In Web service environment, the interacting entities usually cannot be predetermined and may be in different security domains. To address the access authorization for unknown users across domain borders, access control of Web service should be implemented based on domain-independent access control information but not the identities. A context-based access control policy model which can be appropriate for Web service environment was proposed. The main idea of the model was that, various access control information was abstracted and represented as a concept of context which was adopted as the center to define and perform access control policies. The context concept here acted as an intermediary between requesters and the access permissions, which was similar to the role of Role-Based Access Control (RBAC) in a way. Context-based access control policy axioms were defined based on Description Logic (DL), on the basis of these axioms, the access control policy knowledge base with the capacity of reasoning about the access control policies was put forward. Finally, the effect of access control policy enforcement was verified in Racer reasoning system, and the experiment result proved the feasibility and validity of the presented method.
Restricted Velocity Particle Swarm Optimization (RVPSO) and Self-Adaptive Velocity Particle Swarm Optimization (SAVPSO) are two recently proposed Particle Swarm Optimization (PSO) algorithms specially for solving Constrained Optimization Problem (COP), but to our knowledge, no research has been done on the applications of the two algorithms to Unconstrained Optimizations Problem (UOP). To this end, the effectiveness and performance characteristics of the two algorithms in UOP were investigated. Moreover, in view of their relatively strong conservativeness, the algorithms were improved by combining chaos factor and random strategy respectively with the search mechanism to enhance their global exploration ability. Also, the effects of different parameter settings on the performance of all these algorithms were studied. The performance of all these algorithms was evaluated on 5 typical benchmark functions. Experimental and comparison results show that the improved RVPSO is better than RVPSO in terms of robustness and global exploration ability, but it may easily get trapped into local optima when solving high-dimensional multi-modal functions; the improved SAVPSO has stronger exploration ability and faster convergence rate than improved RVPSO, and it can achieve more accurate solutions when applied to high-dimensional multi-modal functions. Therefore, the improved SAVPSO has competitive ability of global optimization, and thus is an effective algorithm for solving unconstrained optimization problems.
Dynamic Contrast-Enhanced Magnetic Resonance Imaging (DCE-MRI) demonstrates that malignant tumors generally show faster and higher levels of enhancement than they are seen in benign or normal tissue, after an intravenous injection of the contrast agent Gd-DTPA, DCE-MRI has played important roles in diagnosis and detecting malignant tumor. However, it is still a challenge on the fast reconstruction of DCE-MR images. Based on the idea of group sparse and the theory of Compressed Sensing (CS), a conjugate gradient algorithm combined with variable density random sampling method was employed to get samples from the local k-spaces (Fourier coefficient) sampling data. Then traditional l1 norm conjugate gradient descent algorithm was extended to l2,1 norm to jointly reconstruct multiple DCE-MR images simultaneously. Compared with conventional Multi-Measurement Vector (MMV) algorithm, the proposed approach yields a faster and more accurate reconstruction result. The experimental results show that when the sampling rate is less than 40%, the joint reconstruction time based on conjugate gradient algorithm almost decreased by 30% compared with the MMV algorithm. In addition, compared with the uniform random sampling, the variable density random sampling method improves the accuracy rate about 70%.