unit 11: gsm (dcs1900) - esde.alesde.al/fti/sistemet e telekomunikacioneve/books/gsm.pdf · common...

RF Engineering

Continuing Education and Training

Unit 11

GSM Based Networks

GSM 900 – DCS 1800 – PCS 1900

Prepared by:

TEC CELLULAR, Inc. a Division of SAFCO Corporation

7619 Emerald Drive

West Melbourne, FL 32904 USA

Phone (407) 952-8300

Fax (407) 725-5062

www.tecc.com

http://www.tecc.com/

RF Engineering Continuing Education

GSM Based Networks

Copyright © 1998 by TEC CELLULAR

ii

GSM Based Networks

Target Audience

This class is intended for both experienced and novice RF engineers.

Course Description

This one-day course covers one of the international TDMA digital standards, GSM.

Through the course students should gain practical knowledge of GSM features and

system operation.

Objectives

Upon completion of the course students will:

Be able to identify GSM system features and organization

Have a detailed knowledge of:

Common air interface, handovers, voice and error control coding, radio interface

and modulation, time advancing, equalization, sleep mode, network services,

MAHO, DTX, power control, etc.

Be able to perform essential tasks of RF System design such as:

Link budget analysis, frequency reuse efficiency, frequency planning, capacity

calculation and system optimization.

Length: 8 Hours


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Table of Contents

1 Introduction. ................................................................................................................ 1

1.1 History and Development of GSM, DSC1800 and DCS1900 ................................ 1

1.2 Usage of GSM across the world ............................................................................. 3

1.3 GSM Standards ....................................................................................................... 6

2 System Organization ................................................................................................... 7

2.1 What is TDMA? ...................................................................................................... 7

2.2 GSM as a TDMA system ........................................................................................ 8

2.3 GSM Efficiency ...................................................................................................... 9

2.4 GSM System Components .................................................................................... 10

2.5 RF Carrier ............................................................................................................. 12

2.5.1 GSM - Europe ............................................................................................... 12

2.5.2 Extended GSM -Europe ................................................................................ 12

2.5.3 DCS-1800 ..................................................................................................... 12

2.5.4 PCS-1900 ...................................................................................................... 13

2.6 Time Slots and TDMA Frames ............................................................................. 14

2.7 Physical channels and bursts ................................................................................. 15

2.7.1 Dedicated Control Channels ......................................................................... 16

2.7.1.1 Stand Alone Dedicated Control Channel (SDCCH) ................................. 16

2.7.1.2 Slow Associated Control Channel (SACCH) ........................................... 16

2.7.1.3 Fast Associated Control Channel (FACCH) ............................................. 16

2.7.2 Common Control Channels........................................................................... 16

2.7.2.1 Random Access Channel (RACH) ............................................................ 16

2.7.2.2 Paging Channel (PCH) .............................................................................. 16

2.7.2.3 Access Grant Channel (AGCH) ................................................................ 17

2.7.3 Broadcast Channels ....................................................................................... 17

2.7.3.1 Broadcast Control Channel (BCCH) ........................................................ 17

2.7.3.2 Frequency Correction Channel (FCCH) ................................................... 18

2.7.3.3 Synchronization Channel (SCH)............................................................... 18

2.7.4 Bursts ............................................................................................................ 18

2.7.4.1 Normal burst (NB) .................................................................................... 18

2.7.4.2 Frequency correction burst (FB) ............................................................... 19

2.7.4.3 Synchronization burst (SB) ....................................................................... 19

2.7.4.4 Dummy Burst ............................................................................................ 20

2.7.4.5 Access burst (AB) ..................................................................................... 20

2.7.5 Guard period ................................................................................................. 21

2.8 Call Processing Messages ..................................................................................... 21

2.9 Processing the Voice Signal.................................................................................. 22

2.9.1 The GSM codec ............................................................................................ 23

2.10 Data Coding ...................................................................................................... 25

2.11 Interleaving ....................................................................................................... 27

2.12 Equalization ...................................................................................................... 29

2.13 Modulation ........................................................................................................ 31

2.13.1 Binary Frequency Shift Keying (BFSK)....................................................... 31

2.13.2 Minimum Shift Keying (MSK) ..................................................................... 31


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2.13.3 Gaussian Minimum Shift Keying (GMSK) .................................................. 33

2.13.4 GMSK specifics for GSM ............................................................................. 35

3 GSM RF Planning ..................................................................................................... 38

3.1 Link Budget .......................................................................................................... 38

3.1.1 Example of a GSM Link Budget .................................................................. 38

3.1.1.1 Calculation of Receiver Sensitivity .......................................................... 38

3.1.1.1.1 Noise Figure of the Receiver ............................................................ 39

3.1.1.1.2 Other Sources of Noise ..................................................................... 39

3.2 Calculation of the Nominal Cell Radius ............................................................... 41

3.3 Reuse Efficiency ................................................................................................... 43

3.4 Capacity Calculations ........................................................................................... 44

3.5 Frequency Planning .............................................................................................. 46

3.6 Interference Reduction Strategies ......................................................................... 48

3.6.1 Hierarchical Cell Structures .......................................................................... 48

3.6.2 Overlay/Underlay .......................................................................................... 48

3.6.3 Frequency Hopping ....................................................................................... 50

3.7 MAHO .................................................................................................................. 52

3.7.1 Signal Strength Measurement Technique ..................................................... 52

3.7.2 BER Measurement Technique ...................................................................... 53

3.8 Power Control ....................................................................................................... 54

3.9 Time Delay Estimation ......................................................................................... 56

3.10 Discontinuous Transmission ............................................................................. 58

3.11 Mobile Station ................................................................................................... 59


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Index of Figures

Figure 1: An illustration of the TDMA concept ................................................................. 7

Figure 2: GSM as a TDMA/FDMA system ........................................................................ 8

Figure 3: GSM System Architecture................................................................................. 11

Figure 4: PCS Spectrum Allocation in the United States ................................................. 13

Figure 5: TDMA Frame Hierarchy ................................................................................... 14

Figure 6: GSM - Time Division Duplex ........................................................................... 14

Figure 7: Logical Channels ............................................................................................... 15

Figure 8: Control Multi-Frame ......................................................................................... 17

Figure 9: Normal Burst ..................................................................................................... 19

Figure 10: Frequency Correction Burst ............................................................................ 19

Figure 11: Synchronization Burst ..................................................................................... 20

Figure 12: Dummy Burst .................................................................................................. 20

Figure 13: Access Burst .................................................................................................... 21

Figure 14: Messages Sent during Call Processing ............................................................ 21

Figure 15: Sampling and Quantization of Analog Signal ................................................. 22

Figure 16: Scheme of audio signal processing in telephony............................................. 23

Figure 17: Block diagram of GSM speech encoder .......................................................... 24

Figure 18: Block diagram of GSM speech decoder .......................................................... 24

Figure 19: Block diagram of channel coding for GSM technology.................................. 25

Figure 20: Illustration of the interleaving process ............................................................ 27

Figure 21: Interleaving for the traffic channel in GSM .................................................... 28

Figure 22: An example of a Power Delay Profile (PDP). ................................................. 29

Figure 23: Principals of the GSM equalizer operation ..................................................... 30

Figure 24: Example of MSK modulation.......................................................................... 33

Figure 25: Shape of the time and frequency response of the „Gaussian‟ filter ................. 34

Figure 26: Spectral characteristics of GMSK and MSK ................................................... 34

Figure 27: Bandwidth of the GSM signal versus the percentage of signal power. ........... 36

Figure 28: Modulation scheme for GSM technology ....................................................... 36

Figure 29: N=3 Reuse Scheme ......................................................................................... 46

Figure 30: N=4 Reuse Scheme ......................................................................................... 47

Figure 31: Area of interference ......................................................................................... 49

Figure 32: Signal strength at point of interference ........................................................... 49

Figure 33: Baseband Hopping .......................................................................................... 51

Figure 34: Synthesizer Hopping ....................................................................................... 52

Figure 35: Illustration of the MAHO Measurement Process ............................................ 52

Figure 36: Regulation of MS Power Control .................................................................... 54

Figure 37: TDMA MS transmission Time Alignment ...................................................... 56


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Index of Tables Table 1: Percentage of total signal energy (%E) carried by the bandwidth (B E) ............. 35

Table 2: Example of GSM Budget – Rural Environment ................................................. 40

Table 3: Example of GSM Link Budget – Suburban Environment .................................. 40

Table 4: Example of GSM Link Budget – Urban Environment ....................................... 41

Table 5: Typical Slope and Intercept Values. ................................................................... 42

Table 6: Capacity of Network Using 3/9 Reuse Scheme .................................................. 45

Table 7: Capacity of Network Using 4/12 Reuse Scheme ................................................ 45

Table 8: Signal Strength / RXLEV Mapping .................................................................... 53

Table 9: BER /RXQUAL Mapping .................................................................................. 53

Table 10: Power Control Levels ....................................................................................... 55

Table 11: Analysis of Voice Activity Factors................................................................... 58

Table 12: PCS-1900 Mobile Class and Power .................................................................. 59

Table 13: DCS-1800 Mobile Class and Power ................................................................. 59

Table 14: GSM-900 Mobile Class and Power .................................................................. 59


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1 Introduction.

The concept of cellular theory was introduced by Bell Labs and studied around the world

during the 1970‟s. Cellular communications consists of numerous base stations with

transmission and reception capabilities, whose individual coverage areas partially

overlap. The frequencies that are transmitted and received at each base station are reused

between clusters of cells. The United States first implemented the cellular concept in

1979 with the development of the Advanced Mobile Phone System (AMPS), where a pre-

operational network was deployed in Chicago, Illinois. Meanwhile, Northern Europe

countries worked together to develop the Nordic Mobile Telephone (NMT) system. The

system was operational in Sweden by 1981, and later on other European countries.

Networks based on these two analog specifications accounted for most cellular networks

during the 1980‟s.

This successful deployment and operation of NMT systems throughout Northern Europe

drove other countries to develop systems such as United Kingdom‟s Total Access

Communications System (TACS, derived from United States‟ AMPS), France‟s

Radiocom 2000, Italy‟s RTMS, and Germany‟s C-450. Of all these, NMT and TACS

systems gained the most popularity in the European community. Eventually, most

countries established their local services using either NMT or TACS based systems. This

created problems for many European countries later on since these two systems (and

variations of them, obviously) were not compatible, and some operated in different

frequency bands (450 MHz & 900 MHz).

1.1 History and Development of GSM, DSC1800 and DCS1900

Throughout the early 1980‟s, European cellular networks, with nation-wide coverage and

several hundreds of thousands of subscribers, experienced enormous growth. The

cellular networks used an analog air interface (NMT and TACS) that by today‟s

standards was not spectrally efficient. Because the demand was so high, the operators

were forced to install more analog infrastructure to handle this constantly increasing

capacity. As networks were launched the availability of additional spectrum became

scarce. Service providers even borrowed frequencies from the spectrum that had been

allocated to a future “European Cellular System”. Therefore, adding more systems was

no longer a feasible resolution to the demand problem.

Along with the capacity problems, the European community was working toward

standardization of the wireless telecommunications arena. Most networks were

developed specifically for operation within a particular country. Since the air interfaces

were incompatible, it was impossible for wireless subscribers to roam in neighboring

countries. At the 1982 Conférence Européenne des Postes et Télécommunications

(CEPT - Conference of European Posts and Telecommunications), a new standardization

group was formed and tasked with the specification of a unique 900 MHz radio

communication system for Europe. More than 20 European countries were represented at

this conference to insure the definition of a standard that made sense and suited everyone.

The group responsible for writing the new standard was named “Groupe Spéciale


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Mobile” (GSM). The standard was later renamed Global System for Mobile

Communications (GSM).

From its inception, the group had several clear objectives for the GSM standard including

roaming and improved capacity. Roaming was desirable so that the subscribers could

travel freely throughout Europe while using a single mobile. This was a very ambitious

goal since it required the alliance and commitment of wireless industries across the

continent, some of which were already providing wireless service at different

frequencies. Capacity improvements were desired by using a spectrally efficient

technology relative to the existing analog network. The objective was for GSM to

offload the analog networks without encountering capacity problems soon after. Many

basic objectives were stated at the onset of the GSM standard. However, it took years for

these to become firm requirements and then a reality.

The original requirements for the GSM standard did not specify the technology to be used

for speech transmissions. However, the trend in telecommunications was toward digital

technology. Since the GSM group was interested in developing a standard that would

serve the needs of the fast growing European subscriber base, they concentrated their

efforts in comparing analog and digital technical specifications, with respect to spectral

efficiency. The group opened the forum for technological proposals from industry,

business, and government agencies across Europe. All prototype systems submitted were

digital, and completely compatible with the new wire-line digital standard, Integrated

Services Digital Network (ISDN). Given the fact that wireless communications relies on

the wire-line infrastructure, it made sense to take advantage of the latest advances in

wire-line technology.

Some of the important factors that led to the development of the GSM are:

The need for a system without the capacity limitations of the existing

cellular networks.

The spectrum was already reserved in the 900 MHz band throughout

Europe.

The need to deregulate mobile telephony.

The unification of the European community politically, economically and

socially.

The incentive for European equipment manufacturers to develop and

produce new infrastructure and mobile terminals for a market that is much

larger than the small markets in single countries.

Exportation of technology and products to countries outside Europe.

Some of the requirements the standardization group set for the new system were:

To be able to economically cover vast areas as well as densely populated

urban and suburban territory.

It should work reliably at high speeds (in fast moving vehicles, i.e. cars,

trains)


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It should work reliably in urban jungles in the hands of pedestrians among

tall buildings, and within large buildings, parking structures, airports and

trains stations.

The system must not be based on existing technologies or systems to avoid

favoritism toward regional institutions or industries.

1.2 Usage of GSM across the world

The following tables list the nations currently committed to GSM technology. Most of

the countries have already launched commercial systems, although some of the countries

have only committed to GSM deployment.

Africa Ghana GSM 900 96 October Scancom

Libya GSM 900 97 March MADAR Telephone Company

Morocco GSM 900 98 Itissalat

South Africa GSM 900 94 June MTN

Zimbabwe GSM 900 98 Econet

North America Canada GSM 1900 96 November Microcell Telecom Inc.

USA GSM 1900 95 November American Personal Communications

USA GSM 1900 96 November Pacific Bell Mobile Services

USA GSM 1900 Pocket

USA GSM 1900 96 October Powertel

USA GSM 1900 96 November Omnipoint Communications Inc.

USA GSM 1900 97 March Airadigm

Latin America Chile GSM 1900 Entel

Pacific Australia GSM 900 93 April Telstra

Australia GSM 900 93 September Vodafone

Fiji GSM 900 94 July Vodafone Fiji

Middle East Bahrain GSM 900 95 April Batelco

Iran GSM 900 96 January KIFZO

Lebanon GSM 900 95 February FTML / Cellis

Oman GSM 900 GTO

UAE GSM 900 95 December Etisalat

http://www.fido.ca/engl/index.htm

http://www.sprint.com/

http://www.pacbell.com/home.html

http://www.powertel.com/

http://www.omnipoint.com/

http://www.entelchile.net/entel/entel.htm

http://www.telstra.com.au/

http://www.vodafone.co.uk/n3/home.html


http://www.batelco.com.bh/

http://www.etisalat.co.ae/


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Asia Azerbadjan GSM 900 96 November Azercell Telekom B.M.

Bangladesh GSM 900 97 March GrameenPhone

China GSM 900,1800 93 December Guandong Mobile Com. Corp.

China GSM 900 95 August Liaoning PTA

China GSM 900 96 January Guangxi PTA

China GSM 1800 Shanghai PTA

China GSM 900,1800 95 September Heilongjiang PTA

China GSM 900 95 August Jiangsu PTA

China GSM 900 95 September Shandong PTA

China GSM 900 Tibet PTA

China GSM 900 96 January Hebei PTA

China GSM 900 96 August Sichuan PTA

China GSM 900 95 September Unicom (Jiangsu)

China GSM 900 96 May Chongqing PTB (Sichuan)

China GSM 900 Unicom (Anhui)

China GSM 900 Unicom (Sichuan)

China GSM 900 Unicom (Hainan)

China GSM 900 Unicom (Jilin)

China GSM 900 93 January SmarTone

(Hong Kong) China GSM 1800 97 January Peoples Telephone Company

(Hong Kong) Georgia GSM 900 97 March Geocell

India GSM 900 95 September Bharti Cellular (New Delhi)

India GSM 900 95 September Hutchison Max Telecom

(Mumbai) India GSM 900 95 September RPG Cellular Services

(Chennai) India GSM 900 97 January Birla AT&T Communications

(Maharshtra) India GSM 900 97 March Birla AT&T Communications

(Gujarat) India GSM 900 97 February RPG Cellcom (Madhya Pradesh)

India GSM 900 97 January JT Mobiles (Karnataka)

India GSM 900 97 January JT Mobiles (Andhra Pradesh)

India GSM 900 97 January Bharti Televentures

(Himachal Pradesh) India GSM 900 97 May Hexacom (Rajasthan)

India GSM 900 Reliance Telecom (Bihar)

India GSM 900 Reliance Telecom (Orissa)

India GSM 900 Reliance Telecom (West Bengal)

India GSM 900 Reliance Telecom (Assam)

India GSM 900 Reliance Telecom (North East)

India GSM 900 Reliance Telecom

(Madhya Pradesh) India GSM 900 Reliance Telecom

(Himachal Pradesh) Indonesia GSM 900 94 July Telkomsel

Indonesia GSM 900 96 October PT. Excelcomindo Pratama

Laos GSM 900 94 December EPTL / Shinawatra

Macau GSM 900 95 October CTM

Malaysia GSM 900 95 July Celcom

Malaysia GSM 1800 5 July Mutiara Telecommunications

Singapore GSM 900 94 April Singapore Telecom

Taiwan GSM 900 94 October Shinawatra (AIS)

Uzbekistan GSM 900 Daewoo Corporation

Vietnam GSM 900 94 April VMS / CIV

http://www.telkomsel.com/

http://www.macau.ctm.net/

http://www.celcom.com.my/

http://www.mutiara.com.my/

http://www.singnet.com.sg/

http://www.ais.co.th/


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Europe Bosnia-Herzegovina GSM 900 96 November PTT

Bosnia-Herzegovina Cyprus GSM 900 95 April CYTA - PTT

Denmark GSM 900,1800 92 March Tele Danmark Mobil

Denmark GSM 1800 Telia Mobile

Estonia GSM 900,1800 95 January Eesti Mobiltelefon

Finland GSM 900,1800 92 June Telecom Finland

France GSM 1800 96 May Bouygues Telecom

France GSM 900 92 July France Telecom Itineris

France GSM 1800 96 November FTM 1800

Germany GSM 900 92 July Mannesmann Mobilfunk

Gibraltar (UK) GSM 900 95 January Gibtel

Greece GSM 900 93 June Tele STET

Greece GSM 900 93 July Panafon

Guernsey (UK) GSM 900 96 March Guernsey Telecom

Hungary GSM 900 94 March Westel 900

Iceland GSM 900 94 August Postur OG Simi

Ireland GSM 900 93 June Telecom Eireann

Isle Of Man (UK) GSM 900 96 March Manx Telecom

Italy GSM 900 92 October Telecom Italia

Lithuania GSM 900 95 November Mobilios Telekomunikacijos

Luxembourg GSM 900,1800 98 Millicom Luxembourg

Macedonia GSM 900 96 October PTT Macedonia

Netherlands GSM 900 95 September Libertel

Netherlands GSM 900 94 July Netherlands PTT

Norway GSM 900 93 May Telenor Mobil (PTT)

Poland GSM 900 96 September Polska Telefonia Cyfrowa(PTC)

Portugal GSM 900 92 October Telecel

Portugal GSM 900 92 October TMN

Romania GSM 900 97 April MobiFon

Russia GSM 900 Udmurt Telecom

Russia GSM 900 98 Tatarian-American Investment &

Finance

Slovakia GSM 900 97 February Eurotel

Slovenia GSM 900 96 September Mobitel

Spain GSM 900 95 July Telefonica MoviStar

Spain GSM 900 95 October Airtel

Sweden GSM 900 92 September Europolitan

Sweden GSM 900,1800 92 November Telia Mobitel

Switzerland GSM 900,1800 93 March Swiss PTT

Turkey GSM 900 94 March Turkcell

Ukraine GSM 900 98 Kiev Star

United Kingdom GSM 900 92 July Vodafone

United Kingdom GSM 900 94 July Cellnet

United Kingdom GSM 1800 93 September One-2-One

Yugoslavia (Montenegro) GSM 900 96 July ProMonte GSM

Yugoslavia (Serbia) GSM 900 96 November Mobile Telecom

http://www.cytanet.com.cy/

http://www.teledanmark.dk/english/english.htm

http://www.emt.ee/

http://www.tfi.net/

http://www.bouyguestelecom.fr/

http://www.francetelecom.fr/

http://www.d2privat.de/

http://www.telestet.gr/

http://www.panafon.gr/

http://www.westel900.net/index_e.html

http://www.simi.is/pti.html

http://www.telecom.ie/

http://www.manx-telecom.com/

http://www.tim.it/

http://lotus.mpt.com.mk/

http://www.libertel.nl/

http://www.ptt-telecom.nl/

http://www.telenor.no/

http://www.eragsm.pl/

http://www.telecel.pt/

http://www.eurotel.sk/

http://www.mobitel.si/

http://www.telefonica.es/

http://www.airtel.es/

http://www.europolitan.se/

http://www.telia.se/

http://www.vptt.ch/

http://www.turkcell.com.tr/


http://www.cellnet.co.uk/

http://www2.one2one.co.uk/one2one/


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1.3 GSM Standards

GSM is an international standard for wireless voice and data communications. The

following standards are of primary interest to RF network design engineers and are

frequently referenced throughout this document:

GSM 04.01 “European digital cellular telecommunication system (Phase 2);

Mobile Station – Base Station System (MS – BSS) interface

General aspects and principles”


Mobile Station – Base Station System (MS – BSS) interface

Channel structures and access capabilities”


Multiplexing and multiple access on the radio path”


Channel coding”


Modulation”


Radio transmission and reception”


Radio subsystem link control”


Radio subsystem synchronization”


Full rate speech processing function”


Full rate speech transcoding”


Substitution and muting of lost frames for full rate speech

channels”


Comfort noise aspects for full rate speech channels ”


Discontinuous Transmission (DTX) for full rate speech channels ”


Voice Activity Detection (VAD) ”


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2 System Organization

2.1 What is TDMA?

GSM is a Time Division Multiple Access (TDMA) technology. In TDMA systems,

multiple users operate on the same frequency. However, they do not operate

simultaneously. Each user is given access to the system at a different time. The principle

of the TDMA scheme is presented in Figure 1.

Uplink ( From MS to BS)

Base Transceiver Station

fu0,

Ts1

fd0,

Ts1, Ts

2, Ts

3, ...,TS

6,TS

7

TS1

TS2

TS3

.... TS6 TS

7

TS2

TS3

.... TS1

TS2

TS3

fu0,

Ts2

fu0,

Ts3

fu0,

Ts4

fu0,

Ts5

fu0,

Ts6

fu0,

Ts7

Downlink ( From BS to MS)

Wireless Communication Channel

Figure 1: An illustration of the TDMA concept

Since users are accessing the channel one at a time, the transmission is discontinuous.

This effectively means that modulation used in TDMA systems must be digital. TDMA

based communications are conducted in an accumulate and burst fashion. While waiting

for the time slot assignment, each user accumulates the portion of the data stream to be

transmitted. When the slot is assigned, all the accumulated data is transmitted over the

channel at higher data rate. The ratio of the channel data rate and user data rate is

directly proportional to the number of users sharing the communication channel. From

Figure 1 we see that slot assignment commonly follows a cyclic pattern. One cycle of the

slot assignments is commonly referred to as frame. In the case of the TDMA system, a

channel can be thought of as a particular slot in the frame. Some advantages of the

TDMA scheme are:


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It has a relatively low complexity.

Because of the discontinuous transmission, the hand-off process is greatly

simplified and the reliability improved. In periods when it is not transmitting or

receiving, the mobile can perform the channel measurements and assist in the hand-

off process – Mobile Assist Hand-off (MAHO).

It is possible to allow different number of slots per frame to different users and

therefore, accommodate users with different data rate requirements.

Some disadvantages of the TDMA are:

High synchronization overhead is required.

Guard times have to be incorporated in order to minimize probability of collision

between the different users.

Only digital modulations can be used.

TDMA, in general, is heavily effected by the selective fading properties of the mobile

channel. For that reason, it is mandatory for TDMA systems to incorporate channel

equalization.

2.2 GSM as a TDMA system

Although GSM is considered a TDMA system, GSM is essentially a combination of two

access schemes TDMA and FDMA. The manner in which mobiles access the GSM

system is illustrated in Figure 2. It can be seen that before the communication

commences, the user has to be assigned with a unique carrier frequency as well as the

time slot. In the case of full rate implementation, seven users share the same carrier

frequency. Since the communication is full duplex, separate frequencies are assigned to

forward and reverse links. This is commonly referred to as frequency division duplexing

(FDD). In short, GSM is a TDMA/FDMA system with FDD.

BTS

USER 1 USER 2 .... USER 8

USER 9 USER 10 .... USER 16

USER 1,

F1

USER 2,

F1

USER 8,

F1

USER 9,

F2

USER 10,

F2

USER

16,

F2

Frequency F1

Frequency F2

Figure 2: GSM as a TDMA/FDMA system


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2.3 GSM Efficiency

There are several types of bits transmitted over the channel in a TDMA based system.

Some of the bits transmitted are control bits necessary for synchronization, training or to

provide guard periods at the end of each time slot‟s transmission cycle. Synchronization

bits are used during access of the network, and training bits are sequences of bits used by

the mobile‟s equalizers. In addition, consecutive bits are dedicated to guard periods to

prevent the collision of transmissions coming from different transmitters.

Control bits present an overhead relative to the data passed over a channel and therefore

decrease the efficiency of the GSM technology. The efficiency of the technology is

calculated as a ratio of the number of bits dedicated to the overhead and total number of

bits transmitted over the GSM channel i.e.[1]:

1001

T

OHf

b

b (1)

where:

f = GSM modulation efficiency.

OHb = Number of bits per frame allocated to overhead channels.

Tb = Total number of bits.

Reliable digital communication over the mobile wireless channel has to be protected with

error control coding. The coding process deliberately introduces redundancy in the

transmitted bit stream that helps correct errors caused by the harsh propagation

environment. As such, the coding can be viewed as a special case of the overhead and

therefore reduces the efficiency of the GSM scheme as well.

Example 1. Consider a digital technology applying a GSM access scheme where the

communication channel is shared by 8 users. Assume that the bit rate coming out of the

vocoder for each of the users is 13 Kb/s. The actual channel data rate is 270.833 Kb/s.

What is the efficiency of the particular GSM access scheme.

Total rate dedicated to the user information is:

sKbbu /104138 (2)

The efficiency of the TDMA is given by:

%4.38100104

104833.2701

f (3)


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2.4 GSM System Components

The GSM system architecture consists of four major parts as depicted in Figure 3. The

four major sub-systems are defined as follows:

1. The Switching Sub-System – controls the call processing and subscriber related

functions. The most important building blocks of the switching system are:

a) MSC (Mobile Switching Center) – Performs the telephony switching

functions for the mobile network. It provides the connection of the network to

the PSTN.

b) AUC (Authentication Center) – Provides authentication and encryption

parameters that verify the identity of every user and insures the privacy of the

conversation. The authentication center also stores information regarding lost,

stolen, or fraudulent phones in the EIR (Equipment Identity Register)

c) HLR (Home Location Register) – Database that contains all subscriber

information, including location for each user within the coverage area of the

MSC.

d) VLR (Visitor Location Register) – Data containing temporary subscriber

information needed for serving of the visiting – roaming customers. Once a

roaming subscriber has been identified within the coverage area of a particular

MSC / VLR, the serving MSC sends information to the subscriber‟s home

network, so that incoming calls are routed to the serving MSC.

2. The Base Station Sub-System – Controls the radio equipment providing the

connection between the mobile user and the land communication network. The

building blocks of the base station sub-system are:

a) BSC (Base Station Controller) – connects the MSC to a network of up to

several hundred base stations.

b) BTS (Base Station) – infrastructure that provides the RF link to the mobile

subscriber.

3. The Operation and Support Sub-System – Supports the maintenance activities of

the network including the following three functions:

a) providing call-processing statistics and troubleshooting functionality

b) provides billing information

c) manages all mobile equipment in the system

4. Mobile Station – End-user device allowing the subscriber to access the system.

The interface between the BSC and the BTS is referred to as the Abis interface. This

interface is GSM specified, although each infrastructure provider has subtle differences

requiring network operators to purchase their equipment from a single manufacturer.

Likewise, GSM has specified an interface between the MSC and the BSC, the A interface

which uses an SS7 protocol. The A interface allows network operators to use different

base stations and switching equipment from different infrastructure providers.


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11

MSC/

VLR

HLR

AUC

EIR GMSC

GIWU

PSTN

Switching

System

BSC

MXE/

MIN

Cellular

Networks

PCS

Networks

BTS

Base

Station

System

Operational

Support

System

OMC

NMC

AUC - Authentication Center

BSC - Base Station Controller

BTS - Base Transceiver Station

EIR - Equipment Identity Register

GIWU - GSM Interworking Unit

GMSC - Gateway MSC

HLR - Home Location Register

MSC - Mobile Switching Center

MIN - Mobile Intelligent Network

MXE - Message Center

NMC - Network Management Center

OMC - OPeration & Maintenance Center

VLR - Visitor Location Register

Figure 3: GSM System Architecture


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2.5 RF Carrier

The RF carrier is the physical frequency assigned to the channel. The following four

sections describe the frequency band and the channel number schemes for the various

applications of GSM.

2.5.1 GSM - Europe

GSM, as initially launched in Europe, uses two – 25 MHz frequency blocks in the 900

MHz band. Since the technology allows full-duplex operation, the base station transmit

spans from 935-960 MHz, while the mobile station transmit spans from 890-915 MHz.

The frequency band is divided into individual 200 kHz channels. Each 200 kHz channel

is numbered according to the Absolute Radio Frequency Channel Number (ARFCN)

scheme. Although there is a capability to use 125 channels, the first channel is used as a

guard band. Therefore the channel assignments are from 1-124. The procedure for

calculating the uplink and downlink frequency, relative to the channel numbers is as

follows:

Uplink

frequency = 890 Mhz + (0.2 Mhz) x ARFCN

Downlink

frequency = 935 Mhz + (0.2 Mhz) x ARFCN

2.5.2 Extended GSM -Europe

As GSM networks matured, an additional 10 MHz of spectrum was allocated, providing

an additional 50 channels of capacity. The channel numbering for the extended GSM

band ranges from 974-1024. Again, the first channel, 974, is reserved for a guard band.

In order to calculate the frequency of a particular channel in the extended band, the

procedure is as follows:

Uplink

frequency = 890 Mhz + (0.2 Mhz) x (ARFCN-1024)

Downlink

frequency = 935 Mhz + (0.2 Mhz) x (ARFCN-1024)

2.5.3 DCS-1800

GSM has also been used as the air interface for the Personal Communications Networks

(PCN) in Europe. Officially referred to as DCS-1800, the technology is essentially the

same as GSM although the channels used are at a higher frequency. DCS-1800 ranges

from 1,805-1,880 MHz in the downlink and 1,710-1,785 MHz in the uplink. Although

the frequency band and the duplex distance are larger, the channel bandwidth remains at

200 kHz. The ARFCN scheme for DCS-1800 ranges from 512-885, where the frequency

of a particular channel can be calculated as follows:


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Uplink

frequency = 1,710 Mhz + (0.2 Mhz) x (ARFCN-511)

Downlink


2.5.4 PCS-1900

GSM has also been used as the air interface for the Personal Communications Networks

(PCN) in United States. Officially referred to as PCS-1900, the technology is essentially

the same as GSM in Europe although the channels used are at a different frequency.

PCS-1900 ranges from 1,850-1,910 MHz in the downlink and 1,930-1,990 MHz in the

uplink (see Figure 4). Although the frequency band and the duplex distance are larger,

the channel bandwidth remains at 200 kHz. The ARFCN scheme for PCS-1900 ranges

from 512-812, where the frequency of a particular channel can be calculated as follows:

Uplink


Downlink


Figure 4: PCS Spectrum Allocation in the United States


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2.6 Time Slots and TDMA Frames

The time frame structure of GSM is divided into time slots, frames, multiframes,

superframes, and hyperframes (see Figure 5). The hyperframe has the longest repetitious

time period, and is composed of 2048 superframes. Fifty-one multiframes comprise one

superframe, and twenty-six voice traffic frames are grouped into one multiframe. Each

hyperframe contains 2,715,647 individual TDMA frames. The GSM timing structure is

setup in this manner to ensure encryption methods provide secure communications.

1 hyperframe = 2048 superframes = 2,715,648 frames

0 1 2 3 4 5 6 2043 2044 2045 2046 2047

1 superframe = 51 (26 frame) multi- frame or 26 (51 frame)multi-frame

0 1 2 3 4 5 6 46 47 48 49 50

0 1 2 3 4 5 6 21 22 23 24 25

1 (26 frame) multi-frame

0 1 2 3 23 24 25

1 (51 frame) multi-frame

0 1 2 3 23 24 25

1 TDMA Frame

0 1 2 3 4 5 6 7

TB Encrypted Data Flag 1 Training Sequence Flag 1 Encrypted Data TB GP

TB Fixed Bits TB GP

TB Encrypted Data Synchronization Sequence Encrypted Data TB GP

TB Synchronization Sequence Encrypted Data TB GP

TB Mixed Bits Training Sequence Mixed Bits TB GP

Normal Bursts

Frequency Correction Burst

Synchronization Burst

Access Burst

Dummy Burst

TB = Tail Bits

GP = Guard Period

Figure 5: TDMA Frame Hierarchy

Eight time slots form a TDMA frame, where each frame has a duration of 4.62 mS.

Therefore each time slot has a period of 577 microseconds. The time alignment of the

TDMA frames is constant at the base station, although adjusted at the mobile station to

compensate for propagation delays. A detailed discussion is provided in Section 3.9.

However, there is a delay of three time slots between the alignment of the uplink and

downlink signal. This allows the mobile station time to process incoming information

before replying back to the base station. The time delay also prevents the mobile from

transmitting and receiving at the same time (see Figure 6).

Base Station Transmits

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0

Mobile Station Transmits

5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5

Figure 6: GSM - Time Division Duplex


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2.7 Physical channels and bursts

The description of a physical channel will be made in terms of timeslots and TDMA

frames and not in terms of bursts. This is because there is not a one-to-one mapping

between a particular physical channel and the use of a particular burst.

In a GSM network, there are two types of channels that are commonly referred to as

physical and logical channels. A physical channel uses a combination of frequency and

time division multiplexing and is defined as a sequence of radio frequency channels and

time slots. The complete definition of a particular physical channel consists of a

description in the frequency domain, and a description in the time domain.

Physical channels are frequency-specific carriers over which the mobile and base stations

communicate. Each carrier is 200 kHz wide and the number of physical channels varies

according to the bandwidth of the spectrum allocation; all channels are divided into eight

time slots.

Logical channels carry specific types of data such as control and traffic information.

Control channels are defined by their specific functions used in call processing, and

traffic channels are defined by their data rate. Figure 7 illustrates the individual

characteristic of the logical channels.

Control

Channels

Logical

Channels

Traffic

Channels

Broadcast

Channels

Common Control

Channels

Dedicated Control

Channels

Figure 7: Logical Channels

Traffic channels carry voice information while control channels are used in the setup of

calls. Control channels are broken up into three categories:

Dedicated Control Channels (DCCH)

Common Control Channels (CCCH)

Broadcast Channels (BCH)

Each of these logical channels performs unique call control functions and is described in

the following subsections.


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2.7.1 Dedicated Control Channels

Dedicated control channels are further broken up into logical channels with unique

functions. All three of these channels are bi-directional, meaning that the channels are

used for uplink and downlink messaging. Their designation and definition are listed as

follows:

2.7.1.1 Stand Alone Dedicated Control Channel (SDCCH)

The Stand Alone Dedicated Control Channel is a temporary channel used to connect the

mobile and base station. After the mobile has originated on the BCH the link is

maintained by the SDCCH before subscriber verification and channel assignment is

complete. Authentication and alert messages are sent via the SDCCH as the mobile

aligns itself with the frame structure.

2.7.1.2 Slow Associated Control Channel (SACCH)

The Slow Associated Control Channel transmit call control data, typically handover

measurement information such as RXLEV and RXQUAL on the uplink, and power

control or timing advance information on the downlink.

2.7.1.3 Fast Associated Control Channel (FACCH)

Fast Associated Control Channel carry urgent signaling information. Messaging

associated with the FACCH is accomplished by replacing frames from a traffic channel.

Two designated bits, dubbed “stealing” bits, are set so that the mobile realizes that the

subsequent data is related to the FACCH.

2.7.2 Common Control Channels

Common control channels are broken up into logical channels with unique functions. The

Random Access Channel (RACH) is used by the mobile to access the network, therefore

it is used only on the uplink. The Paging Channel (PCH) and Access Grant Channels

(AGCH) are used by the base station to communicate with the mobile; therefore, it is

used only on the downlink. Their designation and definition are listed as follows:

2.7.2.1 Random Access Channel (RACH)

The Random Access Channel is used by the mobile to seek access to the network on call

origination or as an acknowledgement of system page. The RACH uses a slotted ALOHA

access scheme, because of the random nature of mobiles accessing the network. RACH

messaging occurs during time slot 0. However, all 51 TDMA frames are available (see

Figure 8 - b). The base station will respond by allocating a traffic channel and assigning

a Stand-Alone Dedicated Control Channel during call setup.

2.7.2.2 Paging Channel (PCH)

The Paging Channel is used by the network for two functions. The primary function of

the PCH is to page individual mobiles, and the secondary function is to send ASCII text

messages as part of the short messaging services feature of GSM networks. When paging

mobile stations, the PCH sends the targeted mobile‟s IMSI and a request for page

confirmation. As mentioned above the mobile responds via the RACH.


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2.7.2.3 Access Grant Channel (AGCH)

The Access Grant Channel is used by the base station to respond to a particular mobile‟s

RACH message. Contained in this logical channel is information that will direct the

mobile to the correct physical channel and time slot.

(a)

Control Multi-Frame Downlink = 51 TDMA Frames

0

F

1

S

2

B

3

B

4

B

5

B

6

C

7

C

8

C

9

C

10

F

11

S

12

C

13

C

14

C

15

C

…

20

F

21

S

…

49

C

50

I

F: Frequency Correction Channel (FCCH)

S: Synchronization Channel (SCH)

B: Broadcast Control Channel (BCCH)

C: Paging Channel / Access Grant Channel (PCH/AGCH)

I: Idle

(b)

Control Multi-Frame Uplink = 51 TDMA Frames

0

R

1

R

2

R

3

R

4

R

5

R

…

…

…

…

…

…

…

…

…

…

…

…

…

…

49

R

50

R

R: Random Access Channel (RACH)

Figure 8: Control Multi-Frame

2.7.3 Broadcast Channels

Broadcast control channels are sub-divided into three logical channels with unique

functions. The Broadcast Control Channel (BCH) operating in time slot 0, is used to

broadcast cell and network identity information to all mobiles. The Synchronization

Channel (SCH) and Frequency Correction Channel (FCCH) also operate in time slot 0.,

Both channels are used to synchronize the mobile station with the base station in

frequency and time. Their designation and definition are listed as follows:

2.7.3.1 Broadcast Control Channel (BCCH)

The Broadcast Control Channel is used to relay the cell and network identity to the

mobile station, as well as neighboring cell frequency information, cell options and access

parameters. In a control multi-frame, the BCCH information is passed to the mobile in

four out of fifty-one specific frames (see Figure 8 – a).


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18

2.7.3.2 Frequency Correction Channel (FCCH)

The Frequency Correction Channel passes information that allows every mobile station to

synchronize its internal oscillator with the base station‟s frequency. It occupies the first

piece of control information in the Control Multi-Frame (51 TDMA frame), and is

repeated every 10 frames (see Figure 8 – a).

2.7.3.3 Synchronization Channel (SCH)

The Synchronization Control Channel follows the FCCH frame and contains the BSIC

and TDMA frame number. Coarse timing advance information is passed to the mobile

over the synchronization channel as well (see Figure 8 – a).

2.7.4 Bursts

Bursts are instantaneous transmissions of data over an RF channel, with voice or data

information. There are five types of burst in GSM which are defined as follows:

Normal – carries voice and control information.

Frequency Correction Burst – used for frequency synchronization.

Synchronization Burst – used for timing synchronization between base station

and mobile station.

Dummy Burst – mixture of bits sent when there is not any other

communications taking place on the BCCH.

Access Burst – used on access and handover where timing advance

information is unknown.

Each timeslot is divided into 156.25 bit periods, where every bit period is referenced to a

bit number. The first bit period is numbered 0 and the final .25 bit period is numbered

156. Bit 0 is transmitted first and continues through to bit 156. The description of bit

numbering is important because the following sections describe the transmission timing

and data associated with bit period during a burst. Guard periods occur between

consecutive bursts appearing in successive timeslots.

2.7.4.1 Normal burst (NB)

The normal burst is used for carrying control and traffic information. Tail bits are located

before and after the encrypted data, and occupy four consecutive bit periods. Twenty-six

bits form the training sequence, allowing the receiver‟s equalizer to adapt its filters to the

local RF environment. Before and after the training sequence bits, is a stealing flag bit.

Stealing flags are used only for traffic channels and are used during FACCH messaging.

Bits 148 through 156 provide a guard period to avoid collisions with the next subsequent

burst.


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0 TB

1

TB

2

TB

3

ED

… ED

… ED

60 F

61 TS

62 TS

… TS

… TS

86 TS

87 F

88 ED

89 ED

… ED

… ED

144 ED

145 TB

146 TB

147 TB

148 GP

149 GP

…

156 GP

Figure 9: Normal Burst

Where:

TB = Tail Bits

ED = Encrypted Data

F = Flag

TS = Training Sequence

GP = Guard Period

2.7.4.2 Frequency correction burst (FB)

The frequency correction burst is used for frequency alignment of the mobile. This is

accomplished by adjusting the mobile‟s internal oscillator, or frequency source, to the

same frequency of the serving base station. Tail bits are located before and after the

encrypted data, and occupy four consecutive bit periods. 142 bits form the sequence of

frequency correction information, which essentially appears as an unmodulated carrier

with a frequency offset of 67.7 kHz. The frequency correction channel is made up of

consecutive frequency correction bursts. Bits 148 through 156 provide a guard period to

avoid collisions with the next subsequent burst.

0 TB

1

TB

2

TB

3

ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

144 ED

145 TB

146 TB

147 TB

148 GP

149 GP

…

156 GP

Figure 10: Frequency Correction Burst

Where:

TB = Tail Bits

ED = Encrypted Data

GP = Guard Period

2.7.4.3 Synchronization burst (SB)

The synchronization burst is used to adjust the time reference of the mobile station. This

is accomplished by adjusting the mobile‟s clock to the relative time of the serving base

station. This will allow the mobile unit‟s transmission to be received by the base station

in the correct time slot. Contained within the burst is a long synchronization sequence, as

well as TDMA frame number and Base Station Identity Code (BSIC) information. Tail

bits are located before and after the encrypted data, and occupy four consecutive bit

periods. Bits 148 through 156 provide a guard period to avoid collisions with the next

subsequent burst.


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0

TB

1

TB

2

TB

3

ED

4

ED

… ED

… ED

41 ED

42 SS

43 SS

… SS

… SS

105 SS

106 ED

107 ED

… ED

… ED

144 ED

145 TB

146 TB

147 TB

148 GP

149 GP

…

156 GP

Figure 11: Synchronization Burst

Where:

TB = Tail Bits

ED = Encrypted Data

SS = Synchronization Sequence

GP = Guard Period

2.7.4.4 Dummy Burst

The dummy burst is used on the BCCH when there are no other channels sending

information. This will allow the mobile scanning the BCCH to receive valid power

measurements. Contained within the burst is a mixture of bits carrying no relevant data

and twenty-six training sequence bits. Tail bits are located before and after the encrypted

data, and occupy four consecutive bit periods. Bits 148 through 156 provide a guard

period to avoid collisions with the next subsequent burst.

0 TB

1

TB

2

TB

3

MB

4

MB

…

MB

…

MB

60

MB

61 TS

62 TS

… TS

… TS

86 TS

87

MB

88

MB

…

MB

…

MB

144 MB

145 TB

146 TB

147 TB

148 GP

149 GP

…

156 GP

Figure 12: Dummy Burst

Where:

TB = Tail Bits

MB = Mixed Bits

TS = Training Sequence

GP = Guard Period

2.7.4.5 Access burst (AB)

The access burst is used for random access and handover access by the mobile. The burst

is characterized by a long guard period, required when the mobile does not have timing

advance information during access or handover. Access bursts are used by the RACH

and TCH after a handover. Contained within the burst is a series of forty-one

synchronization bits, followed by twenty-six encrypted data bits. Tail bits are located

before and after the encrypted data, the first series of tail bits occupy eight bit positions

and the final four occur after the final encrypted bits. Bits 88 through 156 provide a guard

period to avoid collisions with the next subsequent burst.


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0 TB

1

TB

… TB

… TB

7

TB

8

SS

9

SS

… SS

… SS

48 SS

49 EB

50 EB

… EB

… EB

84 EB

85 TB

86 TB

87 TB

88 GP

89 GP

90 GP

… GP

… GP

… GP

156 GP

Figure 13: Access Burst

Where:

TB = Tail Bits

SS = Synchronization Sequence

EB = Encrypted Bits

GP = Guard Period

2.7.5 Guard period

The guard period is provided to allow mobile station‟s transmissions to be attenuated for

some time between bursts. This allows the mobile time to ramp-up and ramp-down

during the guard periods. The base station is not required to have the same ramping

capabilities between adjacent bursts, but is required to have the ramping capabilities for

non-used time-slots.

2.8 Call Processing Messages

In order to grasp the significance of the logical channels, consider the following messages

passed between the base and mobile stations during call processing (see Figure 14).

Mobile Station Base Station PCH Network pages Mobile

Channel Requested RACH

AGCH Channel Assigned

Answer page SDCCH

SDCCH Authentication Requested

Authentication Response SDCCH

SDCCH Request Cipher Mode

Acknowledge Cipher Mode SDCCH

SDCCH Setup Message

Message Confirmed SDCCH

SDCCH Assign Traffic Channel

Acknowledge Traffic Channel FACCH

FACCH Alert

Connect Message FACCH

FACCH Connect Acknowledge

Speech TCH Speech

Figure 14: Messages Sent during Call Processing


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2.9 Processing the Voice Signal

The voice signal processing in any digital communication system occurs in three stages:

Sampling

Quantization

Encoding

The process of voice signal sampling assumes taking instantaneous measurements of a

continuous voice waveform. According to one of the fundamental concepts of

communication (Sampling theorem), the signal is to be sampled at the rate that is at least

two times greater than the highest component in the signal‟s spectrum. Since the voice

signal consists of frequency components up to 4KHz, the sampling has to be performed at

the rate of 8000 samples per second.

Every sample can take a value within the dynamic range of the signal. Consider the

analog signal represented in Figure 15, the process of quantization maps the amplitude of

the signal according to a predetermined number of discrete values. In this simplified

example, each value is represented by a three-bit word. However, the process of

quantization introduces degradation in the signal due to the finite number of samples and

resolution of the discrete values. The digital representation of the analog signal does not

have infinite resolution, therefore the quantization levels are rounded to the nearest

discrete value. The effect of rounding to the nearest quantization level introduces an

error when the signal is reproduced. Any error in the reproduced analog signal has the

same effect as noise; therefore it is referred to as “quantization noise”.

However, if the number of quantization levels is sufficiently large, the quantization noise

can be made negligible. For the quantization of the speech signal, the number of levels

that need to be used, so that degradation of signal quality becomes insignificant, is 256

for a logarithmic A or -law compression, or 4096 levels if linear compression is used.

111 +3V

110 +2V

101 +1V

0V

001 -1V

010 -2V

011 -3V

111 +3V

110 +2V

101 +1V

0V

001 -1V

010 -2V

011 -3V

Analog Signal

Sampling Pulse

PAM

101 110 101 100 010 010 010 100 111 111

PCM

Figure 15: Sampling and Quantization of Analog Signal


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About 64Kb/sec of data is required to digitize a voice signal with sufficient quality.

Transmitting such a large amount of data for each conversation will lead to inefficient

bandwidth requirements for each voice channel. In mobile communication systems,

spectrum is a very expensive commodity and high speech compression is necessary for

its effective usage. Speech compression is accomplished with speech coders that remove

redundancy in the speech signal. They typically extract the properties of the signal so

that it can be reconstructed at the receiver side. By coding the signal properties rather

than the actual signal waveform, vocoders achieve high compression rates. Today‟s

commercial systems use vocoders that compress speech signals to bit rates from 8Kb/sec

to 13Kb/sec. In the future we can expect even greater compression capabilities.

2.9.1 The GSM codec

The Codec is a device that transforms the human voice into a digital signal and

regenerates an audible analog signal from the received digital data. Therefore, it

performs encoding and decoding of the speech signal. Its place in the speech processing

order is illustrated in Figure 16.

BPFA/D

converter

SPEECH

ENCODER

CHANNEL

CODING

TO

MODULATOR

MICROPHONE

BAND-PASS

300 Hz-3.4 kHz

SPEECH

DECODER

CHANNEL

DECODERLP

LOW-PASS

4 kHz

D/Aconverter

Figure 16: Scheme of audio signal processing in telephony

The GSM system uses a simplified regular pulse excited (RPE) codec, with long-term

prediction (LTP), operating at 13 kbits/s to provide toll quality speech. It is a „full rate‟

speech codec. The latest „half rate‟ codec allows two times as many users per physical

channel. A block diagram of the encoder is shown in Figure 17.

The input speech is pre-emphasized, split up into frames 20 ms long, and for each frame,

a set of 8 short-term predictor (STP) coefficients are found. Each frame is then further

split into four 5 ms sub-frames. It is a function of the codec‟s long-term predictor to find

a delay and gain for each sub-frame.

The residual signal after both short and long term filtering is quantified for each sub-

frame as follows. The 40-sample residual signal is decimated into three possible


GSM Based Networks


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excitation sequences, each 13 samples long. The highest energy sequence is chosen as

the best representation of the excitation sequence, and each pulse in the sequence has its

amplitude quantized with three bits.

Pre-emphasis

Segmentation

Hamming

Window

Pre-processing

Short-Term

Prediction

(STP) Analysis

Long-Term

Prediction (LTP)

Analysis

+ LPF

LTP Filter

MU

X

Grid

Selection

Regualar

Pulse

Excitation

rn

pitch pn

LAR coefficients

gain gn

INPUT SPEECH SIGNAL CODED SPEECH

Figure 17: Block diagram of GSM speech encoder

Reconstruction of the signal at the decoder is performed through the long term and then

the short-term synthesis filters. A post-filter is used to improve the perceptual quality of

this reconstructed speech. This is schematically shown in Figure 18.

DM

UX

RPE Decoding

LTP Sythesis

Filter

STP Synthesis

Filter

Post-Processing

o

u

t

p

u

t

An

Pn, g

n

LARn(k)

Figure 18: Block diagram of GSM speech decoder

The output bits do not have the same importance for speech reconstruction. A total of

260 bits are ordered by their importance into three groups (50, 132, and 78 bits each.

These groups of bits are protected differently by error coding control algorithms. The

process of bit-protection by introducing redundancy into the bit stream will be explained

in the next section.


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2.10 Data Coding

Once the voice data is compressed, control information, along with error correction bits

are added. Adding control and error correction bits introduces additional redundancy and

significantly increases the data rate of the channel. However, coding also increases the

robustness of the communication channel necessary in the difficult mobile propagation

environment.

At the output of the vocoder, data is separated into three bit streams. The separation is

performed in accordance to its relative importance. The first group contains the most

significant bits (type Ia bits), the second group contains important bits (type Ib bits) and

the last 78 bits are called type II bits. There are 50, 132 and 78 bits in group Ia, Ib and II,

respectively.

Fifty bits of class Ia are considered vital for quality reconstruction of the speech signal

and they are coded with a rate ½ convolution encoder. Furthermore, those bits are

additionally protected by the three parity bits for error detection. Type Ib bits are

protected by rate ½ convolution encoder while „type II‟ bits are considered less important

and they are not error protected. Illustration of error protection scheme is given in Figure

19. Total of 456 bits per frame at the output of channel coding make 75% overhead bits.

It is a price to be paid for more reliable transmission.

TYPE Ia

BITS

TYPE II

BITS

TYPE Ib

BITS

CONVOLUTIONAL

ENCODER

r=1/2

K=5

MU

X

ERROR DETECTING CODE50

132

78

3

4

189

189

378

456

0TO

INTERLEAVER

FR

OM

V

OC

OD

ER

Figure 19: Block diagram of channel coding for GSM technology.

Two important entities in the process of channel coding are Convolutional Coding and

Error Detecting Codes (as illustrated above in Figure 19).

Convolutional coding consists of convolution of input a bit-sequence and a pre-defined

digital filter. The filter is defined by binary polynomials. In the case of GSM, the

convolution algorithm assumes:

1. The addition of 4 tail bits (set to zero),

2. two different convolutions (polynomials are 134

1 XXXXC and

134

2 XXXC )


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26

3. and „bit puncturing‟, that presumes the transmission of only certain bits of output

sequence.

The process of convolution defines two output sequences of 189 bits. However, before

signal modulation, the 78 “unprotected” (type II) bits are added to the „protected‟ bit

sequence (2189) which results in 456 bits per 20ms (22.8 kbps).

The Viterbi algorithm performs convolutional decoding, where every possible user data

sequence is explored. The algorithm is a maximum likelihood decoder and has the ability

do decimate the number of „possible input bit combinations‟ by discarding sequences that

cannot belong to the maximum likelihood path.


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2.11 Interleaving

Rayleigh fading characterizes the mobile propagation environment. In essence, Rayleigh

fading manifests itself through deep short fades. Approximately 1% of the time a signal

falls 20dB below its mean value and about 0.1% of the time it suffers fades in excess of

30dB. Depending on the velocity of the receiver, deep fades can completely erase an

entire segment of the channel data causing bursts of errors that cannot be corrected

efficiently through error control coding. Optimal performance in error correction is

obtained when errors are more or less uniformly spaced throughout the received bit

sequence.

The process of interleaving provides an easy way to increase performance of the error

control coding without adding any data overhead. There are two methods for

implementation of the interleaver: a block interleaver and a convolution interleaver. Due

to its simplicity, a block interleaver is the most frequently encountered structure in

mobile communications. The block interleaving process is illustrated in Figure 20. The

data is written in the block interleaver matrix column by column and read row by row.

This ensures separation between the adjacent bits during the transmission. As illustrated

in Figure 20, errors caused by Rayleigh fading have burst characteristics and therefore

several consecutive bits get destroyed. However, after the de-interleaving process, the

burst errors caused by the channel are evenly spaced so that most of them can easily be

corrected using the Forward Error Control Coding (FECC).

252015105

24191494

23181383

22171272

21161161

bbbbb

bbbbb

bbbbb

bbbbb

bbbbb

Data is written

column-wise

Data is read

row-wise

Interleaver

b1 b

2 b

3 b

4 b

5 b

6 b

7 b

8 b

9 b

10 b

11 b

12 b

13 b

14 b

25 b

16 b

17 b

18 b

19 b

20...

b1 b

6 b

11 b

16 b

21 b

2 b

7 b

12 b

17 b

22b

3 b

8 b

13 b

18 b

23 b

4 b

9 b

14 b

19 b

24..

Burst Error

Caused by

Rayleigh Fading

Errors are spread over the bit stream

Figure 20: Illustration of the interleaving process

In GSM, full rate speech bits are interleaved on 8 bursts: the 456 bits of one code word

are split into 8 sub-blocks of 57 bits each. For example, bit #0, #8, #16, …,#448 will

belong to the first sub-block and bits #7, #15, …, #455 will belong to the eight sub-block.

One burst in GSM consists of 114 bits and, therefore, it includes two sub-blocks. In

GSM, the interleaving algorithm uses sub-blocks from two consecutive code words and

combines them into eight bursts.

As illustrated in Figure 21, four sub-blocks of code word „C‟ and four sub-blocks of the

previous code word „C-1‟ are combined into four bursts. Moreover, bit-positions („even‟

or „odd‟) within the burst are defined by the number of used sub-blocks. For example,

four bursts (N, N+1, N+2 and N+3) consist of eight sub-blocks: sub-blocks #1, #2, #3


GSM Based Networks


28

and #4 from code word „C‟ occupy „even‟ positions while sub-blocks #5, #6, #7 and #8

from the previous code word „C-1‟ occupy „odd‟ positions.

CODE WORD 'C-1' CODE WORD 'C' CODE WORD 'C+1'

#1

#5

#4

#3

#2

#6

#7

#8

8 s

ub

-blo

ck

s

57 bits

#1

#5

#4

#3

#2

#6

#7

#8

8 s

ub

-blo

cks

57 bits

#1

#5

#4

#3

#2

#6

#7

#8

8 s

ub

-blo

cks

57 bits

COMBINED INTO

FOUR BURSTS

COMBINED INTO

FOUR BURSTS

Figure 21: Interleaving for the traffic channel in GSM

The process of bit interleaving introduces a delay in the data processing. Transmission

time from the first burst to the last one for one complete code word, having in mind the

additional burst for the SACCH is (98)-7=65 bursts which is about 37 msec. One

should have in mind that the described scheme of interleaving is used for full rate speech

transmission or fast signaling mode. Other channels and transmission modes use

different interleaving schemes.


GSM Based Networks


29

2.12 Equalization

On its way from transmitter to receiver, signal travels several different propagation paths.

Multipath propagation results in several replicas of the original signal, which arrive at the

receiver with different time offsets. Multipath properties of the mobile channel can be

examined using channel-sounding devices. Over the past couple of decades, a lot of

research has been done in the multipath characterization of the mobile propagation

environment. A typical shape of a power delay profile is shown in Figure 22.

Figure 22: An example of a Power Delay Profile (PDP).

Multipath propagation is the main source of the Inter Symbol Interference (ISI), which

has been recognized as a major obstacle for high-speed communication over mobile radio

channels. Equalization can be defined as a set of techniques used in communication

systems to combat the damaging effects of the ISI. Equalizers implemented in mobile

communications have to be adaptive. The reason for this is the random and time varying

nature of mobile channel.

There are two operating modes of the adaptive equalizers implemented in mobile

communication systems: training and equalization. Over the training process, the

equalizer receives a known bit sequence sent by the transmitter. By comparing what the

transmitter has sent with what was actually received, the equalizer can determine the

effects of the propagation environment were over the channel. It is reasonable to assume

that the degradation in bits of the user data is the same as the one suffered by the training

sequence. Since the degradation effects on the training sequence are known, the


GSM Based Networks


30

equalizer can be adjusted so that it compensates for the ISI. During equalization, the user

data is passed through the equalizer and most of the channel degradation gets removed.

Equalizers implemented in mobile communication must have the following properties:

1. Small adjustment time (fast convergence) – The algorithm must be fast enough that it

can operate on a relatively short training sequence. Sending the training sequence

reduces the throughput of the user information, so there is an interest in keeping it

small.

2. Relatively small complexity – A small complexity equalizer is needed in order to

reduce the processing burden imposed on the receiver. Also, an equalizer with small

processing requirements consumes less power, which is a very important factor,

especially from the standpoint of subscriber unit‟s battery life.

There is a conflict between the above two requirements and performance of the equalizer.

The tradeoff between the two is used to decide on the actual equalization algorithm and

hardware used.

A diagram that describes the functionality of the equalizer in a GSM system is presented

in Figure 23.

RF

Processing

Adaptive

Equalizer

Equalization

AlgorithmExtraction of

Synchronization

Bits

Unequalized

Data

Equalized

Data

Figure 23: Principals of the GSM equalizer operation

GSM sends a 28-bit synchronization word to every time slot. Out of 28 synchronization

bits, the last four are unique so that each slot can be identified. This causes a 9%

overhead in terms of bit rate. In addition, when the equalizer is functional there is an

increase in the mobile power consumption. In a site with a small cell radius and few

multipaths, the base station can command the mobile to disable the equalizer and hence

reduce its power consumption.


GSM Based Networks


31

2.13 Modulation

The process of digital modulation serves to superimpose a binary digital waveform on the

RF carrier. Modulation may affect any of the carrier‟s characteristics: amplitude (AM),

frequency (FM) or phase (PM). In GSM technology, frequency modulation is used.

More specifically, it uses a distinctive type of continuous phase frequency modulation –

Gaussian Minimum Shift Keying. This section explains GMSK modulation starting from

the basic theory of binary frequency shift keying and minimum shift keying.

2.13.1 Binary Frequency Shift Keying (BFSK)

This type of frequency modulation uses two distinct carrier frequencies ( ffC and

ffC ) according to the binary symbols 1 and 0:

tffKtr CH )22cos()( , bTt 0 , „high tone‟ for binary 1 (4a)

tffKtr CL )22cos()( , bTt 0 , „low tone‟ for binary 0 (4b)

Generally, the waveform composed from )(trH and )(trL

has discontinuous phase that

results in spectral spreading and spurious transmissions. The more common method of

generating a FSK waveform is accomplished by the modulation of a single carrier by the

digital waveform )(tm :

t

fC dmktfKtr )(22cos)( (5)

When the frequencies of signals )(trH and )(trL

are appropriately chosen, the product of

those signals is zero:

dttrtr H

T

H

b

)()(0

= 0 (6)

Hence, )(trH and )(trL are orthogonal. This leads into MSK modulation that uses

orthogonal waveforms.

2.13.2 Minimum Shift Keying (MSK)

When the two frequencies used in BFSK are minimally separated and provide

orthogonality, the modulation is called minimum shift keying. Using (3), it can be shown

that the minimum frequency spacing is determined according to equation (4)

b

LHT

ftftf

4

122)()( (7)


GSM Based Networks


32

where bT is the bit period, fff CH and fff CL . In GSM, the binary

message signal is divided into two sets of bits: “odd bits” sequence tmI and “even

bits” sequence tmQ . This is illustrated in Figure 24, where 10 original data bits are

mapped to 5 bits of “odd-bit” stream and 5 bits of “even-bit” stream. The bit-period in

those two sequences is two times larger than original bit period bT . Additionally, two

sequences have a relative offset of bT . As a result of this bit separation, the two signals

may differ at any time period bT . Since the possible values of tmI and tmQ are

1,1 , the mapping to the frequencies and phases of the RF signal, given in Table 1,

guarantees relatively constant phase of MSK signal. The smoothness of the MSK signal

is illustrated at the bottom of the Figure 24.

Table 1: Message bit to frequency / phase mapping

Odd bit Even bit Frequency (high/low) Phase*

1 1 L +

1 -1 H +

-1 1 H -

-1 -1 L - (same as reference signal / opposite of the reference signal)

Since the message bits are either 1 during a bit interval, the desired frequency spacing

is bT41 around the carrier, and the RF signal is

ttmtm

TtfKtr QI

b

C )()(4

122cos)( (8)

according to equation (2). Coefficient fk is bT41 , and message )(tm , consists of an odd

and even bit stream as is illustrated in Figure 24.


GSM Based Networks


33

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

DATA

ODD BITS

EVEN BITS

HIGH FREQUENCY

LOW FREQUENCY

MSK SIGNAL

Figure 24: Example of MSK modulation

Note that at a particular instant, both high and low frequency may have a + or –

orientation on the waveform. This provides relatively smooth phase transitions between

consecutive bits. In GSM, the two frequencies (Hf and

Lf ) differ by one half of bit rate

which is 135.4 kHz. In other words, those frequencies are 67.7 kHz above and below the

assigned carrier frequency. There is one important modification of MSK that defines

GMSK modulation: filtering of the baseband signal.

2.13.3 Gaussian Minimum Shift Keying (GMSK)

If the rectangular pulses corresponding to the data bit stream are filtered using a

Gaussian-shaped impulse response filter, we get Gaussian MSK (GMSK). The time and

frequency responses of the „Gaussian filter‟ are illustrated in Figure 25. The resulting

signal has low sidelobes compared to MSK, as is illustrated in Figure 26. Since it has

low spectral sidelobes, the GMSK signal provides good spectral efficiency. The GMSK

signal also has a constant envelope and continuous phase as every MSK signal. For

3.0TB which is used in GSM, the spectral density has a negligible sidelobe that is

more than 30 dB below the peak. Compared with the MSK spectral density (sidelobe 20

dB below peak), that is a substantial improvement.


GSM Based Networks


34

The price that is paid for that benefit is a fairly large inter-symbol interference (ISI)

introduced by Gaussian filtering. Namely, a baseband message signal consists of

symbols that occupy a single bit period. Hence, ISI is negligible. After Gaussian

filtering, each symbol spans over several consecutive bits. The harm induced by ISI does

not have an effect on overall performance as long as the GMSK error rate is less than the

error rate produced by the mobile channel.

h(t) H(f)

t/f

Figure 25: Shape of the time and frequency response of the ‘Gaussian’ filter

MSK

Filtered MSKGMSK

(f-fo) / Rb0 1 2 3

-80

-60

-40

-20

0

POWER SPECTRAL

DENSITYdB

Figure 26: Spectral characteristics of GMSK and MSK


GSM Based Networks


35

2.13.4 GMSK specifics for GSM

The Gaussian filter used by GSM systems has a BT product of 0.3. The “BT” is a

product of the 3dB bandwidth of the filter and bit period of the filtered data. Since T is

about 3.7 sec, the 3 dB filter bandwidth is 81.3 kHz.

It would be interesting to describe the spectral density of the resulting baseband GSM

signal. Due to use of the Gaussian filter, the signal will have approximately a Gaussian

power spectral density (PDF(f)):

2

2

2

22

2exp

1

2exp)(

ffHfPDF (9)

where 2 is the variance, related to the 3 dB bandwidth of the filter through the value of

:

2

2 2ln2

B

;

2ln82ln84

1222

2

2

bRBTB

. (10)

This description provides an opportunity to characterize the width of the spectrum that

contains E% of the signal energy, as is listed in Table 2 or Figure 27. It can be seen that

90% of signal power falls into 113 kHz (56.5% of the total channel bandwidth).

Table 2: Percentage of total signal energy (%E) carried by the bandwidth (B E)

Percentage of

signal energy (%E)

Bandwidth (BE) that

contains %E [kHz]

BE/200 kHz (%)

99% 177.1 88.5%

95% 135 67.5%

90% 113 56.5%

80% 88 44%

70% 71 35.3%

60% 57.8 29%

50% 46 23.2%

40% 36 18%

30% 26.5 13.3%

20% 17.4 8.7%

10% 8.6 4.3%


GSM Based Networks


36

Figure 27: Bandwidth of the GSM signal versus the percentage of signal power.

The implementation of the GMSK modulator may be expressed by the diagram in Figure

28. A differentially coded bit sequence is integrated to obtain a signal that will be used as

the phase ( ) of the carrier. A convolution with a Gaussian filter smoothes this signal

and introduces a considerable amount of inter-symbol interference to consecutive bits.

The corresponding „I‟ and „Q‟ components of the signal are modulated by nTfo 2cos

and nTfo 2sin before being added to obtain an output signal.

INTEGRATORGAUSSIAN

FILTER

COS ( )

SIN ( )

+1

-1 NRZ format

data

S(nT)

COS(T)

SIN(T)

'NRZ' is non-return to zero data format

Figure 28: Modulation scheme for GSM technology

The GSM specifications do not impose one particular demodulation algorithm. However,

they define the minimal performance measured after the correction of errors by channel

decoding. The algorithm used must be able to support two multipaths of equal power


GSM Based Networks


37

received at an interval of up to 16 sec (i.e. more than four symbols). With such a level

of intersymbol interference, simple demodulation techniques are ineffective and an

equalizer is required.


GSM Based Networks


38

3 GSM RF Planning

RF planning of a cellular network involves providing adequate coverage and network

capacity over a certain geographic area. As such, it is somewhat technology independent.

However, each technology has a set of specific properties that need to be taken into

consideration in this stage of network design. Some the most important issues related to

GSM systems are covered in this section.

3.1 Link Budget

In the cellular system design process, the link budget is used to determine the maximum

allowable path loss required to achieve balanced forward and reverse link coverage. In

most mobile cellular communication systems, the reverse link is the limiting link due to a

relatively small ERP from the mobile terminals. Therefore, the forward ERP has to be

adjusted so that the maximal allowable path loss on the forward link matches the reverse

link path loss. Once the path loss is calculated, a propagation model can be used to

estimate the nominal coverage radius of the particular cellular site.

3.1.1 Example of a GSM Link Budget

This is an example of a typical link budget for a GSM system. The entries used in the

link budget represent generic values and do not reflect exact specifications of any

particular manufacturer. When performing a real world system design, parameters are to

be obtained from the actual data sheets of the equipment vendor.

3.1.1.1 Calculation of Receiver Sensitivity

In general, the receiver sensitivity is calculated according to:

otherrequired

NI

CdBFkTWRxSens log10 (11)

where:

RxSens - Receiver sensitivity (dBm)

kT - Power Spectrum density of the thermal noise

W - Operating bandwidth of the particular technology

dBF - Noise figure of the receiver expressed in dB

requiredI

C - Required carrier to interference ratio for sufficient voice quality

otherN - Noise coming from other sources, such as man made noise,

atmospheric noise, etc.


GSM Based Networks


39

Since the bandwidth of a GSM channel is 200KHz, the thermal noise floor can be

calculated as:

dBmkHz

Hz

mWkTW 121200101.4log10log10

18

(12)

Determining the Required C/I Ratio

The required carrier to noise ratio is a technology specific parameter and depends on

many different things such as: performance of vocoder, velocity of the users,

performance of the equalizer, multipath profile of the environment, etc. Taking all of

these factors into account in some analytical sense is both tedious and impractical, and, in

practice the required carrier to noise ratio is determined through experimental field

testing. In a GSM network, it is industry-accepted practice to design a non-frequency

hopping network based on a 12 dB C/I ratio and 9 dB C/I ratio for a frequency hopping

network.

3.1.1.1.1 Noise Figure of the Receiver

The noise figure of the receiver is a measure of the noise floor increase due to the active

devices in the receiver circuitry. Good quality base station receivers can have a noise

figure as small as 5 dB. Very often, specially designed low noise, tower-mounted

amplifiers achieve a noise figure of 2-3 dB. Due to the size and cost constraints, mobile

phones have noise figures in the range of 8-10 dB.

3.1.1.1.2 Other Sources of Noise

The thermal noise floor, noise figure, and required C/I, determine the sensitivity of the

receiver operating in an „ideal’ environment. In actuality, there are various sources

producing energy that fall within the frequency band of cellular systems. For example, a

car ignition, an industrial environment, power lines, etc., are known sources of RF energy

in the frequency band from 100 KHz to 2 GHz. In short, all these other sources of RF

energy produce a net increase to the thermal noise floor. The amount of increase is very

specific to the environment that the cellular system is operating, and is usually from 0 dB

in some quiet rural areas up to several dB in highly urban industrial districts.

Examples of GSM link budgets prepared for three different morphological types and

three coverage requirements are presented in Table 3 through Table 5


GSM Based Networks


40

Table 3: Example of GSM Budget – Rural Environment

In vehicle Outdoors

Parameter DL UL DL UL

Maximum TX Power

[dBm]

44.5 33 44.5 33

MS Antenna Gain [dBd] -2.16 -2.16 -2.16 -2.16

Head Loss [dB] -2 -2 -2 -2

Maximum Cable and

Connector Losses [dB]

-4.5 -4.5 -4.5 -4.5

BS Antenna Gain [dBd] 16 16 16 16

In Vehicle Loss [dB] -8 -8 0 0

Noise Floor Adjustment

[dB]

0 0 0 0

Fade Margin (90%

Coverage Reliability)

-6.05 -6.05 -6.05 -6.05

Diversity Gain [dB] 0 3 0 3

RX Sensitivity [dBm] -102 -108 -102 -1086

Maximal Path Loss [dB] 139.79 137.29 147.79 145.29

Table 4: Example of GSM Link Budget – Suburban Environment

In Building In Vehicle Outdoors

Parameter DL UL DL UL DL UL

Maximum TX Power

[dBm]

44.5 33 44.5 33 44.5 33

MS Antenna Gain [dBd] -2.16 -2.16 -2.16 -2.16 -2.16 -2.16

Head Loss [dB] -2 -2 -2 -2 -2 -2

Maximum Cable and


-4.5 -4.5 -4.5 -4.5 -4.5 -4.5

BS Antenna Gain [dB] 16 16 16 16 16 16

In Vehicle Loss [dB] 0 0 -6 -6 0 0

In Building Loss [dB] -15 -15 0 0 0 0

Fade Margin (90%


-6.2 -6.2 -5.53 -5.53 -5.12 -5.12

Diversity Gain [dB] 0 3 0 3 0 3

RX Sensitivity [dBm] -102 -108 -102 -108 -102 -108

Maximal Path Loss [dB] 132.64 130.14 140.97 139.81 147.38 146.22


GSM Based Networks


41

Table 5: Example of GSM Link Budget – Urban Environment

In Building In Vehicle Outdoors

Parameter DL UL DL UL DL UL

Maximum TX Power

[dBm]

44.5 33 44.5 33 44.5 33

MS Antenna Gain [dBd] -2.16 -2.16 -2.16 -2.16 -2.16 -2.16

Head Loss [dB] -2 -2 -2 -2 -2 -2

Maximum Cable and


-4.5 -4.5 -4.5 -4.5 -4.5 -4.5

BS Antenna Gain [dB] 16 16 16 16 16 16

In Vehicle Loss [dB] 0 0 -6 -6 0 0

Building Penetration

Loss [dB]

-18 -18 0 0 0 0

Fade Margin (90%


-10.9 -10.9 -7.3 -7.3 -7.1 -7.1

Diversity Gain [dB] 0 3 0 3 0 3

RX Sensitivity [dBm] -102 -108 -102 -108 -102 -108

Maximal Path Loss [dB] 124.94 122.44 140.54 138.04 146.74 144.24

3.2 Calculation of the Nominal Cell Radius

Based on the maximum allowable path loss obtained from the link budget, and known

parameters of a particular propagation environment, we can calculate the nominal radius

of the cell. As an example, consider the link budget provided in Table 4, in the case of

providing in-vehicle coverage. To calculate the nominal cell radius we will use Lee‟s

propagation model for which the formula for RSL prediction is given as[4]:

tref

t

mref

m

tref

tm

P

P

h

h

h

hdmPdBmRSL log10log10log15log][ 1 (13)

Where:

RSL - Received signal level

mP1 - One mile intercept

m - Slope

th - Height of the BS antenna

mh - Height of the MS antenna

tP - Transmit power

trefmreftref Phh ,, - Reference conditions


GSM Based Networks


42

Table 6 summarizes some typical values for slope and intercept obtained for the

following reference conditions:

feethtref 150 feethmref 10

WPtref 100

Table 6: Typical Slope and Intercept Values.

Carrier Frequency

[MHz]

Environment Type One Mile Intercept

[dBm]

Slope

[dB/dec]

800 MHz [9] Dense Urban -74 -43.1

Urban -63 -40.0

Suburban -59 -38.4

Rural -49 -43.5

1900[10] Dense Urban -80 -43.1

Urban -69 -40.0

Suburban -65 -38.4

Rural -55 -43.5

Let‟s assume the height of the site is 120 feet and that the height of the mobile is

approximately 6 feet above the ground. From the link budget example we can see that

the transmit power of the site is:

CLGtPERP max (14)

where:

ERP Effective radiated power

maxP Power delivered at the output of the transmitter

CL Cable and other losses

Gt Antenna gain

In this example:

WdBmERP 2905.545.41642 1

Knowing the maximal allowable path loss, the minimum received signal level at the edge

of the coverage region is calculated as:

dBmdBdBmRSLm 64.7514.1305.54

1 Note that the power on the downlink is reduced by 0.4dB to provide balanced path


GSM Based Networks


43

Substituting the minimal RSL into Lee‟s equation, and assuming the typical values for

suburban slope and intercept at PCS frequencies of dBmPm 651 , decdBm /4.38 ,

the nominal cell radius is calculated:

100

290log10

10

6log10

150

150log15log4.386564.75 0d

milesd 44.30

In general, the equation giving the size of the nominal cell radius obtained by using Lee‟s

propagation model is given by:

min10 log10log10log15

1log RSL

P

P

h

h

h

hP

md

tref

t

mref

m

tref

tm

(15)

Similar formulas can be derived using other macroscopic propagation models.

3.3 Reuse Efficiency

The minimum distance that allows the same frequency to be reused will depend on the

number of co-channel cells in the vicinity of the serving cell, the individual cell

configuration, and the C/I requirements for the specific technology. Typical C/I

requirements for a GSM based network is 12 dB C/I without frequency hopping and 9 dB

C/I with frequency hopping. A reasonable estimation of the minimum distance required

for reuse may be made for various C/I requirements for some nominal conditions. These

conditions are:

The level of co-channel interference experienced by the serving cell is attributed

primarily to the first tier of reuse cells.

All cells in the system are of identical configuration.

All cells in the system are evenly and appropriately spaced based on effective

coverage areas.

Therefore the theoretical frequency reuse pattern is derived by:

oi

i

n

i

n

D

RI

C

1

(16)

In practice, a GSM network will use one of the following reuse schemes

Reuse Scheme Theoretical C/I

Ratio

3 8.48 dB

4 11.43 dB


GSM Based Networks


44

3.4 Capacity Calculations

The capacity of a GSM-based network is dependent on the frequency reuse scheme and

the amount of the operator‟s spectrum. Frequency reuse schemes are used to divide the

spectrum into unique groups of channels, so that the frequency planner can methodically

assign channels to a network of cells. The reuse scheme is chosen to minimize the

amount of interference in a network of cells, but also has an impact on the capacity of a

particular site.

The amount of spectrum a license holder has must be considered as well. Since the GSM

channel is 200 kHz wide, five channels can fit into 1 MHz of spectrum. Therefore, a

European operator, with 25 MHz of spectrum, will have 62 channels available; whereas,

a PCS operator in the United States, with 30 MHz of spectrum, will have 75 channels

available. Remember that half of the spectrum is for downlink and half is for uplink.

A GSM market that has just launched commercial service will likely use a 4/12-reuse

scheme. Using a 4/12-reuse scheme will have less capacity (not likely to be a problem as

the network turns on), but more importantly, the larger distance to reuse will minimize

interference. As subscribers are added to the network, the capacity increases to the point

that frequencies must be reused over shorter distances. Using frequency hopping, the

GSM network may be frequency planned using all of the channels within three sites.

Frequency hopping is explained in a later section.

The number of channels available per sector when considering the two reuse schemes is

calculated as follows:

ectorChannels/S 2.79

1

kHz 200

Mhz 5Channels Scheme 9Reuse3 (17)

ectorChannels/S 2.112

1

kHz 200

Mhz 5Channels Scheme 12Reuse4 (18)

To calculate the capacity of the network, the Erlang capacity of each sector must be

identified. The Erlang capacity of each sector is calculated by considering the desired

Grade Of Service (GOS) and the number of voice channels per sector. It is industry-

accepted practice to design a network with 1 - 2% GOS. The number of voice channels

per sector is calculated with respect to the number of RF carriers per sector. Generally,

eight time slots are considered for each carrier except the first carrier, which dedicates

one or two time slots for control and messaging. By using an Erlang-B table, the Erlangs

per sector are determined.

The Erlang capacity of the network is calculated as follows:

Network

Sites

Site

Sectors

Sector

Erlangs Network ofCapacity Erlang (19)


GSM Based Networks


45

Comparing the total capacity of networks using the two different reuse schemes is

depicted in Table 7 and Table 8.

Table 7: Capacity of Network Using 3/9 Reuse Scheme

Total Number of Cells Required 100

Number of Voice Channels/ Sector 22

Blocking Rate 1 %

Erlangs/Sector 13.65

Sectors per Site 3

Erlangs/Site 40.95

Erlang Capacity of Network 4095

Table 8: Capacity of Network Using 4/12 Reuse Scheme

Total Number of Cells Required 100

Number of Voice Channels/ Sector 14

Blocking Rate 1 %

Erlangs/Sector 7.351681

Sectors per Site 3

Erlangs/Site 22.05

Erlang Capacity of Network 2205

In the case of a network using a 3/9-reuse scheme, there would be 810 channels.

Conversely, the same network of sites using a 4/12-reuse scheme would have 630

channels; however, the capacity has almost doubled. This example is a bit over

simplified, and conservatively assumes that all sites in the network have the same

distribution of traffic; however, it depicts the gains associated with a tighter reuse

scheme.


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3.5 Frequency Planning

Good frequency planning starts with well-chosen sites. Although some designers are

primarily concerned with the site‟s location relative to a hypothetical grid, a much more

critical factor is coverage and reuse potential. Fortunately, a system-wide frequency

retune can be done relatively easily allowing poor or compromised site locations to be

corrected at a reasonable cost.

Frequency planning is customarily accomplished by dividing the frequency band into

frequency groups. The same frequency group is reused in different cells at a distance

sufficient to prevent co and adjacent channel interference. The number of frequency

groups will depend on the standard site configuration and the reuse scheme (N). Each

frequency group is split up into a number of sets that correspond to the number of sectors

at each site.

Since the number of frequencies is fixed by the bandwidth of the spectrum, an increase in

the number of frequency groups will increase the distance to frequency reuse and reduce

the level of interference. The tradeoff to larger reuse distances is fewer channels are

available per cell. Conversely, reducing the number of frequency groups increases the

capacity of a network. Thereby the distance to reuse is reduced and interference

increases.

For GSM-based networks there are two commonly used cluster designations: 4/12 and

3/9. The first digit indicates the number of cell sites that make up a cluster and the

second number represents the number of frequency groups. As an example a 4/12-reuse

plan would require 12 frequency groups within a four-cell cluster, assuming all cells are a

three-sector configuration.

Figure 29: N=3 Reuse Scheme


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Figure 30: N=4 Reuse Scheme

Regular channel assignments may offer reasonably low levels of interference and simple

network growth opportunities. In actual practice many factors effect the network

configuration and frequency plan. These include terrain undulations that create

irregularities in site coverage areas, and uneven traffic loading in developed population

centers. Often times, frequency assignments based solely on a grid pattern result in

unsatisfactory levels of interference. Typical frequency plans deviate from the ideal

channel layout due to the intolerable levels of interference and the borrowing of channels

from other frequency groups in order to provide capacity.

Some basic guidelines for assigning voice channels are to maintain proper channel

spacing and maximize the distance between reusing cell sites. Likewise, the assignment

of co-channels and adjacent channels at the same site, assigning co-channels and adjacent

channels in adjacent sites, and mixing frequency groups in a cell or sector should be

avoided.


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3.6 Interference Reduction Strategies

GSM-based networks have several features that can be used by the frequency planner to

reduce or eliminate interference. Typical methods used for interference reduction include:

Hierarchical cell layout

Overlaid / Underlaid Subcells

Frequency hopping

3.6.1 Hierarchical Cell Structures

Hierarchical cell structures are used in high traffic areas, where micro-cells, macro-cells

and umbrella-cells co-exist. Micro-cells provide coverage over a small area, but offer a

relatively large amount of capacity. Since micro-cells cover a small area, they are

configured to transmit at lower ERPs; hence, their contribution to co-channel interference

is minimized. Macro-cells provide coverage over a larger area, where the coverage area

is dependent on the environment in which they are located. Umbrella-cells provide

coverage over a larger area, where their primary purpose is to fill-in coverage holes

between micro-cells.

In order to take advantage of this interference reduction strategy, the switch attempts to

process calls first by micro-cells, then macro and umbrella cells. Since a macro-cell or

umbrella-cell transmits at a higher power, the mobile would always tend to use them as

the serving cell. Therefore, the switch must have some knowledge of the type of cell

serving a call. Typically the cells are classified in three unique types, known as layers,

where:

the micro-cell is a layer one cell,

the macro-cell is a layer two cell,

the umbrella-cell is a layer three cell.

Once the switch knows the type of cell, access and handover decisions will be tailored to

favor the lower layer cells. Calls will then originate or handover to a micro-cell even

though the signal from a macro-cell may be stronger from the perspective of a mobile.

As the mobile moves away from the coverage area of a micro-cell, it may handover to the

coverage area of a macro or umbrella cell.

3.6.2 Overlay/Underlay

Overlay/Underlay is another interference reduction strategy, typically used in areas of

high capacity and tight frequency reuse. The scheme may be necessary when more than

one channel set is required at a site, and the level of interference for a particular channel

set is unacceptable away from a site, but not close to the site. Essentially the strategy is

to guarantee a distinct channel set serves mobiles close to a cell, while mobiles away

from a cell are served by another channel set. Using this interference reduction scheme,

the mobile will handover from the underlay channels to the overlay channel set, within

the same cell, before the mobile experiences interference.


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Consider the following example: Two cells are reusing the same channels at a relatively

close reuse distance. The cell and sector experiencing interference has high traffic

demands because of its location relative to two nearby highways; therefore, it requires

two sets of channels. The level at the point of interference is reported as 10 dB C/I (see

Figure 31).

Radio Tower

Radio Tower

Radio Tower

Radio Tower

C/I = 10 dB

Figure 31: Area of interference

Using the overlay / underlay strategy, one channel set is used for the overlay sub-cell and

one channel set is used for the underlay sub-cell. The engineer must ensure that the

mobile served by this sector does not use the underlay channel set in the area it would

experience the interference. Therefore, only the overlay channel set must serve the

mobile in this area.

The signal level, with which the mobile has to handover to the overlay channel set, is

determined by analyzing the signal strength level at the point of interference (see Figure

32). The hand-out threshold for the mobile would be set at a signal strength above this

bin specific signal level. In this case –85 dBm is the serving level, so a hand-out

threshold of –81 would be appropriate, assuming a hysteresis of 4 dB.

Radio Tower

Radio Tower

Radio Tower

Radio Tower

RSS = -85 dBm

Figure 32: Signal strength at point of interference


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Conversely, the underlay channel set (set 12), should carry mobiles served by the site

above –85 dBm. Therefore, the hand-in threshold should be set at –85 dBm.

3.6.3 Frequency Hopping

The network operator can use the frequency hopping capability on all or part of its

network. The two primary advantages of this feature are to reduce the impact of co-

channel interference and to increase the efficiency of coding and interleaving for slow

moving mobile stations. Frequency hopping will also average the voice quality of all

communications through interference diversity.

Frequency hopping may be used in a high capacity network, where the distance to

frequency reuse is relatively small and the probability of interference is large. Generally,

the mobile experiencing co-channel interference will remain in that state until the

interfering signal is attenuated, due to shadowing or log normal fading, or the mobile

retunes to another channel. In either case, the co-channel interference will degrade the

quality of speech and may cause the call to drop.

Frequency hopping is a technique used in high capacity networks to combat the undesired

effects of co-channel interference. Frequency hopping decreases the probability of co-

channel interference because the mobile station is constantly switching channels.

Although the opportunity exists for two mobiles to be tuned to the same channel and

interfere with each other, the mobile is tuned to that channel for only a fraction of a

second, therefore the time the mobile experiences co-channel interference is shortened.

Since the level of interference changes with each channel, the quality of the connection is

improved relative to the mobile that endures interference during the entire duration of the

connection. The short periods of interference experienced on colliding channels can be

dealt with using error correction algorithms and the interleaving process.

Additional benefits of frequency hopping include reductions in adjacent channel

interference, external sources of interference and intermodulation products. The C/I ratio

of a frequency-hopping network improves by approximately 3 dB.

Frequency hopping also can improve the performance of a network by reducing the

harmful effects of multipath. Multipath is a phenomenon related to the combination of

multiple RF signal paths, where the resultant signal waveform may appear as a deep fade

at the mobile station receiver. A deep signal fade can cause degradation in speech quality

or possibly dropped calls. Multipath varies over distance and time, and is also frequency

dependent. Therefore, frequency hopping can counteract the effects of multipath by

switching the mobile from one channel to the next.

GSM uses slow frequency hopping, where every mobile transmits in time slots according

to a sequence of frequencies. The sequence of frequencies is derived from a frequency-

hopping algorithm. Frequency hopping occurs between time slots, occurring as a mobile

station transmits (or receives) during one time slot and hops to the time slot of another

channel before the next TDMA frame.


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The mobile will use parameters broadcast during channel assignment to derive the

hopping sequence. These parameters include the set of frequencies on which to hop, the

hopping sequence number of the cell and the offset index. The offset index is used to

distinguish the different mobiles of the cell using the same mobile allocation. It must be

noted that the basic physical channel supporting the BCCH does not hop

Frequency hopping can be accomplished in two manners, either baseband or synthesizer

hopping. Transmitters operate on a fixed frequency during baseband hopping; however,

the calls are routed from the serving voice channel to the various transmitters associated

with the frequency hopping sequence. Synthesizer hopping is accomplished by having

the voice channel data routed to a single transmitter, and the transmitter tunes to different

frequencies at each burst.

There are advantages and disadvantages to either method of frequency hopping.

Baseband hopping requires the use of narrow-band combiners with up to 16 inputs (see

Figure 33). Therefore, many frequencies can be used in a set of hopping channels. The

drawback is that the number of frequencies is limited to the number of transmitters at the

base station.

Controller

Controller

Controller

Transmitter

Transmitter

Transmitter

Filter Combiner

TRX1

TRX2

TRX3

bus for routing of bursts

Figure 33: Baseband Hopping

Synthesizer hopping allows a larger set of hopping channels because it is not dependent

on the number of transmitters (see ). Since the transmitter retunes to different channels

for consecutive bursts, the range of frequencies available for hopping is expanded.

Obviously, the disadvantage is in combining the transmitters before coupling to the

antennas system, where the use of a wide band synthesizer requires that a wide band

combiner be used. Typically, a large insertion loss is associated with a wide band

combiner, making it unrealistic to cascade combiners.


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Controller

Controller

Controller

Transmitter

Transmitter

Transmitter

Hybrid Combiner

TRX1

TRX2

TRX3

Controller TransmitterTRX4

Hybrid Combiner

Hybrid Combiner

Figure 34: Synthesizer Hopping

3.7 MAHO

In a GSM network, the mobile station aids the handoff process by reporting the RSSI and

BER of the local environment. This process, known as Mobile Assisted Handoff

(MAHO) is a function of the mobile station where the RF channel‟s received signal

strength (RXLEV) and quality information (RXQUAL) is supplied to the base station.

The switch uses this information in order to determine the best serving base station and

possible channel reassignment.

The following three sections describe the handoff message types, the flow of messaging

between the mobile and the base station, and a detailed explanation of the process the

mobile undergoes in assisting with the handoff in a GSM network.

TX RX M TX RX M TX M

FA

BTS A BTS C

BTS B

MS FB

FC

TX - Transmitt Interval

RX - Receive Interval

M - Measurement Interval

Figure 35: Illustration of the MAHO Measurement Process

3.7.1 Signal Strength Measurement Technique

The mobile continuously scans the BCCH of up to 32 neighboring cells, and reports the

relative signal strength of the six strongest to the switch. Neighboring cell information is

passed to the phone via system information messaging on the downlink SACCH.

Likewise, the signal strength measurements are passed back to the BTS via the uplink

SACCH.


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The signal strength measurements and neighbor cell BSIC are made during idle periods

where the phone is neither transmitting nor receiving. Since the measurement report

requires four consecutive SACCH bursts to complete, and there is one SACCH burst per

26 TDMA frames (1 multi-frame), the mobile can make at least 100 measurements. The

number of samples per BCCH is dependent on the number of neighbors.

Neighboring sites are distinguished by their BSIC assignment in order to ensure that the

correct BCCH is being measured. The identification of the BSIC, contained within the

SCH logical channel, is made during the idle frame.

The reported signal strength ranges from –48 to –110 dBm, and is reported to the BTS in

discreet increments of RXLEV. The relative accuracy in estimated RSSI is 3 dB in the

range of –85 to –105 dBm.

Table 9: Signal Strength / RXLEV Mapping

RXLEV dBm

0 Less than –110

1 -110 to –109

2 -109 to -108

…

…

62 -49 to –48

63 Above -48

3.7.2 BER Measurement Technique

In conjunction with the RSSI measurements the mobile station will make an estimate of

BER information by monitoring the accuracy of the data stream at the input to the

channel decoder. BER percentages are reported to the BTS as discreet increments of

RXQUAL. The estimation of BER and its corresponding accuracy are reported in Table

10.

Table 10: BER /RXQUAL Mapping

RXQUAL BER Reporting

Accuracy

0 Less than 0.1 90%

1 0.26 to 0.30 75%

2 0.51 to 0.64 85%

3 1.0 to 1.3 90%

4 1.9 to 2.7 90%

5 3.8 to 5.4 95%

6 7.6 to 11.0 95%

7 Above 15 95%


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3.8 Power Control

The regulation of transmitter power is accomplished on both the uplink and the downlink

of a GSM network. Downlink power control is used to statistically reduce the total

interference in a network, and reduce the probability of a receiver from becoming

saturated. Uplink power control provides the same utility, and also can increase the life

of a mobile station‟s battery.

Power control is performed in 2 dB steps, with 15 discreet increments (see Figure 36).

This allows a 30 dB dynamic range for which power control can be implemented. The

mobile station is capable of power control eight times per SACCH period, or every

thirteenth TDMA frame, thereby providing a maximum adjustment of 16 dB per SACCH

period. The base station is capable of power control on a time slot basis and has the

same dynamic range as the mobile station. All traffic channels can be power controlled

on the downlink; however, since the BCCH is used for mobile assisted handover, it is not

power controlled.

Regulation

Area

distance

Minimum

power level

Maximum

power level

output

power

Figure 36: Regulation of MS Power Control

There are two modes of power control regulation. Initial regulation is accomplished at

immediate assignment and at handover. Stationary regulation occurs during the normal

mode of phone operation. During initial regulation, the phone may transmit at high power

levels and therefore it must respond to power control quickly in order to prevent the

possibility of interference. Stationary regulation does not require a quick adjustment of

power control to control interference, and is used exclusively for the normal mode of

operation.

The process of power control follows the following methodology:

Measurement preparation

Filtering of measurements

Calculation of power order


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The switch records neighboring cell signal strength measurements every SACCH period,

or 480 ms. The use of discontinuous transmissions is evaluated along with the signal

strength measurements. The RXLEV sub measurements are used for a measurement

report that was recorded during the DTX “off” state. Conversely, RXLEV full is used for

a measurement report that was recorded during the DTX “on” state.

Missing measurement reports are estimated according to the previous and subsequent

measurement reports. The missing signal strength value is estimated to be the lowest

signal strength value of the two. The maximum number of missing measurement reports

can be set during optimization, (generally set at three).

Filtering of measurements is accomplished according to the type and duration of desired

filtering. The type and duration of the filter are translatable parameters set to provide a

stable power measurement. Although the default values may differ between GSM

equipment providers, the duration of filtering may be modified to enhance performance

issues. Filtering duration may be decreased in an area where a fast handover is desired

(i.e. the serving signal fades too quickly, and the phone call would otherwise drop).

Conversely, the filtering may be increased in an area where a slow handover is desired

(i.e. the serving and target cells have an area of overlap where handovers between the

cells occur at an undesirable rate).

Table 11: Power Control Levels

Power Class Power

Level

Peak Power

(dBm)

1 0 43 2

1 1 41 3

12 2 39 3

123 3 37 3

123 4 35 3

1234 5 33 3

1234 6 31 3

12345 7 29 3

12345 8 27 3

12345 9 25 3

12345 10 23 3

12345 11 21 3

12345 12 19 3

12345 13 17 3

12345 14 15 3

12345 15 13 3


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3.9 Time Delay Estimation

In a GSM system, the ability of mobile stations to be synchronized in time is critically

important to control interference with adjacent time slot users. Since the distance from

the base station to mobile can vary among multiple users, the various RF propagation

delays can impact the probability of inter-symbol interference. Inter-symbol interference

can be defined as an overlap in time between two signals, where an error occurs in both

signals.

A method of adjusting the time of transmissions must be used to ensure that the integrity

of each individual user‟s time slot is maintained. Timing advance is the process used to

control the mobile station‟s time of transmission. As an example, a mobile station

located far from a base station should start transmitting earlier than a mobile located close

to the base station. The regulation of timing advance is accomplished by offsetting the

occurrence of its RF energy burst. This allows the transmission to arrive at the base

station at the correct time relative to other time slots.

BTS

SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5 SLOT 6

MS2

MS1

d2, Slots 2 &5

d1, Slots 1 &5

d1 > d

2MS2

MS1

T1

T1

T2

T2

Collision

T1 - Delay of MS

1

Signal

T2 - Delay of MS

2

Signal

Figure 37: TDMA MS transmission Time Alignment

The maximum allowable time adjustment is 63 bit periods, and can be adjusted in one-bit

increments. Since each bit is 3.692 microseconds in length the maximum round distance

a mobile can be away from a base station is calculated by:

Maximum round distance = 300,000m x 63 bits x 3.692 x 10-6

bit period

= 69.7788 km


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or 34.89 kilometers away from the base station.

Time alignment is controlled in various phases of call processing including initial channel

designation and handoff. The following two sections describe in detail the time

alignment process at call origination and handoff.


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3.10 Discontinuous Transmission

Discontinuous transmission (DTX) is a feature of GSM that allows the mobile to switch

between two power levels based solely on voice activity. When there is no voice activity,

the transmission of RF ceases. In the presence of voice activity the mobile transmits RF

energy at the power level according to the most recent mobile power order. This feature

benefits the GSM network in two manners, it statistically reduces the level of interference

on both uplink and downlink, and it can increase the battery life of the mobile station.

DTX functionality requires the following functions:

- A Voice Activity Detector on the mobile‟s transmit side,

- Evaluation of the background acoustic noise on the transmit side, in order to

transmit characteristic parameters to the receive side,

- Generation on the receive side of a similar noise, called comfort noise, during

periods where the radio transmission is cut.

DTX is implemented by the use of a Voice Activity Detector (VAD). The voice activity

detector senses the presence of speech, and controls the transmission of RF based on its

measurement in 20 mS intervals. The performance of the voice activity detector may be

impacted due to background noise.

The typical performance of a VAD is illustrated in Table 12. All the values given

represent the percentage of time the radio channel is active. The activity figures

represent a typical voice conversation, where consideration is given to the characteristics

of different talkers, noise attributes and user locations. Obviously, the level of voice

activity is independent of the environment relative to a specific conversation; however

the averages reflect variations in the properties of a speaker‟s voice and the level

dependency of the VAD. As mentioned above, a decreased speech input level increases

the risk of objectionable speech clipping.

Table 12: Analysis of Voice Activity Factors

Mobile Station Environment Typical Voice Activity

Factor

Handset Quiet Location 55%

Handset Moderate Office Noise with

Voice Interference

60%

Handset Strong Voice Interference

(i.e. airport/railway station)

65-70%

Hands-free / Handset Variable Vehicle Noise 60%


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A conversation in a noisy environment (as depicted in Table 12) could be distracting to

the other party of a call if the VAD continuously turned-on and shut-off RF power.

Therefore, part of the voice activity detector‟s functionality is to compare the user‟s voice

against the background noises. The comfort noise evaluation algorithm uses parameters

to estimate the level and spectrum of background noise. The parameters evaluated by the

comfort noise algorithm are sent to the mobile via the Silence Descriptor (SID) frame.

The SID initiates the generation of artificial background noise at the mobile, so that the

perception of background noise levels remains constant. A SID frame is always sent at

the end of a speech burst, before the radio transmission is cut. The level of noise when

DTX shuts off RF power matches the background level of noise that is present when

DTX allows the mobile station to transmit at full power.

3.11 Mobile Station

Subscribers communicate through the wireless network via the mobile station. There are

three types of mobile stations, including:

Vehicle mounted

Transportable

Handheld

Each of these mobiles has a different output power associated with the specific

classification. The common classifications of mobiles and their corresponding output

power vary according to type of GSM network. The mobile class and output power for

various GSM network applications are included in Table 13, Table 14 and Table 15.

Table 13: PCS-1900 Mobile Class and Power

Class Peak Output Power (W)

1 2

2 1

3 .5

Table 14: DCS-1800 Mobile Class and Power


1 1

2 .25

Table 15: GSM-900 Mobile Class and Power


1 20

2 8

3 5

4 2

5 .8


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Each phone must have a Subscriber Identity Module (SIM). The SIM is a plastic card

with a magnetic strip containing subscriber information that is inserted in the phone. The

information stored on the SIM include:

International Mobile Subscriber Identity (IMSI) Number

Mobile Station ISDN Number (MSISDN)

Personal Identity Number (PIN)

Personal Unlocking Key (PUK)

Serial number of the SIM

Ciphering Key (Kc)

Authentication Key (Ki)

Temporary Mobile Subscriber Identity (TMSI)

SIM service table (indicates which optional services are available)

Barred PLMNs

Language Preference

Additional subscriber related information

Of primary importance are the IMSI and PIN numbers. Mobile stations cannot access a

network, except for emergency calls, without a valid International Mobile Subscriber

Identity (IMSI) number. The IMSI number, stored in the SIM identifies each mobile

within a network.

The Personal Identity Number (PIN) is a four to eight digit number that allows a

subscriber to lock or unlock his phone to prevent unauthorized use. If the incorrect PIN

is entered three consecutive times the SIM is locked.

As part of the standardization of GSM mobiles there are certain required features

including:

Display of Called Number

Call Progress Indication

Country/PLMN Indication

Subscription Identity Management

Invalid PIN Indicator

IMSI

Service Indicator

Manufacturers have the opportunity to enhance features in future editions of mobiles as

long as the features do not interfere with the network or other mobiles.

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