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Open Thesis-Cklee-Jul21.Pdf The Pennsylvania State University The Graduate School College of Engineering EFFICIENT INFORMATION ACCESS FOR LOCATION-BASED SERVICES IN MOBILE ENVIRONMENTS A Dissertation in Computer Science and Engineering by Chi Keung Lee c 2009 Chi Keung Lee Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2009 The dissertation of Chi Keung Lee was reviewed and approved∗ by the following: Wang-Chien Lee Associate Professor of Computer Science and Engineering Dissertation Advisor Chair of Committee Donna Peuquet Professor of Geography Piotr Berman Associate Professor of Computer Science and Engineering Daniel Kifer Assistant Professor of Computer Science and Engineering Raj Acharya Professor of Computer Science and Engineering Head of Computer Science and Engineering ∗Signatures are on file in the Graduate School. Abstract The demand for pervasive access of location-related information (e.g., local traffic, restau- rant locations, navigation maps, weather conditions, pollution index, etc.) fosters a tremen- dous application base of Location Based Services (LBSs). Without loss of generality, we model location-related information as spatial objects and the accesses of such information as Location-Dependent Spatial Queries (LDSQs) in LBS systems. In this thesis, we address the efficiency issue of LDSQ processing through several innovative techniques. First, we develop a new client-side data caching model, called Complementary Space Caching (CSC), to effectively shorten data access latency over wireless channels. This is mo- tivated by an observation that mobile clients, with only a subset of objects cached, do not have sufficient knowledge to assert whether or not certain object access from the remote server are necessary for given LDSQs. To address this problem, our CSC requests each client to cache a global data view, that is composed of (i) cached spatial objects and (ii) complementary regions that cover the locations of all the non-cached objects. With a global data view cached, CSC enables clients to assert the completeness of LDSQ results locally. Second, we investigate two new types of complex LDSQs, namely, Nearest Surrounder (NS) Queries and Skyline Queries. Both of them have a wide application base. An NS query returns spatial objects along with dis- jointed angular ranges within which they are the nearest to a given query point. A skyline query retrieves non-dominated spatial objects. An object o is said to be dominated if there is another object o that is strictly better than o for at least one attribute and is not worse than o for all the other attributes. We conduct in-depth analysis and propose novel techniques to efficiently answer these new queries. Third, we propose an LDSQ processing framework, namely ROAD, to support efficient access of spatial objects on a road network. ROAD adopts a search space pruning technique that has not been explored before in this context. In ROAD, a large road network is organized as a hierarchy of interconnected regional sub-networks called Rnets, i.e., search subspaces. Further, with two novel concepts, namely, (i) shortcuts, that allow jumps across Rnets to accelerate the search traversal, and (ii) object abstracts, that provide search guidance during traversals, searches supported by ROAD can bypass those Rnets that contain no object of interest. Also, ROAD is flexible to support various LDSQs and objects. We conduct extensive empirical studies to evaluate the performance of our proposed ap- proaches. The experiment results demonstrate the efficiency of our approaches and their supe- riority over state-of-the-art approaches in corresponding domains. iii Table of Contents List of Tables viii List of Figures ix Acknowledgments xii Chapter 1 Introduction 1 1.1 Location-Dependent Spatial Queries . ..................... 1 1.2 The System Models of Location-Based Services . ............. 3 1.3 LBS Challenges and Opportunities . ..................... 4 1.4 Our Proposals . ................................. 6 1.5 The Organization of the Thesis ......................... 8 Chapter 2 Literature Review 10 2.1 Mobile and Wireless Computing Environments . ............. 10 2.2 Wireless Data Broadcast . ......................... 12 2.2.1 Broadcast Scheduling . ......................... 13 2.2.2 Air Indexing . ............................. 13 2.3 Mobile Client Caching ............................. 17 2.4 Application-Specific Data Access Methods . ................. 19 2.4.1 Time-Critical Data Access . ..................... 19 2.4.2 Semantic-Based Data Access . ..................... 19 2.4.3 Reliable Data Access . ......................... 20 2.5 Spatial Databases . ............................. 20 2.5.1 Spatial Indexes ............................. 21 iv 2.5.2 Spatial Queries ............................. 23 Chapter 3 Complementary Space Caching 24 3.1 Overview of Complementary Space Caching . ................. 24 3.2 LDSQ Processing with CSC . ......................... 27 3.2.1 Cache Initialization . ......................... 27 3.2.2 Query Processing . ......................... 28 3.2.3 Cache Probing ............................. 29 3.2.4 Remainder Query Processing . ..................... 29 3.3 CR Coalescence ................................. 31 3.3.1 Generic CR Coalescence Algorithm . ................. 32 3.3.2 Client Request CR Coalescence . ................. 33 3.3.3 Server Reply CR Coalescence ..................... 34 3.4 Cache Management . ............................. 35 3.4.1 Cache Organization . ......................... 35 3.4.2 Cache CR Coalescence ......................... 36 3.4.3 Cache Replacement . ......................... 36 3.5 Other Client-Side Data Caching Models . ................. 37 3.6 Performance Evaluation ............................. 38 3.6.1 Performance of Caching Schemes . ................. 40 3.6.2 Performance of Different Remainder Query Expressions . 41 3.6.3 Performance of Server Reply CR Coalescence ............. 42 3.6.4 Performance of Cache Management . ................. 43 3.7 Chapter Summary . ............................. 45 Chapter 4 Nearest Surrounder Queries 46 4.1 Definitions of Nearest Surrounder Queries . ................. 46 4.2 Na¨ıve Approaches to NS Searches . ..................... 49 4.3 Angle-Based Bounding Properties . ..................... 50 4.3.1 Edge Angular Bound and Edge Angular Distance . ......... 51 4.3.2 Edge Comparison . ......................... 53 4.3.3 The Brute-Force Algorithm . ..................... 54 4.4 NS Search Algorithms ............................. 57 4.4.1 The Angle-Based Properties of Polygons and MBRs ......... 58 4.4.2 The Sweep Searching Algorithm . ................. 59 4.4.3 The Ripple Searching Algorithm . ................. 66 4.4.4 NS Search Based on Multiple ANS Searches ............. 69 4.5 Performance Evaluation ............................. 71 v 4.5.1 Performance of NS Searching Algorithms . ............. 72 4.5.2 Effectiveness of Optimization Techniques . ............. 78 4.5.3 Improvement Gained by ANS Queries ................. 79 4.6 Chapter Summary . ............................. 80 Chapter 5 Skyline Queries 81 5.1 Analysis of Skyline Search Problems . ..................... 82 5.2 Existing Skyline Search Approaches . ..................... 86 5.2.1 Skyline Query Processing . ..................... 86 5.2.2 Skyline Query Variants ......................... 88 5.3 Skyline Search Based on Z-Order Curve . ................. 92 5.3.1 Dominance Relationships and Z-Order Curve ............. 92 5.3.2 ZBtree Index Structure ......................... 95 5.3.3 ZBtree Index Manipulation . ..................... 96 5.4 Skyline Search on Z-order Curve . ..................... 98 5.4.1 RZ-Region Based Dominance Test . ................. 98 5.4.2 The ZSearch Algorithm ......................... 101 5.5 Skyline Query Variants ............................. 103 5.5.1 Extended ZBtrees . ......................... 103 5.5.2 The ZBand Algorithm ......................... 105 5.5.3 The ZRank Algorithm ......................... 108 5.5.4 The k-ZSearch Algorithm . ..................... 114 5.5.5 The ZSubsapce Algorithm . ..................... 117 5.6 Performance Evaluation of Skyline Query Processing ............. 120 5.6.1 Experiment Settings . ......................... 121 5.6.2 Experiments on Skyline Queries . ................. 123 5.6.3 Experiments on Skyband Queries . ................. 126 5.6.4 Experiments on Top-Ranked Skyline Queries ............. 127 5.6.5 Experiments on k-Dominant Skyline Queries ............. 129 5.6.6 Experiments on Subspace Skyline Queries . ............. 130 5.7 Chapter Summary . ............................. 132 Chapter 6 LDSQs on Spatial Road Network 134 6.1 State-of-the-Art Approaches . ......................... 134 6.1.1 Network Expansion Based Approaches . ............. 135 6.1.2 Euclidean Distance Based Approaches ................. 135 6.1.3 Solution Based Approaches . ..................... 135 6.2 The ROAD Framework ............................. 136 vi 6.2.1 Basic Idea . ............................. 137 6.2.2 Rnets, Shortcuts and Object Abstracts ................. 137 6.2.3 Rnet Hierarchy ............................. 139 6.2.3.1 Network Partitioning Methods . ............. 140 6.2.3.2 Create of Shortcuts and Object Abstracts . ......... 141 6.2.4 Route Overlay and Association Directory . ............. 143 6.3 Processing LDSQs on ROAD . ......................... 145 6.4 ROAD Framework Maintenance . ..................... 149 6.4.1 Object Update . ............................
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