The recent IEEE 802.16j-2009 adds multi-hop relay capabilities to IEEE 802.16 systems, which aims to offer improved coverage and capacity over single-hop radio access systems. In this paper, we point out several security issues, including non-authenticated privacy and key management messages, insecure relay multicast rekeying algorithm and insecure Multicast and Broadcast Rekeying Algorithm (MBRA), of the key management protocols in IEEE 802.16j-2009 and give some solutions. In particular, we propose a new Secure MBRA (SMBRA) based on identity and logical key tree to solve the security issues of MBRA. SMBRA can not only provide backward and forward secrecy of communications but also avoid key forgery. Furthermore, our theoretical analysis and simulation indicate that SMBRA is much more efficient and adequate, especially in a large group.

Visibility culling techniques have been studied extensively in computer graphics for interactive walkthrough applications in recent years. In this paper, a visibility culling approach by exploiting hardware-accelerated occlusion query is proposed. Organizing the regular grid representation of input scene as an octree-like hierarchy, a 2-tier view frustum culling algorithm is to efficiently cull away nodes invisible from a given viewpoint. Employing the eye-siding number of nodes, we can quickly enumerate an occlusion front-to-back order and effectively maximize the number of parallelizable occlusion queries for nodes while traversing the hierarchy. As experimental results show, our approach improves the overall performance in the test walkthrough.

The development of information/communication technology has made it possible to access substantial amounts of data and retrieve information. However, it is often difficult to locate the desired information, and it becomes necessary to spend considerable time determining how to access specific available data. This paper describes a method to quantitatively evaluate the usability of large-scale information-oriented websites and the effects of improvements made to the site design. This is achieved by utilizing the Cognitive Walkthrough for the Web and website modeling using Markov chains. We further demonstrate that we can greatly improve usability through simple modification of the link structure by applying our approach to an actual informational database website with over 40,000 records.