boosting wireless efficiency

Third generation mobile communications

UMTS

Publisher: Willtek Communications GmbHGutenbergstr. 2-485737 IsmaningGermanye-mail: info@willtek.comhttp://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 registeredtrademarks are the property of their respective owners.

UMTS – a communications revolution 6Three generations of mobile communications 8Services – ensuring the success of UMTS 10New technology, new roles 12

The fundamentals of UMTS 153G frequencies 15Frequency bands for UMTS 18Minimum bandwidth requirements 20

Inside the UMTS architecture 22Network overview 24From handset to network – the user equipment 25Virtual home environment 30Service capabilities and APIs 32UMTS system architecture 34UTRAN components 35Node B 38Serving radio network subsystems and drift radio network subsystems 40Handovers 41Hard handovers 42Softer and soft handovers 43Role of the Iur interface during handover 45UMTS logical planes 46Transport network control plane 46Control plane 46Control plane components 47User plane 50User plane components 50ATM in the core network 53The future – all-IP networks? 55Key terms 57

Content

UMTS air interface 62Multiple access routes 63Frequency division duplex (FDD) 65Time division duplex (TDD) 66FDMA-based networks 67CDMA – it’s party time 68CDMA-based networks 70CDMA cells 72Orthogonal codes and multiplexing 73Features of the UMTS radio interface 74Frequency, code, and phase 75CDMA air interface challenges 76The “near-far” problem 76Cell breathing 78Variable rate transmission 79

Glossary 80

Bibliography 86

Information sources 87

6

Almost everyone today seems to own at least one mobile device. The growth

in phone ownership is a relatively new phenomenon and is largely attributa-

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.

UMTS – a communications revolution

7

figure 1 Development of subscriber count and applications for UMTS

UMTS – a guide to the third generation of mobile communicationsis 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.

8

Three generations of mobile communications

First generation (1G) – analog mobile radio networksThese are still commonplace in some parts of the world, but lack the

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 networksThe 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 networksThe first phase in the development of broadband mobile communications is

2.5G, that means an interim step based on 2G technologies.

9

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

10

Services – ensuring the successof UMTS

The mobile communications industry has agreed most of the technical

prerequisites for mobile multimedia communications. Soon users will

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

11

figure 3 Potential 3G services

12

Despite some early attempts by new entrants to secure the lucrative

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.

SubscriberA person or entity deemed as such by law, who has a legal agreement with a

service provider on behalf of one or more users.

UserA 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.

New technology, new roles

13

Service providerIn 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 operatorThe 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 providerThese 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.

figure 4 How the roles fittogether

14

Content providerA 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 brokerThese 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.

15

Sections three and four of this guide provide an overview of UMTS architec-

ture and air interface. This section focuses on fundamental aspects of UMTS

– frequencies, frequency bands, and bandwidth requirements.

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)

The fundamentals of UMTS

3G frequencies

16

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.

17

figure 5 Worldwide 3Gfrequency allocations

18

Frequency division duplex (FDD)2 x 60 MHz

Uplink 1920 to 1980 MHz

Downlink 2110 to 2170 MHz

Time division duplex (TDD) standard band1 x 20 MHz

Uplink and Downlink 1900 to 1920 MHz

TDD optional band1 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.

Frequency bands for UMTS

19

figure 6 UMTS frequency bands

20

The UMTS Forum carried out studies to determine the minimum required

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.

Minimum bandwidthrequirements

21

figure 7 Minimum bandwidth requirements forUMTS

22

The general UMTS network architecture can be divided into two main

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

Inside the UMTS architecture

23

figure 8 UMTS networkoverview. Notice howthe core GPRS infra-structure powers the core UMTS network

24

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 interfacesCu 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

Network overview

figure 9 UMTS domains and interfaces

25

The terminal equipment (TE) forms the interface to the user and holds all of

the applications. The mobile termination (MT) acts as the last radio interface

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

From handset to network – the userequipment

26

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.

27

figure 11 Inside the user’s terminal

28

User profilesA 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.

29

figure 12 A look inside auniversal integratedcircuit card

30

Virtual home environment

UMTS provides subscribers with a virtual home environment (VHE) in which

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.

31

figure 13 The virtual home environment (VHE)

The illustration below demonstrates the architecture necessary to enable

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

Service capabilitiesand APIs

33

figure 14 The key to develop-ing UMTS-basedservices – servicecapabilities and APIs

34

The core network is connected to the UTRAN via the lu interface. The

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 systemslevel

UMTS system architecture

35

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.

Utran Components

36

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).

37

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 theUTRAN

38

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

Node B

39

figure 17 An RNC can control multiple node Bs, which in turn can cover multiple cells

40

Serving radionetwork subsystemsand drift radio net-work subsystems

figure 18 The DRNS and SRNS– connecting theradio terminal to theUTRAN

For each connection between terminal and UTRAN, there is a serving radio

network subsystem (SRNS).

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.

41

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.

42

figure 19 GSM employs hard handovers from cellto cell

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.

Hard handovers

43

UMTS uses a CDMA multiple access technique on the radio interface. This

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.

Softer and soft handovers

44

figure 20 Soft handovers makemacrodiversity possible

45

figure 21 The lur interfacemakes handoversstraightforward

Role of the Iur interface during handover

In UMTS, a handover procedure can be administered solely by the UTRAN

with the lur interface. This interconnects the individual radio network

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.

46

Within the UMTS network architecture, there are three logical planes:

Transport network control plane, control plane, and user plane.

The transport plane moves data generated by UMTS users and control

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.

The control plane manages signaling protocols and procedures.

UMTS logical planes

Transport network control plane

Control plane

47

Control plane components

Radio resource control (RRC)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.

48

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

49

Signaling bearerSCCP 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 controlplane MS-SGSN

50

The user plane enables users to generate various types of bearer traffic,

including voice (8 and 16 Kbps), packet, and unrestricted digital data.

Medium access control (MAC)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.

User plane

User plane components

51

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

52

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.

53

UMTS is capable of transporting narrowband speech connections and

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.

ATM in the core network

54

figure 24 ATM in UMTS networks

55

UMTS specifications are being developed further and are maintained in

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

The future – all-IP networks?

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

58

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.

59

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.

60

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.

61

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.

62

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.

UMTS air interface

figure 26 The different facesof 3G

63

Frequency division multiple access (FDMA)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/TDMAThis 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.

Multiple accessroutes

64

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 withthe maximum possi-ble bandwidth

65

In FDD, the bandwidth for the uplink and downlink is 5 MHz in each

direction. The duplex spacing is 190 MHz.

Frequency divisionduplex (FDD)

figure 28 The main disadvan-tage of FDD is thatbecause it uses sepa-rate bands for uplinkand downlink,operators cannotdistribute resourcesflexibly

66

The available time slots can be used differently for the uplink and downlink.

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.

Time division duplex(TDD)

figure 29 Using TDD makesasymmetrical usageof uplink and down-link possible,but this can lead tointerference

67

In order to suppress interference from cells using the same frequencies,

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).

FDMA-based networks

figure 30 With FDMA net-works, the key toeffective resourceusage and network performance is pre-cise planning

68

Code division multiple access (CDMA) is the radio access technology

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.

CDMA – it’s partytime

69

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 everyvoice is speaking atthe same level –or in CDMA terms –that every handset isset to the samepower level, everyseparate “voice” – ineach different lan-guage will be“heard” – and “understood”.

70

CDMA systems use the same frequency for all users within a cell, which

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.

CDMA-based networks

71

figure 32 Users within CDMA networks are distin-guished by virtuallyunique orthogonalcodes

72

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.

CDMA cells

figure 33 CDMA cells support soft handover and macrodiversity

73

To separate the diverse user data streams, orthogonal variable spreading

codes (OVSF) must be used. Orthogonal codes are codes for which the cross

correlation is equal to 0. Data streams dn(1) and dn

(2) from two different

users can be separated on the receiving end using the orthogonal codes ci(1)

and ci(2).

Orthogonal codesand multiplexing

figure 34 Orthogonal variablespreading factor(OVSF) codes increasedownlink capacity

74

FDD TDD

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

Features of theUMTS radio interface

75

On OSI layer 1, physical channels are used to transmit data. Transport chan-

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.

Frequency, code and phase

figure 35 The properties ofphysical channels

76

As the main radio access interface in UMTS, code division multiple

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 capacity of a CDMA cell depends on the signal-to-noise ratio at the

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.

CDMA air interfacechallenges

The “near-far”problem

77

figure 36 An illustration of the near-far problem

78

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.

Cell breathing

figure 37 Cell breathing cangreatly degrade qual-ity of service withincreased traffic load

79

Variable rate transmission can be used for fast, variable adaptation of the

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.

Variable rate transmission

figure 38 How data rateschange during aconnection

80

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

Glossary

81

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

82

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

83

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

84

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

85

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

86

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

Bibliography

87

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

www.willtek.com Willtek

Information sources

UMTS/CT801/1102/EN

Willtek Communications GmbHGutenbergstrasse 2–485737 IsmaningGermany

info@willtek.com

West Europe/Middle East/Africa/Asia Pacific

Regional Sales Headquarters

Worldwide Headquarters

Willtek Communications Ltd.Roebuck PlaceRoebuck RoadChessington Surrey KT9 1EUUnited Kingdom

willtek.uk@willtek.com

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willtek.us@willtek.com

To find your local sales office, go towww.willtek.com

United Kingdom/Ireland/Benelux