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Self Organizing Networks
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.pdf  1462232240-201412PerformanceSimulationHetNetsAnimutDemekeMasterThesis.pdf (Size: 5.52 MB / Downloads: 4)



Abstract
It is not a hidden fact that, in today’s wireless and cellular networks,
businesses and end users have clearly become dependent on mobility and
the freedom of being always online. Smart phones, tablets and other
devices are expected to be connected all the time and the applications
on these devices are required to run smoothly. That is, for a mobile
network to even be considered to have the minimum requirements from
its users today, network problems like dropped calls, choppy videos and
slow downloads should be out of the question. However, with the fast
growth rate of wireless communication devices competing for spectrum,
the mobile access networks that we have today will soon reach their
maximum capacy. To address these high expectations, mobile network
operators are employing new strategies in densely populated areas and
commercial buildings where spectrum scarcity is highest. 3GPP has been
working on LTE Advanced (Long Term Evolution - Advanced) in order
to improve the spectral efficiency by employing Heterogeneous Networks
(HetNets). HetNets improve the business model for mobile network
operators and users by introducing network topologies which are less
costly and are able to increase the capacity and coverage provided by the
traditional macro cell mobile networks. HetNets are comprised of macro
cells and low cost - low power base stations like picocells, femtocells,
relays and other small cells along with WiFi-APs and distributed antenna
systems (DAS). With HetNets a significant network capacity gain and
uniform broadband experience can be provided to the users anywhere
and at low cost, since the spectrum can be re-used across the multiple
tiers in the network. However, HetNet deployments come with a lot of
challenges and advanced interference control and management techniques
are required to get the highest possible benefit from these networks.
The enhanced Inter Cell Interference Coordination (eICIC) technique has
been studied previously to address the interference problem that these
small cells experience from macro cells. This thesis will concentrate
on the performance improvement achieved when eICIC mechanisms are
introduced to a simple HetNet. The eICIC approach as described in 3GPP’s
Release 10 involves two mechanisms, which this thesis also tries to take into
consideration. These are the CRE (Cell Range Expansion) acheived through
cell bias adjustments and ABS (Almost Blank Subframe) ratio. A discussion
about the importance eICIC in SON - Serlf Organizing Networks will also
be done


Introduction
This Chater will be an introductory part outlining the basic reasons of why
we needed HetNets. This will be followed by a short problem statement
about the work done in this thesis and finally, a description of the structure
of the rest of the thesis will be presented.
The continuous exponential growth in distribution and usage of
smart phones, tablets and other mobile communication devices together
with the electronic devices we use in our everyday activities (like smart
TVs, ATMs, indoor and outdoor appliances and transportation devices) are
generating a continuous and enormous growth of bandwidth requirements
for mobile networks and applications. Although, there has been considerable
improvements in the broadband technologies today(like WiMAX and
LTE) aiming at a much better performance in the 3G and 4G cellular technologies
that are currently being used, they will definitely be unable to
compensate or meet the expected increase of mobile traffic in the very near
future. As shown in Figure 1.1, the monthly mobile voice and data traf-
fic measured by ericsson proves that the data traffic increase observed in
Quarter 1 of 2014 is more than 3X compared to how it was in Quarter 1 of
2012, while the voice traffic change is not as significant as the data traffic.
Following the direction 3GPP has introduced in Release 10 [[16] Section
16], within the long-term evolution-advanced (LTE-A) specifications,
heterogeneous networks (HetNets) which involve the use of existing macro
- cells and small-range (i.e., pico, micro, and femto) cells, seem to be one of
the very few possibilities to follow towards finding the future solution in
order to address the enormously growing bandwidth demand. Small cells
could maneuver the enhancement in the coverage and capacity in densely
populated areas inside the traditional macro-cell deployment. 3GPP has
also made special consideration to femto-cells for providing a better broadband
coverage in home and office installations, and also for outdoor deployments
where a small geographical coverage is required. Femto-cell
deployments have actually become of more interest for mobile network operators,
since studies show that home and commercial environments will
be the the ones that would generate most of the data and voice communications
in the near future. However, the deployment of HetNets does not
come without inferring bigger challenges. In fact, HetNets nature of including several type of low power small-cells overlaid in the macro-network,
would create a great deal of macro to small - cell or small - cell to small
- cell interferences.[4] Apparently, an accurate and precise simulation and
performance evaluations of the proposed Inter-cell Interference coordination
and management mechanisms has become inevitable in order to decide
the most appropriate deployment scenarios of small-cells in the existing
broadband cellular networks.
1.1 Statement of the problem
A lot of studies have been made in the past to point out the importance
of eICIC (Enhanced ICIC) algorithms in achieving the best possible performance
improvement of Heterogeneous networks (HetNets). However, the
exact implementation of the ABS (Almost Blank Subframe)mechanism for
eICIC is left for operators. And since the Self Organizing approaches followed
by each network operators differ greatly in different parts of their
network, so does the appropriate association and decision rules for CRE
(Cell Range Expansion) and Resource Block allocations also. Determining
the radio resource allocation schemes and patterns by using ABS techniques
and evaluating the results that could be obtained by using these
schemes is, therefore, a very important part of HetNet deployments. To
achieve acceptable results, the methodology followed in this thesis work
is:
• Background study to get a good understanding of the problem
and learn about the different approaches that are being used/are
proposed to be used for inter - cell interference coordination and
management in HetNets.

Get familiar with the LENA - LTE/EPC network simulator which is based on the popular open source internet systems simulation
platform ( NS - 3).
• Implement ABS mechanisms in a simple HetNet scenario to address
the Inter - cell interference problems in these type of networks, (eICIC
mechanism), in LENA - LTE simulator.
• Evaluate and compare the simulation results obtained with the
theoretical assumptions or other previously done studies on this
topic.
• Draw conclusions based on the observations made and suggest a
preferable path to future enhancements and HetNet deployments.
1.2 Description of remaining chapters
The rest of the thesis is organised as follows:
Chapter 2 - Background and Review of Literature: will be focused
on the background study that has been carried out as a basis for this thesis
work. LTE network architecture, LTE HetNets and the importance of eICIC
mechanisms in LTE HetNets has been studied.
Chapter 3 - LENA LTE/EPC and NS3: This section of the thesis is
about getting to know what the LENA project and the LTE/EPC simulator
done in conjunction with NS - 3 was about. An X2 interface or messaging
module modification has been done and some time was spent to go through
these changes and understand what has been done in this context.
Chapter 4 - Methods of Inter - Cell Interference Coordination: This
chapter discusses about the different Inter - Cell Interference coordination
and management mechanisms that have been proposed and standardized
by 3GPP. The different types of ICIC mechanisms mentioned could be
implemented to give acceptable results in different deployment scenario,
but ABS technique for eICIC has been proven to be better in HetNets and
it has been discussed in detail.
Chapter 5 - ABS and CRE for eICIC implementation in NS3:
This chapter concentrates on the implementation details that has to be
carried out in order to realize ABS for eICIC. The topics outlined in this
section include: Cell selection and Cell Range expansion (CRE), Victim
UE selection and Protection, packet scheduling strategies and Scheduler
modification made in the course of this thesis, and finally the ABS
mechanism and implementation in detail.
Chapter 6 - Simulation Results and Conclusion: This chapter
covers the results obtained in the simulation process of ABS technique for
a simple HetNet scenario and make some conclusions based on the results.
A suggestion of what could be done in the future has also been done.
Appendices are also included in the end:
Appendix A - Glossary: A list of abbreviations used in this thesis.
Appendix B - Files: A description of files that follow with this thesis.


Background and review of
litrature
This chapter will focus on describing the basic theoretical aspects which are
relevant for this thesis, which are LTE Advanced network, Heterogeneous
Networks (HetNets) and enhanced Inter Cell Interference Coordination
techniques as standardized by 3GPP.
In Heterogeneous Networks (HetNets), both the macro-cell base
stations and the underlying small-cell (low powered) base stations are
often called eNodeBs or eNBs; and these base stations can be using similar
technologies (like LTE) or different technologies (such as, the Macro using
LTE while the small-cell base stations would be using WiFi). In order to
achieve the purposes of HetNets in increasing the overall network capacity,
provide enhanced coverage and customer experience, some mechanisms
should be in place to allow operators to dynamically deploy small-cells
for better coverage and efficiently offload the Macro eNB data traffic. This
is done by considering different parameters of the network such as traffic
characteristics, QoS demand, network congestion and so on. In order
to make a good basis for the interference coordination techniques in the
coming chapters, the next sections of this chapter will discuss some of the
most basic theoretical aspects of Heterogeneous Networks and eICIC. It’s
worth mentioning that by the time this thesis is conducted there has already
been extensive studies and towards the deployment of Heterogeneous
Networks and evaluations of the techniques to cell coordination for these
type of networks.[4][24][14]
2.1 LTE Network Architecture
In order to address the problems or demands caused by the enormously
growing mobile data traffic, the 3rd Generation Partnership Project (3GPP)
standardized a new broadband technology in its release 8 in march 2009
and named the technology as Long Term Evolution (LTE). The introduction
of new features (like flat network architecture and flexible spectrum) in
LTE made it possible to give the current cellular networks a much better
performance compared to the networks which were based on previous standards from 3GPP (like HSPA+ which is also called Evolved 3G),
especially when considering down link (DL) and up link (UL) peak data
rates, spectral efficiency and latency.[21] At the end of 2010, 3GPP made
a lot of improvements to the previous release of LTE and came up with
LTE-Advanced (LTE-A). This release is submitted in order to meet the
requirements of the International Mobile Telecommunications - Advanced
requirements that were published in 2008.[8, 26]
The enhancements that were made in order to come up with the LTE
technologies include the modification of the Radio Access Network (RAN)
and the core network itself. This resulted in a new core network called
as Evolved Packet Core (EPC) and a new and modified RAN called as
Evolved Universal Terrestrial Radio Access Network (E-UTRAN). Figure
2.1 shows a high – level overview of the LTE architecture along with the
standardized interfaced. Here, we will only discuss only some of the
components making up the LTE architecture which are specifically related
to the work in this thesis.


E - UTRAN: carries out the task of providing the radio communication
need to connect the UEs (User Equipments) to the Evolved Packet
Core (EPC). And the Evolved Node Bs (eNodeBs) are equivalent to the the
traditional base stations but with much more enhanced features. The eNodeBs
use the S1 interface in order to relay data from the UEs to the EPC.
In addition to macro eNodeBs (MeNB), which have a high Tx power, the
E - UTRAN also has other alternatives of eNodeBs with lower Tx power.
This includes eNodeB variations like Pico eNode (PeNB), Femto eNodeB
(FeNB) and so on. These lower power eNodeBs are underlaid in the E –
UTRAN in such a way that they would complement the high transmission
power eNodeBs. The PeNBs for example are deployed outdoors in city center
or metro areas where as FeNBs are deployed indooers to provide and
enhance coverage in home and office installations.[16] In addition to relaying
data to the EPC, the following are some of the additional functionalities
of eNodeBs:


• Radio Resource Management (RRM): this functionality is to carry
out the administration of radio resources, scheduling (dynamica
allocation of UL or DL resources).
• Radio Mobility Management (RMM): this is to carry out tasks
of measuring, and analyzing mobility in order to make handover
decisions.
• Radio Resource Control (RRC): is about the functionality of facilitating
the reserving, modifying or allocating resources for the transmission
between the UEs and eNodeBs.
The X2 Interface: is a logical interface that interconnects two
eNodeBs. In the practical E – UTRAN the logical point to point link
between two eNodeBs should be possible even if there is no physical direct
connection between the eNodeBs. This interface is mainly used for the
exchange of signaling information between the eNodeBs, which helps the
network acheive some of the features of the E - UTRAN mentioned above.
The X2 interace model that has been implemented in the LTE – LENA
simulator depicts a point to point link between the eNodeBs. That means
a point – to – point device is created in both eNodeBs and both point – to
– point devices are attached to the point – point – link.[2] More on the X2
interface and the modification made to the LENA X2 interface messaging
module in the course of this thesis is presented in chapter chapter 3 - The
LENA LTE Project and NS-3.
2.2 LTE HetNets
As it has been mentioned in the previous chapter, a systematically
studied network planning is the key approach to addressing the enormous
increase in the mobile broadband subscribers and services that are
continuously becoming bandwidth – sensitive in terms of the limited radio
resources available. Operators have been reacting to these challenges by
continuously enhancing the capacity by introducing new radio spectrum,
adopting modulation and coding scenarios that are more efficient and
also finding a ways to implement new multi-antenna schemes. However,
these techniques and scenarios alone will not suffice in addressing the
challenges in areas where mobile broadband subscribers are densely
populated or around cell edges where the network performance decreases
very fast. Therefore, operators have also started actively researching and
deploying small – cells and carefully integrate them with their existing
macro networks to help with traffic offloading, performance improvement
and QoS while re – using the spectrum in the most efficient way.
Keeping the macro – network in its homogeneous state and trying to
expand it by adding more macro – cells (MeNBs) could have been one way
of adding more performance to the network. But, decreasing the macro –
to – macro distance is only possible to a certain limit. And Moreover, it
would be very difficult to deploy more MeNBs, both cost wise and location wise. Specially, in city centers and metro areas. An alternative solution
operators are implementing to address these problems is using small –
cells by using low – power (low – range) base stations ( such as eNBs,
PeNBs, HeNBs or Relay Nodes (Rns)) and Remote Radio Heads (RRHs)
in to the existing Macro – cell (MeNB). The resulting network with this
kind of approach will be a Heterogeneous Network (HetNet) which brings
a new kind of topology to the network, where MeNBs covering large area
with underlaying small – cells that help the macro – cell to achieve higher
bit rates in a unit area. Figure 2.2 shows a typical LTE HetNet architecture.


As it is depicted in Figure 2.2, In decreasing transmission power
these small – cells of HetNets are called as macro - , micro - , pico -
and femto – cells and the corresponding cell sizes doesn’t only depend
on the transmission powers of the eNodeBs, but also on the deployment
environments and antenna positioning. (such as indoor, outdoor, metro,
city and rural sites). The Femto eNodeBs (FeNBs) that were introduced
in LTE Release 9 [21] are designed with a primary purpose of serving and
enhancing indoor coverages (such as commercial building deployments).
The unplanned mass deployment nature of FeNBs and the fact that
they are privately owned by home and office owners makes them create
the so called Closed Subscriber Group (CSG); and thus FeNBs are only
accessed by specific set of UEs in that subscriber group. This creates
another level of interference between the surrounding FeNBs combined
with the interference from other cells in the area. A variety of femto –
cell interference management solutions have been studied depending of
the physical layer technology and specific deployment scenarios. Most
of the approaches followed in this case require the FeNBs to frequently
communicate with the MeNB and the PeNB so that UEs suffering from
interference could be identified through the backhaul and an interference management will be done. This thesis will not consider the femto – cell
interference coordination scenarios but there are a lot of references that
could help in the implementation if one is interested on that area.[27]
Another type of small – cell in HetNets is the Relay Node (RN),
which is also a low – power eNB which was standardized in LTE Release
10 [16]. The RN is connected via the Un radio interface to the eNB from
which it relays the signal. The RN thus faces interference when the Uu and
Un are using the same frequency. RN interference management is also not
covered in this thesis, but the reader can get more information about this
from the the studies and implementations which have already been done
in this aspect.
2.3 LTE HetNets and eICIC
As we have discussed in the previous sections of this chapter, the true
advantage of HetNets over the traditional homogeneous networks comes
with the use of different types of low – power small – cells effectively
deployed with macro - cells. However, the expected performance
improvement of HetNets would almost be impossible to achieve if the
proper interference coordination mechanisms are not in place. In Release 8
and 9 of 3GPP, an Inter – Cell Interference Coordination (ICIC) mechanism
was introduced. And ICIC helped to some extent in achieving the high
frequency reuse factor set by the LTE standards. Cell edge users in
homogeneous networks benefited a lot more when ICIC was implemented
since the mechanism implemented different types of frequency reuse
schemes to allow cell edge UEs use different sub carriers or resource blocks
(RBs). Which resulted in an improved SINR levels to the cell edge UEs. But
since ICIC technique allocates different sub carriers only when delivering
data channels and control channels are not transmitted through different
sub carriers, this caused interference from the control channels used by the
neighboring cell edge users. Enhanced ICIC (eICIC) on the other hand is
not a frequency domain interference coordination mechanism, instead it
uses the time domain to allow cell edge UEs to use the resource blocks in
different time domains. Therefore, eICIC is a more preferred technique in
the interference management in HetNets. A detailed comparison between
the ICIC and eICIC mechanisms is made in the section 4.1.
 


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