EENG473 Mobile Communications Module 2: Week # (4) : The Cellular Concept - System Design Fundamentals

EENG473 Mobile Communications
Module 2 : Week # (4)
The Cellular Concept –

System Design Fundamentals
2.2 Frequency Reuse
•  Cellular radio systems rely on an intelligent allocation
and reuse of channels throughout a coverage region.
•  Base stations in adjacent cells are assigned channel groups

which contain completely different channels than
neighboring cells. By limiting the coverage area to within
the boundaries of a cell
•  A Cell : is a small geographic area within which each

cellular base station is allocated a group of radio channels
to be used.
•  Frequency reuse or frequency planning : The design

process of selecting and allocating channel groups for all of
the cellular base stations within a system. © 2007 by M.A.Mangoud
The Cellular Concept
© 2016 by M.A.Mangoud
Why Hexagonal model ?(1)
The Footprint: The actual radio coverage of a cell and is
determined from field measurements or propagation
prediction models. Although Real footprint is formless in
nature, a regular cell shape is needed for systematic system
design.
Why circle can not be used to represent the coverage area of a

base station? because adjacent circles can not be overlaid
upon a map without leaving gaps or creating overlapping
regions.
Thus, when considering geometric shapes which cover an

entire region without overlap and with equal area, there are
three sensible choices:
a square; an equilateral triangle; and a hexagon.
Why Hexagonal model ?(2)
(1) A cell must be designed to serve the weakest mobiles
within the footprint, and these are typically located at the
edge of the cell. For a given distance between the center of
a polygon and its farthest perimeter points, the hexagon
has the largest area of the three.
(2) By using the hexagon geometry, the fewest number of

cells can cover a geographic region,
(3) The hexagon closely approximates a circular radiation

pattern which would occur for an omni-directional base
station antenna and free space propagation.
Why Hexagonal model?(3)
The hexagonal cell shape has been universally adopted since :
•  conceptual and is a simplistic model of the radio coverage

for each base station.
•  the hexagon permits easy and manageable analysis of a

cellular system.
•  are attractive shapes since hexagons can be fitted together

without gaps or overlap so that an area of arbitrary size is
completely covered by a set of hexagons.
•  In addition, a hexagon is a relatively good approximation

to a circle (the locus of constant received signal power).
Cells and Clusters
•  The following viewgraph illustrates an area covered by hexagons.

•  sets of N=7 hexagons are grouped together.
•  These groups are referred to as clusters.
•  In the following viewgraph clusters are designated by the heavy

lines. (There are 7 complete clusters.)
•  The complete “system” is composed of multiple clusters.
•  Corresponding cells within clusters share a given system resource

such as frequency slots, time slots, or code sequences.
What are appropriate cluster sizes?
there are only certain cluster sizes and cell layouts which are possible.
Acceptable cluster sizes are defined by the following expression
(N, can only have values which satisfy) :
where i and j are integers.

In many of the examples we will assume cluster sizes defined by
N = 7. For the N = 7 case, i = 2 and j = 1.
19-cell reuse example (N=19)
i= 3 and j = 2 (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.)
•  A large cluster size (N) indicates that the ratio between the cell radius
and the distance between co-channel cells is large.
•  Conversely, a small cluster size (N) indicates that co-channel cells are
located much closer together.
•  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 viewpoint, the smallest possible value of N

is desirable in order to maximize “C”.
The 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.
Example 2.1
If a total of 33 MHz of bandwidth is allocated to a particular
FDD cellular telephone system which uses two 25 kHz simplex
channels to provide full duplex voice and control channels,
(1) compute the number of channels available per cell if a

system uses
(a) 4-cell reuse, (b) 7-cell reuse (c) 12-cell reuse.
If 1 MHz of the allocated spectrum is dedicated to control

channels,
(2) determine an equitable distribution of control channels
and voice channels in each cell for each of the three
systems.
Total bandwidth 33 MHz
Channel BW=25 kHz: 2 simplex channels=50 kHz/duplex channel
Total available channels = 33,000/50 = 660 channels
(a)ForN= 4,
total number of channels available per cell 660/ 4 : 165 channels.
(b)ForN= 7,
total number of channels available per cell = 660/ 7 : 95 channels.
(c)ForN= 12,
total number of channels available per cell = 660 /12 : 55 channels.
A 1 MHz spectrum for control channels implies that there
are 1000K/50K = 20 control channels out of the 660
channels available.
To evenly distribute the control and voice channels,

simply allocate the same number of channels in each
cell wherever possible. Here, the 660 channels must be
evenly distributed 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,
4cells : (5 control channels + 160 voice channels)
In practice, however, each cell only needs a single control channel
(20 channel), 4 cells : (160 voice channels). Total = 640
(b) For N = 7,
4 cells :(3 control channels + 92 voice channels),
+ 2 cells : (3 control channels + 90 voice channels),
+ 1 cell :(2 control channels + 92 voice channels)
In practice, however, each cell would have one control channel,
4 cells :( 91 voice channels),
+ 3 cells : (92 voice channels). Total = 640
(c) For N = 12,

8 cells :(2 control channels + 53 voice channels),
+ 4 cells :(1 control channels + 54 voice channels),
In practice, however, each cell would have one control channel,
8 cells :(53 voice channels),
+ 4 cells :(54 voice channels). Total = 640 © 2016 by M.A.Mangoud

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EENG473 Mobile Communications

Module 2 : Week # (4)

The Cellular Concept –

• Base stations in adjacent cells are assigned channel groups

• A Cell : is a small geographic area within which each

• Frequency reuse or frequency planning : The design

Why circle can not be used to represent the coverage area of a

Thus, when considering geometric shapes which cover an

(2) By using the hexagon geometry, the fewest number of

(3) The hexagon closely approximates a circular radiation

• conceptual and is a simplistic model of the radio coverage

• the hexagon permits easy and manageable analysis of a

• are attractive shapes since hexagons can be fitted together

• In addition, a hexagon is a relatively good approximation

• The following viewgraph illustrates an area covered by hexagons.

• These groups are referred to as clusters.

• In the following viewgraph clusters are designated by the heavy

• The complete “system” is composed of multiple clusters.

• Corresponding cells within clusters share a given system resource

where i and j are integers.

• The value for N is a function of how much interference a mobile or

• From a design viewpoint, the smallest possible value of N

(1) compute the number of channels available per cell if a

If 1 MHz of the allocated spectrum is dedicated to control

To evenly distribute the control and voice channels,

In practice, only the 640 voice channels would be

(c) For N = 12,

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•  Base stations in adjacent cells are assigned channel groups

•  A Cell : is a small geographic area within which each

•  Frequency reuse or frequency planning : The design

•  conceptual and is a simplistic model of the radio coverage

•  the hexagon permits easy and manageable analysis of a

•  are attractive shapes since hexagons can be fitted together

•  In addition, a hexagon is a relatively good approximation

•  The following viewgraph illustrates an area covered by hexagons.

•  These groups are referred to as clusters.

•  In the following viewgraph clusters are designated by the heavy

•  The complete “system” is composed of multiple clusters.

•  Corresponding cells within clusters share a given system resource

•  The value for N is a function of how much interference a mobile or

•  From a design viewpoint, the smallest possible value of N