cellular concepts r

7/29/2019 Cellular Concepts r

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CELLULAR CONCEPTS


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DEFINITION OF A CELL

Mobile Communication System is basically a Radio

Communication System where the coverage area is broken

into a number of sub-areas called Cells.

At the the centre of each Cell , Radio Equipments called BTS (

Base Trans-Receiver Station ) are placed to serve the

customers currently lying in the Cell.

Broadly, a Cell may be of two types-

-A Micro Cell with typical radius equal to 500m approx.

-A Macro Cell with typical radius equal to 30 Km. (approx.).


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NECESSITY OF CELLULAR SYSTEMS

Constraintsof a Radio Communication System:-1. RF(Radio Frequency) is a Limited Natural Resource.

2. Transmitter Power has Upper Limit.

Objective :-

-To extend the service up-to maximum possible area withmaximum no. of Subs..

Achievements with Cellular Structures :-

1. We shall be able to re-use the RFs. This will enable us

(a) To cover wider areas.

(b) To serve more number of subscribers

2. Low Power transmitter will be sufficient.

3. Roaming facility to the Customers can be assured.


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SHAPE OF THE CELLS

A Cell is taken as Hexagonal in shape for the purpose ofEngineering Calculations.

In reality, a Cell may of any shape. It depends upon thephysical condition of the area of coverage.

We do not take a Cell shape as Circular. In that case therewill be some uncovered areas and to cover these areas ,overlapping of Cells will be required.

A Cell may be taken as Triangular, Square, Rectangular etc.in shape. For a given radius , the area covered by a

Hexagonal Cell is maximum w r t Triangular / Square /Rectangular Cells.

It is convenient to do design calculations with hexagonalgeometry.


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BASIC DEFINITIONS IN CELLULAR STRUCTURE

Radius (R) of a Cell :-It is the distance between the Centreand any extreme point on the boundary of a Cell.

Co-Channel Cells :-Cells having the same set of RFs are

termed as Co-Channel Cells.

Co-Channel Distance (D) of separation :-This is thedistance between any two nearest Cells with same set of

RFs.

Co-Channel Interference :-Interference caused in one Cell

due to a Co-Channel Cell. Co-Channel Interference Reduction Factor (q) :-

q=D/R


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DEFINITIONS ( Contd.)

Received Power (C) :- This is the power received from thedesired transmitter.

Noise Power (I) :- It is the power received from un-desirable transmitters.

Path Loss Exponent (n) :- This is a parameter whichdecides at what rate the loss of signal from a transmitteroccurs before it reaches the desired receiver.

Relationship between C, n & R :-

C R-n

Relationship Between I, n & D :-

I D-n


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CELL CLUSTERING

& FREQUENCY REUSING

Cell Clustering :- In order to reuse the RFs, we makegroups of Cells in certain fashions and the groups are

termed as Clusters. The No. of Cells in a Cluster is known as

Cluster Size (N). N may be 4, 7,9 etc.

Frequency Reusing :-The total allotted RFs are distributedamong the Cells of a Cluster such that each Cell of the

Cluster gets a unique set of RFs. We form similar other

Clusters where the same RFs will be re-used among the

member Cells of each Cluster keeping in mind that twoadjacent Cells do not have the same RFs. If N is the Cluster

size, then 1/N is termed as Frequency re-usefactor.


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LOCATING CO-CHL CELLS IN GSM

-Let us introduce a formula-N= i2 + j2 + ij,

where N= Cluster Size

i= Non-negative integer

j= Non-negative integer

For N=4, some of the accepted values for i & j are

respectively- 2 & 0

For N=7, some of the accepted values for i & j are

respectively- 2 & 1


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CELLULAR PRINCIPLE

CELLULAR MOBILE CONCEPTS


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CASE STUDYI

( with higher D & q )

We have q=D/R

With higher D, q increases. This means Co-Channel

Interference is decreased.

Again q=(3N)

Any increase in value of q will increase N. This makes less

nos. of RFs available per Cell of the Cluster leading to less

traffic handling capacity.


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CASE STUDY-II

(with lower d & q)

We take q=D/R

With decrease in value of D, q comes down. This makes

increase in Co-Channel Interference.

Again with q=(3N)

lowering of q will reduce Cluster size and hence more

number of RFs available per Cell of the Cluster leading to

increase in traffic handling capacity.


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A TYPICAL CASE WITH N=7

We take a N=7 and Antenna as omni-directional.

A particular Cell is likely to interfered by six nearest Co-

Channel Cells from six sides of the Hexagonal Cell.

With C/I=(1/6)(R/D)-n

, we have -

q=D/R and n=4Also q=(3N)

Taking both the values of q & considering N=7, it is found

that the signal level is above 18dB if the MS lies within the

boundary of the Cell. However, if he goes very close to theboundary of the Cell , the signal level comes below 18 dB (

worst case 14.47 dB ) which is unacceptable for proper

receptions.


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SOLUTIONS WITH N=7

Let us take N=7 and let us also further sub-divide the Cell into sectors.

If we put Directional Antenna in each sector

instead of Omni Directional ones in the cellas a whole, the C/I value in worst casecomes to be 24.5 dB.

This is is quite acceptable.

In fact, it is being used in many of Cellularapplications.


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4 CELL CLUSTERING IN GSM

Using an Omni directional Antenna and 4 CellClustering, C/I comes below 18 dB as the

Customer moves towards the Cell Boundary.

On the other hand , if each Cell of the 4 CellCluster is divided into 3 Sectors and each Sector is

provided with a Directional Antenna, then C/I

comes to be as much as 20 dB in the worst case.

The RFs allotted for each Cell is distributed amongthe Secors of the Cell. In such scenarios GSM uses

4/12 pattern.


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THREE SECTORED CELL


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MEANING OF 4/12 PATTERN

There will be 4 Cell Clustering.

Each Cell would have 3 Sectors.

There would be 12 (=4*3) Sectors and each Cell

would get 1 RF . Total No. of RFs in a Cluster would be 12 ( Min.)

The 4 Cell Cluster would be repeated to cover the

entire area.


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FREQUENCY PLANNING WITH 4 CELL

CLUSTERING

30

Frequency Planning Aspects

A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

1 2 3 4 5 6 7 8 9 10 11 12

13 14 15 16 17 18 19 20 21 22 23 2425 26 27 28 29 30 31 32 33 34 35 36

A1

A2

A3 B1B2

B3

D1D2

D3 C1

C2

C3


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ERLANGS FORMULA OF TRAFFIC

The basis for Cell Planning is the Traffic calculation. It is thetraffic study which tells us how the subs use the network

and how much traffic they generate.

Estimation of such traffic can be made with the help of

Erlangs formula which is as under-

A= (n*T)/3600 Erlang,

where-

A= Offered Traffic

n= Number of simultaneous callsT=Average call duration in Secs


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MOBILE TRAFFIC MODELAccording to TRA guidelines-

Subscriber Traffic-

-25 mE for normal Subscribers

-40 mE for High Traffic Subscribers

Call Mix-

-Mobile to Mobile = 73% (Loc=59%) & (STD=14%)

-PSTN to Mobile = 27% (Loc=20%) & (STD=7%) Average Call duration - 90 Secs


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GSM TRAFFIC CALCULATINS

1E Traffic means 1 Sub holds a Circuit for 1 Hr.

25 mE Traffic means 1 Sub holds a Circuit for

=(60*60)*25/1000=90 Secs.

1E serves 1 Sub for 1 Hr. 1E serves 10 subs for 60/10=6 Mins (Each Sub)

1E serves 20 subs for =6/2=3 Mins=180 Secs

( Each Sub ) 1E serves 40 subs for =180/2=90 Secs

( Each Sub )


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THANKS

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