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◆ Multi-Radio Infrastructure for 4GRolf Sigle, Oliver Blume, Lutz Ewe, and Wieslawa Wajda
Future multi-radio infrastructure for the fourth generation (4G) will combineexisting radio network technologies into a common communication system.For such an infrastructure, we propose a multi-radio management (MRM)that enables optimized access selection and handover control for mobileusers and increases the overall efficiency of network resources. This paperpresents the MRM concept by its functional architecture, its main andsupporting functions, the migration of the 3rd Generation PartnershipProject (3GPP) system architecture evolution, and last but not least, resultsfrom a demonstrator and simulations. MRM comprises multi-radio resourcemanagement (MRRM) and multi-radio mobility management (MRMM),which are tightly coupled to corresponding legacy subsystems through anabstraction and adaptation layer (AAL). In this way, MRM interworks withdifferent radio access technologies (RATs) and extends their legacy functionswith multi-radio control and sophisticated management based on genericmechanisms and virtualized interfaces. MRM is scalable due to thedistribution of MRM across network nodes, and future RATs can be addedseamlessly due to the AAL concept. © 2009 Alcatel-Lucent.
Bell Labs Technical Journal 13(4), 257–276 (2009) © 2009 Alcatel-Lucent. Published by Wiley Periodicals, Inc.Published online in Wiley InterScience (www.interscience.wiley.com) • DOI: 10.1002/bltj.20348
IntroductionToday, many different radio access technologies
(RATs) such as Global System for Mobile Communica-
tions* (GSM*), Universal Mobile Telecommunications
System (UMTS), code division multiple access 2000
(CDMA2000), or Worldwide Interoperability for
Microwave Access (WiMAX) [22] coexist in parallel—
quite often in an overlaying deployment by the same
operator. These operators demand network solutions
that provide their subscribers with ubiquitous mobile
services in overlay cells, hotspots, indoor-coverage, and
added cells with newly introduced wireless technolo-
gies. The benefit from an interworking of those differ-
ent technologies is constrained, however, by the fact
that inter-RAT handovers of ongoing services are inade-
quately supported today. An interworking between
3rd Generation Partnership Project (3GPP*) RATs, i.e.,
GSM/General Packet Radio Service (GPRS), UMTS, and
the upcoming Long Term Evolution (LTE), is currently
being standardized [5, 7, 8], but inter-RAT mobility with
selected non-3GPP technologies (CDMA2000 and
WiMAX) is only envisaged for the latest LTE technol-
ogy [6]. The interworking between different non-3GPP
technologies is even more limited. Furthermore, these
developed or prepared solutions only deal with the
interworking between two considered technologies at
a time. Some standardization bodies, such as the
Institute of Electrical and Electronics Engineers (IEEE)
with standard draft 802.21 [26], started to address the
problem of a general interworking solution. This paper
presents a multi-radio management concept developed
Panel 1. Abbreviations, Acronyms, and Terms
3GPP—3rd Generation Partnership Project3GPP2—3rd Generation Partnership Project 24G—Fourth generationAAL—Abstraction and adaptation layerANDSF—Access network discovery and
selection functionAP—Access pointBMBF—German Federal Ministry of Research
and EducationBSC—Base station controllerCDMA2000—Code division multiple access
2000CMM—Communication and module managerCN—Core networkCODEC—Coder decoderCQI—Channel quality indicatorDAD—Duplicate address detectionDHCP—Dynamic Host Configuration ProtocolDL—DownlinkeNodeB—Evolved node BePDG—Enhanced packet data gatewayEU—European UnionFMIP—Fast Mobile IPFTP—File Transfer ProtocolGSM—Global System for Mobile
CommunicationsGPRS—General packet radio serviceHSDPA—High speed downlink packet accessHTTP—Hypertext Transfer ProtocolIEEE—Institute of Electrical and Electronics
EngineersIMS—IP Multimedia SubsystemIP—Internet ProtocolIPsec—Internet Protocol securityIPv4—Internet Protocol version 4IPv6—Internet Protocol version 6L2—Layer 2L3—Layer 3LLC—Link layer controlLTE—Long Term EvolutionMIP—Mobile IPMIPv4—MIP version 4
MIPv6—MIP version 6MRM—Multi-radio managementMRM-HAM—MRM heterogeneous access
managementMRMM—Multi-radio mobility managementMRM-NET—MRM radio access networkMRM-TE—MRM terminal entityMRRM—Multi-radio resource managementmsec—MillisecondO&M—Operations and maintenanceOFDM—Orthogonal frequency division
multiplexingOFDMA—Orthogonal frequency division
multiple accessPMIP—Proxy Mobile IPQoS—Quality of serviceRAN—Radio access networkRAS—Radio access selectionRAT—Radio access technologyRel—ReleaseRF—Radio frequencyRNC—Radio network controllerRRC—Radio resource controlRSSI—Received signal strength indicatorRTP—Real Time Transport ProtocolSAE—System architecture evolutionScaleNet—Scalable, Efficient and Flexible
NetworksSINR—Signal-to-interference-plus-noise ratioSIP—Session Initiation ProtocolTDMA—Time division multiple accessUE—User equipmentUL—UplinkUMTS—Universal Mobile Telecommunications
SystemWAG—Wireless access gatewayWIGWAM—Wireless Gigabit with Advanced
Multimedia SupportWiMAX—Worldwide Interoperability for
Microwave AccessWLAN—Wireless local area network
258 Bell Labs Technical Journal DOI: 10.1002/bltj
in Alcatel-Lucent Bell Labs in the framework of coop-
erative national and international projects, such as the
European Union’s (EU’s) Ambient Networks [12–14,
28] project, and the German Federal Ministry of
Research and Education’s (BMBF’s) Scalable, Efficient
and Flexible Networks (ScaleNet) [16] and Wireless
Gigabit with Advanced Multimedia Support (WIG-
WAM) projects [15, 17, 18].
DOI: 10.1002/bltj Bell Labs Technical Journal 259
MRM OverviewToday’s multi-radio interworking approach,
which specifies individual solutions between two indi-
vidual RATs, leads to increasing complexity when
additional technologies are envisaged for interwork-
ing. As an alternative, we propose the creation of a
generic management mechanism for all inter-RAT rele-
vant tasks between any considered radio access tech-
nologies. This solution minimizes the impact on
existing radio technologies, reduces the efforts for
standardization and implementation, and improves
time-to-market for operator deployment.
The multi-radio management (MRM) concept
presented in this paper envisions such a framework
for the integration of multiple RATs in an operator
network. MRM aims to provide the multi-radio infra-
structure with:
• Flexibility for operators to deploy different RATs
in their network,
• Full mobility support between RATs to allow
selection of the most appropriate RAT at any time,
• A common resource management platform
enabling the operator to optimize network usage
and revenue, and
• Minimized requirements for user equipment (UE).
MRM does not aim to be a replacement but
instead an extension of existing heterogeneous radio
access deployments, and thus creates an overlay per-
forming the integration of different RATs. Figure 1shows the functional separation of MRM into multi-
radio resource management (MRRM), which is inter-
working with technology-specific radio resource
management, and multi-radio mobility management
(MRMM), which is in charge of the interworking
between existing technology-specific mobility man-
agement and the Internet Protocol (IP)-based mobil-
ity towards non-3GPP air interfaces. An intermediate
abstraction and adaptation layer (AAL) performs the
interworking of these generic MRM procedures with
the “legacy” RAT functions, hiding the RAT-specific
mechanisms and control functions from the MRM.
Furthermore, it provides a translation of RAT-specific
parameters into generalized parameters that are han-
dled by the MRM. Thus, the management of the
multi-radio infrastructure for fourth generation (4G)
is decoupled from the evolution of air interfaces
defined by different standardization bodies.
MRM FunctionsThe new MRM functionality enables the man-
agement of multiple radio technologies in future 4G
networks. In the following sections, the different com-
ponents and functions of the MRM are described in
detail.
MRM Main TasksMulti-radio management can be separated into
four main tasks which constitute the MRM function-
ality on top of the RAT-specific procedures:
• Collection and provisioning of neighborhood informa-
tion. The MRM collects and provides information
on the network topology.
• Radio access advertisement. The MRM informs idle
mobile terminals about available neighbor cells
of the technology in use and of other RATs.
• Radio access selection. The MRM provides mobile
terminals in connected mode with measurement
Abstraction
Radio equipment/modem
MRM
RAT control
MRMM MRRM
MRM—Multi-radio managementMRMM—Multi-radio mobility managementMRRM—Multi-radio resource managementRAT—Radio access technology
Figure 1.MRM concept for integration of different accesstechnologies by abstraction of the RAT-specificfunctions and parameters.
260 Bell Labs Technical Journal DOI: 10.1002/bltj
configuration and triggers handovers. This
process, denoted as RAS, takes all available radio
technologies into consideration and regards qual-
ity of service (QoS) requirements for the ongo-
ing service, as well as the radio resource usage in
the serving and in candidate technologies.
• Mobility management. The MRM controls the hand-
over sequences that are applied during a change
of the serving RAT.
The primary tasks listed above are supported by
supplemental tasks which, for instance, handle the
configuration, monitoring, and reporting of measure-
ments. In subsequent sections, the MRM tasks will be
considered in detail.
Neighborhood information collection and provisioning.Although the distribution of information about neigh-
boring cells of the same network/technology is already
a part of each 3GPP radio access technology, there is no
mechanism that covers this information distribution at
the inter-RAT level. In order to perform multi-radio
management, comprehensive knowledge about the
present state of all overlapping RANs is indispensable.
This task is achieved by informing the MRM about the
registration and de-registration of cells and their respec-
tive properties. The MRM collects the data, calculates
neighborhood relations from coverage information, and
provides the result as neighborhood information
together with RAT-specific access information.
MRM supplies the managed RANs with informa-
tion about adjacent cells of different RATs which is
used as input for the advertisement and measurement
configuration functions. This enables the terminal to
scan efficiently for pilot channels of neighboring cells,
minimizing battery consumption.
Advertisement. A terminal in idle mode has no
active connection to the network. Hence, the initial
access selection in idle mode must be performed by
the terminal itself. The advertisement process supplies
a terminal with neighborhood information, allowing
it to scan other frequencies and to make an access
selection autonomously. This advertisement can be
done using one of two alternative mechanisms: 1) a
broadcast in RAT-specific system broadcast channels,
or 2) a request/response mechanism between the ter-
minal and the MRM entity in the network.
A generic mechanism is proposed to avoid stan-
dardization issues regarding changes to the system
broadcast for different RATs. Instead, the radio access
information of advertised cells is sent on a broadcast
channel as a container list where the RAT-specific
neighbor cell information is included transparently.
Each container is equipped with a RAT-indicating tag
for identification by the decoding terminal. The bene-
fit of such a generic cell broadcast list is an intrinsic
extensibility to further RATs, since a decoding termi-
nal does not have to have knowledge about the con-
tainer content of unsupported technologies.
Additional measurement parameters or assistance
data that parameterizes the access selection algorithm
in the terminal may be distributed by the advertise-
ment as well, according to the operator deployment
and user subscription model.
Radio access selection. Generally, radio access
selection denotes the process of choosing for a termi-
nal the most appropriate access technology and cell
within a heterogeneous network for a requested ser-
vice at a given location, in terms of configured opera-
tor and user criteria.
RAS is a challenging task as the suitability of dif-
ferent radio technologies varies by service. For mov-
ing users, detection of entering or exiting broadband
hotspot coverage has to be performed fast and effi-
ciently. However, inter-technology handover at
hotspot detection is not always the most reasonable
alternative [9]. Real time services may not be sup-
ported by a wireless local area network (WLAN) [21]
hotspot or fast moving mobile terminals may reside
within a hotspot for only a small duration of time.
Next to the provisioning of best connectivity to
the user, access selection also offers operators the
potential of optimizing network usage and maximiz-
ing revenue. It is known that an inefficient service
distribution between the different access systems leads
to overload on some access systems, even if resources
in alternative access systems are still available.
Moreover, depending on the link performance and
the service type, the resource costs of different access
technologies may differ, leading to a potential for
overall resource usage optimization and hence a
capacity increase.
DOI: 10.1002/bltj Bell Labs Technical Journal 261
The RAS function comprises the access selection
in idle mode (including initial access selection), and
access selection in connected mode.
During the switch-on procedure, the terminal scans
for radio networks and selects a cell of a certain RAT on
the basis of a stored RAT preference list. The terminal
will stay in idle mode if no service is requested. Because
a terminal in idle mode has no active connection to
the network, the access selection in idle mode must
take place in the terminal itself using the neighborhood
information and assistance data received via network
advertisement as mentioned above.
After the terminal requests a service, any further
RAS will be performed in the access network since it
can apply network status information and policies in
addition to the measurements received from the ter-
minal to assess the most suitable radio access.
Additionally, the network receives threshold-based
triggers and events indicating relevant changes in the
system that require making a new RAS decision. Those
events include detection of a hotspot, dropping of a
session, or crossing of link performance/load threshold.
The algorithms applied for the RAS decision make
use of various parameters such as cell load and capac-
ity, candidate link characteristics, user and operator
preferences, and radio efficiency of different services,
which are obtained from different sources in RANs,
the terminal, and by operations and maintenance
(O&M) or policy databases.
The application of a sophisticated RAS algorithm
is decisive for MRM operation. Load balancing RAS
algorithms focus mainly on the resource usage in each
RAT in an effort to exploit all available resources. The
utility maximization based RAS algorithm [10]
designed in this work additionally takes into account
the resource costs of each RAT, based on service qual-
ity and link performance. This leads to a further
increase of system throughput, compared to load bal-
ancing RAS algorithms. Moreover, the approach has
low signaling needs even for decentralized operation,
while being auto adapting and operator tuneable
(throughput-fairness trade-off).
Heterogeneous mobility management. When a RAS
decision is made that changes the serving technology,
the handover will involve a change of the radio interface
and a change of the IP connectivity. The execution of
such an IP layer handover is the task of MRMM.
Depending on the capabilities of the terminal and of the
source and target RANs, a variety of mobility protocols
may be available, e.g., MIP version 4 (MIPv4) [31], MIP
version 6 (MIPv6) [27], Proxy Mobile IP (PMIP) [19],
or Fast Mobile IP (FMIP) [29]. Furthermore, different
handover sequences can be selected: If the mobile device
is capable of sustaining multiple radio connections simul-
taneously, the current user plane connectivity may be
retained until the new connectivity is ready (make-
before-break) instead of disrupting the current radio link
(break-before-make).
When MRRM indicates an RAS event, MRMM
selects the proper handover protocol from the availa-
ble protocols. The MRMM handles the IP mecha-
nisms, while the MRRM is charged with assignment
of radio resources. For initial attachment, as shown in
Figure 2, MRRM attaches the radio link before ini-
tialization of IP. For a handover, MRMM requests
attachment and detachment of the radio links during
the selected handover sequence. As an example,
Figure 2 shows the sequence for a make-before-break
handover.
For both connection establishment and handover
execution, the MRMM must interact with IP kernel
functions for the selection of a valid IP locator on the
new interface, and with the Mobile IP protocol for a
“binding update” of the IP address known as a “care of
address” at the terminal’s home agent. The IP
kernel performs the IP address configuration with
Stateless Address Autoconfiguration, Dynamic Host
Configuration Protocol (DHCP), and Duplicate
Address Detection (DAD). In addition, the handover
event may result in a change of QoS which is reported
to the session layer for coder decoder (CODEC) adap-
tation, i.e., MRM interacts with Session Initiation
Protocol (SIP) and IP Multimedia Subsystem (IMS) to
benefit from the increased bandwidth after handover
into a hotspot or to change to a transmission CODEC
with lower bandwidth before handover to a large
overlay cell. Thus, it can be seen that for a smooth
handover execution, MRMM and MRRM are closely
cooperating and are performing cross layer synchro-
nization between layer 2 (L2), layer 3 (L3), and even
262 Bell Labs Technical Journal DOI: 10.1002/bltj
the application layer, to minimize packet loss and hand-
over latency.
Using neighborhood information and configura-
tion of multi-radio measurements for candidate and
serving link access selection, decisions can be made
proactively before radio connectivity is lost. This
enables further optimization of handover perfor-
mance: some time-consuming parts of the handover
sequence may be performed proactively before the
actual handover event. In this case, the handover
sequence is separated into three phases.
1. The pre-registration phase comprises authentication
and authorization. Advantageously, for single-
radio terminals these steps are supported by the
serving access network with a transparent con-
tainer over the current radio link [6], so that pre-
registration and preparation can start even before
reaching coverage area of the target system. The
preregistration state does not consume resources
and can be sustained for a long time.
2. The handover preparation phase can include radio
resource preparation (e.g., obtaining a descrip-
tion of the target radio channel or UMTS
downlink (DL) scrambling code), as well as IP-
connectivity preparation (IP address assignment).
Handover preparation is performed on a timelier
basis than preregistration, and may have a
restricted lifetime. If there is no assurance that
the ensuing handover will be executed shortly
or at all, then critical resources, like codes, time
slots, or memory for context transfer shall not
yet be reserved.
3. During handover execution the final steps are taken.
This phase involves context transfer, resource
assignment and data forwarding mechanisms in
the access network.
A variety of triggering events can cause the three
phases to be executed. They can be executed consecu-
tively or repeatedly on demand, as coverage of target
cells becomes available or unavailable. Simulation
To abstraction:attach newradio link
MRMM MRRM
HOexecution
Locator
HO protocol select
To session manager:HO occurred, new QoS
Locator select
LinkAttach
LinkDetach
HO finalized
To MIPv6:binding update
To abstraction:detach oldradio link
MRRMLocator selection MRMM
IPConnectionrequest
Locator
LocatorSelect
IP initialization
To abstraction:attach radio link
IPConnectionavailable
To MIPv6:binding update
IP protocol select
HO protocol exec.
exec.—ExecutionHO—HandoverIP—Internet ProtocolMIPv6—Mobile IP version 6
MRM—Multi-radio managementMRMM—Multi-radio mobility managementMRRM—Multi-radio resource managementQoS—Quality of service
(a) Connection establishment (b) Make-before-break handover
Locator selection
Figure 2.Signaling for initial attachment and handover.
DOI: 10.1002/bltj Bell Labs Technical Journal 263
results show that proactive preparation of the most
attractive target cell can significantly increase the
probability of seamless execution of the handover [9].
Thus, using MRM with proactive RAS and a separa-
tion of handover phases reduces the need for cell
overlap at network planning, and can thereby reduce
deployment costs.
MRM measurements. Up-to-date information is
essential to enable well-informed MRM decisions.
This information includes the current state of the
radio access networks and the current state of the radio
links of the mobile terminals served within these
networks.
Measurements of link performance include both
the quality of established links, and the signal strength
and quality of candidate links of multiple technologies
(inter-RAT measurements). The measurement tasks
comprise:
• Determination of potential candidate cells of a
neighboring RAT,
• Setting of thresholds and measurement periods,
• Configuration and initiation of the measure-
ments,
• Measurement result monitoring including filter-
ing, averaging, and threshold supervision, and
• Measurement reporting according to configured
events.
The reported measurements are used on the net-
work side for decisions about establishing and chang-
ing of radio access, and collected for statistics and
predictions in RAS algorithms. The mobile-based
MRM measurements above are also used in the ter-
minal for RAS in idle mode mobility.
In addition to measurements of radio link quality,
MRM is in charge of determining up-to-date system
load states, since the load is considered for RAS strat-
egy. For this purpose, resource measurements are per-
formed by the radio access network nodes. Analogous
to the link measurements, the MRM either makes use
of existing resource measurement procedures in the
considered RAT, or it supplies the functionality by
itself. Resource measurements performed in the net-
work nodes, e.g., cell load measurements, are
reported to the serving MRM component according to
the configured reporting events, such as load thresh-
olds exceeded in a cell.
For both radio link measurements and network
resource measurements, the abstraction and adapta-
tion mechanisms described below provide independ-
ence of MRM functionality from the controlled radio
technology.
Distribution of MRM FunctionalityModern wireless networks distinguish between
entities such as the mobile terminal, the radio access
network, and the core network (CN), which in turn
consist of a number of hierarchical nodes, such as the
base station, the radio network controller (RNC), and
the mobility and session controller. Likewise, MRM
functionality is distributed over those nodes and over
particular access technologies.
MRM functionality is distributed to:
• The MRM terminal entity (MRM-TE) subsystem,
• The MRM radio access network (MRM-NET) sub-
system, and
• MRM heterogeneous access management (MRM-
HAM).
The MRM-TE subsystem handles access selection
in idle state, measurement reporting, configuration
of routing towards the selected air interface, and
finally, the interface to terminal-based IP mobility
(e.g., Mobile IPv4/6) for handover support between
3GPP and non-3GPP air interfaces.
The main task of the MRM-NET subsystem is to trig-
ger the access selection, based on terminal and network
measurements in cooperation with MRM-HAM. Other
functions include triggering the radio access bearer setup
and release, configuring inter-RAT measurements, and
also advertising and discovering accesses (in coopera-
tion with HAM). The MRM-NET has an interface func-
tion toward 3GPP resource and mobility management in
RAN controllers to trigger handover between 3GPP air
interfaces. MRM-NET decisions are based on static con-
figuration or dynamic policies loaded from MRM-HAM.
MRM-HAM is mainly responsible for RAS and
support of network advertisement and network neigh-
borhood information collection and provisioning. The
RAS decision is based on link performance, service
requirements, operator policies, terminal capabilities,
and user profiles. Moreover, it utilizes network status
information collected from all of the involved RATs, as
well as statistical and prediction information.
264 Bell Labs Technical Journal DOI: 10.1002/bltj
The distribution of MRM components in the ter-
minal, radio network, and core network and their
links to other functions is shown in Figure 3. In
this layered model, MRM operates on top of the
technology-specific radio management layer functions
and interacts with higher layer functions such as the
session and mobility management, as well as with
application layer functions. Different solutions are
envisaged for the communication between MRM
components over the respective air interfaces,
depending on whether a 3GPP or non-3GPP technol-
ogy is considered. We discuss this issue in the next
section.
Figure 4 illustrates the information flow between
the MRM-TE, the MRM-NET, and the MRM-HAM.
Once the cell and network information has been col-
lected from all RANs, the MRM-HAM processes it and
provides neighborhood information to the MRM-NET,
which in turn advertises available RATs to the MRM-
TE. Based on this information, the MRM-TE carries
out candidate link performance measurements and
performs initial access selection.
The distributed MRM measurement function in
MRM-HAM and the MRM-NET is responsible for
measurement configuration. MRM-HAM requests the
MRM-NET for cell load/cell capacity measurement.
User planeL3/RNL
Air
inte
rfac
e
Sessionlayer
MRM layer
Abstractionand
adaptationlayer
RAT andtransport
layer
Applicationlayer
E2E—End-to-endIP—Internet ProtocolL3—Layer 3MAP—Mobility anchor pointMM—Mobility managementMRM—Multi-radio managementMRM-HAM—MRM heterogeneous access managementMRM-NET—MRM radio access network
MRM-TE—MRM terminal entityPCRF—Policy and charging rules functionRAT—Radio access technologyRNL—Radio network layerRRM—Radio resource managementSIP—Session Initiation ProtocolSM—Session managementTNL—Transport network layer
Terminal
Application,E2E-SM
MRM-TE
SM/MM
Radio access network
RRM
User planeRNL/TNL
MRM-NET
Evolved packet core
User planeTNL/including MAP
MRM-HAM
SM/MM/PCRF
E2E-SM(SIP Proxy)
IP network
Application,E2E-SM
User planeTNL/including MAP
Generic
RAT specific
RRMRRM
User planeL3/RNL
Generic
RAT specific
Transport of user dataLegacy control interfaces
MRM control interfacesMRM signaling
Figure 3.Location of MRM entities in a mobile network layer model.
DOI: 10.1002/bltj Bell Labs Technical Journal 265
The MRM-NET performs, collects, and analyzes the
measurements of the RAN, and reports them to
MRM-HAM.
The MRM-NET configures link performance
measurements for active and candidate RATs to the
terminal. The terminal, in connected mode, collects,
generalizes, preprocesses, and reports the measure-
ments to the MRM-NET.
Once the RAS triggering function of the MRM-
NET has detected the need for a new access selection
decision, the MRM-NET sends an RAS request to the
MRM-HAM, which calculates the metrics for the suita-
bility of available access alternatives. MRM-HAM
then returns this result to the MRM-NET. If client-
based Mobile IP is used as the handover protocol,
the MRM-NET finally instructs the MRM-TE accordingly
for handover execution.
Abstraction and AdaptationMRM is designed as a generic concept operating
with any kind of radio access technology, which can dif-
fer significantly in design and capabilities. The parame-
ters of radio access networks for different RATs are not
directly comparable, and mechanisms and procedures
differ as well. Therefore, an abstraction and adaptation
layer is proposed, hiding the RAT-specific properties
from the generic MRM. The main functions of this layer
are the translation of generic MRM procedures into
RAT-specific procedures and the mapping of RAT-
specific parameters to generic values and vice versa.
MRM-TE MRM-NETMRM-HAM
MRM—Multi-radio managementMRM-HAM—MRM heterogeneous access managementMRM-NET—MRM radio access network
MRM-TE—MRM terminal entityUE—User equipment
Access advertisement
Radio access selectionin idle mode
Radio access selectiontriggering in connected
mode
Radio access selectiondecision in connected mode
HandoverdecisionHandover
execution
Access advice
Handover initiation
Access selection
Measurement abstraction
Measurement monitoring
Downlink performancemeasurement configuration for
UE in connected mode
Measurement reporting
Measurement reporting
Cell load/capacitymeasurements
Resource usagemeasurement configuration
Link performancemeasurements
Measurement collection
Measurement abstraction
Measurement monitoring
Link performancemeasurements in idle mode
Neighborhood information provisioning
Neighborhood information collection
Measurement collection
Figure 4.Information flow between the functions of the MRM entities.
266 Bell Labs Technical Journal DOI: 10.1002/bltj
The advantage of these abstraction and adapta-
tion functions becomes particularly visible when new
radio technologies are integrated into existing sys-
tems. In this case, the MRM as well as the AAL for
existing RATs remain unchanged, while only the
adaptation and abstraction functions for the new RAT
must be defined.
Parameter Abstraction. Parameter abstraction
denotes the transformation of RAT-specific quantities
to a common denominator so that they become com-
parable by generic MRM algorithms for measurement
evaluation, access selection triggering, and access
selection decision. Before this abstraction takes place,
RAT specific processing can be performed by support-
ing functions such as parameter collection, frequency
domain filtering, averaging, threshold supervision
including hysteresis, and timeouts.
The reverse translation of generalized quantities into
RAT-specific values is required, too. This applies
especially to generic measurement where thresholds
have to be configured, and as a consequence, RAT-
specific measurements must be requested.
The main parameters to be evaluated by MRM
are link performance and access resource usage.
Link performance abstraction. The link perfor-
mance describes at what level of efficiency a link can
fulfil the users’ service requirements. This applies to
both an active link and alternative candidate links.
The abstracted link quality is derived from access-
specific measured quantities such as the:
• Received signal strength indicator (RSSI),
• Signal-to-interference-plus-noise ratio (SINR),
and
• Channel quality indicator (CQI).
A variety of access-specific measures have to be
used for different access technologies. The abstract-
ing functions map the RAT-specific quantities on
abstracted, generalized dimensions. A mapping
function can also consider implementation-specific
variations. For example, simple or complex receivers
of the same access technology achieve different levels of
link performance in terms of data rate for the same
received signal strength.
In order to evaluate the link performance and to
decide if a considered session can be served by an
available RAT, we propose using parameters similar
to the main application QoS requirement, i.e., the link
data rate. Further parameters, such as the required
residual error rate or the maximum delay, serve as
input for the abstraction function. Additionally, they
are used as constraints, which may rule out some of
the candidate links, e.g., if a service such as gaming
requires short packet transfer delays, then a RAT with
high transfer delays like GSM/GPRS cannot be used
for this service.
Only the downlink performance of candidate
links can be estimated. The QoS that each provides
may differ from the estimated link performance based
on terminal measurements. Resource management
in the network may apply additional constraints, such
as change of transmission power or available codes,
which results in a different provided QoS. These con-
straints have to be taken into account by MRM for a
final evaluation.
Resource abstraction. A resource-aware MRM also
needs a resource abstraction model of the access
technologies’ resource structures. This information
includes the critical resources available, their geo-
graphical and node-dependent distribution, as well as
bearer-dependent resource demands.
The critical resources differ depending on the
medium access technology, e.g., time division multi-
ple access (TDMA), code division multiple access
(CDMA), or orthogonal frequency division multiple
access (OFDMA). In TDMA-based systems such as
GSM, the number of time slots is a limiting factor,
while for CDMA-based systems such as UMTS, the
channelization codes and quantity of power used
describe the current resource usage. Hence, the
resource usage descriptive parameters differ in nature
and number for a considered technology.
For each of these parameters, an abstracted usage
value (e.g., given as a percentage) is defined. For its cal-
culation, the MRM has to distinguish between resources
for guaranteed bit-rate traffic and for best-effort traffic,
since assigned resources of the former cannot be freed
to serve another request. Hence, two resource levels
indicating both types of traffic are required.
Moreover, depending on the current service mix
and user distribution a certain amount of extra
resources may be necessary to maintain ongoing ser-
vices, since resource usage may change due to a change
DOI: 10.1002/bltj Bell Labs Technical Journal 267
in channel quality or due to handover. Hence, a third
resource level is defined to indicate the maximum
amount of resources which may be assigned in a cell.
Procedure MappingThe key design principle of the MRM in general
and abstraction and adaptation in particular is the
reuse of existing functions and procedures. This is
especially important in a 3GPP network environment
where specific interworking solutions between some
RATs have already been defined and will be reused
instead of being replaced by MRM.
Figure 5 shows a model of the abstraction and
adaptation layer. Requests from the MRM are for-
warded by the communication and module manager
(CMM) to the procedure mapping of the relevant
RAT, which translates the generic request in RAT-
specific procedures. This may also require parameter
abstraction as discussed above. In the opposite direc-
tion, triggers and measurement reports that are
received from the RAT are translated into generic
answers and forwarded by the CMM to the MRM.
Depending on the capabilities of the underlying
RATs, requests for remote information (e.g., measure-
ments in the terminal) may either be processed locally
by existing RAT-specific procedures or are forwarded
via MRM signaling to the remote side. There, the
CMM sends the request to the corresponding local
“procedure mapping and parameter abstraction” mod-
ule. The response to a request follows the reverse path.
The CMM plays a key role in routing requests and
answers between the MRM and the corresponding
procedure mapping and parameter abstraction mod-
ules and between remote sites. The routing table can
be configured during the registration of the RAT-
specific modules using a “default route” through the
MRM signaling module for remote requests, and
thereby also provides a location abstraction, i.e., the
MRM does not need to know whether its request is
answered locally or by a remote service.
“Transport adaptation” provides message trans-
port for MRM signaling on layer 2 or layer 3. The
mechanism that actually is applied depends on the
specific radio access technology. For 3GPP technolo-
gies, existing message transport mechanisms are
reused, such as the radio resource control (RRC)
direct transfer mechanism of UMTS or the corre-
sponding mechanism on the link layer control (LLC)
level for GSM/GPRS. For non-3GPP technologies, IP-
based transport mechanisms are applied, providing
MRM signaling to connected mobile terminals.
Besides the transport issue, access to MRM-related
radio resource parameters also has to be adapted sepa-
rately for each interworking RAT considered. Again,
for 3GPP technologies, reuse of existing radio resource
control mechanisms can be applied, such as the com-
mon measurements and dedicated measurements
available for UMTS systems. For non-3GPP technolo-
gies, a utilization of standard enhancements is envis-
aged. The IEEE has passed enhancements to WLAN
standards such as 802.11k [23] and 802.11u [25],
which provide measurements on both link layer level
and neighbor cell information provisioning.
Furthermore, the IEEE passed the 802.16g stan-
dard amendment to the WiMAX system [24].
Utilization of these additional management plane pro-
cedures and services allows for neighbor information
provisioning, intra-RAT measurements, and the sup-
ply of handover primitives.
Finally, the IEEE is preparing a cross-technology
standard—IEEE 802.21—to address inter-RAT inter-
working through a media-independent handover
Adaptation/abstraction
Proceduremapping/parameterabstraction
Proceduremapping/parameterabstraction
Communication and module manager
MRMSignalingProtocol
Transportadaptation
MRM
RAT 1 RAT 2
MRM—Multi-radio managementRAT—Radio access technology
Figure 5.Model of the adaptation and abstraction layer.
268 Bell Labs Technical Journal DOI: 10.1002/bltj
signaling framework [26]. Its functionality covers much
of the MRM communication and adaptation demands,
however, access selection, parameter abstraction, and
handover control are neither defined nor intended,
which prevents 802.21 from offering a complete solu-
tion for multi-radio management. Moreover, adequate
support for 802.21 is still imponderable for radio tech-
nologies other than those of IEEE.
MRM CommunicationThe principle of using either RAT-specific com-
munication mechanisms or generic MRM procedures
is presented in Figure 6, which shows MRM signaling
between a terminal and an access network in a 3GPP
UMTS network.
Network-side information and services like inter-
RAT measurements for GSM/GPRS and LTE can be
requested via the network-side procedure mapping
and parameter abstraction function. Other informa-
tion, such as measurements of the link performance of
non-3GPP candidate cells, is requested via MRM sig-
naling. It is envisaged to transport signaling messages
using the existing RRC direct transfer mechanism. A
similar situation arises if a terminal is served by a non-
3GPP network, but with somewhat different mapping
to RAT-specific procedures and MRM message
Adaptation/abstraction
MRM-TE MRM-NET
3GPPL1/L2
Messagetransport
Messagetransport
Signaling
Radio bearer
RRMRAC/RRC
3GPP†
modem
RRMRAC/RRC
Proceduremapping/parameterabstraction
Proceduremapping/parameterabstraction
Non3GPPmodem
Non3GPPdriver
RRCdirecttransfer
RRCdirect
transfer
TCP/IP
MRM-HAM
TCP/IP
MRMSignalingProtocol
Transportadaptation
Communication and module managerMRM communication
manager
RRC Protocol RRC Protocol
Generic elements
UMTS-adaptation/abstraction
Non3GPP-adaptation/abstraction
MRM scope
Logical interfaces Physical interfaces
UMTS-specific
Non3GPP-specific
Adaptation
3GPP—3rd Generation Partnership ProjectIP—Internet ProtocolL1—Layer 1L2—Layer 2MRM—Multi-radio managementMRM-HAM—MRM heterogeneous access managementMRM-NET—MRM radio access network
MRM-TE—MRM terminal entityRAC—Radio access controllerRRC—Radio resource controlRRM—Radio resource managementTCP—Transmission Control ProtocolUMTS—Universal Mobile Telecommunications System
†Trademark of the European Telecommunications Standards Institute.
Communication and module manager
MRMSignalingProtocol
MRMSignalingProtocol
MRMSignalingProtocol
Transportadaptation
Transportadaptation
Transportadaptation
Proceduremapping/parameterabstraction
Adaptation/abstraction
Operator IP network
Figure 6.MRM signaling between terminal and 3GPP UMTS access network.
DOI: 10.1002/bltj Bell Labs Technical Journal 269
transport. Examples of the mapping of MRM proce-
dures onto RAT-specific procedures in a 3GPP system
are discussed below.
In Figure 7, the MRM-NET uses a generic LOAD
REQUEST message to obtain cell load information. The
abstraction and adaptation function interprets this
request and maps it onto RAT-specific procedures (DL
TxPOWER REQUEST, UL NOISE REQUEST and DL
DELAY REQUEST) that request measurements accord-
ingly from the corresponding RNC, which then pro-
vides the input for the load abstraction. For illustration
purposes each measurement is requested using a sepa-
rate procedure, while a combined request may be sent
to the RNC by implementation. The RNC either already
holds the measurement values or it must request meas-
urements from the respective node B. The measure-
ment values are transferred to the abstraction function
which translates the UMTS-specific values into generic
resource usage and reports the result to the MRM-NET.
Figure 8 demonstrates a signaling flow of a ter-
minal currently served by UMTS and the MRM-NET
that is requesting LTE candidate link measurements.
By using neighborhood information received from
MRM-HAM, the MRM-NET sends a generic LINK
MEASUREMENT REQUEST to the AAL located in the
UMTS RNC, which in turn triggers a UMTS RRC
measurement configuration. The requested measure-
ment results are returned to the AAL on the network
side which translates them into a generic value of a
link data rate, and forwards the information to the
MRM-NET.
As indicated above, for other candidate measure-
ments which are not directly supported by UMTS
(e.g., WLAN measurements) the AAL on the network
side forwards the generic measurement request to the
AAL on the terminal side which translates this request
to local RAT-specific procedures (e.g., as provided by
802.11k standard extensions).
Adaptation andabstraction
NET
RESOURCE USAGEMEASUREMENT REQUEST
DL TxPOWER REQUEST(NBAP) CONFIGURE
MEASUREMENT REQUEST
MEASUREMENT REPORTDL TxPOWER
DL TxPOWER INDICATION
UL NOISE RISE REQUEST
…
DL DELAY REQUEST
DL DELAY INDICATION RESOURCE USAGEINDICATION
UL NOISE INDICATION
MRM-NETNode B RNCRRC/RAC
…
DL—DownlinkMRM—Multi-radio managementNBAP—Node B application partNET—Network
RAC—Radio access controllerRNC—Radio network controllerRRC—Radio resource controlUL—Uplink
Figure 7.Signaling flow obtaining resource usage information.
270 Bell Labs Technical Journal DOI: 10.1002/bltj
Migration Towards the MRM Concept: A 3GPPApproach
MRM, as represented by its functions and com-
ponents, has to be distributed among the radio access
and core network infrastructure to obtain the
intended inter-RAT mobility. The 3GPP is concerned
with a system architecture evolution (SAE) [8] intro-
ducing the interworking of different RATs. Since this
is the most advanced approach to a multi-radio archi-
tecture, the MRM concept aims to match to that
approach. Figure 9 shows a distribution of MRM as it
would be integrated in the SAE architecture.
The MRM-TE and the MRM-NET components
operate in close cooperation with technology-specific
radio resource management. Since the MRM-TE com-
ponent handles MRM terminal-side operations, it
must be linked to the corresponding radio resource
and mobility control as implemented technology-
specifically in the terminal. The MRM-NET compo-
nent in turn is placed as close as possible to the
network-side radio resource management controller.
For trusted domains such as a 3GPP or 3GPP2 net-
works, this would be within the corresponding radio
network controller, i.e., the RNC, base station con-
troller (BSC), or evolved node B (eNodeB). For
untrusted domains, however, the MRM-NET should
be located in the closest trusted network node within
that untrusted domain. Since the MRM signaling takes
place at the IP-level in these non-3GPP networks,
communication with the terminal-side MRM is
secured through application of Internet Protocol secu-
rity (IPsec) tunnelling. In the case of an interworking
WLAN, the MRM-NET component would therefore
be placed in the corresponding enhanced packet data
MRM-TE MRM-NET MRM-HAMTerminal Node B RNC
MRM: NEIGHBOUR CELL INFORMATION REQUESTRetrieve cell
informationon accessible RATs:
Adaptation andabstraction
NET
MRM: NEIGHBOUR CELLINFORMATION RESPONSE
LINK MEASUREMENT REQUEST (setup)
MEASUREMENTCOMMAND
(SNR, path loss)
LINK MEASUREMENTREPORT (data rate)
MEASUREMENTREPORT
(SNR, path loss)
LINK MEASUREMENTREPORT (data rate)
Can be a trigger for a potentialchange of RAT
MEASUREMENTREPORT
(SNR, path loss)
LTE—Long Term EvolutionMRM—Multi-radio managementMRM-HAM—MRM heterogeneous access managementMRM-NET—MRM radio access networkMRM-TE—MRM terminal entityNET—Network
RAT—Radio access technologyRNC—Radio network controllerRRC—Radio resource controlSNR—Signal-to-noise ratioUMTS—Universal Mobile Telecommunications System
(repeatedly)
~ ~ ~ ~ ~ ~ ~
RRC: MEASUREMENT REPORT
RRC: MEASUREMENT REPORT
RRC: MEASUREMENT CONTROL(setup)
Figure 8.Signaling flow obtaining LTE candidate link measurements for a terminal currently served by UMTS.
DOI: 10.1002/bltj Bell Labs Technical Journal 271
the same radio network controller. As a result, any
signaling between both components would be node-
internal, thus avoiding additional signaling load on
external links and reducing delay times. However, in
this case, configuration information and the accumu-
lating up-to-date load information have to be exchanged
between neighboring MRM-HAM entities, causing addi-
tional signaling traffic. In a centralised scenario, as
shown in Figure 9, the MRM-HAM resides outside the
RAN of any technology, i.e., within the core network.
gateway (ePDG) node, but not in the wireless
access gateway (WAG) or the access point (AP), since
these nodes are parts of the untrusted domain.
There is no similarly obvious placement for the
MRM-HAM component within the network, since
the MRM-HAM is focused on radio resources yet func-
tions on top of managed radio technologies. Two alter-
native placements can be considered, each having
different benefits. In a collocated scenario the MRM-
HAM is placed near the MRM-NET component within
Figure 9.Example of the mapping of MRM functions onto the 3GPP architecture.
GERAN
GbBTS BSC
Abis
Rx�
Iu
S7
SGSN
GPRS core
S3S6
UTRAN
RNCNode B
PCRFS4
MME
HSSIur
SGi
X2
E-UTRAN
S11
SAE servinggateway
SAE PDNgateway
S1-MMES5/8
MRM-HAM
X2ePDGWi
eNodeB
eNodeB
Evolved packet coreS1-U
MRM-NET
S2a S2beNodeB
3GPP—3rd Generation Partnership ProjectASN—Access service networkBSC—Base station controllerBTS—Base transceiver stationEDGE—Enhanced data rates for GSM evolutioneNodeB—Evolved node Be-PDG—Enhanced packet data gatewayE-UTRAN—Evolved UTRANGERAN—GSM EDGE radio access networkGPRS—General packet radio serviceGSM††—Global System for Mobile Communications††
HSS—Home subscriber serverIMS—IP Multimedia SubsystemIP—Internet Protocol
MME—Mobility management entityMRM—Multi-radio managementMRM-HAM—MRM heterogeneous access managementMRM-NET—MRM radio access networkMRM-TE—MRM terminal entityMRRM—Multi-radio resource managementPCRF—Policy charging rules functionPDN—Packet data networkSAE—System architecture evolutionSGSN—Serving GPRS support nodeUMTS—Universal Mobile Telecommunications SystemUTRAN—UMTS terrestrial radio access networkWiMAX—Worldwide Interoperability for Microwave AccessWLAN—Wireless local area network
†Trademark of the European Telecommunications Standards Institute.††Registered trademarks of the GSM Association.
MRM-TE
MRM-NET
MRM-NET
MRM-NET
IMS/non-IMS PDN
(e.g., Internet)
MRM-NET
MRM-NET
Trusted non-3GPP†
IP access, e.g.,WiMAX
ASNMRM-NET
Untrusted non-3GPPIP access, e.g., WLAN
272 Bell Labs Technical Journal DOI: 10.1002/bltj
Here, communication to the served MRM-NET enti-
ties takes place on wired links between different
network nodes. Since the focus of a considered MRM-
HAM entity is spread across several neighboring RANs
of different technologies, less configuration and load
information has to be exchanged with other neighbor-
ing MRM-HAM entities focusing on different adjacent
RANs in this scenario.
The first steps towards an MRM architecture have
been taken with the 3GPP LTE/SAE Release (Rel.) 8
standardization. However, only a terminal based solu-
tions has been defined, supported by a standardized
access network discovery and selection function
(ANDSF) [8]. Compliant with this approach, the
ANDSF is to be extended towards an MRM-HAM com-
ponent with network control of the terminal’s RAS,
leading to a so-called “policy based terminal triggered
ANDSF decided access selection” [2–4]. This solution
combines advantages of network-based decisions with
terminal-based decisions, where terminal decisions
reflect policies distributed by the network. The solution
is characterized by a significant accuracy of handover
decisions and a low amount of signaling traffic between
processing network nodes. This distribution of the opera-
tional effort between the UE and the ANDSF considers
all relevant information without increased signaling
effort, because decision steps are prepared and calcu-
lated at the origin of the information.
This extended ANDSF collects information about
RANs; provides the terminal with policies, neighbor-
hood information, and further assistance data; and
carries out access selection decisions based on infor-
mation received from the terminal, RANs, and from
other databases. The terminal provides measure-
ments, conducts measurement evaluation, and trig-
gers access selection based on link measurements and
configurable thresholds. As a result, the terminal
either receives a handover command and initiates
handover execution, or requests new assistance data
from ANDSF.
This approach was presented to 3GPP and
received support [2–4]. Due to imminent finalization
of 3GPP Rel. 8, however, it was deferred to 3GPP Rel.
9. An aspect of the MRM concept that proposes a
generic framework for the signaling and handling of
UE capability information, promising a simplification
in the treatment of RAT-specific capability informa-
tion, has been adopted by 3GPP [1].
Proof of Concept by DemonstratorIn order to validate the MRM concept, a demon-
strator [14, 18] has been set up comprising two dif-
ferent radio access networks and a dual-radio mobile
terminal. Generic MRM instances are integrated in
the terminal, the core network, and the access net-
works of each RAT. The demonstrator supports the
Internet Protocol version 6 (IPv6) mobility protocol
[27] during ongoing services of different QoS classes:
file transfer via File Transfer Protocol (FTP), Web
browsing via Hypertext Transfer Protocol (HTTP),
video streaming via Real Time Transport Protocol
(RTP) and video calls via SIP/IMS. MRM interfaces
are specified and implemented between the MRM
instances based on the Diameter protocol, an inter-
face for cross-layer interaction to the MIP kernel func-
tions [20] of the terminal, and an interface to the
AAL. The latter has been implemented for an Alcatel-
Lucent 3GPP high speed downlink packet access
(HSDPA) base station (NodeB) as an overlay cell and
an orthogonal frequency division multiplexing
(OFDM) 802.11a-based hotspot. Real radio signal
strength measurements are acquired. The movement
of the dual-radio UE is emulated through changes to
link performance induced by a radio frequency (RF)
attenuator at the OFDM antenna. Compatibility
between Internet Protocol version 4 (IPv4) and IPv6
devices is achieved by IP tunnelling mechanisms. The
demonstrator setup for the distributed MRM scenario
is shown in Figure 10.
The feasibility of the MRM concept and the scala-
bility by MRM distribution were verified successfully.
The MRM-NET triggers handover decisions in a central
or co-located MRM-HAM based on abstracted radio
link measurements, radio bearer type, preferences and
cell load. MRMM instructs the UE to perform a MIPv6
handover between the two heterogeneous RATs. The
trigger levels, decision algorithm, and the number of
active radio interfaces during handover (make-before-
break versus break-before-make) can be configured.
The resulting handover performance is determined by
user-perceived service interruption and by measure-
ments of packet loss and transmission time.
DOI: 10.1002/bltj Bell Labs Technical Journal 273
The usage of MRM neighborhood information has
been shown to decrease OFDM scanning time from
about half a minute to only a few seconds for a full
scan. But this alone cannot prevent the interruption of
running services during MIP handover. Demonstrator
experiments reveal that proactive measurements and
a timely triggered handover preparation (make-
before-break) are essential for seamless handover. For
a completely lossless handover, optimized MIP imple-
mentations are required, since a binding update can
overtake packets on the fly by as much as 80 mil-
liseconds (msec) between the OFDM and the UMTS
uplink of our demonstrator. This finding resulted in a
proposal for the optimization of make-before-break
handover in PMIP. [11, 30].
ConclusionFourth generation wireless networks will consist of
multiple radio technologies requiring intelligent inter-
working solutions. The multi-radio management
concept presented here is one necessary step towards
a pervasive and effective integration of current and
future access technologies.
MRM handles the differences between hetero-
geneous access technologies in a unified way by
abstraction from RAT-specific parameters and adaptation
OFDM access networkOFDM RAN IPv4 domain
UMTS/HSDPA access network
Core networkIPv6 domain
SignalionOFDM access point
IPv4
IPv6
MIPv6home agent
IPv4
IPv6
MRM-NET
MRM-HAM
SignalionOFDM station
MRM-TE
OFDMcontrol
OFDMcontrol Access
router
Qualcommmobile
IPv6 Core domain SIP@Alice
Accessrouter
UMTS/HSDPAcontrol
HSDPA—High speed downlink packet accessIP—Internet ProtocolIPv4—Internet Protocol version 4IPv6—Internet Protocol version 6IMS—IP Multimedia SubsystemMIPv6—Mobile IP version 6MRM—Multi-radio management
Node B
IPv4
Laptop
MRM-HAM—MRM heterogeneous access managementMRM-NET—MRM radio access networkMRM-TE—MRM terminal entityOFDM—Orthogonal frequency division multiplexingRAN—Radio access networkSIP—Session Initiation ProtocolUMTS—Universal Mobile Telecommunications System
UMTS RAN IPv4 domain
Applicationserver
IMS SIP@ Bob
User terminalTerminal IPv4 domain
MRM-NET
MRM-HAM
UMTS/HSDPAcontrol
Figure 10.Setup of the MRM demonstrator with HSDPA and OFDM access networks and a dual-radio terminal.
274 Bell Labs Technical Journal DOI: 10.1002/bltj
to RAT-specific functionality in an adaptation and
abstraction layer. The proposed general approach
reuses existing protocols where available and com-
plements them with generic MRM protocols where
necessary. This leads to a significant reduction of stan-
dardization and implementation efforts compared to
bilateral interworking solutions between each pair of
radio access technologies.
The MRM functions and architecture, as well as a
possible integration of MRM into the 3GPP SAE archi-
tecture were discussed in detail. The key functionality
of MRM is a network-based radio access selection
mechanism that considers radio link performance,
resource usage, and user and operator preferences.
For seamless inter-technology handovers, MRM syn-
chronizes IP mobility protocols with link and session
layer procedures. This approach provides the follow-
ing benefits for multi-radio infrastructure for 4G:
• Flexibility for operators to deploy in their net-
work the RATs best suited to provide their sub-
scribers with mobile services in each location (city,
rural, or on the road),
• Common resource and mobility management,
offering the operator the potential to optimize
network usage and revenue by selecting the most
efficient RAT for each service,
• Scalable generic algorithms for network assisted
inter-RAT access selection, tunable according to
the operator’s strategy (e.g., to maximize resource
usage, capacity, user satisfaction, fairness, and rev-
enue), and
• Minimizing measurement requirements on UEs
by utilizing network status knowledge for neigh-
borhood indications and measurement configu-
ration.
The improvement of network performance by
multi-radio access selection algorithms has been vali-
dated in multi-cell multi-RAT system simulations.
The feasibility of the MRM concept and the distributed
MRM architecture have been proven by network simu-
lations investigating signaling and processing efforts.
Additionally, simulations have shown that proactive
preparation of the most attractive target cell can sig-
nificantly increase the probability of seamless execu-
tion of the handover. Finally, the MRM concept has
been realized and validated in an MRM demonstrator
offering optimized network-based access selection and
seamless inter-technology IP-based mobility between
a cellular UMTS/HSDPA network and a WLAN
hotspot during ongoing multi-media sessions.
This MRM concept is proposed as a basis for stan-
dardization and realization of fourth generation wire-
less networks.
AcknowledgementsThe authors would like to acknowledge the con-
tribution of the following former and present mem-
bers of the Multi-Radio Management team and
cooperating partners: Anton Ambrosy, Ulrich Barth,
Harald Eckhardt, Dirk Hofmann, Ingo Karla, Edgar
Kühn, Ingmar Blau, and Christian Müller.
This work has been partly funded by the
European Commission within the Sixth Research
Framework Program Ambient Networks project
and the German Federal Ministry of Research and
Education projects ScaleNet and WIGWAM.
*Trademarks3GPP is a trademark of the European Telecommunications
Standards Institute.GSM and Global System for Mobile Communications are
registered trademarks of the GSM Association.
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[18] Germany, Federal Ministry of Education andResearch (BMBF), WIGWAM: Wireless Gigabitwith Advanced Multimedia Support project,“System Concept Final Version—Part 5:Network Layer,” June 2007, �http://www.wigwam-project.com/�.
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[23] Institute of Electrical and ElectronicsEngineers, “Draft Standard for Local andMetropolitan Area Networks—SpecificRequirements—Part 11: Wireless LAN MediumAccess Control (MAC) and Physical Layer(PHY) Specifications—Amendment: RadioResource Management of Wireless LANs,”IEEE P802.11k/D7.0, Jan. 2007.
276 Bell Labs Technical Journal DOI: 10.1002/bltj
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[25] Institute of Electrical and Electronics Engineers,“Draft Standard for Local and MetropolitanArea Networks—Specific Requirements—Part11: Wireless LAN Medium Access Control(MAC) and Physical Layer (PHY)Specifications—Amendment: Enhancements forInterworking with External Networks,” IEEEP802.11u/D1.0, May 2007.
[26] Institute of Electrical and Electronics Engineers,“Draft Standard for Local and MetropolitanArea Networks: Media Independent HandoverServices,” IEEE P802.21/D8.1, Feb. 2008.
[27] D. Johnson, C. Perkins, and J. Arkko, “MobilitySupport in IPv6,” IETF RFC 3775, June 2004,�http://www.ietf.org/rfc/rfc3775.txt�.
[28] F. Kalleitner and J. Eisl (eds.), “MobilitySupport: System Specification, Implementationand Evaluation,” Ambient Networks Phase 2,Project IST-2004-027662, Deliverable D20,European Union 6th Framework Program (FP6)IST-2004-2.4.5, Dec. 2007, �http://www.ambient-networks.org/deliverables.html�.
[29] R. Koodli (ed.), “Mobile IPv6 Fast Handovers,”IETF RFC 5268, June 2008, �http://www.ietf.org/rfc/rfc5268.txt�.
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[31] C. Perkins (ed.), “IP Mobility Support for IPv4,”IETF RFC 3344, Aug. 2002, �http://www.ietf.org/rfc/rfc3344.txt�.
(Manuscript approved August 2008)
ROLF SIGLE is team manger in the Radio SystemOptimization department within the BellLabs Wireless Access Domain in Stuttgart,Germany. He received a diploma degree inelectrical engineering from the Universityof Stuttgart. He is responsible for the Multi-
Radio Infrastructure for 4G project. His researchinterests are radio access network architecture, radioresource management, and system performanceoptimization.
OLIVER BLUME is a research engineer in the RadioSystem Optimization department within theBell Labs Wireless Access Domain inStuttgart, Germany. He holds a degree inphysics from the University of Hamburg,Germany, and a Dr.-Ing. degree in
integrated optical amplifiers from the TechnicalUniversity of Hamburg-Harburg. Dr. Blume’s currentresearch interests are in wireless communications, radio research management, and IP mobility protocols.
LUTZ EWE is a member of technical staff in the RadioSystem Optimization department within theBell Labs Wireless Access Domain inStuttgart, Germany. He received a diplomain physics at the University of Giessen, and adoctoral degree in micro system technology
at the University of Duisburg, Germany. Dr. Ewe’scurrent research focus includes collaboration ofheterogeneous radio access technologies and selfoptimization strategies in radio access systems.
WIESLAWA WAJDA is a research engineer in the RadioSystem Optimization department within theBell Labs Wireless Access Domain inStuttgart, Germany. She received a degreein electrical engineering from the TechnicalUniversity Wroclaw, Poland. Her research
interests are focused on system architectures andtelecommunication system concepts. ◆