system communicaiton chapter 2

8/13/2019 System Communicaiton Chapter 2

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The Cellular Concept

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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


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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


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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


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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


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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.


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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.


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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


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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


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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


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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.


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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


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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


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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


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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:


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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


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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

system communicaiton chapter 2

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