ABSTRACT
The present day cellular network faces an increasing problem of providing capacity and coverage for users. The Long Term Evolution (LTE) system provides a number of ways in
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mitigating these problems among one of which is the deployment of femtocell technology. The femtocell is an example of a heterogeneous network which comprises of different layers of different cell sizes ranging from microcell, picocell and radio relay nodes.The femtocell is the least in size of network densification and is deployed at indoor environments. Deployment of femtocells is not without its challenges, among which is frequent handover. The problem of frequent handover arises from the nature of the femtocell owing to its unplanned deployment, small cell size and access control techniques. Thefrequent handovers in femtocells reduces the user‟s call quality. This research work addressed thefrequent handover problem by considering the users motion as it changes, using the Link Expiration Time (LET) method and theinter-femtocell handover scheme of Rajabizadeh and Abouei in order to accommodate mobile users whose speeds are varying.This scheme was designed to establish a communication link with the nearest femtocell when the users speed undergoes an abrupt change. The developed handover scheme was implemented using a developed Graphical User Interface (GUI) in MATLAB and its performance was evaluated with the traditional handover scheme based on the number of handovers and the time interval between handovers as performance metrics. It was observed from the results obtained that the developed handover scheme performed fewer handovers in comparison to the 3GPP handover schemewhich was better by 24.17%.
TABLE OF CONTENTS
TITLE PAGE I
DECLARATION II
CERTIFICATION III
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DEDICATION IV
ACKNOWLEDGEMENT V
ABSTRACT VI
TABLE OF CONTENTS VII
LIST OF FIGURES X
LIST OF TABLES XI
LIST OF ABBREVIATIONS XII
CHAPTER ONE: INTRODUCTION
1.1Background 1
1.2 Motivation3
1.3 Statement of Research Problem3
1.4 Aim and Objectives4
1.5 Scope of Dissertation 4
1.6 Significance of Research 4
CHAPTER TWO: LITERATURE REVIEW
2.1 Introduction 5
2.2 Review of Fundamental Concepts 5
2.2.1 Long Term Evolution System Architecture5
2.2.2 Handover7
2.2.3 Femtocells17
2.2.4 Link Expiration Time Mobility Prediction 23
2.4 Review of Similar Works 24
CHAPTER THREE: RESEARCH METHODOLOGY
3.1 Introduction 32
3.2 Replication of Inter-Femtocell Handover33
3.2.1 Development of Femtocell Block33
3.2.2 Development of Macrocell-Femtocell Architecture34
3.2.3 Mobility Model35
3.2.4 Path Loss Model36
3.2.5 Received Signal Strength36
CHAPTER ONE
INTRODUCTION
1.1 Background
Intense consumer demand for mobile data and poor signal quality within buildings are two issues mobile network operators grapple with. In mobile communication, it is estimated that nearly two-thirdsof the calls made and over 90% of data usage occurs indoors (Zhang & De la Roche, 2010) and according to a study carried out by Cisco, it observed that the global mobile data traffic will increase nearly 11-fold between 2013 and 2018(Cisco, 2014).
This rapid increase in mobile data activity has led to the development of innovative new technologies and cellular topologies that can meet these demands(Andrews et al., 2012).In order to achieve high data rate communications, the transmitter and receiver can be brought closer to each other to improve system capacity. An example of such idea is the femtocell technology(Chandrasekhar et al., 2008).
A femtocell, also known as a home base station or femtocell access point (FAP), is a short-range, low-cost, low-powered base station installed by a user for better indoor voice and data reception(Chandrasekhar et al., 2008). Compared to microcells and Picocells, femtocells are deployed at indoor environments such as homes, offices, shopping malls and airports etc. to extend the coverage and improve the capacity of a mobile network(Li et al., 2016).The FAP can be classified into two types depending on the capacity and number of users. They are classified as home FAP which can support 3-5 users and enterprise FAP which can support 8-16 users (Shanavas et al., 2013). The femtocell operates in a licensed spectrum and communicates with the operator‟s network over a broadband connection such as a digital subscriber line (DSL), cable modem or a separate radio frequency backhaul channel (Mahmud et al., 2013).
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Femtocells are operated with low transmission power (maximum of 20 dBm)(Rose et al., 2011) and this transmitted signal power defines thefemtocell coverage area. It also has an impact on the interferencethe user experiences, the user equipment (UEs) service off rate, the handover process and signalling(Shbat & Tuzlukov, 2012).The basic difference between a macro cell and a femtocell is the respective backhauls. The femtocell backhaul is simply an interface to the operators core network through the public internet network, while the macro cell backhaul is a dedicated link to the core network (Shbat & Tuzlukov, 2012).There are many advantages for the deployment of femtocells to both the user and the mobile network operator. For the user, the use of a femtocell within the home enables far better coverage and capacity to be enjoyed, the battery life of a UE is also improved because of the low power radiation(Ali-Yahiya, 2011). For the network operator, the cost of deploying extra infrastructure to increase capacity is substantially reduced,as there is no added cost in maintaining and running the femtocells. It simply provides a cost effective means of improving capacity and macro cell reliability.
Despite these advantages of the femtocell technology, there are challenges associated with its deployment. These include interference management, resource allocation, and seamless handover (Li et al., 2016).Cell handover has been considered as one of the most challenging issue in Long Term Evolution-Advanced (LTE-A) macrocell-femtocell network (Xenakis et al., 2014). This is due to the following reasons:
a) The unplanned nature of femtocell deployment
b) The small femtocell radius
c) The denser network layout
d) The employment of access control mechanisms.
The apparent lack of seamless handovers leads to redundant handovers which decreases the user‟s throughput because of interruptions(Becvar & Mach, 2013).
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1.2 Motivation
In recent times as mobile network operators continually upgrade and expand their network infrastructure to meet demand and generate new revenue streams. There are attendant challenges to the attainment of these objectives. More so the unintentional disruption of the conventional network operation. Furthermore with the explosion data intensive applications and smartphone devices,that these changes brings makes thefemtocella favourite option to increase capacity in areas with high user demand and to fill-in areas not covered by the macro cell network.
1.3 Statement of Research Problem
When femtocells are densely deployed, problems such as inadequate system capacity,increase in the signalling overhead (Badri et al., 2013) and poor quality of service arises. These Problems most times are a result of the frequent handovers that will occur due to the small cell size of the femtocell and the speed of the UE. 1.4 Aim and Objectives The aim of this research work is to develop a modified handover decision algorithm using the LET mobility prediction technique to mitigate the problem of frequent handover. The objectives of the research are as follows:
a) Replicate and implement the traditional and inter-femtocell handover decision algorithm ofRajabizadeh and Abouei, (2015)
b) Develop a modified handover decision algorithm using the LET method.
c) Compare the performance of the modified handover decision algorithm to that of the traditional handover decision algorithmbased on the number of handovers and time between handovers.
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1.5 Scope of Dissertation
This dissertation addresses the problem of handover in a LTE macrocell-femtocell heterogeneous network. The basic concepts of handover and an LTE Network in general, are discussed. Previous research works that treated the handover problem are reviewed. Furthermore, the methods adopted in solving the problem is discussed and finally the performance of the proposed method is evaluated using the number of handover and average time before handover metric.
1.6 Significance of Research
This research work is important because it enables the development of a modified handover decision algorithm in order to determine the effect of user velocity on handovers forfemtocells in close proximity. The research then provides a mechanism that offers improved call quality and reduced handovers.
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