UMA/GAN resembles the femtocell
approach, though femtocell extends
the licensed 2G/3G wireless spectrum
to the customer premises instead of
bridging licensed and unlicensed wireless
technologies. So, the underlying
UMA/GAN architecture can likewise
be extended to support the femtocell
approach by overloading the GAN controller
(GANC) with Iu FGW functionality.
This architecture best suits service
providers who have an existing GAN
infrastructure, but want to offer innovative
high-value, high-bandwidth services
using High Speed Packet Access
(HSPA) capability. Such capability
requires higher bandwidth than what’s
offered by current 802.11 deployments.
The FAP presents the Up interface to
the FGW, which also acts as the GANC.
The FGW communicates with the CN
using the Iu interface. At startup, the
FAP establishes a security association
with the FGW to avoid compromising
subscriber information over the public
IP network. The FAP could use TR-069
or some other similar mechanism to
discover and obtain IP addresses from
the ACS for the FGW. The FGW treats
the femtocell as an IP-based device,
and these nodes communicate using IP
address and port numbers.
The FAP converts voice packets to
RTP packets and forwards them to the
FGW, which may have to convert the
RTP packets back to voice, based on
the CN transport network. Transcoding
multiple times due to different transport
networks may lead to loss of voice
quality, which can be fixed by implementing
strict QoS at the Up interface.
IMS-BASED FEMTOS
Proponents of the IMS-based architecture
are looking to take advantage
of the mobile operator’s move to an
all-IP based core network (Fig. 4). The
evolved IMS CN provides an excellent
platform for service innovation by
exploiting easily extensible technologies
like the Session Initiation Protocol (SIP).
The IMS-based architecture suffers
from the lack of a proven MM and CC
model and limited standards around
handover. However, the business case
for femtocells is intertwined with the
ability to deliver new services in IMSbased
femtocells.
In this approach, the FAP interworks
the UMTS signaling plane with the SIP
signaling protocol over the public IP
network. On the IMS core side, the FAP
may interface directly with softswitches
providing call session control function
(CSCF) functionality using SIP and
interface directly with the home subscriber
server (HSS) using the Diameter
protocol for authentication, authorization,
and accounting (AAA) functionality.
Alternatively, the FAP may choose
to interface with these devices through
an aggregating packet data gateway.
On the bearer plane, the FAP forwards
voice traffic toward the IMS core as
Real-Time Protocol (RTP) packets. QoS
depends on the public IP network’s
capablities, including reliability and
minimization of packet delays and loss.
TR-069 could again be used for zerotouch
initial system configuration and
service provisioning of the FAP.
Handovers in the IMS-based
approach are inter-CN in nature, i.e.,
between the mobile switching center
(MSC) and the serving GPRS support
node (SGSN). The FAP will handle most
RM (bearer and control) functionality
within the femtocell environment and
would defer to the CN during femtocellto-
macrocell handover. The latter is a
key issue to resolve from a standardization
perspective if this model is to reach
widespread operator acceptance.
CONCLUSION AND CHALLENGES
Femtocells provide a potent weapon
for mobile operators as they compete
for additional minutes used within the
home. They let operators improve
coverage, increase capacity, reduce
customer churn, and provide innovative
high-quality services, driving increased
average revenue per user (ARPU).
Yet challenges remain. How much
interference will occur between closely
located femtos in apartment buildings,
and what can be done about it? How
much of a carrier’s backhaul can be
relieved of the massive increases in 3G
data traffic and services like video?
Success will depend on further innovation
and architecture standardization.
The good news is that significant
strides toward a common standard
have been made within the 3rd Generation Partnership Program (3GPP) and 3GPP2. Specifically,
3GPP has announced that an agreement has been reached
on a standard architecture that leverages existing UMTS
capabilities as well as innovations from the UMA approach.
The new interface between the FAP and FGW, known as
Iu-h, still needs to be specified further, but this represents
a significant step forward toward harmonization around a
single architecture. There is more work yet to be done on the
details of the new Iu-h interface, as well as open actions to
drive a next-generation standard based around SIP/IMS. The
rapid move toward an initial standard, though, represents the
industry’s strong desire for successful femtocell rollout.
Until the standard is ironed out, however, femtocell device
manufacturers must be able to support multiple architecture
approaches, since operators want to perform trials now, and
each carrier has divergent requirements for the architecture it
wants to utilize. This choice often depends more on an operator’s
existing network assets and evolution plans than on
the independent merits of the various femtocell architectures.
As a result, femtocell device manufacturers need to develop
significant breadth of support for different protocols inhouse,
which is a complex and time-consuming endeavor, or
partner with key telecom technology experts who can support
all of the different approaches immediately.