1,720,971 research outputs found
MIMO underwater acoustic communications over time-varying channels: from theory to practice
Get PDFDespite more than 70% of our planet surface is covered by water, today the underwater world can still be considered largely unknown.
Rivers, lakes, seas and oceans have always been a fundamental resource for human life development, but at the same time they have often represented natural obstacles very hard to surmount.
The most impressive example is probably given by the ocean, whose vastness severely limited geographical explorations and discoveries for tens of centuries.
Anyway, the growing curiosity about what happens below the water surface has gradually led man to immerse in this unknown environment, trying to overcome its inaccessibility and figure out its secrets.
Underwater investigation and exploring have been increasingly supported by technology, advanced over time for different purposes (military, commercial, scientific).
In this regard, providing a communication link between remote users has been recognized as one of the main issues to be addressed.
The first significant solutions derived from the radio-frequency world, subject of study since the 19th century.
Unfortunately both wired and wireless RF inspired signal propagation strategies were not evaluated as successful.
The former ones, since considering the deployment of meters (up to kilometers) of cable in depth, were too costly and difficult, while the latter ones did not offer good performance in terms of communication range due to signal attenuation.
An alternative way, examined with particular interest from the beginning of the 20th century, has been that one offered by acoustics.
Actually, the study of sound and its propagation through different media has been an intriguing topic since the Old World Age, hence the attempt of messaging underwater has seemed to be a great opportunity to convey theoretical principles in a real application.
In addition, not only humans but also marine animals use acoustic waves to communicate, even over several kilometers distances as demonstrated by whales.
So, since already existing in nature, acoustic communications have been considered as potentially successful, furthermore representing an effective trade-off between feasibility and performance, especially if compared to the other electromagnetic signals-based methods.
Communication over RF channels has been extensively investigated so as to become a mature technology.
The thorough knowledge about OSI (Open Systems Interconnection) model physical layer issues has allowed the researchers attention to be drawn to the upper layers.
Following this direction, the recent advances in technology in this field have been accomplished mainly due to novelties in networks managing rather than to enhancements in the signal propagation study.
Moving to acoustics, unfortunately this approach results to be failing if applied in the underwater scenario, as the major challenges rise indeed from physics matters.
The underwater environment is varied and variable, so understanding the mechanisms that govern the propagation of sound in water is a key element for the design of a well-performing communication system.
In this sense, the physical layer has therefore regained the centrality that has been diminished in other contexts.
The underwater acoustic communications can be adopted in a wide range of applications.
The best-known are coastal monitoring, target detection, AUVs (Autonomous Underwater Vehicles) remote control, tsunami alarm, environmental data collection and transmission.
Those ones are very specific activities, so the devices to be employed must sometimes meet very strict requirements.
In this regard, the solutions commercially available provide good performance (that are paid in terms of high costs).
On the other hand, the fact that hardware and software are usually copyrighted leads to have a closed system.
Having reconfigurable devices is instead an opportunity to match the technology with the environment features and variations, especially in real-time applications.
Recently, the need to overcome these constraints has encouraged the debate about underwater technology challenges.
The work by Demirors et al. [1] reports an interesting discussion about the implementation of software-defined underwater acoustic networks (UWANs), highlighting how this solution can provide enhancements in terms of software portability, computational capacity, energy efficiency and real-time reconfigurability.
Furthermore, the authors propose the architecture of a software-defined acoustic modem and evaluate its performance and capabilities with tank and lake experiments.
Considering the comments outlined above, the following dissertation deals with the design of an acoustic communication system.
The preliminary theoretical analysis regarding physical layer concerns, such as signal propagation and channel behavior, represents the starting point from which several proposals regarding the implementation of UWANs are introduced.
In particular the context of Multiple-Input Multiple-Output (MIMO) communications is investigated, presenting several solutions about transmission schemes and receiver implementation.
Furthermore, concerning UWANs management, some strategies for access and error control, established at the data link layer level, are detailed.
It is worth highlighting that the goal of this contribution is not to present a disjointed discussion about the topics just listed.
The objective is instead to propose practical solutions developed hand in hand with theory, making choices firstly by looking at what nature allows
On cellular networks supporting healthcare remote monitoring in IoT scenarios
Get PDFIn the next few years, fundamental technological transitions are expected both for wireless communications, soon resulting in the 5G era, and for the kind of pervasiveness that will be achieved thanks to the Internet of Things. The implementation of such new communication paradigms is expected to significantly revolutionize people’s lives, industry, commerce, and many daily activities. Healthcare applications are considered to be one of the most impacted industries. Sadly, in relation to the COVID-19 pandemic currently afflicting our society, health remote monitoring has become a fundamental and urgent application. The overcrowding of hospitals and medical facilities due to COVID-19, has unavoidably created delays and key issues in providing adequate medical assistance. In several cases, asymptomatic or light symptomatic COVID-19 patients have to be continuously monitored to prevent emergencies, and such an activity does not necessarily require hospitalization. Considering this research direction, this paper investigates the potentiality of cloud-based cellular networks to support remote healthcare monitoring applications implemented in accordance with the IoT paradigm, combined with future cellular systems. The idea is to conveniently replace the physical interaction between patients and doctors with a reliable virtual one, so that hospital services can be reserved for emergencies. Specifically, we investigate the feasibility and effectiveness of remote healthcare monitoring by evaluating its impact on the network performance. Furthermore, we discuss the potentiality of medical data compression and how it can be exploited to reduce the traffic load
Underwater Acoustic Channel Modeling in Harbors
No full textThe performance of underwater acoustic communications are strictly influenced by the behavior of the channel whose features are typically defined by resorting to both empirical and stochastic approaches. Modeling the channel becomes very challenging especially when considering shallow water scenarios where the intensity and variability of signal reflections make the multipath effect hardly predictable. This contribution deals with the study about acoustic signal propagation in harbor areas, presenting an empirical analysis of the multipath channel based on the measurements collected during some trials
Petroni, Andrea Antonio
No full textBiografia e dettagliato profilo storico-critico del pittore Andrea Antonio Petroni (1863-1943), attivo a Napoli, a Roma e in Basilicata tra la fine dell'Ottocento e i primi decenni del Novecento. E' stato un interessante interprete del gusto verista, simbolista e liberty, fino ad oggi poco studiato
Second-order statistics driven LMS blind fractionally spaced channel equalization
No full textA novel cooperative least mean square adaptation rule is proposed with application to fractionally spaced blind channel equalization schemes used in quadrature amplitude modulation data links. A novel cost function based on second-order statistics only of the received signal is defined and jointly used with another chosen among classical blind cost functions such as decision directed and other Bussgang-type ones. Numerical results show that severe misconvergence observed in classical blind equalization algorithms is drastically mitigated using the here described technique
On rethinking cognitive access for underwater acoustic communications
No full textIn this paper, we investigate how to reformulate the concepts of cognitive access, originally developed for radio communications, in the framework of underwater acoustic communications. A straightforward application of the classical energy-detection-based cognitive approach, such as the one employed for radio communications, would result in a reduced spectrum utilization in an acoustic scenario. Actually, in the underwater scenario, acoustic signals sensed by a network node are likely to be due to communication sources as well as natural/artificial acoustic sources (e.g., mammals, ship engines, and so forth), differently from classical cognitive radio access, where each signal at the receiver is generated by a communication source. To maximize the access probability for cognitive acoustic nodes, we focus on understanding the nature of sensed interference. Toward this aim, we try to discriminate among natural and communications sources by classifying the images representing the time and frequency features of the received signals, obtained by means of the Wigner-Ville transform. Two different classifiers are considered here. The first one is targeted on finding natural interference while the second one looks for communication. Simulation results show how the herein described approach drastically enhances the access probability in an acoustic scenario with respect to a direct rephrasing of classical cognitive access. A possible protocol for implementing cognitive access is also described and its performance evaluated.In this paper, we investigate how to reformulate the concepts of cognitive access, originally developed for radio communications, in the framework of underwater acoustic com- munications. A straightforward application of the classical energy- detection-based cognitive approach, such as the one employed for radio communications, would result in a reduced spectrum utiliza- tion in an acoustic scenario. Actually, in the underwater scenario, acoustic signals sensed by a network node are likely to be due to communication sources as well as natural/artificial acoustic sources (e.g., mammals, ship engines, and so forth), differently from classical cognitive radio access, where each signal at the receiver is generated by a communication source. To maximize the access probability for cognitive acoustic nodes, we focus on under- standing the nature of sensed interference. Toward this aim, we try to discriminate among natural and communications sources by classifying the images representing the time and frequency features of the received signals, obtained by means of the Wigner– Ville transform. Two different classifiers are considered here. The first one is targeted on finding natural interference while the sec- ond one looks for communication. Simulation results show how the herein described approach drastically enhances the access proba- bility in an acoustic scenario with respect to a direct rephrasing of classical cognitive access. A possible protocol for implementing cognitive access is also described and its performance evaluated
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
- …
