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UMTS Femtocell as
Wireless Residential Gateway
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June, 2007
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CONTENTS
1. Abstract 4
2. Introduction 4
3. Key Business Drivers for UMTS Femtocell 4
4. Networks Integration Models 5
5. Key Challenges 6
6. Competition and Threats 7
7. Aricent Edge 7
8. Summary 8
9. References 8
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1. ABSTRACT
Mobile operators today are trying hard to overcome the challenges
posed by intense competition in the wireless communication space.
There is an ever-increasing demand from operators to increase
ARPU while decreasing cost. New competitors entering the market
are posing a big threat to existing players apart from the alternative
technologies such as Voice over WiFi. Vendors are under intense
pressure to come up with innovative solutions for providing better
quality and coverage to mobile subscribers at lower cost.
The discussion about UMTS based Femtocell (also called 3G
Access Point`) that is doing rounds for quite sometime to counter
the challenge posed by unlicensed access technologies like WiFi
and WiMAX. The concept is of a UMTS Femtocell is to make a
UMTS NodeB cheap enough to be deployed for residential use by a
very limited number of users. This NodeB is connected to the
backhaul network via subscribers broadband connection and is
deployed in huge volumes. This would deliver the same services to
a subscriber as other alternative technologies and, most importantly,
would use the existing UMTS handset, with no need to upgrade to a
dual-mode handset.
While there are several challenges faced by vendors in making sucha system like network integration, security, provisioning, radio
interference and cost; solutions to these problems exist enabling the
vendors and operators to build a strong and viable business case for
low cost UMTS Femtocell. The subscriber gets a benefit of reduced
call costs, increased quality and convenience of using an existing
UMTS handheld device. The operator benefits by increase in ARPU
and customer loyalty.
This paper takes a closer look at the key business drivers in this
space as well as advantages/opportunities available with the
operators and vendors in this nascent market. The paper also
captures the key challenges that need to be focused on for
developing a Femtocell solution viable for mass-market deployment
and the various network integration models for the Femtocell.
2. INTRODUCTION
WCDMA is an interference-limited technology, in which the cell
capacity is limited by the interference caused by transmission from
each user in the cell. To increase cell capacity, all transmissions
(from base station as wellhandsets) are kept at minimum transmit
power. However, in the in-building coverage area, there is significant
attenuation of the radio signal from the macro cells, which results in
the lower voice quality and higher call drops for CS services and
lower user data rate for PS services. To maintain the QoS, UMTS
indoor users causing significant interference to the outdoor users,
thereby further reducing the macro cell capacity.
UMTS operators believe that the killer applications for their networks
in future would be the high bandwidth streaming applications like
IPTV, video telephony, movie-on-demand. Studies indicate that
majority of usage of these applications would be when the user is
indoors. Clearly, there is a compelling requirement for mobile
network operators for providing an effective coverage for these
applications where it is needed the most inside the buildings.
Using a UMTS femtocell in the home environment will help
minimizing the transmit power levels both for the femtocell and the
UE as there would be a significant decrease in the signal
attenuation. The lower output power from the UE will reduce the
interference generated while decreasing the drain on battery. High
signal attenuation through walls further reduces the impact of
interference of the indoor traffic on the outdoor environment.
A few years ago, an indoor base station would have seemed
impossible especially because of the lack of appropriate hardware
and also considering the impression of a typical base station. Todaythe reality is different, with different companies thinking seriously
about developing an extremely low-power femtocell (home base
station), of size comparable to an existing WiFi access point and a
price of less than $200. It is predicted that there will be 102 million
worldwide users on 32 million home base stations by 2011 [1].
Overall, UMTS has a tremendous potential to take up a significant
share of this indoor wireless access market.
3. KEY BUSINESS DRIVERS FOR UMTS FEMTOCELL
What are the business drivers for this kind of solution? To answerthis, consider the following points:
. Competiion: Mobile Operators today are under tremendous
pressure of balancing the relentless technology advances and
new services with increasing costs, reducing revenues and
growing competition from other service providers. As mobile and
fixed-line markets become saturated and voice revenues decline,
the home is the primary focus for capturing market share by
offering attractive service bundles: voice, data and television as
the so-called triple play. Mobile access adds another dimension for
some
. Backward Compatbility: This solution does not require the
subscriber to purchase an expensive dual-mode handset for
getting indoor and outdoor coverage from a single handheld
device
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. Threat from voice over IP, voice-over-WiFi: The threat from
unlicensed technologies, which are currently eating up the
revenue share of licensed spectrum operators, will get reduced if
cheaper wireless coverage can be provided indoors
. Threat from wireless technologies like WiMax: WiMax based
solutions, which are able to provide higher data rates than UMTS
are also gaining ground for indoor wireless access market but
UMTS have some clear advantages over WiMax. Apart from being
able to work without requiring a dual-mode handset, UMTS would
be able to provide better security on the radio interface and a
unified billing solution to the subscriber for outdoor and indoor
access
. Increased Network Coverage and Capacity: Providing high QoS
in indoor coverage from macro/micro cells has inherent limitations
due to a significant radio signal path loss that happens while
permeating walls. The indoor solution will be much better
equipped to provide a better coverage and high QoS (due to
reduced path loss). At the same time, if the UMTS Femtocell takes
care of the bulk of traffic generated by indoor users, the load on
macro network will get eased significantly. This would simplify the
RF Planning issues and increase cell capacity for the UMTS
macro network. Increased Customer Loyalty: A customer that purchases a
femtocell will not abandon the operators service so easily. Apart
from creating a psychological barrier, there are other benefits to
the customer, such as improved in-home coverage, improved
in-home data rates, and cheap or maybe free calls. If a femtocell
can deliver on these benefits, the probability of a subscriber
switching the service provider will reduce significantly
4. NETWORK INTEGRATION MODELS
An operators Femtocell network must be connected into its mobile
core to interconnect the Femtocell user to the PSTN and people on
the operators macro network. Several options are available for
network integration. Each option comes with a tradeoff in terms of
scale (proprietary gateways and UMA) and/or time to market (IP Iub
and IMS). The best option will depend on an operators existing
network capabilities and their network evolution roadmap. Some of
the network integration models possible have been described here:
. Iub over public IP: This architecture is quite similar to the existing
3G-network architecture where each femtocell (equivalent of
NodeB) is connected to the RNC over an IP interface as shown in
figure below. The Iub signaling takes place over an IP network.
Network security can be handled by the IP security (IPsec)
protocol between the femtocell and the security gateway node.
This architecture is suitable for operators who have an existing
UMTS infrastructure deployed and are looking for a fast
integration of femtocell into this existing infrastructure
Illustration 1: Iub over public IP
. IP Concentrator with standard RNC:An alternative architecture
is possible that uses an aggregator node to connect multiple
femtocells to the existing RNC. The Concentrator node is
connected to the RNC over the standard Iub interface and looks
like a normal Node B
. UMA based network with modified UTRAN architecture: TheUMA approach provides an option of using the existing Core
Network infrastructure while re-architecting the UTRAN to support
the unlicensed spectrum deployment [2]. The interface to the
existing Core Network is through the UMA Network Controller
(UNC), which is the equivalent to an RNC. This concept can be
extended to support a UMTS femtocell through the UNC [3]. The
UMA client function moves into the femtocell allowing any
standard UMTS handset to communicate with the network through
the UMA network. The advantage in this architecture is that it uses
most of the existing network elements and will be able to support
integration of a large number of end-user devices into the mobile
network
Illustration 2: UMA based on modified UTRAN architecture
Mobile Operators Network
ADSLlub over
secured IP
SecurityGateway
lub
Tunneling
RNC
Femtocell
ADSL
Femtocell
luPS SGSN
MSC
Femtocell
Femtocell
luCS
ADSL
Femtocell
Femtocell
ADSL
ADSL
ADSL
ISPNetwork
ISPNetwork
Mobile Operators Network
ADSLUMA
Signalling
UNC
UMTS
Femtocell
UMTS Femtocell
UMTS Femtocell
UMTS Femtocell
UMTS Femtocell
UMTS Femtocell
ADSL
luPS SGSN
MSCluCS
ADSL
ADSL
ADSL
ADSL
ISP
Network
ISP
Network
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. IMS core based with Integrated RNC, SGSN and GGSN:
Illustration 3 shows another approach that moves away from the
existing network architecture completely to an all-IP network
defined in 3GPP IP Multimedia Subsystem (IMS). The voice is
carried using Voice over IP (VoIP) protocol using Session Initiated
Protocol (SIP). The RNC and packet core functionality is now fully
integrated into the femtocell and the Gateway node, which
enables proper interworking with the IMS or a pre-IMS core
network. The SIP signalling can happen directly between the IMS
core and IMS enabled handset in which case the UMTS Femtocell
just provides the UTRAN and packet core functionality. In case UE
is not SIP enabled, the Femtocell shall additionally function as an
IMS client and terminate the SIP signalling on behalf of UE. The
Femtocell shall then interwork the IMS session towards the IMS
core to a standard CS Call on the Radio side towards the UE. The
main advantage of using this architecture is that it doesnt need
the packet core infrastructure, which might get congested in an
eventual large-scale deployment of femtocells. This approach is
more suitable as a long-term strategy in the femtocell evolution
path
Illustration 3: IMS based architecture
5. KEY CHALLENGES
Like any new technology, the femtocell space is also facing a
number of challenges. The ones that are concerning operators and
vendors the most for a successful commercial deployment of
femtocell, are discussed below:
. Subscriber BoM cost: Pricing of the femtocell needs to be kept in
the range of $200 or even lesser to make it a viable FMC solution.
This is not going to be an easy task considering its typical
performance requirements (4 HSPDA/HSUPA users with
cumulative data rate above 1 Mbps).
. Network Integration Model: The network integration model
adopted for femtocell (some models shown above) will also play a
key role in enabling easier integration with existing network
infrastructure while at the same time being able to meet the end
user requirements.
. Upgrades: With the new technologies like HSUPA evolving over a
period of time, a femtocell has to be made upgradeable to support
these new requirements. This can be achieved by relying on
software-defined radio or DSP based solutions but it has cost
implications associated with it.
. Access Control: The radio access cannot be open to everyone in
the coverage area of a femtocell. The subscriber would pay for the
femtocell installation and backhaul (lets say DSL) access charges,
therefore the subscriber would need to have an option to control
the set of users (UEs) authorized to make or receive calls through
that femtocell. Anyone else should be barred from using the
services of that femtocell. This kind of access restriction
mechanism is not provided by the standard UMTS protocols and
therefore some innovation is required to make this happen.
. RF Planning: The RF planning needs to be done very carefully for
deriving the desired benefits from femtocell environment. While aseparate carrier for the femto environment may help mitigate
interference with the macro network, this option may not be
available with operators who are tight on the allocated spectrum
and want to maximize the capacity by frequency reuse. Other
consideration could be the use of such scrambling codes that are
not in use in the neighboring cells. Configuration of scrambling
codes from the operator could be a problem because of the
ad-hoc nature of femtocell deployment. The femtocell could
emulate the UE behavior to listen the neighbor cell environment
and read the primary scrambling codes of the cells around to
select a scrambling code that is not in use
. Network Planning: Network configuration with respect to
allocation of cell identifiers, location and routing area identifiers
etc. to the femtocell needs to be done in such a way that it does
not create integration issues with the core network. Careful
planning is also needed for providing seamless handover and
mobility between the femtocell and macro network. These
aspects, if not taken care properly, can create operational issues
or the UMTS network operator
. Radio Resource Management: The radio resource management
strategy and algorithms need to be developed specifically for the
indoor deployment model. Smart algorithms need to be defined
which enable high radio resource utilization and better service
(lower call drops, higher QoS, seamless handovers) while
minimizing the interference to the neighboring environment
ADSL
SIP Enabled UE
SIP Signalling
Gateway
UMTSFemtocell
UMTS Femtocell
UMTS Femtocell
ADSL
UMTS Femtocell
ADSL
ADSL
ISP
NetworkIMS CORE
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. Radio Emission Aspects: Radio Emission levels in the indoor
environment needs to be kept much lower to avoid any
undesirable impact on human beings. This is achievable due to
much lesser signal attenuation in the indoor environment, which
requires both the UE and the femtocell to transmit at very low
power for achieving good signal strength on the receiver side. This
actually leads to a safer radio environment as compared to macro
network environment and there is a need for educating the end
users for whom the idea of an indoor base station might create
some amount of fear.
. Automatic Configuration: It will be necessary for the femtocell to
get configured, seamlessly connected with the subscribers ISP
network and operators core without requiring much manual
intervention either from the subscriber or from the operator. This
plug-and-play feature is needed firstly to enable the subscriber to
install a femtocell in his/her home independently without any
hassles and secondly for lowering the operators customer support
cost.
. Customer Support Model: Though the customer expectations of
providing support for a residential access point have been
minimized by WiFi access points, still there would be some
requirement for diagnosing the problems that could occur in afemtocell. Network monitoring tools need to be put in place to
ensure that a femtocell is able to deliver good coverage and
reliable service to the end user without which the operator will not
be able to get the maximum value from such deployment.
. Security Aspects: Registration and authentication of the
femtocell need to be carried out in some way to provide a secure
connection with the core network through subscribers ISP
network. This could be achieved by including a SIM in the
femtocell, which will store the subscriber identity and subscription
preferences. Authentication can take place when the femtocell is
powered on and tries to connect with the backhaul network. There
could be other ways of achieving the same results depending
upon the operators deployment scenario
. Regulatory Restrictions: Mechanism needs to be provided in the
femtocell to adhere to regulatory restrictions imposed in a
geographical location/country For example, before the femtocell
starts radio transmission, it needs to ensure that the carrier
frequency/band could be used in that geographical location. The
location where the end user purchases the femtocell cannot be
used for this purpose since the user can easily carry the femtocell
to a distant location/country and try to install the device in that
location
. Synchronization: There is a requirement of timing accuracy of
0.1 parts per million (ppm) for picocells as per the 3GPP Release
6 specifications [7] . A severe synchronization and accuracy
requirements, such as this, can make the equipment costly and
therefore, unviable. Some innovative mechanism for providing
synchronization needs to be in place until there is a relaxation for
picocells in future versions of the 3GPP standards or an operator
specific relaxation is provided.
6. COMPETITION AND THREATS
The competition to the UMTS Femtocell market from other
technologies like Voice over WiFi, WiMAX, UMA is for real.
. Time-to-Market: The existing WiFi based solutions have got a
major head start in terms of product development, trials, marketing
etc. Potential Femtocell vendors need to start with an aggressive
development plan and a marketing campaign to overcome this
lead and position their product credibly with a fast
Ztime-to-market.
. High Speed Packet Data Services: WiMax, which uses a
much-advanced radio interface with higher spectral efficiency,
poses a big threat to the UMTS space for the residential market as
well. WiMax technology, although theoretically boasting of
supporting up to 75 Mbps of peak data rate per 20 MHz channel,
might be able to provide a real world performance of around 45
Mbps in some fixed broadband applications [8] . The introduction
of HSDPA and HSUPA channels in UMTS is
trying to overcome this threat to some extent but the data rates
achievable in WiMax still remain much higher as compared to
UMTS.
7. ARICENT EDGE
Aricent has more than 20,000 man years of telecom experience and
expertise in different wireless systems to help vendors in
architecting and developing femtocell solutions that have viable
deployment scenario and a faster time-to-market. Aricent has
developed a strong partnership with different eco-system vendors to
keep ahead of the evolving standards and provide quicker solutions
to OEMs.
Aricent service offerings in the Femtocell space includes:
. Technical consulting - Migration to newer technologies, Defining
and selecting right architectures
. defining system architecture including system module definition,
interface definition, definition for MIB, fault management,
performance management, system dynamic behavior for a high
availability solution.
. designing high-level and low-level architectures for call control,
resource management, user plane protocols and applications
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. Product development Compltete end-to-end software
development, feature development
. development of high availability, carrier grade solutions for
UMTS Access Network nodes
. Testing Test automation, system integration, interop testing
. Integration of Femtocell with 3rd party UE (both Test UE & actual
UE) to execute functional test scenarios
Extensive Load, Soak & Performance Testing experience in the
UMTS domain
Aricent IPRs in the Femtocell space include:
. Control Plane Stacks RANAP, RNSAP, NBAP, RRC,RLC,
MAC, MAC-es, MAC-hs, MAC-e
. Transport Place Stacks SS7 stacks- SCCP, MTP3-
B/SAAL-NNI, M3UA/SCIP, ALCAP, SAAL-UNI
. User Plane Stacks Iu-UP, Iub-FP, Iur-FP, GTP-U, PDCP
8. SUMMARY
Every solution being conceived or developed will bring its own
complexity and challenges. A mature solution to meet these
challenges is evolving and industry experts are expecting the UMTS
to come as a front-runner in the indoor solution.
9. REFERENCES
. http://www.telecomredux.com/index.php?option=com_content&
task=view&id=3478&Itemid=35
. Kineto Wireless, The Role of UMA in Mobile Network Evolution,
White Paper, August 2006
. Kineto Wireless, The Case for UMA-Enabled Femtocells, White
Paper, August 2006
. 3GPP TS 25.104,Base Station (BS) radio transmission and
reception (FDD)
. http://www.wimax.com/education/faq/faq38
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