1. Handover between WCDMA and GSM
Gertie Alsenmyr, Joakim Bergström, Mattias Hagberg, Anders Milén, Walter Müller, Håkan Palm,
Himke van der Velde, Pontus Wallentin and Fredrik Wallgren
Handover between WCDMA and GSM allows the GSM network to be used mode mobile terminals in a live network. In
to give fallback coverage for WCDMA technology. This means that sub- the demonstration, Ericsson was a total sys-
scribers can experience seamless services—even with a phased build-out tem provider, supplying the network infra-
of WCDMA—which is of importance to the commercial launches in 2003. structure and the mobile terminals. Hand-
As the leading total system provider, Ericsson has developed technolo- over is a key feature for ensuring inter-
working between WCDMA and GSM tech-
gies that overcome the challenges of interworking between WCDMA and
nologies. Ericsson has been a main driver
GSM. For example, Ericsson was first to demonstrate handover from during each phase of the development of in-
WCDMA to GSM in a live network. terworking between WCDMA and GSM,
In this article, the authors describe features such as cell re-selection which included research, standardization,
between WCDMA and GSM, compressed mode measurements, the product development and multi-vendor
WCDMA-to-GSM cell-change order, handover from WCDMA to GSM, and testing. Below, we describe in greater detail
handover from GSM to WCDMA. the key features of this interworking.
Main challenges
Several challenges had to be overcome to
Introduction achieve interworking between WCDMA
Third-generation mobile services are now and GSM. First, to get feasible technical so-
being introduced throughout the world. lutions for the mobile terminal and network
Wideband code-division multiple access implementations, some restrictions had to
(WCDMA) technology is an excellent be set. For example, in the early discussions
framework for providing these services, it was proposed that the mobile terminal
since it meets the performance demands of should be able to have a voice call in
the Mobile Internet, including Web access, WCDMA while sending data in GSM.
audio and video streaming, and video and However, this capability was restricted in
IP multimedia calls. Ericsson has been a the standard, allowing the mobile terminal
main contributor during the development to communicate with only one of the tech-
of WCDMA technology, and many opera- nologies at a time (WCDMA or GSM).
tors have selected Ericsson as their vendor of Another challenge was to minimize the
WCDMA infrastructure. changes to the existing GSM infrastructure.
Although the WCDMA technology will The solution encapsulates messages in “con-
initially only be deployed to cover urban tainers.” As seen in Figure 1, when the net-
areas, many operators still feel the need to work sends a message in WCDMA, to order
provide nationwide coverage from the very handover to GSM, part of the WCDMA
start. The GSM networks have a global foot- message includes a GSM message, which
print that provides access to mobile services, looks exactly the same as if it had been sent
such as voice, circuit-switched and packet- on the GSM radio interface. This part of the
BOX A, TERMS AND ABBREVIATIONS switched data, short message service (SMS) WCDMA message is extracted in the mo-
and multimedia messaging service (MMS). bile terminal and processed as if it had been
3GPP Third-generation Partnership Those operators who already have a GSM received as a regular GSM message in GSM.
Project network want to capitalize on their invest- The same principle is used for handover from
BSC Base station controller
BSS Base station subsystem ments when migrating to WCDMA tech- GSM to WCDMA, and when information
CN Core network nology. A third-generation mobile terminal is passed on the interfaces between network
GSM Global system for mobile equipped with both WCDMA and GSM nodes.
communication technology would put the end-user in con- For handover from GSM to WCDMA, the
IP Internet protocol
MMS Multimedia messaging service
tact with seamless, (practically) worldwide, length of the handover message is also an
MT Mobile terminal mobile service. important factor, since handover perfor-
PDU Protocol data unit Dual-mode WCDMA-GSM mobile ter- mance deteriorates with larger handover
RAT Radio access technology minals of this kind require an interworking messages. Likewise, the bit rate of the GSM
RBS Radio base station mechanism between the WCDMA and radio interface limits its ability to carry large
RF Radio frequency
RNC Radio network controller GSM technologies. For instance, if the user WCDMA handover messages. To handle
SMS Short message service has established a voice call using WCDMA this situation, instead of signaling each pa-
SRNS Serving radio network subsystem technology, and then moves outside of rameter of the actual configuration, the net-
UMTS Universal mobile WCDMA coverage, the voice call needs to work can signal a small size reference to a
telecommunications system
UTRAN UMTS terrestrial radio access
be handed over to GSM without any per- pre-defined WCDMA radio channel con-
network ceived disturbance. figuration, which is either stored in the mo-
WCDMA Wideband code-division multiple Ericsson was first to demonstrate bile terminal (default configuration) or sent
access WCDMA-to-GSM handover using dual- to the mobile terminal in broadcast mes-
6 Ericsson Review No. 1, 2003

2. Common WCDMA
and GSM part of
WCDMA handover message to the terminal mobile
GS M
GSM-specific
WCDMA info part of the
GSM handover mobile terminal
message WCDMA-specific part
of the mobile terminal
GSM handover message is
extracted in WCDMA part... ...and passed on to GSM part
G SM
h a n d o v er m e s s a g e
Figure 1
Encapsulation of the GSM handover mes-
sage in a “container” that is part of the
WCDMA handover message.
sages. The pre-defined WCDMA radio results are compared with a technology-
channel configuration describes bit rates, specific threshold. Furthermore, additional
data block sizes and other radio parameters parameters, such as adjustable offsets, are
of voice or video call service. provided to control the selection between
Although the network solely communi- WCDMA and GSM cells. Where the GSM
cates with the mobile terminal using one ac- measurement of WCDMA cells is con-
cess technology at a time, the mobile ter- cerned, the main challenge is to fit the in-
minal needs to perform measurements on formation into existing GSM messages.
GSM while communicating in WCDMA
and vice versa. Since WCDMA uses contin-
uous transmission and reception in active Figure 2
mode, a regular mobile terminal cannot Compressed mode creates gaps or idle spaces in time that WCDMA mobile terminals use
measure GSM cells while communicating in to perform measurements on GSM cells.
WCDMA. To overcome this obstacle,
Ericsson has introduced what it calls the
compressed mode method. As seen in Fig-
ure 2, a short gap is created in transmission
and reception. To maintain a perceived con-
stant bit rate, the actual transmission bit
rate is increased just before and after the gap.
A constant bit rate is required for services Bit rate=2*R
such as voice, but for Web browsing and
similar services, a constant bit rate is not
necessary. In the latter case, the transmis-
sion can thus be delayed to create a gap. Bit rate=R
Bit rate=R
When the mobile terminal is not in active
mode it uses discontinuous transmission
and reception, and can therefore measure
GSM cells.
Since WCDMA and GSM are different Radio frame
technologies, it is difficult to compare the
measurement results from these technologies. Idle time for GSM
measurements
To overcome this challenge, the measured
Ericsson Review No. 1, 2003 7

3. mobile terminal selects the cell to which it
will connect. However, the network can
broadcast various parameters to influence
this process.
Figure 3 In the network-controlled mode, the net-
Interoperability of mobile terminals and work explicitly orders the mobile terminal
network equipment from different vendors to connect to a specific cell. Ordinarily, the
is crucial for handover between WCDMA
and GSM. Ericsson network equipment, in
network bases its decisions on measurement
commercial operation in Japan since 2002 information provided by the mobile termi-
(here represented by the Indoor Macro nal. For either mode of operation, the net-
RBS 3202), has been publicly shown (at work should consider cells that use each ac-
3GSM World Congress in Cannes, Febru- cess technology. Besides radio link quality,
ary 18-21) to interoperate in WCDMA with
all major mobile terminal vendors—here the network might also consider other as-
represented by two Sony Ericsson Z1010 pects when selecting the cell, for example,
terminals. the current load of the established service.
Two procedures have been defined by
which the network can order the mobile ter-
minal to connect to a cell using another tech-
nology, namely the handover and cell
As mentioned above, a mobile terminal in change order procedures. These are em-
WCDMA makes use of compressed mode to ployed when the mobile terminal uses a ded-
measure GSM. That is, if the mobile termi- icated channel. The handover procedure
nal has a single radio receiver, it requires provides a higher level of service, since it in-
compressed mode. If, on the other hand, the volves a preparation phase in which re-
mobile terminal contains separate sources in the target cell are reserved prior
WCDMA and GSM radio receivers, it can to the actual handover. Accordingly, the
use each receiver in parallel, performing handover procedure is employed when the
GSM measurements without compressed mobile terminal is providing circuit-
mode in the downlink. Notwithstanding, switched service—for instance, voice. The
each solution—compressed mode and dual cell change order procedure applies when
receivers—reduces talk time due to higher the mobile terminal is providing packet-
power consumption in the terminal. switched service, such as Web browsing.
In idle mode, standby time of the mobile
terminal is mainly affected by how often it Cell re-selection between
needs to wake up to monitor radio channels WCDMA and GSM
and perform measurements for cell re- While in WCDMA, the mobile terminal
Figure 4
Overview of the GSM and WCDMA nodes
selection. Since a dual-mode terminal must performs cell re-selection
and interfaces involved in the cell re- measure WCDMA and GSM cells, this has • in idle mode; and
selection and handover procedures. a negative effect on standby time compared • in connected mode when common chan-
to GSM-only mobile terminals. To improve nels are used for packet-switched service.
standby time, the mobile terminal is al- The dual-mode mobile terminal re-selects a
BSS lowed to inhibit measurements on the other GSM cell when that cell is ranked higher
access technology (for example, WCDMA than the current WCDMA cell or any other
RBS BSC when in GSM) when the quality of the cur- WCDMA cell. WCDMA and GSM cells are
rent access technology is adequate for the ranked together according to signal
A network settings. Furthermore, compared strength. This same type of ranking applies
Gb
Um
to re-selection between GSM cells, the mea- in GSM.
surement requirements in the standard are When performing cell re-selection in
more relaxed for re-selection between WCDMA, the mobile terminal either mea-
Core network WCDMA and GSM cells. sures GSM cells continuously or when the
MT
quality of the serving WCDMA cell falls
Uu Mobility procedures for interworking below a given threshold. The mobile termi-
between WCDMA and GSM nal is solely allowed to select a new
Iu There are two basic modes of operation for WCDMA or GSM cell when the average re-
UTRAN handling mobility: ceived quality and average signal strength
• the mobile terminal-controlled mode; exceed a minimum threshold. The
Node RNC and minimum-quality threshold (signal-to-
B
• the network-controlled mode. noise ratio) ensures that the mobile termi-
In the mobile terminal-controlled mode, the nal can receive the information transmitted
8 Ericsson Review No. 1, 2003

4. by the potential target cell. The minimum
threshold for signal strength ensures that MT UTRAN CN BSS
the network can receive the information for
cell re-selection transmitted by the mobile Re-selection
terminal in the target cell. This criterion of a GSM cell
also takes into account Routing area update
• the maximum transmit power that the Routing area update
mobile terminal is allowed to use when
SRNS context request
accessing the cell; and
• the maximum radio frequency (RF) out-
SRNS context response
put power that the mobile terminal can
transmit.
SRNS data forward command
Frequent re-selections can be avoided with
mechanisms such as penalty time and tem-
Forward PDUs
porary offset. Likewise, mechanisms are de-
fined to keep fast-moving mobile terminals
Iu release command
from re-selecting small-sized cells when a
large overlay cell has been configured.
Iu release complete
The network can configure these options
by broadcasting parameters in the
Routing area update accept
WCDMA cell.
When the mobile terminal is actively pro-
viding packet-switched data service in a Figure 5
WCDMA cell and re-selects a GSM cell, it WCDMA-to-GSM cell re-selection.
establishes the radio connection to the GSM
base station subsystem (BSS) and then ini-
tiates the routing area update procedure.
During this procedure, the core network the mobile terminal has a circuit-switched
may retrieve information from the UMTS service and the signal strength falls below a
terrestrial radio access network (UTRAN) given threshold, the WCDMA network or-
on the context of the mobile terminal, which ders the mobile terminal to perform GSM
includes any data packets waiting in the measurements. Typically, the mobile ter-
downlink queue. When complete, the con- minal is instructed to send a measurement
nection to UTRAN is released. Finally, the
core network confirms the routing area up-
date. Figure 5 shows the message sequence Figure 6
after cell re-selection from WCDMA to a Handover from WCDMA to GSM.
GSM cell in idle mode.
MT UTRAN CN BSS
WCDMA-to-GSM cell change order
The mobile terminal measures GSM cells Measurement report
and sends measurement reports to the net- Relocation required
work, which orders the mobile terminal to (Inter-RAT handover info) Handover request
switch to GSM. The measurement proce- (Inter-RAT handover info)
dure and the use of compressed mode are
Handover request ack
identical to that described below for the Handover from Relocation command (Handover command)
WCDMA-to-GSM handover procedure. UTRAN command (Handover command)
The signaling in the cell-change-order (Handover command)
procedure is identical to that in the cell re- Handover access
selection procedure described in Figure 5 ex- Handover access
cept that the network selects the target GSM
cell and initiates the procedure by sending Handover detect
a cell-change-order from the UTRAN mes-
sage. This message includes the information Handover complete
on the target GSM cell. Handover complete
Iu release command
Handover from WCDMA to GSM
Iu release complete
Figure 6 shows the message sequence for
handover from WCDMA to GSM. When
Ericsson Review No. 1, 2003 9

5. report when the quality of a neighboring
MT UTRAN CN BSS
GSM cell exceeds a given threshold and the
Measurement information quality from WCDMA is unsatisfactory.
Measurement report When UTRAN receives the measure-
ment report message, it initiates the hand-
Handover required
Relocation request over, given that all the criteria for handover
Relocation request
have been fulfilled—for example, provided
acknowledge Handover command the mobile terminal is not involved in ser-
(Handover to UTRAN
(Handover to UTRAN command)
command)
vices that require WCDMA. UTRAN then
Inter-system to UTRAN handover command
asks the target BSS to reserve resources. The
(Handover to UTRAN command) target BSS prepares a handover command
message, which includes the details of the
Physical layer allocated resources. This GSM message,
synchronisation Relocation detect which is sent to the mobile terminal via the
WCDMA radio interface, is transferred
within a container that is transparently
Handover to UTRAN
complete Relocation complete
passed on by the different network nodes.
When the mobile terminal receives the
Clear command
handover command, it moves to the target
Clear complete GSM cell and establishes the radio connec-
tion in accordance with the parameters in-
cluded in the handover command message.
Figure 7 The mobile terminal indicates successful
Handover from GSM to WCDMA.
completion of the handover by sending a
handover complete message to the BSS, after
which the GSM network initiates the release
of the WCDMA radio connection.
Handover from GSM to WCDMA
Figure 8 Figure 7 shows the message sequence for
System capacity can be increased by selecting service-based radio access technology.
The blue and red triangles describe system capacity to handle voice and data traffic in two handover from GSM to WCDMA. The net-
separate access technologies, blue and red. If we combine these technologies and enable work orders the dual-mode mobile terminal
service-based handover between them, the capacity can vary depending on how the ser- to perform WCDMA measurements by
vice is allocated. To achieve maximum capacity, all data users should be allocated to the sending the measurement information mes-
blue technology and all voice users to the red (dashed black line). Minimum capacity will
result if all data users are allocated to the red technology and all voice users to the blue
sage, which contains information on neigh-
(solid black line). boring WCDMA cells and the criteria for
performing and reporting measurements.
Data users When the criteria for handover to
Service mix allocation WCDMA have been met, the BSS initiates
(maximum capacity) the allocation of resources to the WCDMA
cell. Encapsulated in these messages, the BSS
also sends information to UTRAN on the
WCDMA capabilities of the mobile terminal.
Service mix allocation When the resources of the WCDMA tar-
(minimum capacity) get cell have been allocated, UTRAN com-
piles the handover-to-UTRAN-command
message, which typically includes the iden-
tity of the pre-defined configuration for the
service in use. This message is then sent
transparently to the mobile terminal
through the core network and BSS.
When the mobile terminal receives the
handover-to-UTRAN command message it
tunes to the WCDMA frequency and begins
radio synchronization. The mobile terminal
then indicates that the handover was suc-
cessful by sending the handover-to-
UTRAN-complete message, after which the
Voice users resources in GSM are released.
10 Ericsson Review No. 1, 2003

6. Roadmapping/customer interaction/product management
System function Node design
design RNC, RBS,
BSC, MSC, GSN
Ericsson Research
Handover between Integration and
WCDMA and verification
GSM
Standardization Interoperability
• 3GPP TSG RAN Terminal vendors, • Test specification
• 3GPP TSG T • System plays
test equipment
• GCF (Global Certification • Test execution
Forum) • World’s 1st WCDMA-GSM
• Ericsson Mobile Platforms handover, Sep 23, 2002 Figure 9
• Sony Ericsson by Ericsson Successful handover between WCDMA
• All other major terminal vendors and GSM requires a holistic perspective.
From coverage fallback to can be obtained, provided the system—on
the basis of requested services—can allocate
network optimization traffic on access technologies.
As described above, the first WCDMA com-
mercial networks provide basic coverage
fallback to GSM. This fallback is merely the
Conclusion
first step on the way toward a true seamless Thanks to interworking between WCDMA
network2, where WCDMA and GSM, to- and GSM, users of third-generation mobile
gether with other access technologies, com- terminals can enjoy seamless coverage from
bine to form a single network. the very start. The challenges of inter-
There are two important areas in this evo- working between WCDMA and GSM have
lution. The first relates to minimizing the been overcome using
perceived impact on the user when the mo- • dual-mode mobile terminals;
bile terminal changes between WCDMA • compressed mode channel measurements;
and GSM. The Third-generation Partner- • cell re-selection between WCDMA and
ship Project (3GPP) is currently working on GSM;
enhancements to the WCDMA and GSM • WCDMA-to-GSM cell change order; and
standards (such as inter-system packet • handover between WCDMA and GSM.
handover) that will reduce the actual inter- Ericsson has successfully demonstrated hand- REFERENCES
ruption in user data transfer from seconds to over between WCDMA and GSM using dual-
fractions of a second during packet-switched mode mobile terminals in a live network. This
1 Birkedal, A., Corbett, E., Jamal, K. and
service. event, which required a holistic perspective, Woodfield, K.: Experiences of operating a
The other area relates to the ability of the was the result of a long-term effort (ten years) pre-commercial WCDMA network.
system to select the access technology that in research, standardization, system develop- Ericsson Review Vol. 79(2002): 2, pp. 50-
is best capable of providing the requested ment and interoperability testing. 61.
2 Heickerö, R. Jelvin, S. and Josefsson, B.:
service and quality. This includes the trig- Ericsson is also a total system provider in Ericsson seamless network. Ericsson
ger criteria for moving between GSM and the area of WCDMA-to-GSM interwork- Review Vol. 79(2002):2, pp.76-83.
WCDMA access technologies. By trigger- ing. The set of features described in this ar- 3 Hedberg, T. and Parkvall, S.: Evolving
ing a change of radio access technology on, ticle, such as compressed mode and hand- WCDMA. Ericsson Review Vol.
for example, the requested service type, it is over from WCDMA to GSM, is available in 78(2001):3, pp. 124-131.
4 Almers, P., Birkedal, A., Seungtai, K.
possible to provide the appropriate quality the Ericsson WCDMA and GSM network Lundqvist, A. and Milén, A.: Experiences
of service for the call, and to increase the infrastructure and in the mobile terminal of the live WCDMA network in Stockholm,
overall capacity of the system. Figure 8 platform products being commercially Sweden. Ericsson Review Vol. 77(2000):4,
shows the increase in system capacity that launched in 2003. pp. 204-215.
Ericsson Review No. 1, 2003 11