1,721,071 research outputs found
A MAC layer traffic-priority management technique in CDMA based ad-hoc networks
No full textRecently, ad-hoc networks have obtained a growing interest due to their advantages in many practical applications. One of the most critical points is the definition of an efficient medium access control (MAC) protocol that allows the transmission of packets generated by a node and routing of packets arriving from other nodes. Moreover the growth of several applications with different requirements in terms of QoS has raised the problem to manage their priority also at the MAC layer. This paper deals with a traffic priority management technique at the MAC layer based on the code division multiple access (CDMA) scheme, where an adaptation of the used spreading factor to the network congestion is foreseen in order to minimize the energy consumption and maximize the network throughpu
DiffServ on-board satellite switching based on cellular neural networks
No full textIn modern satellite communication systems, the quality of service (QoS) management has became a crucial topic due to the increasing interest in multimedia traffic. The actual trends consider the satellite networks as an integrated part of the terrestrial data networks. In IP networks, the differentiated service (DiffServ) approach seems to be the best that satisfies the QoS constraints, due to its end-to-end philosophy. Actual trend considers the satellite on-board switching capabilities for managing multibeam inputs and outputs. In particular this paper deals with the proposal of a new cellular neural network (CNN) for the on-board switching problem to reduce the computational complexity; several traffic classes, according to the DiffServ approach, have been considered and the switch takes into account their priority, queue length and time spent inside queues. Numerical results have shown that the performance is similar to the optimal switching solution of the flexible cellular neural network. Simulation results have been driven with a memoryless distribution and heavy-tailed distribution for several input buffer size and switch dimension
Adaptive Subcarrier Allocation Algorithms in Wireless OFDMA Systems
No full textWiMAX is one of the most important technologies for providing a Broadband Wireless Access (BWA) in a metropolitan area. The use of OFDM transmission has been proposed to reduce the effect of multipath fading in wireless communications; moreover, multiple access is achieved by resorting to the use of OFDMA. Adaptive subcarrier allocation techniques have been selected to exploit the multiuser diversity, leading to an improvement of performances by assigning subchannels to the users accordingly with their channel responses. In this paper, three adaptive subcarrier allocation algorithms have been proposed; they are based on channel capacity and aim to assign an equal amount of capacity to all the users or to distribute the capacity proportionally to the users' channel conditions. The algorithms performances have been valued and compared in terms of bit error rate and throughput. The proposed strategies show a substantial improvement with respect to a static allocation, turning the multiuser diversity into a significant increase of data rate
A Neural Network Approach For Blind Multiuser Detection in DS-CDMA Communication Systems
No full textIn order to enhance data rate for multimedia services, new advanced receivers for next generation mobile com-munications are developed. Adaptive blind multiuser detection has been widely proposed for applications in CDMA (Code Division Multiple Access) wireless com-munication systems for its principal advantage of elim-inating training sequence to set-up receiver filter coeffi-cients. Main drawback of this technique is that it reaches the optimum behavior after a certain number of bit times, which precludes its use in typical time-varying environ-ments. In this paper, a new neural network approach is proposed in order to solve this drawback. In particular, this paper considers the use of a modified Kennedy-Chua neural network, based on the Hopfield model. Simulation results are given to demonstrate the effectiveness of the proposed approach in different time-varying application scenarios
Efficient Adaptive Modulation and Coding Techniques for WiMAX Systems
No full textThe demand for high-speed services and multimedia applications in wider wireless environments leads to the growth of wireless communications, due to the simplicity of installation and the reduction of costs with respect to traditional cabled links. In particular, WiMAX technology is considered one of the most prominent solutions capable to provide a Broadband Wireless Access in metropolitan areas. In this paper two schemes of adaptive modulation and coding are proposed with the aim of improving performances in Non Line-of-Sight communications, typical of urban environments. Through these techniques it is possible to switch the order of the modulation and the coding rate to better match the channel conditions, obtaining comforting results in terms of probability of error and throughput. The system has been modeled with a finite state structure in which every state consists in a possible scheme of transmission (i.e. a specific modulation and coding rate), and the switch among different states happens when multiple thresholds on channel attenuation are reached. The adaptation is realized at the physical level of the transmission for a WiMAX OFDMA structure. The two proposed techniques are suitable for different kind of traffic and, therefore, can be used for respecting different QoS requirements
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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