3G Mobile Cellular Technologies 3

History of UMTS WCDMA
The inception of UMTS standard can be traced back to the early 1990s when ETSI initiated one
UMTS research project in RACE1, seven projects in RACE2 and 14 projects in the ACTS Program.
RACE projects were funded by Commission of European Communities (CEC). ETSI also organized
Future Advanced MObile Universal Telecommunications Systems (FAMOUS) meetings 3 times a year
between Europe, the United States and Japan.
From 1991 to 1995, two CEC funded research projects called Code Division Testbed (CODIT)
and Advanced Time Division Multiple Access (ATDMA) were carried out by the major European
telecom manufacturers and network operators. The CODIT and ATDMA projects investigated the  suitability of wideband CDMA and TDMA-based radio access technologies for 3G systems. This
work was later continued in the Future Radio Wideband Multiple Access System (FRAMES) project
and became the basis of the further ETSI UMTS work until decisions were taken in 1998.
In February 1992 the WRC in Malaga, Spain, allocated frequencies for future UMTS use. Frequencies
1885–2025 MHz and 2110–2200 MHz were identified for IMT-2000. The UMTS Task
Force was established in February 1995, issuing “The Road to UMTS” report.
The UMTS Forum was established at the inaugural meeting, held in Zurich, Switzerland, in
December 1996. Since then, the planned “European” WCDMA standard has been known as the
UMTS. In June 1997 the UMTS Forum produced its first report entitled A regulatory Framework for
UMTS. The UMTS core band was decided in October 1997.
In January 1998 ETSI SMG meeting in Paris, both WCDMA and TD-CDMA proposals were combined
to UMTS air interface specification. In June 1998, Terrestrial air interface proposals (UTRAN,
WCDMA, CDMA2000, EDGE, EP-DECT, TD-SCDMA) were handed into the ITU-R as possible IMT-2000 candidate proposals. The first call using a Nokia WCDMA terminal in DoCoMo’s trial
network was completed in September 1998 at Nokia’s R&D unit near Tokyo in Japan.
On December 4, 1998, ETSI SMG, T1P1, ARIB, TTC, and TTA created 3GPP in Copenhagen,
Denmark, and the first meeting of the 3GPP Technical Specification Groups was held in Sophia
Antipolis, France, on December 7 and 8, 1998.
On April 27 and 28, 1999, Lucent Technologies, Ericsson, and NEC announced that they were
selected by Nippon Telegraph and Telephone (NTT) DoCoMo to supply WCDMA equipment for
NTT DoCoMo’s next generation wireless commercial network in Japan. This was the first announced
WCDMA 3G infrastructure deal.
3GPP approved the UMTS Release 4 specification in March 2001 in a meeting that took place
in Palm Springs.
NTT DoCoMo launched a trial 3G service, an area-specific information service for i-mode on
June 28, 2001. On September 25, 2001, NTT DoCoMo announced that three 3G phone models were
commercially available. NTT DoCoMo launched the first commercial WCDMA 3G mobile network
on October 1, 2001.
On March 14, 2002, UMTS Release 5 was issued.11 UMTS Release 6 was issued on December
16, 2004, which was delayed from its initial target date of June 2003.
Ericsson demonstrates 9 Mbps with WCDMA, High Speed Downlink Packet Access (HSDPA)
phase 2, on February 14, 2005. Ericsson and several operators in three Scandinavian countries demonstrated
the 1.5 Mbps enhanced uplink in the live WCDMA system on May 10, 2005.12 In fact, the
peak data rate for HSDPA can reach up to 8–10 Mbps (and 20 Mbps for MIMO systems) over a
5 MHz bandwidth in WCDMA downlink. HSDPA implementations include Adaptive Modulation and
Coding (AMC), Multiple-Input Multiple-Output (MIMO), Hybrid-Automatic Request (HARQ), fast
cell search, and advanced receiver design. In the 3rd generation partnership project (3GPP) standards,
Release 4 specifications provide efficient IP support, enabling the provision of services through an
all-IP CN and Release 5 specifications focus on the HSDPA to provide data rates up to approximately To better comprehend where the UMTS standard stands in the ITU IMT-2000 proposals, we
provide Figure 3.18, where we have plotted all major ITU endorsed IMT-2000 candidate proposals
which are later called 3G standards. From among all the proposals or standards that were listed, we
classified them into (1) TWO core technologies (TDMA and CDMA); (2) THREE systems (UMTS,
CDMA2000 and UWC-136 or EDGE); and (3) FIVE radio interfaces, which include (a) IMT-DS
(Direct Spread), used in UTRA-FDD; (b) IMT-MC (Multi-Carrier), used in the CDMA2000 system;
(c) IMT-TC (Time Code), used in UTRA-TDD and TD-SCDMA; (d) IMT-SC (Single Carrier), used
in UWC-136 or EDGE technology; and (e) IMT-FT (Frequency Time), used in the DECT system.
3.2.2 ETSI UMTS versus ARIB WCDMA
In this section, we focus our discussions on the ETSI UMTS WCDMA [425] technology due to the
reason that it was a standard release issued by 3GPP. All 3GPP parties should make their best effort
to commit to full compatibility to the 3GPP releases, whose major versions are listed in Table 3.19.
On the other hand, the Japanese version of WCDMA launched by NTT DoCoMo in October 2001, is
also called the ARIB WCDMA system [431] or the Freedom of Mobile Multimedia Access (FOMA)
service.13 It has some technical differences in comparison to UMTS standard, and we offer some
explanations in this subsection.
Members of the 3GPP include organizations such as ETSI of Europe and ARIB of Japan, and so
on, and individual members and market representatives, such as the GSM Association, and the like. Virtually every major OEM is included with the organizational and market representation partners.
The 3GPP, similar to 3GPP2, is divided into several technical standards for their respective areas.
Once these standards are written, the 3GPP endorses the standards and submits them to the ITU.
One aspect of 3G standard development that is often misunderstood by the public is the concept
of releases, a system that also applies to 2G and 2.5G networks. 3G, in this case UMTS, does not
consist only of one release, but a series of releases that build upon the previous releases. Initially,
releases were noted by the year. For instance, Release’99, Release’00, and so on. However, later
releases are no longer tied up to the year in which they are finalized. Instead, the 3GPP has defined
the requirements in Release 4, has practically finalized Release 5, and has recently begun working
on Release 6, all of which are subsequently the releases of the UMTS Release’99 standard.
What makes it even more complex is that within each release (e.g., Release’99) there are multiple
versions. For instance, Release’99 began with the March 2000 version of the Release’99 standard, and
has since evolved every three months since that time in conjunction with the quarterly 3GPP plenary
meetings. Although the basic functionality of Release’99 does not change every quarter, the technical
definition of how the functionality is implemented does change. Specifically, 3GPP members submit
Change Requests (CRs), which identify changes to the baseline documentation. CRs can include
anything from typographical or grammatical errors to additional/changed text that is inserted/replaced
to clear up an ambiguity or correct an error, both of which could prevent a successful launch, in the
documentation.
An interesting issue on the evolutional path of WCDMA technology is the compatibility between
ARIB WCDMA technology [431] that was developed by NTT DoCoMo and UMTS-FDD [425] and
proposed by ETSI.
In October 2001, NTT DoCoMo launched commercial FOMA services. Much has been reported
on the launch, but we still believe that there are some widespread misunderstandings about what
happened in Japan in comparison to European activities on UMTS-FDD.
FOMA networks use WCDMA, like the UMTS standard being deployed in Europe. However,
FOMA uses an earlier version of the UMTS release. Without going into details, NTT DoCoMo got
tired of waiting for Release’99 to become a standard. Instead, it elected to pursue 3G on its own, based
upon the prefinalized Release’99 to meet its own particular technical requirements and subsequently
required its suppliers to provide equipment (infrastructure and handsets) that met those requirements. However, NTT DoCoMo publicly committed to bring its FOMA in line with the UMTS Release’99
in mid-2003.
FOMA has two operational modes: a dedicated 64 kbps circuit connection and a 384 kbps downlink,
and a 64 kbps uplink best-effort connection. The dedicated 64 kbps circuit is currently intended
for real-time services like video conference. It is important to note that UMTS and CDMA2000 do not
have this 64 kbps circuit-switched mode. Thus, subscribers should not expect real-time high-data-rate
services when the launches first occur. UMTS carriers will begin providing real-time services when they
deploy a later release of the UMTS standard. For CDMA2000 carriers, they will not provide real-time
services until they deploy 1xEV-DO (which is a data only scheme) or 1xE-DV (see Sections 3.1.13
and 3.1.12 for the details), which support higher peak data rates and have an all-IP core.
FOMA is a hybrid version of Release’99, but it will evolve to a higher compatibility with the
UMTS standard. Before we move on to the details of the UMTS standard, we need to clear up any
misunderstandings one may have about NTT DoCoMo’s FOMA service and its relationship with the
UMTS Release’99. NTT DoCoMo is a member of 3GPP and it is still involved in the 3GPP process.
However, it was decided to deploy 3G services before the Release’99 standard was frozen for its
own commercial consideration. Its FOMA service, therefore, was based on a prerelease version of the Release’99 standard. Since DoCoMo went at it alone, its 3G solution has evolved, and was not
fully compatible with Release’99. However, DoCoMo has promised to make its FOMA service fully
compatible with the UMTS standard in the next two years after its launch in October 2001.
3.2.3 UMTS Cell and Network Structure
UMTS can offer a different coverage or scale to different users. There are four different UMTS
hierarchical cell structures altogether which include (1) picocell, which covers only a small area such
as one office room; (2) microcell, which can cover a vicinity of several buildings to provide local
UMTS services; (3) macrocell, which will span an area as large as a few kilometers in radius as a
regional service provider; and finally (4) global cell, which will be covered by satellites and will be
available to any place around the world. Under such a hierarchical cell structure, UMTS can provide
services to the users located in various geographical regions on the earth. It is to be noted that the
formation of a global cell needs to use technology other than UTRAN due to the nature of a long
propagation delay in a satellite air-link sector.
Figure 3.19 shows a conceptual diagram of the UMTS hierarchical cell structure, which include
all the four different cells.
A very basic UMTS network structure consists of three fundamental components: (1) Access
Network, in which base stations play a key role in managing the air interface access between the
UMTS network and UE; (2) CN, also called Fixed Network, which is responsible for handling all
internal connections; and (3) Intelligent Network (IN), which is in charge of billing, subscriber location
registration, roaming, handover, and so on. Figure 3.20 shows the UMTS basic network structure and
the UMTS general reference architecture in the UMTS network. It can be seen from the figure that a
UMTS Terrestrial Radio Access Network (UTRAN) contains several radio subsystems, so called Radio
Network Subsystems (RNS) and contains functions for mobility management (MM). RNS controls the
handover whenever a mobile changes cell, implements functions for encoding and administrates the
resources of the UMTS radio interface. The Uu interface connects UTRAN with mobile end devices,
or UE, and is comparable with Um in a GSM network. UTRAN is connected over the Iu interface with the CN, comparable with the A interface in GSM between BSC and MSC. CN contains the
interfaces to other networks and mechanisms for a connection handover to other systems.
A UMTS network can also be explained using a way commonly referred to in the literature, as
shown in Figure 3.21, where there are four basic components, explained as follows:
• Cell: which specifies a basic coverage area. Hardware associated with the cell includes an
antenna system, a high power amplifier (HPA), a transmitter, a receiver, and so on. The cell in
UMTS is equivalent to a sector in GSM or CDMA2000.
• Node B: which is a common equipment at a cell site to control the cells, and is thus equivalent
to RBS, BTS, or Base Station in GSM or CDMA2000. • RNC: which is an equipment to control the Node Bs and interface them to the CN. This is
equivalent to BSC in GSM or CDMA2000.
• UE: which is a subscriber equipment and equivalent to a mobile station in GSM or CDMA2000.
All of these naming conventions or acronyms will be extensively used in this section and followed
whenever UMTS standard will be discussed.
3.2.4 UMTS Radio Interface
As mentioned earlier, the radio interface technology used in UMTS is called UMTS Terrestrial Radio
Access (UTRA), in which two operating modes have been defined: UTRA frequency division duplex
(FDD) and UTRA time-division duplex (TDD) modes.
In general, the UTRA-FDD operation mode is mainly suitable for suburban areas where a symmetrical
transmission of speech and video is required. The data transmission rate in UTRA-FDD mode
can go up to 384 kbps. The UTRA-FDD mode can also work for circuit- and packet-switched services
in urban areas. On the other hand, the UTRA-TDD operation mode works mainly in households and
other restricted areas, such as a company premises, and so on, similar to DECT. The UTRA-TDD is
particularly suitable for the broadcast of speech and video, in both symmetrical (up to 384 kbps) and
asymmetrical (up to 2 Mbps) ways.
Figure 3.22(a) draws a simple diagram to illustrate how the UTRA-FDD operation mode works
in terms of its carrier frequency allocation. Similarly, Figure 3.22(b) depicts the operation principle
for the UTRA-TDD scheme in the time and frequency domains.
It is seen from Figure 3.22 that UTRA-FDD places wideband CDMA (WCDMA) along with
DSSS as a bandwidth expansion technique. All the channels are separated by carrier frequencies,
spreading codes, and phase positions (only for uplink). There are 250 channels created for user data
transmission in total, whose rates can go up to 384 kbps for mobile terminals. It has to be admitted
that UTRA-FDD needs a relatively complex performance control mechanism due to the nature of
FDD, which is particularly suitable for coverage driven roll-out.
UTRA-TDD makes use of wideband TDMA/CDMA techniques along with the DSSS scheme.
Data signals can be sent and received on the same carrier due to the use of TDD to separate uplink
and downlink transmissions. There are 120 channels created for user data transactions in total, whose
rates can go up to 2 Mbps, which is higher than the UTRA-FDD scheme. Channel separation is
implemented by using different spreading codes and time slots, and thus a lower spreading factor
is required than that in the UTRA-FDD scheme. However, a UTRA-TDD operation needs cellwise
precise synchronization in order to keep the same reference clock among different UEs. UTRA-TDD
technology is suitable for small cells with more asymmetric traffic, as well as for unlicensed cordless
and public wireless local loops. TDD technology is best suited for indoor use where interference from
base stations is manageable and the lower range does not matter.
It is noted that FDD and TDD Node Bs can operate at the same RNC, as shown in Figure 3.23.
The differences between UTRA-FDD and UTRA-TDD are listed in Table 3.20. Figure 3.24 depicts
the frame structure used in UTRA-TDD.
To understand the UTRAN specifications better, one can find much more information from the
3GPP RAN documentations, which have been given in [309, 313]. One can easily download all of
those specifications free of charge, with the need to register only once.
The general description of UTRAN specifications of 3GPP RAN are given in the following
documentations:
• 3GPP TS 25.301: Radio Interface Protocol Architecture;
• 3GPP TS 25.302: Services provided by the physical layer;
• 3GPP TS 25.304: UE Procedures in Idle mode and Procedures for Cell Reselection in Connected
Mode. The Layer 3 (RRC) protocols for both FDD and TDD operation modes are discussed in
• 3GPP TS 25.331: Description of the RRC Protocol.
The Layer 2 (MAC/RLC) specifications for both FDD and TDD operation modes are given in
• 3GPP TS 25.321: MAC Protocol Specification;
• 3GPP TS 25.322: Description of the RLC protocol.
The Layer 1 (physical layer) specifications for FDD operation mode are given in the documentations:
• 3GPP TS 25.211: Transport Channels and Physical Channels (FDD);
• 3GPP TS 25.212: Multiplexing and Channel Coding (FDD);
• 3GPP TS 25.213: Spreading and Modulation (FDD).
The Layer 1 (physical layer) specifications for TDD operation mode are given in the documentations:
• 3GPP TS 25.221: Transport Channels and Physical Channels (TDD);
• 3GPP TS 25.222: Multiplexing and Channel Coding (TDD);
• 3GPP TS 25.223: Spreading and Modulation (TDD).
3GPP UTRA specifications cover both the FDD and TDD operation modes. However, due to the
limited space in this book, we shall not discuss them all in a very detailed manner. Therefore, in the
discussions given in this book we focus on the FDD operation mode of UTRA. We discuss the TDD
mode only if it is absolutely necessary, or if we need to compare the FDD mode with the TDD mode
in terms of their performance and complexity, and so on.
3.2.5 UMTS Protocol Stack
The UMTS protocol stack architecture is shown in Figure 3.25, in which three main layers, the
physical layer (L1), the MAC sublayer (L2), the Radio Link Control (RLC) sublayer (L2), and the
Radio Resource Control (RRC) layer (L3) are illustrated. The Control Plane and User Plane in the
UMTS protocol stacks are also shown.
Layer 1: Physical layer
Of course, the forward most part of the protocol stack in UMTS layered architecture is the physical
layer, which offers information transfer services to the MAC layer. These services are denoted as
Transport channels. There are also Physical channels, which is comprised of the following major
functions:
• Various handover functions;
• Error detection and report to higher layers;
• Multiplexing of transport channels;
• Mapping of transport channels to physical channels;
• Fast Close loop Power control;
• Frequency and Time Synchronization;
• Other responsibilities associated with transmitting and receiving signals over the radio media.
The complete list and explanations on all physical channels will be given in the subsection
followed.
Layer 2-1: MAC sublayer
The MAC sublayer offers data transfer to RLC and higher layers. The MAC sublayer is comprised
of the following functions:
• The selection of appropriate TF (basically bit rate), within a predefined set, per information
unit delivered to the physical layer;
• Priority handling between “data flows” of one user as well as between data flows from several
users, the latter being achieved by means of dynamic scheduling;
• Access control on RACH;
• Address control on RACH and FACH;
• Contention resolution on RACH.
It is to be noted that the term “sublayer” is used here to distinguish the full layers, such as
“physical layer,” and so on, as MAC sublayer itself does not form a full layer. Instead, it is only
a part of Layer 2 in the UTRA protocol stack structure. The MAC sublayer along with the RLC
sublayer will form Layer 2.
Layer 2-2: RLC sublayer
The Radio Link Control (RLC) sublayer offers the following services to the higher layers:
• Layer 2 connection establishment/release;
• Transparent data transfer, that is, no protocol overhead is appended to the information unit
received from the higher layer;
• Assured and unassured data transfer.
The RLC sublayer is comprised of the following functions:
• Segmentation and assembly;
• Transfer of user data;
• Error correction by means of retransmission optimized for the WCDMA physical layer;
• Sequence integrity (used by the control plane at the very least);
• Duplicate detection;
• Flow control;
• Ciphering.
Layer 3: RRC layer
The Radio Resource Control (RRC) layer is also called Layer 3 in the UMTS protocol stack and
offers the CN the following services:
• General control service, which is used as an information broadcast service;
• Notification service, which is used for the paging and notification of a selected UEs;
• Dedicated control service, which is used for the establishment/release of a connection and
transfer of messages using the connection.
The RRC layer is comprised of the following functions:
• Broadcasting information from the network to all UEs;
• Radio resource handling (e.g. code allocation, handover, admission control, and measurement
reporting/control);
• QoS Control;
• UE measurement reporting and control of the reporting;
• Power Control, Encryption, and Integrity protection.
3.2.6 UTRA Channels
The UTRA-FDD radio interface has logical channels, which are mapped to transport channels. The
transport channels are further mapped to physical channels. Logical to Transport channel conversion
happens in the MAC layer, which is a lower sublayer in the Data Link Layer (Layer 2).
There are six different Logical Channels in total, which are listed as follows:
• Broadcast Control Channel (BCCH), (DL);
• Paging Control Channel (PCCH), (DL);
• Dedicated Control Channel (DCCH), (UL/DL);
• Common Control Channel (CCCH), (UL/DL);
• Dedicated Traffic Channel (DTCH), (UL/DL);
• Common Traffic Channel (CTCH), (broadcasting).
where the abbreviations of “DL” and “UL” stand for downlink and uplink, respectively.
There are seven different Transport Channels in total:
• Dedicated Transport Channel (DCH), UL/DL, mapped to DCCH and DTCH;
• Broadcast Channel (BCH), DL, mapped to BCCH;
• Forward Access Channel (FACH), DL, mapped to BCCH, CCCH, CTCH, DCCH, and DTCH;
• Paging Channel (PCH), DL, mapped to PCCH;
• Random Access Channel (RACH), UL, mapped to CCCH, DCCH, and DTCH;
• Uplink Common Packet Channel (CPCH), UL, mapped to DCCH and DTCH;
• Downlink Shared Channel (DSCH), DL, mapped to DCCH and DTCH.
There are 13 different Physical Channels in total:
• Primary Common Control Physical Channel (P-CCPCH), mapped to BCH;
• Secondary Common Control Physical Channel (SCCPCH), mapped to FACH, PCH;
• Physical Random Access Channel (PRACH), mapped to RACH;
• Dedicated Physical Data Channel (DPDCH), mapped to DCH;
• Dedicated Physical Control Channel (DPCCH), mapped to DCH;
• Physical Downlink Shared Channel (PDSCH), mapped to DSCH;
• Physical Common Packet Channel (PCPCH), mapped to CPCH;
• Synchronization Channel (SCH);
• Common Pilot Channel (CPICH);
• Acquisition Indicator Channel (AICH);
• Paging Indication Channel (PICH);
• CPCH Status Indication Channel (CSICH);
• Collision Detection/Channel Assignment Indication Channel (CD/CA-ICH).
Figure 3.26 shows all the physical, transport, and logical channels, as well as their mapping
relations among them in UMTS UTRA. It is noted that the mapping from logical channels to transport
channels happens on the MAC layer (L2); while the mapping from transport channels to physical
channels happen on the physical layer (L1). Figure 3.27 shows the general architecture of Layers 1,
2, and 3 in 3GPP UTRA standard. It is also clearly shown that the mapping from the logical channels
to transport channels happens on the MAC layer.
Transport channels
More detailed information about the transport channels are given in the subsequent text. As shown
in Figure 3.27, all the transport channels carry services offered by Layer 1 to the higher layers. A
transport channel is defined by how and with what characteristics data is transferred over-the-airinterface.
There are two groups of transport channels: Dedicated Transport Channels, and Common Transport
Channels (CTC). Only one Dedicated Transport Channel exists, that is, DCH, which is a downlink
or an uplink transport channel, and is transmitted over the entire cell or only over a part of the cell
using, for example, beam-forming antennas. DCH carries both the service data, such as speech frames
and higher-layer control information, such as handover commands or measurement reports from the
terminal.
The content of the information carried on the DCH is not visible to the physical layer, and
thus higher-layer control information and user data are treated in the same way. The physical layer
parameters set by UTRAN may vary between control and data. DCH supports possible fast rate
change (every 10 ms), fast power control, as well as soft handover.
However, there are six CTC, which are divided between all or a group of users in a cell. It
is noted that they do not support soft handover, but some of them can support fast power control.
They include: (1) BCH: Broadcast Channel; (2) FACH: Forward Access Channel; (3) PCH: Paging
Channel; (4) RACH: Random Access Channel; (5) CPCH: Common Packet Channel; and (6) DSCH:
DL Shared Channel.
BCH is a downlink transport channel that is used to broadcast system and cell-specific information.
BCH is always transmitted over the entire cell. The most typical data needed in every network is the
available random access codes and access slots in the cell, or the types of transmit diversity. BCH
is transmitted with relatively high power. Single transport format offers a low and fixed data rate for
the UTRA BCH to support low-end terminals.
PCH is also a downlink transport channel. PCH is always transmitted over the entire cell. PCH
carries data relevant to the paging procedure, that is, when the network wants to initiate communication
with the terminal. The identical paging message can be transmitted in a single cell or in up to a few
hundreds of cells, depending on the system configuration.
Random Access Channel (RACH) is an uplink transport channel. RACH is intended to be used to
carry control information from the terminals, such as requests to set up a connection. RACH can also
be used to send small amounts of packet data from the terminal to the network. The RACH is always
received from the entire cell. The RACH is characterized by collision risk. RACH is transmitted
using open-loop power control.
Forward Access Channel (FACH) is a downlink transport channel. FACH is transmitted over
the entire cell or over only a part of the cell using beam-forming antennas. FACH can carry control
information, for example, after a random access message has been received by the base station. FACH
can also transmit packet data. FACH does not use fast power control. FACH can be transmitted using
slow power control. There can be more than one FACH in a cell. The messages transmitted need to
include in-band identification information.
CPCH is an optional uplink transport channel. CPCH is an extension to the RACH channel that
is intended to carry packet-based user data. CPCH is associated with a dedicated channel on the
downlink that provides power control and CPCH Control Commands (e.g. Emergency Stop) for the
uplink CPCH. The CPCH is characterized by initial collision risk and by using inner-loop power
control. The CPCH may last several frames.
DSCH is an optional downlink transport channel shared by several UEs to carry dedicated user
data and/or control information. The DSCH is always associated with one or several downlink DCH.
The DSCH is transmitted over the entire cell or over only a part of the cell using beam-forming
antennas. DSCH supports fast power control and variable bit rate on a frame-by-frame basis.
Physical channels
The mapping between the transport channels and physical channels happens in the UTRA physical
layer, as shown in Figure 3.26. We would like to discuss uplink physical channels, followed by the
downlink physical channels.
There are two Dedicated Uplink Physical Channels: Uplink Dedicated Physical Data Channel (UL
DPDCH) and Uplink Dedicated Physical Control Channel (UL DPCCH). Also, there are two Common
Uplink Physical Channels, which are Physical Random Access Channel (PRACH) and Physical
Common Packet Channel (PCPCH).
The UL DPDCH carries the DCH transport channel (generated at Layer 2 and above). There
may be zero, one, or several uplink DPDCHs on each radio link. The UL DPCCH carries control
information generated at Layer 1. Only one UL DPCCH exists on each radio link.
The PRACH is used to carry the RACH. The random access transmission is based on a slotted
ALOHA approach with fast acquisition indication. The UE can start the random-access transmission
at the beginning of a number of well-defined time intervals, denoted as access slots. There are 15
access slots per two frames and they are spaced 5120 chips apart. Information on what access slots
are available for random-access transmission is given from higher layers.
The PCPCH is used to carry the CPCH, whose transmission is based on Digital Sense Multiple
Access – Collision Detection (DSMA-CD) approach14 with fast acquisition indication. The UE can
start transmission at the beginning of a number of well-defined time intervals.
There is only one type of downlink Dedicated Physical Channel (DPCH), that is Downlink DPCH
(DL DPCH). Within one downlink DPCH, dedicated data generated at Layer 2 and above, that is,
the dedicated transport channel (DCH), is transmitted in the time-multiplex with control information
generated at Layer 1 (known pilot bits, Transmit Power Control (TPC) commands, and an optional
TFCI).
Common Pilot Channel (CPICH) is a fixed rate (30 kbps, SF = 256) downlink physical channel
that carries a predefined bit /symbol sequence. In case transmit diversity (open or closed loop) is used
on any downlink channel in the cell, the CPICH should be transmitted from both antennas using
the same channelization and scrambling code. There are two types of Common pilot channels: The
Primary CPICH and the Secondary CPICH.
The Primary Common Pilot Channel (P-CPICH) has the following characteristics: The same
channelization code is used for the P-CPICH; The P-CPICH is scrambled by the primary scrambling
code; There is one and only one P-CPICH per cell; The P-CPICH is broadcast over the entire
cell. The Primary CPICH is a phase reference for the following downlink channels: SCH, Primary
CCPCH, AICH, PICH AP-AICH, CD/CA-ICH, CSICH, DL-DPCCH for CPCH and the SCCPCH.
By default, the Primary CPICH is also a phase reference for downlink DPCH and any associated
PDSCH. The Primary CPICH is always a phase reference for a downlink physical channel using
closed-loop transmit diversity.
A Secondary Common Pilot Channel (S-CPICH) has the following characteristics: An arbitrary
channelization code of SF = 256 is used for the S-CPICH; A S-CPICH is scrambled by either the
primary or a secondary scrambling code; There may be zero, one, or several S-CPICHs per cell; A
S-CPICH may be transmitted over the entire cell or only over a part of the cell; A Secondary CPICH
may be a phase reference for a downlink DPCH. The Secondary CPICH can be a phase reference
for a downlink physical channel using open-loop transmit diversity, instead of the Primary CPICH
being a phase reference.
PCCPCH bears the following characteristics: it has a fixed rate: 30 kbps, SF = 256, and is used
to carry the BCH transport channel. Neither TPC commands, nor TFCI nor pilot bits will be sent in
P-CCPCH Secondary Common Control Physical Channel (SCCPCH) is used to carry the FACH and
PCH. Two types of SCCPCHs exist: those that include TFCI and those that do not include TFCI. It
is the UTRAN that determines if a TFCI should be transmitted, hence making it mandatory for all
UEs to support the use of TFCI.
Synchronization Channel (SCH) is a downlink signal used for cell search. The SCH consists of
the Primary and Secondary SCH. The 10 ms radio frames of the Primary and Secondary SCH are
divided into 15 slots, each having a length of 2560 chips.
Physical Downlink Shared Channel (PDSCH) is used to carry the DSCH. A PDSCH corresponds
to a channelization code below or at a PDSCH root channelization code. A PDSCH is allocated on
a radio frame basis to a UE. Within one radio frame, UTRAN may allocate different PDSCHs under
the same PDSCH root channelization code to different UEs based on code multiplexing. Within the
same radio frame, multiple parallel PDSCHs, with the same spreading factor, may be allocated to a
single UE. All the PDSCHs are operated with radio frame synchronization.
The Acquisition Indicator Channel (AICH) is a fixed rate (SF = 256) physical channel used to
carry Acquisition Indicators (AI), which correspond to signatures on the PRACH.
CPCH Access Preamble Acquisition Indicator Channel (AP-AICH) is a fixed rate (SF = 256)
physical channel used to carry the AP acquisition indicators (API) of CPCH. AP AI APIs correspond
to the AP signature transmitted by UE.
CPCH Collision Detection /Channel Assignment Indicator Channel (CD/CA-ICH) is a fixed rate
(SF = 256) physical channel used to carry the CD Indicator (CDI) only if the CA is not active, or to
carry CD Indicator/CA Indicator (CDI/CAI) at the same time if the CA is active.
Paging Indicator Channel (PICH) is to provide terminals with efficient sleep mode operation. In
order to detect the PICH, the terminal needs to obtain the phase reference from the CPICH, and as
with the AICH, the PICH needs to be heard by all the terminals in the cell and thus needs to be sent
at a high power level without power control. The PICH is a fixed rate (SF = 256) physical channel
used to carry the paging indicators. The PICH is always associated with an SCCPCH to which a PCH
transport channel is mapped.
CPCH Status Indicator Channel (CSICH) is a fixed rate (SF = 256) physical channel used to carry
CPCH status information. The CSICH bits indicate the availability of each physical CPCH channel
and are used to tell the terminal to only initiate access on a free channel, but, on the other hand, to
accept a channel assignment command to an unused channel. A CSICH is always associated with a
physical channel used for the transmission of CPCH AP-AICH and uses the same channelization and
scrambling codes.
3.2.7 UTRA Multiplexing and Frame Structure
The frame structure is associated with specific transport or physical channels. In UTRA specifications,
different transport channels are generally given distinct frame structures to fit particular requirements
in terms of their contents, data rates, multiple access schemes, duplex techniques (FDD or TDD),
downlink or uplink, and so on. Therefore, providing detailed information about the frame structures
for all the different transport and physical channels will take up too much room, and thus we are not
allowed to do so in this section. Instead, we should only present some discussions on the generic
UTRA multiplexing and frame structures here, along with some examples.
As shown in Figure 3.20, the detailed operational parameters used in both FDD and TDD modes
are given and then compared. In January 1998, ETSI has decided that the UMTS should be given an
option to operate on two different duplex modes, FDD and TDD modes. The WCDMA technology
was chosen for wide-area services and will use paired FDD bands: 1920–1980 MHz for uplink and
2110–2170 MHz for downlink. On the other hand, TD/CDMA was chosen for private, indoor services
in unpaired TDD bands, that is, 1900–1920 MHz and 2010–2025 MHz.
UTRA-FDD
The UTRA-FDD operation mode is the multiplexing scheme, which separates downlink and uplink
transmissions in different carriers, to implement full-duplex operation in radio links. Several salient
features exist in the UTRA-FDD operation mode as listed below:
• It uses wideband DS-CDMA technology;
• It uses 4.096 Mcps chip rate, which is expandable to 8.192/16.384 Mcps; • It supports asynchronous base stations;
• It uses variable spreading and multicode operation;
• It enables coherent detection in both up- and downlinks;
• It offers optimized packet access on common or dedicated channels.
Figure 3.28 illustrates the frame structure used for the UTRA-FDD operation mode. It is seen
from the figure that the frame length is 10 ms within total 16 slots. The downlink and uplink will
use different multiplexing schemes. The downlink uses time multiplexed control and data frames,
which will be transmitted via I and Q channels in quadrature digital modem. On the other hand, the
uplink will use I/Q code multiplexed control and data frames, in which data signals will be sent
in the Dedicated Physical Data Channel (DPDCH) channel, and control signals will be sent in the
Dedicated Physical Control Channel (DPCCH) channel.
UTRA-FDD can select various ways to map transport channels into physical channels, depending
on the different operational requirements, as shown in Figure 3.29. Figure 3.30 shows the super frame,
frame and slot structures for the UTRA-FDD physical channels, the DPCCH, and the DPDCH.
UTRA-TDD
Time Division Duplex (TDD) mode should use harmonized parameters to UTRA-FDD, such as
chip rate, frame length and slot size, modulation, and so on. Because of the relatively low average
transmission power, UTRA-TDD is intended first and foremost for private, uncoordinated systems,
which can be deployed in unpaired UMTS bands. This is of extreme importance especially for those
countries where the spectral allocation has become very difficult.
Each 0.625 ms slot in UTRA-TDD can be allocated to either uplink or downlink transmissions,
as shown in Figure 3.24. However, at least one slot should be assigned to downlink (BCCH) and
one to uplink (RACH). The same asymmetry and frame synchronization is needed within continuous
areas in coordinated systems. Up to eight codes are used for multiple access in each slot. It allows
multicode transmission. Different users can share the same time slot. Since only a few codes are used
in each time slot, joint detection is supported easily in the UTRA-TDD operation mode.
3.2.8 Spreading and Carrier Modulations
Like any CDMA system, UMTS also needs spreading and carrier modulations. The carrier modulation
is for sending baseband signals into the air through the radio frequency carrier. The spreading
modulation functions as a vehicle to span the spectrum and implement multiple access.
OVSF codes
Both UTRA-FDD and UTRA-TDD use Orthogonal Variable Spreading Factor (OVSF) codes for
spreading modulation. The chip duration of the OVSF codes is Tc. The OVSF codes can be generated
from its unique tree structure. The OVSF codes in UTRA systems perform the following functions:
• Widen the band from 1/Tb to 1/Tc;
• Characterize the users and user services in downlink;
• Characterize the user services in uplink.
The tree structure of OVSF codes is shown in Figure 3.31. There are several important characteristic
features for the OVSF codes:
• They can maintain orthogonality among all the leaves (which is defined as the “end” of each
branch);
• The number of available OVSF codes is exactly equal to the spreading factor (SF);
• They can be completely orthogonal if they operate in exactly synchronized channels.
In the discussions given in this section, we will concentrate on the UTRA-FDD operation mode
due to the constraint on space.
Uplink spreading and modulation
Figure 3.32 illustrates the spreading and modulation for a single uplink DPDCH. Data modulation is
carried out by dual-channel QPSK, where the uplink DPDCH and DPCCH are mapped to the I and
Q branches, which are then spread to the chip rate with two different channelization codes cD/cC
and subsequently complex scrambled by a mobile station–specific complex scrambling code cscramb.
or the Q branch. For each branch, each additional uplink DPDCH should be assigned its own
channelization code. Uplink DPDCHs on different branches may share a common channelization
code.
The spreading and modulation of the message part of the random-access burst is basically the
same as for the uplink dedicated physical channels, as shown in Figure 3.32, where the uplink DPDCH
and uplink DPCCH are replaced by the data part and the control part, respectively. The scrambling
code for the message part is chosen based on the base station-specific preamble code, the randomly
chosen preamble sequence, and the randomly chosen access slot (random-access time-offset). This
guarantees that two simultaneous random-access attempts that use different preamble codes and/or
different preamble sequences will not collide during the data part of the random-access bursts.
The channelization codes of Figure 3.32 are the same type of OVSF codes as used in the downlink,
as shown in Figure 3.33. Each connection is allocated at least one uplink channelization code, to be
used for the uplink DPCCH. In most cases, at least one additional uplink channelization code is
allocated for a uplink DPDCH. Further uplink channelization codes may be allocated if more than
one uplink DPDCH is required. As different mobile stations use different uplink scrambling codes,
the uplink channelization codes may be allocated with no coordination between different connections.
Therefore, the uplink channelization codes are always allocated in a predefined order. The mobile
station and network only need to agree on the number and length (spreading factor) of the uplink
channelization codes.
Either short or long scrambling codes should be used on uplink. The short scrambling code is a
complex code cscramb = cI + jcQ, where cI and cQ are two different codes from the extended Very
Large Kasami set of length 256. The network decides the uplink short scrambling code. The mobile
station is informed as regards the type of short scrambling code to use in the downlink Access Grant
Message that is the base-station response to an uplink Random Access Request. The short scrambling
code may, in rare cases, be changed during a connection.
The long uplink scrambling code is typically used in cells without multiuser detection in the
base station. The mobile station is informed if a long scrambling code should be used in the Access
Grant Message following a Random-Access Request and in the handover message. The type of long
scrambling code to be used is given directly by the short scrambling code. No explicit allocation
of the long scrambling code is thus needed. The scrambling code sequences are constructed as the
position wise modulo-2 sum of 40,960 chip segments of two binary m-sequences generated by means
of two generator polynomials of degree 41. Let x and y be the two m-sequences respectively. The x
sequence is constructed using the primitive (over GF(2)) polynomial 1 + X3 + X41. The y sequence
is constructed using the polynomial 1 + X20 + X41. The resulting sequences thus constitute segments
of a set of Gold sequences. The scrambling code for the quadrature component is a 1024-chip shifted
version of the in-phase scrambling code. The uplink scrambling code word has a period of one radio
frame of 10 ms.
For random access channels, the spreading code for the preamble part is cell-specific and is
broadcast by the base station. More than one preamble code can be used in a base station if the traffic
load is high. The preamble codes must be code planned, since two neighboring cells should not use
the same preamble code. The code used is a real-valued 256 chip Gold code. All 256 codes are used
in the system. The preamble codes are generated in the same way as the codes used for the downlink
synchronization channel.
The modulating chip rate is 4.096 Mcps. This basic chip rate can be extended to 8.192 or
16.384 Mcps. The pulse-shaping filters are root-raised cosine (RRC) with a roll-off factor of α = 0.22
in the frequency domain. QPSK modulation is used.
Downlink spreading and modulation
Figure 3.33 illustrates the spreading and modulation for the downlink DPCH. Data modulation is
QPSK, in which each pair of two bits are serial-to-parallel converted and mapped to the I and Q
branches, respectively. The I and Q branches are then spread to the chip rate with the same channelization
code cch (real spreading) and subsequently scrambled by the same cell specific scrambling
code cscramb (real scrambling). For multicode transmission, each additional downlink DPCH should also be spread/modulated
according to Figure 3.33. Each additional downlink DPCH should be assigned its own channelization
code.
The channelization codes of Figure 3.33 are OVSF codes that preserve the orthogonality between
downlink channels of different rates and spreading factors. The OVSF codes have been defined in
Figure 3.31. Each level in the code tree defines channelization codes of length SF, corresponding to a
spreading factor of SF in Figure 3.33. All codes within the code tree cannot be used simultaneously
within one cell. A code can be used in a cell if and only if no other code on the path from the specific
code to the root of the tree or in the subtree below the specific code is used in the same cell. This
means that the number of available channelization codes is not fixed but depends on the rate and
SF of each physical channel. The channelization code for the BCCH is a predefined code that is the
same for all cells within the system. The channelization code(s) used for the SCCPCH is broadcast
on the BCCH. The channelization codes for the downlink dedicated physical channels are decided
by the network. The mobile station is informed about the type of downlink channelization codes to
receive in the downlink Access Grant Message that is the base-station response to an uplink Random
Access Request. The set of channelization codes may be changed during a connection, typically as a
result of a change of service or an intercell handover. A change of downlink channelization codes is
negotiated over a DCH.
The total number of available scrambling codes is 512, divided into 32 code groups with 16 codes
in each group. The grouping of the downlink codes is done in order to facilitate a fast cell search.
The downlink scrambling code is assigned to the cell (sector) at the initial deployment. The mobile
station learns about the downlink scrambling code during the cell search process. The scrambling code
sequences are constructed as the position wise modulo-2 sum of 40,960 chip segments of two binary
m-sequences generated by means of two generator polynomials of degree 18. Let x and y be the two
sequences respectively. The x sequence is constructed using the primitive (over GF(2)) polynomial
1 + X7 + X18. The y sequence is constructed using the polynomial 1 + X5 + X7 + X10 + X18. The
resulting sequences thus constitute segments of a set of Gold sequences. The scrambling codes are
repeated for every 10 ms radio frame.
The modulating chip rate is 4.096 Mcps. This basic chip rate can be extended to 8.192 or
16.384 Mcps. The pulse-shaping filters are root-raised cosine (RRC) with a roll-off factor of α = 0.22
in the frequency domain.