Handover Strategies in GSM System

Table of Contents

Abstract

List of Figures

List of Tables

Abbreviations

Acknowledgment

Chapter 1
1. Introduction
1.1 History of GSM
1.2 Overview of GSM handover
1.2.1 Between calls
1.2.2 During a call
1.3 The connection establishment in WLAN and the scanning concept
1.3.1 Discovery
1.3.2 Authentication
1.3.3 Association, Disassociation and Re-association
1.3.4 Confidentiality
1.3.5 The AP discovery and association
1.3.5.1 Passive scanning
1.3.5.2 Active scanning

Chapter 2
2. Literature review
2.1 Definition of the handover process
2.2 Types of handover technique
2.3 Reasons of handover failure
2.4 The differences between hard, soft and softer handoffs
2.5 SYNC handovers and asynchronous handover
2.6 Emergency handovers
2.7 Vertical and horizontal handover
2.8 “Multilayer Handover” Strategy, “Ping pong effect” and “take-back”
2.9 Handover decisions in GSM
2.10 GSM handover solutions
2.11 Mathematical calculations and modulations

Chapter 3
3. The focus- calculate the optimal timing and distance for GSM vertical handover
3.1 Time Before Vertical Handover (TBVH)
3.1.1 Time Before Vertical Handover (for outdoor environments)
3.1.2 Time Before Vertical Handover (for indoor environments) - Mobile node movement from a normal BS to BBS

Chapter 4
4. The Simulation and Result for the TBVH
4.1 Why Matlab program
4.2 Modulation
4.3 Decision algorithm

Chapter 5
5. Conclusion and Future work

References

List of figures

1.2 Fig (1): The reference model of GSM

1.3 Fig (2): The connection establishment in WLAN

2.1 Fig (3): handover procedure

2.2 Fig (4): Types of handover

2.4 Fig (5): Hard handover

2.4 Fig (6): Soft handover

2.4 Fig (7): Softer handover

2.7 Fig (8): Example of vertical and horizontal handovers

3. Fig (9): The scenarios of handover in 4G networks

3.1.1 Fig (10): MN under the coverage of the BBS

3.1.2 Fig (11): MN moves towards the BBS

3.1.2 Fig (12): The mobile node in the indoor environment

4.2 Fig (13): TBVH block diagram

4.2 Fig (14): flow chart of TBVH choices

4.2 Fig (15): The relation between Distance & TBVH for the outdoor envirenment

4.2 Fig (16): The relation between Distance & TBVH for the indoor envirenment

4.3 Fig (17): The decision algorithm for vertical handover

List of tables

2.7 Table 1: The differences between vertical and horizontal handovers

2.7 Table 2: The differences in the capability of both the horizontal and the vertical handovers

4.2 Table 3: Calculating TBTH for the first case

4.2 Table 4: Calculating TBVH for the first case before changing and after changing the direction

4.2 Table 5: Calculating TBVH for the second case

Abbreviations

1. HO: Handover (in British language ) or Handoff (in American language )

2. GSM: old name (Groupe Special Mobile), new one (Global System for Mobile communications)

3. Matlab: matrix laboratory

4. WLAN: Wireless Local Area Network

5. SMS: Short Message Service

6. SIM: Subscriber Identity Module

7. GPRS: General Packet Radio Services

8. ESS: Extended Service Set

9. SSID: Service Set ID

10. EDGE: Enhanced Data Rate for GSM Evolution

11. AMPS: Advanced Mobile Phone System

12. TACS: Total Access Communications

13. NMT: Nordic Mobile Telephone system

14. ISDN: Integrated Services Digital Network

15. TDM: Time Division Multiplexing

16. MS: Mobile Station

17. MSL: Mobile Station Location

18. BTS: Base station Transceiver Station

19. BSC: Base station Controller

20. EIR: Equipment Identity Register

21. MSC: Mobile Switching Centre

22. HLR: Home Location Register

23. GSMSC: Gateway Mobile Switching Centre

24. IWMSC: Interworking Mobile Switching Centre

25. AUC: Authentication Centre

26. GCR: Group Call Register

27. SIWF: Shared Interworking Function

28. IWF: Interworking Function

29. VLR: Visitor Location Register

30. IMSI: International Mobile Subscriber Identity

31. SACCH: Slow Access Control Channel

32. FACCH: Fast Access Control Channel

33. BSSMAP: Base Station Subsystem Mobile Application Part

34. FDMA: Frequency Division Multiplexing Access

35. TDMA: Time Division Multiplexing Access

36. CDMA: Code Division Multiplexing Access

37. VOIP: Voice over Internet protocol

38. ITU: International Telecommunication Union

39. MSCTP: Mobile Stream Control Transmission Protocol

40. MTSO: Mobile Telephone Switching Office

41. RSS: Received Signal Strength

42. NCOH: Network Controlled Handoff

43. MAHO: Mobile Assisted Handoff

44. DECT: Digital Enhanced Cordless Telecommunications or Digital European Cordless Telephone

45. QoS: Quality of Services

46. MT: Mobile Terminal

47. TBVH: Time Before Vertical Handover

48. AP: Access Point

49. MN: Mobile Node

50. TD: Time Division

51. SPPQ: Signal Prediction Priority Queuing

52. CBS: Current Base Station

53. NXBS: next Base station

54. ETSI: European Telecommunication Standards Institute

55. TG: Telecommunication Group

56. IP address: Internet Protocol address

57. VHO: Vertical Handover

Acknowledgment

I would like to thank the almighty Allah most graceful, most compassionate for the strength He has given me to pursue this task and stand firm in the face of challenges and there were so many. Thanks are to Him for the opportunity He has blessed me and enabled to do my MSC in Telecommunication Engineering, and for the faith He has implanted in my heart and soul.

This dissertation has been written with the support and assistance of a number of people, I would like to personally thank. At my university, I owe my deepest gratitude to my supervisor, Dr. Aboubaker Lasebae, whose expertise, understanding and patience added considerably to my graduate experience.

Moreover, I would like to thank the other members of my committee for the knowledge and the huge amount of information that I gained from their lectures and seminars, Dr. Leonardo Mostarda, Dr. Shahedur Rahman, Dr. Purav Shah and Mr. Ihab Arusi.

Finally, I would like to thank my family for the support they provided me with through my entire life and through my dissertation because without their love, encouragement and financial support and moral assistance, I would not have finished this course.

Chapter 1

GSM HO overview and scanning concept through WLAN

1. Introduction

The mobile telephone has become an indispensable part of human life because it endows it with mobility and availability. The continuous development in the field of GSM technology connects mobile telephone to networks. The functionality to ensure an acceptable quality of a mobile call when moving from one coverage area to another is called Handover. As a matter of fact, Handover is a process used when the quality of the signal decreases as a result of moving from one coverage area to another. Handover technique transfers the signal of the call transparently from one stationary antenna to another during the call to maintain the signal and to avoid any cut that might happen because of the weakness in both signal and power. There are different types of GSM handover, each one of these types has its own properties which make it unique and different from others. Handover has its own advantages and disadvantages (problems) which are under both investigation and development. In this project, there will be an explanation of the mechanism of the signal transmission and handover from one antenna to another. Moreover, a comparison of the different types of handovers will be made in order to select the best type that can reserve the signal from any cut or interference. Moreover, to resolve the problems and the obstacles that might face the transmitted signal from any cut or interference, this project will calculate the optimal timing and distance for vertical handover between handover facilities to render a solution that can solve these problems.

As an introduction for this project, it will be necessary to understand few concepts which give a general overview before starting the focus of this paper such as:

1. History of GSM.
2. Overview of GSM handover.
3. The connection establishment in WLAN and the scanning concept.

1.1 History of GSM

Nowadays, GSM cell or mobile phone system has become more popular than any other system because of the widely usage in both the local and global levels. GSM is a system which is rich with several features and applications such as: SMS text messages, SIM cards, international roaming and internet. The internet applications have been improved dramatically since new technologies have been introduced such as GPRS and EDGE.

The first cell phone system was developed in 1989. Actually, it was an analogue system. Typically, the old system used the frequency modulated carriers for the voice channel and the data carried on a separate shared control channel. A report from Radio-Electronics.com (25) states that two major systems existed at that time; those were the AMPS that was used in USA and TACS that was used in UK. Moreover, another system was developed called NMT. In fact, it was the first commercial system to be developed in the area of cellular systems; the significant financial benefits were the motivating factor behind the maximization of the GSM system.

In 1982, the basic criteria of the new cellular system were listed to meet the user needs; these criteria included the good voice quality, low cost, international roaming, the ability to support handheld terminals, new services and facilities, efficient spectral, large capacity and ISDN compatibility. The technological developments and related experiments that took place at that time evinced that TDM offered a good performance with the new technology than other techniques did.

In 1991, the GSM system saw its first dawning. It was indeed a farfetched target to investors; however the real beginning was in 1992. By the end of 1993, the GSM had attracted more than one million subscribers; the number was increasing rapidly and constantly. Nowadays the number is being increasing and it has indeed exceeded three billion subscribers.

Because of the wide usage of the GSM in many world countries, it was crucially necessary to change its name from Group Special Mobile to a Global System for Mobile communications.

1.2 Overview of GSM handover

To begin with, it is quite necessary to mention the basic entities of the GSM system as depicted in the following figure:

illustration not visible in this excerpt

Fig (1): The reference model of GSM (11)

The random movement of the mobile node from the coverage area of one cell to another has made it necessary to find a new mechanism by which calls can be transferred from one cell to another without cutting off the call. A report from phonehistory.co.uk (21) proposed that there are two situations for this mechanism to function; these situations are between calls and during calls.

1.2.1 Between calls

In the idle mode, the mobile reports only when it transfers to another VLR by doing a location update. Since each mobile has to report its new position by sending a position update, the network might mislay some of these calls as a result of the huge database or a signalling error. Therefore, to avoid this problem it is better for the mobile to avoid sending more than one report at the same time. In this way, the user might receive an old SMS message or being told to wait for a long voicemail when the process of location update is taking place. When the mobile phone is closed, it sends a log-off signals to the network so that it will not look for a switched off mobile known as an IMSI Detach. It is impossible to avoid such situation specially when the switched off mobile is out of the coverage area. So in this situation the network will not take notice the switched off mobile until the next scheduled location update has been missed.

1.2.2 During a call

As long as the call is in progress and during the time between sending and receiving data, the handset has the responsibility of monitoring the signal it gets from the 16 nearby cells and are listed in the currents neighbour list of the required cell. Every second, it reports the level signal of one of the best six neighbouring cells to the BSC by using SACCH. The decision of switching cells can vary but in general the process accrues to the cell that has the best signal and this subsequently economises the power in the mobile. The organization and coordination of the handover process is a time- critical function; therefore, it is necessary to use FACCH.

Moreover, the decision can be made either by the mobile itself or the BTS but since the BTS sometimes might be too busy, the handover fails and the mobile needs to start scanning again the network for a fresh start. This can occur because of the signal propagation over far distances over hilltops or mountains where no neighbouring listed cells exist near the mobile.

1.3 The connection establishment in WLAN and the scanning concept

Bing (2008) (3) states in his book that because of the absence of a stable physical network connection in WLANs, the MN spends more time and resources in order to scan the surrounding area and to look for optimum wireless coverage. Even when the MN is connected to an AP, the mobile does not guarantee the connectivity within the required period of time due to the mobility and the fluctuation in the channel conditions. In this case, it might be necessary to migrate the mobile data to another AP in order to avoid any connection loss.

It is to be mentioned here that the main aim of wireless devices is not only looking for APs, but also offering the best optimum connectivity .The establishment of this connectivity can be described in the following figure:

illustration not visible in this excerpt

Fig (2): The connection establishment in WLAN (3)

The connection establishment in WLAN processes through the following FOUR steps:

1.3.1 Discovery

To establish a connection between the MN and the AP, the wireless device scans all the radio channels in the surrounding area to detect the AP and other stations. However, this scanning might be either passive or active. In the passive case, the MN listens to the radio channels for beacon transmitted by the APs and after that decides the best choice. However, in the active scan, the MN sends an explicit probe request for each AP having the ability to transmit beacons in response.

1.3.2 Authentication

This can be achieved through exchanging special authenticate packets. The association request starts its progress only when authentication is verified. Otherwise, the recipient sends a de-authentication notifying the requested device.

1.3.3 Association, Disassociation and Re-association

This step seeks to establish an association between the stations or between the station and the AP. When the MN intends to connect to an AP in a BSS, it sends an association request. When handing occurs between the old AP and the new one, the MN sends the disassociation request to the old AP and a re-association to the new AP. Each MN has the ability to connect only with one AP at a particular time. It also consists of the mechanisms responsible for the QoS and the call admission control.

1.3.4 Confidentiality

This is a necessary step to ensure the encrypted form of communication between the two devices that share the public wireless interface. The AP in the BSS has the full responsibility of enforcing the security policies and to advertise these policies in beacon and probe response packets.

1.3.5 The AP discovery and association

In WLAN MN, deciding the AP implies scanning for the availability of different APs before making the final choice of the most appropriate AP. According to Bing (2008) (3), scanning can be initiated in two ways; manually by the subscriber or automatically by the deployment of system selection algorithm. The above mentioned types of scanning namely the passive and the active will be explained in details in the following subsections.

1.3.5.1 Passive scanning

Bing (2008) (3) states that in passive scanning, joining a particular ESS, the MN requires listening to different channels consecutively searching for beacon frames which match the service ID (SSID) of the ESS. However, passive scanning is more time consuming than active scanning.

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