WCDMA Design Handbook available in Hardcover, Paperback, eBook
- ISBN-10:
- 0521828155
- ISBN-13:
- 9780521828154
- Pub. Date:
- 03/17/2005
- Publisher:
- Cambridge University Press
- ISBN-10:
- 0521828155
- ISBN-13:
- 9780521828154
- Pub. Date:
- 03/17/2005
- Publisher:
- Cambridge University Press
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Product Details
| ISBN-13: | 9780521828154 |
|---|---|
| Publisher: | Cambridge University Press |
| Publication date: | 03/17/2005 |
| Pages: | 592 |
| Product dimensions: | 7.56(w) x 9.96(h) x 1.26(d) |
About the Author
Dr Andrew Richardson has many years of experience in digital communication systems, having worked for Philips, Nokia and Simoco on both 2nd and 3rd generation mobile 'phone systems. Since 1999 he has run his own consultancy, Imagicom Ltd, offering design and training services in telecommunication systems technology.
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Cambridge University Press
0521828155 - WCDMA Design Handbook - by Andrew Richardson
Excerpt
1 Introduction
1.1 Concepts and terminology
This book is concerned with an exploration of the design and operation of a technology referred to as wideband code division multiple access (usually abbreviated to WCDMA or W-CDMA).
Before starting the journey exploring the design and operation of the WCDMA technology, we first need to define what we mean by WCDMA. To do this, it is useful to examine first the terminology that surrounds the WCDMA system and which is put into context in Figure 1.1. To understand the diagram, we need to start at the outer ring.
The outer ring encompasses all of the technology that is viewed as being part of the third generation (often shortened to the term 3G) of mobile phone technology. This third generation follows (obviously) from second-generation (2G) technologies such as: the Global System for Mobile (GSM) communications, which is deployed in Europe and many other countries throughout the world; IS54/IS136, which is a 2G standard developed in the USA and also used in a number of countries throughout the world; Personal Digital Cellular (PDC) developed and deployed in Japan; and IS95, the USA developed CDMA standard, which is deployed in the USA and many other countries.
3G is a term used to reflect a number of different technologies being defined as successors to 2G technology. In many cases, the 3G technologies can be seen as an evolution of the 2G networks. Another general term for 3G is International Mobile Telecommunications 2000 (IMT2000).
IMT2000 is a name that was defined by Task Group 8/1 (TG8/1), a standardisation group set up in 1985 by the International Telecommunication Union (ITU) to define world standards for 3G mobile technologies. The role of TG8/1 was essentially to define the requirements for 3G, and to facilitate the process of defining a radio technology that meets them. The term IMT2000 represents the family of technologies that are contained within the outer ring, which have been defined to meet these requirements. The outer ring in Figure 1.1, therefore, is intended to illustrate the extent of 3G technologies and the different terms that are commonly applied to define 3G.
Figure 1.1 3G technology relationship.
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Within the outer ring we can see a number of smaller rings. These smaller rings represent the technologies that have been defined and which form 3G. The original objective for TG8/1 was to define a single standard for the 3G radio access technology (RAT). The resulting situation of having multiple technologies was a consequence of the very difficult environment that existed prior to the definition of 3G. As mentioned previously, there are a number of 2G technologies throughout the world, each of which should have some evolutionary path to 3G. Due to the differences in the technologies as well as some political differences, the best compromise was the creation of a number of different 'technology modes' for 3G leading to a family of technologies.
1.1.1 Universal Mobile Telecommunications System (UMTS)
UMTS is the name of the telecommunication system that is being defined by the 3rd Generation Partnership Project (3GPP). The functionality included within UMTS encompasses all that is required to support the service requirements that were outlined for 3G by TG8/1 and also includes some that are new and evolved.
UMTS represents the complete system, including elements such as the user's mobile equipment, the radio infrastructure necessary to support a call or data session, and the core network equipment that is necessary to transport that user call or data from end to end, as well as billing systems, security systems etc.
Within Figure 1.1, as in this book, we are focussing predominantly on RAT (we will see later that this is not strictly correct, as we will also be considering upper layer protocols that extend beyond the radio access network into the UMTS core network). As a consequence, the core network equipment, which is equally a part of UMTS, is not shown in the diagram.
In UMTS there are two different types of RAT. The first is the subject of this book, namely WCDMA. The second is an evolution of the 2G GSM system that is generically referred to as the GSM/EDGE radio access network (GERAN). Different rings illustrate these two technologies. The focus of this book is WCDMA and as a consequence we won't consider GERAN in much more detail, except where interworking between the two technologies requires us to do so.
Within the WCDMA ring there are two additional rings called frequency division duplex (FDD) mode and time division duplex (TDD) mode. These two modes represent two quite different (different from the perspective of the physical layer of the protocol) technologies. The FDD mode of WCDMA separates users employing codes and frequencies with one frequency for the uplink and a second frequency for the downlink. The TDD mode on the other hand separates users employing codes, frequencies and time and uses the same frequency for both the uplink and the downlink. The emphasis of our considerations is on the FDD mode, although we will consider aspects of the TDD mode.
1.1.2 Other IMT2000 technologies
To complete the picture illustrated by Figure 1.1, we should also consider the other technologies that form a part of our 3G family of technologies referred to as IMT2000. They are cdma2000, UWC136 and DECT.
Cdma2000 has evolved from the 2G technology commonly referred to as IS95. Cdma2000 is a CDMA technology that supports the service requirements defined by TG8/1 for 3G mobile technology. Although cdma2000 and WCDMA have elements within them that are similar, the two systems are sufficiently different to require separate consideration. As a consequence, we will not consider cdma2000 in any greater depth except when we need to address issues that relate to intersystem operation.
UWC136 is a technology derived from the IS54 and IS136 2G digital cellular standards defined in the USA by the Telecommunications Industry Association (TIA). UWC136 is a time division multiple access (TDMA) technology, similar in concept to that used by GSM. In fact UWC136 in one of its modes of operation includes elements of the evolved GSM technology to provide for high data rates. Again, however, we will not consider UWC136 in any greater depth as it is a subject that lies beyond the scope of this book.
Digitally enhanced cordless telephone (DECT) is the final member of the 3G-technology family. DECT was defined by the European Telecommunication Standards Institute (ETSI) as a low power wireless communications system. DECT uses a combination of FDMA, TDMA and TDD to allow users to access the services provided by DECT. Again, DECT lies outside the scope of this book and as a consequence will not be considered in greater detail.
1.2 Major concepts behind UMTS
A question that is often asked regarding UMTS is why there needs to be a new radio interface technology and why an evolution of 2G technology such as enhanced data rates for GSM evolution (EDGE) cannot be used in its place.
The answer, as could be anticipated, is not a simple one. There are many issues that we could consider, such as spectrum efficiency, that could be used (arguably) to justify a new radio technology solution for 3G. There is, however, another line of reasoning, which is less controversial as to the reasons for the existence of WCDMA within the UMTS technology family. When TG8/1 set out the objectives and requirements for 3G, there were a number of elements that made the reuse of 2G technology in a 3G context very difficult without significant evolution of the 2G technology itself (this is the case for cdma2000 and UWC136 described earlier). Three of the most significant elements are outlined below and defined in greater detail in the subsections that follow:
•Support of general quality of service (QoS).
•Support of multimedia services.
•Support of 2 Mb/s.
1.2.1 Support of general QoS
We will see in Section 13.9 that a vast majority of services provided by UMTS use a technique known as packet switched communications. The mechanism behind the success of packet switched communications is dynamic resource sharing, whereby a number of users can share the same transmission resource (this could be anything from a radio signal to an optical fibre). One consequence of dynamic resource sharing in a packet switched network is the dynamic variation in the resources that are available to a user. If nothing is done to overcome the effects of dynamic resource allocation, then this will lead to queuing delays and inefficient use of resources in the network. To help circumvent this problem, packet switched communication systems define a concept known as quality of service (QoS). QoS cannot, on its own, overcome the problem of queuing delays, but it can quantify the requirements that need to be achieved using other procedures.
From the outset, UMTS was designed to support services with an arbitrary QoS. Figure 1.2 illustrates an example of such a service as well as the basic quantities that are used to define the service characteristics. Before considering the specifics of the example service illustrated in Figure 1.2, we must first consider how we define QoS. QoS in general is defined in terms of three quantities: data rate, delay and error characteristics. We will see in Section 13.8 that UMTS defines QoS using a number of parameters, but that broadly speaking these parameters reduce to these three quantities. Let us begin by considering what is meant by these three quantities, and then we can consider the specific example shown in the figure.
Figure 1.2 Example service and QoS relationship.
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Data rate
In a packet switched network, data rate is usually defined in terms of average data rate and peak data rate (both measured over some defined time period). It is the objective of the packet network to offer a service to a user that lies within the requested data rate. If the service were a constant data rate service such as a 57.6 kb/s modem access, then the peak and the average data rates would tend to be the same. If, on the other hand, the service is an Internet access data service, then the peak and average data rates could be quite different.
Delay
The second element of QoS is some measure of delay. In general, delay comprises two parts. First, there is the type of delay, and second there is some measure of the magnitude of the delay.
The type of delay defines the time requirements of the service such as whether it is a real-time service (such as voice communications) or a non-real-time service (such as e-mail delivery). We will see in Chapter 9 that in UMTS the delay characteristic is referred to as the traffic class, and that there are four traffic classes currently defined.
The magnitude of the delay defines how much delay can be tolerated by the service. Bi-direction services such as voice communications in general require low delay, typically in the region of tens of milliseconds. Uni-directional services such as e-mail delivery can accept higher delays, measured in terms of seconds.
The objective for the delay component of QoS, therefore, is to match the user's service requirements for the delay to the delay that can be delivered by the network. In a system operating correctly, the delay of the data should correspond to the delay specified within the QoS.
Error characteristics
The final component of a typical QoS definition is the error characteristics of the end-to-end link (by end-to-end we are assuming that the QoS is defined for the link between two communicating users). The error characteristic defines items such as the bit error rate (BER) or the frame error rate (FER). This is a measure of how many errors can be introduced across the link before the service degrades below a level defined to be acceptable.
The error characteristic is variable and dependent upon the service. Services such as speech, for example, have been found to be quite tolerant to errors. Other services, such as packet data for Web access, are very sensitive to errors.
Example service
Let us return to the example service shown in Figure 1.2. This example is not applicable to the UMTS circuit switched services that carry voice and video; rather it applies to the evolved services based on packet switched connections. In the example we are considering some type of video service (for instance as part of a video conference). For this specific service, we require a peak data rate of 64 kb/s. Although a low data rate, this is typical of what could be required for video conferencing using the very small display screens likely on pocket UMTS video phones. For video communications, the peak and average data rates are not necessarily the same. Most video coding algorithms operate by sending a complete image of data infrequently, and frequently sending an update to this complete image. This procedure results in a data rate whose peak and average values are different.
The delay characteristic for the video service reflects the fact that the service is a conversational real-time service with a need for a low end-to-end delay. The delay that is acceptable will depend on user perceptions of the delay, but we could imagine that end-to-end delays, typically, must be less than a few hundred milliseconds.
The error characteristics of the service are defined by the sensitivity of the video codecs to errors. Typically, a video codec operates with an error rate in the region of 1 × 10-6. This terminology means that the video codec can operate acceptably as long as there is (on average) not more than one error for every million bits received.
1.2.2 Support of multimedia services
The second significant element introduced by UMTS that pushes the requirement for a new radio interface technology further is the support of multimedia services.
Multimedia is one of those terms that is never clearly and consistently defined. Here, and throughout this book, we define multimedia as a collection of data streams between the user and some other end user(s) or application(s). The data streams that comprise this multimedia connection will, in general, have differing QoS characteristics.
It is the objective of the UMTS standard to allow the user to have such a multimedia connection, which comprises a number of such data streams with different QoS characteristics, but all of them multiplexed onto the same physical radio interface connection.
| Data Stream A data stream in the context of this book refers to a flow of information from a user to a destination user or possibly to some destination application. For a specific user there could be a number of these data streams, but not necessarily going to the same destination end-point. |
Figure 1.3 Example multimedia service.
Image not available in HTML version
Figure 1.3 illustrates an example of a multimedia connection comprising a number of data streams. In this example, we see that there are three components to this multimedia service - voice, video and packet data. For each of these services, the QoS is defined and different.
For the voice service, we need a low data rate (typically 12.2 kb/s), low and constant delay and a moderate error characteristic (a BER in the region of 10-3 should be acceptable for most voice communication systems).
For the video service, the data rate needs to be greater (maybe in the region of 64 kb/s as defined earlier), the delay perhaps similar to the voice connection and the error characteristic probably in the region of 10-6, reflecting the greater sensitivity that the video service has to errors.
For the packet data communications, we could envisage data rates in the region of 128 kb/s for high speed Internet access, a delay characteristic that reflects the type of Internet access and an error characteristic with a BER better than 10-9.
Obviously, this is a somewhat artificial example, but what it serves to illustrate is that the UMTS system needs to be able to combine services with totally different QoS requirements onto the same physical radio connection, but do so in a very efficient manner.
1.2.3 Support for 2 Mb/s
The final significant element that has contributed to the decision to create a new radio technology for UMTS relates to the peak data rates that are likely for UMTS. From the very outset TG8/1 within the ITU defined a requirement for 3G to support high data rates of up to 2 Mb/s. Although not intended for use in all application areas (by this we mean in all locations and conditions that the user is likely to be in), this high data rate is required for certain types of application such as high quality video transmission and high speed internet access.
Current 2G technology such as GSM can support data rates of the order of 100 kb/s. With enhancements, such as the change in modulation schemes proposed by EDGE, this provides an upper data rate in the region of 384 kb/s.
| EDGE EDGE is part of the ongoing evolution of the GSM standard, in this case to support higher data rates. EDGE achieves the higher data rates through the use of a higher order modulation scheme based on a technique called 8-PSK (phase shift keying). |
EDGE, therefore, can achieve data rates approaching those required for UMTS. To achieve the 2 Mb/s data rate, however, we need to reconsider the design of the radio interface.
The limits of the GSM radio interface technology are perhaps some of the main reasons why the decision was made to reconsider the definition of a new radio technology. Ultimately, within UMTS, this decision led to the definition of the radio interface technology that we now know as WCDMA.
1.3 Release 99 (R99) network architecture
The UMTS specifications are being issued in different releases. The purpose of this is to stagger the introduction of new services and technologies, which in turn will reduce the problems with installation and commissioning of the system as a whole. The first release of the UMTS specifications is referred to as the R99 specifications. The sub- sequent releases are referred to as Release 4 (R4), Release 5 (R5) and Release 6 (R6).
Perhaps the main objective of R99 was the introduction of the WCDMA radio technology within the radio interface. WCDMA introduces a significant degree of complexity in the design and operation of the radio interface, and is the main focus of this book. R4, R5 and R6 are subsequent releases that add additional functionality and services to the R99 standard.
In this section we want to examine the basic network architecture for the R99 series of specifications. A good understanding of the architecture of the UMTS network is very important when it comes to understanding the design and operation of the remainder of the network.
1.3.1 Fundamental architecture concepts
Basic network structure
Figure 1.4 shows, from a very high level, the basic structure of the UMTS system. The structure is split into three main components: the core network (CN); the UMTS terrestrial radio access network (UTRAN); and the user equipment (UE).
Figure 1.4 High level representation of UMTS network elements.
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The CN is responsible for the higher layer functions such as user mobility, call control, session management and other network centric functions such as billing, security control. The UTRAN is responsible for functions that relate to access, such as the radio access, radio mobility and radio resource utilisation. The decision to divide the fixed network into two distinct networks was a very deliberate and considered step in the overall design of the UMTS system.
In separating radio access from the other network functions, it becomes feasible to evolve the radio access network and the CN independently from each other. In doing this, as part of R99, we can introduce a new radio access technology (WCDMA) and reuse existing CN technology from the GSM and general packet radio service (GPRS) networks. Subsequent releases of the CN (e.g. R4 and R5) can then modify the CN, without necessarily introducing significant changes to the UTRAN.
Access stratum (AS) and non-access stratum (NAS)
A consequence of this decision to separate the UTRAN and the CN is the division in the layers of the protocol stacks, which is illustrated figuratively in Figure 1.5. The diagram illustrates a division in the protocol architecture between the UE, the UTRAN and the CN. The protocols are separated into what are called the AS and NAS. This division applies to both the signalling messages and the user data messages.
Figure 1.5 AS and NAS protocol split.
Image not available in HTML version
| Signalling and user data Throughout the book we will be referring to different types of data that are passed through the system. The term signalling is often used to define the type of data that is used to send special control messages that are used to control the system in some manner. The other type of information, the user data, refers to the actual user information that is sent from some source user to a destination user or destination application. In general, the signalling data passes through what is called the control plane. The user data passes through the user plane. |
The AS carries all of the signalling and user data messages that relate to the access technology used across a specific interface in that part of the system. Across the radio interface, the AS protocols are the lower level protocols between the UE and the UTRAN, and between the UTRAN and the CN.
The NAS carries the signalling messages and user data messages that are independent of the underlying access mechanism. These signalling and user data are passed between the UE and the CN and, conceptually, pass transparently through the UTRAN.
Examples of the types of signalling messages that are carried via the AS are messages that control the power control loops in the system, that control the handover procedures or that allocate channels to a user for use, for instance in a speech call. An example of an NAS signalling message would be one associated with a call setup request, where the call setup messages are independent of the underlying access mechanism. In this example, the call setup message would come from the CN, and be routed transparently through the AS.
1.3.2 Service domains
Figure 1.6 illustrates the functional decomposition of the UMTS fixed network architecture into the UTRAN and the CN service domains. For the R99 architecture there are three of these service domains, referred to as the circuit switched domain (CS domain), the packet switched domain (PS domain) and the broadcast control domain (BC domain).
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