Femtocells: The HOMESNET vision

Abstract

This paper presents the vision shared by partners from the CELTIC project HOMESNET about Femto Base Station deployed at Home, i.e. Home Base Station (HBS). Three of the main aspects under study are presented. The challenges in HBS self-organizing, including femto-macro coexistence, are explained and methodology for successful self-organization is proposed. To achieve very low radio emission at home or constrained environment like hospitals, an architectural option for HBS relying on Radio over Fiber approach is proposed. Last, an innovative use case for HBS, allowing indoor emergency telemedicine scenarios, whereby, paramedics are able to seamlessly exploit HBS at the patient or neighboring apartment, is described. The expected impacts of the HOMESNET project and the planned next steps toward this vision are presented.

Full text

Femtocells: The HOMESNET Vision
Zwi Altman3, Carine Balageas2, Pablo Beltran6, Yossef Ben Ezra5, Eric Formet3, Jyri Hämäläinen1,
Olivier Marcé2 , Edward Mutafungwa1, Sergio Perales6 , Moshe Ran4 , Zhong Zheng1,
1Department of Communications and
Networks
Aalto University School of Science and
Technology
Espoo, Finland
{edward.mutafungwa, zhong.zheng,
jyri.hamalainen}@tkk.fi
2Alcatel-Lucent Bell Labs
Route de Villejust
91461 Nozay, France
{Olivier.Marce, Carine.Balageas}
@alcatel-lucent.com
3Orange Labs
Issy les Moulineaux
France
{Zwi.Altman, Eric.Formet}@orange-
ftgroup.com
4MostlyTek ,
58 Keshet St. Reut, Israel
mran@hit.ac.il
5Optiway,
16 Hamelacha St., New Industrial Zone,
Park Afek Rosh Haayin 48091, Israel
yossef@optiway.biz
6TELNET Redes Inteligentes S.A.
La Muela, Spain
{pbeltran, sperales}@telnet-ri.es
Abstract—This paper presents the vision shared by partners from
the CELTIC project HOMESNET about Femto Base Station
deployed at Home, i.e. Home Base Station (HBS). Three of the
main aspects under study are presented. The challenges in HBS
self-organizing, including femto-macro coexistence, are explained
and methodology for successful self-organization is proposed. To
achieve very low radio emission at home or constrained
environment like hospitals, an architectural option for HBS
relying on Radio over Fiber approach is proposed. Last, an
innovative use case for HBS, allowing indoor emergency
telemedicine scenarios, whereby, paramedics are able to
seamlessly exploit HBS at the patient or neighboring apartment,
is described. The expected impacts of the HOMESNET project
and the planned next steps toward this vision are presented.
Keywords-component; femtocells,Home Base Station SON,
macro-femto coexistence, Radio over Fiber, Public Safety
Communications
I. INTRODUCTION
As the Femto Base Station (BS) approach starts to hit the
market[12], it is now time to consider its evolution toward the
concept of Home Base Station (HBS). The Celtic project
HOMESNET started on July 2009 with objectives to develop
new approaches and new applications for the Femto BS
deployed at home, so-called HBS. Various in-building
solutions have conventionally been employed in order to
provide acceptable indoor coverage. These include repeaters,
distributed antenna systems and micro base stations that have
been used to improve indoor coverage, and recently, the so-
called Femto BS has also been introduced for this purpose.
Femto BS is characterized by very low cost, plug-and-play
installation, and reduced transmission power that can be even
some decibels lower than that of handheld terminals. Femto
BS can be deployed in enterprise premises, mainly for
capacity and coverage reasons, or at home where it is called
Home BS. Home deployment opens the door to new value
added applications, as well as to new challenges. The main
character of the concept is that the HBS can be deployed in
private or customer premises by a user in the same way as
digital subscriber line (DSL) modems have been deployed for
several years. A HBS is typically owned by user and may
offer access only for a certain user group containing e.g.
family or household members. In fact, due to embedded
wireless local area network (WLAN), DSL backhaul, and
reasonable pricing, the difference between current DSL
modem and HBS might be visible to the user only through
additional capabilities.
The HOMESNET project gathers 16 partners from 5 countries
(Finland, France, Israel, Spain, Turkey), including 2
manufacturers and 3 operators. It aims at addressing the
challenges related to the HBS and to their users initiated
deployment characteristics. This paper presents three of the
aspects that are currently under study within the project.
The intrinsic unmanaged way the HBS are deployed, leads to a
loose knowledge about their exact physical position, and
consequently about their exact coverage. Due to the tight
integration of the HBS with the operator’s network, it is
critical to ensure safe and efficient coexistence between HBS
and macro BS. To reach this objective, the HBS must comply
with Self-Organizing Network (SON) concepts that are
described in section II.
Several architectural options exist for HBS design, aiming at
satisfying requirements in terms of integration with operator
network, shared management with the user or energy
consumption reduction. One of the promising directions is to
rely on cable or fiber distribution of radio signal in different
locations within the home or other indoor environment. This
achieves a very low indoor radio emission that complies with
the strictest constraints, e.g. in hospital environment. This
approach is described in section III.
Section IV describes an innovative use case for HBS, allowing
indoor emergency telemedicine scenarios, whereby,
paramedics are able to seamlessly exploit HBS at the patient’s
or neighboring apartment. This kind of use case and associated
This work has been supported by the CELTIC project
HOMESNET (CP6-009)

concept are strong motivations for a wide adoption of HBS as
this opens the door for new services and business cases.
The last section concludes the paper and presents the expected
impacts of the HOMESNET project.
II. SON PESPECTIVES FOR HOME BASE STATIONS
A. SON mechanisms in HBS
As plug and play devices, HBS should be self-managed.
Hence classical operation tasks, from device configuration to
optimization and troubleshooting, should be performed in a
fully autonomic or self-organized manner. In this context,
SON is considered as a key lever to reduce cost of operation
(OPEX), to simplify the management and to enhance the
performance of the HBS technology.
The important SON mechanisms for HBS identified by the
HOMESNET project are [1]:
• Self-configuration – the process which brings a network
element into service in deployment. It encompasses
setting of transport parameters and establishing
connection to the broadband network, download of
relevant information and software/firmware updates,
setting radio parameters configuration, etc.
• Self-optimization – the process where User Equipment
(UE) and HBS measurements are used to auto-tune the
network. The objective is to limit interference and adapt
the HBS to its environment, namely neighboring HBSs
and overlaying macro base stations.
• Self-healing – encompasses fault detection, fault
diagnosis and fault recovery. Recovery could be the
relocation of users from a faulty HBS to the neighboring
ones by adjusting their coverage and favoring
handover/cell reselection. It aims at minimizing the fault
impact on end user (EU) perceived QoS.
Self-organization of HBS has been extensively studied in
main standards and organizations, including 3GPP [2] and
WiMAX Forum [3]. The NGMN Alliance [4] has published
detailed recommendations for Home eNB (HeNB) for LTE
technology, encompassing self-configuration and self-
optimization.
B. Dense HBS deployement SON scenario
To illustrate the challenges of SON in HBS networks,
consider a dense HBS deployment scenario. Figure 1 presents
an office building with a large number of deployed HeNB,
within a LTE coverage zone of an eNB. The large number of
HeNB results in important coverage overlapping.
Furthermore, outdoor eNB transmissions penetrate the
building while HeNBs' transmissions leak outside it. The self-
optimization mechanisms can mitigate interference while
taking advantage of the multiple transmission sources to
improve the system performance and perceived QoS in both
indoor and cell-edge zone.
C. Methodology and challenges in self-optimization
Among the important self-optimization functionalities are
interference mitigation, mobility management and energy
saving:
• Interference mitigation can make use of different
(dynamic) resource allocation mechanisms such as power
allocation; dynamic soft-frequency reuse; fractional load
[5] in which each HeNB turns off some of its interfered
PRBs; and specific scheduling strategies [6]. In this
context, advanced antenna techniques can be very useful.
It is noted that interference mitigation concerns all SON
mechanisms listed above.
• Mobility optimization concerns both camping and
connected mobiles, and can be combined with
interference mitigation and energy saving management.
• Energy saving management aim at ensuring QoS
provisioning while minimizing energy consumption of the
network.
Figure 1: Macro-femto overlaid coverage in a dense
deployment scenario
The coordinated operation of the self-optimizing
functionalities and of self-organizing mechanisms is one of the
challenges in SON, which is needed to guarantee the network
stability.
Different methodologies for self-optimization have been
investigated. In certain use cases, cognition capabilities are
required, including sensing the environment, processing data,
acting/decision making and learning from cumulated
experience. Recent studies in self-optimization have used
reinforcement learning, stochastic approximation, and more
and more often, game theory framework with different
dynamics, such as potential games, team games, Markov
games and others.
This challenging area is with no doubt in its embryonic
phase, and is one of the challenging topics of the HOMESNET
project.

III. LOW RADIO EMISSION HBS
A. HBS over hybrid wired-wireless
Lowering energy consumption of future wireless radio
systems along with great reduction of the wireless radiation
indoor are important keys to next generation of mobile radio
access systems. As transmitted data volume increases by factor
10 every 5 years, new technologies and solutions are becoming
essential to support this trend while meeting the energy
consumption along with the wireless radiation. In
HOMESNET we address these issues through a system
concept named Very-Low-Radiation Distributed Antenna
System (VLR-DAS). This concept targets the distribution of
various radio-protocols supported in HOMESNET including
3G, LTE, IMT-Advanced and WiMAX at a very-low radiation
based on radio-over-X (optical fibre, Coax, Cat-5, power-line
etc.). Simply stated, the VLR-DAS addresses the potential
consumer health and safety concerns through scalable hybrid
wired-wireless topologies that enable reduction of the emitted
power to minimum level needed for coverage of small cell. We
consider configurations for two use cases namely:
• Residential: In-house distribution network. For
home users the concept implies using the distribution
of target wireless signals inside small number of
rooms (say less than 8) based on existing and future
home infrastructure. The target radio signals are
transmitted transparently from one HBS to target
rooms with possibly frequency translate over wired
home infrastructure. The wired part is serving as a
"range extension" unit over 10's of meters to a set of
repeaters called Home Access Nodes (HAN). Each
HAN transmits at minimum power to cover a single
room.
• Enterprise: "Green Hospital". The Green Hospital
represents a corporate use case of citizen-to-authority
and authority-to-citizen (C2A-A2C) where the
exchange of large amount of medical data over large
number of rooms is considered. Here the range
extension over the wired media is 100'm, and 100's of
users are supported through a multi-femtocells
architecture.
The typical characteristics of the VLR-DAS when applied
to the use cases relevant to HOMESNET are:
• Adding a functional device, HAN, between the HBS
and UE. The HAN is used to reduce the indoor
wireless radiation; perform adaptation of the radio
signal to the wired media and, perform initial
identification and filtering of the target radio signal.
• Supporting multi-radio transparently over wide radio
frequencies 1.8GHz to 10.6GHz. Transparency
means that the radio signals are "relayed" over the
wireless-wired domains with a negligible delay and
minimal degradation in signal quality.
• Basically VLR-DAS is applied to indoor
applications. Hence, issues like mobility management
and handover are less dominant while interference
mitigation and coexistence of multiple wireless
standards in-house are crucial.
A typical residential configuration of VLR-DAS over 4 rooms
is given in the Figure 2 below. It is based on simplified serial
concatenation of several HAN's connected through a
multimode fibre (MMF). The use of separate fibre for the
uplink and downlink aimed to simplify and reduce the cost of
the HAN implementation.
In this example HAN#1 serves as the node that communicates
with the HBS over the air to simplify the installation of the
home network. The other HAN's transmit the radio signals
over the air to the target UE.
Figure 2: Typical residential configuration
The specific requirements for 3G Femto cell are the
following.
• The transmission of the various radio signals from
the HBS through the HAN and the wired-wireless
infrastructure to the UE should introduce minimum
delay and negligible distortion to the quality of the
communication link between HBS and UE.
• Coexistence of target radio signals are required both
over wired-wireless indoor infrastructure.
The introduction of the HAN as an intermediate medium
between HNB and the UE should not affect the protocol stack
between the HBS to core network
B. Radio Over Fiber and Gpon
Gigabit Passive Optical Network (GPON) is the ITU
standard (G.984 series) for gigabit capable passive optical
networks. It is commonly deployed to support 2.5Gb/s in
downstream (1490 nm) and 1.25Gb/s in upstream (1310 nm).
Thus two wavelengths are used for digital communications in
GPON. It is reasonably to think that a broadcast service such as
Digital Video Broadcasting (DVB) would be better distributed
out of these two wavelengths, in order to keep bandwidth. The
current trend is to use the so-called third wavelength (1550 nm)
to broadcast DVB.
DVB requires just the downstream signal, as no
information must be sent from subscribers to the central office
or to the internet.
Femtocell
HAN 1
HAN 2
F
d1
F
u1
F
d1,
F
dN
F
u1
F
d1
F
u1,
F
uN
6 -10 m
6 -10 m
10 – 100 m
HAN 3 HAN 4
10 – 100 m 10 – 100 m

HNB HNB GW
SeGW
HMS
Iuh
Iu
Uu
Core
Network
HAN
Figure 3: 3G femto cell logical architecture Deployment configuration/Options with HAN function.
However, if we want to use the same broadcast
philosophy for extending UMTS, WiMAX or LTE coverage
in home, we need another wavelength for the upstream, as
well as a system able to handle the fiber sharing.
Optical Time Division Multiplexing by Locking Lasers
(OTDMLL) is an analog multiplexing technique in time
domain, which is suitable for multiplexing different types of
analog signals, digitally modulated analog signals or data
signals. By integrating this technology into in-building
wireless platforms one can increase the efficiency by at least
by a factor of 4. Furthermore, by employing a technology
agnostic approach that supports both digital and analog
signals, the project design is significantly simplified.
The ability of Time Division multiplexing of both analog
and digitally modulated signals allows the convergence of
the mobile and fixed communications.
OTDMLL samples RF inputs at 10 Gb/s and higher
sampling rates, utilizing several laser transmitters, each
sampling a different RF signal. This is accomplished by
dividing the optical signal into very short time segments and
multiplexing optical data streams to a single optical path, and
then reassembling them at the receiving end.
The addition of OTDMLL will enable another level of
multiplexing (on top of a WDM multiplexing scheme, for
example). This approach allows the delivery of at least 4
times as many channels on a single fiber. Furthermore, the
OTDMLL technology is agnostic to the original signal’s
frequency and technology, enabling the multiplexing of any
desired mix.
The OTDMLL technology can be used for the
transmission of radio signals over Passive Optical Networks.
Homesnet project will provide the framework to explore the
applications of this novel technology in a femtocell or HBS
deployment
IV. FEMTOCELLS FOR PUBLIC SAFETY SERVICES
A. Public Safety Use Case Scenarios
Routine emergency situations (e.g. medical emergency)
and rare high-impact hazard events (e.g. impending
hurricane) result in a significant heightened risk to human
wellbeing, life and/or property. In both situations public
safety communication services are a critical part of the
preparedness and response to the emergency and risk
reduction. The critical nature of the public safety services
imposes stringent requirements on the underlying
communication systems, such as, legacy macrocellular
mobile (GSM/GPRS, UMTS etc.) and WLAN (IEEE
802.11x) networks. This is attributed to the fact that the
tolerance to service unavailability and sudden performance
degradations are much lower during an emergency event
whereby any significant shortcomings in provisioned public
safety communication services may hinder emergency
response actions.
The increasing HBS penetration promises the benefit of
improved mobile femtocellular coverage and capacity to
support critical multimedia public safety communication
services in indoor environments. As such, the HBS provides
a secure, high-throughput and assured-QoS gateway between
local public safety service users and remote public safety
service providers or control centers. To that end, the
HOMESNET project is studying the potential benefits of
seamless leveraging of available HBS assets for public safety
services based on a set of realistic use case scenarios.
The public safety use cases developed in HOMESNET
are based on the main areas of interactions (authority-to-
authority, authority-to-citizens and citizen-to-authority) that
are specified by European Telecommunications Standards
Institute (ETSI) Special Committee (SC) on Emergency
Telecommunications (EMTEL) [7]. In EMTEL the term
authority/organizations collectively refers to emergency first
responders (fire-fighters, police, paramedics etc.), emergency
control centers, public safety answering points, local
administration or any other organization with the
responsibility of providing services that ensure safety and
security of citizens under risk. The next sub-section describes
an example authority-to-authority use case on emergency
telemedicine that benefits from HBS deployments.
B. The Emergency Telemedicine Case Study
Emergency telemedicine encompasses the utilization of
information and communication systems by emergency
medical service (EMS) personnel (usually, paramedics) for
the complete duration of the emergency event cycle [8]. That
starts from the time the paramedics are initially dispatched to
the emergency site, through process of paramedics providing

pre-hospital care on the site and in-transit (in ambulance) and
ends at the patient’s arrival at a receiving medical facility. To
that end, emergency telemedicine is a contributing factor in
improving overall EMS quality. It enables real-time remote
medical consultations for improved decision-making and
enhancement of preparedness at the receiving medical
facility by advance sharing of medical data of incoming
patient. Furthermore, the increased adoption of broadband
mobile communication services by EMS organizations is
enabling multimedia applications for emergency
telemedicine, such as: high-resolution medical image
transfer, high volume medical data telemetry, real-time
streaming video from emergency site and interactive video
consultations [8]. These applications further enhance EMS
clinical value and boost productivity of EMS personnel.
Mobile Operator Core Network
Subscriber and
Equipment Registers
Location Centers,
Application Servers
MBS
RNC
Femto
Gateway
Internet
Local IP Access (FTTP/cable/DSL etc.)
Medical Facility,
Remote Consultation Site
Multiple Dwelling Unit
Paramedic
EUE
Patient
Notes: CSG = Closed Subscriber Group (of a HBS ), DSL = Digital Subscriber Lines, FTTP = Fiber-to-the-Premises, EUE = Emergency User Equipment (Paramedic’s UE, temporary CSG member), GGSN =
Gateway GPRS Support Node, GPRS = General Packet Radio Service, HBS = Home Base Station, HUE = Home User Equipment (UE member of CSG), MBS = Macrocellular Base Station, MGW = Media
Gateway, MUE = Macrocellular User Equipment (UE non-member of any CSG), RNC = Radio Network Controller, PSTN = Public Switched Telephone Network, SGSN = Serving GPRS Support Node.
Emergency Site
Neighbouring
HBSs
Physician/
Emergency Department
HUE
Patient’s HBS
MUE
EUE uplink via Patient’s HBS
EUE uplink via Neighbouring HBS
EUE uplink via MBS
EUE Uplink Connectivity Options
Internet, PSTN,
Other Mobile Network
Patient Records, Resource
Planning, Decision Support etc.
MGW/SGSN
MGW/
GGSN
Figure 4: General end-to-end system architecture
Previous statistical surveys of routine emergency medical
events (e.g., [9]) noted that well over two thirds emergency
events occur in indoor residential environments. These indoor
locations therefore represent the first direct point of contact
between the paramedics and patient, at a moment whereby
emergency telemedicine applications are also required. This
fact highlights the acute need for ubiquitous broadband mobile
data bearer service. This lets paramedics to have continuous
access to their complete suite of emergency telemedicine
applications, particularly in those indoor emergency sites. End-
to-end multimedia service quality also has to be maintained for
an emergency telemedicine session. In this usage any
significant service outages or noticeable quality degradations
may compromise the ultimate objective of providing a reliable
and efficient EMS service [10]. It is noted that contemporary
macro mobile networks may have limited indoor coverage and
capacity for the emergency telemedicine purposes. On the
other hand private (residential) WLAN networks lack the
centralized operator management (e.g. to override user-set
WPA/WEP access controls) required for rapid and prioritized
service provisioning in emergency events.
The HOMESNET project is studying a complementary
solution based on the HBS concept for indoor emergency
telemedicine scenarios. In this approach paramedics are able to
seamlessly exploit HBS resources in the patient’s or
neighboring apartment for emergency telemedicine purposes
(see Figure 4). Extensive simulations are carried out within the
project to compare the performance of the proposed
complementary HBS approach to the conventional
Macrocellular Base Station (MBS)-only approach. Some early
performance simulation results indicate at least one order of
magnitude reduction in service outage rates when paramedics
are able to utilize locally available HBS resources, in the

comparison to the MBS-only case, particularly for multiple
dwelling unit buildings located on the cell edge [11].
Moreover, further studies are planned to quantify the
performance benefits (from the perspective of this case study)
of the various interference mitigation techniques (e.g.,
beamforming, scheduling, etc.) in co-channel macro- and
femtocellular network environments.
V. CONCLUSION
This paper presented three topics that are investigated within
the HOMESNET project. The results on these 3 axes will be
keys to ease HBS deployment, allowing a low impact
integration of the Femto BS at home, a smooth coexistence
with the macro BS, and a high value added service that will
help the adoption of HBS by end users. Beyond coverage and
capacity enhancement that are the primary target of the Femto
BS as defined today on the market, this concept allows to
revolutionize the way the wireless connectivity is provided to
the users at home. By gathering partners from various
horizons that bring into the project their visions and skills,
HOMESNET will help in keeping R&D competence as well
as Intellectual Property Rights (IPRs) owned by European
industry. The next steps will consist in the definition of a HBS
prototype that will demonstrate technological advances in
Femto cell deployment and usage at home.
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