A Survey on UAV-Aided Maritime Communications: Deployment Considerations, Applications, and Future Challenges

Maritime activities represent a major domain of economic growth with several emerging maritime Internet of Things use cases, such as smart ports, autonomous navigation, and ocean monitoring systems. The major enabler for this exciting ecosystem is the provision of broadband, low-delay, and reliable wireless coverage to the ever-increasing number of vessels, buoys, platforms, sensors, and actuators. Towards this end, the integration of unmanned aerial vehicles (UAVs) in maritime communications introduces an aerial dimension to wireless connectivity going above and beyond current deployments, which are mainly relying on shore-based base stations with limited coverage and satellite links with high latency. Considering the potential of UAV-aided wireless communications, this survey presents the state-of-the-art in UAV-aided maritime communications, which, in general, are based on both conventional optimization and machine-learning-aided approaches. More specifically, relevant UAV-based network architectures are discussed together with the role of their building blocks. Then, physical-layer, resource management, and cloud/edge computing and caching UAV-aided solutions in maritime environments are discussed and grouped based on their performance targets. Moreover, as UAVs are characterized by flexible deployment with high re-positioning capabilities, studies on UAV trajectory optimization for maritime applications are thoroughly discussed. In addition, aiming at shedding light on the current status of real-world deployments, experimental studies on UAV-aided maritime communications are presented and implementation details are given. Finally, several important open issues in the area of UAV-aided maritime communications are given, related to the integration of sixth generation (6G) advancements

Η πολιτική του καθεστώτος της 4ης Αυγούστου απέναντι στο Πανεπιστήμιο Αθηνών 1936-1941

Η ιδιοτυπία του καθεστώτος της 4ης Αυγούστου και οι ευρύτερες ιδεολογικές του αναζητήσεις, όπως αποτυπώθηκαν στην πολιτική απέναντι στο Πανεπιστήμιο Αθηνών, είναι μια σελίδα της σύγχρονης ιστορίας που δεν έχει μελετηθεί επαρκώς. Η περίοδος του Μεσοπολέμου έχει ξεχωριστή σημασία για τον χώρο της πανεπιστημιακής εκπαίδευσης, καθώς ιδρύεται το Πανεπιστήμιο της Θεσσαλονίκης, ανωτατοποιείται το Εθνικό Μετσόβιο Πολυτεχνείο και δημιουργούνται άλλες ανώτατες σχολές, όπως η Πάντειος Σχολή Πολιτικών Επιστημών, η Ανωτάτη Σχολή Οικονομικών και Εμπορικών Επιστημών και η Ανωτέρα Γεωπονική Σχολή. Η παρούσα εργασία επικεντρώνεται στο Πανεπιστήμιο Αθηνών και επιχειρεί να αποτυπώσει τα βασικά δομικά χαρακτηριστικά του, καθώς αυτό αποτέλεσε σημείο αναφοράς για την ευρύτερη πνευματική και πολιτιστική ζωή του τόπου. Είναι το μεγαλύτερο και αρχαιότερο πανεπιστήμιο της χώρας, το οποίο σε όλη τη διάρκεια της ιστορίας του διατήρησε μια ισχυρή σχέση με την εκάστοτε πολιτική εξουσία, επιχειρώντας να διατηρήσει την αυτοτέλειά του. Ιδιαίτερα στην περίοδο του Μεσοπολέμου είχε σταθεί εχθρικό και είχε προσπαθήσει να ματαιώσει κάθε εκπαιδευτική μεταρρυθμιστική απόπειρα που προερχόταν από το βενιζελικό στρατόπεδο. Σκοπός της εργασίας είναι να μελετηθεί η εκπαιδευτική πολιτική του καθεστώτος Μεταξά και να αναδειχθεί ο βαθύτερος χαρακτήρας των παρεμβάσεών του απέναντι στο Πανεπιστήμιο κατά τη διάρκεια της δικτατορίας της 4ης Αυγούστου. Περιγράφοντας τα κυρίαρχα ιδεολογικά και πολιτικά στοιχεία του μεταξικού καθεστώτος, που είχε δεχτεί την επιρροή των φασιστικών ιδεών οι οποίες μεσουρανούσαν στην Ευρώπη του Μεσοπολέμου υιοθετώντας πάρα πολλά στοιχεία τους, σκιαγραφείται ο τρόπος με τον οποίο οι προσωπικές γλωσσικές και εκπαιδευτικές απόψεις του Μεταξά μετουσιώθηκαν στην επίσημη εκπαιδευτική πολιτική του Υπουργείου Παιδείας. Πιο συγκεκριμένα, εξετάζοντας τις επιλογές του καθεστώτος στη διοίκηση της Ανώτατης Εκπαίδευσης μέσα από το πρίσμα της γενικότερης πολιτικής της 4ης Αυγούστου, προσπαθώ να αναδείξω την υποδοχή τους από το Πανεπιστήμιο Αθηνών. Το κεντρικό ερώτημα αφορά στον τρόπο, με τον οποίο το Πανεπιστήμιο υποδέχθηκε το νέο καθεστώς και πώς η 4η Αυγούστου, στο πλαίσιο της γενικότερης πολιτικής της το αντιμετώπισε, προσπαθώντας επί της ουσίας με διάφορα μέτρα να το ελέγξει και να το χειραγωγήσει. Τέλος επιχειρείται μια συνολική ανασκόπηση της πολιτικής του Μεταξά απέναντι στο Πανεπιστήμιο Αθηνών σε σύγκριση και με τις αντίστοιχες μεθόδους και πρακτικές για την ανώτατη εκπαίδευση άλλων αυταρχικών και φασιστικών καθεστώτων στην Ευρώπη εκείνη την εποχή.The particular character of the regime of ‘August 4’ and its broader ideological pursuits, as these were reflected in its policy towards the University of Athens, have never been adequately studied. The interwar period marks a watershed in the history of Greek higher education, since it saw the foundation of the University of Thessaloniki, the raising of the NTUA to the status of a university and the creation of other higher education institutions like Panteion Political Science School, the Higher School of Economics and Business, and the Higher School of Agriculture. This paper focuses on the University of Athens and attempts to define its basic structural characteristics, since it was the reference point for the broader intellectual and cultural life. It is the largest and oldest university in the country, which throughout its history has maintained a strong relationship with the current political power in an attempt to maintain its autonomy. Especially in the interwar period it remained hostile and tried to thwart any attempt at educational reform that came from the Venizelist camp. The purpose of this paper is to study the educational policy of the Metaxas regime and to highlight the nature of its interventions towards the University. Describing the dominant ideological and political elements of the regime, which was genuinely authoritarian and influenced from the fascist ideas current in interwar Europe, this study explores the manner in which Metaxas’ own linguistic and educational ideas were translated into the official policy of the Ministry of Education. More specifically, examining the choices of the regime in the administration of higher education through the prism of the general policies of ‘August 4’ this study tries to showcase their reception at the University of Athens. The central question concerns the way in which the University welcomed the new regime and how ‘August 4’ dealt with the University, trying to control and to manipulate it. Finally this study attempts to offer an overview of Metaxas' policy towards the University of Athens compared with the higher education policies of other authoritarian and fascist regimes in Europe at that time

Electrical Vehicles: Current State of the Art, Future Challenges, and Perspectives

The goal of the study presented in this article is to provide a general overview of the various aspects related to electric vehicles (EVs), along with all associated emerging challenges and perspectives. In this context, the basic types of EVs and the corresponding charging technologies are analyzed. Since EVs are expected to be a key component of future smart electrical grids (SEG), connection to the grid issues, along with advanced charging techniques (i.e., wireless power transfer), are analyzed as well. To this end, the main features, the requirements of vehicle to grid (V2G) communications, as well as future developments and scenarios of electrification, are also presented and analyzed. Moreover, integration issues with currently deployed fifth generation (5G) mobile wireless networks are also outlined, in order to ensure optimum transmission and reception quality in V2G communications and improved user experience. This integration is also expanded in autonomous vehicles (AVs) technology (self-driving objects), since optimized information processing from various diverse sources is required in order to ensure advanced traffic management aspects. Document type: Articl

A New Subcarrier Allocation Strategy for MIMO-OFDMA Multicellular Networks Based on Cooperative Interference Mitigation

The goal of the study presented in this paper is to investigate the performance of a new subcarrier allocation strategy for Orthogonal Frequency Division Multiple Access (OFDMA) multicellular networks which employ Multiple Input Multiple Output (MIMO) architecture. For this reason, a hybrid system-link level simulator has been developed executing independent Monte Carlo (MC) simulations in parallel. Up to two tiers of cells around the central cell are taken into consideration and increased loading per cell. The derived results indicate that this strategy can provide up to 12% capacity gain for 16-QAM modulation and two tiers of cells around the central cell in a symmetric 2×2 MIMO configuration. This gain is derived when comparing the proposed strategy to the traditional approach of allocating subcarriers that maximize only the desired user’s signal

An artificial intelligence-based collaboration approach in industrial IoT manufacturing : key concepts, architectural extensions and potential applications

The digitization of manufacturing industry has led to leaner and more efficient production, under the Industry 4.0 concept. Nowadays, datasets collected from shop floor assets and information technology (IT) systems are used in data-driven analytics efforts to support more informed business intelligence decisions. However, these results are currently only used in isolated and dispersed parts of the production process. At the same time, full integration of artificial intelligence (AI) in all parts of manufacturing systems is currently lacking. In this context, the goal of this manuscript is to present a more holistic integration of AI by promoting collaboration. To this end, collaboration is understood as a multi-dimensional conceptual term that covers all important enablers for AI adoption in manufacturing contexts and is promoted in terms of business intelligence optimization, human-in-the-loop and secure federation across manufacturing sites. To address these challenges, the proposed architectural approach builds on three technical pillars: (1) components that extend the functionality of the existing layers in the Reference Architectural Model for Industry 4.0; (2) definition of new layers for collaboration by means of human-in-the-loop and federation; (3) security concerns with AI-powered mechanisms. In addition, system implementation aspects are discussed and potential applications in industrial environments, as well as business impacts, are presented

Multi-user interference mitigation under limited feedback requirements for WCDMA systems with base station cooperation

One of the techniques that has been recently identified for dealing with multi-user interference (MUI) in future communications systems is base station (BS) cooperation or joint processing. However, perfect MUI cancellation with this technique demands severe synchronization requirements, perfect and global channel state information (CSI), and an increased backhaul and signaling overhead. In this paper, we consider a more realistic layout with the aim of mitigating the MUI, where only local CSI is available at the BSs. Due to synchronization inaccuracies and errors in the channel estimation, the system becomes partially asynchronous. In the downlink of wideband code division multiple access based systems, this asynchronism stands for the loss of the orthogonality of the spreading codes allocated to users and thus, for an increase in the MUI level of the system. In this contribution, we propose a framework for mitigating the MUI which builds in three main steps: definition of a cooperation area based on the channel characteristics, statistical modeling of the average MUI power experienced by each user and a specific spreading code allocation scheme for users served with joint processing. This code allocation assigns spreading codes to users in such a way that minimum average cross-correlation between active users can be achieved. Interestingly, these steps can be performed with a limited amount of extra feedback from the user's side

Resource management in OFDMA heterogeneous network

In this study, a Long Term Evolution Advanced (LTEa) ??? based multi-user Orthogonal Frequency Division Multiple Access (OFDMA) heterogeneous network has been simulated and a resource allocation strategy has been proposed. The strategy under consideration can inherently mitigate electromagnetic interference, hence increases the mean number of terminals, and requires no channel state information (CSI). To evaluate the performance of the network platform and the proposed strategy, the system is studied for different network orientations. According to the results, the platform is a good reality simulator, whereas owning to the proposed Radio Resource Management (RRM) algorithm the mean capacity can reach a 12-fold increase especially for highly noisy operating environments

Reduced complexity BER calculations in large scale spatial multiplexing multi-user MIMO orientations in frequency selective fading environments

In this article, a computationally-efficient approach is presented for Bit Error Rate (BER) calculations in multi-user Multiple Input Multiple Output (MIMO) frequency selective fading environments, operating in spatial multiplexing transmission mode. To this end, theoretical expressions for the equivalent Signal to Interference plus Noise Ratio (SINR) per active user and transmission mode, as well as mean BER, are derived. The key idea is that all parameters related to BER calculations can be expressed as sums of identically distributed random variables (RVs). Hence, computational burden can be reduced, since summation formulas take into account the parameters of a standalone RV along with the correlation of an arbitrary pair of RVs. As results indicate, the proposed approach can accurately estimate mean BER in multiuser MIMO orientations with increased reception diversity order, for arbitrary number of transmit/receive antennas, a modulation scheme, and a number of resolvable multi-path components. © 2019 by the authors. Licensee MDPI, Basel, Switzerland