Cellular Topology

Cellular System

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MDF

TAX – Trunk Automatic Exchange MDF – Main Distribution FrameDP – Distribution Point RLU – Remote Location Unit

RSU – Remote Switch Unit

RLU/RSU

Digital Trunk Exchanges

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SCP

Last Mile

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BSNL LL

Airtel LL

TATA LL

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MDF

RLU/RSU

Mobile Network Evolution

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Last Mile

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RLU/RSU

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SCP

Wirelessin

Local Loop

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Mobile Network Evolution

WiLL – Wireless in Local Loop

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Mobile Network Evolution

Wireless Local Loop

WLL – Wireless Local Loop

Primitive Wireless Phone System

Multiplexing - FDMACarrier BW - 30 KHzFrequency Reuse Factor = 0Modulation - FM

WLL – Will Local Loop

• 1G system developed by Bell Labs for deployment in America

• Analog system used FDMA/FDD• Frequency Range-800-900Mhz• Originally only 2 carriers allowed• Each carrier allocated 20 Mhz• 30Khz uplink and downlink channel separated by 45 Mhz

Advantages

High QOS

Disadvantages

Wastage of BW

Primitive Wireless Phone System

Need for Cellular System

Frequency Reuse

Bandwidth Optimization

Seamless Mobility

Reduce Interference

The Cellular Topology was developed by Dr.Martin Cooper of Motorola in 1973

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

• Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

• Capacity within cell limited by available bandwidth and operational requirements

• Each network operator has to size cells to handle expected traffic demand

Cell structure

• Implements space division multiplex: base station covers a certain transmission area (cell)

• Mobile stations communicate only via the base station• Advantages of cell structures:

– higher capacity, higher number of users– less transmission power needed– more robust, decentralized– base station deals with interference, transmission area etc.

locally• Problems:

– fixed network needed for the base stations– handover (changing from one cell to another) necessary– interference with other cells

• Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

Capacity of a Cellular System

• Frequency Re-Use Distance

• The K factor or the cluster size

• Cellular coverage or Signal to interference ratio

• Sectoring

Design Considerations

Four Cell Architecture Five Cell Architecture

Too muchoverlapping

Too muchoverlapping

Seven Cell Architecture

Design Considerations

Less overlapping

Ideal cells Fictitious cells

Concept of Cellular Networks

• A single high power transmitter services one larger area multiple low power transmitters service multiple smaller areas (Cells)

• Frequency can be reused by cells far away from each other improve usage

• A set of cells that do not share frequency form a cluster

• The cluster is then replicated throughout the desired communication area

Frequency re-use distance is based on the cluster size K

The cluster size is specified in terms of the offset of the center of a cluster from the center of the adjacent cluster

K = i2 + ij + j2

K = 22 + 2*0 + 02

K = 4 + 0 + 0

K = 4

D = 3K * R

D = 3.46R i

D

R

The Frequency Re-Use for K = 4

K - Factor

K = i2 + ij + j2

K = 22 + 2*1 + 12

K = 4 + 2 + 1

K = 7

D = 3K * R

D = 4.58R

3

i1

4

5

6

7

1

2

35

6

7

D

R

Frequency Re-Use Distance

K - Factor

1

2

3

4

1

1

1

1

1

12

2

2

2

2

3

3

3

3

3

4

4

4

4

4

4

3

2

Cell Structure for K = 4

12

34

5

6

7

12

34

5

6

7

2

1

12

34

5

6

7

12

34

5

6

7

1

2

3

4

5

6

7

The Cell Structure for K = 7

1

11

1

2 2

22

3

3

3

3

4

4 4

45

5 5

5

6

6 6

6

7

7

7

7

8 8

889

99

9

10

1010

10

1111

1111

1212

12 12

Cell Structure for K = 12

One Cell

24 Cells

Re-use of the frequency

72 Cells

Cell splitting

Increasing cellular System Capacity

• Cell sectoring– Directional antennas subdivide cell into 3

or 6 sectors

– Might also increase cell capacity by factor of 3 or 6

Sect 2

Sect 1

Sect 3

Cell Sectoring

3 Sectors per Cell

Increasing cellular system capacity

Cell splitting

– Decrease transmission power in base and mobile

– Results in more and smaller cells– Reuse frequencies in non-contiguous cell

groups– Example: ½ cell radius leads 4 fold

capacity increase

Tri-Sector antenna for a cell

Highway

TownSuburb

Rural

Cell Distribution in a Network

Macro Cell – 25 to 35 Km

Micro Cell – 1 to 5 Km

Pico Cell – 100 m to 1 Km

ClustersCorridors

Advantages of Cellular Networks

• More capacity due to spectral reuse• Lower transmission power due to smaller

transmitter/receiver distances• More robust system as Base Station problem only

effects the immediate cell• More predictable propagation environment due to

shorter distances

Disadvantages of Cellular Networks

• Need for more infrastructure• Need for fixed network to connect Base

Stations• Some residual interference from co-channel

cells• Handover procedure required

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