UMTS Guide English Print Version4.0.Qxd
UMTSThird generation mobile communications
boosting wireless efficiency Publisher: Willtek Communications GmbH Gutenbergstr. 2-4 85737 Ismaning Germany e-mail: [email protected] http://www.willtek.com
Co-Author: Helmut Visel, Acterna Eningen GmbH
© Copyright 2002 Willtek Communications GmbH. All rights reserved. Willtek Communications, Willtek and its logo are trademarks of Willtek Communications GmbH. All other trademarks and registered trademarks are the property of their respective owners.
Content UMTS – a communications revolution 6 Three generations of mobile communications 8 Services – ensuring the success of UMTS 10 New technology, new roles 12
The fundamentals of UMTS 15 3G frequencies 15 Frequency bands for UMTS 18 Minimum bandwidth requirements 20
Inside the UMTS architecture 22 Network overview 24 From handset to network – the user equipment 25 Virtual home environment 30 Service capabilities and APIs 32 UMTS system architecture 34 UTRAN components 35 Node B 38 Serving radio network subsystems and drift radio network subsystems 40 Handovers 41 Hard handovers 42 Softer and soft handovers 43 Role of the Iur interface during handover 45 UMTS logical planes 46 Transport network control plane 46 Control plane 46 Control plane components 47 User plane 50 User plane components 50 ATM in the core network 53 The future – all-IP networks? 55 Key terms 57 UMTS air interface 62 Multiple access routes 63 Frequency division duplex (FDD) 65 Time division duplex (TDD) 66 FDMA-based networks 67 CDMA – it’s party time 68 CDMA-based networks 70 CDMA cells 72 Orthogonal codes and multiplexing 73 Features of the UMTS radio interface 74 Frequency, code, and phase 75 CDMA air interface challenges 76 The “near-far” problem 76 Cell breathing 78 Variable rate transmission 79
Glossary 80
Bibliography 86
Information sources 87 UMTS – a Almost everyone today seems to own at least one mobile device. The growth communications in phone ownership is a relatively new phenomenon and is largely attributa- revolution ble to the quality of the digital radio services like cdmaOne, US-TDMA, PDC and GSM. A new, high capacity mobile infrastructure, universal mobile telecommunications system (UMTS) is poised to change the face of mobile communications. With UMTS, the possibility of making narrowband voice calls and exchanging broadband multimedia content simultaneously becomes reality.
In some countries, usage of mobile radio devices has already exceeded 80 percent of the population. The number of mobile telecommunications users looks set to exceed the number of fixed network lines in a number of countries. Meanwhile, the number of Internet users is growing at almost 18 million new subscribers per month, while data traffic is doubling approxi- mately every six months. Given these rates of growth, the mobile Internet underpinned by UMTS transport technology will perform a vital role in mod- ern, high bandwidth communications.
6 UMTS – a guide to the third generation of mobile communications is designed to provide an insight into fundamental aspects of UMTS technology, how it works, and some of the issues that face the industry.
Willtek is one of the world’s leading providers of wireless network testing equipment. As a leading member of the telecommunications industry bodies such as ITU-T, ETSI and ANSI, the company is well placed to comment on the emerging broadband mobile market. Willtek provides solu- tions that meet the needs of high bandwidth radio communications today and in the future. figure 1 Development of subscriber count and applications for UMTS
7 Three generations First generation (1G) – analog mobile radio networks of mobile These are still commonplace in some parts of the world, but lack the communications features of modern, digital networks. Because data has to be adapted for analog transmission using a modem, analog networks introduce losses and demand intensive management. Added to this, mobile data traffic is growing at a much faster rate than speech traffic, which means that analog networks are no longer suitable for mobile multimediacommunications.
Second generation (2G) – digital mobile radio networks The difference between analog and digital networks is that with digital networks, users are guaranteed a consistently high quality of speech. 2G opened the door to a range of data services such as facsimile, email, text messaging (SMS), and PC connection. In addition, users can exploit features like call forwarding and international roaming. The typical GSM network is a common example of a second-generation mobile radio network.
Third generation (2.5G and 3G) – broadband digital mobile networks The first phase in the development of broadband mobile communications is 2.5G, that means an interim step based on 2G technologies.
8 The main technologies here are General Packet Radio Service (GPRS) and 1xRTT. These technologies embrace packet switching as opposed to the more traditional circuit-switched networks. GPRS has been built onto existing GSM network infrastructures, so it does not have the bandwidth possibilities of the next phase in broadband mobile: third generation mobile networks (or 3G).
While there are other 3G network standards, this guide focuses on just one – UMTS. figure 2 Three generations of mobile radio
9 Services – The mobile communications industry has agreed most of the technical ensuring the success prerequisites for mobile multimedia communications. Soon users will of UMTS combine speech, text, and video in a single call.
However, at the beginning of this new era in mobile communications, companies will need to work hard to convince potential users of the benefits of 3G. If UMTS is to be successful, potential subscribers will want access to a much wider range of exciting, cost-effective, and innovative services. The main factors that need to be addressed before this can happen are: – Bandwidth requirements – The need for realtime capabilities – The distinction between point-to-point services for individual communications and point-to-multipoint broadcast services such as mobile TV
10 figure 3 Potential 3G services
11 New technology, Despite some early attempts by new entrants to secure the lucrative new roles services segment of the 1G and 2G mobile communications market, network operators have delivered most of these services alone. With UMTS, the scope for innovative services is such that a redefinition of the traditional roles of many stakeholders is taking place.
Subscriber A person or entity deemed as such by law, who has a legal agreement with a service provider on behalf of one or more users.
User A person or entity deemed as such by law, who possesses an authorization for usage from a subscriber. In the simplest case, the subscriber is the same as the user.
12 Service provider In Europe, these are the organizations that deliver services to subscribers. A subscriber and service provider enter into a commercial agreement regard- ing specific service provision. The service provider requires the cooperation of the network operator to deliver its services to the subscriber. Service providers manage the profile for each subscriber. The profile details the serv- ices in the customer contract, for example, rates and quality of service.
Network operator The network operator combines their own transport, and possibly access services, with those offered by service providers. Network operators may choose to manage the backbone and access networks alone, or to work with another operator. In the US, this term is interchangeable with service provider.
Value-added service provider These providers deliver services that extend beyond telecommunications services. Examples of value-added services include mailbox functions and location-based services. Invoices for value-added services may be addressed directly to end-users, or handled by service providers.
13 Content provider A content provider is responsible for content delivery. One example of this would be a video store that provides movies through a streaming video-on- demand service.
Service broker These are organizations that act as resellers of products from different serv- ice providers to the end user. They invoice their services directly to the customer. figure 4 How the roles fit together
14 The fundamentals Sections three and four of this guide provide an overview of UMTS architec- of UMTS ture and air interface. This section focuses on fundamental aspects of UMTS – frequencies, frequency bands, and bandwidth requirements.
3G frequencies The body responsible for worldwide radio frequencies’ allocation, the World Administrative Radio Conference (WARC), designated the following frequency bands for worldwide third generation mobile radio systems under the International Mobile Telephony 2000 (IMT-2000) framework thus far:
1885 to 2025 MHz (ITU-specified band for 3G) 2110 to 2200 MHz (ITU-specified band for 3G) 1710 to 1885 MHz (extends the current 2G band for GSM 1800 for 3G use) 2500 to 2690 MHz (new band for future use) 806 to 960 MHz (extends current 2G bands for 3G use)
15 It is expected that new bands will become available around 2010. In Europe, the first 15 MHz of the lower band overlaps with the frequencies reserved for digital enhanced cordless telecommunications (DECT). The industry divided the remaining spectrum into a pair-based division for frequency division duplex (FDD) with 2 x 60 MHz. The range 1920 to 1980 MHz was reserved for the uplink and the range 2110 to 2170 MHz for the downlink. An unpaired spectral band of 35 MHz was reserved for (asymmetrical) time divi- sion duplex (TDD) mode from 1900 to 1920 MHz and from 2010 to 2025 MHz. The total available bandwidth for UMTS in Europe is thus exactly 155 MHz for terrestrial systems. The paired bands at 1980 to 2010 MHz and 2170 to 2200 MHz were dedicated to satellite systems.
Differences in the ITU definition of spectra also exist elsewhere besides Europe.
16 figure 5 Worldwide 3G frequency allocations
17 Frequency bands Frequency division duplex (FDD) for UMTS 2 x 60 MHz Uplink 1920 to 1980 MHz Downlink 2110 to 2170 MHz
Time division duplex (TDD) standard band 1 x 20 MHz Uplink and Downlink 1900 to 1920 MHz
TDD optional band 1 x 15 MHz Uplink and Downlink 2010 to 2025 MHz
FDD is ideal for services that require a symmetrical transmission capacity for both uplink and downlink because the directions use different frequency bands. With TDD, the same frequency band is used for uplink and downlink and the directions are separated by time. Switching between uplink and downlink can be configured for optimum performance with services that require asymmetrical resources, for example Internet browsing. In UMTS, both FDD and TDD are used; however, the first implementations will be based on FDD.
18 figure 6 UMTS frequency bands
19 Minimum bandwidth The UMTS Forum carried out studies to determine the minimum required requirements radio resources for a UMTS operator. The studies used the following assumptions: – Between 2000 and 2005, UMTS will carry most of the innovative multimedia services – UMTS will transport only 10 percent of speech services and services with low data rates. 2G systems will carry the remaining 90 percent – The minimum channel spacing is 5 MHz, for example this is the smallest bandwidth unit that can be allocated to an operator – In Europe, the entire frequency band from 155 MHz will be available for 3G systems – Specific bandwidth allocated for urban areas with high traffic volume (4 Mbps/km2 for the uplink and 37 Mbps/km2 for the downlink)
Study conclusions: A UMTS operator requires at least a paired spectral band of 2 x 15 MHz and an unpaired band of 5 MHz.
20 figure 7 Minimum bandwidth requirements for UMTS
21 Inside the UMTS The general UMTS network architecture can be divided into two main architecture segments: The radio access network and the core network. The UMTS terrestrial radio access network (UTRAN) provides the radio access network. Of course, the GSM base station subsystem (BSS) still exists in parallel for narrowband speech or data services and components of the BSS may be reused in the UTRAN. The actual core network can also be divided into two subnetworks: the circuit-switched GSM core network based on mobile switching centers (MSCs) and the packet-switched GPRS core network based on GPRS support nodes (GSNs). Circuit-switched core network = Circuit-switched domain (CS domain) Packet-switched core network = Packet-switched domain (PS domain)
The CS and PS domains interconnect via a number of newly defined interfaces.
Transmission technology in the CS domain ISDN protocols (Q.931, ISUP) Transmission technology in the PS domain IP protocols
22 figure 8 UMTS network overview. Notice how the core GPRS infra- structure powers the core UMTS network
23 Network overview Below are the basic domains of the UMTS architecture. These domains exist as a result of developments within existing network infrastructures. The core network domain is based on the GSM and ISDN infrastructure.
UMTS interfaces Cu Between USIM and mobile unit Uu Between user equipment domain and infrastructure domain Iu Between access network domain and serving network domain [Zu] Between serving network domain and home network domain [Yu] Between serving network domain and transit network domain figure 9 UMTS domains and interfaces
24 From handset to The terminal equipment (TE) forms the interface to the user and holds all of network – the user the applications. The mobile termination (MT) acts as the last radio interface equipment in the UMTS network. Together, the TE and MT are called the user equipment. They can be implemented as separate devices or together in a single device. In the latter case, the interface between the TE and MT is not accessible.
figure 10 User equipment components
25 User equipment is split into the user services identity module (USIM) and the mobile equipment (ME). The mobile equipment contains the mobile termination (MT) and the terminal equipment. The MT forms the interface to the UTRAN and provides access to network resources. The terminal equip- ment represents the user interface and contains applications such as an Internet browser. Typically, a TE might be a portable computer or hand- held personal digital assistant (PDA). A PDA with integrated UMTS capabilities forms a complete UE.
The USIM is a logical entity containing data and procedures to allow unique, secure identification of the subscriber to the network. It is physically located on a stand-alone smart card. The USIM is assigned to a user and makes distinction between the terminal and user identity.
Besides multiple USIMs, other applications can also be stored on a universal integrated circuit card (UICC). Mobile banking is one example of a UICC application. There are many advantages to the UICC, such as allowing all of its applications to use a common address book.
26 figure 11 Inside the user’s terminal
27 User profiles A subscriber can select on a per-call basis the appropriate profile from several possible user profiles within a USIM. A user profile contains data and settings for personalized services. If several subscribers use the same device, each subscriber can save his or her settings in a separate profile. It is also possible for several profiles to be active at once. This means a user can simultaneously set up or receive calls that are associated with different profiles. User profiles can be protected against unauthorized usage using a PIN. Each user profile is linked to at least one user address also known as the mobile station ISDN number (MSISDN number). This is important for incoming calls and charge computation.
28 figure 12 A look inside a universal integrated circuit card
29 Virtual home UMTS provides subscribers with a virtual home environment (VHE) in which environment they can access subscribed services from any network and any terminal in the same way. In terms of the terminal’s interface, users should believe that they are using the same UMTS terminal that is familiar to them, even if they are connected to a different terminal or network. Many industry experts believe that the VHE is a key selling point when it comes to the mass marketing of UMTS.
The home environment (HE) is responsible for the entire provision of services to the subscriber. The HE is also the personal service environment of a subscriber.
30 figure 13 The virtual home environment (VHE)
31 Service capabilities The illustration below demonstrates the architecture necessary to enable and APIs the development of new UMTS-based services.
UMTS applications can access service capability servers via open, standardized interfaces, known as application programming interfaces (APIs). These servers provide service capabilities via the API interface. These service capabilities are elementary functions that can be used to develop complex applications. To use a programming analogy, they can be compared to macros or subprograms. To provide service capabilities via the network interface, the service environment accesses all of the resources and functions available in the network such as the SIM application toolkit, intelligent network and customized application for mobile network enhanced logic.
API Application programming interface IN Intelligent network SAT SIM application toolkit MEXE Mobile execution environment CAMEL Customized application for mobile network enhanced logic SSF Service switching function (in conjunction with an IN function) WAP Wireless application protocol
32 figure 14 The key to develop- ing UMTS-based services – service capabilities and APIs
33 UMTS system The core network is connected to the UTRAN via the lu interface. The architecture lu comprises two different interfaces: the Iu-CS interface transmits circuit-switched traffic between the UTRAN and mobile switching center (MSC); and the Iu-PS interface transmits packet data traffic between the UTRAN and the serving GPRS support node (SGSN). The SGSN and MSC communicate with the same home location register (HLR) via the mobile application part (MAP). The Gs interface is available as an option in a UMTS core network. figure 15 UMTS at systems level
34 Utran Components The UTRAN consists of several radio network subsystems (RNS). All of the RNSs are connected via the Iu interface directly to the UMTS core network. Each RNS consists of a radio network controller (RNC) and one or more node Bs. A node B contains one or more radio stations, each of which covers a radio cell or sector. It manages a group of radio cells that can be operated in FDD mode, TDD mode or in both duplex modes. It is capable of controlling soft handovers and macrodiversity within its cells independently of the RNC.
Macrodiversity describes the ability to maintain an ongoing connection between the mobile terminal and network through more than one base station. It is important because investigations have shown that mobile stations often maintain a connection to more than one base station up to 80 percent of the time.
The RNC handles control of handovers and functions related to macrodiver- sity between different node Bs. The individual RNSs can interchange data directly via the corresponding RNC using the Iur interface. A node B is connected to its RNC via the Iub interface.
35 To a certain extent, the Iu interface has a dual function. It transmits circuit-switched traffic, for example, speech and packet-switched traffic, for example, Internet browsing, between the RNC and the corresponding subnetworks of the core network.
The Iur interface provides mobility management between different RNCs without incorporating the core network. Soft handovers and macrodiversity thus become possible across RNS boundaries. The Iur interface can be imple- mented using direct physical connections or virtual connections based on any desired transport networks.
Between different RNSs, handover control is also possible via the Iu interface by incorporating the core network. In this case, however, macrodiversity is not possible since this function is an exclusive feature of radio protocols that do not reach into the core network (they terminate in the RNC).
36 The standards bodies have specified the corresponding protocol layers and functions for each UTRAN interface, lu, lur, and lub. The transport protocol layers provide services for transporting user data, signaling data, and specif- ic operations and maintenance data. To achieve the required bandwidth flexibility, asynchronous transfer mode (ATM) with its adaptation layers (AAL2 and AAL5) has been chosen as the UTRAN transmission technology for the lower transport layers. The network architecture itself is not part of 3GPP, but depends on the network operator. figure 16 Components of the UTRAN
37 Node B A node B contains one or more radio stations, each of which covers a radio cell (sector). One node B frequently covers three sectors or cells. However, there are variations in which 1, 2, 4 or 6 cells are covered by a single node B. Multiple carriers using multiple frequencies can be present per node B – each with a bandwidth of 5 MHz. UMTS is based on code division multiple access (CDMA) technology, which means that neighboring cells can use the same carrier frequency. Multiple access is achieved through the use of code sequences. To increase the capacity, however, different carriers can be distributed among the individual cells.
The capacity of an individual cell can be greatly increased by using multiple carrier frequencies (5 MHz bands) in a cell, and not just one. The available CDMA codes can then be reused on each carrier.
Channels per cell = number of CDMA codes × number of carrier frequencies
38 figure 17 An RNC can control multiple node Bs, which in turn can cover multiple cells
39 Serving radio For each connection between terminal and UTRAN, there is a serving radio network subsystems network subsystem (SRNS). and drift radio net- work subsystems If necessary, the SRNS is supported by one or more drift radio network subsystems (DRNS). In this case, the terminal also uses DRNS radio resources in addition to SRNS radio resources. The collection of parallel data streams takes place within the UTRAN via the Iur interface. This mechanism is based on the frequency equality of different base stations and is a special feature of CDMA technology. It can be equated with the macrodiversity feature. figure 18 The DRNS and SRNS – connecting the radio terminal to the UTRAN
40 Handovers Users are constantly moving within mobile networks. To ensure that they are not cut off mid-call, all networks adopt a handover control procedure. With the introduction of UMTS, come soft handovers and macrodiversity – proce- dures that enable terminals to switch from cell to cell without changing frequencies when operating purely on UMTS networks.
41 Hard handovers A hard handover is required if the frequency, protocol or network has to be changed when moving from one cell to another. This is the case in UMTS if there is a need to change to another non-UMTS band – for example to GSM – when switching cells. Within the typical GSM mobile radio system, this was the only possible type of handover. Since GSM is based on a combina- tion of FDMA and TDMA, neighboring cells always use different frequencies. At the most basic level, a hard handover occurs in the following cases: Interfrequency handover; Handover between FDD and TDD; Handover between UMTS and GSM. figure 19 GSM employs hard handovers from cell to cell
42 Softer and soft UMTS uses a CDMA multiple access technique on the radio interface. This handovers means that the same frequency can be reused in neighboring cells and all mobiles can communicate with the UTRAN at the same time and on the same frequency. Channels are separated using orthogonal codes.
A mobile can simultaneously maintain connections to multiple base stations operating on the same frequency without problems. This results in a significant improvement in transmission quality. Poor connections to an individual base station and fading effects can be compensated through the spatially different antenna positions within the respective base stations.
The macrodiversity feature in CDMA systems makes handover very straight- forward. A user moving through the network communicates simultaneously with the best-received base stations. In the boundary area between two cells, this is – at least – the base stations of the two closest cells. If the reception of a new base station worsens once again, then the handover is simply halted.
During softer handover, a mobile station overlaps the cell coverage area of two adjacent sectors of a base station. Communications between mobile station and base station take place simultaneously via two air interface channels, one for each separate sector.
43 figure 20 Soft handovers make macrodiversity possible
44 Role of the Iur In UMTS, a handover procedure can be administered solely by the UTRAN interface during with the lur interface. This interconnects the individual radio network handover subsystems (RNSs). The UMTS mobile switching center (UMSC) in the core network does not have to be synchronized with the mobile station’s direction of movement. In other words, the transfer of a connection to the mobile from one lu interface to the direct lu interface is not timing-critical and can take place later. The transfer of the connection on the lu interface takes place at the same time as the process of SRNS relocation in which the control function for a mobile is transferred from one RNC to another SRNC. figure 21 The lur interface makes handovers straightforward
45 UMTS logical planes Within the UMTS network architecture, there are three logical planes: Transport network control plane, control plane, and user plane.
Transport network The transport plane moves data generated by UMTS users and control control plane planes. It consists of the following components: – ATM physical layer (E1, T1, OC3c, and STM-1 physical interfaces) – ATM layers (CELL, SAR) – AAL2 interface – AAL5-NNI interface – AAL5-UNI interface
The transport layer is automatically invoked. Users control the adaptation layer procedures from higher layer protocols.
Control plane The control plane manages signaling protocols and procedures.
46 Control plane Radio resource control (RRC) components RRC handles various functions, including: – System information broadcast – Setup, cleardown, and maintenance of RRC connections between UE and UTRAN – Mobility functions, for example, handovers and cell updates – Paging – Routing of data from higher protocol layers – Monitoring and control of the requested quality of service – Control and reporting for UE measurements – Outer loop power control – Ciphering control – Distribution of the uplink DCH transport channel resources among different UEs
Radio access network application protocol (RANAP) This protocol encapsulates and transmits data from higher protocols between the UTRAN and SGSN and transports signaling between the end points. RANAP controls the GTP connections for user data on the Iu interface.
47 Signaling connection control part (SCCP) Part of the SS#7 signaling system, SCCP expands upon the MTP functions and enables end-to-end routing based on different addresses (SPC, global title, subsystem numbers). The SCCP provides two connectionless and two connection-oriented modes.
GPRS mobility management (GMM) GPRS mobility management includes functions such as: – GPRS attach – GPRS detach – Security – Routing area update
Session management (SM) Session management includes functions such as: – PDP context activation – PDP context modification – PDP context deactivation
48 Signaling bearer SCCP makes use of signaling bearer services. In other words, SCCP data frames are transported via signaling bearers (SBs). These can be structured differently. Here, the operator can decide to establish a SS#7-based stack or switch over to IP-based protocols. In the case of SS#7, the SB consists of MTP3 and the adaptation layers SSCF and SSCOP. In the case of the IP solu- tion, the signaling bearer is composed of the IP protocol with the adaptation layers SCTP and M3UA.
figure 22 The UMTS control plane MS-SGSN
49 User plane The user plane enables users to generate various types of bearer traffic, including voice (8 and 16 Kbps), packet, and unrestricted digital data.
User plane Medium access control (MAC) components The MAC protocol controls access to the common radio channels and allocates the radio resources.
Radio link control (RLC) This protocol provides logical connections between the mobile station and UTRAN. Setup, cleardown, and monitoring of connections are part of the RLC.
50 Packet data convergence protocol (PDCP) PDCP behaves as an adaptation layer between the higher transport protocols and the special requirements of the RLC/MAC layer. PDCP delivers the higher layers with a transparent transport service and supports key Internet proto- cols such as IP. Because PDCP protocols are transparent, any possible subsequent introduction of additional higher protocols has no impact upon the radio interface protocols in lower layers. PDCP enables protocol header compression. Online compression of user data is not supported since it is generally already handled by the applications. figure 23 Overview of UMTS protocol layers
51 GPRS tunneling protocol for user plane (GTP-U) This protocol tunnels user data between the UTRAN and the SGSN and between the GSNs of the backbone network, for example between SGSN and GGSN. All the data to be transported is encapsulated by GTP. GTP can be seen as a protocol-transparent tunnel between protocol entities.
User datagram protocol (UDP) / Internet protocol (IP) These protocols are used in the GSN backbone network. The GSN backbone network is an IP-based network with a private address space. UDP provides a connectionless, non-acknowledged transport service.
ATM adaptation layer 5 (AAL5) The AAL5 enables the segmentation of long IP frames and the division of these segments among ATM cells. AAL5 also provides a connection- oriented or connectionless transport service.
52 ATM in the core UMTS is capable of transporting narrowband speech connections and network broadband data connections equally well. That is why it is important that the network transport system is flexible. For this reason, ATM is the best choice. ATM can group data streams efficiently with very different band- widths and route them separately via logical connections.
An ATM data stream consists of ATM cells with a constant length. An ATM cell consists of a 5-byte header and a 48-byte payload field. In UMTS, ATM is used with two different adaptation layers: AAL5 for broadband data streams (ATM adaptation layer 5) This enables the transport of long user data frames (IP frames with up to 65536 bytes) in a series of ATM cells. The main function of AAL5 is thus to segment and reassemble long user data frames. AAL2 for narrowband speech (ATM adaptation layer 2) AAL2 makes multiplexing very low bit rate data streams into common ATM cells possible and efficient. It eliminates the problem of resource wastage caused by ATM cells that are only partially filled with narrowband speech, for example speech at 8 kbps.
53 figure 24 ATM in UMTS networks
54 The future – all-IP UMTS specifications are being developed further and are maintained in networks? yearly releases. In this solution, the duplicate backbone structure for speech and data (GSM/GPRS) is abandoned in favor of a pure IP architecture. Exist- ing circuit-switched speech services will also be transported over this unified IP backbone network in the future using voice over IP (VoIP).
Key terms
CSCF Call state control function MGCF Media gateway control function MGW Media gateway function MRF Multimedia resource function SGW Signaling gateway function
55 figure 25 The future of mobile architects – Internet protocol (IP)
56 Key terms
Asynchronous transfer mode ATM is a high-performance, (ATM) cell-oriented switching and multiplexing technology. Base station controller (BSC) BSCs manage the radio resources of one or more base transceiver stations. Base station subsystem (BSS) The GSM BSS consists of a base station, base station controller, transcoder submultiplexer and cellular transmission. Base transceiver station (BTS) The BTS holds the radio transceivers that define a cell and coordinates the radio-link protocols with the mobile device. Core network The core network provides the inter- face from users to the wider telecommunications network.
57 Key terms
GPRS support nodes (GSN) GSN constitute the parts of the core network that switch packet data. The two main nodes are the serving GPRS support node (SGSN) and the gateway GPRS support node (GGSN). Hard handover GSM systems use hard handover between cells. This means that the mobile device is passed from one base station to another as it moves across the network. Also used in UMTS. Macrodiversity Macrodiversity is the result of soft handovers and is an efficient and comprehensive fading migration technique. It is also known as cell overlap. Node B Node B is the physical unit for radio transmission/reception with cells.
58 Key terms
Orthogonal variable spreading OVSF codes are important to UMTS factor (OVSF) codes because they allow the base station to increase downlink capacity signifi- cantly. The properties of these codes are such that within specific limita- tions, they do not interfere with each other. This means that a mobile device receiving data on one of these codes will not perceive interference from transmissions to other mobiles using different codes. Packet switching network Packet switching is a technique whereby the network routes individual packets of data between different destinations based on addressing in the packet.
59 Key terms
Radio network controller RNCs interface with the core network, (RNC) control radio transmitters and receivers in node Bs. They also perform other radio access and link maintenance functions, such as soft handover within UMTS networks. A RNC is similar to a BSC. Soft handover The concept of soft handover was developed for CDMA so that the user's transmission can be received at two or more base stations and combined during the changeover. UMTS mobile switching center UMSC integrates the functions of a (UMSC) mobile switching center (MSC), visitor location register (VLR), and service switching point (SSP) into a single unit. It is responsible for all call handling as well as the interfaces to other switching elements, both in 3G, GPRS, and GSM networks.
60 Key terms
User services identity module USIM is the smart card for 3G (USIM) mobile phones. UMTS terrestrial radio UTRAN is the conceptual term used access network (UTRAN) for describing the radio component of a UMTS network. Code division multiple access WCDMA is the main third generation (CDMA) or wideband code interface in the world. Using the same division multiple access frequency band across the globe, (WCDMA) 2 GHz, it offers variable bit rates of up to 2 Mbps, on-demand service multiplexing within a single connection, and flexibility.
61 UMTS air interface Despite the global framework, known as ITU IMT-2000, different radio interfaces were defined for 3G. This became necessary after no global accord was reached despite long negotiations. Selection of the right radio interface is critical since this determines the capacity of a radio system as well as other general points, including interference, multipath propagation, and handovers. In addition, the choice of a specific radio interface has a sizable influence on the cost of the overall system.
figure 26 The different faces of 3G
62 Multiple access Frequency division multiple access (FDMA) routes In FDMA systems, the available bandwidth is divided into frequency channels. Users occupy a complete frequency channel over the entire time.
Time division multiple access (TDMA) With this technique, a transmission medium is available to a user only for a certain time. During the remaining time, other users are able to use the medium.
FDMA/TDMA This is a mix of both multiple access techniques. It is commonly used in mobile radio systems like GSM. Within FDMA/TDMA, a group of carrier frequencies are available and are subdivided into time slots for improved efficiency. In GSM 900, there are 124 frequency channels and each has a bandwidth of 200 kHz. The individual frequency channels are subdivided into eight time slots each. Each time slot is 577 µs wide, and a TDMA frame thus lasts 4.615 ms.
63 Code division multiple access (CDMA) Here, the available frequency channel is broken down by different code sequences that are multiplied by the user signals of the individual subscribers. All of the subscribers transmit on the same frequency and at the same time. If the transmission bandwidth is much wider than the user signal bandwidth, then this is known as direct spread CDMA.
figure 27 CDMA provides oper- ators and infrastruc- ture providers with the maximum possi- ble bandwidth
64 Frequency division In FDD, the bandwidth for the uplink and downlink is 5 MHz in each duplex (FDD) direction. The duplex spacing is 190 MHz.
figure 28 The main disadvan- tage of FDD is that because it uses sepa- rate bands for uplink and downlink, operators cannot distribute resources flexibly
65 Time division duplex The available time slots can be used differently for the uplink and downlink. (TDD) This allows great flexibility for asymmetrical allocation of uplink and downlink resources. In UMTS, 15 time slots are grouped together into a frame with a length of 10 ms. A time slot thus has a length of 667 µs.
figure 29 Using TDD makes asymmetrical usage of uplink and down- link possible, but this can lead to interference
66 FDMA-based In order to suppress interference from cells using the same frequencies, networks these cells must have a minimum distance between them. This frequency reuse factor (FRF) is a limitation of GSM. The signal-to-noise ratio between user signal and neighboring cell interference determines the quality of a GSM network. To enable better exploitation of radio resources, cells can be subdivided into sectors (3 or 6) in which directional antennas are then used. Exact frequency planning is very important in FDMA systems (as in GSM systems).
figure 30 With FDMA net- works, the key to effective resource usage and network performance is pre- cise planning
67 CDMA – it’s party Code division multiple access (CDMA) is the radio access technology time used in UMTS networks. Instead of using just frequencies, or time and frequencies, CDMA adds another dimension – orthogonal codes. These iden- tifiers enable operators to carry more users on the same cells at the same frequency and time. To understand how CDMA works, consider this simple analogy.
Imagine that people of different nationalities are at the same party. Four people are speaking at the gathering simultaneously, but in different languages. Each individual partygoer can listen to one of the four in their native language by synchronizing to that particular speaker. The listener’s brain blocks out all the parallel presentations in other languages. To the lis- tener, these other talks are just background noise, as long as none of the speakers are shouting.
68 The different languages in this example correspond to the different codes in CDMA, and the background noise represents levels of interference in CDMA. If the background noise increases significantly, it becomes very difficult to filter out individual signals. The “Off” point for a CDMA system is characterized by the maximum interference threshold. This analogy also illustrates how essential it is in CDMA to control the transmit power to extend the analogy – nobody speaks louder than is absolutely necessary for the different user signals to be separated. In CDMA systems, the power control function has a far more important role than it does in FDMA/TDMA systems like GSM. figure 31 The CDMA party. Provided that every voice is speaking at the same level – or in CDMA terms – that every handset is set to the same power level, every separate “voice” – in each different lan- guage will be “heard” – and “understood”.
69 CDMA-based CDMA systems use the same frequency for all users within a cell, which networks means that all users send their data at the same time. The same frequency is also used in all other cells. With CDMA, the frequency reuse factor (FRF) is 1. No frequency planning is required. The channels are separated from each other using different code sequences.
In CDMA systems, the cell capacity, which is the maximum number of simul- taneously active users, depends solely on the signal/interference (S/I) ratio at the receiving location. Unlike GSM, CDMA is not strictly limited by the number of available channels, frequencies, and time slots. Every new sub- scriber slightly reduces the S/I ratio at the receiving BTS since the subscriber generates additional interference.
70 figure 32 Users within CDMA networks are distin- guished by virtually unique orthogonal codes
71 CDMA cells In a CDMA system, a mobile can be simultaneously connected to different base stations since the same frequency is used in all cells. This improves the radio properties considerably. Also, fading effects and attenuation on a given propagation path can be partially compensated for on another propa- gation path. Where there is only one carrier frequency, the handover is very simple because of real-time, fast switchover of resources (channels) when changing cells. This is known as a soft handover.
figure 33 CDMA cells support soft handover and macrodiversity
72 Orthogonal codes To separate the diverse user data streams, orthogonal variable spreading and multiplexing codes (OVSF) must be used. Orthogonal codes are codes for which the cross (1) (2) correlation is equal to 0. Data streams dn and dn from two different (1) users can be separated on the receiving end using the orthogonal codes ci (2) and ci . figure 34 Orthogonal variable spreading factor (OVSF) codes increase downlink capacity
73 Features of the FDD TDD UMTS radio interface Multiple access Direct sequence CDMA TDMA with DS-CDMA per TS Frequency bands UL 1920-1980 MHz UL/DL 1900-1920 MHz DL 2110-2170 MHz optional 2010-2025 MHz Bandwidth 5 MHz 5 MHz Channel spacing 200 kHz 200 kHz Chip rate 3.84 Mchip/s 3.84 Mchip/s Frame length 10 ms 10 ms Time slots/frame 15 15 Slot length 667 µs 667 µs BS synchronization Not required Required Multirate/variable rate Multicode, Multislot, multicode, variable spreading factor variable spreading factor Spread factor DL: 512 - 4 DL: 16 - 1 UL: 256 - 4 UL: 16 - 1 Channel coding Convolutional, turbo Convolutional, turbo
74 Frequency, code On OSI layer 1, physical channels are used to transmit data. Transport chan- and phase nels are used above layer 1. The function of layer 1 is to transfer the transport channels via the physical channels. Multiple transport channels can be transmitted via a physical channel. Separation of the physical chan- nels involves the frequency, the code and, on the uplink, the phase shift. Between the I phase and the Q phase, the phase angle is 90 degrees. In other words, the physical “frequency” resource is used multiple times through the code and phase. figure 35 The properties of physical channels
75 CDMA air interface As the main radio access interface in UMTS, code division multiple challenges access (CDMA) or wideband CDMA (W-CDMA) presents its own unique set of problems that the industry must address. The main challenges are described over the following pages.
The “near-far” The capacity of a CDMA cell depends on the signal-to-noise ratio at the problem receiving site. The signals of mobile terminals located at different distances must arrive at the BTS with the same receive power level. A handset located close to the BTS must therefore transmit with less power than a terminal located further away. If all devices transmitted with the same power level, remote handsets would not be heard since their signal would just disappear into the noise. This effect is known as the “near-far problem”. Whereas power control is an option in FDMA/TDMA systems to reduce interference to neighboring cells and preserve the terminal’s batter- ies, it is a basic function for proper operation of CDMA systems. CDMA systems cannot work without very precise and effective power control mechanisms. Power control information is transmitted from the network to the mobile terminal 1500 times per second.
76 figure 36 An illustration of the near-far problem
77 Cell breathing CDMA cells can overlap if the traffic volume remains within reasonable limits. However, heavy traffic load continuously reduces the signal-to-inter- ference ratio. To compensate for this, the transmission power of all of the mobiles must be increased, but there are physical limits. In other words, there is insufficient power available for the mobiles at the edge of the cell. Signals from distant mobiles arrive at the BTS with insufficient power level and can no longer be reconstructed. This is known as cell breathing. In extreme cases, there can be areas between cells where coverage is no longer certain. To avoid these problems, an exact plan for the base station’s loca- tion is necessary. figure 37 Cell breathing can greatly degrade qual- ity of service with increased traffic load
78 Variable rate Variable rate transmission can be used for fast, variable adaptation of the transmission data transmission speed. To do this, the spread factor is changed during transmission. The change in spread factor and the associated usage of a new OVSF code apply to at least one complete radio frame. In other words, the data rate can be varied only with a 10-ms resolution during an ongoing transmission.
figure 38 How data rates change during a connection
79 Glossary Abbreviation In full
2G 2nd generation 3G 3rd generation 3GPP 3rd generation partnership project AAL2 ATM adaption layer 2 AAL5 ATM adaption layer 5 API Application programming interface ATM Asynchronous transfer mode AuC Authentication center BG Border gateway BPSK Binary PSK BSC Base station controller BSS Base station subsystem BTS Base transceiver station CAMEL Customized application for mobile network enhanced logic CDMA Code division multiple access CN Core network CS Circuit switched CSCF Call state control function
80 Abbreviation In full
DECT Digital enhanced cordless telecommunications DL Downlink DRNC Drifting radio network controller DRNS Drifting radio network subsystem DSS1 Digital subscriber signaling no.1 EDGE Enhanced data rates for GSM evolution EGPRS Enhanced GPRS EIR Equipment identity register FDD Frequency division duplex FDMA Frequency division multiple access GERAN GSM/EDGE radio access network GGSN Gateway GPRS support node GMM GPRS mobility management GMSC Gateway MSC GPRS General packet radio service GSM Global system for mobile communications GSN GPRS support node GTP GPRS tunneling protocol HLC High layer compatibility HLR Home location register
81 Abbreviation In full
HSCSD High speed circuit switched data IMT 2000 International mobile telecommunications 2000 IN Intelligent network IP Internet protocol ISDN Integrated services digital network ISUP ISDN user part ITU International telecommunication union IuUPP Iu user plane protocol LLC Low layer compatibility MAC Medium access control MAP Mobile application part ME Mobile equipment MGCF Media gateway control function MGW Media gateway function MRF Multimedia resource function MS Mobile station MSC Mobile switching center MSS Mobile satellite system MT Mobile termination MTP Message transfer part
82 Abbreviation In full
Node-B UMTS base station O&M Operation and maintenance OVSF Orthogonal variable spreading factor PCS Personal communication system PCU Packet control unit PDCP Packet data convergence protocol PDN Packet data network PDP Packet data protocol PHS Personal handyphone system PLMN Public land mobile network PS Packet switched PSK Phase shift keying PSTN Public switched telephone network QoS Quality of service QPSK Quarternary PSK R97, R98, R99, R4, R5 Release version RANAP Radio access network application protocol RLC Radio link control RNC Radio network controller RNS Radio network subsystem
83 Abbreviation In full
RRC Radio resource control SB Signaling bearer SCCP Signaling connection control part SCTP S common transport protocol SF Spreading factor SGSN Serving GPRS support node SGW Signaling gateway function SM Session management SMS-SC Short message service – service center SRNC Serving radio network controller SRNS Serving radio network subsystem SS#7 Signaling system no. 7 SSCF Service specific co-ordination function SSCOP Service specific connection oriented protocol SSF Service switching function TDD Time division duplex TDMA Time division multiple access TE Terminal equipment UDP User datagram protocol UE User equipment
84 Abbreviation In full
UICC Universal integrated circuit card UL Uplink UMSC UMTS-MSC UMTS Universal mobile telecommunications system USIM Universal subscriber identity module UTRA Universal terrestrial radio access UTRAN Universal terrestrial radio access network VHE Virtual home environment VLR Visitor location register WAP Wireless application protocol WARC World administrative radio conference W-CDMA Wideband-CDMA
85 Bibliography UMTS Mobile Communications for the Future. Edited by Flavio Muratore. Published by John Wiley & Sons. ISBN 0 471 49829 7
WCDMA for UMTS. Edited by Harri Holma and Antti Toskala. Published by John Wiley & Sons. ISBN 0 471 48687 6
86 Information sources www.etsi.org European Telecommunications Standards Institute www.umts-forum.org UMTS-Forum www.3gpp.org 3rd Generation Partnership Project www.itu.int International Telecommunications Union www.umts-dp.com UMTS Development Partnership www.imst.de Institut für Mobil- und Satellitenfunktechnik GmbH
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