Cellular Architecture Design Concepts
-
Upload
t-l-singal -
Category
Education
-
view
2.892 -
download
6
description
Transcript of Cellular Architecture Design Concepts
May 21-23, 2012
Chitkara University, Punjab
National-Level Faculty Development Workshop
“Challenges for Research in Wireless Communication Technologies”
Presented by
Prof. T. L. Singal
1
Principles of Cellular
Communications
Cellular Terminology Cell structure and
cluster
Frequency Reuse Concept Cluster Size and System Capacity
Method of Locating Co-channel Cells
Frequency Reuse Distance
Co-channel Interference and Signal Quality
Co-channel Interference Reduction Methods
2
A cell is the basic geographic unit of a cellular
system.
It is also called a footprint.
A cell is the radio area covered by a cell-site
that is located at its center
A large geographical area is divided into a number of contiguous smaller geographical coverage areas called cells
3
Determined by the desired
received signal level by the
mobile subscribers from
its base station transmitter
in its operating area
Ideal, actual and
fictitious cell models
R
R
R
R
R
(a) Ideal Cell
(b) Actual Cell
(c) Fictitious Cell
4
5
6
7
Offers best possible non-
overlapped cell radio
coverage
Multiple hexagons can be
arranged next to each
other
For a given radius (largest
possible distance between
the polygon center and its
edge), the hexagon has the
largest area
Simplifies the planning
and design of a cellular
system 8
A group of cells that use a different set of
frequencies in each cell
Only selected number of cells can form a
cluster
Can be repeated any number of times in a
systematic manner
The cluster size is the number of cells
within it, and designated by K
9
10
Limitations:
High power
consumption
Low capacity
Large size of the
mobile
One very powerful transmitter located at the
highest spot in an area would broadcast in a
radius of up to 50 kilometers.
11
Main limitation of a conventional mobile wireless
communication system is:
Limited availability of FREQUENCY
SPECTRUM !!!
So the big challenge is …
to serve large number of mobile users ..
within limited allocated frequency spectrum
with a specified system quality
12
System capacity at required QoS with conventional frequency plan
Subscriber growth
Time
Out of Capacity!!!
The need: •Optimum spectrum usage
•More capacity •High quality of service (QoS)
•Low cost
How to increase capacity & Spectrum utilization
? 13
Reuse allocated RF spectrum or a given set of
frequencies (frequency channels) in a given
large geographical service area without
increasing the interference
Divide the service area into a number of small
areas called cells
Allocate a subset of frequencies to each cell
Use low-power transmitters with lower height
antennas at the base stations
14
Large coverage area, efficient spectrum
utilization and enhanced system capacity are
the major attributes of cellular communication
Frequency reuse is the core concept of cellular
communications
The design process of selecting and allocating
channel groups for all the cellular base
stations within a system is called frequency
reuse
15
16
Cells, which use the same set of frequencies,
are referred to as cochannel cells
Co-channel cells are located sufficiently
physically apart so as not to cause cochannel
interference
The space between adjacent cochannel cells is
filled with other cells that use different
frequencies to provide frequency isolation
17
Suppose the system has RF
spectrum for 100 voice
channels
Scenario 1: A high power
base station covering entire
area – system capacity = 100
channels
Scenario 2: Divide spectrum
into 4 groups of 25 channels
each; cells (1, 7), (2, 4), (3, 5),
6 are assigned distinct
channel groups – system
capacity = 175 channels
1
2
6
3
4
5
7
HPT – High Power Transmitter
LPT – Low Power Transmitter
18
Technical issues for proper design and planning
of a cellular network:
Selection of a suitable frequency reuse pattern
Physical deployment and radio coverage
modeling
Plans to account for the expansion of the
cellular network
Analysis of the relationship between the
capacity, cell size, and the cost of the
infrastructure
19
In a cellular system, the whole
geographical service area is divided into a
number of clusters having finite number
of cells.
The K number of cells in a cluster (cluster
size) use the complete set of available
frequency channels, N.
Each cell in the cluster contains J = (N/K)
number of channels only
Therefore, N = J x K; where J ≤ N
20
The cluster can be replicated many times to
cover the desired geographical area by a
cellular communication system
Let M be the number of clusters in the
system, then overall system capacity, C is
given as
C = M x N → C = M x J x K (N = J x K)
When K is reduced, J is proportionally
increased since N = J x K is constant.
To increase C, M is increased. But reducing K
will increase co-channel interference!
21
Cells which use the same set of
frequencies are referred to as
cochannel cells.
The interference between cochannel
cells is referred to as cochannel
interference.
The space between adjacent
cochannel cells are filled with cells
using different frequencies.
22
The shift parameters i
and j are separated by
60 degrees in a
hexagonal geometry
It can have any
integer value 0, 1, 2,
….
These can be used to
determine the location
of cochannel cells
23
Rules for determining the nearest cochannel cell
using “Shift parameters” (i, j) to lay out a
cellular system is:
Step 1: Move i cells along any side of a
hexagon.
Step 2: Turn 60 degrees anticlockwise
Step 3: Move j cells.
where i and j are shift parameters and can have
integer value 0, 1, 2, 3, and so on …
24
25
Distance between
two adjacent cells,
d = √3 R
Let D be the distance
between two
co-channel cells.
D = √{3R2(i2+j2+ixj)}
D2/(3R2) = i2+j2+i x j
26
By joining the
centers of the six
nearest cochannel
cells, a large
hexagon is formed
with radius equal
to D, which is also
the cochannel cell
separation.
27
Asmall hex = (3√3/2)xR2
Alarge hex = (3√3/2)xD2
Number of cells in large
hexagon, L = D2∕R2
Number of cells enclosed
by large hexagon,
L = K+6x[(1 ∕3)x K] = 3 x
K
K = D2∕ (3 x R2)
K = i2 + j2 + i x j
28
29
30
1
1 1
1
2 2
2 2
3
3
3
3
4
4 4
4 5
5 5
5
6
6 6
6
7
7
7
7
8 8
8 8
9
9 9
9
10
10 10
10
11 11
11 11
12 12
12 12
31
Reusing an identical frequency channel in
different cells is limited by co-channel
interference between cells.
The co-channel interference can become a
major problem in cellular communication.
It is desirable to find the minimum frequency
reuse distance D in order to reduce this co-
channel interference.
32
The number of co-channel cells in the vicinity
of the center cell
The type of geographic terrain contour
The antenna height
The transmitted power at each cell site
NOTE: As long as the cell size is fixed, co-
channel interference is independent of
transmitter power of each cell.
33
q = D ∕ R The frequency reuse ratio, q is also referred as
the co-channel reuse ratio
the co-channel reuse factor
co-channel interference reduction factor
R
C1
R
C1
D
34
7
6
1
2
3
4
5 7
6
1
2
3
4
5 R
D
• The real power of the cellular concept is that
Interference is not related to the absolute distance between cells
but related to the ratio of the distance between co-channel (same frequency) cells to the cell radius
35
The frequency reuse factor, q can be
determined from the cluster size K as
q = D / R = (3K)
The frequency reuse factor determines the
minimum distance for repeating a set of
frequencies
The spectrum efficiency is most significantly
influenced by the frequency reuse factor
36
37
Theoretically, a large value of D is desired to
reduce co-channel interference. Large D can
be obtained by choosing large cluster size K.
However, when K is too large, the number of
channels assigned to each of K cells becomes
small since N is fixed.
As K increases, system capacity per cell and
hence spectrum efficiency reduces
significantly.
38
Now the challenge is to obtain the smallest K which can still meet the desired system performance requirements in terms of capacity per cell, spectrum utilization efficiency and received signal quality (minimum co-channel interference).
This involves estimating co-channel interference and selecting the minimum frequency reuse distance D to reduce co channel interference (and hence small K).
39
Advantage: The
frequency reuse
system can
drastically increase
the spectrum
efficiency, thereby,
increasing the
system capacity.
Disadvantage: If the
system is not
properly designed,
co-channel
interference may
occur due to the
simultaneous use of
the same channel.
Co-channel Interference is the major
concern in the concept of frequency reuse.
40
Sources of Interference
Another mobile in the same cell
A call in progress in a neighboring cell
Other base stations operating in the same frequency
band or
Any non cellular system which inadvertently leaks
energy into the cellular frequency band
41
Interference is more pronounced in urban areas due to greater RF noise floor and the large number of base stations and mobiles.
Capacity cannot be increased.
On Control Channels
Interference leads to missed calls and blocked calls due to error in the digital signaling
On Voice Channels
Causes cross talk
where the
subscriber hears
interference in the
background due to
an undesirable
transmission.
42
43
A channel scanning mobile receiver records
three received signals while moving in any one
co-channel cell, under following conditions:
When only serving cell transmits (signal
recorded is termed as C)
Cell-sites of all six co-channel cells only
transmit (signal recorded is termed as I)
No transmission by any cell site (signal
recorded is termed as N)
44
If C/I > 18 dB in most of the area being served
by a cell, the system is said to be properly
designed.
If C/I < 18 dB and C/N > 18 dB in some areas,
the system is said to have co-channel
interference problem.
If both C/I and C/N < 18 dB and C/I is
approximately same as C/N in a given area,
the system is said to have radio coverage
problem.
If both C/I and C/N are < 18 dB and C/I <
C/N in a given area, the system is said to have
both co-channel interference as well as radio
coverage problem. 45
46
When only the mobile unit in the serving cell
transmits (signal recorded is termed as C)
Up to six interference levels are obtained at
the serving cell site from six mobile units
transmitting in six co-channel interfering
cells. (the statistical average signal recorded is
termed as I)
No transmission by any mobile unit (signal
recorded is termed as N)
47
Increasing the
separation
between two co-
channel cells (D)
Lowering the
antenna heights
at the cell site
Using directional
antennas at the
cell site
Use of diversity
scheme at
receiver
48
The essential principles of cellular
communication include frequency reuse, co-
channel cells, and co-channel interference
Various cell parameters based on regular
hexagonal cellular pattern include cluster,
frequency reuse distance, reuse factor
Co-channel interference reduction methods
aim at maintaining desired signal quality
and cell capacity
49
He executed international assignment as Senior Network Consultant with Flextronics International Inc. USA during 2000-02. He was associated with Nokia, AT&T, Cingular Wireless and Nortel Networks, for optimization of 2G/3G Cellular Networks in USA. Since 2003, he is in teaching profession in engineering colleges in India. He has number of technical research papers published in the IEEE Proceedings, Journals, and International/National Conferences. He has authored two text-books `Wireless Communications’ and `Analog & Digital Communications’, published by renowned publisher Tata McGraw-Hill.
T. L. Singal graduated from National Institute of Technology, Kurukshetra in 1981 . He began his illustrious career with Avionics Design Bureau at HAL, Hyderabad and worked on design and development of Air-Route Surveillance Radar Communication System. Then he led R&D group in a Telecom company and successfully developed Multi-Access Wireless Communication Systems.
50
T L Singal, Wireless Communications,
ISBN: 978-0-07-068178-1,
Tata McGraw-Hill, First Edition, 2010.
www.mhhe.com/singal/wc
51
[email protected] Contact No: +91-98145-42041
52
53