Third-generation mobile services are now [616885]

Introduction
Third-generation mobile services are now
being introduced throughout the world.Wideband code-division multiple access(WCDMA) technology is an excellentframework for providing these services,since it meets the performance demands ofthe Mobile Internet, including Web access,audio and video streaming, and video andIP multimedia calls. Ericsson has been amain contributor during the developmentof WCDMA technology, and many opera-tors have selected Ericsson as their vendor ofWCDMA infrastructure.
Although the WCDMA technology will
initially only be deployed to cover urbanareas, many operators still feel the need toprovide nationwide coverage from the verystart. The GSM networks have a global foot-print that provides access to mobile services,such as voice, circuit-switched and packet-switched data, short message service (SMS)and multimedia messaging service (MMS).Those operators who already have a GSMnetwork want to capitalize on their invest-ments when migrating to WCDMA tech-nology. A third-generation mobile terminalequipped with both WCDMA and GSMtechnology would put the end-user in con-tact with seamless, (practically) worldwide,mobile service.
Dual-mode WCDMA-GSM mobile ter-
minals of this kind require an interworkingmechanism between the WCDMA andGSM technologies. For instance, if the userhas established a voice call using WCDMAtechnology, and then moves outside ofWCDMA coverage, the voice call needs tobe handed over to GSM without any per-ceived disturbance.
Ericsson was first to demonstrate
WCDMA-to-GSM handover using dual-mode mobile terminals in a live network. In
the demonstration, Ericsson was a total sys-tem provider, supplying the network infra-structure and the mobile terminals. Hand-over is a key feature for ensuring inter-working between WCDMA and GSM tech-nologies. Ericsson has been a main driverduring each phase of the development of in-terworking between WCDMA and GSM,which included research, standardization,product development and multi-vendortesting. Below, we describe in greater detailthe key features of this interworking.
Main challenges
Several challenges had to be overcome toachieve interworking between WCDMAand GSM. First, to get feasible technical so-lutions for the mobile terminal and networkimplementations, some restrictions had tobe set. For example, in the early discussionsit was proposed that the mobile terminalshould be able to have a voice call inWCDMA while sending data in GSM.However, this capability was restricted inthe standard, allowing the mobile terminalto communicate with only one of the tech-nologies at a time (WCDMA or GSM).
Another challenge was to minimize the
changes to the existing GSM infrastructure.The solution encapsulates messages in “con-tainers.” As seen in Figure 1, when the net-work sends a message in WCDMA, to orderhandover to GSM, part of the WCDMAmessage includes a GSM message, whichlooks exactly the same as if it had been senton the GSM radio interface. This part of theWCDMA message is extracted in the mo-bile terminal and processed as if it had beenreceived as a regular GSM message in GSM.The same principle is used for handover fromGSM to WCDMA, and when informationis passed on the interfaces between networknodes.
For handover from GSM to WCDMA, the
length of the handover message is also animportant factor, since handover perfor-mance deteriorates with larger handovermessages. Likewise, the bit rate of the GSMradio interface limits its ability to carry largeWCDMA handover messages. To handlethis situation, instead of signaling each pa-rameter of the actual configuration, the net-work can signal a small size reference to apre-defined WCDMA radio channel con-figuration, which is either stored in the mo-bile terminal (default configuration) or sentto the mobile terminal in broadcast mes-
6 Ericsson Review No. 1, 2003Handover 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
to give fallback coverage for WCDMA technology. This means that sub-scribers can experience seamless services—even with a phased build-outof WCDMA—which is of importance to the commercial launches in 2003.As the leading total system provider, Ericsson has developed technolo-gies that overcome the challenges of interworking between WCDMA andGSM. For example, Ericsson was first to demonstrate handover fromWCDMA to GSM in a live network.
In this article, the authors describe features such as cell re-selection
between WCDMA and GSM, compressed mode measurements, theWCDMA-to-GSM cell-change order, handover from WCDMA to GSM, andhandover from GSM to WCDMA.
3GPP Third-generation Partnership
Project
BSC Base station controllerBSS Base station subsystemCN Core networkGSM Global system for mobile
communication
IP Internet protocolMMS Multimedia messaging serviceMT Mobile terminalPDU Protocol data unitRAT Radio access technologyRBS Radio base stationRF Radio frequencyRNC Radio network controllerSMS Short message serviceSRNS Serving radio network subsystemUMTS Universal mobile
telecommunications system
UTRAN UMTS terrestrial radio access
network
WCDMA Wideband code-division multiple
accessBOX A, TERMS AND ABBREVIATIONS

Ericsson Review No. 1, 2003 7sages. The pre-defined WCDMA radio
channel configuration describes bit rates,data block sizes and other radio parametersof voice or video call service.
Although the network solely communi-
cates with the mobile terminal using one ac-cess technology at a time, the mobile ter-minal needs to perform measurements onGSM while communicating in WCDMAand vice versa. Since WCDMA uses contin-uous transmission and reception in activemode, a regular mobile terminal cannotmeasure GSM cells while communicating inWCDMA. To overcome this obstacle, Ericsson has introduced what it calls thecompressed mode method. As seen in Fig-ure 2, a short gap is created in transmissionand reception. To maintain a perceived con-stant bit rate, the actual transmission bitrate is increased just before and after the gap.A constant bit rate is required for servicessuch as voice, but for Web browsing andsimilar services, a constant bit rate is notnecessary. In the latter case, the transmis-sion can thus be delayed to create a gap.When the mobile terminal is not in activemode it uses discontinuous transmissionand reception, and can therefore measureGSM cells.
Since WCDMA and GSM are different
technologies, it is difficult to compare themeasurement results from these technologies.To overcome this challenge, the measured results are compared with a technology-
specific threshold. Furthermore, additionalparameters, such as adjustable offsets, areprovided to control the selection betweenWCDMA and GSM cells. Where the GSMmeasurement of WCDMA cells is con-cerned, the main challenge is to fit the in-formation into existing GSM messages.
Bit rate=2*R
Bit rate=R Bit rate=R
Radio frame
Idle time for GSM
measurementsFigure 2
Compressed mode creates gaps or idle spaces in time that WCDMA mobile terminals useto perform measurements on GSM cells.
WCDMA info
WCDMA-specific part
of the mobile terminalGSM-specific
part of themobile terminalCommon WCDMA
and GSM part ofthe terminal mobile
GSM handover
message
GSM handover message is
extracted in WCDMA part… …and passed on to GSM partWCDMAhandovermessagetoGSM
GSMhandovermessage
Figure 1
Encapsulation of the GSM handover mes-sage in a “container” that is part of theWCDMA handover message.

As mentioned above, a mobile terminal in
WCDMA makes use of compressed mode tomeasure GSM. That is, if the mobile termi-nal has a single radio receiver, it requirescompressed mode. If, on the other hand, themobile terminal contains separateWCDMA and GSM radio receivers, it canuse each receiver in parallel, performingGSM measurements without compressedmode in the downlink. Notwithstanding,each solution—compressed mode and dualreceivers—reduces talk time due to higherpower consumption in the terminal.
In idle mode, standby time of the mobile
terminal is mainly affected by how often itneeds to wake up to monitor radio channelsand perform measurements for cell re-selection. Since a dual-mode terminal mustmeasure WCDMA and GSM cells, this hasa negative effect on standby time comparedto GSM-only mobile terminals. To improvestandby time, the mobile terminal is al-lowed to inhibit measurements on the otheraccess technology (for example, WCDMAwhen in GSM) when the quality of the cur-rent access technology is adequate for thenetwork settings. Furthermore, comparedto re-selection between GSM cells, the mea-surement requirements in the standard aremore relaxed for re-selection betweenWCDMA and GSM cells.
Mobility procedures for interworking
between WCDMA and GSM
There are two basic modes of operation for
handling mobility: • the mobile terminal-controlled mode;
and
• the network-controlled mode. In the mobile terminal-controlled mode, themobile terminal selects the cell to which it
will connect. However, the network canbroadcast various parameters to influencethis process.
In the network-controlled mode, the net-
work explicitly orders the mobile terminalto connect to a specific cell. Ordinarily, thenetwork bases its decisions on measurementinformation provided by the mobile termi-nal. For either mode of operation, the net-work should consider cells that use each ac-cess technology. Besides radio link quality,the network might also consider other as-pects when selecting the cell, for example,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 cellchange order procedures. These are em-ployed when the mobile terminal uses a ded-icated channel. The handover procedureprovides a higher level of service, since it in-volves a preparation phase in which re-sources in the target cell are reserved priorto the actual handover. Accordingly, thehandover procedure is employed when themobile terminal is providing circuit-switched service—for instance, voice. Thecell change order procedure applies whenthe mobile terminal is providing packet-switched service, such as Web browsing.
Cell re-selection between
WCDMA and GSM
While in WCDMA, the mobile terminal
performs cell re-selection • in idle mode; and • in connected mode when common chan-
nels are used for packet-switched service.
The dual-mode mobile terminal re-selects aGSM cell when that cell is ranked higherthan the current WCDMA cell or any otherWCDMA cell. WCDMA and GSM cells areranked together according to signalstrength. This same type of ranking appliesin GSM.
When performing cell re-selection in
WCDMA, the mobile terminal either mea-sures GSM cells continuously or when thequality of the serving WCDMA cell fallsbelow a given threshold. The mobile termi-nal is solely allowed to select a newWCDMA or GSM cell when the average re-ceived quality and average signal strengthexceed a minimum threshold. The minimum-quality threshold (signal-to-noise ratio) ensures that the mobile termi-nal can receive the information transmitted
8 Ericsson Review No. 1, 2003
Figure 3
Interoperability of mobile terminals andnetwork equipment from different vendorsis crucial for handover between WCDMAand GSM. Ericsson network equipment, incommercial operation in Japan since 2002(here represented by the Indoor MacroRBS 3202), has been publicly shown (at3GSM World Congress in Cannes, Febru-ary 18-21) to interoperate in WCDMA withall major mobile terminal vendors—hererepresented by two Sony Ericsson Z1010terminals.
Core network
UuUmA
Gb
IuMTBSS
RBS
Node
BBSC
RNCUTRANFigure 4
Overview of the GSM and WCDMA nodesand interfaces involved in the cell re-selection and handover procedures.

Ericsson Review No. 1, 2003 9by the potential target cell. The minimum
threshold for signal strength ensures thatthe network can receive the information forcell re-selection transmitted by the mobileterminal in the target cell. This criterionalso takes into account• the maximum transmit power that the
mobile terminal is allowed to use whenaccessing the cell; and
• the maximum radio frequency (RF) out-
put power that the mobile terminal cantransmit.
Frequent re-selections can be avoided withmechanisms such as penalty time and tem-porary offset. Likewise, mechanisms are de-fined to keep fast-moving mobile terminalsfrom re-selecting small-sized cells when alarge overlay cell has been configured.
The network can configure these options
by broadcasting parameters in theWCDMA cell.
When the mobile terminal is actively pro-
viding packet-switched data service in aWCDMA cell and re-selects a GSM cell, itestablishes the radio connection to the GSMbase station subsystem (BSS) and then ini-tiates the routing area update procedure.During this procedure, the core networkmay retrieve information from the UMTSterrestrial radio access network (UTRAN)on the context of the mobile terminal, whichincludes any data packets waiting in thedownlink queue. When complete, the con-nection to UTRAN is released. Finally, thecore network confirms the routing area up-date. Figure 5 shows the message sequenceafter cell re-selection from WCDMA to aGSM cell in idle mode.
WCDMA-to-GSM cell change order
The mobile terminal measures GSM cellsand sends measurement reports to the net-work, which orders the mobile terminal toswitch to GSM. The measurement proce-dure and the use of compressed mode areidentical to that described below for theWCDMA-to-GSM handover procedure.
The signaling in the cell-change-order
procedure is identical to that in the cell re-selection procedure described in Figure 5 ex-cept that the network selects the target GSMcell and initiates the procedure by sendinga cell-change-order from the UTRAN mes-sage. This message includes the informationon the target GSM cell.
Handover from WCDMA to GSM
Figure 6 shows the message sequence forhandover from WCDMA to GSM. Whenthe mobile terminal has a circuit-switched
service and the signal strength falls below agiven threshold, the WCDMA network or-ders the mobile terminal to perform GSMmeasurements. Typically, the mobile ter-minal is instructed to send a measurement
SRNS context requestMT UTRAN CN BSS
Re-selection
of a GSM cell
Routing area update
Routing area update
SRNS context response
SRNS data forward command
Forward PDUs
Iu release command
Iu release complete
Routing area update accept
Figure 5
WCDMA-to-GSM cell re-selection.
Relocation commandMeasurement report
Handover from
UTRAN command
Handover access
Handover access
Handover detect
Handover complete
Handover complete(Handover command)(Handover command)MT UTRAN CN BSS
Handover request
(Inter-RAT handover info)Relocation required
(Inter-RAT handover info)
Handover request ack
(Handover command)
Iu release command
Iu release completeFigure 6
Handover from WCDMA to GSM.

report when the quality of a neighboring
GSM cell exceeds a given threshold and thequality from WCDMA is unsatisfactory.
When UTRAN receives the measure-
ment report message, it initiates the hand-over, given that all the criteria for handoverhave been fulfilled—for example, providedthe mobile terminal is not involved in ser-vices that require WCDMA. UTRAN thenasks the target BSS to reserve resources. Thetarget BSS prepares a handover commandmessage, which includes the details of theallocated resources. This GSM message,which is sent to the mobile terminal via theWCDMA radio interface, is transferredwithin a container that is transparentlypassed on by the different network nodes.
When the mobile terminal receives the
handover command, it moves to the targetGSM cell and establishes the radio connec-tion in accordance with the parameters in-cluded in the handover command message.The mobile terminal indicates successfulcompletion of the handover by sending ahandover complete message to the BSS, afterwhich the GSM network initiates the releaseof the WCDMA radio connection.
Handover from GSM to WCDMA
Figure 7 shows the message sequence forhandover from GSM to WCDMA. The net-work orders the dual-mode mobile terminalto perform WCDMA measurements bysending the measurement information mes-sage, which contains information on neigh-boring WCDMA cells and the criteria forperforming and reporting measurements.
When the criteria for handover to
WCDMA have been met, the BSS initiatesthe allocation of resources to the WCDMAcell. Encapsulated in these messages, the BSSalso sends information to UTRAN on theWCDMA capabilities of the mobile terminal.
When the resources of the WCDMA tar-
get cell have been allocated, UTRAN com-piles the handover-to-UTRAN-commandmessage, which typically includes the iden-tity of the pre-defined configuration for theservice in use. This message is then senttransparently to the mobile terminalthrough the core network and BSS.
When the mobile terminal receives the
handover-to-UTRAN command message ittunes to the WCDMA frequency and beginsradio synchronization. The mobile terminalthen indicates that the handover was suc-cessful by sending the handover-to-UTRAN-complete message, after which theresources in GSM are released.
10 Ericsson Review No. 1, 2003
Measurement information
Handover to UTRAN
completePhysical layer
synchronisationMT UTRAN CN BSS
Handover requiredMeasurement report
Relocation request
Relocation request
acknowledge
(Handover to UTRAN command)
Relocation completeRelocation detectHandover command
(Handover to UTRAN
command)
Clear command
Clear completeInter-system to UTRAN handover command
(Handover to UTRAN command)
Figure 7
Handover from GSM to WCDMA.
Data users
Voice usersService mix allocation
(minimum capacity)Service mix allocation(maximum capacity)Figure 8
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 twoseparate access technologies, blue and red. If we combine these technologies and enable
service-based handover between them, the capacity can vary depending on how the ser-vice is allocated. To achieve maximum capacity, all data users should be allocated to theblue technology and all voice users to the red(dashed black line). Minimum capacity will
result if all data users are allocated to the redtechnology and all voice users to the blue
(solid black line).

Ericsson Review No. 1, 2003 11From coverage fallback to
network optimization
As described above, the first WCDMA com-
mercial networks provide basic coveragefallback to GSM. This fallback is merely thefirst step on the way toward a true seamlessnetwork
2, where WCDMA and GSM, to-
gether with other access technologies, com-bine to form a single network.
There are two important areas in this evo-
lution. The first relates to minimizing theperceived impact on the user when the mo-bile terminal changes between WCDMAand GSM. The Third-generation Partner-ship Project (3GPP) is currently working onenhancements to the WCDMA and GSMstandards (such as inter-system packet handover) that will reduce the actual inter-ruption in user data transfer from seconds tofractions of a second during packet-switchedservice.
The other area relates to the ability of the
system to select the access technology thatis best capable of providing the requestedservice and quality. This includes the trig-ger criteria for moving between GSM andWCDMA access technologies. By trigger-ing a change of radio access technology on,for example, the requested service type, it ispossible to provide the appropriate qualityof service for the call, and to increase theoverall capacity of the system. Figure 8shows the increase in system capacity thatcan be obtained, provided the system—on
the basis of requested services—can allocatetraffic on access technologies.
Conclusion
Thanks to interworking between WCDMAand GSM, users of third-generation mobileterminals can enjoy seamless coverage fromthe very start. The challenges of inter-working between WCDMA and GSM havebeen overcome using • dual-mode mobile terminals;• compressed mode channel measurements;• cell re-selection between WCDMA and
GSM;
• WCDMA-to-GSM cell change order; and • handover between WCDMA and GSM.Ericsson has successfully demonstrated hand-over between WCDMA and GSM using dual-mode mobile terminals in a live network. Thisevent, which required a holistic perspective,was the result of a long-term effort (ten years)in research, standardization, system develop-ment and interoperability testing.
Ericsson is also a total system provider in
the area of WCDMA-to-GSM interwork-ing. The set of features described in this ar-ticle, such as compressed mode and hand-over from WCDMA to GSM, is available inthe Ericsson WCDMA and GSM networkinfrastructure and in the mobile terminalplatform products being commerciallylaunched in 2003.
1 Birkedal, A., Corbett, E., Jamal, K. and
Woodfield, K.: Experiences of operating apre-commercial WCDMA network.Ericsson Review Vol. 79(2002): 2, pp. 50-61.
2 Heicker ö, R. Jelvin, S. and Josefsson, B.:
Ericsson seamless network. EricssonReview Vol. 79(2002):2, pp.76-83.
3 Hedberg, T. and Parkvall, S.: Evolving
WCDMA. Ericsson Review Vol.78(2001):3, pp. 124-131.
4 Almers, P., Birkedal, A., Seungtai, K.
Lundqvist, A. and Mil én, A.: Experiences
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Figure 9
Successful handover between WCDMAand GSM requires a holistic perspective.