3. cellular technology

8/8/2019 3. Cellular Technology

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Dept. of Electronic and Information Engg. Page 1 Comm. Products / Cellular Technology

CELLULAR MOBILE DESIGN PRINCIPLES

1 A General Trunking

In order to utilize the switching network moreeffectively, the low-usage local loops (subscribers

lines) are grouped into high-usage trunk-groups before

switching.

Router

ConcentratorSubscribers'lines

Expander

Incoming Outgoing

Central control

trunk-groups

2 Traffic Theory

Traffic theory is used to perform cost-effectivedimensioning of switching and transmission equipment.

2.1 Traffic Unit

Traffic unit is a measure of traffic intensity. The

international unit of traffic intensity is the erlang. Itrepresents the proportion of time within an hour that a

circuit is occupied.


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Trunk-group traffic is the product of the number of calls

and the mean duration of calls handled by the trunk-

group.

A = M Tc (2.1)

where A = Offered traffic in erlangs

M = Number of calls during the busy hour

Tc = Mean call-holding time in hours

2.2 Busy Hour

It is the given period within a day that bears the highest

traffic intensity. The 'busy hour' traffic is used to work

out the equipment quantities of the network. The

reason to use busy hour traffic is that this period usuallyhas the highest amount of blocked or lost calls. If the

dimensioning of equipment at this period is correct and

blocked calls can be minimized, all other non-busy hour

traffic should then be handled satisfactorily.

2.3 Congestion

If all the trunks in a group are busy, it can accept no

further calls. This state is known as congestion (Fig.

2.1).

Traffic carried = traffic offered - traffic lost (2.2)


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traffic

offered

Call

Set-up ConversationCall

Attempt

Call

Cleared

no channel

with channel

traffic

lost

traffic

carried

Figure 2.1 Lost traffic

To specify the performance of a network, the term

grade of service (GOS) is used. It is defined as

B = traffic lost / traffic offered

= proportion of time for which congestion

exists

= probability of congestion or blocking

probability

= probability that a call will be lost due tocongestion (2.3)


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2.4 Erlang-B Distribution

The Erlang-B formula is used in block-call-clear

condition and is given in Eq. 2.4:

B =

=

N

0k

k

N

!k

A

!N

A

(2.4)

where B = Erlang-B loss probability

A = offered traffic intensity

N = available number of circuits

2.4.1 Erlang-B Traffic Table

Erlang-B traffic table (Table 2.1) is used to determine

the maximum amount of offered traffic to a group of N

trunks under a specified grade of service.

2.5 Trunking Efficiency

It is a measure of channel utilization and is defined as

Carried trunk-group traffic / number of channel in the

trunk-group (2.5)


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Number 1 lost call in Number 1 lost call inof 50 100 200 1000 of 50 100 200 1000

trunks (0.02) (0.01) (0.005) (0.001) trunks (0.02) (0.01) (0.005) (0.001)

E E E E E E E E

1 0.02 0.01 0.005 0.001 51 41.2 38.8 36.8 33.4

2 0.22 0.15 0.105 0.046 52 42.1 39.7 37.6 34.2

3 0.6 0.45 0.35 0.19 53 43.1 40.6 38.5 35.0

4 1.1 0.9 0.7 0.44 54 44.0 41.5 39.4 35.85 1.7 1.4 1.1 0.8 55 45.0 42.4 40.3 36.7

6 2.3 1.9 1.6 1.1 56 45.9 43.3 41.2 37.5

7 2.9 2.5 2.2 1.6 57 46.9 44.2 42.1 38.3

8 3.6 3.2 2.7 2.1 58 47.8 45.1 43.0 39.1

9 4.3 3.8 3.3 2.6 59 48.7 46.0 43.9 40.0

10 5.1 4.5 4.0 3.1 60 49.7 46.9 44.7 40.8

11 5.8 5.2 4.6 3.6 61 50.6 47.9 45.6 41.6

12 6.6 5.9 5.3 4.2 62 51.6 48.8 46.5 42.5

13 7.4 6.6 6.0 4.8 63 52.5 49.7 47.4 43.4

14 8.2 7.4 6.6 5.4 64 53.4 50.6 48.3 44.1

15 9.0 8.1 7.4 6.1 65 54.4 51.5 49.2 45.016 9.8 8.9 8.1 6.7 66 55.3 52.4 50.1 45.8

17 10.7 9.6 8.8 7.4 67 56.3 53.3 51.0 46.6

18 11.5 10.4 9.6 8.0 68 57.2 54.2 51.9 47.5

19 12.3 11.2 10.3 8.7 69 58.2 55.1 52.8 48.3

20 13.2 12.0 11.1 9.4 70 59.1 56.0 53.7 49.2

21 14.0 12.8 11.9 10.1 71 60.1 57.0 54.6 50.1

22 14.9 13.7 12.6 10.8 72 61.0 58.0 55.5 50.9

23 15.7 14.5 13.4 11.5 73 62.0 58.9 56.4 51.8

24 16.6 15.3 14.2 12.2 74 62.9 59.8 57.3 52.6

25 17.5 16.1 15.0 13.0 75 63.9 60.7 58.2 53.5

26 18.4 16.9 15.8 13.7 76 64.8 61.7 59.1 54.327 19.3 17.7 16.6 14.4 77 65.8 62.6 60.0 55.2

28 20.2 18.6 17.4 15.2 78 66.7 63.6 60.9 56.1

29 21.1 19.5 18.2 15.9 79 67.7 64.5 61.8 56.9

30 22.0 20.4 19.0 16.7 80 68.6 65.4 62.7 58.7

31 22.9 21.2 19.8 17.4 81 69.6 66.3 63.6 58.7

32 23.8 22.1 20.6 18.2 82 70.5 67.2 64.5 59.5

33 24.7 23.0 21.4 18.9 83 71.5 68.1 65.4 60.4

34 25.6 23.8 22.3 19.7 84 72.4 69.1 66.3 61.3

35 26.5 24.6 23.1 20.5 85 73.4 70.1 67.2 62.1

36 27.4 25.5 23.9 21.3 86 74.4 71.0 68.1 63.0

37 28.3 26.4 24.8 22.1 87 75.4 71.9 69.0 63.9

38 29.3 27.3 25.6 22.9 88 76.3 72.8 69.9 64.8

39 30.1 28.2 26.5 23.7 89 77.2 73.7 70.8 65.6

40 31.0 29.0 27.3 24.5 90 78.2 74.7 71.8 66.6

41 32.0 29.9 28.2 25.3 91 79.2 75.6 72.7 67.4

42 32.9 30.8 29.0 26.1 92 80.1 76.6 73.6 68.3

43 33.8 31.7 29.9 26.9 93 81.0 77.5 74.3 69.1

44 34.7 32.6 30.8 27.7 94 81.9 78.4 75.4 70.0

45 35.6 33.4 31.6 28.5 95 82.9 79.3 76.3 70.9

46 36.6 34.3 32.5 29.3 96 83.8 80.3 77.2 71.8

47 37.5 35.2 33.3 30.1 97 84.8 81.2 78.2 72.6

48 38.4 36.1 34.2 30.9 98 85.7 82.2 79.1 73.5

49 39.4 37.0 35.1 31.7 99 86.7 83.2 80.0 74.4

50 40.3 37.9 35.9 32.5 100 87.6 84.0 80.9 75.3

Table 2.1 Traffic table


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3 Limitation of Radio Telephone

Limited coverage. Poor service performance. Inefficient spectrum utilization.4 Background of Cellular Mobile System

4.1 Network Configuration

In a cellular system (Fig. 4.1), the area to be covered is

divided into a number of small areas called cells. A

base station is used in the center of the cell to serve the

mobiles within its coverage. Each base station is

connected by a fixed data link to a MSC. The MSCs

are interconnected to each other and to the publicswitched telephone network (PSTN).

BS

MSCBS BS

BSMSC

PSTN

BSMScell

Figure 4.1 Cellular Network Infrastructure

Abbreviation: MS ----- Mobile station

BS ----- Base station

MSC -- Mobile switching centre


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4.2 Radio Cells

Hexagonal-shaped cell layouts are used in initial design

to partition coverage areas. They provide seamlesscoverage.

In flat terrain, circular cell layouts are used. They

provide sufficient overlapping regions for call

handovers in addition to seamless coverage.

In most urban area, irregular layouts are used. Noticethat overlapping regions and seamless coverage still

maintained.

Fictitious Ideal Real

Figure 4.2 Cell coverage

4.3 Cluster and Frequency Re-use

Each cellular network has assigned two bands of radio

spectrum for duplex operation. The duplex bands are

divided into a number of carrier pairs and are then

assigned to a defined number N of cells. The N cellsform a group known as the cluster. The cluster repeats

itself over the whole coverage area and therefore


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frequencies are re-used as many times as possible

depending upon the number of clusters. The pattern of

the cells within a cluster is fixed (Fig. 5.1) as it is

optimized for minimizing interference and therefore N

is also known as reuse pattern.

4.4 Frequency Channels

There are two groups of channel operate in forwardor

down (BS to MS) direction and reverse or up (MS to

BS) direction.

Traffic channels are for speech or data

communication.

Control channels are for management purposes.

BS MS

FCCH

RCCH

FTCH

RTCH

FCCH - Forward control channeRCCH - Reverse control channe

FTCH - Forward traffic channelRTCH - Reverse traffic channel

Figure 4.3 Communication between mobile and base

station


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4.5 Location Registration

When a mobile is not engaged in a call, it tunes to the

control channel of the situated cell and monitors theforward signaling information. As the mobile moves

across the network, it will scan all control channels and

lock onto the strongest one which is usually the situated

cells control channel.

The mobile checks the broadcasted location information

and if this differs from the previous cell, the mobileautomatically updates with the network of its location.

Therefore, the network continues to maintain an

updated location database of all mobiles.

4.6 Handoff / Handover

It is a process of transferring a call to another base

station. Handoff occurs when the mobile is at the cell

boundary or in signal-strength holes within a cell.

Handoff decision is based on one or more of the

following conditions:

Received power is below certain threshold (e.g.-95 dBm).

Received Carrier-to-Interference power ratio, C/I isbelow certain threshold (e.g. 18 dB).

A better channel is available from adjacent cell. The local cell is too congested while the adjacent

cell is not (forced handoff).


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4.7 Soft Handoff

The handoff described above belongs to the break-

before-make type. It has higher probability of call dropdue to unsuccessful handover.

In CDMA system the mobile station can use one

frequency but with different codes to communicate with

two base stations at the same time. Handover can be

carried out in a make-before-break condition. It

guarantees higher successful rate of handover.

5 Frequency Reuse Pattern Selection

Because of the frequency re-use, nearby mobiles may

use the same frequency and cause co-channelinterference. The key objective of planning a cellular

radio system is to design the reuse pattern N and

frequency allocation in order to maximize the capacity

of the network whilst controlling co-channel

interference and other interference to within acceptable

limits.

Typical values of N are 3, 4, 7 and 12 (Fig. 5.1).

Reducing N will increase the trunking efficiency.

However, as N decreases, the distance between cocells

(cells with same channels) also reduces, which

increases the co-channel interference.


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AB

DC

A

BC

DE

F

G

4-cell cluster 7-cell cluster

A

GB

CD

E

F

12-cell cluster

HI

J

K

L

BA

C

3-cell cluster

Figure 5.1 Cell reuse patterns

The minimum distance which allows the same

frequency to be reused is called reuse distance D. D isrelated to the cell radius Ras shown in Fig. 5.2 and the

ratio of D to R, called the reuse ratio, is a function of N,

namely

N3R

D= (5.1)

DR

Figure 5.2 D and R of a cluster


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5.1 Co-channel Interference

During a call, the mobile receives wanted carrier signal

C from the base station in which it is located, and alsointerfering signal I from other cells. The carrier-to-

interference ratio C/I ratio is related to the D/R ratio.

For the 3-cell, 4-cell and 7-cell cluster there could be up

to six immediate interferes, as shown in Fig. 5.3.

4-cell clusters 7-cell clusters3-cell clusters

Figure 5.3 The geometry associated with interfering

cells using 3-cell 4-cell and 7-cell clusters

Assuming the fourth power propagation law, an

approximate value of C/I ratio is

C/I =4

4

D6

R

I6

C

I

C

==

(5.2)

Using the D/R ratio,

C/I = 22 N5.1)N3(6

1

I

C== (5.3)


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This shows that the C/I ratio is a function of the reuse

pattern N. For example, suppose N = 7, then C/I 73,

or 18 dB.

Cellular radio systems are designed to tolerate a certain

amount of C/I ratio. Beyond this level, speech quality

will be severely degraded. This lower limit on C/I ratio

effectively sets the minimum D/R ratio that can be used.

The C/I ratio requirement has two other factors may

need to be taken into account:

Adjacent channel interference from near channelsin neighbouring cells.

Multipath fading which may weaken C ascompared to I.

The C/I ratio requirement for analogue cellular systems

varies from 18 to 21 dB. The C/I ratio requirement for

GSM is 9 dB in theory and is 15 dB in practice to

provide quality services.

5.2 Frequency Reuse Pattern of CDMASystem

In CDMA system, because of spread spectrum

technique, much high C/I ratio can be acceptable.

Therefore, all frequencies are re-used in every sector of

cell.


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1

11

1

1

1

111

Figure 5.4 CDMA reuse pattern

5.3 Power Control

Both the base station and the mobile transmission

power needed to be controlled. This has multiple

effects of

(i) Reducing power consumption of the mobile.

(ii) Reducing the co-channel and adjacent channel

interference.

(iii) Reducing the generation of intermodulation

product.

(iv) Conforming the coverage of cell.

5.4 Cell Size

Cell size need not be the same. The cells are

subdivided into smaller cells (microcells) towards the

middle of the city to permit management of a higher

density of users. The peripheral is served by large cells

(macrocells). The underlying principle is if each cell

(whether large or small) has the same amount of

frequency channels, a small cell in size can carry moretraffic per unit area than a large cell.


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5.5 Sectorization

Sectorization is a standard practice in most cellular

systems. In a regular cellular layout, co-channelinterference will be received from six surrounding cells.

One way of cutting the interference is to use several

directional antennas at the base stations, with each

antenna illuminating a sector of the cell and with a

separate channel set allocated to each sector.

60o and 120o cell sectorization are commonly employed(Fig. 5.5). It reduces the number of prime interference

sources to one and two respectively (Fig. 5.6).

120o

60o 120

o

Figure 5.5 60o and 120o sectors

D

Mobile

D+0.7R

Mobile

D+0.7R

R

Figure 5.6 Interference in 7-cell cluster with sectors


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From Fig. 5.6 and Eq. 5.2, worst case C/I ratio:

(7-cell cluster with six sectors per cell)

C/I =4

4

)R7.0D(

R

I

C

+= = 779 = 29 dB

(7-cell cluster with three sectors per cell)

C/I =44

4

D)R7.0D(

R

I

C

++

= = 282 = 24.5 dB

A disadvantage is that the channel sets are divided

between sectors and trunking efficiency is reduced.

However, improved C/I ratio allows the system to use a

smaller reuse pattern N. The net effect of sectorizationis an increase in the total system capacity (Table 5.1).

After sectorization, the original cell coverage is no

more valid. Fig. 5.7 shows the radiation pattern and

coverage of the new 120o sectored cell. Fig. 5.8 shows

the 3-site, 9-cell cluster and the 4-site, 12-cell cluster

which are evolved from the 3-cell and 4-cell clusters.


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System

(396 ch)

N Channels

per sector

Offered

load / site

Mean

C/I (dB)

4 99 86.7 13.8

Omni 7 56 / 57 45.9 / 46.9 18.712 33 24.7 23.3

4 33 74.1 19.8

120o 7 19 36.9 24.5

Sector 12 11 17.4 29

4 16 / 17 64.2 / 58.8 24.8

60o 7 9 25.8 28.9

Sector 12 6 13.8 33GoS = 2%

Table 5.1 Omni vs. sectorized cellular system

performance

d d

d= maximum service distance

Figure 5.7 Cell and 120o sectored cell


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3-cell cluster 3/9-cell cluster

4-cell cluster

4/12-cell cluster

Figure 5.8 Examples of reuse patterns


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5.6 Frequency Channel Assignment

Frequency assignment scheme is used to control

adjacent channel interference (ACI). Main assignmentcriterion is to maintain frequency separation between

channels in the same cell and in the adjacent cells.

Consider the 4-cell cluster in Fig. 5.1 as an example, a

simple frequency assignment will be:

f1 cell A, f2 cell B, f3 cell C, f4 cell D,

f5 cell A, f6 cell B, f7 cell C, f8 cell D .

The main advantage of this arrangement is that all

frequencies within one cell are widely separated (e.g.,

in B: f2, f6, f10 ). It will greatly reduce the ACI.

ACI also appears when two adjacent frequencies (e.g.,

f3 and f4) are assigned to two adjacent cells. It can be

shown that a careful frequency assignment provides

smaller ACI but cannot eliminate ACI. In general, ACI

decreases with the increase of cluster size N.

For sectored cases, the cells are numbered as shown in

Fig. 5.9. Consider the 4/12-cell cluster as an example, a

simple frequency assignment will be:


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f1 cell A1, f2 cell B1, f3 cell C1, f4 cell D1,




f17 cell A2, f18 cell B2, f19 cell C2, f20 cell D2 .

It can be shown that there is no adjacent frequency in

adjacent cells when cluster is equal or larger than 4/12-

cell cluster.

3/9-cell cluster 4/12-cell cluster

A1

A3 A2

B3 B2

C1

C3 C2

B1C2

A3 A2

C3 C3

B2 A1

A3

B1

C1

C2 B1

B2 A1 B3

D1 D2

B2 A1 C3 C2

C1 A3 A2 B1 D1

C2 D3 B3 B2 A1

B1 D1 D2 C1 A3

D3C2C3A1B2

D1B1A2

Figure 5.9 Frequency assignments of 3/9, 4/12-cell

clusters


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5.7 Reuse Pattern Selection

7/21 reuse pattern:

It provides ideal isolation of co-channel and adjacentchannel interference.

4/12 reuse pattern:

Most GSM systems are planned around this reuse

pattern over flat terrain.

3/9 reuse pattern:This pattern can be used by GSM in theory, but it has

the problem of the adjacent channel interference.

5.8 Antenna Arrangement

Fig. 5.10a is a typical antenna configuration layout for a

3-sector cell site. Two receive antennas provide space-

diversity against multi-path fading. Fig. 5.10b shows

the 6-sector antenna array.

Figure 5.10 Antenna array of cell site

R RT R RT

R RT

R

R

T

R

R

T

R

R

T

R

R

T

R

R

T

R

R

T


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6 Adding Capacity

Once a cellular network has been planned to provide

overall coverage, there are a number of ways of addingadditional capacity.

6.1 Cell Architecture Approach

6.1.1 Sectorization

60o and 120o cell sectorization are commonly

employed. It reduces the number of prime interference

sources to one and two respectively. A better overall

C/I ratio then allows a smaller reuse pattern, N to be

used to increase the overall traffic capacity.

6.1.2 Regular Grid

A regular and consistent reuse pattern provides better

overall C/I ratio as compared with non-regular reuse

pattern.

6.1.3 Cell Splitting

By reducing the size of cells, more cells (hence

channels) per area will be available and traffic capacity

will increase. But as cell sizes decrease, it becomes

difficult to find suitable base station sites.


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Figure 6.1 Example of cell splitting for omni-cell.

Figure 6.2 Example of cell splitting for 3-sector cell.


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6.1.4 Microcell, Picocell and Hierarchical

Structure

Marcocells refer generally to cells with large coveragethat serve fast moving vehicle.

Minicells refer generally to cells with antennas just

above local clutter and therefore their coverages are

smaller than that of the macrocells.

Microcells refer generally to cells of small coverage

that are able to serve slower moving pedestrian.

Picocells refer generally to cells that serve indoor area.

Smaller cell size brings up the traffic density per unit

area but the problems are:

It needs new techniques to handle handover. Site locations are difficult to find.Hierarchical layered cell structure with macrocells on

the top and picocells in the bottom provides (i) widearea coverage, (ii) capacity, and (iii) less handover.

However, a careful frequency planning is needed.


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6.2 Dual Networks with Dual-mode Mobiles

Network can be combined for example, GSM800 +

PCS1800, GSM800 + DECT, AMPS + CDMA.Alternative routing between the two networks will

increase trunking efficiency and capacity. Of course, it

needs dual-mode mobiles and the underlay

infrastructure should be able to handle the handover.

6.3 Frequency Utilization Approach

Half rate voice codec is available in GSM specification.

It doubles the channel capacity but the voice quality of

such codec is poorer than the full rate codec and

therefore is still not employed.

6.4 Interference Reduction Approach

Frequency hopping and DTX can be utilized together to

reduce interference and a smaller reuse pattern can be

used.

6.4.1 Frequency Hopping

Frequency hopping has a time averaging effect which

provides extra protection against both channel fading

and co-channel interference. It will reduce dropped-callrate but increase average Bit-Error-Rate, BER.


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6.4.2 Discontinue Transmission (DTX)

Transmission is stopped during the silent period of

conversation. This may account for 60% of thetransmission time in a conversation and a 60%

interference reduction can be obtained. In addition,

mobile power consumption is reduced.

6.5 Frequency Reuse Approach

6.5.1 Intelligent Underlay/Overlay (IUO)

Fig. 6.3 shows the IUO operation. There are additional

overlay cells at the centers of normal underlay cells.

When a mobile moves into the overlay cell coverage

region, the call will be handover from the underlay cell

to the overlay cell and vice versa.

All the overlay cells use some sets of channel known as

super channels. These super channels are arranged in

smaller re-use pattern (e.g. 1/3, 2/6 as shown in Fig.

6.4) and will increase the overall traffic capacity. On

the other hand, co-channel interference among overlaycells is also increased with the short reuse distance.

However, the small overlay cells provide stronger field

strengths and give satisfy C/I ratio.

The network will measure the co-channel interference

and adjust the overlay cell boundaries dynamically and

this is what we call intelligent. This allows maximumutilization of overlay cells to increase system capacity.


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Main advantage of using IUO is that there is no extra

hardware added to the system.

MS

MS

MS

Overlay

Underlay

Overlay

Underlay

BS

BS

Co-channel

interference fsfs

fr

Figure 6.3 Intelligent Underlay-Overlay in Omni

cells

Figure 6.4 Intelligent Underlay-Overlay in sectorized

cells


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7 Cellular System Design Procedure

7.1 Coverage Planning

From the terrain data and selected propagation model,

determine the path loss and fade margin at cell edge.

With the maximum allowable transmitter power and

receiver sensitivity, determine the maximum allowable

distance which gives the maximum coverage of the cell

or sector. The total number of cells or sectors is then

determined by repeating the process until full coverage

is achieved.

7.2 Capacity Planning

From population, land usage and other statistics,

forecast the traffic distribution of the covered region.Together with the coverage plan, determine the traffic

density of each cell in the region.

7.3 Frequency Planning

Select a reuse pattern and then allocate the channels toeach cell. This process should satisfy the required cell

traffic densities and the C/I ratio requirements at the

same time. Several iterations of the above processes

may be needed to finalize a basic cellular plan.

7.4 Traffic Growth

Use the mentioned techniques to handle traffic growth.


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Appendix 1 Erlang-B Formula

Let A = mean traffic (Erlangs)

= mean arrival rate of all callsh = mean service time of a call

N = number of trunk lines

A = h

For a group of N trunks the number of calls in progress

follows a birth and death process known as regular

Markov chain.

The probability of a call arriving during t:

P(a) = At/h

The probability of a call ending during t when k calls

are in progress:

P(e) = kt/h

The probability of a transition from k-1 to k busy trunksduring t is:

P(k-1 k) = P(k-1)P(a) = P(k-1) At/h

The probability of a transition from k to k-1 busy trunks

during t is:

P(k k-1) = P(k)P(e) = P(k) kt/h


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The assumption of statistical equilibrium requires that

P(k k-1) = P(k-1 k)

P(k) =k

AP(k-1)

and in general

P(x) = !x

Ax

P(0)

where P(x) is equal to the probability of x calls in

progress, then

=0xP(x) =

=0x !x

Ax

P(0) = 1

P(0) = 1/=0x !x

Ax

then

P(x) =!x

AxP(0) =

=0k

k

x

!k

A

!x

A


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If the system has N circuits, then

P(x) =

=

N

0k

k

x

!k

A!x

A

A call will not find a free trunk line when all N circuits

are occupied, then

Blocking Probability, B = P(N) =

=

N

0k

k

N

!k

A

!N

A

References

1. CY Lee, Mobile Cellular Telecommunications Systems, McGraw-Hill,

2nd edition, 1995

2. JE Flood, Telecommunications Switching, Traffic and Networks, Prentice

Hall, 1995

3. cellular technology

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