Tetsuji Satoh

An anonymous communication technique using dummies for location-based services

Recently, highly accurate positioning devices enable us to provide various types of location-based services. On the other hand, because such position data include deeply personal information, the protection of location privacy is one of the most significant problems in location-based services. In this paper, we propose an anonymous communication technique to protect the location privacy of the users of location-based services. In our proposed technique, such users generate several false position data (dummies) to send to service providers with the true position data of users. Because service providers cannot distinguish the true position data, user location privacy is protected. We also describe a cost reduction technique for communication between a client and a server. Moreover, we conducted performance study experiments on our proposed technique using practical position data. As a result of the experiments, we observed that our proposed technique protects the location privacy of people and can sufficiently reduce communication costs so that our communication techniques can be applied in practical location-based services.

Protection of Location Privacy using Dummies for Location-based Services

Recently, highly accurate positioning devices enable us to provide various types of location-based services. On the other hand, because position data obtained by such devices include deeply personal information, protection of location privacy is one of the most significant issues of location-based services. Therefore, we propose a technique to anonymize position data. In our proposed technique, the psrsonal user of a location-based service generates several false position data (dummies) sent to the service provider with the true position data of the user. Because the service provider cannot distinguish the true position data, the user’s location privacy is protected. We conducted performance study experiments on our proposed technique using practical trajectory data. As a result of the experiments, we observed that our proposed technique protects the location privacy of users.

Shape-Based Similarity Query for Trajectory of Mobile Objects

In this paper, we describe an efficient indexing method for a shape-based similarity search of the trajectory of dynamically changing locations of people and mobile objects. In order to manage trajectories in database systems, we define a data model of trajectories as directed lines in a space, and the similarity between trajectories is defined as the Euclidean distance between directed discrete lines. Our proposed similarity query can be used to find interested patterns embedded into the trajectories, for example, the trajectories of mobile cars in a city may include patterns for expecting traffic jams. Furthermore, we propose an efficient indexing method to retrieve similar trajectories for a query by combining a spatial indexing technique (R+-Tree) and a dimension reduction technique, which is called PAA (Piecewise Approximate Aggregate). The indexing method can efficiently retrieve trajectories whose shape in a space is similar to the shape of a candidate trajectory from the database.

Clustering Multidimensional Trajectories based on Shape and Velocity

Recently, the analysis of moving objects has become one of the most important technologies to be used in various applications such as GIS, navigation systems, and locationbased information systems, Existing geographic analysis approaches are based on points where each object is located at a certain time. These techniques can extract interesting motion patterns from each moving object, but they can not extract relative motion patterns from many moving objects. Therefore, to retrieve moving objects with a similar trajectory shape to another given moving object, we propose queries based on the similarity between the shapes of moving object trajectories. Our proposed technique can find trajectories whose shape is similar to a certain given trajectory. We define the shape-based similarity query trajectories as an extension of similarity queries for time series data, and then we propose a new clustering technique based on similarity by combining both velocities of moving objects and shapes of objects. Moreover, we show the effectiveness of our proposed clustering method through a performance study using moving rickshaw data.

Rinda: a relational database processor with hardware specialized for searching and sorting

Rinda, a database processor that accelerates nonindexed relational database queries, is described. Rinda is composed of content search processors and relational operational accelerating processors; the former, search rows stored in disk storage, and the latter sort rows stored in the main memory. The processors connect to a general-purpose host computer with channel interfaces. Performance results show that Rinda executes queries three to 100 times faster then conventional database-management system software in a benchmark system.<<ETX>>

An index structure for parallel database processing

In relational database management systems, indexing is a popular and powerful technique to accelerate database processing. There are two main research issues on indexing; index structures and access controls. The studies of index structures, which are usually trees or hashing, aim at lower access cost and smaller storage overhead. On the other hand, the studies of access controls focus on concurrency of transactions accessing the same index. The root node of a tree-structured index, is a hot hot spot accessed by several transactions concurrently. If index keys are inserted or deleted frequently, many conflicts occur and the performance of the system is degraded. This problem is serious in multiprocessor environments where several processors access the same database in parallel. Similar problems occur in intra-transaction parallelism. The response time of a transaction cannot be reduced enough if parallel portions of the transaction conflict with each other while updating an index. A new index structure and its access control are discussed to achieve inter- and intra-transaction parallelism on shared memory multiprocessors. They dominate the utilization of processors in the systems.<<ETX>>

A multiversion concurrency control algorithm for concurrent execution of partial update and bulk retrieval transactions

A novel multiversion concurrency control algorithm is presented which substantially improves concurrency between batch and online transactions. A bulk retrieval transaction which reads all data objects and partial update transactions which read and write a few data objects are studied as batch and online transactions, respectively. The algorithm is based on a mixture of multiversion time-stamp ordering and modified two-version two-phase locking. It can remove the interference between the retrieval and the update transactions, and reduce the interference between the update transactions themselves. Results of a simulation study show that the completion ratio of those transactions with this algorithm is much better than that with existing algorithms, when the number of transactions is large and the data contention between those transactions is high.<<ETX>>

Load balancing algorithms for parallel database processing on shared memory multiprocessors

This paper describes new load balancing algorithms for parallel database processing on shared memory multiprocessors. The goal of load balancing is to reduce overhead as well as load imbalance, but there is a tradeoff between them in ordinary algorithms. Unfortunately, optimum performance can hardly be obtained using ordinary algorithms because their performances depend on several factors such as database size, the number of processors and data distribution. The proposed algorithms solve these problems by varying the number of tasks allocated at a time ( which was fixed in ordinary algorithms ) according to the number of remaining tasks and the maximum and minimum processing times of a task. Performance evaluations show that the proposed algorithms achieve fair load balancing with lower overhead independent of the above factors.<<ETX>>

Extendible hashing for concurrent insertions and retrievals

Proposes an improved extendible hashing and bucket multi-versioning method, achieving a higher concurrency. In our improved extendible hashing, the global depth and directory entries are asynchronously modified to reduce lock conflicts on the directory. Furthermore, bucket multi-versioning enables read-only access to a bucket which is being split. Simulation studies show that these two methods provide speedup in proportion to the number of processors and enable concurrent insertions and retrievals to be performed without either one affecting the other.

Obtaining Factors Describing Impression of Questions and Answers and Estimation of Their Scores from Feature Values of Statements

First, this paper evaluates the impression of questions and answers at Questions and Answers (Q & A) sites in order to avoid the problem of mismatch between the questioner and the respondent. Fifty impression words effective in evaluating impressive expression of statements are selected from a dictionary. An impressive evaluation experiment is then conducted for sixty questions and answers posted at Yahoo! Chiebukuro by using those impression words. Nine factors are obtained by applying factor analysis to the scores obtained through the experiment. Then factor scores of any other statements are tried to be estimated by using multiple regression analysis. This result, however, shows that the estimation accuracy is insufficient. To improve the estimation accuracy, the multiple regression analysis considering quadratic terms is applied. The result of the analysis shows that the estimation accuracy can be improved.