Upload
tahir-hussain
View
219
Download
0
Embed Size (px)
Citation preview
8/13/2019 System Communicaiton Chapter 2
1/17
The Cellular Concept
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s
CChhaapptteerr22
TThhee CCeelllluullaarr CCoonncceepptt
Engr Munawwar nwarEngr NaveedJ an
8/13/2019 System Communicaiton Chapter 2
2/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 1
2.1 The Cellular Concept
Everyone is familiar with the usage of the term cellular in describing mobile radio systems. You
probably know that it is called cellular because the
network is composed of a number of cells. Mobile
radio systems work on the basis of cells for tworeasons. The first reason is that radio signals at the
frequencies used for cellular travel only a few
kilometers (kms) from the point at which they are
transmitted. They travel more or less equal
distances in all directions; hence, if one
transmitter is viewed in isolation, the area around
it where a radio signal can be received is typically
approximately circular. If the network designer
wants to cover a large area, then he must have a
number of transmitters positioned so that whenone gets to the edge of the first cell there is a second cell overlapping slightly, providing radio signal.
Hence the construction of the network is a series of approximately circular cells. This is shown in
Figure-2.1
The second reason has to do with the availability of something called radio spectrum. Simply, radio
spectrum is what radio signals use to travel through space. Whenever a conversation takes place using
a mobile radio system, it consumes a certain amount of radio spectrum for the duration of the call. An
analogy here is car parks. When you park your car in a car park it takes up a parking space. When you
leave the car park, the space becomes free for someone else to use. The number of spaces in the car
park is strictly limited and when there are as many cars as there are spaces nobody else can use the car
park until someone leaves. Radio spectrum in any particular cell is rather like this. However, there is
an important difference. Once you move far enough away from the first cell, the radio signal will have
become much weaker and so the same bit of radio spectrum can be reused in another cell without the
two interfering with each other. By this means, the same bit of radio spectrum can be reused several
times around the country. So splitting the network into a number of small cells increases the number
of users who can make telephone calls around the country. This is explained in much greater detail
later on. So, in summary, cellular radio systems are often called cellular because the network is
composed of a number of cells, each with radius of a few kilometers, spread across the country. This
is necessary because the radio signal does not travel long distances from the transmitter, but it is also
desirable because it allows the radio frequency to be reused, thus increasing the capacity of the
network.
In the beginning, mobile systems were developed much like radio or television broadcasting
(i.e., a large area was covered by installing a single, high-power transmitter in a tower situated at the
highest point in the area). A single high-power transmitter mobile radio system gave good coverage
with a small number of simultaneous conversations depending on the number of channelsNc. The (Nc
+1) caller was blocked. Those systems were also characterized by the lack of handoff. To increase the
number of simultaneous conversations, a large area can be divided into a large number of small areas,
Na. Each small area is called a cell. To cover a cell, a single low-power transmitter is required. If
every cell uses the same frequency that is available for a large area, and its available bandwidth is
divided into the number of channels,Nc, then instead ofNc simultaneous conversations for a large
Figure -2.1
8/13/2019 System Communicaiton Chapter 2
3/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 2
area, there would beNc simultaneous conversations for each cell. Thus, now there can be (Na *Nc)
simultaneous
conversations in the entire large area as compared with onlyNc The idea of using the same frequency
in all the cells does not work because of the interference between mobile terminals operating on the
same channel in adjacent cells. Therefore, the same frequency cannot be used in each cell, and it isnecessary to skip a few cells before the same frequency is used. Cellular concept is illustrated in
Figure-2.2
The cellular concept, therefore, is a wireless system designed by dividing a large area into
several small cells, replacing a single, high-power transmitter in a large area with a single, low-power
transmitter in each cell, and reusing
the frequency of a cell to another
cell after skipping several cells.
Thus, the limited bandwidth is
reused in distant cells, causing a
virtually infinite multiplication of
the available frequency.
2.2 Frequency Reuse
The cellular structure was
introduced due to capacity problems
of mobile communication systems.
In a cellular radio system, the area
covered by the mobile radio system
is divided into cells. In theory, the cells are considered hexagonal, but in practice they are less regular
in shape. The hexagon shape is conceptual and is a simplistic model of the radio coverage for each
base station, but it has been universally adopted since the hexagon permits easy and manageable
analysis of a cellular system. The actual radio coverage is known as thefootprint and is determined
from the field measurements or propagation prediction model. Although the real footprint is
amorphous in nature, a regular cell shape is needed for systematic system design and adaptation for
future growth. While it might seem natural to choose a circle to represent the coverage area of base
station, adjacent circles cannot 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, a square, an equilateral triangle and a hexagon. 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 the polygon and its farthest perimeter
points, the hexagon has the largest area of the three. Thus by using the hexagon geometry, the fewest
number of cells can cover a geographic region and the hexagon closely approximates a circular
radiation pattern which would occur for an omni-directional base station antenna and free space
propagation. Of course, the actual cellular footprint is determined by the contour in which a given
transmitter serves the mobiles successfully. Each cell contains a base station, which is connected to
the mobile switching center (MSC). This MSC is connected to the fixed telecommunication system
the public switched telephone network (PSTN). MSC serves as the central coordinator and controller
for the cellular radio system and as the interface between mobile and PSTN. The cellular radio user in
Fig ure 2.2
8/13/2019 System Communicaiton Chapter 2
4/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 3
a car or train or in the street picks up a handset, dials a number, and immediately can talk to the
person he or she called.
Each cell is assigned a part of the available frequency spectrum. Cellular radio systems offer
the possibility of using the same part of the frequency spectrum more than once. This is called
frequency reuse. Cells with identical channel frequencies (i.e., the same part of the frequencyspectrum) are called co-channel cells. The co-channel cells have to be sufficiently separated to avoid
interference. The distance between these co-channel cells is achieved by the creation of a cluster of
cells. As explained earlier, cells with identical numbers make use of the same part of the frequency
spectrum.
To understand a frequency reuse concept, 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
8/13/2019 System Communicaiton Chapter 2
5/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 4
An important design parameter denoting the amount of frequency reuse in a certain area is called the
normalized reuse distance. The normalized reuse distance,Ru is defined as the ratio of the reuse
distance,D, between the centers of the nearest co-channel cells and the cell radius,R, as shown in
Figure-2.4. Hence,
The relationship between Ru and N can be
given by
Figure -2.4
2.3 Signals-to-Noise Ratio
The interfacing caused by neighboring cells is measured as the signal-to-noise ratio:
This ratio of the useful signal to the interfering signal is usually measured in decibels (dB)
and called the Signal-to-Noise Ratio (SNR). The intensity of the interference is essentially a
function of co-channel interference depending on the frequency reuse distance D. From the
viewpoint of a mobile station, the co-channel interference is caused by base stations at
distance D from the current base station. A worst-case estimate for the signal-to-noise ratio
Fig ure 2.3
N
8/13/2019 System Communicaiton Chapter 2
6/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 5
W of a mobile station at the border of the covered area at distance R from the base station
can be obtained, subject to propagation losses, by assuming that all six neighboring
interfering transmitters operate at the same power and are approximately equally far apart
(distance D large against cell radius R)
By neglecting the noise N we obtain the following approximation for the Carrier-to-
Interference Ratio C/I (CIR):
Therefore the signal-to-noise ratio depends essentially on the ratio of the cell radius R to the
frequency reuse distance D. From these considerations it follows that for a desired or needed
signal-to-noise ratio W at a given cell radius, one must choose a minimum distance for the
frequency reuse, above which the co-channel interference fall below the required threshold.
2.4 Why different systems have a different cluster size
Up till now it has been said that if you travel far enough for the radio signal to become
undetectable and then move as far again, then you can put in another cell. It might be
imagined that this distance will be the same regardless of the radio system because it is
related to propagation laws, not radio system design. In fact, this is a slight simplification.
You only need to travel far enough for the radio system to fall to a level where it will not
interfere with another radio system. This is the same effect as the two speakers in the same
room. You do not need to move the other speaker so far away that you cannot hear them at all
in order to be able to have a conversation. Moving them away so that they are much quieter
than you, although still audible, is quite sufficient. Mobile radio systems have a key
specification, called the signal-to-interference ratio (SIR) that specifies just how quiet the
other speaker needs to be before they do not pose a problem. A typical SIR might be around
10. So when you have moved far enough away that the signal level has fallen to a tenth of the
minimum signal level that would be experienced at the edge of the cell, if you move as faraway again then you can put in another base station reusing the same frequency.
It so happens that the distance you need to move is very sensitive to the SIR. A system with a
SIR of 100 would have a reuse distance much greater than one with a SIR of 10, resulting in a
much greater cluster size and hence less efficient use of radio frequencies. The actual SIR
that a system can tolerate depends on a number of factors, key amongst which is the tolerance
of the voice coder to errors on the radio channel and the power of the error correction system
that is used. So it can be that different systems can have quite different SIRs hence require
quite different distances between the frequencies being reused and hence have quite different
cluster sizes.
8/13/2019 System Communicaiton Chapter 2
7/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 6
2.5 Why one channel can serve many users
If you were designing a supermarket, how many checkouts would you have? You might start
with the total population of the town. But you know that not all of them will go shopping at
the same time. However, you do know that perhaps one in every five will do their shopping
on a Friday night. Only a few of those in the supermarket at any time are actually queuing at
the checkout. You might go to a neighboring town of similar size and count the number of
people going into the supermarket every minute on Friday night. The checkouts need to be
able to handle this many people per minute otherwise queues will develop. If you count an
average of 10 people going in per minute and time the average person to take 2 minutes at the
checkout, then you need 20 minutes of checkout time for every minute of real time, or 20
checkouts.
Then you notice that in one minute 15 people go in while in the next minute only five
go in. The average, as you noted earlier, over a period of an hour is still 10, but people do notarrive perfectly evenly spaced apart. What should you do now? You could increase the
number of checkout minutes to 30 (i.e., have 30 checkouts) to cope with the peak demand,
but then when there were only five people, two-thirds of the checkouts would be idle. It is at
this point that you might start thinking that a little science would be useful.
This is an identical problem to the world of mobile radio. Not everyone makes a phone call at
the same time; some make a lot, others hardly ever call. If the average user is only on the
phone for10%of the time, then you could share a single channel amongst 10 users. But if you
did this you run the risk that two of them will try to use the channel at the same time and one
will get a network busy message. If this happens too often your users will migrate to acompetitors network.
This problem was studied in detail by Swedish engineer A. K. Erlang in the early part
of the twentieth century. The results he obtained are used in the design of all
telecommunication networks. Erlang studied what happened as you varied the number of
users that you tried to fit onto a channel and discovered, not unsurprisingly, that the more
users you tried to fit onto the channel, the higher the chance that each user would not be able
to access the channel, which he termed being blocked. He went much further than this. He
found that if you had a set of channels, perhaps 10, and you grouped them all together so that
if a subscriber wanted to make a call they were able to use any one of these channels that
were free, then the probability of them being blocked was reduced. (This is equivalent to
being able to use any of the checkouts in the supermarket.)
This seems intuitively reasonable. Say in your supermarket, for no good reason, you
decided to split the checkouts into two groups. When a customer came into the supermarket
you alternatively assigned them to the left group or the right group of checkouts. Now in
some cases, a lot of the shoppers in the right group will finish at the same time and there will
be queues in the right group but checkouts free in the left group. If you had not restricted the
shoppers in any way, this would not have happened and there would have been fewer queues.
8/13/2019 System Communicaiton Chapter 2
8/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 7
In the same way, the more radio channels and users that can be put together in one pool, the
less the likelihood that they will be blocked.
2.6 Handover/Handoff Mechanism
Handover, also known as handoff, is a process to switch an ongoing call from one cellto the adjacent cell as a mobile user approaches the cell boundary.
Figure-2.5 shows that as the user moves from cell 1 to cell 2, the channel frequencies will be
automatically changed from the setf1to the setf2. Handover is an automatic process, if the
signal strength falls below a threshold
level. It is not noticed by the user
because it happens very quicklywithin
200 to 300 ms
The need for a handover may be causedby radio, operation and management
(O&M), or by traffic. Radio causes the
majority of handover requests. The
parameters involved are low signal level
or high error rate. This can be caused by
a mobile moving out of a cell or signal
blocking by objects.
O&M-generated handovers are rare. They evolve from the maintenance of equipment,
equipment failure, and channel rearrangement. Handovers due to unevenly distributed traffic
may cause some mobiles at the border of a cell to be handed over to an adjacent cell.
The performance metrics used to evaluate handover algorithms are handover blocking
probability, call blocking probability, handover probability, call dropping probability, rate of
handover, probability of an unnecessary handover, duration of interruption, and delay
(distance).
A handover is performed in three stages. The mobile station (MS) continuously gathers
information of the received signal level of the base station (BS) with which it is connected,
and of all other BTSs it can detect. This information is then averaged to filter out fast-fadingeffects. The averaged data is then passed on to the decision algorithm, which decides if it will
request a handover to another station. When it decides to do so, handover is executed by both
the old BS and the MS, resulting in a connection to the new BS.
As stated earlier, the received signal level suffers from fading effects. To prevent handover
resulting from temporary fluctuations in the received signal level, the measurements must be
averaged. An averaging window whose length determines the number of samples to be
averaged is used. Longer averaging lengths give more reliable handover decisions, but also
result in longer handover delays. Detailed studies were done to determine the averaging
window shapethat is, to determine whether recent measurements should be treated as more
Figure -2.5
8/13/2019 System Communicaiton Chapter 2
9/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 8
reliable than older ones. The averaging window is used to trade off between handover rate
and handover delay.
The time over which a call may be maintained within a cell, without handoff, is called
the dwell time. The dwell time of a particular user is governed by a number of factors,
including propagation, interference, distance between the subscriber and the base station and
other time varying effects. Even when a mobile user is stationary, ambient motion in the
vicinity of the base station and the mobile can produce fading; thus even a stationary
subscriber may have a random and finite dwell time. Analysis indicates that the statistics of
dwell time vary greatly, depending on the speed of the user and the type of radio coverage.
For example in mature cell which provide coverage for vehicular highway users, most users
tend to have a relatively constant speed and travel along fixed and well-defined paths with
good radio coverage. In such instances, the dwell time for an arbitrary user is a random
variable with a distribution that is highly concentrated about the mean dwell time. On the
other hand, for users in dense, cluttered, microcell environments, there is typically a largevariation of dwell time about the mean and dwell times are typically shorter than the cell
geometry would otherwise suggest. It is apparent that statistics of dwell time is important in
the practical design of handoff algorithms.
In first generation analog cellular system, signal strength measurement are made by
base station and supervised by MSC. Each base station constantly monitors the signal
strengths of all of its reverse voice channels to determine the relative location of each mobile
user with respect to base station tower. In addition to measuring the RSSI of calls in progress
within a cell, a spare receiver in each base station, called the locator receiver, is used to scan
and determine signal strengths of mobile users which are in neighboring cells. The locatorreceiver is controlled by the MSC and is used to monitor the signal strength of users in
neighboring cells which appear to be in need of handoff and reports all RSSI values to the
MSC. Based on the locator receiver signal strength information from each base station, the
MSC decides if a handoff is necessary or not.
In todays second generation system, handoff decisions are mobile assisted. In mobile
assisted handover (MAHO), every mobile station measures the received power from the
surrounding base stations and continually reports the result of these measurements to the
serving base station. A handoff is initiated when the power received from the base station of a
neighboring cell begins to exceed the power received from the current base station by a
certain level or for a certain period of time. The MAHO method enables the call to be handed
over between the base stations at much faster rate than in first generation analog systems
since the handoff measurements are made by each mobile and the MSC no longer constantly
monitors signal strength. MAHO is practically suited for microcell environments where
handoffs are more frequent.
During a course of a call, if a mobile moves from one cellular system to a different
cellular system controlled by different MSC, an intersystem handoff becomes necessary. An
MSC engages in an intersystem handoff when a mobile signal becomes weak in a given celland the MSC cannot find another cell within its system to which it can transfer the call in
8/13/2019 System Communicaiton Chapter 2
10/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 9
progress. There are many issues that must be addressed when implanting an intersystem
handoff. For instance, a local call may become a long distance call as the mobile moves out
of its home system and becomes a roamer in a neighboring system. Also compatibility
between the two MSCs must be determined before implementing an intersystem handoff.
Different systems have different policies and methods for managing handoff requests. Some
systems handle handoff requests in the same way they handle originating calls. In such
systems, the probability that a handoff request will not be served by a base station is equal to
the blocking probability of incoming calls. However from users point of view, having a call
abruptly terminated while in the middle of a conversation is more annoying than being
blocked occasionally on a new call attempt. To improve the quality of service as perceived by
the users, various methods have been devised to prioritize handoff requests over call initiation
requests when allocating voice channels.
2.6-1 Decision Algorithm for Handover Timing
8/13/2019 System Communicaiton Chapter 2
11/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 10
2.6-2 Umbrella Cell Approach
In practical cellular system, several problems arise when attempting to design for a
wide range of mobile velocities. High speed vehicles pass through the coverage region of a
cell within a matter of seconds, whereas pedestrian users may never need a handoff during a
call. Particularly with the addition of microcells to provide capacity, the MSC can quickly
become burdened if high speed users are constantly being passed between very small cells.
Several schemes have been devised to handle the simultaneous traffic of high speed and low
speed users while minimizing the handoff intervention from the MSC. Another practical
limitation is the ability to obtain new cell sites.
Although the cellular concept clearly provides additional capacity through the
addition of cell sites, in practice it is difficult for cellular service providers to obtain new
physical cell site locations in urban areas. Zoning laws, ordinances, and other non technical
barriers often make it more attractive for a cellular provider to install additional channels andbase stations at the same physical location of an existing cell, rather than find new site
locations. By using different antenna heights (often on the same building or tower) and
different power levels, it is possible to provide large and small cells which are co-located
at a single location. This technique is called the umbrella cell approachand is used to
provide large area coverage to high speed users while providing small area coverage to users
travelling at low speeds. The umbrella cell approach ensures that the number of handoffs is
minimized for high speed users and provides additional microcell channels for pedestrian
users. The speed of each user may be estimated by the base station or MSC by evaluating
how rapidly the short-term average signal strength on the RVC changes over time, or more
sophisticated algorithms may be used to evaluate and partition users. If a high speed user in
the large umbrella cell is approaching the base station and its velocity is rapidly decreasing,
the base station may decide to hand the user into the co-located microcell without MSC
intervention.
8/13/2019 System Communicaiton Chapter 2
12/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 11
2.6-3 Cell Dragging
Another practical handoff problem in microcell systems is known as cell dragging.
Cell dragging results from pedestrian users that provide a very strong signal to the base
station. Such a situation occurs in an urban environment when there is a line of sight radio
path between the subscriber and the base station. As the user travels away from the base
station at a very slow speed, the average signal strength does not decay rapidly. Even when
the user has travelled well beyond the designed range of the cell, the received signal at the
base station may be above the handoff threshold, thus a handoff may not be made. This
creates a potential interference and traffic management problem since the user has meanwhile
travelled deep within a neighboring cell. To solve the cell dragging problem, handoffs
threshold and radio coverage parameters must be adjusted carefully.
2.7 IMPROVING CAPACITY & COVERAGE IN CELLULAR SYSTEMAs the demand for wireless service increases the number of channels assigned to a
cell eventually becomes insufficient to support the required number of users. At this point
cellular design techniques are needed to provide more channels per unit coverage area. Three
popular techniques are discussed below.
2.7-1 Cell Splitting
Unfortunately, economic considerations made the concept of creating 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
into smaller ones. In this way, urban centers can be split into as many areas as necessary to
provide acceptable service levels in heavy-traffic regions, while larger, less expensive cells can
be used to cover remote rural regions as shown in figure-2.6
Figure -2.6
8/13/2019 System Communicaiton Chapter 2
13/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 12
This technique is used to increase the number of cell. When a cell becomes congested it
divides the cell into smaller cell. By this way the subdivided cell has its own base station
known as BTS. Also the antenna
size becomes small and low
transmitted power is reduced. Cellspitting increases the capacity of
cellular system and the number of
time that channels are reused.
By this way the cell has
smaller radius and the new smaller
cell called the micro cell should be
installed between the existing cells.
So the number of capacity increases
due to the additional number ofchannel per unit area.
The given figure-2.6 shows the
spitted cell and the large cell. In this figure every cell were reduced in such a way that the
every cell is cut in half, in order to cover the entire service area with smaller cell
approximately four time as many cell is required. Considering a circle with can show this a
radius R. The area covered by such a circle is four times as large as the area covered by the
circle with radius R/2. The increase number of cell will increase the number of cluster over
the coverage area, which would increase the number of channel and thus the capacity in the
coverage area increases. Cell splitting allows a system to grow by replacing large cell withsmaller, while not upsetting the channel allocation scheme required maintaining the minimum
number of co-channel reuse ratio.
An example of cell splitting is shown in figure-1.7. The base station are placed at the corner of
the cell, and the area served by the base station A is assumed to be saturated traffic i, e blocking
of base station A exceeds acceptable rates. New base stations are therefore needed in the
region to increase the number of channel in the area and to reduce the area served by the single
base station. From the figure B comes to know that the original bas station has been
surrounded by the six new micro-cells. In the figure the smaller cell were added in such away
as to preserve the frequency reused plan of the system. For example the microcell base stationlabeled G was placed half way between two larger stations utilizing the same channel set G.
For the new cell to be smaller in size, the transmit power of the cell must be reduced. The
transmit power of the new cell with radius half that of the original cell can be found by
examining the received power Pat new and old cell boundaries and setting them equal to each
other. This is necessary to ensure that the frequency reuse plan for the new microcells behaves
exactly as the original cell.
In example the smaller cell were added in such a way that to preserve the frequency reuse plan
of the system, for example the micro cell base station labelled G was placed half way between
two larger base stations utilizing the same channel set G. This is also case for the other micro
Figure -1.8
Figure -2.7
8/13/2019 System Communicaiton Chapter 2
14/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 13
cells in the figure. As can be seen from the figure-1.8, cell splitting merely scales the geometry
of the cluster. In this case the radius of each micro cell is half that of the original cell.
2.7-2 Cell Sectoring
Cell splitting achieves capacity improvement by essentially rescaling the system.By decreasing the cell radius R and keeping the co-channel reuse ratio D/R unchanged, cell
splitting increases the number of channel per unit area. Another method to increase the
capacity is to keep the cell radius unchanged seek methods to decrease the D/R ratio.
Sectoring increase s the SIR so that the cluster size may be reduced. In this approach, first the
SIR is improved using the directional antennas, then capacity improvement is achieved by
reducing the number of cells in the cluster, thus increasing the frequency reuse. However to
do this successfully, it is necessary to reduce the relative interference without decreasing the
transmit power.
The co-channel of interface in a cellular system may be decrease by replacing a single
omni-directional antennas at the base station by several directional antennas, each traditional
within a specific sector. By using directional antennas, a given cell will receive interference
and transmit within only a fraction of the available co-direction cells. The technique for with
the co-channel interference is reduced depends on the amount of sectoring used.
Figure 2.8
A cell is normally partitioned into 120 degree sector or six 60 degree sectors as shown in the
figure-2.8
When sectoring is employed, the channels used in a particular cell are broken down
into sectored groups and are used only within particular sector. Assuming seven-cell reuse,
for the case of 120 degrees sectors, the number of interferers in the first tier is reduced from
six to two. This is because only two of the six co-channel cells receive interference with a
particular sectored channel group. The resulting S/I is a significant improvement over the
omni-directional case, where the worst case S/I was proved to be 17 dB. This S/I
improvement allows the wireless engineer to then decrease the cluster size N in order to
improve the frequency reuse, and thus the system capacity. In practical system further
8/13/2019 System Communicaiton Chapter 2
15/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 14
improvement in S/I is achieved by down tilting the sector antennas such that the radiation
pattern in the vertical plane has a notch at the nearest co-channel cell distance.
The improvement in S/I implies that with 120 degrees sectoring, the minimum
required S/I Of 18 dB can be easily achieved with seven-cell reuse as compared to 12-cell
reuse for the worst possible situation in the unsectored case. Thus sectoring reduces
interference, which amounts to an increase in capacity by a factor of 12/7or 1.714. In
practice, the reduction in interference offered by sectoring enable planners to reduce the
cluster size N and provides an additional degree of freedom in assigning channels.
The penalty for improved S/I and the resulting capacity improvement from the
shrinking cluster size is an increased number of antennas at the base station and a decrease in
trunking efficiency due to channel sectoring at the base station. Because sectoring uses more
than one antenna per base station, the available channels in the cell must be subdivided and
dedicated to a specific antenna. This breaks up the available trunked channel pool into several
small pools and decreases trunking efficiency. Since sectoring reduces the coverage area of a
particular group of channels, the number of handoffs increases as well. Fortunately many
modern base stations support sectorization and allow mobiles to be handed off from sector to
sector within the same cell without intervention from MSC, so the handoff problem is often
not a major concern.
2.7-2-i Using Sectored Sites
The distribution of RF carriers, and the size of the cells, is selected to achieve a balance
between avoiding co-channel interference by geographically separating cells using the same
RF frequencies, and achieving a channel density sufficient to satisfy the anticipated demand.
By sectoring a site we can fit more cells into the same geographical area, thus
increasing the number of MS subscribers who can gain access and use the cellular network.
This sectorization of sites typically occurs in densely populated areas, or where a high
demand of MSs is anticipated, such as conference centers /business premises.
2.7-2-ii 4 Site/3 Cell
A typical re-use pattern used in GSM planning is the 4 site/3 cell. For example, the network
provider has 36 frequencies available, and wishes to use the 4 site/3 cell re-use pattern hemay split the frequencies up as follows:
Cell
A1
Cell
B1
Cell
C1
Cell
D1
Cell
A2
Cell
B2
Cell
C2
Cell
D2
Cell
A3
Cell
B3
Cell
C3
Cell
D3
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
25 26 27 28 29 30 31 32 33 34 35 36
In this configuration each cell has a total of 3 carriers and each site has a total of 9 carriers. If
the provider wished to reconfigure to a 3 site/3 cell then the result would be:
8/13/2019 System Communicaiton Chapter 2
16/17
The Cellular Concept Chap:2
CECOS Unive rsity o f IT & Em e rg ing Sc ie nc e s Page 15
Cell
A1
Cell
B1
Cell
C1
Cell
A2
Cell
B2
Cell
C2
Cell
A3
Cell
B3
Cell
C3
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18
19 20 21 22 23 24 25 26 27
28 29 30 31 32 33 34 35 36
As can be seen from the table, each cell now has 4 carriers and each site has 12 carriers.
This has the benefit of supporting more subscribers in the same geographic region, but
problems could arise with co-channel and adjacent channel interference.
Figure -2.9
8/13/2019 System Communicaiton Chapter 2
17/17
The Cellular Concept Chap:2
2.7-3 Micro Cell Zone Concept
When the load on the switching and control link of the mobile system increases, the number of
Handoff will be required for the sectoring. The solution for this problem was presented by Lee.
This concept is based on a micro cell concept for seven cell reuse. In this scheme each of the
three zone sites represented by Tx and Rx in the figure. As the mobile travels from one zone to
zone to other zone within the cell, it retains the same channel. Thus unlike in sectoring, a
handoff is not required at the MSC when the mobile travels between zones within the cell. The
base station simply switches the channel to a different zone site. In this way a given channel
active only in the particular zone in which the mobile is traveling and hence the base station
radiation is localized and interference is reduced.
The advantage of the zone cell technique is that while the cell maintains a particular
coverage radius, the co-channel interference in the cellular system is reduced since a large
central base station is replaced by several lower powered transmitter on the edges of the ofthe cell. Decrease co-channel interference improves the signal quality and also leads to an
increase in capacity without the degradation in trucking efficiency caused by the sectoring
Figure -2.10