Hong Va Leong

Nearest Surrounder Queries

In this paper, we present a new type of spatial queries called Nearest Surrounder (NS) queries. An NS query determines the nearest polygon-shaped spatial objects (referred to as nearest surrounder objects) and their orientations with respect to a query point from an object set. Besides, we derive two NS query variants, namely, multitier NS (m-NS) queries and angle-constrained NS (ANS) queries. An m-NS query searches multiple layers of NS objects for the same range of angles from a query point. An ANS query searches for NS objects within a specified range of angles. To evaluate NS queries and their variants, we explore angle-based and distance-based bound properties of polygons, and devise two efficient algorithms, namely, Sweep and Ripple, based on R-tree. The algorithms access objects in an order according to their orientations and distances with respect to a given query point, respectively. They are efficient as they can finish a search with one index lookup. Besides, they can progressively deliver a query result. Through empirical studies, we evaluate the proposed algorithms and report their performance for both synthetic and real object sets.

Tracking Nearest Surrounders in Moving Object Environments

This paper presents a system framework to support continuous nearest surrounder (NS) queries in moving object environments. NS query finds the nearest objects at individual distinct angles from a query point. This query distinguishes itself from other conventional spatial queries such as range queries and nearest neighbor queries by considering both distance and angular aspects of objects with respect to a query point. One of NS query applications is to monitor the nearest objects around an observation point. In our framework, a centralized server is dedicated to collect object location updates, to determine affected NS queries of each object location update, to compute the incremental result change of affected queries and to deliver result updates to corresponding interested users/applications that initiate the queries. In particular, we propose algorithms namely, safe region formation and partial query evaluation, that can significantly improve the system performance. Through simulations, we validate our proposed algorithms over a wide range of settings

Efficient valid scope computation for location-dependent spatial queries in mobile and wireless environments

In mobile and wireless environments, mobile clients can access information with respect to their locations by submitting Location-Dependent Spatial Queries (LDSQs) to Location-Based Service (LBS) servers. Owing to scarce wireless channel bandwidth and limited client battery life, frequent LDSQ submission from clients must be avoided. Observing that LDSQs issued from similar client positions would normally return the same results, we explore the idea of valid scope, that represents a spatial area in which a set of LDSQs will retrieve exactly the same query results. With a valid scope derived and an LDSQ result cached at the client side, a client can assert whether the new LDSQs can be answered with the maintained LDSQ result, thus eliminating the LDSQs sent to the server. As such, contention on wireless channel and client energy consumed for data transmission can be considerably reduced. In this paper, we design efficient algorithms to compute the valid scope for common types of LDSQs, including nearest neighbor queries and range queries. Through an extensive set of experiments, our proposed valid scope computation algorithms are shown to significantly outperform existing approaches.

Privacy enhancement in mobile data broadcasting environments

In mobile and wireless environments, clients will request for information by submitting queries to the server, which delivers the required data via point-to-point connection or broadcast channels. The broadcast paradigm possesses the advantage of scalability. However, the broadcast data is vulnerable to eavesdropping and the adversary may be able to associate the requested data items and hence the original query with a specific client, a form of privacy threat that we would like to protect against. In this paper, we propose to protect the privacy of queries generated by individual clients by allowing them to hide themselves behind other clients. We assume a possibly untrustworthy server and make use of a trustworthy anonymizer for the queries. At the query anonymizer, user queries are clustered and consolidated into subsuming queries, to obfuscate the queries from being recovered. The consolidated queries are then expanded to improve the obfuscation effect, making it harder for the adversary to deduce the original queries. We define various performance metrics on the query privacy, by studying the ability that the adversary could crack the client queries, and propose algorithms to obfuscate a collection of user queries.

Mobile Data Access

Next generation wireless networks are going to use IP capable mobile terminals and support both voice/multimedia and data services. VoIP is a promising technology allowing a converged core network for all services, and provides a service platform for easier and quicker service creation. Application of VoIP in wireless networks, however, is not so straightforward as people may expect. Among other issues, mobility support is perhaps the most challenging one. The different levels of mobility support (terminal, personal and service mobility) highlight the gap between the existing VoIP framework, which is so far focusing on fixed networks, and the requirements in a wireless environment. After analyzing the existing solutions/proposals for this problem, a framework is proposed in which the mobility-related services are decomposed into several levels so that mobility support is made available to many applications. This layered approach would help to lead to a mobility enabled wireless IP infrastructure in which voice service is just an application.