cellular architecture design concepts

May 21-23, 2012

Chitkara University, Punjab

National-Level Faculty Development Workshop

“Challenges for Research in Wireless Communication Technologies”

Presented by

Prof. T. L. Singal

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Principles of Cellular

Communications

Cellular Terminology Cell structure and

cluster

Frequency Reuse Concept Cluster Size and System Capacity

Method of Locating Co-channel Cells

Frequency Reuse Distance

Co-channel Interference and Signal Quality

Co-channel Interference Reduction Methods

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A cell is the basic geographic unit of a cellular

system.

It is also called a footprint.

A cell is the radio area covered by a cell-site

that is located at its center

A large geographical area is divided into a number of contiguous smaller geographical coverage areas called cells

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Determined by the desired

received signal level by the

mobile subscribers from

its base station transmitter

in its operating area

Ideal, actual and

fictitious cell models

R

R

R

R

R

(a) Ideal Cell

(b) Actual Cell

(c) Fictitious Cell

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6

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Offers best possible non-

overlapped cell radio

coverage

Multiple hexagons can be

arranged next to each

other

For a given radius (largest

possible distance between

the polygon center and its

edge), the hexagon has the

largest area

Simplifies the planning

and design of a cellular

system 8

A group of cells that use a different set of

frequencies in each cell

Only selected number of cells can form a

cluster

Can be repeated any number of times in a

systematic manner

The cluster size is the number of cells

within it, and designated by K

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Limitations:

High power

consumption

Low capacity

Large size of the

mobile

One very powerful transmitter located at the

highest spot in an area would broadcast in a

radius of up to 50 kilometers.

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Main limitation of a conventional mobile wireless

communication system is:

Limited availability of FREQUENCY

SPECTRUM !!!

So the big challenge is …

to serve large number of mobile users ..

within limited allocated frequency spectrum

with a specified system quality

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System capacity at required QoS with conventional frequency plan

Subscriber growth

Time

Out of Capacity!!!

The need: •Optimum spectrum usage

•More capacity •High quality of service (QoS)

•Low cost

How to increase capacity & Spectrum utilization

? 13

Reuse allocated RF spectrum or a given set of

frequencies (frequency channels) in a given

large geographical service area without

increasing the interference

Divide the service area into a number of small

areas called cells

Allocate a subset of frequencies to each cell

Use low-power transmitters with lower height

antennas at the base stations

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Large coverage area, efficient spectrum

utilization and enhanced system capacity are

the major attributes of cellular communication

Frequency reuse is the core concept of cellular

communications

The design process of selecting and allocating

channel groups for all the cellular base

stations within a system is called frequency

reuse

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Cells, which use the same set of frequencies,

are referred to as cochannel cells

Co-channel cells are located sufficiently

physically apart so as not to cause cochannel

interference

The space between adjacent cochannel cells is

filled with other cells that use different

frequencies to provide frequency isolation

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Suppose the system has RF

spectrum for 100 voice

channels

Scenario 1: A high power

base station covering entire

area – system capacity = 100

channels

Scenario 2: Divide spectrum

into 4 groups of 25 channels

each; cells (1, 7), (2, 4), (3, 5),

6 are assigned distinct

channel groups – system

capacity = 175 channels

1

2

6

3

4

5

7

HPT – High Power Transmitter

LPT – Low Power Transmitter

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Technical issues for proper design and planning

of a cellular network:

Selection of a suitable frequency reuse pattern

Physical deployment and radio coverage

modeling

Plans to account for the expansion of the

cellular network

Analysis of the relationship between the

capacity, cell size, and the cost of the

infrastructure

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In a cellular system, the whole

geographical service area is divided into a

number of clusters having finite number

of cells.

The K number of cells in a cluster (cluster

size) use the complete set of available

frequency channels, N.

Each cell in the cluster contains J = (N/K)

number of channels only

Therefore, N = J x K; where J ≤ N

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The cluster can be replicated many times to

cover the desired geographical area by a

cellular communication system

Let M be the number of clusters in the

system, then overall system capacity, C is

given as

C = M x N → C = M x J x K (N = J x K)

When K is reduced, J is proportionally

increased since N = J x K is constant.

To increase C, M is increased. But reducing K

will increase co-channel interference!

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Cells which use the same set of

frequencies are referred to as

cochannel cells.

The interference between cochannel

cells is referred to as cochannel

interference.

The space between adjacent

cochannel cells are filled with cells

using different frequencies.

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The shift parameters i

and j are separated by

60 degrees in a

hexagonal geometry

It can have any

integer value 0, 1, 2,

….

These can be used to

determine the location

of cochannel cells

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Rules for determining the nearest cochannel cell

using “Shift parameters” (i, j) to lay out a

cellular system is:

Step 1: Move i cells along any side of a

hexagon.

Step 2: Turn 60 degrees anticlockwise

Step 3: Move j cells.

where i and j are shift parameters and can have

integer value 0, 1, 2, 3, and so on …

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Distance between

two adjacent cells,

d = √3 R

Let D be the distance

between two

co-channel cells.

D = √{3R2(i2+j2+ixj)}

D2/(3R2) = i2+j2+i x j

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By joining the

centers of the six

nearest cochannel

cells, a large

hexagon is formed

with radius equal

to D, which is also

the cochannel cell

separation.

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Asmall hex = (3√3/2)xR2

Alarge hex = (3√3/2)xD2

Number of cells in large

hexagon, L = D2∕R2

Number of cells enclosed

by large hexagon,

L = K+6x[(1 ∕3)x K] = 3 x

K

K = D2∕ (3 x R2)

K = i2 + j2 + i x j

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1

1 1

1

2 2

2 2

3

3

3

3

4

4 4

4 5

5 5

5

6

6 6

6

7

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7

7

8 8

8 8

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11 11

11 11

12 12

12 12

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Reusing an identical frequency channel in

different cells is limited by co-channel

interference between cells.

The co-channel interference can become a

major problem in cellular communication.

It is desirable to find the minimum frequency

reuse distance D in order to reduce this co-

channel interference.

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The number of co-channel cells in the vicinity

of the center cell

The type of geographic terrain contour

The antenna height

The transmitted power at each cell site

NOTE: As long as the cell size is fixed, co-

channel interference is independent of

transmitter power of each cell.

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q = D ∕ R The frequency reuse ratio, q is also referred as

the co-channel reuse ratio

the co-channel reuse factor

co-channel interference reduction factor

R

C1

R

C1

D

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7

6

1

2

3

4

5 7

6

1

2

3

4

5 R

D

• The real power of the cellular concept is that

Interference is not related to the absolute distance between cells

but related to the ratio of the distance between co-channel (same frequency) cells to the cell radius

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The frequency reuse factor, q can be

determined from the cluster size K as

q = D / R = (3K)

The frequency reuse factor determines the

minimum distance for repeating a set of

frequencies

The spectrum efficiency is most significantly

influenced by the frequency reuse factor

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Theoretically, a large value of D is desired to

reduce co-channel interference. Large D can

be obtained by choosing large cluster size K.

However, when K is too large, the number of

channels assigned to each of K cells becomes

small since N is fixed.

As K increases, system capacity per cell and

hence spectrum efficiency reduces

significantly.

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Now the challenge is to obtain the smallest K which can still meet the desired system performance requirements in terms of capacity per cell, spectrum utilization efficiency and received signal quality (minimum co-channel interference).

This involves estimating co-channel interference and selecting the minimum frequency reuse distance D to reduce co channel interference (and hence small K).

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Advantage: The

frequency reuse

system can

drastically increase

the spectrum

efficiency, thereby,

increasing the

system capacity.

Disadvantage: If the

system is not

properly designed,

co-channel

interference may

occur due to the

simultaneous use of

the same channel.

Co-channel Interference is the major

concern in the concept of frequency reuse.

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Sources of Interference

Another mobile in the same cell

A call in progress in a neighboring cell

Other base stations operating in the same frequency

band or

Any non cellular system which inadvertently leaks

energy into the cellular frequency band

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Interference is more pronounced in urban areas due to greater RF noise floor and the large number of base stations and mobiles.

Capacity cannot be increased.

On Control Channels

Interference leads to missed calls and blocked calls due to error in the digital signaling

On Voice Channels

Causes cross talk

where the

subscriber hears

interference in the

background due to

an undesirable

transmission.

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A channel scanning mobile receiver records

three received signals while moving in any one

co-channel cell, under following conditions:

When only serving cell transmits (signal

recorded is termed as C)

Cell-sites of all six co-channel cells only

transmit (signal recorded is termed as I)

No transmission by any cell site (signal

recorded is termed as N)

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If C/I > 18 dB in most of the area being served

by a cell, the system is said to be properly

designed.

If C/I < 18 dB and C/N > 18 dB in some areas,

the system is said to have co-channel

interference problem.

If both C/I and C/N < 18 dB and C/I is

approximately same as C/N in a given area,

the system is said to have radio coverage

problem.

If both C/I and C/N are < 18 dB and C/I <

C/N in a given area, the system is said to have

both co-channel interference as well as radio

coverage problem. 45

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When only the mobile unit in the serving cell

transmits (signal recorded is termed as C)

Up to six interference levels are obtained at

the serving cell site from six mobile units

transmitting in six co-channel interfering

cells. (the statistical average signal recorded is

termed as I)

No transmission by any mobile unit (signal

recorded is termed as N)

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Increasing the

separation

between two co-

channel cells (D)

Lowering the

antenna heights

at the cell site

Using directional

antennas at the

cell site

Use of diversity

scheme at

receiver

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The essential principles of cellular

communication include frequency reuse, co-

channel cells, and co-channel interference

Various cell parameters based on regular

hexagonal cellular pattern include cluster,

frequency reuse distance, reuse factor

Co-channel interference reduction methods

aim at maintaining desired signal quality

and cell capacity

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He executed international assignment as Senior Network Consultant with Flextronics International Inc. USA during 2000-02. He was associated with Nokia, AT&T, Cingular Wireless and Nortel Networks, for optimization of 2G/3G Cellular Networks in USA. Since 2003, he is in teaching profession in engineering colleges in India. He has number of technical research papers published in the IEEE Proceedings, Journals, and International/National Conferences. He has authored two text-books `Wireless Communications’ and `Analog & Digital Communications’, published by renowned publisher Tata McGraw-Hill.

T. L. Singal graduated from National Institute of Technology, Kurukshetra in 1981 . He began his illustrious career with Avionics Design Bureau at HAL, Hyderabad and worked on design and development of Air-Route Surveillance Radar Communication System. Then he led R&D group in a Telecom company and successfully developed Multi-Access Wireless Communication Systems.

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T L Singal, Wireless Communications,

ISBN: 978-0-07-068178-1,

Tata McGraw-Hill, First Edition, 2010.

www.mhhe.com/singal/wc

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tl.singal@chitkara.edu.in Contact No: +91-98145-42041

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