NET
`
`Geoff Sanders I Lionel Thorens I Manfred Reisky
`Oliver Rulik I Stefan Deylitz
`
`~WILEY
`
`Page 1 of 21
`
`GOOGLE EXHIBIT 1022
`
`

`

`GPRS Networks
`
`Geoff Sanders, Lionel Thorens, Manfred Reisky,
`Oliver Rulik and Stefan Deylitz
`
`All of
`
`tfk GmbH, Germany
`
`~ WILEY
`~ ............. ---------------~~-
`
`Page 2 of 21
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`

`

`•
`
`Copyright © 2003
`
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`
`ISBN 0-470-85317-4
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`
`Page 3 of 21
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`

`

`Contents
`
`Preface
`
`Introduction
`
`1 Mobile Radio Evolution
`1.1 Trend from Speech to Data Transmission
`1.2 The Third Generation
`1.3 GSM - The Global System for Mobile Communications
`1.4 GSM - Evolutionary Concept
`1.5 The Standards
`
`2 The General Packet Radio Service
`2.1 GPRS Objectives and Advantages
`2.2 GPRS Architecture
`2.3 Characteristics of a GPRS Connection
`2.4 Logical Functions
`
`3 Interfaces and Protocols
`3.1 Introduction
`3.2 Layer Model
`3.3 The Names of the GPRS Interfaces
`3.4 GPRS Procedures
`3.5 GPRS Attach
`3.6 Activation of a PDP Context
`3.7 Data Transfer
`3.8 Physical Implementation in the GPRS Network
`3.9 GPRS Signalling
`3.10 GPRS Protocol Planes
`
`4 GPRS Procedures
`4.1 GPRS Mobility Management Procedures
`4.2 Session Management Procedures
`4.3 Packet Transfer Procedures
`
`vii
`
`ix
`
`1
`1
`3
`4
`13
`15
`
`17
`17
`18
`30
`40
`
`59
`59
`59
`61
`62
`63
`64
`66
`67
`74
`83
`
`87
`87
`103
`106
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`~ ............... ---------------~~
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`Page 4 of 21
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`vi
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`Contents
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`•
`
`5 Changes in the Radio Subsystem for GPRS
`5.1 Overview and Key Architecture
`5.2
`Introduction of EDGE, ECSD and E-GPRS
`
`6 Core Network
`6.1 Serving GPRS Support Node (SGSN)
`6.2 Gateway GPRS Support Node (GGSN)
`6.3 Access Network PCU - SGSN (Gb Interface)
`6.4 Core Network SGSN, GGSN (Gn Interface)
`6.5 Additional Elements in the Core Network
`6.6 Additional Elements at the Gi Interface
`6.7 Connections Towards the GSM Network
`
`7 Terminal Equipment
`7.1 Types of Terminal Equipment
`7.2 Multi-slot Classes and GPRS MS Classes
`7 .3 The Settings in a GPRS-enabled Mobile Device
`
`8 Planning and Dimensioning
`8.1
`Introduction
`8.2 Network Dimensioning
`8.3 GPRS Radio Subsystem
`8.4 GPRS Core Network
`8.5 User Aspects
`8.6
`Indoor Radio Networks
`
`9 Towards All-IP Networks
`9.1 The TCP/IP Protocol Suite
`9.2 Convergence of Fixed, Mobile and Data Networks
`9.3 The Roles of GSM, GPRS and UMTS in Converged Networks
`
`10 Applications
`10 .1 Services
`10.2 Multimedia Messaging Service (MMS)
`10.3 GSM-R
`10.4 m-Business and m-Commerce
`
`11 Roaming and GRX
`11.1 Introduction
`11.2 Why do we need Roaming in GPRS?
`11.3 Architecture
`11.4 GPRS Roaming eXchange (GRX) Network
`11.5 Procedures
`11.6 Quality Aspects of GRX
`
`Glossary and Abbreviations
`
`Index
`
`109
`109
`120
`
`125
`127
`130
`133
`134
`136
`139
`140
`
`143
`143
`146
`152
`
`155
`155
`156
`162
`166
`189
`190
`
`195
`195
`205
`207
`
`217
`217
`234
`236
`240
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`245
`245
`245
`246
`247
`248
`250
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`251
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`283
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`Page 5 of 21
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`1
`
`Mobile Radio Evolution
`
`1.1 Trend from Speech to Data Transmission
`Beginning with the first single cell mobile telephone services in the 1940s and continuing
`through 'the first and second generations of mobile telephone services, the primary function
`of the mobile device was to enable speech calls to be set up between the mobile user and
`a fixed network of base stations (BTS) and telephone exchanges.
`This scenario has changed dramatically in recent years. With the introduction of first
`SMS and then WAP, few people now support the idea that a mobile device is only
`for speech calls, and just as the rate of change in mobile applications is accelerating,
`so is the rate of acceptance. As more and more people make use of mobile services,
`the multifunction portable device is becoming ever more indispensable in our daily
`lives.
`This is an enormous departure from accepted norms. The trend from mobile speech to
`data is gaining acceptance faster than any other technology ever invented. The Internet
`took decades to reach its present form, and only since the mid-1990s has it enjoyed
`such widespread utilization from home users. GSM however, in just 10 short years,
`achieved over 80 % market penetration in some European countries! Compared with other
`industries, the rate of change in mobile technology is unprecedented. The automobile
`industry introduces a new model in about 5 years; the fashion industry also measures
`change in terms of years. Even the computer industry, which has experienced quantum
`leaps in terms of hardware technology and applications software, has not had the kind of
`acceptance which mobile telecommunications are having.
`The fact of the matter is that all these industries - one could almost say all indus(cid:173)
`tries - are going to be directly affected by the trend towards applications based on mobile
`data transfer. It is already possible to send an SMS during a normal speech call, download
`information from the Internet via WAP, and access web sites and e-mail via a notebook or
`PDA connected to a mobile phone. This is only the beginning: it is predicted that by 2010
`the volume of new mobile phone sales in Europe and North America will start to level
`off. This is the first indicator of the coming boom in mobile services based on packet data
`transmission. Already some manufacturers have started to develop and market solutions
`for m-services and m-commerce as they are already aware that future revenues will be
`
`GPRS Networks. G. Sanders, L. Thorens, M. Reisky, 0 . Rulik, S. Deylitz
`© 2003 John Wiley & Sons, Ltd.
`ISBN: 0-470-85317-4
`
`~--------------------
`
`Page 6 of 21
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`•
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`2
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`GPRS Networks
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`generated not through equipment sales, but by providing the customer with new ways to
`make use of that equipment.
`We can already see the start of this trend. The SIM tool kit introduces extra functionality
`onto the SIM card, which allows the user to subscribe to various service providers who
`use the tool kit to run their own proprietary software. This represents a major step forward
`in that it is no longer necessary to modify the mobile device or the SIM card in order to
`introduce new or improved services. This in turn means that the user can take advantage
`of new services at any time without having to invest in new equipment.
`The automobile industry is a good example of current trends. Almost from the beginning
`of GSM, manufacturers at the top end of the market have been installing mobile phones in
`their cars. More recently, navigation systems based on GPS satellite positioning systems
`combined with electronic maps stored on CD-ROMs have become standard items on such
`vehicles. In recent years, these two systems have been linked together (converged) so that
`the navigation system receives traffic reports via SMS and uses this data to avoid traffic
`jams. A further enhancement is to store such things as hotel and tourist information on the
`navigation CD. Several manufacturers have now moved on to using DVDs as a storage
`medium due to the limited storage capacity of a CD-ROM.
`What is the next step? The answer is to do away with on-board storage completely
`and use the general packet radio service (GPRS) to download the required information as
`and when required. Once this step has been taken, the on-board systems only need to be
`slightly upgraded to enable full Internet access. Such systems are in fact currently available
`as custom solutions, so it is only a matter of time before they become options/standard
`features offered by the manufacturer. This step is seen by many as essential to the con(cid:173)
`tinued evolution of in-car systems. The problem with CD/DVD systems is that the data
`is out of date almost as soon as it is released and cannot easily be updated. Further, such
`storage media offer a very limited amount of data as far as hotel and tourist information is
`concerned, added to which price and availability details cannot be included for the reason
`stated previously. A GPRS based system, which accesses a centralized server network,
`would always have access to the latest traffic information and could provide web links
`to relevant hotel, restaurant and tourist information web sites, based on GPS location
`information provided via the mobile.
`As can be seen from the above example, there is a general trend towards increasing
`convergence of previously independent systems. In this we can also observe a trend in
`the way the Internet is being utilized. Previously, servers attached to the World Wide
`Web (www) enabled information exchange across the net. Later, e-commerce changed
`the face of the Web as it evolved from being purely an 'Information superhighway' (data,
`e-mail, gaming) to a way of doing business on a global scale. Now we see a further trend
`towards centralized, Web-based, servers replacing discrete/portable storage media. Such
`servers already provide business users with storage for their documents, presentations and
`other files . They can be accessed/updated online, avoiding the risk of having important
`information corrupted or stolen from a portable device. Such systems could replace in(cid:173)
`car data storage - or any other application that depends on information stored on CD or
`DVD - using GPRS to give mobile access to the Internet.
`The trends towards applications which can use GPRS are plainly visible. The success
`of i-mode in Japan and its subsequent release in Europe has demonstrated that there is
`a significant demand for mobile services based on high-speed packet data transfer. In its
`
`Page 7 of 21
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`Mobile Radio Evolution
`
`3
`
`first year of business, i-mode attracted over 20 million users in Japan. If this success is
`repeated in Europe (where GPRS is the bearer) it will not only prove the technology and
`the business model, but will also provide a driving force for new services based on the
`general packet radio service.
`
`1.2 The Third Generation
`Among the many questions asked about 3G, one of the most frequent is 'Why do we
`need it?'. The answer lies in the planning. 2G was a .response to a need for cheaper,
`more reliable, mobile communications which could easily cross international boundaries.
`The technology evolved through different phases which can be roughly characterized
`as: speech ~ speech related services ~ data services. Mobile Internet access was not
`initially envisaged, and this is why GSM has had to be supplemented with such things
`as HSCSD, GPRS and EDGE in the final phase (phase 2+ ). 3G, on the other hand, was
`intended from the outset to enable high bit-rate mobile data services (in addition to speech
`calls and traditional services) and has been planned accordingly.
`The evolution towards 3G and beyond is happening on different levels. On the tech(cid:173)
`nological level we have a new radio access network, the UTRAN (UMTS radio access
`network), which interfaces with the GSM circuit switched (CS) core network for speech
`calls and the GPRS packet oriented (PO) core network for data transfer. On the services
`level, GSM required the addition of first the intelligent networks (IN) platform, to enable
`a greater variety of national services, and then the CAMEL (customized applications for
`mobile network enhanced logic) platform to make these services available while roaming
`outside the home PLMN. Modifications were needed to the radio access technology to
`enable high speed data services to be implemented. UMTS has been designed so that the
`services provided to the end-user are independent of the radio access technology.
`On the network level, the current evolutionary trend is towards so called 'all-IP'
`networks. The convergence of CS and PO technologies already begun with the imple(cid:173)
`mentation of GPRS will reach its logical conclusion at some point in the future when the
`radio access networks, GERAN (GSM/EDGE radio access network) and UTRAN, will
`connect directly to a packet data network. Speech will be carried as voice over IP (VoIP)
`and will be routed from the source to the destination in the same way that other data
`packets are routed from one application to another.
`The technological evolution from FDMA in lG, through the combination ofFDMA and
`TDMA in 2G, and on to CDMA in 3G, is a clear progression with each new development
`seeking to solve the problems of the previous generation. The evolution of the services
`offered to the end-user, however, is not such a linear process and thus a clear development
`path is difficult to identify. Providers are using the Internet as a base to offer their content
`and services to end-users with a great variety of terminal equipment. To maximize their
`revenues, they need to reach the maximum possible number of people and so must serve
`users with equipment from different phases and different generations. SMS, HSCSD,
`GPRS and WAP can all be used as means to bring value added services to the user and
`here is where the real strength of GPRS can be seen. The GPRS infrastructure can be
`utilized to deliver SMS and WAP services to both GSM and UMTS users. Here again
`we see evolution resulting in convergence. The implementation of GPRS was originally
`seen as a bridge between 2G and 3G but in actual fact has resulted in developments that
`have a direct affect on both generations.
`
`Page 8 of 21
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`•
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`4
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`GPRS Networks
`
`1.3 GSM - The Global System for Mobile Communications
`GSM is currently going through a period of optimization that is likely to continue for some
`time, as developers refine and enhance the system to get the best possible performance
`from existing GSM networks. Many of these refinements directly or indirectly affect
`GPRS and so will be described in the following sections.
`
`1.3.l Reasons for Success
`Originally GSM stood for 'groupe speciale mobile' and was intended to be a new telecom(cid:173)
`munications standard for Europe. The success of the standard meant that the name had
`to be changed to reflect the truly worldwide application of GSM and became the 'global
`system for mobile communications'. The basis of GSM' s success can be summarized
`as follows:
`
`• an open standard - anyone can have access to the specifications,
`• standardized interfaces - multi-vendor solutions possible,
`• designed with roaming as a prerequisite - network architecture and procedures,
`• encrypted transmission - ensures privacy/security of speech/data,
`• subscriber identity module (SIM) - enables effective subscriber control,
`• efficient use of available frequency spectrum - FDMA and TDMA,
`• low power requirement - small battery size also means small mobiles,
`• digital transmission - gives high speech quality and enables signalling for services,
`• evolutionary implementation concept - upgradeable with downwards compatibility.
`
`1.3.2 The GSM Air lnteiface
`It is important to have an appreciation of the air interface Um (unlimited mobility) as
`GPRS shares this interface with GSM. In earlier implementations, GSM and GPRS chan(cid:173)
`nels were kept separate - i.e. separate frequencies were reserved for each one. With later
`releases, GSM and GPRS traffic channels can be mixed on the same frequency. The
`following description covers the main structure and terminology of the Um - signalling,
`protocols and the allocation of traffic channels will be covered in detail later.
`The efficient use of the available radio spectrum was a key issue in the development of
`GSM. The uplink (UL) and downlink (DL) frequency blocks are divided into frequency
`bands with a bandwidth of 200 kHz. Each frequency band is divided in the time domain
`to give eight separate channels called 'time slots'. Hence, up to eight users can share
`the same frequency as their individual transmissions are separated in time - this is called
`time division multiple access (TDMA). The eight time slots on a single frequency band
`are called a 'TDMA frame'.
`When a traffic channel (TCH) has been set up - i.e. the mobile station (MS) is assigned
`a frequency band and an UL/DL time slot number (the grey shaded time slots in Figure 1.1
`represent the UL and DL time slots for one subscriber) - the MS can send and receive
`digitized speech, signalling messages, or data via this channel. In the case of speech,
`the analog voice signal must first be digitized and then channel coded (compressed +
`redundancy added) before it can be transmitted. Three coding schemes have been defined
`for this purpose: full rate (FR), enhanced full rate (EFR) and half rate (HR). FR and EFR
`both transmit in consecutive TDMA frames but HR gets its name from the fact that it
`
`Page 9 of 21
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`Mobile Radio Evolution
`
`5
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`y
`TOMA frame
`
`Figure 1.1 Division of radio resources by frequency and time
`
`only makes use of a time slot in every second frame. This means that two mobiles using
`HR can share the same time slot (Figure 1.2).
`The need to implemenr flexible allocation of GSM and GPRS channels on the Um
`has led to a refinement in the BSS software called active HR pairing. As calls start and
`finish using HR, we can be left with a situation where we have no channels available for
`GPRS because they are all occupied by single (unpaired) HR calls. A system is therefore
`required to force intracell handovers so that two unpaired HR calls can be moved onto
`one channel. Active HR pairing ensures that HR calls occupy the minimum number of
`time slots so that channels for FR speech connections and GPRS are not blocked. This
`also reduces co-channel interference as fewer frequencies are in use (Figure 1.3).
`It was mentioned above that time slots can carry speech, signalling, or data - i.e. a
`time slot is just a container - but it is important that we understand how this works as
`there are some differences between GSM and GPRS.
`If the MS is moving around during a phone call, then the distance to the BTS is
`changing. This means that the propagation delay (the 'travel time' for the time slot)
`
`Page 10 of 21
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`6
`
`GPRS Networks
`
`-
`
`... 1010110100101101001111000110100101 ...
`
`In this simplified representation of full rate
`(FR), 20 ms blocks of the digitized analogue
`voice signal are coded into timeslots
`
`... 1010110100101101001111000110100101 .. .
`
`In this simplified representation of half rate
`(HR), the HR coding scheme only uses every
`second time slot. The shaded timeslots can be
`used by another mobile
`
`Figure 1.2 Full rate and Half rate coding
`
`between the MS and BTS, as well as the received signal strength, will also change. To
`ensure that the signal arrives at the BTS at the right time with the right signal strength, the
`BTS sends timing advance (TA) and power control (PC) values to the MS via signalling
`messages, roughly twice every second. These signalling messages are sent in the same
`time slot used for the coded speech information, but not simultaneously. For FR, the BTS
`sends speech in the time slot for the first 12 TDMA frames, but the thirteenth frame is
`used to send a time slot containing signalling for the MS - e.g. PC and TA. Then 12
`more speech time slots are sent, followed by an idle slot containing nothing. This gives
`us the timing structures shown in Figure 1.4.
`
`Page 11 of 21
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`Mobile Radio Evolution
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`7
`
`X = BTS signalling channel
`G = GPRS channel
`H = Half rate voice channel
`h = half rate voice channel
`
`x ~M G l!Sl G l?1
`
`n
`
`x~
`
`In this situation, there are no time slots
`availble for GPRS or full rate calls.
`
`Active HR pairing resolves the problem
`by using intracell handovers to pair up
`HR calls in single timeslots.
`
`The following pairing operation makes three
`time slots available for GPRS or FR calls.
`
`x IZ1YlYJA21Z1YI
`
`I I I
`
`tB:I
`
`Figure 1.3 Active HR pairing
`
`FR
`
`l s l s l s l sls l s l s l s l s ] s l s l slx l s l sls l s l s l s l slsls lslsls l 1 I
`
`HR
`
`I s I s I s l s I s I s I s I s I s I s I s I s I x I s I s I s I s I s I s I s I s I s I s I s I s I x I
`
`For FR: S =speech X =signalling I =idle
`For HR: S =speech for first mobile X =signalling for first mobiles= speech for second mobile
`X = signalling for second mobile
`
`Figure 1.4 Multi-frames for voice traffic channels
`
`Page 12 of 21
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`•
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`8
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`GPRS Networks
`
`As you can see, the idle time slot is necessary because it is used for the signalling to the
`second MS during a HR call. These structures are called 'multi-frames' and are made up
`of 26 time slots (each time slot belongs to one TDMA frame). GPRS uses a 52 time slot
`multi-frame for traffic channels which will be described later in the appropriate section.
`In addition to the traffic channel multi-frame there are also signalling multi-frames. Just
`as the traffic channels (TCH) need to have a structure so that the MS can send and receive
`different kinds of information, the broadcast and control channels also need a structure
`to enable registration, authyntication and call set-up messages to be exchanged between
`the BTS and several mobiles. The UL and DL multi-frames have different structures but
`each is based on a sequence of 51 TDMA frames.
`At this point it is important to be clear about the different kinds of channels. A time
`slot is known as a 'physical' channel as it can be clearly defined in time and space. The
`physical channel assigned to an MS can be used to carry different information flows - i.e.
`speech, signalling or data. These information flows can be thought of as 'logical' channels
`being carried at different times by the physical channel. The multi-frame defined for
`signalling between base station and mobile ('X' in Figure 1.3.) represents a single physical
`channel carrying many different logical channels in a pre-defined sequence which repeats
`every 51 TDMA frames .
`In earlier releases, GPRS makes use of the same logical channels as GSM in the
`signalling multi-frame. Later releases introduce GPRS specific channels. For example,
`the first signalling message from the MS to the BTS is sent on the random access channel
`(RACH), later releases of GPRS implement a PRACH - packet random access channel.
`Depending on the number of carriers supported by the BTS there can be one or more
`time slots reserved for such signalling. All BTS have one carrier configured to trans(cid:173)
`mit at full power. The broadcast channel (BCCH) is transmitted on this carrier in the
`first DL time slot (called 'TSO' as time slots are numbered from 0 to 7). For a BTS
`with only two carriers, TSO on the UL and DL will be used to receive and send all
`the logical signalling channels. As more carriers are added, more time slots must be
`reserved for signalling - i.e. more users = higher signalling load = more signalling chan(cid:173)
`nels required (Figure 1.5). This impacts GPRS directly and will be covered in the GPRS
`signalling section.
`Until now the time slots have been treated as an abstract concept - i.e. subdivisions of a
`carrier frequency - but as we will later be looking at a new modulation method (EDGE),
`it is necessary to understand how GSM actually gets the bits across the air interface.
`Time slots are transmitted in the form of a short duration (577 µ,s) radio signal called
`a 'burst'. Based on the power and timing factors (PC and TA) received from the BTS,
`the MS ramps up its output to the given PC value, transmits the modulated bits with the
`given TA and ramps down again (see Figure 1.6).
`The bits are modulated using Gaussian minimum shift keying (GMSK), a low-noise
`method which makes efficient use of the available 200 kHz channel bandwidth. GMSK
`shifts between two frequencies, each 67.7 kHz above or below the assigned carrier fre(cid:173)
`quency for the channel - the absolute radio frequency carrier number (ARFCN). Simply
`switching between the two frequencies would produce a lot of noise (splash) so phase
`shifting is used both to generate the frequencies and to move smoothly between them by
`varying the rate of phase change. GMSK transmits '1 ' bits using one frequency and 'O'
`bits on the other frequency.
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`The following represents a base station with two
`carriers, i.e. 2UL and 2DL
`
`X contains all BTS to MS signalling channels
`
`TSO
`
`TS1 TS2 TS3 TS4 TS5 TS6 TS?
`
`UL I x I I I I I I I I
`
`TSO
`
`TS1 TS2 TS3 TS4 TS5 TS6 TS?
`
`UL I x I I I I I I I I
`
`The following represents a base station with th ree
`carriers, i.e. SUL and 3DL
`
`X1 contains the broadcast channel (BCCH)
`X2 contains dedicated control channels
`
`TSO
`
`TS 1 TS2 TS3 TS4 TS5 TS6 TS?
`
`LIL I J IJ 1 111 1
`DL I J IJ 1 111 1
`
`TSO
`
`TS 1 TS2 TS3 TS4 TSS TS6 TS7
`
`Figure 1.5 Time slots reserved for signalling channels
`
`The GMSK approach used by GSM transmits the bit stream one bit at a time. EDGE
`however, uses 8PSK which transmits three bits at a time. Combining EDGE with GPRS
`(E-GPRS), bit rates approaching half a megabit per second are theoretically possible.
`
`1.3.3 GERAN and Cell Planning
`As GPRS makes use of the existing GSM radio access network it is important to under(cid:173)
`stand how the cell/frequency planning is performed and how this can be optimized to
`give better service to both speech and data users. The following is a brief review of
`the GERAN.
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`GPRS Networks
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`_n_ radio burst sent with power PC at time TA
`_,
`,- ~\ mobile is moving away from the base station
`\_
`r-··\ mobile is moving towards the base station
`. .J
`\_
`
`+ Qi
`:;:
`0
`>, c_
`ell :;:
`ell
`PC
`en
`"E
`ell :;:
`B
`t
`
`---------,
`I
`\
`I
`'
`I
`I
`I
`I
`
`I
`I
`
`;
`v·
`
`/ _~
`
`time
`
`Figure 1.6 Radio burst characteristics as MS moves towards/away from the BTS
`
`As far as the cellular structure of a GSM mobile network (PLMN) is concerned, we
`are used to seeing neat honeycomb diagrams of hexagonal cells. On a theoretical level
`the cells are round and have regular overlaps where handovers can take place. In actual
`fact the shape of the cells are greatly affected by screening, multi-path propagation and
`interference effects and are not the tidy circles we would like them to be. Such effects
`must be considered by the planners. Propagation models are developed to help plan the
`radio network. Reflection results in attenuation of the signal and can lead to areas with no
`coverage - so called 'dark spots' - as can screening by buildings. External sources can
`also attenuate or distort the radio signals but one of the biggest challenges facing radio
`planners is in optimizing the frequency reuse patterns (as follows). Standard GSM900
`has 124 frequency channels and the planner's job is to ensure that neighbouring cells do
`not use the same frequencies as they will interfere with each other. There are various
`planning models in use, each of which defines a pattern of cells with different frequencies
`in each cell. The pattern - called a 'cluster' - can be repeated to ensure that the distance
`between cells with the same frequency is large enough to minimize interference - this is
`the so called 'frequency reuse distance' (Figure 1.7).
`Until the advent of synchronized frequency hopping, the minimum cluster size was
`seven cells as shown in Figure 1. 7. Using frequency hopping, a user's time slot is not
`always transmitted on the same frequency. Instead, the frequency changes after every
`TDMA frame. This reduces the impact of destructive interference as a user's time slot
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`is only affected when it is sent on a disturbed frequency - i.e. on average the time slot
`will be sent on channels which are relatively clean. The idea of synchronized frequency
`hopping is basically that each BTS is allocated up to 64 carriers which they can use
`(maximum 16 simultaneously) and the hopping is synchronized so that no two BTSs in
`neighbouring clusters use the same frequencies at the same time. This enables cluster size
`to be reduced to just four cells, and also allows the maximum number of active carriers
`per cell to be increased from 16 to 24.
`The limited number of frequencies per cell causes another problem for the planners
`when areas of high population density are considered. Away from towns and cities, large
`
`cluster using nine sector
`(cells three per BTS) each
`with different
`frequencies
`
`Figure 1.7 Cell clusters using seven, nine and twelve frequencies
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`•
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`cluster using 12 sector cells
`(three sectors per BTS} each
`with different frequencies
`
`Figure 1.7
`
`(continued)
`
`cells, up to 35 km in radius, with only a few carriers can be used. As the population density
`increases, more carriers can be planned per BTS. If an area requires more capacity (i.e.
`more carriers per square km) than can be delivered by BTSs with the maximum allowed
`number of carriers, the operator has three main options - any or all of which can be
`implemented:
`
`• smaller cells = more BTS per km2 = more carriers per km2 ,
`• large overlaps = more carriers per km2
`• sector cells = more carriers per km2
`.
`
`,
`
`The first results in fewer additional problems