GSM Based Networks

RF EngineeringContinuing Education and Training

Unit 11

GSM Based NetworksGSM 900 – DCS 1800 – PCS 1900

Prepared by:

TEC CELLULAR, Inc. a Division of SAFCO Corporation7619 Emerald Drive

West Melbourne, FL 32904 USAPhone (407) 952-8300Fax (407) 725-5062

www.tecc.com

http://www.tecc.com/

RF Engineering Continuing EducationGSM Based Networks

GSM Based NetworksTarget 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 Contents1 Introduction..................................................................................................................... iv

1.1 History and Development of GSM, DSC1800 and DCS1900..................................iv1.2 Usage of GSM across the world............................................................................... vi1.3 GSM Standards ......................................................................................................... x

2 System Organization........................................................................................................xi2.1 What is TDMA?........................................................................................................xi2.2 GSM as a TDMA system.........................................................................................xii2.3 GSM Efficiency......................................................................................................xiii2.4 GSM System Components......................................................................................xiv2.5 RF Carrier............................................................................................................... xvi

2.5.1 GSM - Europe..................................................................................................xvi2.5.2 Extended GSM -Europe...................................................................................xvi2.5.3 DCS-1800........................................................................................................ xvi2.5.4 PCS-1900........................................................................................................xvii

2.6 Time Slots and TDMA Frames............................................................................ xviii2.7 Physical channels and bursts...................................................................................xix

2.7.1 Dedicated Control Channels............................................................................. xx2.7.1.1 Stand Alone Dedicated Control Channel (SDCCH)..................................xx2.7.1.2 Slow Associated Control Channel (SACCH)............................................ xx2.7.1.3 Fast Associated Control Channel (FACCH)..............................................xx

2.7.2 Common Control Channels...............................................................................xx2.7.2.1 Random Access Channel (RACH).............................................................xx2.7.2.2 Paging Channel (PCH)..............................................................................xx2.7.2.3 Access Grant Channel (AGCH)................................................................xxi

2.7.3 Broadcast Channels..........................................................................................xxi2.7.3.1 Broadcast Control Channel (BCCH)........................................................ xxi2.7.3.2 Frequency Correction Channel (FCCH)..................................................xxii2.7.3.3 Synchronization Channel (SCH)............................................................. xxii

2.7.4 Bursts.............................................................................................................. xxii2.7.4.1 Normal burst (NB)...................................................................................xxii2.7.4.2 Frequency correction burst (FB).............................................................xxiii2.7.4.3 Synchronization burst (SB).................................................................... xxiii2.7.4.4 Dummy Burst..........................................................................................xxiv2.7.4.5 Access burst (AB)...................................................................................xxiv

2.7.5 Guard period................................................................................................... xxv2.8 Call Processing Messages...................................................................................... xxv2.9 Processing the Voice Signal................................................................................. xxvi

2.9.1 The GSM codec............................................................................................ xxvii2.10 Data Coding........................................................................................................ xxix2.11 Interleaving......................................................................................................... xxxi2.12 Equalization...................................................................................................... xxxiii2.13 Modulation.........................................................................................................xxxv

2.13.1 Binary Frequency Shift Keying (BFSK).....................................................xxxv2.13.2 Minimum Shift Keying (MSK)...................................................................xxxv

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2.13.3 Gaussian Minimum Shift Keying (GMSK)..............................................xxxvii2.13.4 GMSK specifics for GSM..........................................................................xxxix

3 GSM RF Planning......................................................................................................... xlii3.1 Link Budget............................................................................................................ xlii

3.1.1 Example of a GSM Link Budget .................................................................... xlii3.1.1.1 Calculation of Receiver Sensitivity.......................................................... xlii

3.1.1.1.1 Noise Figure of the Receiver............................................................xliii3.1.1.1.2 Other Sources of Noise.....................................................................xliii

3.2 Calculation of the Nominal Cell Radius................................................................. xlv3.3 Reuse Efficiency...................................................................................................xlvii3.4 Capacity Calculations..........................................................................................xlviii3.5 Frequency Planning.................................................................................................... l3.6 Interference Reduction Strategies.............................................................................lii

3.6.1 Hierarchical Cell Structures...............................................................................lii3.6.2 Overlay/Underlay ............................................................................................. lii3.6.3 Frequency Hopping.......................................................................................... liv

3.7 MAHO..................................................................................................................... lvi3.7.1 Signal Strength Measurement Technique......................................................... lvi3.7.2 BER Measurement Technique.........................................................................lvii

3.8 Power Control........................................................................................................lviii3.9 Time Delay Estimation............................................................................................. lx3.10 Discontinuous Transmission..................................................................................lxi3.11 Mobile Station...................................................................................................... lxii

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Index of FiguresFigure 1: An illustration of the TDMA concept................................................................. xiFigure 2: GSM as a TDMA/FDMA system...................................................................... xiiFigure 3: GSM System Architecture................................................................................. xvFigure 4: PCS Spectrum Allocation in the United States................................................xviiFigure 5: TDMA Frame Hierarchy................................................................................ xviiiFigure 6: GSM - Time Division Duplex........................................................................ xviiiFigure 7: Logical Channels...............................................................................................xixFigure 8: Control Multi-Frame......................................................................................... xxiFigure 9: Normal Burst...................................................................................................xxiiiFigure 10: Frequency Correction Burst..........................................................................xxiiiFigure 11: Synchronization Burst...................................................................................xxivFigure 12: Dummy Burst................................................................................................xxivFigure 13: Access Burst...................................................................................................xxvFigure 14: Messages Sent during Call Processing...........................................................xxvFigure 15: Sampling and Quantization of Analog Signal...............................................xxviFigure 16: Scheme of audio signal processing in telephony......................................... xxviiFigure 17: Block diagram of GSM speech encoder..................................................... xxviiiFigure 18: Block diagram of GSM speech decoder..................................................... xxviiiFigure 19: Block diagram of channel coding for GSM technology............................... xxixFigure 20: Illustration of the interleaving process..........................................................xxxiFigure 21: Interleaving for the traffic channel in GSM.................................................xxxiiFigure 22: An example of a Power Delay Profile (PDP)............................................. xxxiiiFigure 23: Principals of the GSM equalizer operation................................................. xxxivFigure 24: Example of MSK modulation.................................................................... xxxviiFigure 25: Shape of the time and frequency response of the ‘Gaussian’ filter.......... xxxviiiFigure 26: Spectral characteristics of GMSK and MSK ........................................... xxxviiiFigure 27: Bandwidth of the GSM signal versus the percentage of signal power............. xlFigure 28: Modulation scheme for GSM technology.........................................................xlFigure 29: N=3 Reuse Scheme............................................................................................. lFigure 30: N=4 Reuse Scheme............................................................................................ liFigure 31: Area of interference......................................................................................... liiiFigure 32: Signal strength at point of interference............................................................liiiFigure 33: Baseband Hopping............................................................................................ lvFigure 34: Synthesizer Hopping........................................................................................ lviFigure 35: Illustration of the MAHO Measurement Process.............................................lviFigure 36: Regulation of MS Power Control.................................................................. lviiiFigure 37: TDMA MS transmission Time Alignment....................................................... lx

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Index of Tables Table 1: Message bit to frequency / phase mapping .................................................. xxxviTable 2: Percentage of total signal energy (%E) carried by the bandwidth (B E)....... xxxixTable 3: Example of GSM Budget – Rural Environment............................................... xlivTable 4: Example of GSM Link Budget – Suburban Environment................................ xlivTable 5: Example of GSM Link Budget – Urban Environment.......................................xlvTable 6: Typical Slope and Intercept Values.................................................................. xlviTable 7: Capacity of Network Using 3/9 Reuse Scheme................................................ xlixTable 8: Capacity of Network Using 4/12 Reuse Scheme.............................................. xlixTable 9: Signal Strength / RXLEV Mapping................................................................... lviiTable 10: BER /RXQUAL Mapping................................................................................lviiTable 11: Power Control Levels ....................................................................................... lixTable 12: Analysis of Voice Activity Factors................................................................... lxiTable 13: PCS-1900 Mobile Class and Power................................................................. lxiiTable 14: DCS-1800 Mobile Class and Power................................................................ lxiiTable 15: GSM-900 Mobile Class and Power..................................................................lxii

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 DCS1900Throughout 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

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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 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.

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•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)•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 worldThe 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 MTNZimbabwe 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

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


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

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

http://www.omnipoint.com/

http://www.powertel.com/

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

http://www.sprint.com/

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


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

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

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


Asia Azerbadjan GSM 900 96 November Azercell Telekom B.M.Bangladesh GSM 900 97 March GrameenPhoneChina 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 CelcomMalaysia GSM 1800 5 July Mutiara TelecommunicationsSingapore 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

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

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

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

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

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

http://www.telkomsel.com/


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

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

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


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

http://www.vptt.ch/

http://www.telia.se/

http://www.europolitan.se/

http://www.airtel.es/

http://www.telefonica.es/

http://www.mobitel.si/

http://www.eurotel.sk/

http://www.telecel.pt/

http://www.eragsm.pl/

http://www.telenor.no/

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

http://www.libertel.nl/

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

http://www.tim.it/

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

http://www.telecom.ie/

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

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

http://www.panafon.gr/

http://www.telestet.gr/

http://www.d2privat.de/

http://www.francetelecom.fr/

http://www.bouyguestelecom.fr/

http://www.tfi.net/

http://www.emt.ee/

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

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


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”

GSM 04.03 “European digital cellular telecommunication system (Phase 2); Mobile Station – Base Station System (MS – BSS) interface Channel structures and access capabilities”

GSM 05.02 “European digital cellular telecommunication system (Phase 2); Multiplexing and multiple access on the radio path”

GSM 05.03 “European digital cellular telecommunication system (Phase 2); Channel coding”

GSM 05.04 “European digital cellular telecommunication system (Phase 2); Modulation”

GSM 05.05 “European digital cellular telecommunication system (Phase 2); Radio transmission and reception”

GSM 05.08 “European digital cellular telecommunication system (Phase 2); Radio subsystem link control”

GSM 05.10 “European digital cellular telecommunication system (Phase 2); Radio subsystem synchronization”

GSM 06.01 “European digital cellular telecommunication system (Phase 2); Full rate speech processing function”

GSM 06.10 “European digital cellular telecommunication system (Phase 2); Full rate speech transcoding”

GSM 06.11 “European digital cellular telecommunication system (Phase 2); Substitution and muting of lost frames for full rate speech channels”

GSM 06.12 “European digital cellular telecommunication system (Phase 2); Comfort noise aspects for full rate speech channels ”

GSM 06.31 “European digital cellular telecommunication system (Phase 2); Discontinuous Transmission (DTX) for full rate speech channels ”

GSM 06.32 “European digital cellular telecommunication system (Phase 2); 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, Ts2, Ts3, ...,TS6,TS7

TS1 TS2 TS3 .... TS6 TS7

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 systemAlthough 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

USER16,F2

Frequency F1

Frequency F2

Figure 2: GSM as a TDMA/FDMA system

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2.3 GSM EfficiencyThere 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

bα (1)

where:fα = 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 =×

−−=fα (3)

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2.4 GSM System ComponentsThe 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 functionalityb) provides billing informationc) 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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MSC/VLR

HLR

AUC

EIR GMSC

GIWU

PSTN

SwitchingSystem

BSC

MXE/MIN

CellularNetworks

PCSNetworks

BTS

BaseStationSystem

OperationalSupportSystem

OMC

NMC

AUC - Authentication CenterBSC - Base Station ControllerBTS - Base Transceiver StationEIR - Equipment Identity RegisterGIWU - GSM Interworking UnitGMSC - Gateway MSCHLR - Home Location RegisterMSC - Mobile Switching CenterMIN - Mobile Intelligent NetworkMXE - Message CenterNMC - Network Management CenterOMC - OPeration & Maintenance CenterVLR - Visitor Location Register

Figure 3: GSM System Architecture

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2.5 RF CarrierThe 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 - EuropeGSM, 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:

Uplinkfrequency = 890 Mhz + (0.2 Mhz) x ARFCN

Downlinkfrequency = 935 Mhz + (0.2 Mhz) x ARFCN

2.5.2 Extended GSM -EuropeAs 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:

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

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

2.5.3 DCS-1800GSM 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:

Copyright © 1998 by TEC CELLULARxvi


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

Downlinkfrequency = 1,785 Mhz + (0.2 Mhz) x (ARFCN-511)

2.5.4 PCS-1900GSM 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:

Uplinkfrequency = 1,850 Mhz + (0.2 Mhz) x (ARFCN-512)

Downlinkfrequency = 1,930 Mhz + (0.2 Mhz) x (ARFCN-512)

Figure 4: PCS Spectrum Allocation in the United States

Copyright © 1998 by TEC CELLULARxvii


2.6 Time Slots and TDMA FramesThe 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 frames0 1 2 3 4 5 6 2043 2044 2045 2046 2047

1 superframe = 51 (26 frame) multi- frame or 26 (51 frame)multi-frame0 1 2 3 4 5 6 46 47 48 49 500 1 2 3 4 5 6 21 22 23 24 25

1 (26 frame) multi-frame0 1 2 3 23 24 25

1 (51 frame) multi-frame0 1 2 3 23 24 25

1 TDMA Frame0 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 BitsGP = Guard Period

Figure 5: TDMA Frame HierarchyEight 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 Transmits0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0

Mobile Station Transmits5 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 burstsThe 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.

ControlChannels

LogicalChannels

TrafficChannels

BroadcastChannels

Common ControlChannels

Dedicated ControlChannels

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.

Copyright © 1998 by TEC CELLULARxix


2.7.1 Dedicated Control ChannelsDedicated 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 ChannelsCommon 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.

Copyright © 1998 by TEC CELLULARxx


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

0F

1S

2B

3B

4B

5B

6C

7C

8C

9C

10F

11S

12C

13C

14C

15C

… 20F

21S

… 49C

50I

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

0R

1R

2R

3R

4R

5R

… … … … … … … … … … … … … … 49R

50R

R: Random Access Channel (RACH)

Figure 8: Control Multi-Frame

2.7.3 Broadcast ChannelsBroadcast 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).

Copyright © 1998 by TEC CELLULARxxi


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 BurstsBursts 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.

Copyright © 1998 by TEC CELLULARxxii


0TB

1TB

2TB

3ED

…ED

…ED

60F

61TS

62TS

…TS

…TS

86TS

87F

88ED

89ED

…ED

…ED

144ED

145TB

146TB

147TB

148GP

149GP

… 156GP

Figure 9: Normal Burst

Where:TB = Tail BitsED = Encrypted DataF = FlagTS = Training SequenceGP = 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.

0TB

1TB

2TB

3ED

…ED

…ED

…ED

…ED

…ED

…ED

…ED

…ED

…ED

…ED

…ED

…ED

…ED

144ED

145TB

146TB

147TB

148GP

149GP

… 156GP

Figure 10: Frequency Correction Burst

Where:TB = Tail BitsED = Encrypted DataGP = 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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0TB

1TB

2TB

3ED

4ED

…ED

…ED

41ED

42SS

43SS

…SS

…SS

105SS

106ED

107ED

…ED

…ED

144ED

145TB

146TB

147TB

148GP

149GP

… 156GP

Figure 11: Synchronization Burst

Where:TB = Tail BitsED = Encrypted DataSS = Synchronization SequenceGP = Guard Period

2.7.4.4 Dummy BurstThe 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.

0TB

1TB

2TB

3MB

4MB

…MB

…MB

60MB

61TS

62TS

…TS

…TS

86TS

87MB

88MB

…MB

…MB

144MB

145TB

146TB

147TB

148GP

149GP

… 156GP

Figure 12: Dummy Burst

Where:TB = Tail BitsMB = Mixed BitsTS = Training SequenceGP = 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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0TB

1TB

…TB

…TB

7TB

8SS

9SS

…SS

…SS

48SS

49EB

50EB

…EB

…EB

84EB

85TB

86TB

87TB

88GP

89GP

90GP

…GP

…GP

…GP

156GP

Figure 13: Access BurstWhere:TB = Tail BitsSS = Synchronization SequenceEB = Encrypted BitsGP = Guard Period

2.7.5 Guard periodThe 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 MessagesIn 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 RACHAGCH Channel Assigned

Answer page SDCCHSDCCH Authentication Requested

Authentication Response SDCCHSDCCH Request Cipher Mode

Acknowledge Cipher Mode SDCCHSDCCH Setup Message

Message Confirmed SDCCHSDCCH Assign Traffic Channel

Acknowledge Traffic Channel FACCHFACCH Alert

Connect Message FACCHFACCH Connect Acknowledge

Speech TCH Speech

Figure 14: Messages Sent during Call Processing

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2.9 Processing the Voice SignalThe 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 µ -law compression, or 4096 levels if linear compression is used.

111 +3V110 +2V101 +1V

0V001 -1V010 -2V011 -3V

111 +3V110 +2V101 +1V

0V001 -1V010 -2V011 -3V

Analog Signal

Sampling Pulse

PAM

101 110 101 100 010 010 010 100 111 111PCM

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 codecThe 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.

BPF A/Dconverter

SPEECH

ENCODER

CHANNEL

CODING

TO

MODULATORMICROPHONE

BAND-PASS300 Hz-3.4 kHz

SPEECH

DECODER

CHANNEL

DECODERLP

LOW-PASS4 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

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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

HammingWindow

Pre-processing

Short-TermPrediction

(STP) Analysis

Long-TermPrediction (LTP)

Analysis

+ LPF

LTP Filter

MU

X

GridSelection

RegualarPulse

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 SythesisFilter

STP SynthesisFilter

Post-Processing

output

An

Pn, gn

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 CodingOnce 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 Figure19. 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 IaBITS

TYPE IIBITS

TYPE IbBITS

CONVOLUTIONALENCODER

r=1/2

K=5

MU

X

ERROR DETECTING CODE50

132

78

3

4

189

189

378

456

0TO

INTERLEAVER

FRO

M 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 )3. and ‘bit puncturing’, that presumes the transmission of only certain bits of output

sequence.

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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 (2×189) 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 InterleavingRayleigh 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

bbbbbbbbbbbbbbbbbbbbbbbbb

Data is writ tencolumn-wise

Data is readrow-wise

Interleaver

b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12 b13 b14 b25 b16 b17 b18 b19 b20...

b1 b6 b11 b16 b21 b2 b7 b12 b17 b22b3 b8 b13 b18 b23 b4 b9 b14 b19 b24..

Burst ErrorCaused by

Rayleigh Fading

Errors are spread o ver the bit strea m

Figure 20: Illustration of the interleaving processIn 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

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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 su

b-bl

ocks

57 bits

#1

#5

#4

#3

#2

#6

#7

#8

8 su

b-bl

ocks

57 bits

#1

#5

#4

#3

#2

#6

#7

#8

8 su

b-bl

ocks

57 bits

COMBINED INTOFOUR BURSTS

COMBINED INTOFOUR 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 (9×8)-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.

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2.12 EqualizationOn 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 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.

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RFProcessing

AdaptiveEqualizer

EqualizationAlgorithmExtraction of

SynchronizationBits

UnequalizedData

EqualizedData

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.

Copyright © 1998 by TEC CELLULARxxxiv


2.13 ModulationThe 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()( ∆+⋅= ππ , bTt ≤≤0 , ‘high tone’ for binary 1 (4a)tffKtr CL )22cos()( ∆−⋅= ππ , 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 ττππ )(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)

bLH T

ftftf⋅

⋅=∆=−4

122)()( (7)

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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

bC )()(

4122cos)( ππ (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.

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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

3.0=⋅ TB 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.

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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 SPECTRALDENSITYdB

Figure 26: Spectral characteristics of GMSK and MSK

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2.13.4 GMSK specifics for GSMThe 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 µsec, 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

12exp)(

σαffHfPDF (9)

where σ2 is the variance, related to the 3 dB bandwidth of the filter through the value of α:

( )2

2 2ln2B

⋅=α ; ⇒ ( )( )

( )2ln82ln841 222

22

⋅⋅

=⋅

=⋅

= bRBTBα

σ . (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%

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0

20

40

60

80

100

120

140

160

180

200

99% 90% 80% 70% 60% 50% 40% 30% 20% 10%

Percentage of signal power

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 Figure28. A differentially coded bit sequence is integrated to obtain a signal that will be used as the phase (φ ) 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 ⋅π2cos and ( )nTfo ⋅− π2sin before being added to obtain an output signal.

INTEGRATOR GAUSSIANFILTER

COS ( )

SIN ( )

+1

-1 NRZ formatdata

S(nT)φCOS(ω T)

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

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of intersymbol interference, simple demodulation techniques are ineffective and an equalizer is required.

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3 GSM RF PlanningRF 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 BudgetIn 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:

( ) [ ] ( ) otherrequiredNI

CdBFkTWRxSens +++= log10 (11)

where:RxSens - Receiver sensitivity (dBm)kT - Power Spectrum density of the thermal noiseW - Operating bandwidth of the particular technology

[ ]dBF - Noise figure of the receiver expressed in dB( )

requiredIC - Required carrier to interference ratio for sufficient voice quality

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

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

() dBmkHzHz

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 ReceiverThe 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 NoiseThe 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

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Table 3: Example of GSM Budget – Rural Environment

In vehicle OutdoorsParameter DL UL DL ULMaximum TX Power [dBm]

44.5 33 44.5 33

MS Antenna Gain [dBd] -2.16 -2.16 -2.16 -2.16Head Loss [dB] -2 -2 -2 -2Maximum Cable and Connector Losses [dB]

-4.5 -4.5 -4.5 -4.5

BS Antenna Gain [dBd] 16 16 16 16In Vehicle Loss [dB] -8 -8 0 0Noise 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 3RX Sensitivity [dBm] -102 -108 -102 -1086Maximal Path Loss [dB] 139.79 137.29 147.79 145.29

Table 4: Example of GSM Link Budget – Suburban Environment

In Building In Vehicle OutdoorsParameter DL UL DL UL DL ULMaximum 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 -2.16Head Loss [dB] -2 -2 -2 -2 -2 -2Maximum Cable and Connector Losses [dB]

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

BS Antenna Gain [dB] 16 16 16 16 16 16In Vehicle Loss [dB] 0 0 -6 -6 0 0In Building Loss [dB] -15 -15 0 0 0 0Fade Margin (90% Coverage Reliability)

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

Diversity Gain [dB] 0 3 0 3 0 3RX Sensitivity [dBm] -102 -108 -102 -108 -102 -108Maximal Path Loss [dB] 132.64 130.14 140.97 139.81 147.38 146.22

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Table 5: Example of GSM Link Budget – Urban Environment

In Building In Vehicle OutdoorsParameter DL UL DL UL DL ULMaximum 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 -2.16Head Loss [dB] -2 -2 -2 -2 -2 -2Maximum Cable and Connector Losses [dB]

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

BS Antenna Gain [dB] 16 16 16 16 16 16In Vehicle Loss [dB] 0 0 -6 -6 0 0Building Penetration Loss [dB]

-18 -18 0 0 0 0

Fade Margin (90% Coverage Reliability)

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

Diversity Gain [dB] 0 3 0 3 0 3RX Sensitivity [dBm] -102 -108 -102 -108 -102 -108Maximal Path Loss [dB] 124.94 122.44 140.54 138.04 146.74 144.24

3.2 Calculation of the Nominal Cell RadiusBased 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

Phh

hh

dmPdBmRSL log10log10log15log][ 1 (13)

Where:RSL - Received signal level

mP1 - One mile interceptm - Slope

th - Height of the BS antenna

mh - Height of the MS antenna

tP - Transmit power

trefmreftref Phh ,, - Reference conditions

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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 -43.1Urban -63 -40.0Suburban -59 -38.4Rural -49 -43.5

1900[10] Dense Urban -80 -43.1Urban -69 -40.0Suburban -65 -38.4Rural -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 transmitterCL Cable and other lossesGt 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 −=−=

Substituting the minimal RSL into Lee’s equation, and assuming the typical values for suburban slope and intercept at PCS frequencies of dBmP m 651 −= , decdBm /4.38−= , the nominal cell radius is calculated:

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

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( )

+

+

+−−=−

100290log10

106log10

150150log15log4.386564.75 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 log10log10log151log RSL

PP

hh

hhP

md

tref

t

mref

m

tref

tm

(15)

Similar formulas can be derived using other macroscopic propagation models.

3.3 Reuse EfficiencyThe 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

ni

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 dB4 11.43 dB

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3.4 Capacity CalculationsThe 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 2.791

kHz 200Mhz 5Channels Scheme 9Reuse3 =×=− (17)

ectorChannels/S 2.1121

kHz 200Mhz 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:

NetworkSites

SiteSectors

SectorErlangs Network ofCapacity Erlang ××= (19)

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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 100Number of Voice Channels/ Sector 22Blocking Rate 1 %Erlangs/Sector 13.65Sectors per Site 3Erlangs/Site 40.95Erlang Capacity of Network 4095

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

Total Number of Cells Required 100Number of Voice Channels/ Sector 14Blocking Rate 1 %Erlangs/Sector 7.351681Sectors per Site 3Erlangs/Site 22.05Erlang 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.

Copyright © 1998 by TEC CELLULARxlix


3.5 Frequency PlanningGood 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 StrategiesGSM-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 StructuresHierarchical 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 interferenceUsing 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 Figure32). 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 HoppingThe 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 CombinerTRX1

TRX2

TRX3

Controller TransmitterTRX4Hybrid Combiner

Hybrid Combiner

Figure 34: Synthesizer Hopping

3.7 MAHOIn 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.

Figure 35: Illustration of the MAHO Measurement Process

3.7.1 Signal Strength Measurement TechniqueThe 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.

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.

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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 dBm0 Less than –1101 -110 to –1092 -109 to -108……62 -49 to –4863 Above -48

3.7.2 BER Measurement TechniqueIn 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 Table10.

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 ControlThe 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.

RegulationArea

distance

Minimumpower level

Maximumpower level

outputpower

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 ± 21 1 41 ± 312 2 39 ± 3123 3 37 ± 3123 4 35 ± 31234 5 33 ± 31234 6 31 ± 312345 7 29 ± 312345 8 27 ± 312345 9 25 ± 312345 10 23 ± 312345 11 21 ± 312345 12 19 ± 312345 13 17 ± 312345 14 15 ± 312345 15 13 ± 3

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3.9 Time Delay EstimationIn 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.

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

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 TransmissionDiscontinuous 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 Interference60%

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 StationSubscribers 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 22 13 .5

Table 14: DCS-1800 Mobile Class and Power

Class Peak Output Power (W)1 12 .25

Table 15: GSM-900 Mobile Class and Power

Class Peak Output Power (W)1 202 83 54 25 .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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GSM Based Networks