11 cellular capacity enhancement

HET452 Wireless CommunicationsHET718 Mobile and Personal Networks

System Design of Cellular Networks (3)

Lecture eleven

Faculty of Information and Communication Technologies

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Outline of Lecture

• Purpose of Lecture is to outline techniques used for improving coverage and capacity in Cellular systems

• Cellular architecture modifications• Frequency allocation schemes


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Capacity Expansion Techniques for Cellular Radio

• Four Methods1. Buy more bandwidth2. Modify cellular architecture3. Modify frequency allocation scheme4. Use a different modem and access technology

• We will consider modifications to the cellular architecture (2) and modifications to the frequenceyallocation scheme (3) in this lecture

• Option 4 was covered in previous lectures


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Modify Cellular Architecture

• Cell splitting– Reduce radius of cell to increase frequency reuse

• Cell sectoring / Smart Antennae– Reduce co-channel interference

• Repeaters– Use retransmitters to cover areas subject to severe fading


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Cell Splitting – A hierarchy of cells

• Can split cells into smaller cells– Cells may be congested– Cell may not have coverage in a particular area

• Urban canyons, indoors

• Hierarchy of cells– Femtocells: metres– Picocells: interiors of buildings with range in 10s of metres– Micro cells: urban canyons. Range 100s of metres– Macro cells: metropolitan areas. Range of kilometres– Megacells: national coverage: Hundreds of kilometres


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Cell Splitting to Minimize Congestion

• Where cells are congested, split the cell into two smaller cells and allocate additional channels to it

• New smaller cell is midway between co-channel cells

A B

D EC

F G A

C D

B

F G

a


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Signal-Interference Ratio if all cells splitLet radius of cell A be R and the distance to each of its neighbouring co-channel cells be Di . Then the minumum SIR experienced by cell A is:

( )∑=

−

−

=0

1

i

i

ni

n

cellA

D

RSIR

Now, the radius of cell a is R/2 and the distances to each of its neighbouring co-channel cells are Di/2Then the maximum SIR experienced by cell a is:

( )

( )cellAi

i

ni

n

i

i

ni

n

cella SIRD

RD

R

SIR ==

=

∑∑=

−

−

=

−

−

00

11 2

2


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Partial upgrade

• Split cells one at a time, as load grows– Don’t halve all Di

• How can we retain SIR at previous levels?– Reduce power

• Microcell becomes very small– Aim microcell antenna down, to reduce range– Use different channels in cells A and a

• Channels in old cell broken down into two groups• A separate group allocated to each of cells A and a


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Overlaid Cells Concept

• Using different frequencies within different sized cells is called the ‘overlaid cells concept’

• It can be generalized to the hierarchy of cells listed earlier.

• Generally, allocate frequencies from the larger cell to mobile stations that are moving rapidly and frequencies from the smaller cells to mobile stations moving more slowly. This minimizes the number of handovers experienced by the mobile station.


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Reuse Partitioning with Overlaid Micro Cells

• Channels are divided among a larger macrocell and a smaller microcell entirely contained within the larger cell

• Channels allocated to the smaller cells have a smaller radius than the larger cells they can use a smaller co-channel reuse distance than the overlay cell

• Typically the macrocell will be in a cluster of 7 cells while the microcell will be in a cluster of 3


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Cell Sectoring

• SIR is proportional to the cell reuse ratio D/R• Cell splitting reduces R and D so increasing the

number of channels per unit area• Cell Sectoring increases SIR so that the cluster size

can be reduced• Makes use of directional antennae to increase the

SIR


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Cell Sectoring

• Replace single omni-directional antenna at the base station by several directional antennae

• Channels used by the base station are broken into separate groups and are used only within a particular sector

• Usually broken into three 120o

sectors or six 60o

sectors 1

2 3 345

621


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Cell Sectoring and SIR

• Sectoring improves SIR by reducing the interference contribution from other cells

• For cluster size 7, where SIR is 17dB, 120o

sectoring improves SIR to 24 dB, enabling reduction in cluster size


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Cell Sectoring and SIR

B

G

D E

G

A

C D

B

F G

A B

E

A

C D

B

F G

A B

C

F A

C

B

F G

A B

D EC

F G A

C D

F

F

120o

Sectors, seven cell clusters

Interference at cell A is now only from two other cells instead of six

m


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Approximation of Worst Case SIR for first tier of co-channel cells (N=7)

M

•Radius of cell is R•Distance between centre of cells is D•Approximate distances shown on diagram•Worst case is when Mobile Station at edge of cell•Cochannel interference from two cells D + R distance

B

B

B

B

B

B

B

D+R

D – RD

D+R

D

D – R


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Approximation of Worst Case SIR for first tier of co-channel cells (N=7)

• For 120o

sectors:

nn

n

QRDRSIR −−

−

+=

+=

)1(21

)(2

• For 60o

sectors:

nn

n

QRDRSIR −−

−

+=

+=

)1(1

)(


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Sectoring Problem• Calculate the SIR (forward channel) for cluster size 7

where 60 degree sectoring is used. Assume a path loss constant n = 4

• Repeat for n = 3• Comments?• Some useful formulae

nn

n

QRDRSIR −−

−

+=

+=

)1(1

)(

NRDQ 3==


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Sectoring Penalties

• Sectoring improves SIR but with some costs– Additional antennas at each base station– Handovers between sectors

• Usually done by the base station– Loss of trunking efficiency

• Main disadvantage of sectoring• Radio channels are allocated to each antenna. • This breaks up the available pool of channels so decreasing

trunking efficiency

• Sectoring doesn’t help with reverse channel SIR– Other techniques needed

• Power control main one


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Sectoring and Trunking ExampleConsider a system where the average call lasts 2 minutes and the probability of blocking is to be no more than 1%.

If there are 395 channels in a 7 cell reuse system then there will be 57 channels per cell. Assuming blocked calls are cleared (Erlang B) then this system can handle 1326 calls per hour.

Now consider 120 degree sectoring. Now there are only 19 channels per antenna sector. Using Erlang B, each sector can handle 336 calls per hour. Since each cell contains three sectors, the system can now handle only 1008 calls per hour. A 24% decrease in call handling capacity!


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Sectoring and Trunking ExampleIf capacity decreases per BS by 24%, why use sectoring?

It allows more frequency reuse (since less interference):Replace 7-cell reuse by 4-cell reuse 75% increase in capacity.

Overall effect:(1 – 0.24) (1 + 0.75) = 1.33. 33% INCREASE in capacity.

(Actually, overall benefit is greater, since more frequency reuse also helps trunking efficiency.)


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Smart Antennas

• Smart Antennae take the idea of a microcell to its logical limit

• Base station directs a narrow beam towards a mobile station

• Enables very high frequency reuse• Known as Space Division Multiple Access


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Smart Antennas• Forms a different beam for each subscriber• Minimizes interference between beams


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Repeaters

• Radio retransmitters connected to a base station• Used to provide coverage in hard-to-reach areas

– Valleys, buildings, tunnels• Can be thought of as a distributed antenna• Repeaters connected to base station through

microwave, coaxial or fibre optic cable


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Example of use of a RepeaterBase station unable to cover a difficult area because of a valley.

Solution is to install a repeater attached to the same base station

Base StationCoverage

X

Additional Base Station Coverage with a Repeater

R


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Summary of Cell Architecture Modifications

• Cell Splitting• Cell Sectoring• Smart antennae• Repeaters


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Problem on Cell Splitting

•How might you arrange the frequency allocation in the split cells above?

1

2

3

67

3

15

4


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Alternative Frequency Allocation Schemes

• Channel allocation is straightforward when traffic is uniform.– When it isn’t, it can be very complicated.

• Fixed Channel Allocation• Centralized Dynamic Channel Allocation• Distributed Dynamic Channel Allocation


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Fixed Channel Allocation

• Predetermined number of channels allocated to each cell

• No transfer or borrowing between cells• Eg GSM with cluster size of 4

– [1, 5, … , 121] for first set of cells– [2, 6, … , 122] for second set of cells– [3, 7, … , 123] for third set of cells– [4, 8, … , 124] for third set of cells


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Advantages and Disadvantages of Fixed Channel Allocation

• Advantages– Easy to implement– Works well if traffic in network is uniform and remains

constant with time• Disadvantages

– Wasteful if traffic is not uniform• Some cells have unacceptable blocking because all channels

used while other cells have unused channels– Doesn’t cater for traffic that varies with time

• Cells on the central business district will have high traffic rates during the day, but low rates in the evening and at weekends


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Channel Borrowing

• Short term allocation of borrowed channels to cells• Allocate unused channels in one cell to another cell

with no available channels• Channels can be borrowed by a neighbour if it is not

in use in any of its co-channel cells• The channel in the co-channel cells is locked until the

channel is returned


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Channel Borrowing

B

G

D E

G

D

F G

A B

E

A B

A B

C

F

C

F

A B

D EC

F G A

C D

F

D

C

E

F

Affected cells(locked channels)

Cell D is congested

Channel is borrowed from G

G


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Dynamic Channel Allocation

• No fixed assignment of channels to cells• All available channels are part of a ‘pool’ that are

allocated to cells as needed• Allocation of channels needs to consider the effect of

channel allocation on SIR– Cannot allocate a channel if it will reduce SIR for other

cells below an acceptable level• Places big demands on MSC to manage channel

allocation


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Dynamic Channel Allocation

• Two approaches– Centralised Allocation

• Allocation decisions made by MSC• Mobile Switching Centre allocates all channels based on

information from all attached base stations regarding signal strength

– Decentralised Allocation• Allocation decisions made by Base Station• Base station measures interference on all channels not yet

allocated and chooses the one with least interference• Slightly like modern “cognitive radio” techniques


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Dynamic Allocation

• Centralised– Very efficient usage of channels– But a high signalling overhead– Single point of failure has robustness consequences

• Decentralised– Less efficient– But more robust since no single point of failure– Smaller signalling overhead


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Summary of Frequency Allocation Schemes

Attribute Fixed Channel Allocation Dynamic Channel AllocationFlexibility in channel allocation Low HighReusability of channels Maximum LimitedTemporal and spatial changes Very sensitive InsensitiveGrade of service Fluctuating StableRadio equipment Covers only the channels

allocated to the cellHas to cover all channels that could be assigned to the cell

Computational effort Low HighCall setup delay Low HighImplementation complexity Low HighFrequency planning Complex None Sigalling load Low HighControl Centralised Centralised or decentralised


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Summary

• Techniques for improving capacity of a cellular system

• Cell based– Cell splitting– Cell sectoring– Repeaters– Microcells

• Frequency Allocation Schemes– Fixed and decentralised

11 cellular capacity enhancement

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