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4G Small Cell Big Gains: Increased Cellular
Capacity in an LTE Environment (July 2015)
Michael L. Pulos, Masters in Cyber Security Management
School of Engineering, Washington University St. Louis
[email protected]
ABSTRACT— With the volatile expansion in mobile data traffic,
small cell/femtocell is regarded as an effective enhancement to
mobile QoS and system capacity of existing cellular networks. I
give a detailed description behind deployment challenges,
including topics as radio interference, scalable security test bed
solutions, backhaul concerns, spectral efficiency guarantees,
scalability impacts and RF propagation control.
I. INTRODUCTION
The rapid proliferation of mobile devices has caused
a significant traffic increase on the wireless infrastructure
that originally was designed to support telephony
operations. This paradigm shift towards smartphones,
tablets, laptops, and IoT (Internet of Things) has caused a
distinct traffic expansion in mobile networks. This
data/traffic expansion has been growing at a rate that
exceeds current deployment capacity of the major carriers
in the United States. In 2014 the total smartphone
subscriptions grew to 2.8 billion [1]. According to the
report by
Cisco [2], smartphones generate 49 times larger
traffic, and tablets generate 127 times compared to
conventional feature phones. Moreover, application
consumption by the use of streaming video, music, P2P
file transfer, and cloud storage has added to the network
congestion in the current wireless infrastructure. The
“Internet of Things (IoT)” and new devices will continue
to increase data and network consumption in the future.
Japan is a great example of an over saturated wireless
network and a window into our very own future in the
United States. In Japan, yearly growth rate of traffic from
2011 to 2013 are about 2.2, 1.8 and 1.6-fold [3]. In
contrast, the United is not far behind; please see charts
below: Fig. 1-5
Fig. 1
Fig. 2
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Fig. 4
A new forecast from Ericsson suggest in its latest mobility
report that in the next three years by 2018, there would be 4.5
billion smartphone subscribers worldwide, with 6- percent of
world’s population covered by LTE (Long Term Evolution).
Other findings include the growth of video traffic by 60
percent annually and traffic volume will grow 12-fold [6].
Given the above statistical information, it is acceptable to
fathom that processing this much traffic/data with the scarce
wireless spectrum will become an increasingly challenging
issue in cellular networks [7].
According to Chandrasekhar et al. [8], more than 60% of
mobile voice traffic and 90% of mobile data traffic originate
in indoor environments. Moreover, for the sake of this
discussion I will use femtocell and small cell interchangeably.
We will focus more on Enterprise Deployment vice home use
but I will occasionally reference some of the home use
deployments as some of the methodology for deployment may
be the same. In general small cell service is provided by low
power, low cost, limited-coverage access points (AP), also
known as NodeB or eNodeB or eNodeB in 3GPP/LTE [9].
In the enterprise deployment, the use of small cells can be a
way to enhance current BYOD programs while providing
better signal quality and a more secure posture within your
workspace. Lastly, I will also touch on the following
concerns/topics of small cell implementation:
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Deployment challenges of small cells in an existing IT
Infrastructure?
Deployment options of small cell architecture
Co-channel assignments of small cell and macro network
deployments
What are the interoperability concerns of small cells?
II. ASSUMPTIONS
Access control mechanism that mobile operators and
users are willing to adopt is crucial to the sustaining and
implementation of small cells. Corporate policies and
procedures will help solidify agreements between
employee and employer.
Deployment in the Enterprise will utilize a closed access
mode managed by corporate IT infrastructure.
Closed access mode is needed to insure QoS in the
corporate environment.
Because the licensed spectrum is limited, it is necessary to
implement co-channel assignment in small cell systems
[10].
Co-channel assignment needs to address the problems
caused by cross-tier
(macrocell with femtocell) interference and co-tier
(femtocell with femtocell) interference [11], [12].
Heterogeneous architectures based on nested tiers of more
and more dense small cells operating at higher and higher
frequencies are expected not only to improve the overall
area spectral efficiency of the cellular network but also to
increase coverage and user signal-to-interference-plusnoise ratio (SINR) in most deployment scenarios [13].
Radio-Interface-Based Synchronization is paramount in
hand-off operations from Macrocell cell to small
cell/femtocell.
Firecycle Model and simulations will be used during
deployment, focusing on its capabilities and ability to
scale up simulations and to model itself over multiple
VMs in a corporate cloud environment.
Firecycle has been designed, implemented, and coded
from scratch using OPNET Modeler [14] as the
underlying platform and simulation engine. All the nodes
and elements of the model are custom coded and
assemble together to run as a network simulation on
OPNET. Set of libraries and definition files provide the
means to run the realistic traffic models.
III. CURRENT LTE DEPLOYEMENT SITUATION
To accommodate the increasing mobile traffic,
network upgrading from HSPA to LTE is one of the most
effective solutions for mobile operators. Compared to
HSPA, LTE can perform 10 times higher in transmission
rate, 3 times higher in spectrum efficiency, and
approximately 1/4 in transmit latency. World’s first LTE
service was launched by TeliaSonera on December 2009.
After that, as of September 17, 2014, more than 331 LTE
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networks have been launched in 112 countries with
accommodating 280.4 million subscribers in the world
[5].
Fig. 8 2014 AT&T 4G LTE Map
Fig. 9 2014 Sprint 4G LTE Map
Fig. 6 HetNet structure of LTE-A [13]
Fig. 10 Global LTE MAP
Fig. 7 2014 Verizon 4G LTE Map
IV. SCALABLE SECURITY TEST BED FOR LARGESCALE LTE DEPLOYMENTS
As most of us know cyber security research has grown in an
exponential rate over the last few years, resulting in many
successful mitigation strategies focused on threat elimination
and containment. In the wireless community the majority of
the work focused on the old GSM (Global System for Mobile
Communications) and UMTS (Universal Mobile
Telecommunications System). Today the LTE networks
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remains behind the times in focused security research which
could combat potential breaches in the LTE networks if
additional research is done.
In the future the LTE landscape will encapsulate certain
critical applications with very strict security guidelines and
requirements. Moreover, the next generation EMS
(Emergency Response Systems) planned by the US
Department of Homeland Security: the Nationwide
Interoperable Public Safety Broadband Network [16]. LTE is
also considered as the underlying technology for advanced
military tactical networks [17].
Parallel to the capital security requirements of LTE
networks, the cyber security backdrop has substantially
advanced over the last few years. In the age of massive DDoS
attacks, Botnet armies for hire, mobile malware and fraud and
the advent of the Advanced Persistent Threat, the importance
of enhancing the security of LTE networks against security
attacks is clear [18]. During the implementation of small cells
at the enterprise level there will need to be extensive security
testing and simulations to offer the benefits of off-loading
traffic from the local macro tower to the corporate owned
small cell providing enhanced security awareness/posture.
The enterprise can gain many benefits from integrating
Firecycle modeler methodologies into the current enterprise
architecture and is required in order to maintain a heighten
security posture. Firecycle would be an added item that would
be utilized during standard deployments/upgrades of
infrastructure, transport, application upgrades and would
follow the engineering V in the lifecycle of the project.
Firecycle is designed and built to be compliant with LTE/3PP
utilizing a standard test bed framework. Please see figure (a)
and (b):
(a)
Fig. 11 (b)
Firecycle will be used to assess the impact of large-scale
security attacks against the enterprise LTE and small cell
corporate environment. Statistical data from the modeling
simulations will analyze QoS, load, frequency, and time
occurrence of simulated attack vectors. Quantitative statistical
analysis will help the CSO and CISO determine applicable
corporate security posture for the enterprise environment.
V. ACCESS AND HAND-OFFS
As discussed before in this deployment, access will utilize
a Closed Mode method.
Before a device transmits a signal of its own and during the
power on/up, a small cell base station searches for a primary
and secondary synchronization signal (PSSs/SSSs) of a
neighbor cell (on the downlink) [13]. Once detected, the base
station obtains the ID and timing of the neighboring cell. The
cell uses the acquired neighbor cell ID to determine the CRS
waveform of the cell, and use the timing to locate the cell
synchronization sub-frames where the cell synchronization
signal is present. This procedure is repeated until the small cell
finds the timing source with the lowest stratum. The
synchronization stratum for this cell is then determined based
on the detected source cell stratum, and its CRS signal is
transmitted on the corresponding radio frame determined by
its stratum. In addition to performing routine periodic
synchronization, once in a while, a small cell has to repeat the
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above procedure to detect whether any change occurs that may
impact its own stratum [13]. For the deployment in the
enterprise the synchronization signal (CRS) will be established
near the immediate vicinity of the corporate building. The
corporate enterprise management office will negotiate with the
local carrier to establish cell radio boundaries to insure the
QoS and minimal radio interference if possible. This
arrangement will be crucial for successful hand-offs when
entering the corporate environment. It is possible to entertain a
hybrid access mode but was omitted due to security liability
and is not recommended in an enterprise deployment.
Fig. 12 a) Illustration of multi-hop synchronization in a small cell network,
where cells M and Q are macrocells, and cells A to K are small cells. A small
cell always looks for and synchronizes to the cell with the lowest stratum level
within its detection range. In this example, cell K neighbors cell E, cell H, and
cell D. It synchronizes to cell E that has the lowest stratum (2) in its range.
Cell K thus has a synchronization stratum 3 derived from cell E. The arrow in
the diagram indicates where the synchronization source from which a small
cell receives its synchronization signals; b) illustration of unsynchronized
cells in multi-hop synchronization due to the limit of the maximum number of
synchronization hops. In this example, the maximum number of
synchronization hops per synchronization chain is three, which leaves cell D
unsynchronized; that is; cell D is not able to synchronize to the required
accuracy [13].
In the design of Wireless Mesh Networks (WMN) for
small cell/femtocell, radio interference, spectral
efficiency, RF propagation control and backhaul concerns
pose the most challenging items when integrating small
cell technology into the corporate enterprise environment.
Another consideration is the cost and reliability which can
dictate network topology. The integration into existing IT
infrastructure will have to also take into account the
lifecycle of the current architecture. Some technologies
may not integrate so well into aging and outdated
infrastructure.
We can assume the use of the above methodologies
and a couple of WMN optimization formulas indicated in
this paper will increase the success rate during
implementation and reduce costs during deployment. The
use of spanning tree (MST) and shortest path (TSP) will
provide a reliable backhaul infrastructure to integrate into
the existing network topology of the corporate enterprise.
The following algorithms will be used to evaluate
network topology and efficiency: Prim’s Algorithm
(Minimum Spanning Tree), Floyd Algorithm (Shortest
Path). The aforementioned algorithms have very fast
reproduction velocity and state-of-the-art principles when
dealing with mesh network in total cost, delay or latency,
accessibility and outages [19].
The deployment options for small cell architecture is
limited; however, during the implementation and design
phases of the project radio interference, spectral
efficiency guaranties and RF propagation controls should
be establish with the local carrier/provider. The enterprise
carrier will insure de-confliction of radio signals and
proper RF propagation within the local area of the
corporate environment /building. If de-confliction cannot
be agreed upon with the carrier proper shielding/radio
jamming equipment can be deployed to protect the
corporate interest. This equipment will not be authorized
to effective EMS signals and must comply with FCC
regulations and guidelines. Co- channel assignment of
small cell and macro network cell deployments in the area
with is orchestrated in conjunction with the local carrier
and the organic corporate IT Department. This marriage
between the two organizations will also entertain all
interoperability concerns. Below are diagrams/charts for
additional considerations during implementation.
Fig. 13 Macrocell Coverage and Congestion
VI. IMPLEMENTATION
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Fig. 14 Macrocell, Femtocells (Small Cell), and Picocell
Synchronization
Fig. 15 3GPP standard schedule for LTE/LTE-A[4]
VII. CONCLUSION
With the rapid expansion of mobile data traffic and the
need to have secure wireless transmissions in the corporate
environment, the use of small cell technology is an attractive
solution for an enterprise environment. When implementing
and engineering a solution for the corporate environment
many things need to be taken into consideration radio
interference, spectral efficiencies, RF propagation and scalable
security test bed solutions are a must before integration into
the current corporate IT infrastructure. Additional, concerns
need to address existing IT infrastructure to ensure capability
with existing 4G LTE technologies and the possibility of 5G
future wireless technologies. In conclusion, the
aforementioned algorithms and software/hardware deployment
methodologies can be used for solid bases to be incorporated
into a deployment strategy for an enterprise solution.
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