Abdisalam Thesis

DECLARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT

Author’s full names: ABDISALAM ABDULAHI MOHAMED

ABDIRAHMAN MOHAMED OMAR

ALISALAD ALASOW MOHAMED

Title : INTRA-BSC HANDOVER OPTIMIZATION FOR GSM

QUALITY OF SERVICE IMPROVEMENT

Academic Session : 2014/2015

I declare that this thesis is classified as:

We acknowledged that University of Hormuud reserves the right as

follows:

1. 1. The thesis is the property of University of Hormuud.

2. 2. The Library of University of Hormuud has the right to make

copies for the purpose of research only.

3. 3. The Library has the right to make copies of the thesis for

academic exchange.

4.

Certified by:

SIGNATURE OF SUPERVISOR

BURHAN OMAR SHEIKH AHMED

NAME OF SUPERVISOR

Date: 1ST August 2015

√

CONFIDENTIAL (Contains confidential information)

RESTRICTED (Contains restricted information as specified

by the organization where research was done)

OPEN ACCESS I agree that my thesis to be published as

online open access (full text)

UNIVERSITY OF HORMUUD

ii

“I hereby declare that I have read this project report and in my

opinion this project report is sufficient in terms of scope and quality for the

award of the degree of Bachelor of Engineering (Telecommunication)”

Signature : ...................................................

Name of Supervisor : BURHAN OMAR SHEIKH AHMED

Date : 1st August, 2015

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INTRA-BSC HANDOVER OPTIMIZATION FOR GSM QUALITY OF SERVICE

IMPROVEMENT

ABDISALAM ABDULAHI MOHAMED

ABDIRAHMAN MOHAMED OMAR

ALISALAD ALASOW MOHAMED

A project report submitted in partial fulfilment of the

requirements for the award of the degree of

Bachelor of Engineering (Telecommunication)

Faculty of Engineering

Department of Electrical and Electronics

University of Hormuud

AUGUST 2015

iv

We declare that this project report entitled “Intra-BSC handover optimization for

GSM quality of service improvement” is the result of our own research except as

cited in the references. The project report has not been accepted for any degree and

is not concurrently submitted in candidature of any other degree.

Signature : ....................................................

Name : Abdisalam Abdulahi Mohamed

Signature : ....................................................

Name : Abdirahman Mohamed Omar

Signature : ....................................................

Name : Alisalad Alasow Mohamed

Date : 1st August, 2015

v

Unusual dedicated to our ALLAH, Parents and friends for support, encouragement

and motivation through our education.

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ACKNOWLEDGEMENT

Thanks to ALLAH for all blesses

The first big thanks and appreciation goes to our brilliant supervisor Eng.

Burhan Omar Sheikh Ahmed for his tremendous work and help on this project, also

for his superb encouragement, provision and useful suggestions throughout this thesis

work.

Our sincere thanks also goes to the administration and the staff of Radio

Network Optimization and Planning department of Hormuud telecom Somalia for

providing us an opportunity to join their teams in their field operations and frankly

giving us access to their live-network vital information and research facilities.

Without their precious support, it could not be possible to conduct this study at all.

Most importantly, we feel great pleasure and honor to express a heart full

gratitude to our beloved parents as their support and encouragement was in the end

what made this final project possible and without their sacrifices in both moral and

financial support, the completion of this thesis would have been a measly dream.

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ABSTRACT

Mobile terminals allow subscribers to access services while on the move. This

exclusive feature has driven the fast growth in the mobile network industry, shifting

it from a new technology into a huge industry within less than two decades.

Handover is the essential functionality for dealing with the mobility of the

mobile users; in GSM network, handover can occur in all network levels, but one kind

with an extra importance is that occurs between different base stations under one base

station controller (intra-BSC handover), this thesis study, however, investigates intra-

BSC handover glitches in GSM network and comes up with invaluable

recommendations to optimize those blenders. The data is collected through drive test

by using TEMS (Test Mobile Systems) investigation conducted in the study area. The

collected data has being analyzed and examined strictly focusing on handover

optimization metrics such as handover failures and unnecessary handovers and lately,

possible way-outs are provided.

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TABLE OF CONTENT

CHAPTER TITLE PAGES

DECLARATION ii

DEDICATION v

ACKNOWLEDGEMENT vi

ABSTRACT vii

TABLE OF CONTENTS viii

LIST OF FIGURES xii

LIST OF TABLES xiii

LIST OF ABREVIATIONS xiv

1 INTRODUCTION

1.1 Background of the study 1

1.2 Problem statement 2

1.3 Research objectives 2

1.3.1 Specific research objectives 3

1.4 Hypothesis of the study 3

1.5 Significance of the study 3

1.6 Significance of the study 3

1.7 Organisation of the thesis 4

2 BACKGROUND AND LITERATURE REVIEW

2.1 Introduction 5

2.2 Background and overview 5

2.3 cellular concept 7

2.4 Global system for mobile communication (GSM) 8

2.4.1 GSM network architecture 9

2.4.2 GSM radio interface 10

2.4.3 GSM Channels 11

2.4.3.1 Physical channels 11

2.4.3.2 Physical channels 12

2.4.3.2.1 Broadcast control channel

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(BCCH) 12

2.4.3.2.2 Common Control Channel

(CCCH) 13

2.5 Radio resource management in cellular system (RRM) 13

2.5.1 Admission Control 13

2.5.2 Channel Allocation and bandwidth management 14

2.5.3 Power control 15

2.5.4 Handover 16

2.5.4.1 Requirements for GSM handover 17

2.5.4.2 Handover Management 17

2.5.4.3 Handover strategies 19

2.5.4.3.1 Network Controlled Handover

(NCHO) 19

2.5.4.3.2 Mobile Assisted Handover

(MAHO) 20

2.5.4.3.3 Mobile Controlled Handover

(MCHO) 21

2.5.4.4 GSM handover measurement 22

2.5.4.5 Handover schemes 23

2.5.4.5.1 Guard Channels 24

2.5.4.5.2 Queuing Handover Calls 24

2.5.4.5.3 Sub Rating Schemes 25

2.5.4.5.4 Generic Handover Schemes 25

2.5.4.5.5 Hybrid Handover Schemes 26

2.5.4.6 Handover Algorithm 26

2.5.4.6.1 Conventional Handover Algorithm 26

2.5.4.6.2 Velocity Based on Algorithm 27

2.5.4.6.3 Direction Biased algorithms 28

2.5.4.6.4 Signal Interference Based

Algorithm 28

2.5.4.6.5 Intelligent Handover Algorithms 28

2.5.4.7 Performance metrics for Handover 29

2.5.4.8 Types of handover 30

2.5.4.8.1 Hard handover (HHO) 30

2.5.4.8.2 Soft handover (SHO) 30

2.6 GSM network Evaluation criteria 32

2.7 Key Performance Indicators assessment and QOS

x

Estimation 32

2.8 Radio network planning and optimization 33

2.8.1 Radio network planning and optimization follow 33

2.8.2 GSM radio network optimization 35

2.8.2.1 Intra-BSC Handover optimization 36

2.8.3 GSM Network optimization tools 36

2.9 Literature review 37

2.10 Summary 39

3 RESEARCH METHODOLOGY AND MATERIALS

3.1 Introduction 40

3.2 Drive Test Overview 42

3.3 GSM Network Drive Testing 42

3.4 Important KPIs and Their Implications 43

3.5 Network optimization tools 44

3.7 Implemented Action Plan 46

3.7.1 Geographic Description of the Propagation Terrain 46

3.7.2 Data Collection Process 47

3.7.3 Drive Test Survey Route 48

3.8 Summary 48

4 RESULTS AND ANALYSIS

4.1 Introduction 49

4.2 Observed cases and their corresponding analysis 51

4.2.1 Hand failure due to lack of resources 51

4.2.1.1 Event description 51

4.2.2 Hand failure due to missing neighbor 52


4.2.2.2 Recommended solution for missing

Neighbor HO failure 52

4.2.3 One way Neighbor 53


4.2.3.2 Recommended solution for one way

Neighbor HO failure 53

4.2.4 Ping Pong 54


4.2.4.2 Recommended solution for Ping-Pong HO 54

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4.2.5 Lack of late Handover 55


4.3 Drive test KPI Reports 56

4.4 Summary 56

5 CONCLUSION AND FUTURE WORK

5.1 Conclusion 57

5.2 Future Work 58

REFERENCES 59

APPENDICES A-B 61-62

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LIST OF FIGURE

FIGURE NO. TITLE PAGES

2.1 GSM network architecture 9

2.2 GSM Cellular Layout for Frequency Reuse 15

2.3 handover process 18

2.4 MAHO inter BS message flow 21

2.5 MCHO inter-BS message follow 22

2.6 Signal levels for handover 27

2.7 Comparison between hard and soft handover 31

2.8 Radio network planning optimization flow 33

2.9 Network optimization procedure 35

3.1 General radio network optimization flow 41

3.2 Drive Test Data Collection Set Up 45

3.3 Aerial view of the target area (Siliga, Mogadishu) 46

3.4 Drive test coverage plot 48

4.1 Handover failure for lack of resources 51

4.2 Hand Failure Due to Missing Neighbor 52

4.3 Hand failure due to one way neighbor 53

4.4 Ping-Pong 54

4.4 smooth handover case 55

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LIST OF TABLE

TABLE NO. TITLE PAGES

2.1 First Generation deployed countries 7

3.1 Selected serving sectors of the base station 47

4.1 Target area and all serving sectors of the base station 50

4.2 TEMS generated report 56

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LIST OF ABBREVIATION

1G First Generation

2G Second Generation

3G Third Generation

3GPP Third Generation Partnership Project

4G Fourth Generation

AMPS Advance Mobile Phone Service

ANSI American National Standards Institute

ARFCN Absolute Radio Frequency Channel Number

AT&T American Telephone & Telegraph

AuC Authentication center

BS Base Station

BSC Base Station Controller

BSIC Base station identity codes

BSS Base Station Subsystem

BTS Base Station Transceiver

C/I Carrier to Interference ratio

CDMA Code Division Multiple Access

DECT Digital Enhanced Cordless

Telecommunications

EIR Equipment Identity Register

ETACS Extended Total Access Communications

ETSI European Telecommunications Standards

Institute

FDMA Frequency Division Multiple Access

FM Frequency modulation

GMSK Gaussian Minimum Shift Keying

GPS Global Positioning System

GSM Global System for Mobile

HLR Home Location Register

HSR Handover Success Rate

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IEEE Institute of Electrical and Electronics

Engineering

IETF Internet Engineering Task Force

IMEI international mobile equipment identity

ITU International Telecommunication Union

KPI Key Performance Indicator

LA Local Area

LTE Long Term Evolution

MAHO Mobile Assisted Handover

MCHO Mobile Controlled Handover

MSC Mobile Switching Center

MSK Minimum shift keying

NCHO Network Controlled Handover

NMT Nordic Mobile Telephone

NPS Non-Prioritized Schemes

NTT Nippon Telephone and Telegraph

OMC Operation and Maintenance Center

PSTN Public Switching Telephone Network

QoS Quality-of-Service

RNO Radio Frequency Network Optimization

RSSI Received Signal Strength Indication

SIM Subscriber Identity Module

SNR Signal to noise ratio

SQI Speech Quality Index

TACS Total Access Communication System

TCH Traffic Channel.

TDMA Time Division Multiple Access

TEMS Transmission Evaluation and Monitoring

System

UMTS Universal Mobile Telephone Service

VLR Visitor Location Register

CHAPTER 1

INTRODUCTION

1.1 Background of the study

More freedom for movement is the competitive advantage for cellular

networks over their preceding ones. For the side of the service providers, customer

satisfaction has a paramount importance whereas subscribers’ need is better quality

service. To meet that need, overall mobile network is subdivide into cells with better

coverage. Another challenge, however has arisen as the mobile subscribers begin to

cross from cell to cell during active session (on call); mobility management become

an agenda with greater importance in the telecommunication arenas. Transferring an

ongoing call from serving cell to a neighboring one become the best solution. Live

session transfer process is called handover or handoff [1].

In GSM, handover is an important entity in the list of network performance

indicators as Mobility is the most important feature of a wireless cellular

communication system. Usually, Continuous service is achieved by supporting

handover from one cell to another [2]. Handover processes are performed by all

cellular telecommunications networks and it is a core element of the whole concept

of cellular telecommunications [3]. If a handover process fails; it causes a

phenomenon called call dropping which means terminating the call as the subscriber

is in the middle of conversation because of lack of/weakening connection to serving

Base Station (BS) [4]. Thus, a study on handover improves system quality.

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1.2 Problem Statement

In our country, the sector of technology have been suffering for so long from

lack of enough human resource to keep up with the unfolding technological

advancements of the world. Telecom industry, however, was not exceptional and there

was almost no prior research or study conducted in the field of GSM optimization

overall and the issue of handover in particular, all these issues give room for the local

researchers to conduct some sort of study on this deprived field.

Handover is extremely vital in cellular network because of the cellular

architecture employed to maximize spectrum utilization as users move through the

coverage area of cellular system. One way to develop the cellular network

performance is to use efficient handover prioritization schemes when a user is

switching between the cells [5, 6]. The continuation of an active call is also considered

as one of the most important quality measurements in cellular systems [1]. The above

mentioned importance of handover asserted by the prior studies justifies the need for

conducting study on handover related scenarios.

The idea behind this study come to exist after members of the research team

attended a vocational training at the Radio Network Optimization (RNO) department

of Hormuud telecom and worked with RNO teams in their field operation. During

their one month training, the research team become convinced with the need of

conducting a study on one of the most frequently occurring network glitches; Intra-

BSC handover, exploring answers for the possible causes of intra-BSC handover and

proposing their corresponding solutions.

1.3 Research objectives

The general objective of this study is to investigate overall handover problems

and propose possible recommendations. Some of the specific study objectives are

listed in the following section.

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1.3.1 Specific research objectives

1. To Study over all handover process in 2G networks.

2. To investigate the problems that arise from intra-BSC handover failure in

GSM (2G) network.

3. To analyze intra-BSC handover related issues in 2G network in terms of voice

quality and interference.

4. To come up with a conclusion based on the study findings.

1.4 Hypothesis of the study

The research team has a lot to consider in commencing their work on the

selected area, the first study hypothesis is if there is a handover related problems in

the selected area, then there is all sorts of handover blunders.

1.5 Significance of the study

Handover is one of the major key performance indicators in every GSM

network and it is linked to the quality of service each service provider strives to attain.

The failure of any handover process is considered as another nail on the coffin of

customer satisfaction goals. This study, however, contributes more on improving

intra-BSC handover problems, helping GSM service providers to easily optimize their

network faults relating to handover. In addition to that, this study allows other

researchers interested in this field to take advantage of the results presented in this

study.

1.6 Scope and limitations of the Study

This study focuses on intra-BSC handover related cases occurring in the

selected area (Siliga area Wadajir district) served by BSs under one BSC for Hormuud

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telecom Somalia. Other unrelated parameters are not considered in this study. The

weakness this study has is that the study is conducted on small number of BSs and for

more insight, the study area should be extended. The other service providers’

networks are not also examined for further generalization on GSM network.

1.7 Organization of the thesis

Chapter two of this thesis work covers some vital concepts and theories related

in wireless mobile communication networks providing background and overview of

wireless networks, recapitulating different generations of wireless communication

systems, and basic concept of the cellular wireless networks and GSM in particular

going down to more handover concepts and related parameters. At last, it concludes

some of the major related works taken as literature and analyzed their approaches as

they relate.

Chapter three clearly portrays the method applied for that collection the

process using drive test methodology and some employed materials for the process.

Chapter four dissects the collected data through drive testing method, and

other relevant and assisting sources such BSC data and subsequently comes up with

some beneficial recommendations.

Chapter five summarizes the work in this thesis, draws a conclusion and

contributes some insights for the forthcoming future work.

CHAPTER 2

BACKGROUND AND LITERATURE REVIEW

2.1 Introduction

This chapter introduces background information on the development of mobile

radio communication starting from the early days of analog communication,

descending down to cellular concept and the deployment of GSM network. It also

presents deeper handover related concepts such as HO types, measurement, schemes

and algorithms. Over all GSM optimization techniques and criteria as well as

handover optimization procedures are also elaborated at the end of this chapter, some

Previous related works were also presented as a literature.

2.2 Background and overview

The history of wireless communications could be traced back to 1897 when

Gugliemo Marconi first experimented radio’s ability for providing continuous contact

with the ships sailing the English Channel and since then, new wireless

communications methods and services have been actively adopted by the people

around the world. Particularly in the last three decades, the mobile radio

communications industry has experienced exponential growth powered by digital and

RF circuit construction developments and other miniaturization technologies which

make handy radio smaller, cheaper and easy-to-use. These trends are anticipated to

progress with greater pace for the following decades [7].

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By the beginning of the 20th century, wireless technologies experienced

greater developments although the capacity and the service quality was limited due to

the analog systems applied in the field. In 1950s and 1960s ATandT, Bell laboratories

and other telecommunication companies throughout the world developed the concept

of dividing the coverage area into smaller parts (cells) that could increase the spectrum

usage. But, that concept was first deployed by Advanced Mobile Phone Systems

(AMPS) in late 1983 [7].

Using frequency division multiple access together with very narrow

bandwidth is considered as the first milestone for the first generation (1G). Nippon

Telephone and Telegraph (NTT) in Japan, Nordic Mobile Telephones (NMT) and

Total Access Communication Systems (TACS) in Europe also made the same

systems. All these systems offered handover and roaming capabilities but couldn’t

interoperate among them [8]. This was marked as one of the major downgrading

factors of 1G [5, 9].

Second-generation (2G) mobile systems supporting low bit rate data services

as well as the traditional speech service were developed in the end of 1980s.

Compared to first-generation systems, second-generation (2G) systems employ digital

multiple access technology, such as TDMA (time division multiple access) and

CDMA (code division multiple access). Therefore, compared with first generation

systems, higher spectrum efficiency, better data services, and more advanced roaming

were offered by 2G systems.

In Europe, Global System for Mobile Communications (GSM) was deployed

to offer a single unified standard. This enabled continuous services throughout Europe

by means of international roaming. Global System for Mobile Communications, or

GSM, uses FDMA together with TDMA technology to support multiple users during

development over more than 25 years, GSM technology has been continuously

improving to offer better services in the market [10]. The following table shows early

systems and the countries first deployed [10].

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Table 2.1: First Generation deployed countries

2.3 Cellular concept

At the beginning of radio mobile communication technology, spectrum

congestion was a major problem but it was not the only one, high power antennas

mounted at the top of high tower consuming more electric energy was another major

setback. The cellular concept became major breakthrough in solving those problems.

System Countries

NMT-450 Slovenia, Spain, Sweden, Thailand, Turkey, and Ukraine

NMT-900 Cambodia, Cyprus, Denmark, Faroe Islands, Finland,

France, Greenland, Netherlands, Norway, Belgium,

Bulgaria, Cambodia, Croatia, Czech Republic, Denmark,

Estonia, Faroe Islands, Germany, Hungary, Iceland,

Indonesia, Italy, Serbia, Sweden, Switzerland, and

Thailand

TACS/ETACS Austria, Azerbaijan, Bahrain, China, Hong Kong, Ireland,

Italy, Japan, Kuwait, Macao, Malaysia, Latvia, Lithuania,

Malaysia, Moldova, Netherlands, Norway, Poland,

Romania, Russia, Slovakia, Malta, Philippines, Singapore,

Spain, Sri Lanka, United Arab Emirates, and United

Kingdom

AMPS

Argentina, Australia, Bangladesh, Brazil, Brunei, Burma,

Cambodia, Canada, China, Georgia, Guam, Hong Kong,

Indonesia, Kazakhstan, Kyrgyzstan, Malaysia, Mexico,

Mongolia, Nauru, New Zealand, Pakistan, Papua New

Guinea, Philippines, Russia, Singapore, South Korea, Sri

Lanka, Tajikistan, Taiwan, and Thailand

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Cellular concept offered very high capacity in a limited spectrum allocation without

any major technological changes. It is system level concept calling for replacing single

high power transmitter with many low power transmitters with small portion of

system coverage area [7].

Each base station is assigned a portion of the total number of channels

available to the entire system, and nearby base stations are assigned different

frequencies to avoid interference. A number of neighboring BSs are grouped together

to form one cluster. Each cluster is allocated the whole channels available for the

entire system. Maintaining a distance called reuse distance, the same frequencies

could be used in different clusters throughout the coverage area [7, 11]. The area

covered by the one BS is called cell and modeled with hexagonal shape.

Mobile subscribers under the coverage area of a specific BS is served by that

BS it is attached to. Mobile radio communication, however, guarantees freedom of

movement for mobile subscribers. These movements may occur both in idle mode

(not on call) and dedicated mode (on call). When mobile subscriber at idle mode

crosses cell boarders, cell reselection process occurs. In contrast, a mobile subscriber

in a dedicated mode crossing cell boarders needs handover to be performed [12, 13].

2.4 Global system for mobile communication (GSM)

GSM is a second generation digital cellular system using digitized

transmission replacing earlier analog systems in order to enhance transmission

quality, system capacity, and coverage area. GSM operates basically on three-

frequencies 900 MHz, 1800MHz and 1900MHz [5]. But the first two spectrums are

mainly used worldwide. To make efficient use of frequency bands, GSM uses a mix

of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access

(TDMA).

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GSM uses Gaussian Minimum Shift Keying (GMSK) modulation scheme to

transmit information (data and signaling) over Air Interface. An MSK signal is created

by applying a half sinusoidal pulse instead of a square pulse. If a Gaussian pulse shape

is used instead then the resultant digital modulation technique is an improved version

of MSK digital modulation technique in terms of bandwidth and spectral efficiency

and is termed as GMSK digital modulation technique (Gaussian Minimum Shift

Keying). Furthermore, the major benefit in this method is the sufficiently lower side

lobe levels and the narrower main lobe as compared to a QPSK and MSK pulse [14].

2.4.1 GSM network architecture

The general architecture of GSM network is shown in figure 2.1. The GSM

system consists of several functional components including mobile switching centers

(MSC), base stations (BSC) with associated base transceivers (BS), an operation and

maintenance center (OMC) and gateway MSC.GSM mobile terminal or mobile

stations communicates across the Um interface, known as the air interface, with a BS

in the small cell in which the mobile unit is located. This communication with a BS

takes place through the radio channels. The network coverage area is divided into

small regions called cells. Multiple cells are grouped together form a cluster or a

locations area (LA) for the mobility management [7].

Figure 2.1: GSM network architecture [5]

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BSC is connected to MSC through direct line or radio communication link.

The BSC holds radio frequencies, manages the handover of mobile station from one

cell to another with in the BSS (base station subsystem).the interface through which

MSC communicates with the PSTN (public switched telephone network) is called the

gateway MSC. MSC’s main components including home location register (HLR),

visitor location register (VLR), authentication register (AuC) and equipment identity

register (EIR) [5, 7].

The HLR and VLR together with MSC provide the call routing and roaming

capabilities of GSM. The HLR stores both permanent and temporary information

about each of the mobile station that belongs to it. The VLR, on the other hand, holds

information about mobile station that is currently physically in the region covered by

MSC. VLR becomes important when user leaves the area served by his home MSC.

The two registers are used for authentication and security purpose. The EIR is a

database that contains a list of all valid mobile equipment on the network, where each

mobile station is identified by its international mobile equipment identity (IMEI). It

helps in security and prevents uses of network by mobile station that have not been

approved. The AuC holds the authentication and encryptions keys that are stored in

each user SIM card for authentication and encryption over radio channel [5].

Mobile station is the mobile subscriber connected to the network by using the

Subscriber Identity Module (SIM), communicates Base station transceiver (BS)

through air interface called Um- interface. Number of BSs are over run by Base station

controller (BSC) through an interface called Abis interface. While number of BSCs

are also controlled by one mobile switching center (MSC) communicating through an

interface called A interface. Number of MSCs together with their underlying network

parts constitute a GSM service provider [4].

2.4.2 GSM radio interface

The ITU-T (International Telecommunication Union) allocated the 890-915

MHz frequency for uplink transmission and 935-960MHz for downlink transmission

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for GSM900 and 1710-1785MHz frequency for uplink and 1805-1880MHz for

downlink transmission for GSM1800. Due to the limited radio bandwidth GSM

utilizes a combination of FDMA and TDMA called Multi-Carrier TDMA to access

the available radio spectrum. Older standards of mobile system use only FDMA. In

FDMA system one specific frequency is allocated for every user during a call where

TDMA allows several users to share the same frequency channel by dividing the

signal into different timeslots. This allows multiple stations to share the same radio

frequency channel while using one part of its bandwidth. GSM system always uses

TDMA with in FDMA structure [5].

2.4.3 GSM Channels

Channel is a medium such us wire, coaxial cable, a wave guide, an optical

fiber or a radio link through which the transmitted output is sent [15]. Generally, GSM

channels are divided into two main categories; physical channels and logical channels

[5, 16]. Number of logical channels are multiplexed onto physical channels, which

assists for the system to run multiple activates in parallel which does not require to

use dedicated line for every slot transmission. Hence, the logical channels improve

the physical channels reuse capabilities to higher level. Logical channels are linked

on the physical channels so they are known as laid over grid of physical channels [5].

2.4.3.1 Physical channels

A physical channel is determined by the carrier frequency or a number of

carrier frequencies with defined hopping sequence and the time slot number. As

mentioned above, GSM uses both frequency division and time division multiple

access. FDMA parts involves the division by frequency of the 25 MHz bandwidth in

to 124 carrier frequencies (Absolute Radio Frequency Channel Number. ARFCN)

guard band of 200 KHz for GSM-900. For GSM-1800, frequency spectrum of 75 MHz

bandwidth is divided in to 374 carrier frequencies spaced 200 KHz. TDMA further

divides each carrier frequencies in to 8 time slots such that each carrier frequency is

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shared by 8 users. Hence, in GSM, the basic radio resource is a time slot with duration

of 577μs. 8 Time slots of 577μs constitutes a 4.615ms TDMA Frame [5].

2.4.3.2 Logical channels

GSM logical Channels are sub grouped into two main parts; traffic channels

and control channels. Traffic channels carry speech and data information while

control channels deal with network management and channel maintenance tasks.

Control channels are in turn subdivided into two main categories; Broadcast control

channels (BCCH) and Common control channels (CCCH) [16].

2.4.3.2.1 Broadcast control channel (BCCH)

Broadcast control channel is a downlink channel used by the Base station

transceiver to broadcast information to mobile station and inform them about the

incoming calls. It is required in initial to provide a time slot for a call. BCCH

broadcasts general information required to set up calls, such information include

power control parameters, access methods, network parameters etc.

Broadcast channels are over three types; FCCH, SCH and BCH. FCCH

(Frequency Correction Channel) Provides MSC with the frequency reference of the

system to-allow synchronization with the network and frequency drift correction.

Second broadcast channel SCH (Synchronization Channel) Provides frame

synchronization for MSC and identification of BSC. The synchronization channel

transmits the sequence that is needed for link quality estimation and equalization.

Third broadcast control channel (BCCH) which is also downlink channel is broadcast

channel (BCH) that is used by BS to broadcast information to mobile station and

inform them about the incoming calls. It is required in initial to provide a time slot for

a call [5, 15].

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2.4.3.2.2 Common Control Channel (CCCH)

The common control channel is a combination of common control channels

that is used between MSC and BSC before a dedicated control channel is allocated.

There are three downlink paging, access grant and cell broadcast channels and one

random access uplink channel. Paging channel (PCH) is activated for selective

addressing of a mobile station during a connect request from the network. Random

access channel (RACH) is transmitted by mobile station as uplink and to access the

network and request channel capacity form the BSC to establish a connection. The

access grant channel (AGCH) channel is transmitted by the BSC in reply to random

access from MSC. According to the call setup mechanism selected by network

operator is allocated for call. Cell broadcast channel (CBCH) channel containing

broadcast messages information about the traffic information etc. [5].

2.5 Radio resource management in cellular system (RRM)

Management has a pivotal role for everything needed to be efficiently utilized.

In communication networks, however, elements such as frequency channels,

timeslots, code channels; transmission power, battery energy and the number of

transceivers are the primary resources. For service providers to save cost and increase

revenue, the radio resources should be managed in a more efficient fashion which can

support in increasing quality of service as well as the efficiency and effectiveness of

wireless networks. Radio resource management’s key points are; admission control,

power control, handover, and load control functionalities [13, 6, 17].

2.5.1 Admission Control

In admission control, new calls and already ongoing calls should be treated

differently to keep the system from being burdened. New calls, however, could be

queued. While Handovers may be prioritized. It is all about customer satisfaction

issue; an ongoing call to be terminated is more customer scaring phenomenon.

14

Therefore, prioritization is inevitable for better service as it increases the capacity of

the system. On the other hand, more capacity means more income for service

providers, and part of the apparent service quality can be credited to the accessibility

of the network [13, 17].

2.5.2 Channel Allocation and bandwidth management

In cellular systems, the bandwidth may be the most precious and important

resource; therefore, it should be well planned and efficiently utilized [13]. A GSM

cellular network is made of number of radio cells or cells served by fixed base station.

These cells are dedicated to cover different areas to provide radio coverage over vast

area.

Radio cells, however, are grouped into clusters and each frequency is used

once per cluster. The capacity in cellular network can be improved because the same

radio frequency can be reused in different area for completely different transmission

in a regular way. The ruse of frequencies enables a cellular system to handle enormous

number of calls with limited numbers of channels. GSM cellular layout typically

involves the frequency reuse factor which is inversely proportional to K (where K is

number of cell per cluster). The co- channel interference is serious problem in this

scheme while adjacent co-channel interference is not a big problem.

𝐷 = 𝑅√3𝑁 (2.1)

Where R is the radius of the cell, D is the distance from the center of the cell to its

neighbor using the same frequency and N is number of cells. [5].

Channel assignment strategies can be classified into fixed, dynamic, and

flexible. Fixed Channel Assignment (FCA) strategy permanently assigns a set of

channels to each cell in a cluster.

15

Figure 2.2: GSM Cellular Layout for Frequency Reuse [7, 5]

Dynamic Channel Assignment (DCA) strategy, makes all the channels in a

cluster available for use within a cluster. The actual channel assignment for a new call

attempt is based on the minimization of a cost function that depends on future blocking

probability, frequency usage of the candidate channel, and reuse distance of the

channel. Dynamic channel allocation does not require a priori frequency planning but

must determine whether co-channel usage is allowed or not, therefore, algorithm used

must guarantee a safe co-channel reuse distance. Hence, a measure of interference for

the handover candidate channel is required as an input to the channel allocation

algorithm. In microcell [7, 13, 17].

The Flexible Channel Assignment (FLCA) strategy permanently distributes

some channels among the cells in a cluster and keeps the remaining channels available

for any cell’s use when that cell’s permanent channels are inadequate to cope with

high traffic demand [17].

2.5.3 Power control

Power control increases battery life, decreases health risks, and mitigates

interference. One way to apply power control is to use signal to interference ratio SIR

as a criterion. In this circumstance, MSs try to attain a target SIR through continuous

power adjustments. If the minimum possible power that meets the required carrier to

16

interference ratio (C/I) limit at the receiver is transmitted, spectrum efficiency will

increase compared to uncontrolled transmit power systems. Increasing transmit power

to increase (C/I) for better transmission quality does not necessarily meet the objective

since other transmitters in the system may also increase their power levels to reduce

their interference, thus increasing the interference level of the whole system.

2.5.4 Handover

Different researchers and writers made different definitions for handover.

Handover is the procedure that transfers an ongoing call from one cell to another as

the user moves through the coverage area of cellular system, as described in [3, 5, 18,

32]. In other words, Handover is the process of changing the channel (frequency, time

slot, spreading code, or combination of them) associated with the current connection

while a call is in progress. In cellular telecommunications, the term handover or

handoff refers to the process of transferring an ongoing call or data session from one

channel connected to the core network to another channel [1]. The later definition is

more meaningful then the first one, because, the transformation is not only for calls

but also for data transmission as well.

Number of causes are considered of been responsible for handover process.

MS movement from one base station coverage area to another base station coverage

area is considered as a paramount cause of handover, however, it is not the only one

but there is a number of other causes that could lead to handover such as bad signal

quality, emergency, Rx level drop, interference, load and many others [19].

Handoff and handover are two words used interchangeably. American English

uses the term handoff, and this is most commonly used within some American

Organizations such as 3GPP2 and in American originated technologies such as

CDMA2000. In British English the term handover is more common, and is used

within international and European organizations such as ITU-T, IETF, ETSI and

3GPP, and standardized with in European originated standards such as GSM and

UMSS. The term handover is more common than handover in academic research

17

publications and literature, while handover is slightly more common within the IEEE

and ANSI organizations. The time over which a call is maintained within a cell

without handover is called dwell time [1, 7].

2.5.4.1 Requirements for GSM handover

The process of handover within any cellular system has a paramount

significance. It is a critical process, if made inaccurately handover can result in the

loss of the call. Dropped calls are particularly annoying to users and if the number of

dropped calls increases, user’s displeasure rises and they are likely to migrate to

another network. Accordingly GSM handover was an area to which particular

consideration was paid when developing standards [4].

Handover may affect wireless networks in many ways such as, quality-of-

service (QoS) and the capacity of the network [13]. So there are a number of desirable

features and requirements to reduce the adverse effects of a handover:

1. The handover should be fast enough.

2. The handover latency should be low.

3. The total number of handovers should be minimal.

4. Successful handovers to total attempted handovers should be maximized.

5. The effect of handover on QoS should be minimal.

2.5.4.2 Handover Management

Handover management, in short, means preserving the traffic connection with

a moving user when crossing cell boundaries. Handover occurs when the quality or

the strength of the radio signal falls below certain parameters (signal quality reason)

it may also occur when the traffic capacity of a cell has reached its maximum level or

18

Figure 2.3: Handover process

Closer (traffic reason). GSM standard identifies about 40 reasons for a handover.

Handover is initialized by the mobile or by the base station [1, 20].

Handover process may be viewed as quite complex, but then, it could be

summarized with the following three major steps involved in handover process:

1. Measurement: During this stage link measurements (e.g. Received Signal

Strength (RSS), Signal to Interference Ratio (SIR), distance measure, Bit Error

Rate (BER)) are carried out at both parts: the BS and the MS [1, 13 ,17].

2. Decision: The objective of this phase is the selection of the new channel, based

On actual resource availability and the network load, the measurement

outcomes are equated with predefined thresholds and then it is decided

whether to initiate the handover or not. Different kinds of handover decision

protocols are of course used [1, 13, 17].

3. Execution: In this phase, the handover process is completed I.e. the network

allows the MS to communicate with a BS in one of its cells, to transfer its

communication into another channel or another cell. During this phase, the

over-the-air and network process signaling is performed, as well as,

authentication, database lookup and network reconfiguration [1, 13, 17].

19

2.5.4.3 Handover strategies

Handover decision is made and instigated based on measurement. Different

systems employ dissimilar methods to accomplish handover processes and these are

characterized by handover protocols. The mobile terminal measures continuously

level of signal in current channels and compare it with some other different channels

[1]. When deciding to handover an MS, it should be assured that the drop in the

measured signal level is not as the result of momentary fading and that the mobile

actually moving away from the serving base station [7] or any other justifiable reason

stands behind the needed handover. Based on the measurement results, the decision-

making process of handover may be centralized or decentralized i.e. handover

decision is made by handset, the network or the association between them, depending

on the handover control protocol [1, 7, 13]. The following three strategies are

proposed for handover detection:

1. Network Controlled Handover (NCHO).

2. Mobile Assisted Handover (MAHO).

3. Mobile Controlled Handover (MCHO)

2.5.4.3.1 Network Controlled Handover (NCHO)

With NCHO, base stations (BSs) measure the signal coming from all mobile

stations (MSs) in its coverage area and network triggers the handover process when

some handover criteria are met. in this method, the base station monitors the signal

strength and quality from the mobile station and when these deteriorate below some

threshold, the network arranges for a handover to another base station. The network

examines all the surrounding base station to monitor the signal from the mobile station

and report the measurement result back to the network. The network then chooses a

new base station for the handover and informs both the mobile station through the old

base station and the new base station as well [1, 6].

20

Signal level measurement is made by base stations and supervised by mobile

switching center (MSC). In addition to radio signal quality of ongoing calls within the

cell, BSs uses reverse voice channels to determine the relative location of each MS.

Moreover, each BS has an extra receiver called locator receiver, dedicated for

monitoring the signal strength of the MSs in the neighboring cells. The locator

receiver is controlled by the MSC to truck the MS in the neighboring cell which tend

to undergo handover process [7].

NCHO is used in first generation cellular systems such as Advanced Mobile

Phone System (AMPS), TACS (total access communication system), and NMT

(Nordic Mobile Telephones). In general, the handover process (including data

transmission, channel switching, and network switching) takes 100–200ms [1, 6, 7].

2.5.4.3.2 Mobile Assisted Handover (MAHO)

In NCHO, the load of the network is high since network handles the whole

process itself; in MAHO, in contrast, the handover is more dispersed. Both the mobile

station and the base station supervise the quality of the link. The network asks the MS

to measure the signal from the surrounding BSs. But the network makes the handover

decision based on reports from the MS. The mobile station is responsible for doing

the received signal strength indication (RSSI) measurement of neighboring base

stations [6, 7].

This handover strategy is used by the GSM cellular standard because it is

easier than NCHO and the MS transmits the measurement results to the BS twice a

second. The decision as to when and where to execute the handover is still made in

the network. In the circuit-switched GSM, the BS controller (BSC) is in charge of the

radio interface management. The handover time between handover decision and

execution in such a circuit-switched GSM is approximately one second [7, 13]. The

following figure shows MAHO inter base station message follow.

21

Figure 2.4: MAHO inter BS message flow [1]

2.5.4.3.3 Mobile Controlled Handover (MCHO)

In MCHO strategy, the mobile station (MS) continuously monitors the signals

of the surrounding BSs and initiates handover process when handover criteria is met.

The MS constantly monitors the signal strength and quality from the accessed base

station and several handover candidate base stations. Reaction time of MCHO

estimated of 0.1 seconds and it is used in DECT (Digital Enhanced Cordless

Telecommunications) standard [1, 6, 7]. The figures below shows inter-BS message

follow.

22

Figure 2.5: MCHO inter-BS message follow [1]

2.5.4.4 GSM handover measurement

Since GSM uses MAHO strategy for handover, the mobile station makes

measurements which are used in triggering of the handover and in the evaluation of

the handover candidate cell. This makes measurement an essential part of the

handover process. In order to make efficient handover, these measurements should be

refreshed as fast as possible. The mobile station measures crucial parameters such as

uplink receiving level and quality and the level of the neighboring cells and sends this

information to the network so that the decision for the handover is available to network

all the times [5].

These measurements reports from the MS is carried on the SACCH signaling

channel after every 0.48 sec, this means at least once per second. In terms of capacity,

the SACCH channel is error free; which means that the measurements reporting is

23

faultless almost [5]. In general measurements report contains parameters that describe

the current network connection, the radio conditions of the neighboring cells and the

targeted cells to handover.

In GSM, one measurement message is sent from a mobile station to the BS

every 0.48 sec. this message contains the uplink and downlink Rx level of the serving

cell and that of up to six neighboring cells. However a mobile station may pre-

synchronize with more than six neighboring cells. In this case, the measurements

corresponding to the six cells with the best signal level are reported to the BS. The

measurements of the neighboring cells is more challenging because a mobile station

has to establish which neighboring cell it can receive and divide the measurement time

among those cells capable of receiving [5, 11].

2.5.4.5 Handover schemes

Handover, as presented in the previous sections, is a very important in a

wireless network for continuation of connections and quality of serves perceived for

the users [1]. Handover schemes can be distinguished into Non-Prioritized Schemes

(NPS) and Prioritized Schemes [13, 5].

Non- prioritized schemes both handover calls and incoming calls are treated

equally. When there are free channels available for the BS, both handover calls and

incoming calls have the same level of importance. If there is no channel available for

the BS, the calls will be blocked; hence there is an increase call drop probability

(CDP). Non-prioritized schemes employ complete sharing (CS) and complete

partition (CP) strategies. CP provides equal chance to access the available channel for

both handover calls and new calls. The CP policy divides the available bandwidth into

sub-pools according to new calls and handover calls [13].

24

For prioritized schemes, the call drop probability (CDP) and call block

probability (CBP) is reduced by increasing the priority of handover calls over arriving

new calls. This implies that the call blocking probability increased whereas the

handover failure probability decreases. Handover prioritizing schemes, however,

leads to better performance at the expanse of the reduction in the total admitted traffic

and an increase in the call block probability of new call [13]. There are several

handover prioritization schemes that have been proposed a few of the most important

are fallows:

2.5.4.5.1 Guard Channels

The guard channel scheme is reserves some fixed or adaptively changing

number of channels dedicated for handover calls only. The remaining number of the

channels are intended for both handover and incoming calls. As a result of reserving

channels for handover call, there is a reduction in number towards forced termination

probability and an increasing in call drop probability [13, 5, 6].

The guard channels are dynamically governed by the neighboring Base

Stations. Each BS determines the number of MSs in pre handover zone (PHZ)

periodically and informs its neighbor BS related to that PHZ. The PHZ is small area

that is located near the handover zone and contains the possible users that will enter

the handover zone soon. If the BS receives the number of MSs in PHZ it reserves

adequate amount of guard channels for handover calls. A new call is assigned a

channel if there is no handover call in the queued list [5, 6].

2.5.4.5.2 Queuing Handover Calls

Queuing handover call prioritization scheme queues the handover calls when

all channels are fully occupied in the BSC. As a channel is released in the BSC, it is

assigned to one of the handover calls in the queue. This scheme reduces the call

25

dropping probability at the cost of the increased call blocking probability .In queuing

schemes, a new call request is assigned a channel if the queue is empty and if there is

at least of free channel in the BSC. The call remains queued until either a channel

available in the new cell or the power by the base station in the current cell drops

below the receiver threshold. If the call reaches the receiver threshold and no free

channel if found then the call is terminated [13].

Queuing handover is probable due to the overlap regions between the

neighboring cells in which the mobile station can communicate with more than one

base station. Queuing is operational only when the handover requests arrive in groups

and traffic is low. First in first out (FIFO) scheme is the most common queuing scheme

where the handover requests are ordered according to their arrival. The Most Critical

First (MCF) policy, which is the first handover calls have highest priority [13].

2.5.4.5.3 Sub Rating Schemes

In order to receive more handover calls, the sub rating scheme reduces the

bandwidth of the existing calls. Under these methods, the ongoing calls are forced to

operate under degraded modes to accommodate calls into an overloaded system.

Certain channels are allowed to be divided into two channels with half original rates

in order to put up more calls into the system. Using these forms, half of the channel is

use to maintain the new handover calls, and other half of the channel is use to maintain

the existing call. To combine with the sub rated channel to form the original full-rated

channel the sub rated channel is released. This scheme reduces the block probability

and forced termination probability for handover calls on contrary with the introduction

of degradation in the system [17].

2.5.4.5.4 Generic Handover Schemes

In order to allocate the channel using the local state- based call request, double

threshold policies; the generic algorithm scheme is applied. The BSC supervises the

26

state information for making decisions based on abbreviation of state information and

a small number of cells. This scheme delivers better admission control policy

comparing with other methods [13].

2.5.4.5.5 Hybrid Handover Schemes

In hybrid scheme, a combination of all handover schemes is used. This policy

is intended to reduce blocking probability and to improve the channel utilization and

of course made a significant progress [13].

2.5.4.6 Handover Algorithm

Based on the handover criteria, handover algorithms are grouped into two

classes:

a- Conventional Handover Algorithm is the algorithm which is based on the

signal strength, distances, velocities and power budget [9].

b- Intelligent Handover Algorithm is the algorithm which is based on all

technology such as natural networks, prediction, fuzzy logic, and pattern

recognition [9].

2.5.4.6.1 Conventional Handover Algorithm

In traditional networks, both the mobile station and the base station frequently

measure the radio signal strength. The mobile station transmits its measurements

reports constantly to the BS. If the BS detect a reduction in radio signal under a

minimal level; it initiates a handover request as shown in the figure above. The BS

then notifies the BSC about the request, which then confirms if it is possible to transfer

the call into a new adjacent cell. In fact the BSC checks weather a free channel is

Figure 2.6: Signal Levels for

Handover [2]

27

Figure 2.6: Signal levels for handover

available in the new adjacent cell or not. In this state, the BSC does not segregate

between the channel requests either for fresh call or handover. If a free channel is

available in the new adjacent cell then handover request can be satisfied, and the

mobile station switch to new cell. But, if there is no free channel in the adjacent cell

then it increases the dropping probability of handover call. The main downside of this

handover procedure is the fact that the handover request for channel is same as used

for fresh calls [9].

In conventional handover algorithm, the relative Signal Strength (RSS) of BSs

are measured over time and the BS with the strongest signal strength is selected to

carry out a handover. To select the strongest signal strength several measurements are

used such as relative Signal Strength with threshold, Relative Signal Strength with

Hysteresis and relative Signal Strength with Hysteresis and Threshold [13, 5, 18].

2.5.4.6.2 Velocity Based Algorithm.

When the user moves fast, the probability of call drop may be high due to

extreme delay during handover. Hence a fast handover algorithm with velocity

adaptation can be recommended for urban communication. Corner detection

28

algorithm is also combined with the handover algorithm to speed up handovers in non-

line of sight state [13].

2.5.4.6.3 Direction Biased algorithms

This algorithm is very important for high mobility. Hence, if the mobile station

(MS) moves fast, the MS is not handed off quickly enough to another BS, the call will

be drop. The basic idea behind this algorithm is that handovers to the BS in the

direction towards which the MS is moving are encourage, while handovers to the BS

from which the MS is receding are discourage. This algorithm is decreasing call drop

probability for hard handover [5, 13, 17].

2.5.4.6.4 Signal Interference Based Algorithm

In this algorithm, Signal to interference ratio (SIR) is used to measure speech

communication quality and system capacity. The toll quality voice, SIR at the cell

boundary should be relatively high. This algorithm allows handover; if the SIR of the

current BS is lower than the threshold and the SIR of the target BS is better. When

actual C/I is lower than the designed C/I, the voice quality becomes poor, and the rate

of call drop increases. SIR also determines reuse distance [13, 17].

2.5.4.6.5 Intelligent Handover Algorithms

Intelligent means smart, an intelligent handover algorithm, all handover

procedures are handled by an intelligent network elements. The fuzzy logic based

approach is one of the eminent algorithms; it allows an ordered tuning of the handover

parameters to provide a balanced compromise among different system characteristics.

And other dominant intelligent algorithm is Neural network based handover algorithm

which suggests neural encoding of the fuzzy logic systems to concurrently achieve

the goals of high performance and reduced complexity. Pattern recognition based

handover algorithms, classifies meaningful regularities in noisy or complex

29

environments. These techniques are based on the concept that, points in a feature

space are mathematically defined and are close enough to represent same kind of

objects [13].

2.5.4.7 Performance metrics for Handover

Performance metrics are primarily used to appraise handover algorithms, some of

them are listed below [18]:

1. Call blocking probability: - the probability that a new call attempt is blocked.

2. Handover blocking probability: - the probability that a handover trial is

blocked.

3. Handover probability: - The probability that an ongoing call needs a handover

before the call terminates while communicating with a particular cell,. This

metric could be translated into the average number of handovers per cell.

4. Call dropping probability: - The probability that a call ends for the reason of

handover failure. This metric can be extracted directly from the handover

blocking probability and the handover probability.

5. Duration of interruption: - The span of time during handover for which the

mobile subscriber is in communication with neither base station. This metric

greatly depends on the particular network topology and the range of the

handover.

6. Probability of an unnecessary handover: - The probability that a handover is

encouraged by a particular handover algorithm when the prevailing radio link

is still adequate.

7. Rate of handover: - the number of handovers per unit time. The combination

of this probability with the average call duration, could be used to determine

the average number of handovers per call, and therefore the handover

probability.

8. Delay: - The distance through which a mobile moves from the point at which

the handover should occur to the point at which it does.

30

2.5.4.8 Types of handover

Handover, however, can be classified into different kinds based on several

factors, like the type of the network, the involved network elements or the number of

active connections and the type of traffic that the network supports [13]. In GSM,

according to the part of the network involved, handover could be; intra-BS, intra-BSC,

intra-MSC or inter-MSC handover. Considering connection establishment and

termination (number of channels that an MS can be connected at once), handover can

be grouped under two main categories; hard handover and soft handover [1, 6, 7, 18].

2.5.4.8.1 Hard handover (HHO)

The definition of a hard handover is one where an existing connection must be

broken before the new one is established [3].it is the condition in which the channel

in the source cell is released and only then the channel in the target cell is tied up.

Thus, the connection to the source is broken before or 'as' the connection to the target

is made. As a result, such handovers are also known as break-before-make [13, 17].

When an MS is between base stations, then mobile can switch with any of the

neighboring base stations once the connection with the old base station is terminated.

Hard handover is used by the systems which use time division multiple access

TDMA and frequency division multiple access FDMA such as WLAN, GSM, GPRS,

LTE and WiMAX where different frequencies are used in adjacent cells. Data do not

have to be replicated and therefore, the data overhead is minimized it is therefore

Simple and cheap and handover event is very short and usually not perceptible by the

user [6, 13].

2.5.4.8.2 Soft handover (SHO)

A soft handover is one in which the channel in the serving cell is retained and

used for a while in parallel with the channel in the target cell. This implies that the

31

Figure 2.7: Comparison between hard and soft handover [16]

connection to the target is established before the connection to the source is

terminated, hence this handovers is called make-before-break [7, 13]. Soft handovers

may sometimes involve using links to more than two cell, e.g. connections to three,

four or more cells can be maintained by one phone at the same time. The latter is more

advantageous, and when such combining is performed both in the downlink (forward

link) and the uplink (reverse link) the handover is termed as softer. Softer handovers

are possible when the cells involved in the handovers have a single cell site.

Soft handover occurs in the systems where it is possible to have neighboring

cells on the same frequency such as CDMA [3]. Consuming simultaneously channels

in multiple cells makes the use of more complex hardware inevitable [13, 21]. The

connection established in this form of handover is more reliable and handover failure

rate is expected to be minimum [6, 17].

Handovers can also be distinguished into horizontal and vertical. The type of

handover occurring in a homogenous network is termed as horizontal and the one in

heterogeneous network is known as vertical handover [6, 13]. Vertical handovers can

be further distinguished into Downward Vertical Handover (DVH) and Upward

Vertical Handover (UVH). In DVH the mobile user handovers to the network that has

higher bandwidth and limited coverage, while in UVH the mobile user transfers its

connection to the network with lower bandwidth and wider coverage [7, 18, 21, 16].

32

2.6 GSM network Evaluation criteria

GSM network performance and Quality Of Service evaluation are the two

most important criteria to be met by every service provider as their customer

satisfaction depends on network performance and quality. Radio frequency network

optimization (RNO) teams, however, have a very significant and vital role in

optimizing an operational network to keep up with the ever increasing demands from

the end users [2, 11].

2.7 Key Performance Indicators assessment and QOS estimation

The following are some of the major GSM network KPIs that give us the real

depiction of the network performance:

1. CSSR (Call Set up Success Rate).

2. CDR (Call Drop Rate).

3. HSR (Handover Success Rate).

4. TCH (Traffic Channel) Congestion Rate.

5. RX Level.

6. RX Quality.

Call set up success rate is a major KPI in GSM network optimization, it

measures number of successful TCH Assignments out of total number of TCH

assignment attempts. [11] It could be affected by; radio interface congestion, lack of

radio resources allocation, Increase in radio traffic in inbound network or Faulty BSS

Hardware. Call drop rate is also indicates the rate of calls not completed successfully.

[2] The handover success rate (HSR), however, shows the percentage of successful

handovers of all handover attempts. A handover attempt is when a handover command

is sent to the mobile. [11] Interference, Missing adjacencies and Hardware faults

mainly contribute HSR degradation.

33

Figure 2.8: Radio network planning optimization flow [11]

Traffic channel congestion rate (TCHCR) and Stand Alone Dedicated Control

Channel (SDCCH) Access Success Rate are other significant indicators that come to exist as

a result of lack of resources and hardware failure. RX LEVEL and RX QUALITY, in similar,

to the previous parameters, project a part of the whole network quality picture.

2.8 Radio network planning and optimization

In planning and optimization radio network, the main goal is to build a radio

network of large capacity and broad coverage as best as possible and make it ready

for future network development and expansion. Network planning and optimization

is an organized project covering the whole process of network building from

technology system comparison to radio transmission theory. Antenna feeder index

analysis, network capability forecast and other minor activities will not be left out.

Network planning and optimization also involves from macro view such as

characteristics of coverage capability and general design idea of radio network to

micro view such as cell parameters [11].

2.8.1 Radio network planning and optimization follow

The following chart summarizes radio network planning and optimization process.

Analysis of traffic and coverage

Emulation

Survey

System design

Installation

Optimization

34

Call service coverage analysis is the first stage of planning optimization. More

crucial information is required to support network planning cost limit. Various maps,

coverage area type, service type, terminal type and proportion, coverage and

capability requests of different services, available band, class of service, population

distribution, the development of system capacity, income distribution and the use of

fixed-line phone; all these are needed at this stage [11].

The second stage is emulation, network dimensioning estimate should be

carried out on the basis of BSS equipment and the mature planning method after call

service coverage analysis to get the coverage areas and the number of base stations

and then obtaining the configuration of all base stations according to the information

provided by the first stage. Planning software is then used to verify the coverage and

capacity results estimated at the first stage [11].

Survey, however, comes at the third stage and it is all about field exploration

in accordance with the emulation results. At this stage, potential base station address

is recorded following the requirement of the base station building including power

supply, transmission, electromagnetic background, land offset and the influence of the

future cell splitting techniques [11].

The fourth phase is system design. It involves deciding frequencies,

neighboring cell plan, and the function parameters of each cell according to the

distribution and the type of base stations. The fifth, on the other hand, is installation,

the system is installed in compliance with the design specified at the prior step [11].

The sixth stage, optimization, the goal of resource optimization is to achieve

the best possible resource utilization. With refined adjustment and a complementary

to the project defects. Analyzing and tracing problems and preparing the current

network for future expansion is the key role for optimizers [11].

35

Figure 2.9: Network optimization procedure [11]

2.8.2 GSM radio network optimization

Radio Network Optimization (RNO) teams for every communication firm are

assigned to improve network performance and maximize the benefit of the existing

network resources through parameter collection, data analysis, parameter adjustment

and necessary technical means [2, 11]. The fundamental assignment of radio network

planning and optimization is to pursue a balance among coverage, capacity and quality

of based on lucid investment and the limited frequency resources, accordingly

attaining the best rate of investment return. The following figure shows the network

optimization procedure [11].

At the first stage, actual network parameters are figured out in regarding with

the radio environment, hot-traffic spot and understanding customer requirements.

Data collection comes next to collect information relating to the traffic statistics, alarm

data and drive test data. After data collection; network tuning step is to be taken which

tune network functional parameters. After accomplishing all above tasks; network

36

optimization report covering fulfilled network performance indexes and suggestions

for network development comes at last [11].

2.8.2.1 Intra-BSC Handover optimization

Intra-BSC handover is one of the most frequently occurring handover since it

occurs in between adjacent BTSs. Optimizing intra-BSC HO simply means increasing

HSR in the level of BTSs under the control of same BSC. What is so interesting in

intra-BSC HO is that the whole process is managed by the BSC it concerns and the

MSC is only informed about the process. In this thesis, however, only intra-BSC HO

is focused and the rest of other types of HO are not mainly considered.

2.8.3 GSM Network optimization tools

Both software and hardware gadgets used for network optimization are

collectively called optimization tools, test MS, drive test software and signaling

analyzer are some of optimization tools used [2, 11].

Test MS is an important tool for engineers to perform network test. It can

display the service cell of mobile telecommunication network and six neighbor cells,

it can be also used to view IMSI of the SIM card, scan BCCH, view network

parameters and it can be forced to cell selection and handover; to analyze network

performance [2, 11].

GSM drive test software, like ANT, TEMS and SAFCO are used for data

collection. A drive test software, however, can be a foreground data collection

software or background data analysis software. Foreground data collection software

is used for uplink and downlink data collection such functions include; interference

test, parameter collection, geographic navigation, SQI (speech quality index)test and

traffic statistics. Background data analysis software, however, guides engineers to

evaluate and optimize the network rationally and effectively by allowing them make

37

radio coverage evaluation, neighbor cell analysis, handover analysis and signaling

analysis. [11]

MA-10 and K1205 are two major signaling analyzers used in GSM

optimization. With the help of MA-10 signaling analyzer, network engineers can

collect and analyze Abis interface data and A-interface data, view the whole signaling

procedure, and obtain the measurement report and compare the information with the

downlink signals obtained from drive test. [11]

2.9 Literature review

Optimization concept could be traced back to the early days of GSM network

deployment. Since then, quite a lot of research has been conducted on optimization

field. According to the relation with this thesis, most published researches can be

divided into two broad categories:

The first type is analyzing general handover issues in an independent study, there are

some studies emphasizing on handover related issues.

Syed Imran Basha and Idrish Shaik investigated reducing Handover Failure

Rate by RF Optimization in which RF performance parameters such as the received

signal strength, receive voice quality, carrier to interference ratio, etc. are defined for

the efficient and effective functioning of the RF network. They also present short-call

and long-call control tests from the drive testing process working on various tools

such as Agilent 15.2 Drive test tool, ACTIX Post processing tool. By analyzing the

drive test results, the main motive behind this study was to identify the causes of

handover failures in a BSNL service test area and necessitate steps to reduce the

handover failure rate [22].

After checking all other GSM parameters and going down to handover issues,

the study has suggested some possible reasons for handover failure, such reasons

38

include; unavailable time slots because of high traffic, congestion, low signal strength

or bad quality on target cell. Hardware problems in target cells is considered as

another major possible cause for handover failure. TRX or time slot problems is

attributed as another more likely cause of handover failure. If handover attempt fails,

MS tries to return to old channel and if it couldn’t, call drops.

Syed Imran Basha and Idrish Shaik concluded that most of the network

problems they came across were caused by increasing subscribers and the changing

environment. And the main purpose of optimization was to increase the utilization of

the network resources, solving both existing and potential problems on the network

as well as identifying the probable solutions for future network planning.

The second category deals with Overall GSM KPI optimization without specific

attention on handover, Olasunkanmi F. Oseni conducted a study aimed at improving

the Quality of Service (QoS) of the GSM radio network set up within Abeokuta City,

Nigeria. A drive test was carried out in dedicated mode with the objective of collecting

measurement data as a function of location and to detect the eventual black spots in

the GSM radio network. The data collected were examined in post-processing

software tool (MapInfo Professional) to identify the causes of problems and determine

how these problems can be solved effectively and efficiently [23].

The study showed that radio network technologies explored should be

optimized for better quality of service delivery in some parts of the city. The poor

radio coverage and decline in quality of service acknowledged in the affected areas of

the city were traced to the land topology and the presence of physical impediments

present in the propagation environment. Also, poor quality samples collected over

some BSs were due to poor coverage in the area. It is therefore recommended that a

base station should be well planned by the Radio Network Planning (RNP) team of

the network provider and vendor to enhance radio coverage in the affected area. The

planned site is also expected to improve the Quality of Service (QoS) offered by the

radio networks. The Hand-Over (HO) failures were mainly due to Base Station

Controller (BSC) Synchronization issue observed. This should be resolved properly

39

and timely. If the optimization is successfully accomplished, the QoS, reliability and

availability of RF Coverage area will be highly improved resulting in more customers

and more profits to the mobile telecom service providers [23].

Baha’ Khaled made evaluation on Jawwal GSM network in Jenien city,

Palestine. In his study, he used two approaches; evaluation of KPIs and drive test

analysis. The city was divided into seven different areas for studying. In [24] RX level

is mainly considered leading to TCH relating issues to be firmly focused. As a result

of his work on Jawwal GSM network, GSM network performance has risen to a

substantial percentage [24].

In similar, Giriraj Sharma, Ashish Kumar proposes some practical cases and

solutions adopted to improve the network QOS during drive test & post processing.

Major QOS parameter Handover, call drop, congestion, interference reasons and

solutions are discussed. Drive test tool ASCOM TEMS 10.2.1 is used to perform drive

test. Finally the study suggested that if optimization is done constantly it will result

better network quality [21].

2.10 Summary

Handover is the process of transferring live session from one part of the

communication system to another part of the system. It has an utmost importance that

makes it play a pivotal role in the system quality of service. In GSM, handover a key

performance indicator as it indicates quality of the network. Optimization is making

a network productive and efficient with the limited resources available and preparing

it for potential forthcoming expansion. To make GSM network more efficient, several

researches were conducted all over the world. The most available literature covers

handover particularly or GSM in general and handover is taken as a subtitle.

According to this thesis, intra-BSC handover is taken as a case study and the

methodology employed for investigation is fully depicted in chapter three of this

thesis.

CHAPTER 3

RESEARCH METHODOLOGY AND MATERIALS

3.1 Introduction

This chapter shades some light on Intra-BSC handover Optimization by

using Drive test methodology together with data recorded by BSC to upsurge the

performance of the current and future live networks. Through Radio Network

Optimization, the service quality and resources usage of the network are critically

enhanced and the balance among coverage and capacity is achieved. When new cell

sites are being deployed (live network), the key issue of GSM mobile operators is

Radio Network Optimization (RNO).

Radio network optimization is carried out in order to improve the network

performance with the available resources. RNO teams are required to keep up with

current standards and demands and to prepare the network to comply with future

requirements as well. The main driving force behind optimization is to rise the

operation of the network resources, resolve the existing and possible glitches on the

network and categorize the probable way-outs for the current (Live Systems) and

future networks hindrances. Providing Easy way for the operation and maintenance

teams in their job of troubleshooting, is one of the principal contributions of

optimization [25]. Handover optimization, however, has a lion share in overall GSM

network Optimization. In this study, looking deep into statistics and

41

collecting/analyzing drive test data is the core method applied for network

evaluation on the bases of handover optimization.

In general, the following key steps are followed in Radio Network Optimization

process:

a) Data Collection and verification

b) Data analysis

c) Parameter and hardware adjustment

d) Optimization result confirmation and reporting.

e) On-going drive testing measurements using Drive testing tool (TEMS

investigation)

The chart below shows the methodology of network optimization

Figure 3.1 General radio network optimization flow

42

Due to the mobility of subscribers and complexity of the radio wave

propagation, most of network problems are caused by increasing subscribers and the

changing environment. Thus, Radio Network Optimization is a continuous process.

3.2 Drive Test Overview

The performance of wireless network infrastructure to be comprehensively

evaluated by drive testing. Factors such as the network coverage, the level of

availability, the call quality and the data capacity of a base station can all be

observed in detail. Drive testing can be done whenever new cell sites are being

deployed (live systems), to confirm that they fit in into the network properly. It can

also be carried out in on a working network to make sure that the operational

effectiveness of the network is sustained, or to examine a particular problem that has

been revealed in order to find the origin of that problem and subsequently resolve it.

it also ensures that its installation and its on-going preservation are both carried out

correctly[24, 25].

3.3 GSM Network Drive Testing

Carrying out drive test operation on GSM network palaces the service

provider at the place of their subscribers making them better armed with real

information to deal with situations which might otherwise result in a decrease of

their customer base and the revenues that they could generate. Similarly,

troubleshooting can be carried out when necessary. Through this process, the source

of a particular issue being experienced by subscribers (such as a high frequency of

dropped calls in a particular cell site or handover failure), which could have

occurred simply because of errors in the course of the deployment process or

through some errors arising later (like component failure), can be found.

In addition, drive testing should be done both before and after the upgrading

of a cell site, so that it can be established whether or not the desired performance

43

optimization have been made. Network optimization is another important practice

enabled via drive testing, allowing operators to ensure that their network resources

are fully consumed and that capital overheads has not been wasted.

3.4 Significant KPIs and Their Insinuations

1. Accessibility - This basically describes how easy it is for the mobile

subscriber to access the network and obtain a service (in order to make data

transfers, etc.). Normally a KPI will be set for a network attach failure ratio

of less than 1%.

2. Retain-ability –is a measure of the Radio networks ability to maintain an

active mobile session until the user terminates. It shows the percentage of

active calls dropped. In general, this describes the capability of a service,

once it has been obtained, to continue for the period required by the mobile

subscriber. With regard to voice based services, it informs engineers about

how well a network performs in keep calls from the setup to their normal call

end. It is invaluable in safeguarding the network against dropped calls,

allowing the operator to identify poor performing cells and resolve the

underlying issues impinging on their operation. Normally a KPI will be set

for a dropped call ratio of less than 2%.

3. Integrity – This tells engineers if the network perceived audio quality

is ‘Good’ in more than 95% of all the samples [11].

4. Power budget handover threshold – The power budget expression (PBGT)

offers a technique of associating a path loss of an MS and serving cell

(PBGT) with a path loss of the MS and a potential handover target cell

(PBGT). A handover may be instigated when (PBGT) goes beyond a

handover threshold value designated by a system operator [11].

44

5. Minimum downlink power for handover candidate cell – It is the minimum

allowed access level for a cell to be a neighbor cell. When the cell level

measured by MS is greater than the threshold, the BSS list the cell into

candidate cell list for handover judgment. The acceptable power level ranges

from –110 dBm to –47 dBm [11].

3.5 Network optimization tools

Network optimization tools are used for data collection, data analysis, and

simulation analysis. These tools are:

Laptop with TEMS investigation 8.0.

Car to carry out the drive test.

Full drive test kit.

Digital camera.

GPS.

Maps.

3.6 Necessary Equipment (Materials)

The basic kit required for completion of a drive test include:

1. A vehicle – It can be any form of a car or even motorbike. It facilitates the

field operative to navigate a pre-defined route while getting test data (in

some cases though testing may even be done on foot).

2. Measurement device – it can be a series of mobile handsets, a data card, or a

scanner. Through the device parameters such as the signal strength, call

quality, transfer rate, success of handovers between adjacent cells, etc. can

all be learnt. Normally two handsets will be used. The first will be active, so

that call-based measurements can be taken, while the second will be inactive,

45

Figure 3.2: Drive test data collection set up

just connected to the network. Through these handsets the ‘call’ mode and

‘idle’ mode performance can be evaluated at various points along the route.

Both long calls and short calls are undertaken as these will allow different

network performance parameters to be tested (short calls can test signal-

based performance characteristics, while long calls can test activities such as

handovers, etc.).

3. A laptop– For the storage and subsequent manipulation of acquired data.

4. A GPS system – Through which the exact location of the operative can be

determined at any stage of the test run. Using the acquired test data and the

GPS data, a visual representation of network’s coverage can be put together,

showing where there are potential areas of concern.

5. Choosing the software package is crucial when looking to embark on drive

test work. for this study , Ascom Network Testing’s TEMS™ Investigation

is used. It is a multi-technology platform that presents mobile operators with

one all-encompassing solution for the collection; analysis and processing of

data that help to optimize and troubleshoot 2G, 3G and also LTE (4G)

network, as well as carry out comparisons with other networks.

46

3.7 Implemented Action Plan

3.7.1 Geographic Description of the Propagation Terrain

Mogadishu is the capital city of Somalia located on Latitude 2.0333° N and

Longitude 45.3500° E. The area under study is Siliga area Wadajir district

Mogadishu Somalia on the Latitude 2.03415°N and Longitude 45.29065°E. Siliga is

categorized as a suburban environment with narrow streets and low buildings mainly

made up of aluminum sheets. Table 3.1 gives information about the target area and

all serving sectors of the base station.

Figure 3.3: Aerial view of the target area (Siliga, Mogadishu)

47

Table 3.1: Selected serving sectors of the base station

BTS Name Longitude Latitude BTS

Configuration

1142_BAN_SILIGA_900

1142_BAN_SILIGA_1800 45.29065° 2.03415° S444/S888

1069_BAN_JIIRO MISKIIN_900

1069_BAN_JIIRO MISKIIN_900 45.2917° 2.0263° S444/S888

1164_BAN_ADC_B 45.29405° 2.03882° S444/S888

1066_BAN_BUULA XUUBEEY_900

1066_BAN_BUULA XUUBEEY_900 45.29829° 2.02697° S444/S888

1167_BAN_JAMACADA

HORMUUD2_1800

45.302735° 2.028789°

S444/S888

1128_BAN_ISB.BANAADIR_1800 45.2995° 2.0358° S444/S888

1046_BAN_SEEY PIANO_900

1046_BAN_SEEY PIANO_1800 45.29964° 2.0395° S444/S888

3.7.2 Data Collection Process

A drive test was conducted to obtain the actual field measurement data which

was later used for analysis process. The drive test equipment set up consists of a

laptop (having Transmission Evaluation and Monitoring System (TEMS)

investigation software installed on it), a power supply unit, TEMS Mobile Station,

Global Positioning System (GPS) and a vehicle. TEMS Investigation software

offered the capabilities of data collection, real-time analysis and post-processing, all

in one. Furthermore, TEMS investigation can provide important features such as

continuous scanning of GPS coordinates, received signal level, channel number and

base station identity codes (BSIC). An illustration of the setup is given in figure ().

Data were collected in the drive test mode as log files and they were played back in

the replay mode for inspection and analysis

48

Figure3.4: Drive test coverage plot

3.7.3 Drive Test Survey Route

The drive test survey route was carefully planned with the aid of road and

vector maps such that the measurement collection process involved all the base

stations earlier marked out for investigation. The routes covered in this study were

Siliga road and Nasteha road; this gives easy access to the coverage areas of the

respective cells investigated.

3.8 Summary

Radio network optimization and planning has a greater importance for live

systems and future network expansion as well. In GSM, optimization is a routine job

intended for attaining good performance. The work in this thesis particularly focuses

on intra-BSC handover related aspects. Drive test is made using Ascom Network

Testing’s TEMS™ Investigation together with necessary materials such as GPS and

measurement Devices, to collect real data from a preplanned rout with several base

stations serving in Siliga area under Hormuud telecom’s 2G network.

CHAPTER 4

RESULTS AND ANALYSIS

4.1 Introduction

In this chapter, the data collected for the study area is deeply examined. The

information was collected and processed through drive testing process using TEMS

(Test Mobile Systems) investigation, the output is portrayed in the form of screen

snap shots of the TEMS log files, BS data in the BSC configuration and TEMS line

charts. The data collected is analyzed in a way to comply with the objectives of the

study. These outputs of the examined HO optimization aspects include, several HO

failures, Ping-Pong HO and positive handover trends as well.

Through the analysis of intra-BSC handover optimization data,

complications related to inter BS handover are located and then proper regulations

are made. Furthermore, the data collection tool consisted of ascom TEMS tool with

an antenna mounted on a moving vehicle; 1.5 meter above ground level, GPS,

personal computer and a piece of compass. The PC houses the operating system and

the data collection software (ascom TEMS Investigation 14.1). The personal

computer serves as the communication hub for all other equipment in the system.

The GPS operates with global positioning satellites to provide the location tracking

for the system during data collection position on a global map which has been

installed on the personal computer.

50

Drive test data is collected from a preplanned route with seven serving sites

(receiving strongest signals from them) under the same BSC. A long call

examination was established observing more closely to handover relating events.

The seven serving sites with their primary descriptions are shown in the in the table

below.

Table 4.1: Target area and all serving sectors of the base station.

BTS Name Longitude Latitude BTS

Configuration

1142_BAN_SILIGA_900

1142_BAN_SILIGA_1800 45.29065° 2.03415° S444/S888

1069_BAN_JIIRO MISKIIN_900

1069_BAN_JIIRO MISKIIN_900 45.2917° 2.0263° S444/S888

1164_BAN_ADC_B_1800 45.29405° 2.03882° S444/S888

1066_BAN_BUULA XUUBEEY_900

1066_BAN_BUULA XUUBEEY_900 45.29829° 2.02697° S444/S888

1167_BAN_JAMACADA

HORMUUD2_1800

45.302735° 2.028789°

S444/S888

1128_BAN_ISB.BANAADIR_1800 45.2995° 2.0358° S444/S888

1046_BAN_SEEY PIANO_900

1046_BAN_SEEY PIANO_1800 45.29964° 2.0395° S444/S888

As the vehicle started moving and the call established, log files started

recording data. More data is recorded by the drive test tool but those concerning

handover and their underlying events were firmly monitored. The following steps

are used for the analysis procedure:

1. Targeting both positive and negative network trends.

2. Identifying the possible causes of each problem points on TEMS.

3. Checking significant parameters such as BSC data configuration, traffic

statistic and BS alarms.

4. Proposing possible solutions and recommendation.

51

Figure 4.1: Handover failure for lack of resources

4.2 Observed cases and their corresponding analysis

In the course of the analysis of intra-BSC handover related data collected

from the selected area, more handover-related cases; both ups and downs immerged

and some of those cases are dissected in the following sections.

4.2.1 Hand failure due to lack of resources

4.2.1.1 Event description

In this case, handover failure occurs due to the unavailability of time slots

because of high traffic, congestion, low signal strength or bad quality on target cell.

This condition occurred between Siliga_B (ARFCN=14, BSIC=20) and Siliga_F

(ARFCN=535, BSIC=05). This case could be solved by increasing the capacity or HO

reserve channels of the BSs to host potential handovers.

52

4.2.2 Hand failure due to missing neighbor


If a handover is not completed to a neighbor cell that seems to be best server,

there is a possibility of a missing neighbor relation. This will happen with sudden

appearance of strong cell in the neighbor list just after a handover. Missing neighbor

scenario exists between Serving cell MDN_CEELQALAW_E (ARFCN: 525 BSIC: 0-3)

and JIIRO MISKIIN_E (ARFCN: 514 BSIC: 7-3).

4.2.2.2 Recommended solution for missing neighbor HO failure

Missing neighbor problem could be resolved by configuring each BS with

proper data concerning neighboring cells that can have potential possibilities of

handover. After all BSs are being introduced each other, each one accepts handover

from others and then no more missing neighbor problem to happen.

Missing neighbor

ARFCN: 514 BSIC: 7-3

Figure 4.2: Hand Failure Due to Missing Neighbor

53

Figure 4.3: Hand failure due to one way neighbor

4.2.3 One way Neighbor


A neighbor relation needs to be defined as mutual. When defining cell A and Cell B

as neighbors to each other, neighbor relation from A to B and from B to A has to be defined.

Otherwise the HO attempt is not possible in both directions. It should be remembered that

there will be rare cases where planner will need one–way neighbor relations.

4.2.3.2 Recommended solution for one way neighbor HO failure

When deploying a new site, many different things should be considered.

Excellent frequency planning together with more cautious configuration is

recommended, at this stage, each cell should be configured with bidirectional

neighboring.

54

Figure 4.4: Ping-Pong

4.2.4 Ping Pong


Ping–pong is termed as a series of successive handovers occurring more

frequently than normal handovers. It of course loads the system. Ping–pong effect

can be as a result of fading, the MS moving in a zigzag pattern between the cells,

incorrect handover margins or by non–linearity in the receiver. All these cases will

cause ping–pong handovers. Ping-Pong scenario is observed in an area served by

XRN_MADIINA, XALANE, TAHLIIL WARSAME and AIRPORT1.

4.2.4.2 Recommended solution for Ping-Pong HO

A hysteresis criteria is one strategy used to prevent the ping–pong effect. First thing

to check whenever Ping-Pong scenario is detected will be handover margins between the

neighbors. And the margins should be matched.

55

Figure 4.5: Smooth handover case

4.2.5 Lack of late Handover


In some abnormal cases, handover process take place a little late. Because of

mismatched handover margins between the neighbors. This study, however, showed that

there is no late handovers in the study area. One example is that occurred in between

Serving cell 1033_XRN_MADINA_E( BCCH ARFCN: 518 and BSIC: 1-7) and

Target cell 1072_TAHLIIL WARSAME_F (BCCH ARFCN: 520 BSIC: 5-3).

This study was not only for finding faults, but also acknowledges positive

trends encountered throughout data collection process. The lack of late handover

scenario falsifies one of the major study hypothesis that was the existence of all sorts

of handover problems in the area explored.

56

4.3 Drive test KPI Reports

The table below shows some key indicators recorded in the log files of the

drive test by establishing long call (Total duration: 02:45:47.66) in BSC5 west rout.

And one key important point is that handover failure rate is 2.21% which is

remarkable. Dropped calls also exist and one of its main contributor is handover

problem.

Table 4.2: TEMS generated report

Event occurrence Number of times

Blocked Call 1

Call Attempt 4

Call Setup 2

Dropped Call 2

Handover 226

Handover Failure 5

4.4 Summary

Intra-BSC handover is an important parameter and being optimized

contributes more to the network quality. This chapter presents some existing cases

such as handover process failures and unnecessary handover blunders as well. It also

contributes possible way-outs and recommendations to resolve those glitches.

Admitting the positive side of the findings, this chapter emphasis on the on the

existence of more miserable than admirable experiences in the study area and calls

more actions to be taken.

CHAPTER 5

CONCLUSION AND FUTURE WORK

5.1 Conclusion

In this thesis, intra-BSC handover related cases has being investigated in the

GSM network of Hormuud telecom carrying out drive test campaign on a given small

study area in Wadajir district called Siliga. At the beginning of the study, the first

hypothesis was the existence of all sorts of handover problems, but the study proved

the existence of some sort of positive handover trends. Some of the HO failures

detected come to exist as a result of missing neighbor relationships, lack of resources,

one way neighbor or Ping–pong effect. Although this study is intended for finding

handover hampering elements, it has also revealed certain positive remarks for some

handover related cases such as the lack of late handover scenario.

The main notion behind this study was to dissect all handover problems

existing in the GSM network of Hormuud telecom serving in the selected area and

eventually come up with possible recommendations to solve detected glitches.

Chapter four of this thesis, however, presents some of the hunted cases in the study

and endorses some recommendations to rectify current and potential possible future

failures. The preliminary experiences of the application of the data provided by this

thesis, however, contributed more to the work of RNO teams by shading some light

on some critical and quality-threatening factors.

58

5.2 Future Work

Countless efforts have been paid on getting this work complete in all aspects.

But as with all projects, there is always a room for improvements and further

enchantments. Same is the case with this thesis work. This study has being basically

conducted on handover on the basis of intra-BSC level; more exploration could be

made on the other levels both higher and lower, such as intra-MSC handover, inter-

MSC handover and intra-BS handover related aspects.

This study, however, covers basic measurement levels, no more algorithms

examined in it. But, since the GSM network components was manufactured by one

specific Company, their particular algorithms play a role on the systems investigated.

For other researchers interested in algorithm exploration, there is a room for them to

conduct a very advanced form of study on algorithms.

59

REFERENCES

1. Paul, L. C. Handoff / Handover Mechanism for Mobility Improvement in

wireless communication. Global Journal of Researches in Engineering.

2013. 16(13): 1-13.

2. B.L.Jagadesh, K. ,. a. N. Handover Analysis. International Journal of

Engineering Research and Applications (IJERA). 2013. 1(2). 287-291.

3. Sumaira Kanwal, M. A. Handoff Issues In Cellular Networks. 1st National

Conference on Security, Computing, & Communication. May 23-25, 2008.

Islamabad: Kohat University of Science & Technology, 2008. 99-103.

4. Harpreet Kaur, Amrit Kaur. Various Handover Management Techniques In

GSM. International Journal of Advanced Engineering Research and Science

(IJAERS). 2014. 1(2). 28-33.

5. khan, J. Handover management in GSM cellular system. International

Journal of Computer Applications. 2010. 8(12).14-24.

6. Nasıf Ekiz, T. S. S. K. a. K. F. An Overview of Handoff Techniques in

Cellular. International Journal of Computer, Information, Systems and

Control Engineering, 2007. 1(6). 1683-1686.

7. Rappaport, T. S. Wirelwss Communications. 2nd ed. Upper Saddel River:

Prentice Hall. 1996.

8. Jörg Eberspächer et al. GSM switching, services and protocols. london: John

Wiley & Sons. 2001.

9. Jahangir Khan, Ali Abbas and Khisro Khan. Cellular Handover Approaches

in 2.5G to 5G Technology. International Journal of Computer Applications.

2011. 21 (2). 28-37.

10. Korhonen, J. Introduction to 3G Mobile Communications. 2nd ed. boston:

Artech House. 2003.

11. 11. Huawei technology. training reference material for RNP and RNO.

Beijing: training center of Huawei. 2009.

12. Shri GSM: Physical & logical Channels, London. John Wiley & Sons. 2011

http://learntelecom.com/author/shri/

http://learntelecom.com/gsm-physical-logical-channels/

60

13. A. Bhuvaneswari, D. E. G. D. P. R. Survey On Handoff Techniques. journal

of Global Research in Computer Science. 2011. 2 (6).140-144.

14. D.K.Sharma. Analog And Digital Modulation Techniques: An Overview.

International Journal of Computing Science and Communication

Technologie. 2010. 3(1).551-561.

15. Tanvir Sing. Channels and Identities in GSM. International Journal of

Electronics & Communication Technology. 2011. 2(3).210-214.

16. Chen, Y. Soft Handover Issues in Radio Resource Management for 3G

WCDMA Networks. Msc. Thesis. Queen Mary, University of London. 2003.

17. Tripathi, N. D., Generic Adaptive Handoff Algorithms Using Fuzzy Logic and

Neural Networks. PhD. Thesis. Virginia Polytechnic Institute and State

University. 1997.

18. Gregory P. pollini. Trends in handover design. IEEE communication magazine

. 1996. 82-90.

19. Amit Kumar. Evolution of Mobile Wireless Communication Networks 1G to

4G. International Journal of Electronics & Communication Technology.

2010.1(1). 62-78.

20. Ouyang Congxing et al. An analysis of radio handover success ratio, Beijing:

Huawei technologies. 2005.

21. Bansal, G. S. a. A. K., A Practical Approach to Improve GSM Network

Quality by RF Optimization. International Journal of Engineering and

Advanced Technology (IJEAT). 2010. 1(1) 95-99.

22. Syed Imran Basha, I. S. Reducing Handover Failure Rate by RF. International

Journal of Engineering and Innovative Technology (IJEIT). 2013 2(11).29-33.

23. Olasunkanmi. et al. Radio Frequency Optimization Of Mobile Networks In

Abeokuta, Nigeria For Improved Quality Of Service. International Journal of

Research in Engineering and Technology. 2014. 3(8) 174-180.

24. Mkheimer, B. K. Evaluation and Enhancement of Jawwal GSM Network in

Jenien City. Jerusalem. Msc. Thesis. Al Quds University; 2011.

25. Goksel, S. Optimization and log file analysis in GSM. 1st ed. Antalya. 2003.

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APPENDIX A

STATISTICAL REPORT FROM THE TEMS LOG FILES

62

APPENDIX B

MAP OF MOGADISHU

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Abdisalam Thesis

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