Mobile communication concepts

Mobile Communications

Prepared by:

R – THANDAIAH PRABU M.E.,
LECTURER / ECE

Mobile Communications 1

Frequency Reuse
Each cellular base station is allocated a
group of radio channels.
Base stations in adjacent cells are
assigned channel groups which contain
different channels than neighboring cells.

Cellular Frequency Reuse
Concept
Cells with the same letter, use
the same set of frequencies.
B
G C
A
A cell cluster is outlined F D
in bold, and replicated over E B
the coverage area. B G C
G C A
In this example, the
A F D
F D E
cluster size, N, is equal to 7; E
and the frequency reuse factor is 1/7,
since each cell contains 1/7 of the total number
of available channels.

Choices of Hexagonal Cell
Factors:

• Equal area
• No overlap between cells

Choices:
S
S S

A1 A2 A3

For a given S
A3 > A1
A3 > A2
Here, A3 provides maximum
coverage area for a given value of S.
Actual cellular footprint is determined by the
contour of a given transmitting antenna.
By using hexagon geometry, the fewest
number of cells covers a given geographic
region.

Frequency reuse
A
A
2 2
7 3 2
7 3
1
1 D7 3 A
6 4 1 A
6 4 A
5 5 6 4
5

A
A
D
= 3N
R
D = distance between cells using the same frequency
R = cell radius
N = reuse pattern (the cluster size, which is 7).

Thus, for a 7-cell group with cell radius R = 3 miles, the frequency reuse
distance D is 13.74 miles.

Channel Capacity
Let a cellular system have total of
S duplex channels for use.
If S channels are divided into N cells (in a cluster) into
unique and disjoint channel groups which each has
the same number of channels, total number of
available radio channels is:
S = KN
Where K is the number of channels / cell.

…Channel Capacity
If a cluster is replicated M times within
the system, the total number of
duplex channels, C, or the capacity, is

C = MKN = MS.
Cluster size N = 4, 7 or 12

Design of cluster size N
In order to connect without gaps between
adjacent cells (to tessellate)

N = i2 + ij + j2

Where i and j are non-negative integers

Example i = 2, j = 1

N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7

To Find the Nearest
Co-channel Neighbor of
Particular Cell:
• Move i cells along any chain
or hexagon.
• Then turn 60 degrees counterclockwise
and
move j cells.

How to Locate Co-channel
Cells in a Cellular System

A
In this example,
A N = 19
( i.e.,
i = 3,
A
j = 2)
A
A
Adapted from
[Oet83] © IEEE.

A
A

Example

If a particular FDD (frequency duplex) cellular telephone
system has a total bandwidth of 33 MHz,and if the phone
system uses two 25 KHz simplex channels to provide full
duplex voice and control channels...
compute the number of
channels per cell if
N = 4, 7, 12.

Solution
Total bandwidth = 33 MHz
Channel bandwidth = 25 KHz x 2 = 50 KHz
Total avail. channels = 33 MHz / 50 KHz = 660

N=4 Channel per cell = 660 / 4 =
165 channels

N=7 Channel per cell = 660 / 7 =
95 channels

N = 12 Channel per cell = 660 / 12 =
55 channels

Interference and
System Capacity

Major limiting factor in performance
of cellular radio systems - two main
types:
– Co-channel interference
– Adjacent channel interference

Co-Channel Interference
Cells that use the same set of
frequencies are called
co-channel cells.
Interference between
the cells is called
co-channel interference.

Co-Channel Interference
Signal to interference ratio
(SIR) or S/ I for a mobile
receiver is given by:

io
S/ I = SIR = S /( ∑ Ii)
i= 1

S = signal power from designated base station

First Tier of Co-channel Cells
for a Cluster Size of N = 7
When the mobile is at the cell boundary (point A),
it experiences worst case co-channel interference on

the forward channel.

The marked distances between the mobile and
different co-channel cells are based on
approximations made for easy analysis.

A
First Tier of
Co-Channel
Cells for a A
D+
A

Cluster Size R
D R
of N = 7 D+
Ii = Interference D- A R
power caused by A R D A
the ith interfering D-
co-channel cell R
A

Assumptions
For any given antenna
(base station) the power
at a distance d is given by:

Po Pr
d

-n
Pr = Po (d / do) ; n is path loss exponent

...Assumptions
io D
∑ (/ i )-n
Hence, S / I = R -n
i= 1

io = total number of first layer interfacing cells

If the mobile is at the center of the cell, Di = D
io
∑ 1
-n i= 1
-n -n
S/I =R / (D) = (R / D) /
io

For a hexagonal geometry

D / R =√(3N) = Q - co-channel
reuse ratio

S / I = [√(3N) ] n / io

Maximum co-channel interface –
when mobile is at cell boundary.
For N = 7

S / I~R-4 /
[ 2(D-R)-4+2(D+R)-4+ 2D-4]

Adjacent Channel Interference
• Interference resulting from
signals which are adjacent in
frequency to the desired signal.
• Due to imperfect receiver filters that allow nearby
frequencies to leak into pass band.
• Can be minimized by careful filtering and
assignments; and, by keeping frequency separation
between channels in a given cell as large as
possible, the adjacent channel interference may be
reduced considerably.

Channel Assignment Strategies:
Fixed Channel Assignments

• Each cell is allocated a pre-determined
set of voice channels.
• If all the channels in that cell are occupied, the call
is blocked, and the subscriber does not receive
service.
• Variation includes a borrowing strategy:
a cell is allowed to borrow channels from a
neighboring cell if all its own channels are
occupied. This is supervised by the MSC.

Channel Assignment Strategies:
Dynamic Channel Assignments

• Voice channels are not allocated to different
cells permanently.
• Each time a call request is made, the serving
base station requests a channel from the MSC.
• The switch then allocates a channel to the
requested call, based on a decision algorithm
taking into account different factors - frequency
re-use of candidate channel, cost factors.

...Channel Assignment
Strategies:
Dynamic Channel Assignments

Dynamic channel assignment is more
complex (real time), but reduces likelihood
of blocking.

Improving Capacity in
Cellular Systems

• As demand for wireless services increases,
the number of channels assigned to a cell is
not enough to support the required number of
users.

• Solution is to increase channels per unit
coverage area.

CELL SPLITTING
• The main aim of cellular mobile systems is to
improve utilization of spectrum efficiency

• The frequency reuse scheme is one concept,
and cell splitting is another concept.

• When traffic density is very high then each cell
cannot provide enough mobile calls. Then the
original cell can be split into smaller cells
resulting in cell splitting.

• Usually the new radius is one-half the original
radius.

Cell Splitting

• Subdivides a congested cell into smaller
cells, each with its own base station.
• Increases the capacity of a cellular
system.

Sectoring
• Achieves capacity improvement by essentially
rescaling the system.
• Cell radius R is unchanged but the
co-channel ratio D / R is decreased.
• Capacity improvement is achieved by reducing the
number of cells in a cluster, and this increases
frequency reuse.
• Replacing a single omni-directional antenna at base
station with several directional antennas, each
radiating within a specified sector.

Micro Cell Zone Concept
• Large control base station is replaced by
several lower powered transmitters on the
edge of the cell.
• The mobile retains the same channel and the
base station simply switches the channel to a
different zone site and the mobile moves from
zone to zone.
• Since a given channel is active only in a
particular zone in which mobile is traveling,
base station radiation is localized and
interference is reduced.

... Micro Cell Zone Concept
• The channels are distributed in time and
space by all three zones are reused in
co-channel cells.

• Advantage is that while the cell
maintains a particular coverage radius,
co-channel interference is reduced due
to zone transmitters on edge of the cell.

Microwave or
fiber optic link

Zone Selector
Zone Selector
Base
station
The Micro
Tx/R Tx/Rx Cell Concept
x
(Adapted from
[Lee91b] ©
Tx/R IEEE)
x

Antennas
• Antenna pattern, antenna gain, antenna
tilting, and antenna height all affect the
cellular system design
• Different antenna patterns and antenna
gains at the cell site and at the mobile
units would affect the system performance
and so must be considered in the system
design.
• Antenna tilting can reduce the interference
to the neighboring cells and enhance the
weak spots in the cell

Trunking and Grade of Service
• Cellular radio system relies on trunking to
accommodate a large number of users in a
limited radio spectrum - How a large population
can be accommodated by a limited number of
services.
• Trunking - each user is allocated a channel on a
per-call basis; and upon termination of the call,
the previously occupied channel is immediately
returned to the pool of available channels
• Initiated by Danish mathematician, Erlang.

Why 800 MHz frequency is selected for mobiles?
•
• Fixed Station Services - 30 MHz to 100 MHz
• Television Broadcasting - 41 MHz to 960 MHz
• FM Broadcasting - 100 MHz
• Air to Ground system - 118 MHz to 136 MHz
• Maritime mobile services - 160 MHz
• Military Aircraft use - 225 MHz to 400 MHz
• Frequency bands between 30 MHz to 400 MHz is crowded with large number of
services and above 10 GHz is not used due to propagation path loss, multipath
fading and improper medium due to rain activity. So 800 MHz is chosen for
mobile communication

Mobile Communications 38

Mobile communication concepts

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Mobile communication concepts