lecture 5 knt4073 cellular
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KNT 4073 Telecommunication Networks andServices 1
Lecture 5
Cellular Network
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Introduction to Cellular
Network
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Introduction to Cellular Network
Mobile Communications Principles
Each mobile uses a separate, temporary radio channel to
talk to the cell site.
The cell site talks to many mobiles at once, using one
channel per mobile.
Channels use a pairoffrequencies for communication
one frequency (the forward link) for transmitting from the
cell site and one frequency (the reverse link) for the cell
site to receive calls from the users.
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Introduction to Cellular Network
Radio energy dissipates over distance, so mobiles
must stay near the base station to maintain
communications.
The basic structure of mobile networks includes:
telephone systems and
radio services.
Where mobile radio service operates in a closed
network and has no access to the telephone system,
mobile telephone service allows interconnection to
the telephone network.
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Introduction to Cellular Network
Early Mobile Telephone System Architecture
Traditional mobile service was structured in a fashion similar to
television broadcasting:
One powerful transmitter located at the highest spot in an area
that would broadcast in a radius of up to 50 kilometers.
The cellular concept structured the mobile telephone
network in a different way. Instead of using one powerful transmitter, many low-power
transmitters were placed throughout a coverage area.
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Introduction to Cellular Network
For example,
by dividing a metropolitan region into one hundred
different areas (cells) with low-power transmitters using
12 conversations (channels) each, the system capacity
theoretically could be increased from 12 conversations
or voice channels using one powerful transmitterto
1,200 conversations (channels) using one hundred low-
power transmitters.
.
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Introduction to Cellular Network
Mobile Telephone System Using the Cellular
Concept
Interference problems caused by mobile units using
the same channel in adjacent areas proved that all
channels could not be reused in every cell.
Areas had to be skipped before the same channel
could be reused.
Even though this affected the efficiency of the original
concept, frequency reuse was still a viable solution tothe problems of mobile telephony systems.
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Introduction to Cellular Network
Engineers discovered that the interference
effects were not due to the distance between
areas, but to the ratio of the distance between
areas to the transmitter power (radius) of theareas.
By reducing the radius of an area by 50
percent, service providers could increase the
number of potential customers in an areafourfold.
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Introduction to Cellular Network
Systems based on areas with a one-kilometer
radius would have one hundred times more
channels than systems with areas 10
kilometers in radius. The smaller the radius => The more channels
Speculation led to the conclusion that by
reducing the radius of areas to a few hundred
meters, millions of calls could be served.
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Introduction to Cellular Network
The cellular concept employs variable low-power
levels, which allow cells to be sized according to the
subscriber density and demand of a given area.
As the population grows, cells can be added to
accommodate that growth.
Frequencies used in one cell cluster can be reused in
other cells.
Conversations can be handed off from cell to cell to
maintain constant phone service as the user moves
between cells.
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Introduction to Cellular Network
The cellular radio equipment (base station) can
communicate with mobiles as long as they are within
range.
Radio energy dissipates over distance, so the
mobiles must be within the operating range of the
base station.
Like the early mobile radio system, the base station
communicates with mobiles via a channel.
The channel is made of two frequencies, one for
transmitting to the base station and one to receive
information from the base station.
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Cellular System Architecture
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Cellular System Architecture
Increases in demand and the poor quality of
existing service => led mobile service
providers to research ways to improve the
quality of service and to support more usersin their systems.
Because the amount of frequency spectrum
available for mobile cellular use was limited,
efficient use of the required frequencies wasneeded for mobile cellular coverage.
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Cellular System Architecture
In modern cellular telephony, rural and urban
regions are divided into areas according to
specific provisioning guidelines.
Deployment parameters, such as amount ofcell-splitting and cell sizes, are determined by
engineers experienced in cellular system
architecture.
Provisioning for each region is planned
according to an engineering plan that
includes cells, clusters, frequency reuse, and
handovers.KNT 4073 Telecommunication Networks and
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Cellular System Architecture
Cells
A cell is the basic geographic unit of a cellular
system.
The term cellular comes from the honeycombshape of the areas into which a coverage
region is divided.
Cells are base stations transmitting over small
geographic areas that are represented ashexagons.
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Cellular System Architecture
Each cell size varies depending on the
landscape.
Because of constraints imposed by natural
terrain and man-made structures, the trueshape of cells is not a perfect hexagon.
Figure 1 shows the cells.
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Cellular System Architecture
Cluster
A cluster is a group of
cells.
No channels arereused within a cluster.
Figure 2 illustrates a
seven-cell cluster.
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Cellular System Architecture
Frequency Reuse
Because only a small numberof radio channel
frequencies were available for mobile
systems, engineers had to find a way to reuseradio channels to carry more than one
conversation at a time.
The solution the industry adopted was called
frequency planning orfrequency reuse.
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Cellular System Architecture
Frequency Reuse
Frequency reuse was implemented by restructuring the
mobile telephone system architecture into the cellular
concept.
The concept of frequency reuse is based on assigning
to each cell => a group of radio channels used within a
small geographic area.
Cells are assigned a group of channels that is
completely different from neighboring cells. The coverage area of cells is called the footprint.
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Cellular System Architecture
Frequency Reuse
This footprint is limitedby a boundary so that the same
group of channels can be used in different cells that
are far enough away from each other so that their
frequencies do not interfere.
Cells with the same number have the same set of
frequencies.
Because the number of available frequencies is 7, the
frequency reuse factor is 1/7. That is, each cell is using 1/7 of available cellular
channels.
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Cellular System Architecture
Cell Splitting
Unfortunately, economic considerations made
the concept ofcreating full systems with many
small areas impractical. To overcome this difficulty, system operators
developed the idea of cell splitting.
As a service area becomes full of users, this
approach is used to split a single area intosmallerones.
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Cellular System Architecture
Cell Splitting
In this way, urban centers can be split into as
many areas as necessary to provide
acceptable service levels in heavy-trafficregions, while larger, less expensive cells can
be used to cover remote rural regions.
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Cellular System Architecture
Cell Splitting
Fig: cell splitting
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Cellular System Architecture
Handoff
The final obstacle in the development of the
cellular network involved the problem created
when a mobile subscriber traveled from onecell to another during a call.
As adjacent areas do not use the same radio
channels, a call must either be droppedor
transferred from one radio channel to anotherwhen a user crosses the line between
adjacent cells.
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Cellular System Architecture
Handoff
Because dropping the call is unacceptable, the
process ofhandoff was created.
Handoff occurs when the mobile telephonenetwork automatically transfers a call from
radio channel to radio channel as a mobile
crosses adjacent cells.
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Cellular System Architecture
Figure of Handoff
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Cellular System Architecture
During a call, two parties are on one voice channel.
When the mobile unit moves out of the coverage area
of a given cell site, the reception becomes weak.
At this point, the cell site in use requests a handoff.
The system switches the call to a stronger-frequency
channel in a new site without interrupting the call or
alerting the user.
The call continues as long as the user is talking, and
the user does not notice the handoff at all.
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Cellular System Components
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Cellular System Components
The cellular system offers mobile and
portable telephone stations the same service
provided fixed stations over conventional
wired loops. It has the capacity to serve tens of thousands
of subscribers in a major metropolitan area.
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Cellular System Components
The cellular communications system consists
of the following four major components that
work together to provide mobile service to
subscribers: public switched telephone network (PSTN)
mobile telephone switching office (MTSO)
cell site with antenna system
mobile subscriber unit (MSU)
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Cellular System Components:
Typical Cellular Network
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Cellular System Components
PSTN
The PSTN is made up of local networks, the
exchange area networks, and the long-haul
network that interconnect telephones andother communication devices on a worldwide
basis.
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Cellular System Components
Mobile Telephone Switching Office
(MTSO) The MTSO is the central office for mobile switching.
It houses the mobile switching center (MSC), fieldmonitoring, and relay stations for switching calls from
cell sites to wireline central offices (PSTN).
In analog cellular networks, the MSC controls the
system operation.
The MSC controls calls, tracks billing information, and
locates cellular subscribers.
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Cellular System Components
The Cell Site
The term cell site is used to refer to the
physical location ofradio equipment that
provides coverage within a cell.A list of hardware located at a cell site
includes power sources, interface equipment,
radio frequency transmitters and receivers,
and antenna systems.
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Cellular System Components
Mobile Subscriber Units (MSUs)
The mobile subscriber unit consists of a control unit
and a transceiverthat transmits and receives radio
transmissions to and from a cell site.
The following three types of MSUs are available:
the mobile telephone (typical transmit power is 4.0
watts)
the portable (typical transmit power is 0.6 watts)
the transportable (typical transmit power is 1.6 watts)
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Cellular System Components
Mobile Subscriber Units (MSUs)
The mobile telephone is installed in the trunk
of a car, and the handset is installed in a
convenient location to the driver. Portable and transportable telephones are
hand-held and can be used anywhere.
The use of portable and transportable
telephones is limited to the charge life of theinternal battery.
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Radio Network Planning and
Optimisation
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Objectives of radio network planning
1)To obtain sufficient coverage over the entire
service area to ensure that high quality voice services
and data services with low error rates can be offered
to the subscribers.
2)To offer the subscriber traffic network capacitywith
sufficiently low blocking and call droppingrate.
3)To enable an economical network implementation
when the service is establishedand a controllednetwork expansion during the life cycle of the
network.
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Radio Network Planning
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Radio Network Planning
The radio network planning process can be
divided into different phases.
Preplanning phase - the basic general
properties of the future network areinvestigated, for example, what kind of mobile
services will be offered by the network, what
kind of requirements the different services
impose on the network, the basic networkconfiguration parameters and so on
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Radio Network Planning
Main phase -A site survey is done about the to-be-
covered area, and the possible sites to set up the
base stations are investigated. All the data related to
the geographical properties and the estimated traffic
volumes at different points of the area will beincorporated into a digital map, which consists of
different pixels, each of which records all the
information about this point. Based on the
propagation model, the link budget is calculated,which will help to define the cell range and coverage
threshold.
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Radio Network Planning
Constant adjustment - will be made to
improve the network planning. Through
driving tests the simulated results will be
examined and refined until the bestcompromise between all of the facts is
achieved. Then the final radio plan is ready to
be deployed in the area to be covered and
served.
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Radio Network Planning
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Network planning from operator perspective
For an operator good network planning =
Less money spend to infrastructure
More satisfied customers (good service quality) Less need for adjustments
For an operator network optimization =
Better return for investment
Less need for costly hardware updates
Less need for new sites (which are very
expensive)
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Radio Network Planning
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Radio Network Optimization
There are four methods that are frequently
used for optimization for solving cost
functions reliably and rapidly:
Random Walk, Simulated Annealing,
Tabu search and
Genetic Algorithms.
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Random Walk:
The Random Walk search method generates new neighbors
at each iteration.
The cost function at the new neighbor is evaluated and
compared to the one at the current point. If the new cost function
is lower than the old one, then the neighbor will be
unconditionally accepted. And if the new function is worse than
the former, it will be conditionally accepted with a probability of
p, with p=0 becoming the greedy search algorithm with no
solution accepted unless it provides a more suitable cost
function, and p=1 leading to the fully random search, thus
evaluating all possible combinations of base stations at allpossible places. From literature surveys, the optimal value of the
probability p of choosing a worse solution than the previous one
is p=0.03
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Simulated Annealing:
The Simulated Annealing search is similar to the random
walk.
It also calculates the cost function of the generated new
neighbor, and compares the result with the already obtained old
result. There is also conditional and unconditional accept. The
difference is that the probability of accepting conditionally is
changing during the search procedure, while in random walk the
probability p is fixed from the beginning to the end of one
search. The change of this probability is controlled using some
method, which can be analogy to the cooling procedure of
annealing, so that the search becomes more and more greedy.
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Tabu Search:
Tabu search is defined as follows:
The already selected base station positions in the last
K iterations will be considered tabu, and so will not
be taken into consideration for the generation of new
neighbors. Usually K is chosen to be 1. The size ofcandidates for each iteration can be reset, and the
larger the size of candidates, the better the final result,
but also it takes longer for the simulations to converge.
The best value for the candidate size can be found tobe equal to 10 in the literature.
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Genetic Algorithm:
The idea of Genetic Algorithm comes from the
phenomena of the human evolution first proposed
by Darwin.
From a set of parent points, the children can be
generated. We select those children points, that showthe best results when evaluating the cost function
compared to the ones of their parents and their
generation. In the Genetic Algorithm, we use a
mutation operator that can be related to the definitionof neighborhood. The definition of population size
and generations are also similar to the candidate size
and iterations mentioned before.
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The End
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