1,720,987 research outputs found
Cooperation and coordination in cognitive networks with packet retransmission
No full textThe cognitive framework, which fosters an efficient use of the channel resource by granting access to smart and adaptive terminals, has been recently integrated with the cooperative paradigm. Accordingly, following a spectrum-leasing approach, licensed (primary) users may let unlicensed (secondary) users access the owned bandwidth in exchange for an increased performance via cooperation. Nevertheless, assuming that secondary transmission is limited to idle primary slots, as the traffic from the primary sources increases, implementation of spectrum leasing via cooperation becomes more challenging. In fact, the opportunities for secondary transmission reduce and the secondary users may find spectrum leasing not advantageous. In this paper, a solution is proposed that exploits fading temporal correlation and retransmission-based error control in order to increase secondary throughput even in high primary traffic conditions. Focusing on a simple network with a primary and a secondary source transmitting to a common destination, it is shown that the primary source may actively leave part of its channel resource to the secondary source without significant performance loss while greatly improving secondary throughput
An evaluation of the hybrid sparse/diffuse algorithm for underwater acoustic channel estimation
No full textThe underwater acoustic channel has been usually modeled as sparse. However, in some scenarios of interest, e.g., shallow water environments due to the interaction with the surface and the seabed, the channel exhibits also a dense arrival of multipath components. In these cases, a Hybrid Sparse/Diffuse (HSD) channel representation, rather than a purely sparse one, may be more appropriate. In this work, we present the HSD channel model and channel estimators based on it. We evaluate these estimation strategies on the SPACE08 experimental data set. We show that the HSD estimators outperform the more conventional purely sparse and least squares estimators. Moreover, we show that an exponential Power Delay Profile (PDP) for the diffuse component is appropriate in scenarios where the receiver is far away from the transmitter. Finally, the HSD estimators and the exponential PDP model are shown to be robust even in scenarios where the channel does not exhibit a diffuse component
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
On Optimal Control of Wireless Networks with Multiuser Detection, Hybrid ARQ and Distortion Constraints
No full textWe present a novel optimization framework based on stochastic control and Markov theory for wireless networks where users concurrently access the channel and implement retransmission-based error control. In order to let users transmit at the same time, we consider an interference mitigation, rather than a collision avoidance approach. Our focus is on the interaction between the stochastic processes modeling the various individual sources of the network. Due to retransmissions, transmission by a user does not only instantaneously interfere with other simultaneous communications, but also biases the future evolution of the stochastic processes describing the other users. We, thus, define a novel interference measure called process distortion, that takes this effect into account. We investigate the optimization of access and power control for a network with two groups of users where transmission by the second group is constrained by the process distortion generated to the first group. We present algorithms to solve the unconstrained and constrained infinite horizon average cost per stage problems modeling this scenario. We discuss in detail the application of this framework to cognitive networks
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
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