1,721,316 research outputs found
Markov models for the physical layer block error process in a WCDMA cellular system
No full textIn this paper, we investigate the possibility of using Markov chains to model the error process in the data blocks delivered by the physical layer of wideband code division multiple access a (WCDMA) cellular system. Suitable Markov models (MM) are designed to fulfil the two following objectives: First, an upper layer protocol supplied by the output obtained from the MM should behave as if it were running on the actual physical layer; second, MM parameters should be linked via simple relationships to the main physical layer parameters. Starting from the results on the error statistics obtained from a suitable simulation tool which jointly performs system and link level analysis, we first classify the users on the basis of performance level and burstiness, and then, we provide some guidelines for the design of Markov models in the different system and channel conditions. The performance of an automatic repeat request (ARQ) (Go-Back N) protocol at the link layer is taken as an example to test the accuracy of the proposed models. It is shown that the perspective of using simple error models in the analysis of upper layer protocols is feasible in many cases
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
A single calibration graph for the direct determination of ascorbic and dehydroascorbic acids by electrogenerated luminescence based on Ru(bpy)3(2+) in aqueous solution
No full textAscorbic (H(2)A) and dehydroascorbic (DA) acids were for the first time directly determined in a single chromatographic run by means of the tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)(3)(2+)) based electrogenerated chemiluminescence (ECL) detection. For the first time, it was demonstrated that DA, a nonelectroactive compound, is ECL active and is responsible for the ECL behavior of H(2)A. This fact, together with the lack of a DA standard, suggested the use of a calibration graph obtained for H(2)A, for determining both analytes. The proven ECL activity of DA, together with literature data relative to the standard redox potentials of the different species coming from H(2)A, led to a reconsideration of the proposed ECL reaction mechanism for H(2)A. The role of the OH- ion in the reaction mechanism of the two analytes appeared to be crucial, H(2)A and DA could be separated by a suitable C-18-reversed-phase HPLC column using an aqueous 30 mN H3PO4 solution as the mobile phase. The optimal ECL response was achieved by polarizing the working electrode at 1.150 V vs SCE (standard calomel electrode) (oxidation diffusion limiting potential for both H2A and Ru(bpy)(3)(2+)). The Ru(bpy)(3)(2+) solution, at pH 10 for carbonate buffer, was mixed to the eluent solution in a postcolumn system, obtaining, still at pH 10, the final 0.25 mM RU(bpy)(3)(2+) concentration. The detection limit found for the two analytes was I x 10(-7) M. The method was successfully applied to the determination of the analytes in a commercially available orange fruit juice
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
Performance of Capture-Division Packetized Access (CDPA) with partial frequency reuse and power control
No full textPreviously, a new method for achieving spectrum reuse in cellular systems, called capture-division packetized access (CDPA), has been introduced. The method uses a single frequency in all cells but, unlike CDMA, allows each transmitter to access the full bandwidth. Practically, the CDPA's way of operation can be seen as an S-ALOHA scheme among different cells in which the mobile terminals belonging to the same cell transmit using a collision-free mechanism, which is easily obtained due to the very short intra-cell propagation delay. Parallel transmission in different cells is achieved through the “capture” capability. Packets that are not captured are almost immediately retransmitted, thus assuring that packets are eventually correctly received. In this paper we analyze an extension of the CDPA that uses partial frequency reuse. In fact, in this case, a trade-off exists between the bandwidth wasted by retransmissions and the bandwidth wasted by subdividing spectrum usage in K parts. It is shown that, depending on the capture threshold ratio b at receivers, an optimal choice exists. In all cases, the spectrum efficiency obtained almost doubles the efficiency of systems, like GSM, with K=7. The gain offered by power-control techniques is also investigate
On the use of rate and power adaptation in V-BLAST systems for data protocol performance improvement
No full textIn this paper we address the issue of optimizing the performance of data-link and transport protocols running on a system including multiple transmit and receive antennas with a V-BLAST architecture. More specifically, we explore the possibility of adaptively selecting which transmit antennas and which per antenna rates to use as well as the antenna transmit powers in order to maximize the rate of data packets successfully delivered through the system. This is a novel approach with respect to the recent literature which is generally focused on channel capacity optimization. We show that the use of this rate/power adaptation technique, when a data transmission protocol is running over the link, may lead to significant throughput improvements especially at small signal-to-noise ratios
Effects of discontinuous transmission on the performance of a WCDMA cellular system
No full textIn this paper, we investigate some side effects of discontinuous transmission (DTX) on the performance of a wide-band code-division multiple-access (WCDMA) radio interface with signal-to-interference-ratio-based closed-loop power control. More precisely, we study how the DTX mechanism affects the transmission of some user signals. It is shown that the mobile terminals, when they turn on their data channels after some frames without any data to transmit, can cause a sudden increase of the interference level, leading, for some neighboring users, to a temporary performance degradation that lasts until the power control has completely reacted to this event. The main parameters that characterize the dynamics of the DTX mechanism are described and the tradeoffs in the choice of their values are evaluated; we also propose a suitable variation to the DTX mechanism that reduces the unwanted effects. For our investigation, a suitable simulation tool, which integrates the system-level and link-level analysis, has been developed, including most of the actual details and features of the WCDMA physical layer
Capture-division packetized access (CDPA) for cellular systems: performance analysis of the inbound and outbound channels
No full textThe paper presents a new cellular architecture for radio access, CDPA, that can be applied to present and future cellular systems, independently of the cell size. It poses as an appealing alternative to systems based on classical bandwidth-subdivision methods, namely TDMA, FDMA or CDMA. In these systems, parallelism of communications is achieved by subdividing the bandwidth “a priori” among cells. In CDPA no bandwidth subdivision is operated. All cells and terminals use a single frequency channel and transmit packets on a slotted channel. Parallel transmission in different cells is achieved through the “capture” capability. A dynamic polling mechanism, C-PRMA, managed by the base station, guarantees almost immediate re-transmission of packets that are not captured, thus assuring that packets are eventually correctly received. Analytical evaluations show that CDPA has the potential to provide larger capacity than the other cited systems in the case of continuous traffic sources. Furthermore, as C-PRMA is inherently apt to sustain bursty traffic, the system capacity is easily doubled in the case of packetized voice transmission using silence suppressio
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