1

PART I A Brief History

PART II Cellular Concepts

PART III GSM- Radio Interface

CELLULAR MOBILE RADIO

2

MOBILE COMMUNICATION

• REVOLUTION IN TELECOM.

• MOBILE COMMUNICATION IS A VERY RAPIDLY GROWING AND A POPULAR SERVICE.

• MORE THAN 40 FOLD INCREASE IN LAST 10 YEARS

• IT HAS BECOME A BACKBONE FOR BUSINESS SUCCESS AND EFFICIENCY

• CHANGED THE LIFE STYLE ALL OVER THE WORLD.

3

History• First mobile service started in 1946 in St.

Louis, Missouri, USA.– Manually operated service, restricted area of

service, only few lucky subscribers.

• Between 1950-60, it evolved to be automatic with decreased cost.

• Mobile telephony service appeared in its useful form in 1960s.

4

Mobile Communications: 1980s Analog systems: 1st Generation

• AMPS: Advanced mobile phone service USA: 800 MHz band

• TACS: Total Access communication system UK : 900 MHz band

• NMT: Nordic Mobile telephone service• Scandinavian: 450 MHz & 900 MHz band

5

Mobile Communications:1990s Digital systems: 2nd Generation

• DAMPS: Digital AMPS• USA: 800 MHz band, IS-54, IS-136

• CDMA: Code division multiple access system: US: 900 MHz band :IS-95

• GSM: Global system for mobile comm. Europe: 900/1800 MHz

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• Any time Anywhere

• Mobility & Roaming

• High capacity & subs. density

• Efficient use of radio spectrum

• Seamless Network Architecture

• Low cost

• Flexibility

• Innovative Services

• Standard Interfaces

MOBILE COMMUNICATION

OBJECTIVES

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MOBILE COMMUNICATION

• 1 G -analog (cellular revolution)

- only mobile voice services

WIRELESS GENERATIONS

• 2 G - digital (breaking digital barrier) - mostly for voice services & data delivery possible

• 3 G - Voice & data ( breaking data barrier)

- Mainly for data services where voice services will also be possible

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CELLULAR MOBILE CONCEPTS

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CELLULAR MOBILE CONCEPTS

•RADIO IN LOCAL LOOP

•LIMITED AVAILABILITY OF RF SPECTRUM

•INTERFERENCE PROBLEM

•INTERFERENCE AND SYSTEM CAPACITY

•FREQUENCY REUSE PATTERN

•TRUNKING EFFICIENCY

•CELLULAR PRINCIPLE

•CELLULAR ENVIRONMENT

•CAPACITY CONSIDERATIONS

•FUTURE TRENDS

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Propagation lossL in dBs

Transmit power PT

and antenna gain GT

Voice Channels Coverage area

Forward path

Reverse path

Or control

channels

Lines toMSC

Site noice

MS

Operating distance d

Planned cell radius R

Cell radio range - radius R max

Radio Cell Parameters

12

Fundamental problems

• Radio range, or coverage

• no. of channels, or voice circuits

• Full, seamless service coverage

• Large no. of subscribers in the range of millions

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•

FERQUENCY SCARCITY PROBLEM

TelEx.

WIRED NETWORK

100,000 50 Khz = 5 Ghz (NOT POSSIBLE)

Hence Individual RF Loop is not extended

But a Common group of channels is shared

CELLULAR MOBILE CONCEPTS

BW forTelephony speech: 3KHz increases to 25 KHz with FM for Radio Trans.

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CELLULAR MOBILE CONCEPTS

CELLRADIUS(KM)

COVERAGEAREA(KM2)

No. OfSUBS.

No. ofCHANNELS

REQD.

1 3.14 100 8

3 28.3 900 38

10 314 10,000 @ 360

25 1960 60,000 @ 2,000

ASSUMPTIONS Traffic /User = 30 mE , GOS = 1%

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CELLULAR MOBILE CONCEPTS

• 360 * 25 KHz * 2 = 18 000 KHz = 18 MHz FOR A CELL OF 10 KM RADIUS ONLY

• WHICH IS IMPOSSIBLE TO BE ALLOCATED

• HENCE FREQUENCY REUSE IS A MUST TO COVER THE TOTAL SERVICE AREA WITH A LIMITED AVAILABLE RF RESOURCES

• HENCE THE NEED FOR A CELLULAR PRINCIPLE

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CELLULAR MOBILE CONCEPTS

WHAT IS A CELL ?

• A base station (transmitter) having a number of RF channels is called a cell

• Each cell covers a a limited number of mobile subscribers within the cell boundaries ( Coverage area)

• Typical Cell Radius Aprrox = 30 Km (Start up), 1 KM (Mature)

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A CLUSTER OF CELLS

4

5

6

7

2

3

1 1 2 3 4 5 6 7

GIVEN FREQ. RESOURCE

CELLULAR MOBILE CONCEPTS

18

14

2

13

5

6

7 2

6

3

4

5

7DR

CELLULAR PRINCIPLE N=7

1 2 3 4 5 6 7Frequency Reuse Pattern “N”=7

Given Freq Resource

CELLULAR MOBILE CONCEPTS

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CELLULAR MOBILE CONCEPTS

CO-CHL INTERFERENCE :Interference caused by another cell/mobile using the same frequency

D

R

Co Chl Interference is a Function of “Q”the re-use ratio:

Q =D / R

Lower Q Increased Co-Chl Interference

Higher Q Reduced Co-Chl Interference

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CELLULAR MOBILE CONCEPTS

CO - Chl Interference

Q = D /R = 3N N =Cluster SizeR = Size (Radius of Cell)D = Distance between two Co- Chl Cells

N Q=D/R

1 1.733 3.004 3.467 4.589 5.2012 6.00

Higher Q Less Interference Higher NMore Cluster SizeLess RF freq/cellLess Traffic HandlingCapacity of the system

LOWER Q Higher Interference Increased System Handling Capacity

Higher Q Less Interference Higher NMore Cluster SizeLess RF freq/cellLess Traffic HandlingCapacity of the system

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CELLULAR MOBILE CONCEPTS

Co- Chl Interference Reduction Technique

• Antenna Front to Back Coupling Reduces Potential Interference

• Use Directional Antennas Instead of Omni Directional Antennas

• Receives interference from lesser directions

*1

23 f1

f2

f3

Three Sectored CellOmni Directional

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CELLULAR MOBILE CONCEPTS

Improvement in Co- Chl Interference

*1

23 f1

f2

f3

Three Sectored Cell

Q= 4.6 ,N=7 , S/I=14 db (Omni) S/I= 24.5 dB (Three Sectored)

Q= 4.6 ,N=7 , S/I=14 db (Omni) S/I= 29 dB (Six Sectored)

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CELLULAR MOBILE CONCEPTS

FREQUENCY REUSE PATTERN

*1

23 f1

f2

f3

Three Sectored Cell

CDMA = 1/3 ; 1 Cell Pattern & each cell with 3 sectors

GSM = 4/12 ; 4 Cell Pattern & each cell with 3 sectors

DAMPS = 7/21 ; 7 Cell Pattern & each cell with 3 sectors

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CELLULAR MOBILE CONCEPTS FREQUENCY REUSE PATTERN

4/12 7/21

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CELLULAR MOBILE CONCEPTS

Extract from Traffic Table - Erlang B Model

Numberof AccessChannels

GoS

0.5% 1.0% 2 %

5 1.13 1.36 1.6610 3.96 4.46 5.0820 11.1 12.0 13.233 21.5 22.9 24.650 36.0 37.9 40.356 41.2 43.3 45.999 80.0 84.1 87.0100 80.9 85.0 88.0

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CELLULAR MOBILE CONCEPTS

TRUNKING EFFICIENCY

• More The Number Of Access Channels In A Cell Further Increase In The System Handling Capacity

• The Number Of Users Served In A Cell Are Directly Proportional To The Access Channels Allocated In A Cell

TRUNKING EFFICIENCY

• It is better to have a single cell than to split into two with half the number of access channels

201010

132 Subs50 subs50 subs

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Cellular Environment• Cellular Environment is quite different from fixed radio

systems

-Cellular approach

-MOBILITY of the user

-Dynamically changing surrounding terrain conditions

RF Signal attenuates, RF d- =4 (generally)

• Multipath Fading Distortions

• Signal fluctuations due to mobility of the user

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CELLULAR ENVIRONMENT

Techniques• Power Control • Channel Coding• Interleaving• Equalization• Slow Frequency Hopping• Antenna Diversity

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GSM CAPACITY CONSIDERATIONS

1

1

1

1

2

8 8 Access Channels 1--Signaling 7- Voice

With 2 % GoS 2.94 E 2.94E/25mE=120 Subs

120 Subs/Sector 3 = 360 Subscribers

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MOBILE COMMUNICATION

WORLD CELLULAR MARKET 1N 2004

• ASIA -PACIFIC 35% 225 M

• WESTERN EUROPE 30 % 192.85 M

• NORTH AMERICA 17 % 109.25 M

• LATIN AMERICA 8 % 51.5 M

• EASTERN EUROPE 5 % 32.25 M

• AFRICA 3 % 19.25 M

• MIDDLE EAST 2 % 12.5 M

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Public Land Mobile Network• INDIA has adopted GSM standard for PLMN.• Digital Cellular System.• Operates at 900 MHz.• International Roaming facility.• Power class 0.8 to 20W.• Cell Radius upto 35 Kms.• Maximum mobility speed 250 Km/hr.

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GSM - RADIO INTERFACE

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Communication - Mobile

TelephoneExchange

SubscriberLine (2W)

Inter-ExchangeJunction

Mobile SwitchingCentre (MSC)

BSC BTSMS

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GSMRADIO INTERFACE

• Most Important Interface

• Full Compatibility between mobile stations of various Manufacturers & Networks of different vendors to help roaming

• To increase spectral efficiency

-- Large number of simultaneous calls in a given

bandwidth

-- Frequency Reuse

-- Interference

-- Use of Interference Reduction Techniques

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GSM

Specifications - II • Frequency Bands -

Mobile to Cell (UP-LINK) - 890 to 915 MHz

Cell to Mobile (DOWN -LINK) - 935 to 960 MHz

• Channel Bandwidth - 200 KHz• Access Method - TDMA/FDMA• Modulation - GMSK

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GSMSpecifications - III

• Number of Channels- 124

• Voice Channel Coding - ( 13 Kbps) RPE-LTP

RPE-LTP - Regular Pulse Excitation Long Term Prediction

FULL RATE - 13Kb/s ; HALF RATE - 6.5 Kb/s

• Bit Rate - 270.833 Kbps

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GSM - MULTIPLE ACCESS• GSM uses both FDMA & TDMA

Freq

Mhz. 890.2

0

890.4

1

890.6

2

890.8

3

891.0

4 5

915

124

• FDMA Access along Frequency axis

• Each RF carrier 200khz apart• Total 124 RF Channels available. One or more carrier assigned to each base station

• Absolute Radio Freq Carrier Number (ARFCN) 0 & 124 not used untill it is co-ordinated with Non -GSM operators in adjacent freq. bands.

• In most cases 124 RF Channels are used

……...

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GSMFDMA

25 MHz 25 MHz

Mobile to Base

0 1 2

890.2 890.4 890.6(MHz)

Base to Mobile

0 1 2

935.2 935.4 935.6

200 kHz45MHz

Channel layout and frequency bands of operation

890 935 960915

200 kHz

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GSMTDMA

8765

43

21

8765

43

21

45 MHz

Frequency

F2’F1’(Cell transmit)

F2F1(Cell Rx)

Amplitude

Typical TDMA/ FDMA frame structure