cellular topology
TRANSCRIPT
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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MDF
RLU/RSU
Switch TAX TAX
SCP
Wirelessin
Local Loop
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Mobile Network Evolution
WiLL – Wireless in Local Loop
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MDF
RLU/RSU
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SCP
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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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