ABSTRACT
Increase in GSM mobile station (MS) density without a corresponding increase in radio channel resource capacity reduces the quality of service (QoS) standard provided to users. The attempts made to improve the QoS standard have resulted in the creation of new base stations (BS) with the consequent reduction in the existing cell sizes. Increased call handovers are experienced in such situation. The increase is usually accommodated by resource reservation. How well the reservation is made greatly influences the QoS a GSM BS provides to MSs. This project work, therefore, presented optimum GSM call handover radio channel resource reservation system model. This model considers the QoS standard of fresh calls while reserving resources for handover calls. Therefore, the developed system model interfaced the handover resource reservation model to a typical cellular network model. This approach is rare due to the simulation complexity that is usually experienced with existing handover resource reservation models. The complexity was reduced in this work by integrating the model of a typical GSM network model with the resource reservation model in a MATLAB block oriented simulation environment – Simulink. The relationships between the handover call blocking probability, the number of resources to be reserved under varying traffic intensity were simulated to validate the model. Results show that the model can be successfully applied in the study of resource reservation systems.
TABLE OF CONTENTS
APPROVAL PAGE.. ii
CERTIFICATION.. iii
DECLARATION.. iv
DEDICATION.. v
ACKNOWLEDGEMENT. vi
ABSTRACT. vii
Contents. viii
LIST OF FIGURES. x
CHAPTER ONE.. 1
INTRODUCTION.. 1
1.0 Background of the study. 1
1.2 Objectives of the study. 3
1.3 Significance of the study. 3
1.4 Scope of the study. 3
1.5 Study methodology. 3
1.6 Project Outline. 4
CHAPTER TWO.. 5
LITERATURE REVIEW… 5
2.0 Introduction. 5
2.1 GSM Architecture. 5
2.1.1 Mobile Station. 6
2.1.2 Base Station Subsystem.. 8
2.1.3 Network Switching System (NSS) 10
2.2 GSM Channel Structure. 12
2.2.1 User Traffic Channel (TCH) 12
2.2.2 Control Channel (CCH) 12
2.2.3 Broadcast channels (BCHs) 13
2.2.4 Common control channel (CCCH) 13
2.2.5 Dedicated control channels (DCCH) 14
2.2.6 GSM Technical Specifications. 17
2.3 GSM Mobility. 21
2.3.1 Location Update. 21
2.3.2 Common Mobility Procedures. 22
2.3.3 Call Handover 23
2.4 PUBLISHED WORKS IN THE AREA.. 25
2.4.1 Radio Channel Resources. 26
2.5 Conclusion. 27
CHAPTER 3. 28
SYSTEM MODEL.. 28
3.0 INTRODUCTION.. 28
3.1 MODEL DESIGN SPECIFICATION.. 28
3.2 TYPICAL CELLULAR NETWORK ARCHITECTURE.. 29
3.3 RADIO CHANNELS RESERVATION MODEL.. 30
3.3.1 Resource Reservation Subsystem.. 30
3.3.2 ANALYTICAL MODULE.. 31
3.3.3 Allocation algorithm module. 32
CHAPTER 4. 35
SYSTEM MODEL SIMULATION AND RESULT ANALYSIS. 35
4.0 INTRODUCTION.. 35
4.1 TRAFFIC SOURCE MODULE.. 35
4.2 MULTIPLE ACCESS MEDIA MODULE.. 37
4.3 ANALYTICAL SUBSYSTEM… 39
4.3 RESULTS AND RESULT ANALYSIS. 42
CHAPTER 5. 46
CONCLUSION.. 46
5.1 RECOMMENDATION FOR FURTHER WORK.. 46
5.2 CONTRIBUTION.. 47
REFERENCES. 48
CHAPTER ONE
INTRODUCTION
1.0 BACKGROUND OF THE STUDY
In every part of the world, presently, mobile communication networks are continuously experiencing increase in MS subscription. Correspondingly, the density of BSs is increased to maintain QoS standard. No doubt, increase in the number of BSs in a given area brings about a corresponding increase in the number of cell areas and then leads to the reduction in the sizes of the existing cells [1]. These situations often result in the increase of handover events between cells since the probability of MSs traversing cell(s) during a typical call connection increases. GSM handover event occurs when a mobile telephone network automatically transfers a call from one radio channel of a BS to another radio channel in a different BS as MS traverses cell boundaries [2]. In other words, handover involves the process of changing radio channel parameters such as frequency, time slot and spreading code associated with an on-going call connection [2]. Cell boundary is actually defined for a given BS by the points in space where minimum signal level required for standard quality reception from the BS are received [3]. The Mobile Switching Center (MSC) and Base Station Controller (BSC) setup monitor signal reception levels as MS moves. Whenever signal strength goes down below the minimum limit, it automatically switches the call to any idle channel in any BS with the strongest received signal above the minimum level required for handover within range.
In the case where the required resources (radio frequencies) for handover are not available, the on-going call(s) requiring handover are dropped, thus degrading the network QoS. Management of the limited and scarce radio channels needed for the handover calls requires a precision oriented network resource management system [4, 5, 6]. It is pertinent to note that GSM network users are more sensitive to handover call blocking than the fresh call blocking [7, 8]. This is because handover calls are on-going and have engaged considerable network resources. Therefore, it would be more productive to see an on-going call to its logical conclusion before accepting fresh calls when there is dearth of resources. Since users are very sensitive to the interruption of on-going calls, appropriate precision radio channel assignment system needed to be deployed to minimize the handover call drops. If radio channels are optimally allocated, handover call drop probability would be appropriately minimized while maximizing channel utilization.
The researches carried out in the area of handover, so far, are mostly directed at the reduction of handover rate, handover probability and the handover time (delay) [1, 9]. There is not much work on the development of the tool required for the development of network management systems needed to manage networks that encounter handover events such as GSM networks. It is on this basis that a resource reservation computer simulation modeling approach was proposed. The proposed modeling approach was tested on handover calls drop probabilities under varying traffic loadings.
Therefore, in this project a computer simulation model for the analysis of handover drop probabilities under varying traffic loadings was developed. The developed system model interfaced handover resource reservation model to a typical cellular network model. This approach is rarely applied in the study of resource reservation for handover calls due to the simulation complexity that is usually experienced with existing handover resource reservation models. Resource reservation models are usually comprised of analytical expressions that are highly difficult to tract especially when appreciable level of detailed insight into the model is required [1, 2, 4, 6, 8]. The complexity is greatly reduced, in this work, by the application of an integrated handover resource reservation model. The integrated model has both analytical and computer simulation modeling approaches combined together in one model. In the case where the analytical model becomes complex the computer simulation approach takes over. A typical GSM network architecture was modeled and interfaced to an integrated resource reservation model in a MATLAB block oriented simulation environment – Simulink. The relationships between the handover call blocking probability, the number of resources to be reserved and varying traffic intensity were simulated. Results show that the model can be successfully applied in the study of resource reservation systems. The model can be used to determine the optimum number of resources that can be reserved for handover calls for a given BS resource capacity while considering fresh call blocking probability under varying traffic loadings.
1.2 OBJECTIVES OF THE STUDY
This work is intended to produce a handover channel reservation modeling approach in MATLAB Simulink environment for the study of handover calls in GSM Cellular networks. Therefore, the objectives of this project include,
- The development of a model that defines the relationships between the handover call blocking probability, fresh call blocking probability, the number of resources to be reserved and traffic intensity.
- using integrated modeling approach [3, 17] in the development of the modeling approach
1.3 SIGNIFICANCE OF THE STUDY
The GSM mobility based model developed could be made available to researchers and GSM network operators working on the issues related to call drops that arise from handovers as a result of radio channel unavailability.
1.4 SCOPE OF THE STUDY
The study involves model development and analysis for a typical GSM cellular network node using MATLAB Simulink object oriented simulation software. There are quality of service, QoS, parameters of interest but this study will focus mainly on the parameters that concerns users of mobile/wireless telecommunication services such as,
- Call handover drop probability
- Fresh call drop probability
1.5 STUDY METHODOLOGY
The study methodology applied in this work is as follows:
- Firstly, the literature review on a typical GSM network architecture state-of-the-art and the review of the researches carried out in GSM network call handover issues were executed. This prepared the ground for the study without re-inventing the wheel. At the end, a choice modeling approach was adopted.
- Secondly, a physical model was developed from an adopted typical GSM network node architecture.
- Thirdly, the resource reservation physical model was developed and interfaced to the GSM node architecture model.
- Fourthly, the physical model was converted into MATLAB computer simulation model and then, validated.
- Finally, the computer simulation model was simulated and the results analyzed.
1.6 PROJECT OUTLINE
This research work report is presented in five chapters. Chapter one is the overall introduction of the research work, followed by Chapter two which presented detailed literature review. Chapter two also presented definition of concepts and the state of the art in cellular networks. This included the explanation of the basic GSM network architecture and infrastructure. It also presented relevant published problems in the area of cellular network call handover and their solutions. In chapter three, the GSM network model was developed and validated. Chapter four presented the simulation process in MATLAB. The results generated were also analyzed and presented in this same chapter. Chapter five presented the conclusion of the work. The significance of the result obtained is also explained in this chapter together with recommendation for further works and contribution to knowledge made by this work.
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