Handover Enhancement for Mobile WiMAX Based on Mobile. Movement and Base Stations Communications. Emran Hassan Mohammed Al-Saleh.

Transcription

1 Handover Enhancement for Mobile WiMAX Based on Mobile Movement and Base Stations Communications By Emran Hassan Mohammed Al-Saleh Supervisor Dr. Ghassan Samara This Thesis was Submitted in Partial Fulfillment of the Requirements for the Master Degree in Computer Science Faculty of Graduate Studies Zarqa University Zarqa, Jordan First Semester December, 2015

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4 ACKNOWLEDGMENTS Prior to acknowledgments, I must glorify Allah the Almighty who gave me courage and patience to carry out this work successfully. I would like to thank my supervisor, Dr. Ghassan Samarah for advice and encouragement. My dear parents, No words can appreciate what you have done for me. All these years, thank you from the bottom of my heart. A special thanks to my lovely wife, for being so understanding, and caring throughout the course of my research work. This thesis truly would not have been possible without her constant support. My dear brothers, sisters and friends! How can I ever thank you? Your endless love and support fills me with life. You are my source of inspiration and I am forever indebted to you. Loads of love and thanks to you all!.

5 TABLE OF CONTENTS Contents List of Tables. vii List of Figures viii List of Acronyms... x List of Publications xi Abstract in Arabic... xii Abstract in English xiii Chapter 1: Introduction Overview Problem Definition Thesis Objectives Thesis Contributions Thesis Outline... 6 Chapter 2: Background and Related Works Introduction Background of WiMAX Technology Mobility Feature (Handover) in WiMAX WiMAX Network Architecture ASN-Anchored Mobility (Intra-ASN Mobility) CSN-Anchored Mobility (Inter-ASN Mobility) Types of Handover in Mobile WiMAX Handover Procedure Network Topology Acquisition Phase (NTAP) Actual Handover Phase (AHOP) Related Work The Scanning Interval Time Frequency of Scanning Operations Distance Estimation and Zones Concept Summary Chapter 3: Proposed An Improved MS-Controlled Handover Algorithm Introduction An Improved Mobile Station Controlled Handover Algorithm Page

6 3.3 The Concept of Main Neighbor Base Station Proposed Improved Mobile Station Controlled Handover Algorithm In Zone of Normally In Zone of Concern Selection the Potential Target Base Stations Based on Main NBS Selecting the Target Base Station Based on Selection the PTBSs The Proposed Improved Mobile Station Controlled Handover Algorithm Summary Chapter 4: Performance Evaluation Introduction Simulation Study on Improved MS-Controlled Handover Technique Choice of simulator Mobility of MS used for simulation Simulation Topology Parameters Used in Simulation Evaluation Metrics Handover Delay Number of Scans Performed in Pre-Handover Phase Throughput Packet Loss Ratio Simulation Results and Discussion Average of NTAP and AHOP Delay Number of Scans Performed in Pre-Handover Phase Throughput Packet Loss Ratio Summary Chapter 5: Conclusion and Future Works Introduction Conclusions Future Works.. 56 References. 57

7 LIST OF TABLES Number Table Caption Page Table 4-1 Key Simulation Parameters 45 vii

8 LIST OF FIGURES Number Figure Caption Page Figure 1-1 Example of Cellular Network 2 Figure 2-1 WiMAX Physical and MAC Layer Architecture 9 Figure 2-2 WiMAX Network Reference Model 11 Figure 2-3 ASN and CSN Anchored Mobility 12 Figure 2-4 Handover Procedure Phases in Mobile WiMAX System 14 Figure 2-5 The Chart of Message Sequence for Network Topology Acquisition Phase 15 Figure 2-6 The Chart of Message Sequence of Actual Handover Phase 18 Figure 2-7 Zones Concept 23 Figure 2-8 Distance Estimation with AOD based Lookahead Scheme 24 Figure 2-9 Flowchart of the AOD-based Fast MAC-Layer Handover Scheme 26 Figure 3-1 Block Diagram of Improved Mobile Station Controlled Handover Technique 30 Figure 3-2 Mobile WiMAX centralized deployment architecture 32 Figure 3-3 Broadcasting the MOB_NBR-ADV Message in Zone of Normally 32 Figure 3-4 The steps that are performed in the Zone of Concern 33 Figure 3-5(a) The Illustrative Example of the Orientation Matching the MS movement direction according to Main Neighbor Base Station 34 Figure 3-5(b) Angle of divergence between the movement of Mobile Station and Main Neighbor Base Station 35 Figure 3-5(c) Angle of divergence between the movement of Mobile Station and Main Neighbor Base Station 36 Figure 3-5(d) Angle of divergence between the movement of Mobile Station and Main Neighbor Base Station 36 Figure 3-5(e) Angle of divergence between the movement of Mobile Station and Main Neighbor Base Station 37 Figure 3-6 Flowchart of the Improved Mobile Station Controlled Handover Algorithm 38 Figure 4-1 The Multi-Cell Simulation Topology 42 Figure 4-2 Average of Pre-Handover (NTAP) Delay Analysis 48 Figure 4-3 Average of Total Handover (AHOP) Delay Analysis 49 Figure 4-4 Average Number of Scanning Iterations in Pre-Handover (NTAP) Phase 50 viii

9 Figure 4-5 Throughput Analysis in Handover Process 52 Figure 4-6 Packet Loss Ratio Analysis in Handover Process 53 ix

10 LIST OF ACRONYMS AHOP AOD BS CTBS HO MOB_MSHO-REQ MOB_NBR-ADV MOB_SCN-REQ MOB_SCN-RSP MS NBS NTAP PTBS QoS RSS SBS TBS WiMAX ZC ZD ZE ZN Actual Handover Phase Angle of Divergence Base Station Candidate TBS Handover Mobile Station Handover Request Mobile Neighbor Advertisement Scanning Interval Allocation Request Scanning Interval Allocation Response Mobile Station Neighboring Base Station Network Topology Acquisition Phase Potential TBS Quality of Service Received Signal Strength Serving Base Station Target Base Station Worldwide Interoperability for Microwave Access Zone of Concern Zone of Doom Zone of Emergency Zone of Normalcy x

11 LIST OF PUBLICATIONS Emran Hassan Al-Saleh and Ghassan Samara, Mobile Station-Controlled Handover Scheme in Mobile WiMAX: Case Study, International Journal of Computer Applications, Vol. 127 (8), pp , October xi

12 تحسين التسليم في شبكات WiMAX باالعتماد على حركة جهاز الخلوي واالتصال بين محطات االتصال إعداد عمران حسن الصالح إشراف د. غسان سمارة الملخص إن عملية التسليم في شبكات الهاتف النقال WiMAX تعتبر نرأ م رم االيرات الةاورمة مالمهمرةي مالتري تسرم باوتم ارية االتصرال عنرا اقتقرال الهراتف النقرال نرأ ننطقرة تاطيهرا صاعرال اتصرال إلري ننطقرة تاطيهرا صاعرال اتصرال مخ ى دمن اققطاع. التسليم الصعب في نعيار شبكات WIMAX إلزانريي بينمرا التسرليم اللريأ اختيرارن. منرأ نراي كان الت كيز في ذا العمل علي التسليم الصعب. تعاقي خوارزنية التسليم التي يتةكم فيها الهاتف النقال نأ عال عيوب مالتي تؤث علي كفاءل النظرا ي متتمرمأ التأخي الناتج عأ عمليات المسر الزادرال مالاير رر مرية لمةطرات االتصرال الم رامرل ي خ يقرة اختيرار نةطرات االتصال المةتملة لعملية التسليم مخ يقة اختيار نةطة االتصال الهاف. علي من حالي تخفيض الوصت الالز لعملية التسليم متةسيأ اختيار صاعال االتصال الهاف ما المشكلتان اللتان تم الت كيز عليهم في ذا العمل المقت ح. نأ مجل تقايم حل لهاتيأ المشكلتيأي في ذه ال والة تم اصت اح قموذج لتةسيأ عملية التسليم ي مصرا مخلرع علير اوم النموذج المةسأ للتسليم الرذن يرتةكم فير الهراتف النقرال. رذا النمروذج يرتم الرتةكم فير بعمليرة التسرليم نرأ خرالل الهاتف النقالي مميما يستطيع الهاتف النقال تتبرع ح كتر بالنسربة لموصرع صواعرا االتصرال الم رامرل. منهر ت النترادج الت يبيرة للنمرروذج تخفريض بنسرربة % 5.7 علرري ن مرل التررأخي فري عمليرة التسرليمي متقليرل % علرري قسربة البياقات المفقودل خالل عملية التسليمي متةسيأ % 4.62 علي كمية البياقات المستلمة. xii

13 Handover Enhancement for Mobile WiMAX Based on Mobile Movement and Base Stations Communications ABSTRACT Handover is one of the most critical mechanisms in Mobile Worldwide Interoperability for Microwave Access networks. It allows a session to be seamlessly maintained when the Mobile Station moves and be handovered between one base station to another base station without any call drop. Two types of handover in Worldwide Interoperability for Microwave Access standard, the hard handover that is mandatory and the soft handover that is optional. Hence, in this work the concentration was on the hard handover. The Mobile Station-Controlled handover procedure encounters many flaws which affect the system's efficiency, that include delay caused by excessive scanning activities, methods of selecting potential target base stations and process of selecting the Target Base Station. However, reducing the handover delay and improving the selecting TBS process are the two problems that this thesis intended to solve it. In order to solve the two problems above, this thesis proposes an Improved Mobile Station-Controlled Handover Technique. This technique is not only fully controlled by the mobile station, but also the mobile station self tracks its own movement with respect to the location of the Main neighboring base station. The experimental results of the proposed technique show that, it reduces the overall handover delay by 5.7 %, a reduction of % in the packet loss ratio is achieved, and the throughput is increased by 4.62 %. xiii

14 Chapter 1 Introduction 1.1 Overview At the present time, in an era of mobile phones, Laptops, tablets, and electronic gadget there is often a necessity to set up a network to enable communication among these devices. Wireless communication is the origin of the active research, the development and growth of it has increased in human lives with the birth of the mobile telephony system. A cellular network is one of the types of wireless networks, that uses the wireless communication links to interconnect wireless host like mobile phones. This type of network is called infrastructure based network, because it needs infrastructure to operate. The base station (BS) is the main part of this wireless network infrastructure, and it is responsible for communicating with the wireless hosts, coordinates simultaneous transmission and reception by many hosts under its control and forwarding packets of data between these hosts. All BSs connected to the telephone network or the internet via a number of mobile switching centers (MSC) through a base station controller (BSC). The geographical area in the cellular network is divided into a large number of coverage areas, each area is called cells, and each cell contains a BS. The coverage area of the cell depends on many factors, like transmitting power, the height of the BS, number of antennas and besides the existence tree, buildings, and other obstructions as shown in figure 1-1 (Borcoci, 2008). 1

15 Figure 1-1: Example of Cellular Network (Borcoci, 2008) A large number of MSs may move around freely in each cell or from one cell to another in cellular network, it needs for management. The managing total mobility of all users constitutes two challenges in a cellular network. The first challenge called Roaming (Teo et al., 2007) which refers to the need of reaching the network for any mobile user, at any time, who can be presented in any cell, for delivering a packet or for initiating a session for voice, data or multimedia communication. A centralized database is responsible for finding or locating a roaming user who can be presented in any cell, and maintain the recent information about the current location of each user. The second important challenge in cellular network is Handoff (also called handover) of an mobile station (MS) (Ray et al., 2010), where the MS currently has an ongoing communication session from the current cell to the next cell. The importance of 2

16 performing handoff seamlessly, is to avoid call drop and throughput degradation. Performing handover seamlessly, efficiently, reliably and fast is still an important area of existing research and the current thesis includes our work on this problem in connection with handover in the Worldwide Interoperability for Microwave Access (WiMAX) network. 1.2 Problem Definition The total handover process in IEEE e standard occurs in two phases, namely, Network Topology Acquisition phase (NTAP) and the Actual Handover phase (AHOP) (Ray et al., 2010). The standardization group of IEEE e has specified a scalable and flexible Layer-2 handover policy, where it allows MS, SBS or the backbone network for initiating the handovers (IEEE, 2005). The decision for making process of handover in Mobile WiMAX by the MS called Mobile-Controlled Handover (Zdarsky and Schmitt, 2004), where the MS makes a handover initiation decision when the received signal strength (RSS) from service base station (SBS) drops below particular threshold, which might disturb the current an ongoing communication session, and MS goes to handover with one of neighbor BS (NBS), called target BS (TBS). Another scheme in (Ray et al., 2011) proposed MS- Controlled Fast Handover algorithm in attempting to improve the handover shortcomings based on some features such as distance estimation and zones concept. The existing WiMAX handover mechanism suggested in the MS-Controlled Fast Handover scheme (Ray et al., 2011) for searching and selecting TBS process needs more scanning time in pre-handover phase when the MS performs scanning for all NBSs during the first cycle scanning in zone of concern even though the BSs locations in the opposite direction of the MS movement, these excessive scanning increases the 3

17 handover delay in Mobile WiMAX networks, which affects the system's efficiency such as throughput degradation, packet loss and call drop. Also, selecting only a single BS as TBS in the second scanning cycle in zone of emergency based on angle of divergence did not take into account that there is a possibility that the MS cannot connect to this BS due to BS out of service or when the MS changes its direction suddenly which causes handover fails. So, there is a necessity to solve the previous problems and the proposed algorithm should obey the following: The number of scanned NBSs from the available must kept as minimum. The handover process must be fast, to avoid interrupted connectivity needed for high speed mobility. The performing handover should occur only when necessary in order to save a lot of network resources and benefit all MSs. The handover must not cause throughput degradation, high packet loss and a call drop in the ongoing connection. However, the work done in this thesis focuses on reducing the handover delay when searching and selecting the TBS in the pre-handover phase, avoiding unwanted scanning for all NBSs and avoiding wasting the resources of the network. In this thesis proposes a solutions to solve these problems is introduced by proposing an Improved MS-controlled handover technique where the role played by the Main NBS, the method of selecting potential TBSs (PTBSs) for scanning purposes and the method of selecting TBS. Thus, the overall system aims to achieve fast and reliable handover in the WiMAX networks. 4

18 1.3 Research Objectives This research aims to achieve the following objectives: 1. To reduce a scanning situations in high signal strength zone by using the Main NBS concept that selected among all advertised NBSs. 2. To improve the method of selecting the PTBSs that can be suggested by the Main NBS for scanning. 3. To improve the method of selecting the TBS for handover purposes using QoS measurement in addition to direction of motion of MS. As a consequent, the proposed algorithm for selecting TBS, in order to reduce the number of scanning situations, hence reducing the total scanning delay, and finally implement and evaluate the proposed method. 1.4 Thesis Contributions The main contribution of this thesis is to treat the handover in Mobile WiMAX networks with focus on delay in handover, number of scanning situations and selecting the TBS, and this will be achieved by: 1. The proposition of a new Main NBS concept to reduce the number of NBSs for scanning purpose. 2. Improving the method of selecting PTBSs for scanning purposes based on the Main NBS. 3. Improving the method of selecting TBS for handover purpose. 5

19 1.5 Thesis Outline The article presented in this thesis has been organized into five different chapters begins with the existing chapter, i.e. Introduction and ends with the conclusion chapter. A brief summary of the contents of the remaining four chapters is as follows: Chapter 2: This chapter provides a general discussion about the WiMAX technology. It includes a discussion some important features and its network architecture. A discussion is provided on the different types of handover techniques supported by it and an overview of the some of the different handover schemes proposed by the WiMAX handover research community towards the removal or reduction of the handover shortcomings. Chapter 3: This chapter discusses the Improved MS-Controlled hard handover scheme in Mobile WiMAX networks based on "Main NBS", distance estimation and zones concepts. The proposed scheme provides solution for the handover delay problem. In the scheme, the MS estimates its current distance and angle of divergence, relative to its Main NBS, by periodically monitoring the signal strength received from the Main NBS through scanning. This enables the MS to select a PTBSs in order to estimate which NBS would come nearest to it and hence should be chosen as its next TBS. The scheme reduce the scanning activities and thus the overall handover delay. The scheme are properly validated through simulation studies discussed in Chapter 4. Reducing the handover delay of the WiMAX network is probably the major contribution of this technique. Chapter 4: This chapter provides a discussion of the simulation scenarios performed to validate the proposed handover scheme. Discussions are 6

20 provided about how the simulations are done and how the results are obtained. The results showing the improvement of using proposed scheme.. Chapter 5: Finally, this Conclusion chapter summarizes the research work along with some of the future works. 7

21 Chapter 2 Background and Related Works 2.1 Introduction This chapter provides an overview of WiMAX technology, contains some features of the MAC-layer. Section 2.3 includes mobility feature of mobile WiMAX network architecture, types of mobility and the handover techniques supported by it. Finally, section 2.4 presents an overview of research issues in Handover techniques of Mobile WiMAX. 2.2 Background of WiMAX Technology WiMAX is a wireless communication technology that is prepared for wireless "metropolitan area network" (WMAN), it based on IEEE working group (Group, 2015) and adopted by both the IEEE and the European Telecommunication Standards Institute's (ETSI). IEEE e (IEEE, 2005) is a wireless broadband standard, provides the features and attributes to the standard required to support mobility, and provides solutions for high speed broadband wireless access in a metropolitan area. Two different versions of WiMAX based on IEEE and IEEE e called fixed WiMAX and Mobile WiMAX version designed by the WiMAX forum (IEEE, 2005). However, in this thesis, emphasis will be placed on the Mobile WiMAX. Mobile WiMAX is a wireless networking system based on the IEEE e standard, that is capable of delivering voice, data, and video services. It provides high throughput broadband connections over long distance. It adopts multi-input multioutput technology to increase the base station coverage area (Lee and Choi, 2008). It 8

22 supports wireless Metropolitan Area Network connectivity at speed up to 70 Mbps and it also supports different modulation schemes (Khanduri et al., 2013). Mobile WiMAX has an interoperable network architecture for efficient treatment of different end-to-end services for users like provision of IP connectivity, seamless mobility and handover management, QoS, session management and security. These networking aspects were standardized and developed by the Network Working Group of the WiMAX Forum (ForumTM, 2006). The IEEE standard defines a protocol architecture including various layers like many other protocols. The protocol stack of the IEEE (Khanduri et al., 2013) shows in Figure 2-1. It includes a Medium Access Control (MAC) layer and a Physical (PHY) layer. Figure 2-1: WiMAX Physical and MAC Layer Architecture (WiMAX, 2008) The sequences of standards specify functionalities of PHY and MAC layer with many advanced features to provide ubiquitous broadband access for all kinds of terminals (Bacioccola et al., 2010). 9

23 The support of QoS in the design of WiMAX is an important part of the connection oriented. It includes several features suitable for a broad range of applications, mobility and handover management, different channel-access mechanisms and security features. Handover procedure contains two phases, the first phase is Network Topology Acquisition Phase (NTAP). The second phase is Actual Handover Phase (AHOP). WiMAX supports three different types of MAC-layer handover, namely, fast base station switching (FBSS), macro diversity handover (MDHO), and hard handover. Detailed discussion of WiMAX system network architecture and these handover activities are provided in the next sections. 2.3 Mobility Feature (Handover) in WiMAX In recent years, the number of mobile devices have increased exponentially by the day. This drive the demand for higher bandwidths to access to many bandwidth demanding applications such as multimedia streaming, web browsing, gaming and many more. However, the development of networks providing mobility determines various requirements for mobile user. The main requirement is the ability of a MS to change the serving BS according to the movements of the user to new BS. This process is called handover. Additionally, the handover has to be performed without disturbances in the connection between the MS and both old and new BSs [(ForumTM, 2006), (IEEE, 2006)] WiMAX Network Architecture In mobile WiMAX, the architecture aimed to support unified range of functionalities for different deployment models, such as flat, hybrid and centralized (Das et al., 2006). The WiMAX Network Reference Model is the popular terminology used for the logical representation of the network architecture, it explains the different 10

24 protocols for the different network entities in the architecture along with the different reference points between them (Andrews et al., 2007). Figure 2.2 (ForumTM, 2008) shows Network Reference Model for the Mobile WiMAX network. Figure 2-2: WiMAX Network Reference Model (ForumTM, 2008) The architecture consists of three main logical parts: Mobile Stations (MS) used by different subscribers to access the underlying network; Access Service Network (ASN) and the Connectivity Service Network (CSN). There are various levels of IEEE air interface mobility in WiMAX, access service network gateway (ASN-GW) re-anchoring mobility, and connectivity service network (CSN) mobility, as shown in Figure 2-3 (Ergen, 2007). The Architecture of WiMAX network defined by WiMAX Forum to describe the multiple level of handover in WiMAX. Besides the handover between BSs, two types of mobility are considered: 11

25 Figure 2-3: ASN and CSN Anchored Mobility (Ergen, 2007) ASN- anchored mobility (intra-asn mobility): In the case of ASN anchored mobility, ASN supports handover case when MS moves under the control of BS to another within the same ASN without the need to changing or updating its care of address (CoA). Handovers occurs due to ASN-anchored mobility are also known as ASN-anchored handover (Ergen, 2007). Figure 2-3 shows ASN-anchored handovers, when an MS moves from under the control of BS1 to BS2 that indicates an ASN-anchored mobility CSN- anchored mobility (inter-asn mobility): In the case of CSN anchored mobility, change of the traffic anchor point in the ASN for MS. So, the MS needs to update its care of address for each handover. Figure 2-3 shows CSN-anchored handovers, when an MS moves from BS1/BS2 under ASN-GW1 to BS3 under ASN-GW2, that indicates a CSN-anchored mobility (Lee and Choi, 2008). 12

26 Therefore, ASN-Anchored mobility is considered as layer2 handover, because it does not need updating of IP address, while CSN-Anchored mobility is considered as layer3 handover, because it has to consider IP mobility between ASN and CSN Types of Handover in Mobile WiMAX As mentioned earlier, the handover technique is the process of transferring an ongoing call or data session from one channel to another through core network. The standardization group of IEEE has defined three types of data-link layer handover for the Mobile WiMAX technology (IEEE, 2005). The two soft handover processes, FBSS and MDHO are the optional types, and the hard handover is the default handover mechanism. In hard handover, the connection with a BS is ended initially before the MS switches to another BS. Handover of hard handover will be occurred when the received signal strength (RSS) from service base station (SBS) drops below particular threshold, which might disturb the current an ongoing communication session, and MS goes to handover with one of neighbor BS (NBS), called target BS. This type of handover also called as break-before-make (Ray et al., 2010). In soft handover techniques, FBSS and MDHO, a connection to the TBS is established before MS leaves the connection from SBS which is called make-beforebreak, unlike the hard handover process in which the MS remains connected to one BS. Both of the soft handover techniques use the concepts of Anchor BS (ABS) which has the most powerful signal strength and a list of candidate BSs called Diversity Set (DS). Each MS maintains a DS of its own. In both the FBSS and MDHO cases, handover occur when new BS have more powerful signal strength than SBS, and shifts in DS when it is updated. In FBSS, handover process is not performed while MS switches 13

27 current ABS to the selected new target ABS. But in MDHO case, the MS transmits or receives traffic from multiple BSs included in DS (Ray et al., 2010) Handover Procedure In Mobile WiMAX, the handover procedure can be divided into two main phases, the first is Network Topology Acquisition Phase (NTAP) also known as pre-handover phase and the second is Actual Handover Phase (AHOP). The pre-handover phase includes the network topology advertisement with the neighboring BS scanning and association. The second phase consists of cell selection, decision and initiation of handover, and network re-entry (Ray et al., 2010). Both phases of handover procedure can be summarized as in Figure 2-4 (Ben-Mubark, 2015). Figure 2-4: Handover procedure phases in Mobile WiMAX system (Ben-Mubark, 2015) Network Topology Acquisition Phase (NTAP) In this phase, the MS and SBS with the help of backhaul network, collect information about the network topology before adoption the actual handover decision. The result of this action is to choose one NBS from the lists of potential NBSs as Target BS (TBS) for handover process. Figure 2-5 shows the chart of the sequence of message for this phase (Ray et al., 2010). 14

28 Figure 2-5: The Chart of Message Sequence for Pre-Handover Phase (Ray et al., 2010) The main steps involved in this phase will be discussed as follow: Step 1: BS advertising the Network Topology: The information of NBSs periodically broadcasts by SBS using MAC-management message Mobile Neighbor Advertisement (MOB_NBR-ADV), in order to prepare for potential handover activity. Step 2: Scanning and Synchronization of Advertised Neighboring BSs by MS: The scanning phase can be activated by SBS or MS to permit MS to scan the advertised NBSs in order to measure the signal qualities of different it such as received signal strength indicator (RSSI), carrier-to-interference plus noise ratio (CINR), Roundtrip delay (RTD) and using the result to select one NBS as TBS. 15

29 MS sends a scanning interval allocation request message (MOB_SCN-REQ) to SBS which consists a list of potential NBSs selected from the MOB_NBR-ADV to start the scanning process. The SBS sends back a scanning response (MOB_SCN-RSP) message to MS specifying the scanning interval (in the frames form) for the scanning process. The response message contains information about starting frame, length of interleaving intervals, and the number of scan iterations. So, the MS scans the selected NBSs within particular time frames in order to select appropriate candidate BSs for the HO. All communication between MS and SBS during Scanning is temporarily stopped over and the arriving packets are thus buffered accordingly. The selection of a list of candidate BSs for handover is the scanning results. MS sends scanning results report (MOB_SCN-REP) message which contains the results of scanning activity to SBS. Step 3: Cell Re-Selection: The step of ranging activities takes place between MS and the different NBSs, through collecting further information on PHY channel such as power adjustments, correct timing offset, and any change in burst profile related with the selected TBSs. Through the association process the ranging information is obtained and plays animated role to select appropriate TBS for a successful and potential handover activity. MSs may get optionally associated to some or all the NBSs in the list according to Mobile WiMAX standards. Activity of successful ranging-association indicates the end of scanning interval, and thus the end of cell reselection process for a handover activity. The MS chooses candidate NBSs as potential candidates for a handover activity. The next phase is the AHOP in which the MS firstly breaks its existing connection with the SBS and reconnects to the TBS Actual Handover Phase (AHOP) 16

30 The steps of different sub-phases to the AHOP are described below. Figure 2-6 shows the chart of sequence of message for this phase (Ray et al., 2010). Step 4: Handover Decision and Initiation: In this step, The MS send HO request (MOB_MSHO-REQ) message to the SBS which indicates the identity of the potential TBS to detect whether they can supply the QoS and other important resources to support connection with the MS after the handover activity. The MS send Mobile Handover Indication (MOB_HO-IND) message to confirm the handover decision and to tell the BS whether it is really proceeding with the handover or not. At this point the connection between MS and the SBS is also discontinued and the SBS or ASN-GW starts buffering packets in order to avoid packet loss. However, the SBS could also send all resources related with MS to TBS over the backbone network if this is needed. Step 5: MS Synchronization with the selected TBS: In this step, MS synchronizes with downlink (DL) transmission of determining the TBS, which means the MS performs frequency and time synchronization with the TBS. In addition to this, the MS decodes the UCD and DCD messages to get the TBS ranging channel-related information. 17

31 Figure 2-6: The Chart of Message Sequence of AHOP (Ray et al., 2010) Step 6: Ranging and Network Re-Entry: The MS can synchronize its UL with the BS by using the ranging channel slots and thus get additional information of the timing and power level. With the UL synchronization operation, the MS gets ready to get in the new network by performs the steps of network re-entry which including the Basic Capabilities Negotiation, MS Authorization, Registration of the MS and Establishing IP Connectivity. Step 7: Termination of MS Contexts: After the MS completing the network re-entry activities, the previous SBS closes all kinds of MS-related connections and contexts associated with them, like counters, state machines, timers, all kinds of queued information. 18

32 2.4 Related work Mobile WiMAX handover techniques encounter some handover performance weakness and each phase has some issues (Ben-Mubarak et al., 2009), the research runs continuously on worldwide to resolve them. Although hard handover technique is the most bandwidth efficient in Mobile WiMAX, but it is still suffers from many problems in pre-handover phase like redundant scanning activity in scanning interval during searching and selecting a NBS as TBS. A discussion of some researches for avoiding excessive handover scanning activities will be presented, in order to reduce the overall hard handover delay during pre-handover phase before initiating handover activity. The solutions can be classified into different research categories: The scanning interval time, frequency of scanning operations and 'distance estimation & zones concept', and it will be presented all of them in more detail as follows: The scanning interval Time This category focused on the scanning interval has to be completed. Authors in (Rouil and Golmie, 2006) proposed an Adaptive Channel Scanning algorithm for IEEE e to estimate the overall required scanning time by MS. The scanning intervals are assigned to multiple MSs by interleaving them with the data transmission intervals. In addition, it preserves the QoS of the application traffic in the system. However, use of unlimited channel buffers leads to increase the problem of channel resource wastage. In this scheme, it actually focuses on minimizing the disruptive effects of excessive scanning activities on the various application traffics instead of minimizing excessive scanning activities. Authors in (Al-Khateeb and Hashim, 2011) utilized movement prediction by using fuzzy logic which allows the MS to execute pre-scanning before the actual handover. 19

33 This prediction will be reserved the required amount of resources for the upcoming handover at the TBS before initiation and execution handover. Another paper in (Winston and Shaji, 2012) proposed a scanning algorithm to improve the scanning parameters, scan interval, interleaving interval and scan period, and therefore it improves the scanning parameters value based on the MS speed and BS density Frequency of scanning operations This category focused on analyzing and evaluating the effect of the excessive scanning activities in [(Lee et al., 2006), (Wang et al., 2007), (Hoon-gyu et al., 2007)]. All these schemes attempt to minimize the delay and reduce frequency of scanning operations. In (Lee et al., 2006), the author proposed, from the MOB_NBR-ADV messages, the MS can obtain the Carrier to Interference-plus-Noise Ratio (CINR) and the arrival time difference of the downlink signal from its NBSs, so, it can select the TBS which have the largest mean CINR and smallest arrival time difference. Then, the ranging, synchronization and association activities performing by MS only with TBS. Thus, TBS estimation algorithm using mean CINR and arrival time difference reduces number of NBSs scanning which leads to reducing the handover delay. Author in (Wang et al., 2007) proposed algorithm to use a single target BS with best CINR is chosen for scanning; fast ranging and pre-registration to improve network reentering process. Also in (Hoon-gyu et al., 2007), the prediction of TBSs based on the required bandwidth and QoS, which lead to reduce the scanning and ranging related activities. 20

34 However, as this scheme does not take into account the MS s movement direction, it might lead to unwanted ping-pong effect. In addition, scanning a single BS as TBS could lead to a problem when it's channel situation is changed and could not meet the requirements of end-user, this leads to handover failure. Another schemes proposed to reduce the effect of excessive scanning based on mobile location prediction to improve the handover scanning performance. Mobile location prediction considers the mobile movement history records during a schedule time like in [(Priya and Raja, 2012), (Zhang et al., 2010)], in which the authors proposed a fast handover scheme using location based on table of mobility pattern. Each MS preserves information about the SBS and TBS and send it to the SBS by Mobile Handover Indication (MOB_HO-IND) message. Based on this message the SBS will update mobility pattern table and chooses the TBS based on the parameters in the table. Therefore, the MS will only performs a scanning activity for the possible NBSs that achieve the QoS in the table instead of scanning all the NBSs. However, the NBSs channel situations and their respective QoS could have changed after the MS sent the information to the SBS. In addition, the authors in (Qi and Maode, 2009) and (Lu and Ma, 2011) proposed a location-aware scanning to reduce the number of scanning operations. They used the arrival-time-difference (ATD) of downlink (DL) MAP messages from three NBSs to estimate the MS location. However, this scheme has to scan at least three NBSs assuming that all NBSs are synchronized. Moreover, authors in (Ben-Mubark et al., 2011) proposed mobile station movement direction prediction based handover scanning algorithm. This algorithm based on the structure of cell sectoring-zoning and the MS position, the SBS will calculate the 21

35 distance between the MS and the NBSs using the accumulative distance function, the SBS can predict the direction of MS movement. In this scheme, only two BSs that the MS moves toward them will be chosen as a candidates for the handover scanning purpose. However, this scheme did not evaluate the status probability when real TBS is not scanned and handover fails Distance Estimation and Zones Concept Authors in [(Ray et al., 2010), (Ray et al., 2011)] proposed schemes based on selftracking. It is based on the distances estimation between the MS and its NBSs based on RSS. Authors in [(Ray et al., 2011), (Ray S. K., 2012)] proposed "MS-Controlled Fast Handover Scheme". In this scheme, the algorithm proposed to select the suitable TBS from NBSs by using the zones concept and the "distance estimation and lookahead" technique. In zones concept, the MS perceives itself as take up one of four possible zones while monitoring the RSS received from the SBS. The MS performs all various steps related to the handover process within these four concentric zones (viz., the Zone of Normalcy (ZN), the Zone of Concern (ZC), the Zone of Emergency (ZE), the Zone of Doom (ZD)) before the RSS becomes too low which makes sure that the process of scanning begins well in advanced. The zones concept depicted in Figure 2-7. The four zones are in fact created to match suitably chosen RSS levels. These levels are chosen in which that the section of TBS process is normally completed within the ZC, and the residual part of the process of handover is completed within the ZE by the SBS and network. Knowing that, when the MS enters the ZC, it begins receiving range of power from the SBS which is still high, hence, it is likely to have been rushed to be prepared for the 22

36 pre-handover activity, especially when the MS performs the scanning-related activities with all NBSs. Figure 2-7: Zones Concept (Ray et al., 2011) As well, by using the "distance estimation and lookahead" technique, The MS can approximately estimate its current distance from any neighbor base station by monitoring the RSS received from the concerned base station (Ray et al., 2007) and using this information of RSS in a suitable pathloss formula (Andrews et al., 2007). This parameter of RSS indicator (RSSI) used in familiar Mobile WiMAX HO framework to reduce the effect of noise and fading and actually obtained after some filtering of the received carrier signal followed by computing its logarithm. The MS performs a lookahead by estimating the angle of divergence of the NBSs with consideration to its own direction of motion. If the illustrated scenario in Figure 2-8 was taken as an example, the computation of cosine angle of divergence with NBSs computed as follows: 23

37 Cosθ = { (Ax) 2 + (xy) 2 - (Ay) 2 } / { 2 (Ax) (xy) } (2.1) Figure 2-8: Distance Estimation with AOD based Lookahead Scheme (Ray et al., 2011) These estimates allows the MS to perform lookahead to decide which NBSs should to continue or discontinue to monitoring, and which NBS the MS comes close to it after leaving the current cell. This technique will allow the MS to choose the TBS among NBSs being scanned based on the MS movement direction. In this scheme, the MS can select the TBS before the signal threshold is reached, thus reducing call breaks. But, it is noted, in this scheme the MS performs two scanning cycles, the first cycle scanning performed in zone of concern for all potential BSs (the MS has excluded the two NBSs, assumed presently highly overloaded) even though the BSs locations in the 24

38 opposite direction of the MS movement. Hence, performing more number of scanning situations takes more scanning time (or delay). Also, selecting only a single BS as TBS in the second scanning cycle in zone of emergency based on angle of divergence did not take into account that there is a possibility that the MS cannot connect to this BS due to BS overload or when the MS changes its direction suddenly which causes handover fails. Consequently, it is noted that all the above proposed schemes are dealing with all advertised NBSs regardless that the MS is moving away from the NBSs, and the number of scanning situations performed on these NBSs. However, there is a need to ideas dealing with the unwanted delays, decreasing number of NBSs scanning and scanning situations, increasing throughput, minimizing packet loss ratio owing to redundant scanning activities during Mobile WiMAX HO operations. The flowchart of the angle of divergence based TBS lookahead is shown in Figure 2-9 which contains the functions for selecting TBS in the scheme of "MS-Controlled Handover Technique" by (Ray et al., 2011). 25

39 Figure 2-9: Flowchart of the AOD-based Fast MAC-Layer Handover Scheme (Ray et al., 2011) 26

40 2.5 Summary In this chapter, an overview of WiMAX technology has been provided including some features of its MAC layer, network architecture and the types of handover supported by it. In cellular network, the mobility and handover related actions can be classified based on the processes performed in physical, MAC and Network layers. It has been discussed the types of handover, namely, Hard HO and Soft HO, supported by WiMAX system. The Hard HO is the default handover mechanism. In the last section, it has provided a study of the different research works of the WiMAX handover. Additionally, more discussion on "MS-Controlled Handover Scheme" by (Ray et al., 2011) were presented, it should be noted that our research work in this thesis will focused on the problems found in this study to achieve less unwanted delays, decrease number of scanning NBSs, increasing throughput and minimizing packet loss ratio owing to redundant scanning activities during Mobile WiMAX handover operations. 27

41 Chapter 3 Proposed An Improved MS-Controlled Handover Algorithm 3.1 Introduction As discussed earlier in the previous chapter, the existing mechanism for searching and selecting TBS process in MS-Controlled handover scheme needs more scanning time in pre-handover phase when the MS performs scanning for all NBSs during the first cycle scanning in zone of concern although in this zone the MS receives high range power from SBS. In addition, in case of two PTBSs having the same angle of divergence and showing a progressive movement, both were chosen as CTBSs and TBS selection process needs performing another scanning cycle to do the final choice of the TBS. Also, selecting only a single BS as TBS in the second scanning cycle in zone of emergency based on angle of divergence did not take into account that there is a possibility that the MS cannot connect to this BS due to BS overload or when the MS changing its direction suddenly which causes handover fails. Therefore, monitoring the RSS from NBSs to perform scanning activity in another way will contribute to solve the problem of large handover delay in Mobile WiMAX networks by reducing the number of scanning situations based on the MS movement direction. Also, consider more than a single NBS to be scanned as a TBS will increase the probability of handover process success. However, in order to enhance the handover operation performance in Mobile WiMAX networks, an Improved Algorithm of MS-Controlled Handover is proposed 28

42 which combined some of the concepts used in the MS-controlled handover algorithm (Ray et al., 2011) with additional concepts to improve the handover operation, where the concepts based mainly on the concept of Main NBS, the method selection of PTBSs for scanning purposes and the method of selection TBS. In this chapter, the phases of the proposed Improved handover algorithm and procedure will be presented and discussed, along with a new concept of Main NBS, how to select the PTBSs and the method of selecting TBS. 3.2 An Improved MS-Controlled Handover algorithm An improved algorithm suggested or based on many ideas, namely: The MS can approximately estimate its present distance from the Main NBS by measuring the RSS received from it. By using two distance estimates for Main NBS, an MS can perform a PTBSs selection process to select its PTBSs through simple computation of its relative angle of divergence with respect to Main NBS. The idea is to reduce the number of scanning situations during the existence of the MS in the zone of concern (range of power received from the SBS is still high). By using another two distance estimates for PTBSs, an MS can perform a TBS selection process to select its PTBSs through simple computation of its relative angle of divergence and QoS offered by the PTBSs. follows: In this scheme, The MS executes various steps for selecting its own TBS as 29

43 i. The MS make sure of the need to a handover by utilizing the RSS received from SBS, and send a scanning request to SBS. ii. By utilizing the RSS received from Main NBS, the MS estimates its current distances from Main NBS. iii. With two distance estimates from Main NBS, the MS determine its movement with respect of its Main NBS, and select the PTBSs. iv. The MS select one of the NBS as the TBS among the PTBSs for the handover, which shows the highest close movement and QoS, finally, v. The MS request the SBS to hand it over to its selected TBS. The main steps included in the Improved MS-controlled handover scheme shown in Figure 3-1 as a block diagram of the fast handover technique. MS receives the MOB_NBR-ADV message periodically from its SBS in Zone Normally. MS make sure if there is any need for initiating a handover ( from the RSS ) Based on two sequential distance estimates, the MS computes its angle of divergence with respect to each PTBSs From the RSS received from the main NBS in zone concern, the MS perform two sequential distance estimates to compute its angle of divergence with it and determine the PTBSs The MS determines which NBS it currently has the lowest angle of divergence and offer satisfactory QoS to select it as a TBS The MS sends MOB_SCN-REQ message to SBS to allow scanning of all PTBSs when need for a handover process The MS sends a MOB_HO-IND message to request its SBS for handing it over to the selected TBS Figure 3-1: Block Diagram of Improved MS-Controlled Handover Technique 30

44 3.3 The Concept of Main NBS Having discussed the MS-Controlled Handover Technique in chapter two, it is noticed that the MS performed scanning operations in the zone of concern all over the NBSs. In new technique, it has been suggested a Main NBS concept where it is selected among all NBSs by SBS for scanning purpose instead of scanning all NBSs, in order to reduce the number of scanning situations on this zone. The Main NBS is selected based on many factors such as the lowest loading and offer satisfactory QoS. The SBS performs a function of selected the Main NBS by collects, through the backbone network, the information about all NBSs and determines the Main NBS and the PTBSs related to it, and the MS is posted via broadcasted MOB_NBR-ADV message by SBS. For the purpose of explaining the Main NBS, the scenario of Mobile WiMAX centralized deployment architecture (Ray et al., 2011) depicted in Figure 3-2 where the scenario has six NBSs 1, 2, 3, 4, 5 and 6, clustered around its SBS combined with zones concept (Ray et al., 2011). In this scenario, any NBS is likely to be the Main NBS depending on the reasons that have been mentioned. 3.4 Proposed Improved MS-Controlled Handover Algorithm In this section, the phases of a proposed algorithm discussed and explained in details. Firstly, explain the steps in each zone, secondly give details of the selecting PTBSs. Finally, selecting a NBS as TBS for handover process, explaining the steps in an illustrated example. 31

45 Figure 3-2: Mobile WiMAX centralized deployment architecture (Ray et al., 2011) In Zone of Normally The MS receives high RSS power from its SBS when it stays in the Zone of Normally. The MS creates a set of its NBSs by monitoring the periodic message MOB_NBR-ADV broadcasted by SBS as shown in Figure 3-3. Figure 3-3: Broadcasting the MOB_NBR-ADV Message in Zone of Normally 32

46 3.4.2 In Zone of Concern After leaving the Zone of Normally, when the MS enters the Zone of Concern, the MS initiates two sequential scanning cycles for the Main NBS that has been obtained from SBS for distances estimation purpose. Thus, by finding the cosine angle of divergence of the Main NBS, the MS can select the PTBSs. Figure 3-4 shows the steps that are performed in the Zone of Concern. Assume that a NBS4 has been selected as a Main NBS, and the MS moving along straight line AB. It is noted that, the MS performs the first distance estimation at the point x, and another in point y, and the length of third common side xy can be computed by using the average of the MS speed during the time T, therefore cosine of angle of a triangle can be computed according to the "Law of Cosines" in trigonometry (see chapter 2). Figure 3-4: The steps that are performed in the Zone of Concern 33

47 3.4.3 Selection the PTBSs based on Main NBS After computed the angle of divergence with Main NBS, the MS sends MOB_SCN- REQ message to SBS to allow scanning of all PTBSs. For explaining the PTBSs selection based on Main NBS, Figure 3-5 (a) shows the illustrated example of the orientation matching the MS movement direction according to Main NBS, assumes that a NBS4 has been selected as a Main NBS, there are several cases can be divided according to the movement of the MS: Figure 3-5: (a) The Illustrative Example of the Orientation Matching the MS movement direction according to Main NBS The 1 st case: If the Angle of Divergence with Main NBS = 0, then the MS is moving exactly towards the Main NBS, and the MS will select the Main NBS4 as a PTBSs. 34

48 The 2 nd case: If the Angle of Divergence with Main NBS > 0 and <= 90, the MS is moving towards near the main NBS but its forward movement towards the NBS2, NBS3 and NBS4 and they have been selected as a PTBSs as shown in the figure 3-5 (b). Figure 3-5: (b) Angle of divergence between the movement of MS and Main NBS The 3rd case: If the Angle of Divergence with Main NBS > 90 and <= 180, the movement of MS is away from main NBS and the MS movement towards the NBS1 and NBS2 and they have been selected as PTBSs, as shown in the figure 3-5 (c). The 4th case: If the Angle of Divergence with Main NBS > 180 and <= 270, that means the MS is moving towards the NBS1, NBS5 and NBS6 and they have been selected as a PTBSs, as shown in the figure 3-5 (d). The last case: If the Angle of Divergence with Main NBS > 270, then The MS is moving towards the NBS4, NBS5 and NBS6 and they have been selected as a PTBSs, as shown in the figure 3-5 (e). 35

49 Figure 3-5: (c) Angle of divergence between the movement of MS and Main NBS Figure 3-5: (d) Angle of divergence between the movement of MS and Main NBS 36

50 SBS Figure 3-5: (e) Angle of divergence between the movement of MS and Main NBS Selecting the TBS based on Selection the PTBSs After leaving the Zone of Concern, when the MS enters the Zone of Emergency, the MS initiates two sequential scanning cycles for the PTBSs that have been obtained from previous step. Thus, by finding the cosine Angle of Divergence of the PTBSs and make a comparison among them, the MS can select the TBS which has a minimum value of the Angle of Divergence from all the PTBSs. Then, the MS sends MOB_HO-IND message to the SBS for executing an urgent handover by passing the ID of the selected TBS. AS mentioned before, entire HO process should be completed before the MS enters the Zone of Doom to avoid excessive packet loss or call drop because of the poor of RSS. However, in the case of selecting two PTBSs that have the same angle of divergence in Zone of Emergency, one from two is selected based on if it shows both a 37

51 progressive movement (compared with its previous distance), QoS is greater than another. 3.5 The Proposed Improved MS-Controlled Handover Algorithm The following flowchart describes the steps of the proposed algorithm as shown in Figure 3-6 where contains the functions implementing of the selecting TBS for handover process. Figure 3-6: Flowchart of the Improved MS-Controlled Handover Algorithm 38

52 3.6 Summary In this chapter, an Improved Algorithm for Handover scheme has been described. This technique employs the principle of distance estimation which uses the pathloss property of the RSS received by MS from its Main NBS to select the PTBSs, followed by selecting TBS technique. The MS performs two scanning cycles processes with main NBS and PTBSs. Many NBSs may be quite disqualified from the scanning session depending on the Angle of Divergence with Main NBS. The MS estimates the corresponding distance with each PTBSs from the RSS received from it and performs a selecting TBS scheme to select which NBS should be selected as the TBS. The MS selects the NBS as the TBS which shows the least angle of divergence with MS direction of motion and offered a satisfactory QoS. 39

53 Chapter 4 Performance Evaluation 4.1 Introduction This chapter presents and discusses the simulation methodology and the results to evaluate the performance of two WiMAX handover schemes (MS-controlled handover scheme and Improved MS-Controlled Handover scheme), and their impact on the handover performance in cellular networks. The two handover schemes have been simulated using the MATLAB R2013b simulator. Firstly, the simulation study on the Improved MS-Controlled Handover scheme were presented followed by the evaluation metrics that performed to analyze the system. Finally, an analysis of simulation results were presented with intensive discussion. 4.2 Simulation Study on Improved MS-Controlled Handover Technique The Improved MS-Controlled Handover scheme that offers handover in Mobile WiMAX networks which is described in chapter 3, will be simulated in this chapter. This technique employs distance estimation (Ray et al., 2011) by using the distance based on pathloss property (Andrews et al., 2007) of the RSS received by the MS from the suitable NBS. The MS performs two scanning operations with Main NB, the MS estimates the corresponding distance samples of Main NBS relative to itself. Based on these distances and the angle of divergence with Main NBS, the MS performs a suitable chosen scheme to select a PTBSs. Then, the NBS shows the least Angle of Divergence among the PTBSs with respect to the MS direction of motion will select as TBS. The next section provides a discussions on many important things used in simulator such as making a suitable choices on the simulator, simulation environment, deployment network and mobility points. 40

54 4.2.1 Choice of simulator For the purpose of this research work, MATLAB R2013b simulator (Etter and Kuncicky, 2011) was chosen. This simulator has options provided a basic implementation of Mobile WiMAX air interface and other features that were required on different roaming and handover-related research in WiMAX and other cellular networks, which provides many facilities like: It provides a support of Mobile WiMAX air interface: MATLAB script files provided a basic implementation of the Mobile WiMAX air interface (Layers 1 and 2). It provides a hard handover framework: The implementation of hard handover in Mobile WiMAX including scanning-ranging and network re-entry activities. It provides a deployment of networks topology: A simulation topology with multiple a suitable placed WiMAX BSs, each has channel frequency. It provides a support of mobility models: By using the random point for the MS mobility. Thus, we use random point for the direction motion of the MS. It provides the pathloss property: By using pathloss property through the equations to estimate distances between the MS and NBSs Mobility of MS used for simulation To simulate the two handover schemes, a user inside a moving vehicle with mobile device were considered. In addition, the vehicles can move in different speeds in a Mobile WiMAX networks based metropolitan area environment. So, it take into consideration the range of vehicle speeds from 20 Km/h to 120 Km/h. Moreover, it also assumes that the MS is moving along highways (i.e. straight and not zigzag or random). 41

55 4.2.3 Simulation Topology To build the simulation topology, environment containing a multiple BSs [(Ray et al., 2011), (Ben-Mubarak et al., 2011)] has chosen instead of an environment containing two or three BSs, because of the handovers simulation among multiple BSs is a good option in a high speed mobility environment supported by Mobile WiMAX, which estimates the performance of the handover scheme by using realistic mobility and pathloss model. Moreover, in Mobile WiMAX technology that spanning over metropolitan areas, there is a need more than two or three BSs to cover the whole city area. Thus, in the simulation topology shown in Figure 5-1, which contains seven different cells, each cell has three MSs and one BS in it. All the BSs are connected through the backbone network. Figure 4-1: The Multi-Cell Simulation Topology 42

56 These 21 nodes are spread over a service area of 1500 m x 1500 m (Ray et al., 2011) terrain for WiMAX infrastructure. This area is divided into many cells, each cell has a BS deployed in a multi-cell environment operating with different radio frequencies within the range 2.4 GHz to 2.45 GHz, and each cell has a set of mobile nodes, and each mobile node communicates with each other's and moves randomly in different direction with some mobility speed, and all MSs communicate simultaneously with their respective BSs. Moreover, the SBS S is selected to provide the services to MS and it will be handovered to the NBS B. The constant bit rate (CBR) is chosen as the nature of traffic type (IEEE, 2005) in the simulation. The effect of noise is also considered to create a real time environment. As per the simulation model, a single MS initially move from gray point and served by SBS S, then moving to black point and performs handover between six BSs whenever needed according to the handover scheme. The path loss propagation is modeled using COST-231 Hata model (Andrews et al., 2007) to calculate the pathloss effects during simulation Parameters Used in Simulation The simulation parameters listed in Table 5.1 according to the WiMAX forum specifications (WiMAX, 2009), that are used to analyze and achieve the correctness of our proposed scheme. Also, in order to analyze the performance of schemes, it has a particular handover related attributes were used in (Ray S. K., 2012), these attributes include the delay of following handover activities: TIni: The time taken for interval of initiation before scanning phase. 43

57 TScan: The time required until the MS complete scanning, synchronization and ranging activities with the different NBSs (IEEE, 2005). This time depends on the number of NBSs to be scanned. THO_Prep: The time required for handover preparation (WiMAX, 2009). This time includes the time for exchanging notification message between the MS and SBS related to pre-handover through scanning phase, like MS handover request (MOB_MSHO-REQ) and BS handover response (MOB_BSHO-RSP) messages which exchanged before finalizing the final TBS for the handover activity. TNormal_Sync: The time of DL and UL synchronization of the MS with different NBSs. TTBS_Sync: The time of DL and UL synchronization of the MS with the new selected TBS. TCont_Rang: The time ranging required for the MS to perform a successful ranging with NBS after contesting with other MSs over available ranging slots (IEEE, 2005). At least two ranging iterations should occurs before a successful ranging operation is accomplished. TCap_Neg: The time required for performing negotiation. TAuth: The time required for successful authorization during network entry through authorization hand-shaking framework. TReg: The time required for completing a successful registration policy during network entry. 44

58 Table 4-1: Key Simulation Parameters Parameters Values Service Area 1500 m x 1500 m Type of Technology IEEE e Number of BSs 7 Number of MSs 21 Number of cells 7 BS max transmit power 20 W MS max transmit power 200 mw Wave Length 10e-2 Loss Factor 1.5 Bandwidth 10 MHz FFT Size 1024 MAC Address Auto Assigned MAC Propagation delay 1 μs BS Link Propagation Delay 1 ms Frequency 2.4 GHz Number of Bits per packet 1024 Bits Time taken to travel 20 Default Frame Length 20 ms BS Antenna Height 15 m QPSK Encoding Rate 0.5 BS Link Propagation Delay 1 ms Scan Interleaving Interval 6 frames MS's Movement Speed 20 Km/h Km/h Evaluation Metrics Handover Delay A single MS has used to select the delay of handover. The nature of traffic used in the simulation is constant bit rate (CBR). According to our simulation model, one MS initially controlled by its SBS, and move from gray to black point between the BSs as shown in figure 4-1, then perform a handover whenever needed during the simulation Number of Scans Performed in Pre-Handover Phase In a long trip by vehicle, in practice, generally choice of the shortest path is most common and natural. Moreover, even if the path becomes non-linear, it becomes so 45

59 only over small distances. In a Mobile WiMAX network, the radius of a cell varies in the range 500 m 2 Km and the MS speed generally varies between 20 Km/hr and 120 Km/hr (Andrews et al., 2007). In order to perform the simulation, assume a cell radius of 1 Km and an MS speed of 90 Km/hr (i.e. 25 m/sec). Also, assume a 10% overlap between the neighboring cells. Further assume that, the zones of SBS (i.e.: ZN, ZC, ZE and ZD) are 450 m, 750 m, 900 m and 1 Km, respectively, all measured with respect to the SBS (Ray S. K., 2012) Throughput Throughput is a measure of how many units of information or number of packets are successfully delivered through the network in a given amount of time. It is measured in terms bits/second during the HO process. The amount of throughput should be high or else it affects every service in Mobile WiMAX. Equation 3 shows how to calculate throughput (Mehta and Gupta, 2012). Throughput = i Packets Delivered ( i Packets Arrival Packet start time i ) (5.1) where i is the number of MSs on the service area Packet Loss Ratio Packet Loss affects on the quality of application. It is caused by several reasons like errors in wireless network, due to noise, MS receives low RSS from its SBS, congestion in network when the channel becomes overloaded, etc. Packet Loss Ratio should be minimum to keep the successful delivery of high QoS. The packet loss value should be preserved at minimum level according to International Telecommunication Union (ITU). Equation 4 shows how to calculate the Packet Loss Ratio (Mehta and Gupta, 2012) during HO process. 46

60 Packet Loss Ratio = Packets Lost i Packets Sent i X 100 (5.2) where i is the number of MSs on the service area. 4.3 Simulation Results and Discussion This section explains the simulation results of the two handover techniques, namely, MS-Controlled Handover Technique and Improved MS-Controlled Handover Technique. The proposed scheme focused on improving the handover performance firstly in terms of reducing the scanning situations, hence reducing handover time (delay) and in choosing the best TBS for handover. Improvements have been proposed in the NTAP Average of NTAP and AHOP Delay For proposed MS-Controlled Handover technique, Figure 4-2 shows the results of the average of pre-handover time when the MS moves at six different speeds ranging from 20 km/h to 120 km/h in comparison to the MS-Controlled Handover technique. However, the overall time of handover procedure in Mobile WiMAX includes the NTAP time and AHOP time. The total time taken in NTAP includes the time of initiating the potential handover process by sensing the broadcasting message from SBS (MOB_NBR-ADV), and then executing the scanning and synchronization activities with NBSs, and then select the TBS (Andrews et al., 2007). The select TBS followed by the processes executes during the AHOP time, which includes the time for handover preparation, synchronization, ranging, authorization and registration time. 47

61 Figure 4-2: Average of Pre-Handover (NTAP) Delay Analysis It is noted, in the proposed scheme, the overall NTAP delay is reduced due to decreasing the number of NBSs, which the MS performs the scanning on it. Also, the decreasing number of NBSs as a PTBSs leading to reduce the number of exchanging MAC management messages between MS and SBS until select the final TBS before the MS goes for the handover. So, this reduction also reduces the overall handover time. The simulation show that, in comparison with the MS-Controlled handover scheme, the proposed Handover scheme reduce the scanning activity time which leads to reduce the NTAP time by 6.9 % because the scanning activity is a part of pre-handover phase. This ratio for this result and next results (AHOP time, number of scans, throughput and packet loss ratio) is computed based on the tool in (Goodwin T. H., 1998). During the NTAP, the time spent for this phase is taken to complete the scanning, synchronization and ranging activities. In the MS-controlled handover scheme, the unnecessary scanning for some NBSs in another direction of the MS movement need 48

62 more time to complete it. So, the overall time spent to complete these activities is high. However, in our scheme, the MS scans the NBSs which are as a PTBSs based on the value of angle with Main NBS. In addition to that, the MS scan only the NBSs in same direction with respect to its movement and present adequate resources. Hence, the number of NBSs that the MS needs to scan it before selecting the TBS is more less in case of Proposed Handover scheme. Hence, that the NTAP time taken by MS to complete a handover activities, whenever the MS speed increased the time decreases. The maximum time taken when the speed of MS is 20 km/h and minimum at the speed of 120 km/h. So, the interscanning interval different according the speed of the MS. With respect to the AHOP time, Figure 4-3 shows the results of the average of total handover delay in comparison to the MS-Controlled Handover technique. Figure 4-3: Average of Total Handover (AHOP) Delay Analysis 49

63 It is noted that the time taken to complete the actual handover phase affected by the pre-handover phase. The results show that the proposed handover scheme reduce the overall handover delay by 5.7 %. This can be explanation by the fact that all handover time (NTAP time + AHOP time) includes the time taken to perform the steps including in handover procedure in different schemes affected by the number of scanning activities executed which leads to reduce the number of MAC management messages between MS and SBS Number of Scans Performed in Pre-Handover Phase For proposed MS-Controlled Handover technique, Figure 4-4 shows the results of the average number of scans performed in pre-handover phase when the MS moves at six different speeds ranging from 20 km/h to 120 km/h in comparison to the MS- Controlled Handover technique. Figure 4-4: Average Number of Scanning Iterations in Pre-Handover (NTAP) Phase 50

64 Moreover, in the worst case, regardless of the MS speeds and Time Frame estimation, the MS needs number of scanning based on the zones concept as follows: Number of scanning(proposed Scheme) = 2 (Main NBS) + 6 (PTBSs) = 8 Number of scanning(ms-controlled Scheme) = 12 (NBSs) + 2 (TBSs) = 14 As a consequence, the number of scans performed during pre-handover phase in proposed Handover Technique is smaller in comparison to that performed in case of the MS-Controlled Handover technique by 2.62 %. This is due to the fact that an MS, in the proposed scheme, scans only the Main NBS in zone of concern and PTBSs selected based on the Angle of Divergence with respect of the Main NBS, which shows progressive movements with respect to the MS. In case of the MS-Controlled Handover scheme, regardless of the movement direction, an MS may scan almost all the different advertised NBSs in zone of concern, before selecting the TBS in zone of emergency for a handover. So, the number of scanning iterations could be up more than six during prehandover phase. On the other hand, in the proposed scheme, the number of scans during pre-handover phase is between three and four, before the MS could select a TBS (refer to Section 3.4) Throughput As have explained in the previous section, throughput is a measure of how many units of information or number of packets successfully delivered through the network in a given amount of time. In our simulation, the calculation of throughput was done during the handover process. Measured throughput for various speeds of MSs are show in Figure

65 Figure 4-5: Throughput Analysis in Handover Process Here, it should be draw attention for the time interrupt after selecting the TBS and complete the handover activities, then the connection cut between the MS and SBS and connect to the new SBS (i.e.: TBS), this time affect in the amount of throughput along with the zone where the MS is and the signal strength in this zone. The simulation show that, in comparison with the MS-Controlled handover scheme, our proposed scheme increase the amount of throughput by 4.62 %. With take in consideration, whenever increased the speed of MS, the throughput is increased, because of the MS complete most of the handover activities in Zone of Emergency and before enters Zone of Doom to avoid throughput degradation or call drop because of the poor of RSS Packet Loss Ratio As was mentioned earlier, Packet Loss Ratio indicates the number of packets ratio lost during transmission from source to destination in the handover activities interval. It 52

66 is in fact the measure of number of packets lost or undelivered in the network. Measured packet loss ratio for various speeds of MSs are show in Figure 4-6. Figure 4-6: Packet Loss Ratio Analysis in Handover Process The simulation show that, in comparison with the MS-Controlled handover scheme, our proposed scheme decrease the amount of packet loss ratio by % approximately. As mentioned in throughput analysis, whenever increased the speed of MS, the Packet Loss Ratio is decreased, because of the MS complete most of the handover activities in Zone of Emergency and before enters Zone of Doom to avoid excessive packet loss or call drop because of the poor of RSS. The results of analysis shows that as the speed of MSs increases, Packet Loss Ratio decreased as well. 4.4 Summary In this chapter, the simulation scenarios and the results for the two handover techniques were discussed, namely, the MS-controlled handover scheme and the Improved MS-controlled handover scheme. In order to simulate two techniques, a 53