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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
Willtek Communications Inc.7369 Shadeland Station Way, Suite 20046256 Indianapolis, IndianaUSA
willtek.us@willtek.com
To find your local sales office, go towww.willtek.com
United Kingdom/Ireland/Benelux
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