1,720,981 research outputs found
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
Analyzing the Signal Strength of 2,946 Clients Operating in 446 WiFi Networks
Full text linkIn this thesis we analyze data that was collected over a 24 hour period from 446 access points that provide connections for 2,946 clients. The data was obtained from deployments of modern commercial Google Wifi access points. A focus of this thesis is an analysis of the Received Signal Strength Indication (RSSI) from messages sent from clients to the access point. The RSSI depends on both the environment in which the signals operate and the distance between the client and the access point.
Our dataset includes 417,122 data points of which 45.1% of the data points are from signals using the 2.4 GHz spectrum and the remaining 54.9% are from the 5 GHz spectrum. The data has been collected by each access point (AP) every 5 minutes over a period of 24 hours. We find from our analysis that across all access points, the average number of clients (across all spectrums) that are simultaneously connected in any 5 minute window is quite small. That is, 65.7% of the APs have on average 3 or fewer clients that are simultaneously connected in any 5 minute window. However, we also find that 6.5% of the APs service on average 9 or more clients.
In this thesis we develop and utilize a methodology to categorize clients and networks using RSSI values (signal strengths) of the messages received by access points from the clients, to study the possible PHY rates which can be used by clients to send messages to the APs. The methodology also helps us to capture and examine the variability in signal strengths.
Several previous studies have characterized WiFi networks using the measured throughput of the clients. However, the throughput experienced and rates used by clients in those studies depend on the capabilities of the clients. We believe that a significant advantage of our methodology is that it is independent of the capabilities of the clients used in the study. In addition, our methodology is also able characterize the environments in which the WiFi devices operate. This is because our methodology primarily uses the signal strength of the messages to characterize devices in a WiFi network and the signal strength changes over time due to the movements of the sender, receiver, or people in the area.
We use our methodology to analyze both clients and networks. We find from our analysis of clients that, over the 24 hour period, 90% of the signals from 84.2% of the clients are received by the APs with either Good or Moderate signal strengths. Thus, for the majority of the clients signal strengths are mostly quite reliable. We also find that clients using the 2.4 GHz spectrum have signals about as good as or better than clients using the 5 GHz spectrum.
However, perhaps one of the most interesting findings is that, when analyzing networks we find that 27% or more of all networks have one or more clients whose signals are received by the AP with unreliable signal strengths. These clients could potentially impede the throughput of all the other clients in the same network and also networks in the vicinity, due to the WiFi performance anomaly problem. We also find that networks with more clients have more clients with unreliable signals and that the fraction of networks with one or more clients with unreliable signals is quite close to what is expected based on probabilities. From the results of our analysis of clients and the analysis of networks, we note that a small number of clients may impact the performance of a considerably large number of networks
NeuRA: Using Neural Networks to Improve WiFi Rate Adaptation
Full text linkAlthough a variety of rate adaptation algorithms have been proposed for 802.11 devices, sampling-based algorithms are preferred and used in practice because they only require frame loss information which is available on all devices. Unfortunately, sampling can impose significant overheads because it can lead to excessive frame loss or the choice of suboptimal rates. In this thesis, we design a novel neural network based rate adaptation algorithm, called NeuRA. NeuRA significantly improves the efficiency of sampling in rate adaptation algorithms by using a neural network model to predict the expected throughput of many rates, rather than sampling their throughput. Furthermore, we propose a feature selection technique to select the best set of rates to sample.
Despite decades of research on rate adaptation in 802.11 networks, there are no definitive results which determine which algorithm is the best or if any algorithm is close to optimal. We design an offline algorithm that uses information about the fate of future frames to make statistically optimal frame aggregation and rate adaptation decisions. This algorithm provides an upper bound on the throughput that can be obtained by practical online algorithms and enables us to evaluate rate adaptation algorithms with respect to this upper bound.
Our trace-based evaluations using a wide variety of real-world scenarios show that NeuRA outperforms the widely-used Minstrel HT algorithm by up to 24% (16% on average) and the Intel iwl-mvm-rs algorithm by up to 32% (13% on average). Moreover, the upper bound given by the offline optimal algorithm shows a throughput up to 58% (30% on average) higher than Minstrel HT and up to 31% (12% on average) higher than NeuRA. Hence, NeuRA reduces the gap in throughput between Minstrel HT and the offline optimal algorithm by half. Additionally, our results show that several-fold improvements over Minstrel HT shown in previous work are unlikely to be obtained in real-world scenarios. Finally, we implement NeuRA using the Linux ath9k driver to show that the neural network processing requirements are sufficiently low to be practical and that NeuRA can be used to obtain statistically significant improvements in throughput when compared with Minstrel HT
Reliable WiFi Backscatter Communication in WiTAG
No full textWiFi backscatter systems offer the potential to provide low-powered WiFi-compatible communication. This technology is especially promising when coupled with low-power sensors to periodically communicate readings from IoT devices. WiTAG is an extremely attractive approach because it greatly reduces power consumption by avoiding the use of WiFi receivers or signal detectors while ensuring compatibility with existing WiFi infrastructure. WiTAG operates at the MAC layer by corrupting or not corrupting subframes (MPDUs) within a transmitted frame (A-MPDU). For example, corruption of an MPDU signals a 0 and non-corruption signals a 1. Because it eschews the use of receivers and signal detectors WiTAG is unable to sense when frames are being sent by nearby WiFi devices that it relies on for communication.
In this thesis, we describe the significant challenges that arise when formulating, transmitting, and reliably detecting and decoding messages transmitted from WiTAG. We design a message encoding framework to overcome these challenges. We show that although WiTAG relies on probabilities for overlapping a tag’s message with an A-MPDU it is possible to increase the odds of an overlap, thus increasing message rates. This permits the transmission of highly reliable messages in a relatively short period of time
Evaluating and Characterizing the Performance of 802.11 Networks
Full text linkThe 802.11 standard has become the dominant protocol for Wireless Local Area Networks (WLANs). As an indication of its current and growing popularity, it is estimated that over 20 billion WiFi chipsets will be shipped between 2016 and 2021. In a span of less than 20 years, the speed of these networks has increased from 11 Mbps to several Gbps. The ever-increasing demand for more bandwidth required by applications such as large downloads, 4K video streaming, and virtual reality applications, along with the problems caused by interfering WiFi and non-WiFi devices operating on a shared spectrum has made the evaluation, understanding, and optimization of the performance of 802.11 networks an important research topic.
In 802.11 networks, highly variable channel conditions make conducting valid, repeatable, and realistic experiments extremely challenging. Highly variable channel conditions, although representative of what devices actually experience, are often avoided in order to conduct repeatable experiments. In this thesis, we study existing methodologies for the empirical evaluation of 802.11 networks. We show that commonly used methodologies, such as running experiments multiple times and reporting the average along with the confidence interval, can produce misleading results in some environments.
We propose and evaluate a new empirical evaluation methodology that expands the environments in which repeatable evaluations can be conducted for the purpose of comparing competing alternatives. Even with our new methodology, in environments with highly variable channel conditions, distinguishing statistically significant differences can be very difficult because variations in channel conditions lead to large confidence intervals. Moreover, running many experiments is usually very time consuming. Therefore, we propose and evaluate a trace-based approach that combines the realism of experiments with the repeatability of simulators. A key to our approach is that we capture data related to properties of the channel that impact throughput. These traces can be collected under conditions representative of those in which devices are likely to be used and then used to evaluate different algorithms or systems, resulting in fair comparisons because the alternatives are exposed to identical channel conditions.
Finally, we characterize the relationships between the numerous transmission rates in 802.11n networks with the purpose of reducing the complexities caused by the large number of transmission rates when finding the optimal combination of physical-layer features. We find that there are strong relationships between most of the transmission rates over extended periods of time even in environments that involve mobility and experience interference. This work demonstrates that there are significant opportunities for utilizing relationships between rate configurations in designing algorithms that must choose the best combination of physical-layer features to use from a very large space of possibilities
- …
