Migration Path to UMTS and the Third Generation Partnership Project (3GPP)

The radio access for UMTS is known as Universal Terrestrial Radio Access
(UTRA). This is a WCDMA-based radio solution, which includes both FDD
and TDD modes. The radio access network (RAN) is known as UTRAN. It
takes more than an air interface or an access network to make a complete
system, however. The core network must also be considered. Because of the
widespread deployment and success of Global System for Mobile communications
(GSM), it is appropriate to base the UMTS core network upon an
evolution of the GSM core network. In fact, as we shall see, the initial
release of UMTS (3GPP Release 1999) makes use of the same core network
architecture as defined for GSM/GPRS, albeit with some enhancements.
Moreover, the core network is required to support both UMTS and GSM
radio access networks (that is, both UTRAN and the GSM BSS).

The evolution of the GSM BSS has not stopped, however. As we shall see,
enhancements such as the Enhanced Data Rates for Global Evolution
(EDGE) have been made. With the requirements for the continued evolution
of GSM and for the GSM to meet UMTS requirements, it makes sense
for the continued maintenance and evolution of GSM specifications to be
undertaken by 3GPP. Consequently, 3GPP, rather than ETSI, is now
responsible for GSM specifications as well as UMTS-specific specifications.
For several years, the various enhancements to GSM have been developed
according to yearly releases. Thus, for a given GSM specification, versions
have been related to Release 1996, Release 1997, and Release 1998.
Initially, 3GPP determined to continue with that approach. Therefore, the
first release of specifications from 3GPP is known as 3GPP Release 1999.
The release includes not only new specifications for the support of a
UTRAN access, but also enhanced versions of existing GSM specifications
(such as for the support of EDGE). The 3GPP Release 1999 specifications
were completed in March of 2000. These, of course, will be subject to some
revisions and corrections as errors and inconsistencies are discovered during
test and deployment.
The next release of 3GPP specifications was originally termed 3GPP
Release 2000. This included major changes to the core network. The
changes were so significant, however, that they could not all be handled in
a single step. Thus, Release 2000 was divided into two releases: Release 4
and Release 5. Going forward, the concept of yearly releases will no longer
apply, and releases will be structured and timed according to defined functionality.
The Release 4 specifications were frozen in the first half of 2001.
This means that no new content is to be added and any changes to the specifications
will occur only to correct errors or inconsistencies. For Release 5,
it is expected that specifications will be frozen in December of 2001.
For the most part (although not exclusively), 3GPP Release 1999 focuses
mainly on the access network (including a totally new air interface) and the
changes needed to the core network to support that access network. Release
4 focuses more on changes to the architecture of the core network. Release
5 introduces a new call model, which means changes to user terminals,
changes to the core network, and some changes to the access network
(although the fundamentals of the air interface remain the same). Given
that the air interface is new in Release 1999 and that it does not drastically
change in later releases, it is best to begin our description of UMTS technology
with the WCDMA air interface. The primary focus in this book will
be on the FDD mode of operation, with less emphasis on TDD. First, however,
a few words about the types of services that UMTS can offer.