse frequency in nearby cells
• 10 to 50 frequencies assigned to each cell
• Transmission power controlled to limit power at that
frequency escaping to adjacent cells
• The issue is to determine how many cells must
intervene between two cells using the same frequency
 So that the cells do not interfere.
Dr. Loai Bani Melhim 21

22. Possible Solution (1)
We define N.
• N is number of cells in a repetitious pattern.
• In a hexagonal cell pattern, only the following values of N
are possible
• N = I2 + J2 + (I  J), I,J = 0,1,2,3,…
• Possible values of N are 1,3,4,7,9,12,13,16,19,21, and so
on
Dr. Loai Bani Melhim 22

23. Dr. Loai Bani Melhim 23

24. Dr. Loai Bani Melhim 24

25. The Cellular Concept
Frequency Reuse Pattern for N = 7
Dr. Loai Bani Melhim
[Rappa P-59]
Frequency Reuse Factor = 1/7
25

26. Dr. Loai Bani Melhim 26

27. Possible Solution (2)
Example:
• For AMPS (American Advanced Mobile Phone System)
• Total number of frequencies allotted to the system = K = 395
• N = 7
• Each cell can have K/ N = 57 frequencies per cell on average
Dr. Loai Bani Melhim 27

28. Possible Solution (3)
If
• D = minimum distance between centers of cells that
use same frequency band (called co-channels)
• R = radius of a cell
• d = distance b/w cells = 3R
• Since d = 3R
N
R
D
Q 3
Ratio
Reuse
Channel
-
Co 


N
d
D

Dr. Loai Bani Melhim
[Rappa P-60]

29. To Find Nearest Co-Channel
To find nearest co-channel neighbors of a particular cell,
one must do the following
• Move i cells along any chain of hexagons and then
• Turn 60 degrees counter-clockwise and move j cells.
• This is illustrated in figure 3.2 for i = 3, and j = 2 (example,
N = 19)
Dr. Loai Bani Melhim
[Rappa P-60]
29

30. 19-cell reuse example (N=19)
Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., I = 3,
j = 2). (Adapted from [Oet83] © IEEE.)
Dr. Loai Bani Melhim
[Rappa P-60]
Dr. Abid
Ali
Minhas

31. Cluster size and Capacity (1)
• Let a cellular system has a total of S duplex
channels available for use.
• If each cell is allocated a group of k channels (k <
S), and
• If the S channels are divided among N cells into
unique and disjoint channel groups which each
have the same number of channels,
Total number of available radio channels S = k N
Dr. Loai Bani Melhim
[Rappa P-58-60]
31

32. Cluster size and Capacity (2)
• CLUSTER: The N cells which collectively use the
complete set of available frequencies is called a
cluster.
• Measure of capacity: If a cluster is repeated M
times within the system, the total number of
duplex channels, C, can be used as a measure of
capacity and is given by
C = M k N = M S
• From above, “the capacity of a cellular system”, is
directly proportional to the number of times a
cluster is replicated in a fixed service area.
Dr. Loai Bani Melhim
[Rappa P-58-60]
32

33. Cluster size and Capacity (3)
If the cluster size N is reduced while the cell size is
kept constant,
• more clusters are required to cover a given area,
and
• hence more capacity (a large value of C) is
achieved.
• A larger cluster size causes the ratio b/w the cell
radius and the distance b/w co-channel cells
(R/D) to decrease, leading to weaker co-channel
interference.
Dr. Loai Bani Melhim 33

34. Cluster size and Capacity (4)
• Conversely, a small cluster size indicates that co-
channel cells are located much closer together.
• RESULT: Smaller N, gives max C but more
interference.
• The value for N is a function of how much
interference a mobile or base station can tolerate
while maintaining a sufficient quality of
communications.
• From a design point of view, the smallest possible
value of N is desirable in order to maximize
capacity over a given coverage area.
Dr. Loai Bani Melhim 34

35. Cluster size and Capacity (5)
Problem
For an area of 1000 m2, total of 100 duplex channels are allocated. By
keeping cell size constant equal to 20 m2, calculate capacity C for
following cluster sizes.
• N = 4
• N = 8
• Solution (N=4)
• No of total cells = 1000/20 = 50 cell
• When N = 4, M = 50/4 = 12.5 which means that the total number of
clusters M = 13
• C= 100* 13 = 1300 channels.
•
• Solution (N=8)
• No of total cells = 1000/20 = 50 cell
• When N = 8, M = 50/8 = 6.25 which means that the total number of
clusters M = 7
• C= 100* 7 = 700 channels.
Dr. Loai Bani Melhim 35

36. Cellular Systems Terms (1)
Frequency Reuse Factor: of a cellular system is
given by 1/N, since each cell within a cluster is
only assigned 1/N of the total available channels
in the system.
Cluster Size: The factor N is called the cluster
size.
Footprint: The actual radio coverage of a cell is
known as the footprint and is determined from
field measurements or propagation prediction
models.
Dr. Loai Bani Melhim
[Rappa P-58-60]
36

37. Cellular Systems Terms (2)
• Mobile Stations (MS): Mobile handsets, which is used by any user to
communicate with any other user.
• Mobile Switching Center (MSC): Each base station is controlled by a
switching office, called mobile switching center.
• Base Station (BS) – includes an antenna, a controller, and a
number of receivers
• Mobile telecommunications switching office (MTSO) –
connects calls between mobile units
• Two types of channels available between mobile unit and BS
• Control channels – used to exchange information having to
do with setting up and maintaining calls
• Traffic channels – carry voice or data connection between
users
Dr. Loai Bani Melhim
[Rappa P-58-60]
37

38. Position of
Base Station Transmitters
• When using hexagons to model coverage areas, BS
transmitters are depicted as either being in the center of
the cell (center-excited cells) or on three of the six cell
vertices (edge-excited cells).
• Normally, omni-directional antennas are used in center-
excited cells and sectored directional antennas are
used in edge-excited cells.
• Practical considerations do not allow BS to be placed
exactly as they appear in the hexagonal layout.
• Most system designs permit a BS to be positioned up to
one-fourth the cell radius away from the ideal location.
Dr. Loai Bani Melhim
[Rappa P-58-60]
38

39. Equations
• consider a cellular system which has a total of S duplex channels
available for use. If each cell is allocated a group of k channels
(k < S), and if the S channels are divided among N cells into unique
and disjoint channel groups which each have the same number of
channels, the total number of available radio channels can be
expressed as
• Equation 3.1 S = k N
• The N cells which collectively use the complete set of available
frequencies is called a cluster. If a cluster is replicated M times within
the system, the total number of duplex channels, C, can be used as a
measure of capacity and is given by
• Equation 3.2 C = MkN = MS
Dr. Loai Bani Melhim
[Rappa P-61]
39

40. Equations
• In order to connect without gaps between adjacent cells (tessellate)
the geometry of hexagons is such that the number of cells per cluster,
N, can only have values which satisfy Equation (3.3).
• Equation 3.3 N = i^2 + ij + j^2 where i and j are non-negative
integers.
• Check the next slide for illustrating example
Dr. Loai Bani Melhim
[Rappa P-61]
40

41. Example
• To find the nearest co-channel neighbors of a particular cell, one must
do the following: (1) move i cells along any chain of hexagons and
then (2) turn 60 degrees counter-clockwise and move j cells.
• Method of locating co-channel cells in a cellular system. In this
example, N = 19 (i.e., i = 3, j = 2).
Dr. Loai Bani Melhim 41

42. Example
If a total of 33 MHz of bandwidth is allocated to a particular
FDD (Frequency Division Duplex, is the first variation of
W-CDMA ) cellular telephone system which uses two
25 KHz simplex channels to provide full duplex voice
and control channels, compute the number of channels
available per cell if a system uses
a) four cell reuse
b) seven cell reuse and
c) Twelve cell reuse
If 1 MHz of the allocated spectrum is dedicated to
control channels, determine an equitable distribution of
control channels and voice channels in each cell for
each of the three systems.
Dr. Loai Bani Melhim
[Rappa P-61]
42

43. Solution1
Total bandwidth = 33 MHz
Channel bw = 25 kHz × 2 simplex channels = 50 kHz/duplex channel
Total available channels = 33,000/50 = 660 channels
a) For N = 4, total number of channels available per cell = 660/4 =
165 channels.
b) For N = 7, total number of channels available per cell = 660/7 = 95
channels.
c) For N = 12, total number of channels available per cell = 660/12 =
55 channels.
Dr. Loai Bani Melhim
[Rappa P-61]
43

44. Solution 2
A 1 MHz spectrum for control channels implies that there are 1000/50 =
20 control channels out of the 660 channels available. To evenly
distribute the control and voice channels, simply allocate the same
number of voice channels in each cell wherever possible. Here, the 660
channels must be evenly distributed to each cell within the cluster. In
practice, only the 640 voice channels would be allocated, since the
control channels are allocated separately as 1 per cell.
a)For N = 4, we can have five control channels and 160 voice channels
per cell. In practice, however, each cell only needs a single control
channel (the control channels have a greater reuse distance than the
voice channels). Thus, one control channel and 160 voice channels
would be assigned to each cell.
Dr. Loai Bani Melhim
[Rappa P-61]
44

45. Solution 3
b) For N = 7, four cells with three control channels and 92 voice
channels, two cells with three control channels and 90 voice
channels, and one cell with two control channels and 92 voice
channels could be allocated. In practice, however, each cell would
have one control channel, four cells would have 91 voice channels,
and three cells would have 92 voice channels.
c) For N = 12, we can have eight cells with two control channels and
53 voice channels, and four cells with one control channel and 54
voice channels each. In an actual system, each cell would have
one control channel, eight cells would have 53 voice channels, and
four cells would have 54 voice channels.
Dr. Loai Bani Melhim
[Rappa P-61]
45