Cross-layer design for single-cell OFDMA systems with heterogeneous QoS and partial CSIT

Abstract— This paper proposes a novel cross-layer scheduling scheme for a single-cell orthogonal frequency division multiple access (OFDMA) wireless system with partial channel state information (CSI) at transmitter (CSIT) and heterogeneous user delay requirements. Previous research efforts on OFDMA resource allocation are typically based on the availability of perfect CSI or imperfect CSI but with small error variance. Either case consists to typify a non tangible system as the potential facts of channel feedback delay or large channel estimation errors have not been considered. Thus, to attain a more realistic resolution our cross-layer design determines optimal subcarrier and power allocation policies based on partial CSIT and individual user’s quality of service (QoS) requirements. The simulation results show that the proposed cross-layer scheduler can maximize the system’s throughput and at the same time satisfy heterogeneous delay requirements of various users with significant low power consumption

A selective delayed channel access (SDCA) for the high-throughput IEEE 802.11n

Abstract— In this paper we investigate the potential benefits of a selective delayed channel access algorithm (SDCA) for the future IEEE 802.11n based high-throughput networks. The proposed solution aims to resolve the poor channel utilization and the low efficiency that EDCA’s high priority stations adhere due to shorter waiting times and consequently to the network’s degrading overall end performance. The algorithm functions at the MAC level where it delays the packets from being transmitted by postponing the channel access request, based on their traffic characteristics. As a result, the flow’s average aggregate size increases and consequently so is the channel efficiency. However, in some situations we notice that further deferring has a negative impact with TCP applications, thus we further introduce a traffic awareness feature that allows the algorithm to distinguish which flows are using the TCP protocol and override any additional MAC delay. We validate through various simulations that SDCA improves throughput significantly and maximizes channel utilization

IEEE 802.11n MAC frame aggregation mechanisms for next-generation high-throughput WLANs [Medium access control protocols for wireless LANs]

IEEE 802.11n is an ongoing next-generation wireless LAN standard that supports a very highspeed connection with more than 100 Mb/s data throughput measured at the medium access control layer. This article investigates the key MAC enhancements that help 802.11n achieve high throughput and high efficiency. A detailed description is given for various frame aggregation mechanisms proposed in the latest 802.11n draft standard. Our simulation results confirm that A-MSDU, A-MPDU, and a combination of these methods improve extensively the channel efficiency and data throughput. We analyze the performance of each frame aggregation scheme in distinct scenarios, and we conclude that overall, the two-level aggregation is the most efficacious

The history of teaching quantum mechanics in Greece

In this work, our goal is to examine the attitude of the Greek scientific community towards Quantum Mechanics and establish the history of teaching of this theory in Greece. We have examined Physics textbooks written by professors of the University of Athens, as well as records of public speeches, university yearbooks from 1923 to 1970, articles in popular scientific magazines, political and philosophical publications and lemmas of encyclopedias that were influencing the public at the time. Our research shows that Quantum Mechanics in Greece was involved in the debate between the political left and right and the church organizations of the period, leading to an extremely idealistic misinterpretation of the theory. As a result, Quantum Mechanics was late to establish itself in Greece, with the first structured and autonomous course on Quantum Mechanics appearing as late as 1962. © Springer Science+Business Media B.V. 2007

Ptolemy's experiments on refraction in science class

In this paper, the inclusion of history and philosophy of science in science teaching is proposed with the goal of depicting pre-service primary teachers' perception of the role of historical experiments in learning science and the nature of science. In order to achieve this, a teaching-learning sequence was designed and conducted in the context of an Introductory Physics Laboratory Course following the integrated approach to teaching the nature of science. Ptolemy's experiments on refraction and the combination of his advanced and erroneous views contributed to pre-service primary teachers' conceptualization of the empirical and tentative nature of science and the relationship between mathematics and science. The majority of the pre-service primary teachers perceived the history of science as important in understanding scientific concepts and aspects of the nature of science. © 2020 IOP Publishing Ltd

What Conceptions do Greek School Students Form about Biological Evolution?

In Greece, since 2000, the teaching of evolutionary theory is restricted solely to lower (junior) high school and specifically to ninth grade. Even though the theory of evolution is included to the 12th grade biology textbook, it is not taught in Greek upper (senior) high schools. This study presents research conducted on the conceptions of Greek students regarding issues set out in the theory of evolution after the formal completion of the teaching of the theory. The sample comprised 411 10th grade students from 12 different schools. The research results show that the students appear to have a positive view of the idea of evolution, the evolution of man, and the common origin of organisms. However, they have retained many alternative views, or else they are completely in ignorance of basic issues in evolutionary theory regarding: what is considered evolution in biology, the main mechanism of evolutionary changes in what is considered natural selection, what the theory of evolution actually explains, and what the word theory means in science. At least in Greece, these views still prevail because the theory of evolution is marginalized in the teaching of biology in Greek schools, and biology education does not help students formulate overall conceptual structures to enable them to understand the question of biological change. © 2008 Springer Science + Business Media, LLC 2008

Pre-Service Elementary Teachers' Conceptions about Wind

This paper presents a study concerning Greek pre-service elementary teachers' (PET) conceptions about wind. The study focuses on their (i) understanding of scientific concepts used to explain wind formation, (ii) conceptions about wind measurement and (iii) ability to read and translate symbols of wind on weather maps. Our research reveals a lack of understanding in wind formation mechanism, and difficulties in orientation and in weather maps reading. Our findings are consistent with previous research carried out in different national and educational contexts. In the final section, we outline the implications of these findings for the design of learning activities concerning wind in primary, secondary and tertiary education including Environmental Science courses attended by PET. © 2013 Taylor and Francis Group, LLC

Recent developments in the digital approach of symbolic dynamics

Purpose: The purpose of this paper is to explore new mathematical results to advance the understanding of the picture of a chaotic unimodal map. Design/methodology/approach: Ever since Poicare, deterministic chaos is ultimately connected with exponential divergence of nearby trajectories, unpredictability and erratic behaviour. Here, the authors propose an alternative approach in terms of complexity theory and transcendence. Findings: In this paper, the authors were able to reproduce previous results easily, due to new theorems. Originality/value: The paper updates previous results and proposes a more complete understanding of the phenomenon of deterministic chaos, also making possible connections with number theory, combinatorics and possibly quantum mechanics, as in quantum mechanics there does not exist the notion on nearby trajectories. © Emerald Group Publishing Limited