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Institutionen för systemteknik Department of Electrical Engineering Examensarbete Algorithm for Handoff in VDL mode 4 Examensarbete utfört i Kommunikationssystem vid Tekniska högskolan i Linköping av Rickard

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Institutionen för systemteknik Department of Electrical Engineering Examensarbete Algorithm for Handoff in VDL mode 4 Examensarbete utfört i Kommunikationssystem vid Tekniska högskolan i Linköping av Rickard Andersson LiTH-ISY-EX--10/4332 SE Linköping 2010 Department of Electrical Engineering Linköpings tekniska högskola Linköping University Institutionen för systemteknik S Linköping, Sweden Linköping Algorithm for Handoff in VDL mode 4 Examensarbete utfört i Kommunikationssystem vid Tekniska högskolan i Linköping av Rickard Andersson LiTH-ISY-EX--10/4332 SE Handledare: Jan-Erik Lundmark CNS Systems Examinator: Mikael Olofsson ISY, Linköpings Universitet Linköping 22 februari 2010 Presentationsdatum Publiceringsdatum (elektronisk version) Institution och avdelning Institutionen för systemteknik Department of Electrical Engineering Språk Svenska x Annat (ange nedan) Engelska / English Antal sidor 65 Typ av publikation Licentiatavhandling x Examensarbete C-uppsats D-uppsats Rapport Annat (ange nedan) ISBN (licentiatavhandling) ISRN LiTH-ISY-EX 10/4332--SE Serietitel (licentiatavhandling) Serienummer/ISSN (licentiatavhandling) URL för elektronisk version urn:nbn:se:liu:diva Publikationens titel Algorithm for Handoff in VDL mode 4 Författare Rickard Andersson Sammanfattning Abstract VDL mode 4 is a digital data link operating in the VHF band, its mainly use is for the aviation industry. VDL4 can as an example provide with positioning data, speed information of aircrafts or vehicles equipped with a VDL4 transponder. A connection between the groundsystem and the airborne system is called a point to point connection, which can be used for various applications. This data link needs to be transferred between groundstations during flights in order maintain the connection, which is called handoff. The handoff process needs to be quick enough to not drop the link and at the same time a low rate of handoffs is desirable. The data link is regarded as a narrow resource and link management data for handoff is considered as overhead. This thesis studies how to make the handoff procedure optimal with respect to involved aspects. Previous research of handoff algorithms and models of the VHF-channel are treated. Standardized parameters and procedures in VDL4 and are explored in order to find an optimal solution for the handoff procedure in VDL4. The studied topics are analyzed and it is concluded to suggest an algorithm based on an adaptive hysteresis including signal quality and positioning data provided in VDL4. Standardized parameters which could be useful in the handoff procedure are commented, since the VDL4 standards are under development. Nyckelord Handoff, VDL4, algorithm, Abstract VDL mode 4 is a digital data link operating in the VHF band, its mainly use is for the aviation industry. VDL4 can as an example provide with positioning data, speed information of aircrafts or vehicles equipped with a VDL4 transponder. A connection between the ground system and the airborne system is called a point to point connection, which can be used for various applications. This data link needs to be transferred between ground stations during flights in order maintain the connection, which is called handoff. The handoff process needs to be quick enough to not drop the link and at the same time a low rate of handoffs is desirable. The data link is regarded as a narrow resource and link management data for handoff is considered as overhead. This thesis studies how to make the handoff procedure optimal with respect to involved aspects. Previous research of handoff algorithms and models of the VHF-channel are treated. Standardized parameters and procedures in VDL4 and are explored in order to find an optimal solution for the handoff procedure in VDL4. The studied topics are analyzed and it is concluded to suggest an algorithm based on an adaptive hysteresis including signal quality and positioning data provided in VDL4. Standardized parameters which could be useful in the handoff procedure are commented, since the VDL4 standards are under development. Acronyms and abbreviations ADS-B Automatic Dependent Surveillance Broadcast 23 ATC Automatic Traffic Control 1 ATM Air Traffic Management 1 BER Bit Error Rate 24 DOS Directory of Service 38 ETSI The European Telecommunications Standards Institute 2 Eurocontrol The European Organization for the Safety of Air Navigation 5 FIS-B Flight Information Service Broadcast 23 FDMA Frequency Division Multiple Access 14 FSP Free Space Propagation 12 GNSS Global Navigation Satellite System 24 GPS Global Positioning System 1 GSC Global Signaling Channel 38 GSIF Ground information frames 38 GTD Geometrical Theory of Diffraction 33 KIAS Knots Indicated Air Speed 45 LME Link Management entity 26 NAC Navigation Accuracy Categories 37 NIC Navigation Integrity Category 37 NM Nautical Mile 31 OSI Open System Interconnection 13 PECT Peer Entity Contact Table 37 RSS Received Signal Strength 16 RADAR RAdio detection And Range 23 SCAA Swedish Civil Aviation Authority 1 SNR Signal-to-Noise Ratio 12 SQP Signal Quality Parameter 36 STDMA Self organizing Time Division Multiple Access 24 TCP Trajectory Change Point 39 TDMA Time Division Multiple Access 14 TIS-B Traffic Information Service Broadcast 23 UTC Coordinated Universal Time 24 VDL2 Very high frequency Digital Link mode 2 26 VDL4 Very high frequency Digital Link mode 4 1 VME VDL Management entity 26 VSS VDL Mode 4 Specific Services 25 Table of contents 1. Introduction Background Problem Purpose and Goal Interested Parties Limitations Disposition Method Scientific Approach Reliability and Validity Collection of Data Source Criticism and Scientific View Work Method Basic Concepts of Wireless Communications Wireless Communication Link Budget OSI Reference Model Handoff Methods for Making a Handoff Decision VDL4 in the Aeronautical Telecommunication Network VDL4 - VHF Digital Link mode Link Management Handoff in VDL VDL4 Link Budget Channel Properties in the VHF band Available Parameters for a Handoff Decision in VDL Algorithm for Handoff in VDL Assumptions and Requirements on the Handoff Algorithm Analytical Approach Evaluation of Available Mandatory Information Requirements on the Handoff Algorithm in VDL Handoff Algorithm Based on Mandatory Information Evaluation of Optional Information Handoff Algorithm with Additional Optional Information Conclusions Fulfillment of the Purpose Further Studies References Appendix... 62 List of figures Figure 1 Disposition of the thesis... 4 Figure 2 Work Method... 6 Figure 3 Basic wireless communication... 7 Figure 4 Multipath in wireless communication... 8 Figure 5 Knife edge diffraction... 9 Figure 6 Link budget in a wireless system Figure 7 Dipole omni-directional antenna pattern Figure 8 Visualization of angles in direction (θ,φ ) Figure 9 OSI reference model Figure 10 Illustration of FDMA Figure 11 Illustration of TDMA Figure 12 Visualization of a handoff scenario Figure 13 General influence of fading Figure 14 Optimal decision point for Handoff Figure 15 The Ping-Pong phenomenon Figure 16 General Overview of thevdl4 Communication system Figure 17 A VDL4 transponder Figure 18 VDL4 in the ATN Figure 19 VDL4 referred to OSI-model Figure 20 The active functions in a point-to-point scenario in VDL Figure 21 Procedure for communication during mobile initiated handoff Figure 22 location of specified receiver sensitivity Figure 23 theoretical transmitting distance, class A transponder Figure 24 Line of sight scenario Figure 25 Empirical studies of received power levels at 6000m Figure 26 Estimated coverage of two ground stations Figure 27 Accuracy of received positioning data Figure 28 Assumed structure of cells in a ground system Figure 29 A Handoff Scenario for Load Balancing Figure 30 Handoff decision based on duration of the point-to-point link and path of flight Figure 31 Relative Change of Distance to Ground Stations Figure 32 Example of holding Figure 33 Example of the adaptive hysteresis Figure 34 3D view of the adaptive hysteresis Figure 35 Scenario for order of preference... 51 1. Introduction This chapter gives a background to the thesis. The studied problem as well as the study s purpose and goal is given. Furthermore the chapter presents information regarding limitations and interested parties for the thesis. Finally the disposition of the study is presented. 1.1 Background There is an expectancy of an increase of air traffic, airspace gets more crowded and ATM, Air Traffic Management, plays an important role for the aviation industry. Air traffic planning or correction of flight routes are some of the functions the ATM handles. Predefined flight routes are not enough with an increasing demand. The SCAA, Swedish Civil Aviation Authority, is conducting the FRAS, Free Route Airspace Sweden project, where predefined flight routes will become a memory. As a first step, a part of Sweden s airspace was recently released, giving pilots the opportunity to plan other paths of flight than the earlier predefined routes. 1 To achieve an efficient management of the air traffic, a reliable communication is a crucial factor. The SCAA is investigating new technologies to handle a future ATM, with improvements of the needed applications and an increased ability to meet future needs. One candidate for the next generation of ATC, Automatic Traffic Control, is a technical platform based on digital communication. The VDL4, VHF Very high frequency Digital Link Mode 4, is a standardized data link technology intended to be used in the civil air traffic. 2 The VDL4 standard gives possibilities to transmit different kinds of information, such as weather information or surveillance of traffic based on GPS, Global Positioning System. The next generation ATM could give a pilot graphical and textual weather information on a moving map and also information of all other traffic. A continuous update of positions and directions of other traffic or changing weather activities on the path of the flight are some of the possibilities. In a scenario where a ground operator wishes to communicate with a specific aircraft, a point-topoint connection is used. To maintain an ongoing transmission between a moving aircraft and the ground station, it is, for different reasons, needed to switch the data link to another ground station. The process of moving a data link from one ground station to another is called handoff. This event could for instance occur if an aircraft flies out of reach from a ground station to which the point-to-point is connected or perhaps in the case if the communication channel is too densely populated. Making a good decision of how and when to make a handoff is crucial to maintain a reliable connection between an aircraft and ground. 1 Airspace/ 2 1 1.2 Problem The standard documentation, provided from ETSI, the European Telecommunications Standards Institute, does not provide any guidelines about algorithms handling the handoff procedure in VDL4. It is therefore a task for different manufacturers to solve the handoff process based on interpretations from the given requirements. The link management is considered as overhead information on the data link i.e. the radio is considered as a narrow resource and should mainly be used to send other type of information, as example positioning data rather than data concerning link management. Therefore it is important to achieve a low number of handoffs and still maintain the point-to-point link between a mobile and a ground-station. 1.3 Purpose and Goal The thesis studies how to optimize the handoff process in VDL4. The scenario where an airborne aircraft and a ground-station have established a point-to-point connection is considered. The purpose of the study is to develop algorithms for an optimal handoff decision between ground-stations. To fulfill the purpose specific goals are set: Develop an algorithm based on VDL4 ETSI standards and use information stated as mandatory as input to a handoff decision. The parameters set as optional in the ETSI standard, are to be examined if the optional transmitted information could lead to an improved handoff process. In case of possible improvements, the handoff algorithm is to be developed with the extension of information. 1.4 Interested Parties The target group of the thesis is mainly providers of VDL4 equipment. Information to understand relevant parts related to the handoff procedure in VDL4, without extensive knowledge of radio communication, is provided. The thesis is of interest for other students, as well as others, with an interest of radio communication or handoff procedures. 1.5 Limitations The study does not consider actual choice of equipment, differences in performance between equipment from various manufacturers is beyond the scope of the thesis. A handoff algorithm could perform different in one system compared to another because of implementation aspects. The equipment is considered to perform according to the requirements provided from VDL4 standards. 2 Much information related to propagation of radio waves concerning the physical channel has been left out. As example polarization or antenna theory etc. although influencing the properties of propagation, it is considered to be outside the scope of the study. The focus of the thesis is the handoff process and the given information is mainly to present some sections involving the complexities of a handoff in VDL4. The study does not consider the actual placement of ground stations. It is assumed a system where the coverage from ground stations are overlapping i.e. more than one ground station is available for an aircraft to connect to during flight. The design of the ground system is beyond the scope of the study. The developed algorithm is intended for a general structure of a ground network for VDL4. Models of the VDL4 channel are examined to determine if a model should be used in the handoff algorithm, any modification or development of a VHF model in an air-ground scenario is not handled, mainly due to a need for empirical tests and which is beyond the scope of the study. Parameters in VDL4, which influence the performance such as throughput of data or transmission delays, are not considered. Optimization of throughput or transfer delays and similar is not handled when already considered in previous studies. 3 The target group using point-to-point communication in VDL4 is mainly considered as traffic flights, such as personal transports or goods transports. 3 Further reading in AMCP WG M2 Appendix M VHF Datalinks for point to point communications, 7 th meeting ACMP jan 1.6 Disposition An overview of the thesis s disposition is presented in Figure 1 Disposition of the thesis. A brief description of each chapter s content is also given. Introduction Chapter 1 Chapter 1 gives an introduction to the topic that is studied, the purpose and limitations are presented. Metodhology Chapter 2 Chapter 2 presents the methodology, presenting how the study has been conducted and the research philosophy. Chapter 3 Basic Concepts of Wireless Communication In chapter 3 a theoretical frame of references is presented.. Topics related to wireless communication are given. This provides with necessary information to understand the studied system and related issues in a handoff process. Chapter 4 VDL4 in the Aeronautical Telecommunication Network Chapter 4 presents the studied system, giving an overview of VDL4. Topics that are important for a further analysis as well as the framework for the handoff procedure in VDL4 is given. Chapter 5 Algorithm for Handoff in VDL4 In chapter 5 an analysis based on the earlier information is made, leading to a solution for an algorithm handling handoff for point-to-point VDL4. Conclusions Chapter 6 Chapter 6 presents the conclusions of the study as well as suggestions for further studies. Figure 1 Disposition of the thesis 4 2 Method This chapter presents an insight of how the study has been made. The approach to the problem as well as the reliability and validity of the study are discussed. How information has been collected and the author s attitude and scientific view are presented. Finally the work method for the study is given. To facilitate a research study different methods are used depending on the design of the research. By applying a method, a structure in the work process is given 2.1 Scientific Approach A study s research problem and purpose is the starting point in deciding what method to use. 4 For the study the approach is that based on theory and previous research a solution will be derived. 2.2 Reliability and Validity Reliability is related to the question whether the results of a study are repeatable or not. 5 Meaning that if the same study is remade the same results would be concluded. It can be argued that used documents and literature possibly have been interpreted incorrect, however assumptions are accounted for in the document providing a high reliability. Other assumptions or interpretations of used sources could perhaps give other results. Validity is to what extent collected data is valid and relevant. 6 The intention has been to gather relevant information and use it properly. Information of the handoff process in VDL4, as well as information about similar systems and related research, has been used to fulfill the relevance criteria. The validity is determined when the result is checked to fulfill the thesis s purpose. 2.3 Collection of Data Handoff procedures have been extensively researched and in order to fulfill the purpose of the thesis secondary data has been used. Secondary data is defined as data gathered for some purpose other than the objectives of this particular study. 7 The literature and articles within the field of the study have been collected at the University of Linköping, information from databases provided at the library and Internet. Documentation regarding VDL4 standards is achievable from the ETSI webpage and also provided at the Eurocontrol, The European Organization for the Safety of Air Navigation, webpage. The used books have for the most part been provided by the library at Linkoping s University and the referred articles have mainly been collected from IEEE, Institute of electrical and electronic Engineers. Key words when searching for information have been: handoff, handover, propagation, VDL, handoff algorithm, VHF channel, algorithm optimization. 4 Ghauri P. Grønhaug K. Research Methods in Business Studies, third edition, Pearson Education Limited, England Bryman A. Bell E Business Research Methods Oxford University Press New York Esaiasson P. Gilljam a.o Metodpraktikan Nordstedts Juridik AB Elanders Ghauri P. Grønhaug K. Research Methods in Business Studies, third edition, Pearson Education Limited, England 2.4 Source Criticism and Scientific View This study is based on theoretical information and a researcher needs to critically analyze and carefully reflect the used sources. 8 The ETSI document is one of the main components for the providers of VDL4 equipment as well as the SARPS documents. These documents set the requirements of the equipment and on the system and unclear statements or definitions are reflected upon in the report. The used articles are published on IEEE, a globally well known and recognized organization. IEEE provides a wide range of professional and technical information. This leads the author to the assumption that the used articles are trustworthy. Written material appearing to be influenced by commercial or nonobjective interests is handled through a humanistic view Work Method The work on the thesis has been divided into different steps. Initial theoretical studies are an important part of the progress. To handle a task properly, a good knowledge of the actual problem is required and an outline of this study s work method is presented in Figure 2 Work Method. Figure 2 W
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