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Dense Areas Femtocell Deployment: Access Types
and Challenges
Afraa Khalifah1, Nadine Akkari2, Ghadah Aldabbagh3
Faculty of Computing and Information Technology
Computer Science Department
King Abdulaziz University
Jeddah, Kingdom of Saudi Arabia
1akhalifah@kau.edu.sa, 2nakkari@kau.edu.sa, 3galdabbagh@kau.edu.sa
Abstract— In large open dense areas, high numbers of people use
their smart phones to share pictures or data and download other
information. This behavior creates traffic profiles that differ
from those typically seen in the traditional network in less dense
areas where less uplink traffic and less frequent packet
transmission will be experienced. Thus planning for these dense
network conditions must consider the uplink capacity and
control plane dimensioning. Reducing the cell size has always
been the best way to increase capacity. But smaller cells
introduce the cost of more interference control, capacity and
mobility management. This paper will compare the access types
of femtocells in case of dense area networks and present the
related challenges in terms of femtocell interference, offloading
and mobility management. A comparison of existing schemes that
consider the above mentioned challenges is presented and a road
map is given to point out the main protocols and access type that
should be adopted in case of dense areas planning.
Keywords— Femtocell; Offloading; Mobility management.
I. INTRODUCTION
To achieve the huge capacity requirements for networks
during massive event, a number of femtocells will be
deployed. The motivation for the deployment of small cells is
that small cells accept offloading of macro-network
infrastructure for mobile Internet users in highly populated
areas, to provide contiguous wide area with higher capacity. In
addition to capacity improvements due to femtocells, capacity
offload (transferring users from macro to femtocells) helps
macro users to achieve higher throughputs since fewer users
share the macro network resources. Therefore, enabling
capacity offload to femtocells is improving the overall system
capacity. The main driver for user is improved coverage and
capacity to offers better Quality of Service (QoS), not only to
indoor users but also to outdoor users by offloading [1]. For
dense areas access, metro femtocells access are considered as
reducing the cell size has always been the best way to increase
capacity. On the other hand, smaller cells will require more
interference control, capacity and mobility management.
In mass events, the uplink can even experience more
traffic than is seen in the downlink. Streaming is not typically
used during mass. The average data volume per channel
allocation is smaller in mass events because the traffic is
generated by smart phones instead of laptops or tablets.
Fig.1a represents a traditional macrocellular deployment
scenario, with macrocell base stations offering service to both
indoor and outdoor users [2]. Fig.1b represents a joint macro-
femtocell deployment, which allows femtocells to serve
indoor and outdoor users while macrocell resources are used
more wisely to serve a larger set of outdoor users compared to
the deployment scenario of Fig. 1a, hence leading to network-
wide capacity enhancements [2].
Femtocells provide near peak data rates, and very high
capacity. Femtocells are usually installed by users, so they
bring substantial cost savings to help improve indoor and
outdoor coverage by off-loading traffic from the macro
network. Femto is actually fairly small — similar to the size of
typical WiFi router. Femtocell consumes less power and easier
to deploy. The femto base station’s small size and light weight
reduces site and infrastructure requirements significantly, and
its large capacity makes it the right choice for handling mass
event capacity.
Various types of femtocell deployment are available.
Public access femto are mainly integrated in the network and
planned to be only in small numbers so that it is easier to
arrange full handover in both directions (to and from macro
cells). Home femtocells have one antenna. This is for
simplicity but performance falls over if you have more than a
few users. More robust femtocells use two antennas (“Receive
Diversity”) so if one antenna has a poor signal, the other one
will have a good signal. The domestic femtocell is a 4 channel
unit, handling 4 concurrent voice calls. The enterprise
femtocell is a larger device handling between 16-32
concurrent voice call in very heavily dense environment. The
metro-femtocell is a new concept where operator themselves
deploy large number of femtocells for high traffic area with a
low cost solution. The metro femtocell is public access so that
any mobile subscriber can use it. While home femtocells are
limited to registered users, the metro femtocell is visible to the
macro network.
This paper is organized as follows: section 2 discussed the
difference femtocell access types. Section 3 presents the
challenges of the small cell deployment and compare the
existing solutions in terms of interference, mobility
management and offloading capacity. Road map for planning
dense areas networks is presented in Section 4. Section 5
concludes the paper with future works.
64ISBN: 978-1-4799-3166-8 ©2014 IEEE
Fig. 1. Femtocell deployment [2]
II. FEMTO ACCESS TYPES
In orthogonal multiple access, the choices of access mode are
highly dependent on the cellular user density, with both operator
and owner preferring open access in medium density, closed
access in high density [3]. The Differences between different
access modes are presented as per [3]:
A. Open Access
Open Access mitigates interference and provides a better
overall network performance in terms of QoS (Quality of
Service) and throughput [4], because all available resources
are shared between users. However, the number of handovers
and signaling in the network is heavily increased.
B. Closed Access
The closed access mode service is only for Closed
Subscriber Group (CSG) users. However, the system operator
will set a different service levels for the needs of CSG users.
In this case, management is more complex seems femtocell
must also be able to known the user's pay level [5].
C. Hybrid Access
In release 9 introducing open access mode and hybrid
access mode [6] Hybrid Access mode: this access mode only
allows particular outside users to access a femtocell. The
conditions of access to a femtocell by an outside user can be
defined by each operator separately and entry to any guest or
new user can be requested by the owner [6].
In hybrid mode, non-CSG service can only get limited
service. Of course access control mode depends on the
management of system operators [5].
Hybrid cell can be used in exactly the same way as an
open access cell by UEs who are not members of the CSG or
who are CSG-unaware. But in addition, a hybrid cell can be
identified as such by CSG-aware UEs and, perhaps more
importantly, by other nearby small cells. This can have a
number of advantages [7]. The femtocell can be deployed
completely under the control of the operator; this would most
likely be the case in outdoor metropolitan scenarios [7]. For
operator-controlled deployments it is possible that a
centralized cell-planning approach is taken, where the location
of the femtocell and its neighbors are known and modelled
within an Radio Frequency (RF) propagation analysis tool to
allow for Operations, administration and management (OAM)
configuration of the cell [7]. When small cells are used in
areas using shared carrier and in hybrid access mode, traffic
offload from the macro is possible [7].
A hybrid access mode is an effective methods for
admission control and handoff management for users need to
be designed to achieve the desired network objectives [8]. The
hybrid access scheme can balance between advantages and
disadvantages of the other two access modes [9]. The simplest
approach in hybrid access is FIFO until N nonsubscriber, then
any incoming one will be rejected [9]. In both hybrid and open
access modes, the amount of traffic that can be off-loaded by a
femtocell depends on the coverage area of the femtocell [10].
It is better for home users to be attached to a femtocell on
closed access while outdoor users benefit more from open
access one [10].
Table I. shows a comparison of access types as per [3] in
terms of interference, number of handovers, and user density,
in addition to other network parameters.
TABLE I. COMPARISON OF FEMTOCELL ACCESS NETWORKS TYPES
Open Access Closed Access Hybrid Access
Deployment
Public places
(airports,
shopping malls)
Residential
deployment
scenarios
Enterprise
deployment
scenarios
Number of
handovers High Small Medium
Provider Cost Inexpensive Expensive Expensive
Owner
preference No Yes Yes
High user
densities No Yes Yes
QoS Low High High
Femto-to-
Macro
interference
Increase Decrease Decreas e
III. ACCESS CHALLENGES
A. Mobility Management
With dense femtocell deployment there would be a need
for mobility management and handover procedures. With the
deployment of the Home eNodeB, the handover between
femtocell and 3GPP macrocell networks is become more and
more important in the LTE based networks. Handover in
femtocells highly depends upon the access mode being used.
The number of handover is very large in the case of open
access, while are reduced in closed and hybrid access modes
[11]. The handover procedure is also different for femtocells
and a number of procedures have been suggested. A femtocell
can have a large number of neighbors and these neighbors are
created on an ad hoc basis, making it difficult to constantly
keep track of neighboring femtocells.
The communication with large number of neighboring
femtocells for handover would also be difficult in limited
radio resources.
65ISBN: 978-1-4799-3166-8 ©2014 IEEE
Mobility management is a key challenge, as in case of
dense deployment, it would not be possible for a femtocell to
keep track of its neighbors for handover.
An effective and efficient mobility management and
handover scheme is necessary for mass deployment of
femtocells in LTE networks [8].
There are three types of handover in femtocell deployment
scenarios: macrocell to femtocell, femtocell to macrocell,
femtocell to femtocell.
In [12], a mobility management scheme is proposed where
an intermediate node called an Home-eNB (HeNB) Gateway
(GW) is introduced to solve the scalability and security
problems raised by mass deployment of femtocells. The paper
proposed two mobility management methods for handover of
femto-to-femto type. In method 1, HeNB GW acts as a
mobility anchor to control handovers among femtocells. Also,
reduces the signaling traffic in Evolved Packet Core (EPC)
and is more suitable for enterprise and campus use. While in
method 2, HeNB GW is more like a relay between HeNB and
EPC. The two methods are compared based on handover
signaling cost.
In [13] a handover mechanism between macrocell and
femtocell for LTE based networks is proposed. This handover
mechanism basically takes into account the QoS and speed of
the UE for handover. Unlike the traditional UMTS femtocell
handover algorithm, the mechanism does not allow the high
speed users handover from macrocell to femtocell while low
speed users will be allowed. At the same time, the mechanism
is differentiating between real-time users and non-real-time
users. The proposed mechanism will reduce the unnecessary
handover especially for high speed users and non-real-time
users thus the total number of handovers is reduced as well.
Another handover algorithm proposed in [5] considered
the received signal strength (RSS), velocity (V), available
bandwidth, QoS, and interference level. The algorithm reduce
unnecessary handover initialize and remove the cross-layer
interference. The paper propose handover algorithm for LTE-
based femtocell networks for handover type (a) and (b). Also,
it considered hybrid access mode. But the algorithm do not
considered the co-layer interference and handover type (c).
In [14] the study tried to solve the challenges of
interference and mobility management of femtocell systems.
The goal was to allocate appropriate radio resources for macro
user data packet transmission in order to mitigate the effect of
macro-femto interference.
TABLE II. COMPARISON OF MOBILITY MANAGEMENT SOLUTIONS
Type of handover Network access
type
Femto Access
type
[12] Femtocell-to-Femtocell Indoors
(enterprise, home) Closed
[13] Macrocell-to-Femtocell
Femtocell-to-Macrocell
Indoors
(enterprise, home) Closed mode
[5] Macrocell-to-Femtocell
Femtocell-to-Macrocell Outdoors Hybrid mode
[14] Macrocell-to-Femtocell Indoors Closed mode
The offloading coordination procedure between macrocells
and femtocells has been described in detail. After simulation
they decide that the improvement mainly depends on the
number of femtocells deployed in the network. Table II. shows
the different mobility management considerations for different
femto access types and handover types.
B. Femtocell Interferene
The mobile industry is looking for new femtocell markets,
e.g., use of femtocells in massive event. However, installing
femtocells in these challenging scenarios, where more than
one femtocell may co-exists and many users may enter
femtocells’ coverage, leads to major interference challenges
never addressed before in residential deployments [15].
Two types of interferences that occur in two-tier femtocell
network architecture (central macrocell with OFDMA
femtocells) are as follows:
Co-tier interference: This type of interference occurs
among network elements that belong to the same tier in the
network. In case of a femtocell network, co-tier interference
occurs between neighboring femtocells use the same sub-
channels [8].
Cross-tier interference: This type of interference occurs
among network elements that belong to the different tiers of
the network, i.e., interference between femtocells and
macrocells [8].
Fig. 2. Possible interference scenarios [8]
Fig.2 illustrates all possible interference scenarios in an
OFDMA-based femtocell network.
66ISBN: 978-1-4799-3166-8 ©2014 IEEE
When effective interference management scheme is
adopted, then the co-tier interference can be mitigated and the
cross-tier interference can be reduced which would enhance
the throughput of the overall network.
To mitigate interference problems in metro-femto
offloading, Femto-Aware Spectrum Arrangement Scheme was
proposed in [16] where macrocells nearby HeNBs that pose
potential threat of cross-tier interference are put into the
femtocell-interference pool by the Macro-eNB (MeNB) and
are assigned a dedicated portion of the total frequency
spectrum in order to mitigate co-channel interference. On the
other hand, since other macrocell UEs are not close to any
HeNB, they share the rest of the frequency spectrum along
with the femtocell UEs. However, this scheme does not
consider inter-HeNB interference and may be inefficient if the
number of macrocell UEs near the HeNB increases. So, this
method could not be considered in case of mass deployment.
Clustering of Femtocells was proposed in [17]. In this
paper, a portion of the entire frequency band is dedicated to
the MeNB users and the rest is reused by the MeNB and
HeNBs. The clustering algorithm allocates HeNBs into
different frequency reuse clusters and UEs of different HeNBs
in the same cluster use the same sub-channels allocated from
the shared frequency band. Based on the geographical
locations of the HeNBs, the threshold distance for clustering
interference is calculated. If the Euclidean distance between
any two HeNBs is less than the threshold distance, then they
are assigned to different clusters to avoid co-tier and cross-tier
interferences.
Power Control Approach is proposed in [8]. Power control
methods for cross-tier interference mitigation generally focus
on reducing transmission power of HeNBs. Dynamic or
adjustable power setting, which is preferred over fixed HeNB
power setting, can be performed either in proactive or in
reactive manner each of which again can be performed either
in open loop power setting (OLPS) or closed-loop power
setting (CLPS) mode . In the OLPS mode, the HeNB adjusts
its transmission power based on its measurement results or
predetermined system parameters (i.e., in a proactive manner).
In the CLPS mode, the HeNB adjusts its transmission power
based on the coordination with MeNB (i.e., in a reactive
manner). A hybrid mode can be used where the HeNB
switches between the two modes according to the operation
scenarios.
Fig. 3. Centralized and distributed sensing eNB [8]
Another related concept is power control for HeNBs on a
cluster basis in which the initial power setting for the HeNBs
is done based on the number of active femtocells in a cluster
[18]. Fig.3a shows centralized sensing which can be used
where MeNB can estimate the number of active femtocells per
cluster and broadcast the interference allowance information
to femtocells for their initial power setting. Distributed sensing
in Fig.3b can be used where each cell senses if the others are
active in the same cluster and adjusts its initial power setting
accordingly.
Fractional Frequency Reuse (FFR) and Resource
Partitioning were described in [19]. The basic mechanism of
this method divides the entire frequency spectrum into several
sub-bands. Then, each sub-band is differently assigned to each
macrocell or sub-area of the macrocell. Since the resources for
MeNB and HeNB do not overlapped, this scheme mitigates
co-tier and cross-tier interference. The FFR use a fixed
partitioning, which would cause a loss in throughput
performance due to inefficient use of the bandwidth resources
[8].
An adaptive FFR scheme is proposed in [19] by using
dynamic partitioning scheme. The location information of the
HeNBs may be obtained and maintained within the network
through using registered physical address associated with the
IP (Internet Protocol) address that HeNB uses. If the HeNB is
situated at a highly dense inner region, then orthogonal sub-
channels are adopted by the HeNBs. Otherwise, the HeNB
selects a sub-channel randomly for a certain period of time,
and then hops to other sub-channel reducing downlink cross-
tier interference.
TABLE III. COMPARISON OF DIFFERENT INTERFERENCE MITIGATION
SCHEME S
Femto-Aware
Spectrum
Arrangement
Clustering
of
Femtocells
Power
Control
Fractional
Frequency
Reuse
(FFR)
Transmission
mode Uplink Downlink Downlink Downlink
eNB
Cooperation
required
Required Required Not
Required
Not
Required
Access mode Closed Closed
Closed and
open and
hybrid
Closed and
open and
hybrid
Efficiency Low Moderate High High
Type of
interference Cross-tier Co-tier and
Cross-tier Cross-tier Co-tier and
Cross-tier
C. Offload capacity
To increase the capacity in massive event we need to
increase the number of the femto access points because we can
avoid the drawback of idle femto access point in case of
massive event.
Macro-offloading also very sensitive with the type of
environment: Rural, Urban. Dense Urban, Home residential.
In [20], the main focus is to investigate offloading via
power control, femtocell deployment, and offloading due to
the favorable channel conditions to the femto access points
67ISBN: 978-1-4799-3166-8 ©2014 IEEE
through quantifying their offloading gain and studying their
effect on the overall network performance. It was shown that
increasing the number of the femto access points has the
greatest impact on the tier association probability but at the
cost of increasing the percentage of idle femto access points.
In [14], after conducting a simulation study, they conclude
that the improvement mainly depends on the number of
femtocells deployed in the network. In [21] the study focuses
on the uncoordinated co-channel deployment of closed
subscriber Femtocell groups. The authors considered the
number of carriers available to the operator, their
configuration, and also how users were assigned to carriers
and cell types. To analyze macro offload of 3G Femtocells,
the study showed the combination of adaptive Femtocell
power calibration with a Macro-user frequency allocation
method that considered the SINR (signal-to-interference-noise
ratio) difference between the mixed and a Macro-only carrier.
In [22], the study employed statistical models for macro Node
base station and femto Access Point (AP) locations to compute
various probabilities that are important in governing the
interaction between a macro cellular network and an overlaid
femto cellular network. Also, the paper measured the fraction
of users that should be offloaded from macro to a newly-
deployed femto cellular network operating under OA. Finally,
the paper concluded that it is difficult in 3G femtocells to
operate under CSG mode if the respective femto APs cannot
regulate their transmit powers (due to the increased co-
channel interference), whereas if these femtocells were
operating as OA there would be many benefits in terms of
improved coverage and macro traffic offloading to the femto
cellular network.
IV. ROAD MAP FOR DENSE AREA NETWORK PLANNING
In mass deployment, we have to satisfy the QoS
requirements of macro and femtocell UEs and at the same
time enhance the capacity and coverage of the network. In
addition, we have come up with the following guidelines for
planning a dense area.
First, to increase the capacity, an increase of the number of
the femto access points is needed with corresponding schemes
for avoiding interference by assigning different sets of
subchannels to macrocells and femtocells.
Second, for femtocell type and its corresponding access
mode, it is required to change the type of femtocell from
residential to enterprise or metro and set the Access control
mode to adaptive depending on the femtocell density as per
Fig.4. This figure presents the flowchart of the adaptive femto
access mode for dense area. This procedure is triggered when
any new femocell is deployed or a network needs to adjust the
access mode due to the variation of femtocell density. The
femtocell adaptive access mode will first check the density of
femtocells. In the case of low density scenarios, femtocells
could use open access mode. However, femtocells should use
hybrid access mode when the density of femtocells is high.
Third, for the interference management scheme, a Hybrid
interference management scheme is required which combines
power control with resource partitioning like adaptive FFR.
Fig. 4. Femto acess mode for dense area
Adaptive FFR is considered as effective interference
scheme for OFDMA-based two-tier femto networks. Adopting
adaptive FFR requires minimal coordination among HeNBs
and MeNB. It also reduces the signaling overhead and system
complexity. Thus, a hybrid approach based on power control
and adaptive FFR will reduce co-tier and cross-tier
interferences.
Fourth, to achieve higher user offloading to the femto
network tier, femto access points should be deployed at places
where there are favorable channel conditions to the femto
access points. In addition, adaptive femtocell access mode
should be adopted.
V. CONCLUSION
In this paper, we have presented the three main challenges
of dense area network planning which are the interference,
mobility management and offloading. A comparative study
was conducted for the related solutions and current works
aiming to mitigate the corresponding problems and challenges.
A road map for network planning is finally presented based on
the discussed solutions. Future work will consist of developing
an algorithm for macro-femto offloading based on the
proposed road map and studying its performance for an
efficient network planning.
ACKNOWLEDGMENT
This paper was funded by the Deanship of Scientific
Research (DSR), King Abdulaziz University, under grant
No.(11-15-1432 HiCi). The authors acknowledge with thanks
DSR technical and financial support. Also, the authors would
like to thank Professor Andreas Polydoros and Dr. Nikos
Dimitriou for their support and guidance.
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69ISBN: 978-1-4799-3166-8 ©2014 IEEE
