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Communication Links (communication + link)
Selected AbstractsLarge-scale site diversity for satellite communication networksINTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 4 2002M. Luglio Abstract The utilization of high frequencies, such as Ka-band and beyond, necessary to avoid the highly congested lower satellite frequencies and to get larger bandwidth availability is considered for many developing satellite systems. The new satellite low-margin systems in Ka-band will need to be designed using fade countermeasures to counteract rain attenuation. One of these techniques foresees the possibility of switching the communication link among different Earth stations spread on a very large territory to reduce the system outage time to the joint outage time of all the stations. The design of such systems depends on the probability that the Earth stations simultaneously exceed their margins. In this paper, a well-assessed model is utilized for the prediction of joint statistics of rain attenuation in multiple locations, using Monte Carlo simulation. The model is based on a pair of multi-variate normal processes whose parameters are related to those characterizing the single-location statistics and whose covariance matrices are assumed to depend only on the distances between locations. The main results concerning both the probability and margin improvement will be presented and discussed. Copyright © 2002 John Wiley & Sons, Ltd. [source] Autonomous capture of a tumbling satelliteJOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 4 2007Ioannis Rekleitis In this paper, we describe a framework for the autonomous capture and servicing of satellites. The work is based on laboratory experiments that illustrate the autonomy and remote-operation aspects. The satellite-capture problem is representative of most on-orbit robotic manipulation tasks where the environment is known and structured, but it is dynamic since the satellite to be captured is in free flight. Bandwidth limitations and communication dropouts dominate the quality of the communication link. The satellite-servicing scenario is implemented on a robotic test-bed in laboratory settings. The communication aspects were validated in transatlantic tests. © 2007 Canadian Space Agency [source] Architecture-based semantic evolution of embedded remotely controlled systemsJOURNAL OF SOFTWARE MAINTENANCE AND EVOLUTION: RESEARCH AND PRACTICE, Issue 3 2003Lawrence Chung Abstract Evolution of a software system is a natural process. In most systems, evolution takes place during the maintenance phase of their life cycles. Those systems that have reached their limit in evolution have usually reached their end of useful life and may have to be replaced. However, there are systems in which evolution occurs during the operational phase of their life cycles. Such systems are designed to evolve while in use or, in other words, be adaptable. Semantically adaptable systems are of particular interest to industry as such systems often times adapt themselves to environment change with little or no intervention from their developing or maintaining organization. Since embedded systems usually have a restricted hardware configuration, it is difficult to apply the techniques developed for non-embedded systems directly to embedded systems. This paper focuses on evolution through adaptation and develops the concepts and techniques for semantic evolution in embedded systems. As the first step in the development of a software solution, architectures of software systems themselves have to be made semantically evolvable. In this paper we explore various architectural alternatives for the semantic evolution of embedded systems,these architectures are based on four different techniques that we have identified for semantic evolution in embedded systems. The development of these architectures follows the systematic process provided by the non-functional requirement (NFR) framework, which also permits the architectures to be rated in terms of their evolvability. As the field of embedded systems is vast, this paper concentrates on those embedded systems that can be remotely controlled. In this application domain the embedded system is connected to an external controller by a communication link such as ethernet, serial, radio frequency, etc., and receives commands from and sends responses to the external controller via the communication link. The architectures developed in this paper have been partly validated by applying them in a real embedded system,a test instrument used for testing cell phones. These architectures and techniques for semantic evolution in this application domain give a glimpse of what can be done in achieving semantic evolution in software-implemented systems. Copyright © 2003 John Wiley & Sons, Ltd. [source] Parallel heterogeneous CBIR system for efficient hyperspectral image retrieval using spectral mixture analysisCONCURRENCY AND COMPUTATION: PRACTICE & EXPERIENCE, Issue 9 2010Antonio J. Plaza Abstract The purpose of content-based image retrieval (CBIR) is to retrieve, from real data stored in a database, information that is relevant to a query. In remote sensing applications, the wealth of spectral information provided by latest-generation (hyperspectral) instruments has quickly introduced the need for parallel CBIR systems able to effectively retrieve features of interest from ever-growing data archives. To address this need, this paper develops a new parallel CBIR system that has been specifically designed to be run on heterogeneous networks of computers (HNOCs). These platforms have soon become a standard computing architecture in remote sensing missions due to the distributed nature of data repositories. The proposed heterogeneous system first extracts an image feature vector able to characterize image content with sub-pixel precision using spectral mixture analysis concepts, and then uses the obtained feature as a search reference. The system is validated using a complex hyperspectral image database, and implemented on several networks of workstations and a Beowulf cluster at NASA's Goddard Space Flight Center. Our experimental results indicate that the proposed parallel system can efficiently retrieve hyperspectral images from complex image databases by efficiently adapting to the underlying parallel platform on which it is run, regardless of the heterogeneity in the compute nodes and communication links that form such parallel platform. Copyright © 2009 John Wiley & Sons, Ltd. [source] Provision of oncology services in remote rural areas: a Scottish perspectiveEUROPEAN JOURNAL OF CANCER CARE, Issue 2 2004S.M. SMITH research assistant There is a paucity of research into rural health care services. In particular little is known about the provision of specialist cancer services for patients who live in remote rural areas of the UK. This study set out to investigate current models of medical and clinical oncology care in Scotland. A national survey with key health professionals was conducted to identify rural oncology schemes currently in operation. Detailed quantitative data about the schemes together with qualitative data on how health professionals view current models of care were collected by a computer-assisted telephone survey. Schemes that currently provide outpatient and chemotherapy oncology services for remote rural patients fell into three categories: central clinics (5); shared care outreach clinics with chemotherapy provision (11); and shared care outreach clinics without chemotherapy provision (7). All radiotherapy was conducted at central clinics (5). Widely varying practices in delivery of cancer care were found across the country. The main issues for professionals about current models of care involved expertise, travelling and accessibility (for patients), communication and expansion of the rural service. Nation-wide consistency in cancer care has still to be achieved. Travelling for treatment was seen to take its toll on all patients but particularly for the very remote, elderly and poor. Most professionals believe that an expansion of rural services would be of benefit to these patients. It is clear, however, that the proper infrastructure needs to be in place in terms of local expertise, ensured quality of care, and good communication links with cancer centres before this could happen. [source] Multiple spot diffusing geometries for indoor optical wireless communication systemsINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 10 2003A. G. Al-Ghamdi Abstract In order to improve the performance of indoor optical wireless communication links, two multispot diffusing geometries based on diamond and line strip spot distribution geometries are proposed, analysed and compared to the known uniform spot distribution. Such geometries combine the advantages of the diffuse and the line-of-sight systems, giving great robustness and ease of use. The novel line strip multibeam transmitter geometry has resulted in a receiver signal-to-noise ratio (SNR) improvement of about 4 dB compared to the conventional diffuse system as well as a significant reduction in the pulse spread. Simulation and comparison results for both the conventional diffuse system and the three multispot diffusing geometries are presented. Further, pulse responses, SNR, and delay spread results at various locations are presented. Copyright © 2003 John Wiley & Sons, Ltd. [source] Video compression for multicast environments using spatial scalability and simulcast codingINTERNATIONAL JOURNAL OF IMAGING SYSTEMS AND TECHNOLOGY, Issue 6 2003Wade K. Wan Abstract A common problem with many video transmission applications is the wide range of available bandwidths between the server and different clients. These environments require efficient multicast video service, the capability to transmit and receive the same video sequence at different resolutions. Two approaches to achieve multicast service are scalable coding (dependent bitstream coding) and simulcast coding (independent bitstream coding). One would expect scalable coding to have higher coding efficiency because a scalable coded bitstream can exploit similar information in another bitstream. This reasoning would suggest that multicast implementations should only use scalable coding for maximum coding efficiency. However, this article shows results where simulcast coding has been found to outperform spatial scalability (one type of scalable coding). In this article, methods are described to select between simulcast coding and spatial scalability for multicast video transmission. These techniques can be used to determine the proper multicast coding approach for providing service to clients with different communication links. The methodology described can also be used to construct decision regions to guide more general scenarios or adaptively switch between the two coding approaches. A number of important results were obtained that may be directly applicable to commercial multicast systems. © 2004 Wiley Periodicals, Inc. Int J Imaging Syst Technol 13, 331,340, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ima.10065 [source] A tutorial on using genetic algorithms for the design of network topologyINTERNATIONAL JOURNAL OF NETWORK MANAGEMENT, Issue 4 2006Bassam Al-Bassam The design of network topology is an important part of network design, since network topology is directly associated with network operational behavior, capacity, reliability, and cost. This paper is a tutorial paper concerned with illustrating how the optimization capabilities of genetic algorithms can be used to design suitable network topologies considering basic topology problems. Simple genetic algorithms have been developed for the topology problem of mesh networks, considering single node and single link failure tolerance. The algorithms are based on criteria of two important measures: minimizing the length of communication links; and minimizing traffic flow through these links for given traffic loads. The first measure contributes to minimizing the cost of cabling, while the second measure contributes to minimizing the cost of link capacity. The work provides a useful approach and tools to network students and professionals concerned with the topology design of backbone networks. The developed software is made available on the Internet.,Copyright © 2006 John Wiley & Sons, Ltd. [source] Prediction of triple-orbital diversity performance in Earth-space communicationINTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 3 2002A. D. Panagopoulos Abstract Orbital diversity is considered to be an effective technique to overcome large fade margins in satellite communication links. This paper discusses triple-orbital diversity, which uses three satellites and an Earth receiving site. A method for calculating the outage probability of a triple-orbital diversity protection scheme is proposed. It is based on a model for convective raincells and the lognormal assumption for point rainfall rate statistics. Numerical results are compared with an available set of experimental data taken from a VSAT Earth-station located in Japan. The agreement was found to be quite encouraging. Some useful conclusions, concerning the relative advantage of using triple- against the double-orbital diversity scheme are also deduced. Copyright © 2002 John Wiley & Sons, Ltd. [source] Planar inverted-F antennas on implantable medical devices: Meandered type versus spiral typeMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 3 2006Jaehoon Kim Abstract Based on a planar inverted-F antenna (PIFA) configuration, two different shaped (meandered and spiral) antennas are designed using finite-difference time-domain (FDTD) simulations and measurement results. The antennas are installed on an implantable medical device in a biological-tissue simulating model. The impedance-matching and radiation characteristics of two PIFAs are compared in order to observe which shape is more appropriate for wireless communication links of implantable medical devices. Additionally, the effects of the human skin's thickness on the antennas are studied in order to consider medical devices which are implanted in various subcutaneous tissues. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 567,572, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21409 [source] Optimization of Internet Protocol network design and routingNETWORKS: AN INTERNATIONAL JOURNAL, Issue 1 2004Kaj Holmberg Abstract We consider network design and routing for Internet Protocol (IP) traffic. The design problem concerns capacity dimensioning of communication links, where the design cost consists of fixed charges and linear capacity expansion costs. The optimization problem also concerns determining the amount of traffic demand to be carried by the network and the metric used by a shortest path routing protocol. We present a novel linear mixed-integer mathematical formulation and two heuristic solution procedures. The first heuristic uses mixed-integer programming to generate a sequence of routing solutions. The second solution approach is a simulated annealing meta heuristic. Computational experiments for synthesized and real-life networks show that high-quality solutions can be obtained by both approaches. © 2003 Wiley Periodicals, Inc. [source] Minimum linear gossip graphs and maximal linear (,, k)-gossip graphs ,NETWORKS: AN INTERNATIONAL JOURNAL, Issue 3 2001Pierre Fraigniaud Abstract Gossiping is an information dissemination problem in which each node of a communication network has a unique piece of information that must be transmitted to all other nodes using two-way communications between pairs of nodes along the communication links of the network. In this paper, we study gossiping using a linear-cost model of communication which includes a start-up time and a propagation time which is proportional to the amount of information transmitted. A minimum linear gossip graph is a graph (modeling a network), with the minimum possible number of links, in which gossiping can be completed in minimum time under the linear-cost model. For networks with an even number of nodes, we prove that the structure of minimum linear gossip graphs is independent of the relative values of the start-up and unit propagation times. We prove that this is not true when the number of nodes is odd. We present four infinite families of minimum linear gossip graphs. We also present minimum linear gossip graphs for all even numbers of nodes n , 32 except n = 22. A linear (,, k)- gossip graph is a graph with maximum degree , in which gossiping can be completed in k rounds with minimum propagation time. We present three infinite families of maximal linear (,, k)- gossip graphs, that is, linear (,, k)-gossip graphs with a maximum number of nodes. We show that not all minimum broadcast graphs are maximal linear (,, k)-gossip graphs. © 2001 John Wiley & Sons, Inc. [source] | ||||||||||||||||