In applications like social networking services and online games, multiple moving users which form a group may wish to be continuously notified about the best meeting point from their locations. A promising technique for reducing the communication frequency of the application server is to employ safe regions, which capture the validity of query results with respect to the users' locations. Unfortunately, the safe regions in our problem exhibit characteristics such as irregular shapes and inter-dependencies, which render existing methods that compute a single safe region inapplicable to our problem. To tackle these challenges, we first examine the shapes of safe regions in our problem's context and propose feasible approximations for them. We design efficient algorithms for computing these safe regions. We also study a variant of the problem called the sum-optimal meeting point and extend our solutions to solve this variant. Experiments with both real and synthetic data demonstrate the effectiveness of our proposal in terms of computational and communication costs.

Online driving direction services offer fundamental functionality to mobile users, and such services see substantial and increasing loads as mobile access continues to proliferate. Cache servers can be deployed in order to reduce the resulting network traffic. We define so-called concise shortest paths that are equivalent to driving instructions. A concise shortest path occupies much less space than a shortest path; yet it provides sufficient navigation information to mobile users. Then we propose techniques that enable the caching of concise shortest paths in order to improve the cache hit ratio.

Web search is ubiquitous in our daily lives. Caching has been extensively used to reduce the computation time of the search engine and reduce the network traffic beyond a proxy server. Another form of web search, known as online shortest path search, is popular due to advances in geo-positioning. However, existing caching techniques are ineffective for shortest path queries. This is due to several crucial differences between web search results and shortest path results, in relation to query matching, cache item overlapping, and query cost variation.

A privacy-aware proximity detection service determines if two mobile users are close to each other without requiring them to disclose their exact locations. Existing proposals for such services provide weak privacy, give low accuracy guarantees, incur high communication costs, or lack flexibility in user preferences. We address these shortcomings with a client-server solution for proximity detection, based on encrypted, multi-level partitions of the spatial domain. Our service notifies a user if any friend users enter the user's specified area of interest, called the vicinity region. This region, in contrast to related work, can be of any shape and can be flexibly changed on the fly. Encryption and blind evaluation on the server ensures strong privacy, while low communication costs are achieved by an adaptive location-update policy. Experimental results show that the flexible functionality of the proposed solution is provided with low communication cost.