120 research outputs found
Mini-review: The role of crop rotation, intercropping, sowing dates and increased crop density towards a sustainable crop and weed management in arable crops
No full textSaabunud / Received 17.05.2020 ; Aktsepteeritud / Accepted 29.05.2020 ; Avaldatud veebis / Published online 30.05.2020 ; Vastutav autor / Corresponding author: Panagiotis Kanatas e-mail: [email protected] extended and in many cases unjustified use of herbicides
has resulted in herbicide resistance development and serious environmental
concerns. Therefore, the need for implementation and wider adoption of
several agronomic and cultural practices is imperative. Ecologically-based
crop management practices like crop rotation, intercropping, delay of
sowing date and increased crop density can be also the basis for effective
and sustainable weed management. In the present review, several cases are
presented and the key points of each method are discussed. Special attention
is given to the fact that the efficacy of each practice is depended on the
specific soil and climatic conditions along with the field history of each site
and crop. Alternative methods of weed management should be further
studied and optimized to include them in both organic and conventional
production systems and ensure the sustainability of agroecosystems
The aromatic plant Melissa officinalis and effects of its aqueous extracts on summer annual and invasive weed species
No full textSaabunud / Received 14.05.2020 ; Aktsepteeritud / Accepted 25.05.2020 ; Avaldatud veebis / Published online 25.05.2020 ; Vastutav autor / Corresponding author: Panagiotis Kanatas e-mail: [email protected] effects of aqueous extracts of the aromatic plant Melissa
officinalis were studied on the seed germination and early seedlings growth
of ten summer annual species in Petri dish bioassays and pot experiments.
The in vitro experiments on the aqueous extracts from M. officinalis on seed
germination shows that the extracts of 5 and 10% were the most active
inhibitors for all the studied weed species. Seed germination reduction by
the aqueous extracts was up to 54% of the untreated seed lot for each species.
On the contrary, the concentration of 1% resulted in germination reduction
ranging from 1 to 11%, while in some of the weed species (P. minor, S.
nigrum, P. angulata and C. albida) the effect was rather stimulatory. This
finding is in full agreement with “novel weapons hypothesis” and supports
that native plants compared with invasive (like C. albida and P. angulata)
are affected more due to the absence of tolerance or resistance to the
allelochemicals. The allelopathy RI of aqueous extracts of lemon balm was
negative in most cases, while in the case of 10% concentration, emergence
was reduced by 58, 54, 48, 46 and 43% for X. strumarium, C. album, S.
faberi, C. canadensis and C. bonariensis, respectively. The allelopathic
activity of M. officinalis could be exploited in future studies, to identify and
isolate the allelochemicals, as models for future herbicides for integrated
weed management
Multiple-Active Multiple-Passive Antenna Systems and Applications
No full textThis chapter focuses on extensions and foreseen applications of the parasitic antenna array technology. Moving beyond the single-active (single-RF) communication setup, hitherto discussed in the previous chapters, the work in this chapter extends the analysis to generalized multiple-active multiple-passive (MAMP) antenna topologies, as explained in Sect. 8.1. Then, Sect. 8.2 proposes MAMP antenna structures with application to reconfigurable MIMO transmission in the presence of antenna mutual coupling under poor scattering channel conditions. For this purpose, the section presents an adaptive MAMP antenna system capable of changing its transmission parameters via passive radiators attached to tunable loads, according to the structure of the RF propagation channel. The hybrid MAMP array structure can be tractably analyzed using the active element response vector (instead of the classical steering vector) and the active element current vector (all being functions of the variable loading). The adaptive MAMP system targets at maximizing tight MIMO ergodic and outage rate bounds, relying on partial channel knowledge when tuning to a different loading state for optimizing the rate of communication. The proposed adaptive MAMP system can be limited to practical dimensions whereas the passive antennas require no extra RF hardware, thus meeting the cost, space, and power constrains of the users’ mobile terminals. The simulation results show that the adaptive MAMP system, thanks to its “adaptivity”, is able to achieve satisfactory performance even in poor scattering environments whereas a significant part of the mutual information that is lost owing to the spatial correlation and the electromagnetic coupling is successfully retrieved. Section 8.3 extends our communication scenario to account for multiuser diversity systems, describing novel parasitic antenna-assisted switched beam array architectures for enhanced selection combining with application to the downlink of cellular systems exploiting multiuser diversity. Specifically, this section deals with the problem of the poor performance of antenna selection for compact user terminals in multiuser diversity systems. Although antenna selection is a simple and efficient technique for enhancing the downlink performance of multiuser diversity systems, the large antenna inter-element spacing required for achieving spatial diversity is prohibitive for user terminals due to size restrictions. In order to allay this problem, miniaturized switched-beam MAMP receiver designs assisted by low-cost passive reflectors are proposed. Unlike conventional spatial receive diversity systems, the proposed angular diversity architectures occupy a small volume, whereas the antenna system properties are optimized by controlling the strong reactive fields present at small dimensions. The systems are designed for maximum antenna efficiency and low inter-beam correlation, thus yielding K practically uncorrelated receive diversity branches. The simulation results show that the proposed enhanced diversity combining systems improve the average throughput of a multiuser network outperforming classical antenna selection especially for small user populations and compact user terminal size
Design and Implementation of Parasitic Antenna Arrays for Beamspace-MIMO
No full textThis chapter discusses different key issues related to the actual design and implementation of parasitic antenna arrays for the novel beamspace-MIMO concept. The proposed design strategies are illustrated by a concrete operational prototype. The chapter also briefly discusses the possibility of considering realistic antenna elements for practical portable applications as well as the effects of the operating environment on the system performance.MNWAV
Capacity Performance of Adaptive Receive Antenna Subarray Formation for MIMO Systems
No full textAntenna subarray formation is a novel RF preprocessing technique that reduces the hardware complexity of MIMO systems while alleviating the performance degradations of conventional antenna selection schemes. With this method, each RF chain is not allocated to a single antenna element, but instead to the complex-weighted and combined response of a subarray of elements. In this paper, we derive tight upper bounds on the ergodic capacity of the proposed technique for Rayleigh i.i.d. channels. Furthermore, we study the capacity performance of an analytical algorithm based on a Frobenius norm criterion when applied to both Rayleigh i.i.d. and measured MIMO channels
A stochastic geometry-based performance analysis of a UAV corridor-assisted IoT network
No full textThe exploitation of unmanned aerial vehicles (UAVs) in enhancing network performance in the context of beyond-fifth-generation (5G) communications has shown a variety of benefits compared to terrestrial counterparts. In addition, they have been largely conceived to play a central role in data dissemination to Internet of Things (IoT) devices. In the proposed work, a novel stochastic geometry unified framework is proposed to study the downlink performance in a UAV-assisted IoT network that integrates both UAV-base stations (UAV-BSs) and terrestrial IoT receiving devices. The framework builds upon the concept of the aerial UAV corridor, which is modeled as a finite line above the IoT network, and the one-dimensional (1D) binomial point process (BPP) is employed for modeling the spatial locations of the UAV-BSs in the aerial corridor. Subsequently, a comprehensive SNR-based performance analysis in terms of coverage probability, average rate, and energy efficiency is conducted under three association strategies, namely, the nth nearest-selection scheme, the random selection scheme, and the joint transmission coordinated multi-point (JT-CoMP) scheme. The numerical results reveal valuable system-level insights and trade-offs and provide a firm foundation for the design of UAV-assisted IoT networks. Copyright © 2024 Armeniakos, Maliatsos, Bithas and Kanatas
LAND MOBILE SATELLITE CHANNEL CHARACTERIZATION
No full textΗ ΔΙΔΑΚΤΟΡΙΚΗ ΔΙΑΤΡΙΒΗ ΠΟΥ ΑΚΟΛΟΥΘΕΙ ΣΤΟΧΕΥΕΙ ΣΤΟ ΧΑΡΑΚΤΗΡΙΣΜΟ ΤΟΥ ΔΟΡΥΦΟΡΙΚΟΥ ΚΑΝΑΛΙΟΥ ΚΙΝΗΤΩΝ ΕΠΙΚΟΙΝΩΝΙΩΝ, ΜΕ ΕΜΦΑΣΗ ΣΕ ΚΑΘΑΡΑ ΑΣΤΙΚΟ ΠΕΡΙΒΑΛΛΟΝ ΚΑΙ ΙΔΙΑΙΤΕΡΑ ΣΤΟ ΕΜΠΟΡΙΚΟ ΚΕΝΤΡΟ ΠΟΛΕΩΝ ΜΕ ΠΥΚΝΗ ΔΟΜΗΣΗ, ΨΗΛΑ ΚΤΙΡΙΑ ΚΑΙ ΜΕΓΑΛΗ ΚΥΚΛΟΦΟΡΙΑΚΗ ΚΙΝΗΣΗ, ΟΠΟΥ ΥΠΑΡΧΕΙ ΕΛΛΕΙΠΗΣ ΓΝΩΣΗ ΤΩΝ ΧΑΡΑΚΤΗΡΙΣΤΙΚΩΝ ΔΙΑΔΟΣΗΣ ΤΩΝΗΛΕΚΤΡΟΜΑΓΝΗΤΙΚΩΝ ΚΥΜΑΤΩΝ. Η ΜΕΛΕΤΗ ΕΠΙΚΕΝΤΡΩΝΕΤΑΙ ΣΕ ΜΕΓΑΛΕΣ ΓΩΝΙΕΣ ΑΝΥΨΩΣΗΣ (>60 ΜΟΙΡΩΝ) ΚΑΙ ΣΤΗΝ L ΖΩΝΗ ΣΥΧΝΟΤΗΤΩΝ. ΑΡΧΙΚΑ ΓΙΝΕΤΑΙ ΣΥΓΚΡΙΤΙΚΗ ΠΑΡΟΥΣΙΑΣΗ ΥΠΑΡΧΟΝΤΩΝ ΕΜΠΕΙΡΙΚΩΝ ΜΟΝΤΕΛΩΝ, ΕΝΩ ΣΤΗ ΣΥΝΕΧΕΙΑ ΠΑΡΟΥΣΙΑΖΕΤΑΙ ΕΝΑ ΝΕΟ ΕΜΠΕΙΡΙΚΟ ΜΟΝΤΕΛΟ ΥΠΟΛΟΓΙΣΜΟΥ ΤΟΥ ΑΠΑΙΤΟΥΜΕΝΟΥ ΠΕΡΙΘΩΡΙΟΥ ΙΣΧΥΟΣ, ΠΟΥ ΠΡΟΕΚΥΨΕ ΑΠΟ ΜΕΤΡΗΣΕΙΣ ΠΟΥ ΔΙΕΞΗΧΘΗΣΑΝ ΣΤΟ ΚΕΝΤΡΟ ΤΗΣ ΑΘΗΝΑΣ ΚΑΙ ΕΧΕΙ ΙΣΧΥ ΓΙΑ ΤΗΝ L ΖΩΝΗ ΣΥΧΝΟΤΗΤΩΝ ΚΑΙ ΓΙΑ ΜΕΓΑΛΕΣ ΓΩΝΙΕΣ ΑΝΥΨΩΣΗΣ (60 ΜΟΙΡΕΣ, 70 ΜΟΙΡΕΣ, 80 ΜΟΙΡΕΣ). ΚΑΤΟΠΙΝ ΑΝΑΠΤΥΣΣΕΤΑΙ ΕΝΑ ΝΕΟ ΤΡΙΣΔΙΑΣΤΑΤΟ ΜΟΝΤΕΛΟ ΣΚΕΔΑΣΗΣ ΠΟΥ ΕΝΣΩΜΑΤΩΝΕΙ ΤΗ ΣΧΕΤΙΚΗ ΚΙΝΗΣΗ ΤΟΥ ΔΟΡΥΦΟΡΟΥ ΩΣ ΠΡΟΣ ΤΟΝ ΚΙΝΗΤΟ ΔΕΚΤΗ ΚΑΙ ΤΟΥΣ ΓΕΙΤΟΝΙΚΟΥΣ ΤΟΥ ΣΚΕΔΑΣΤΕΣ, ΛΑΜΒΑΝΟΝΤΑΣ ΕΤΣΙ ΥΠΟΨΗ ΤΗΝ ΟΛΙΣΘΗΣΗ DOPPLER ΣΤΟ ΛΑΜΒΑΝΟΜΕΝΟ ΑΠΟ ΤΟ ΔΕΚΤΗ ΣΗΜΑ. ΣΤΗ ΣΥΝΕΧΕΙΑ ΑΝΑΠΤΥΣΣΕΤΑΙ ΜΙΑ ΜΕΘΟΔΟΣ ΓΙΑ ΤΗ ΣΥΝΘΕΣΗ ΕΝΟΣ ΣΗΜΑΤΟΣ ΣΕ ΥΠΟΛΟΓΙΣΤΗ, ΠΟΥ ΠΡΟΣΟΜΟΙΩΝΕΙ ΤΙΣ ΔΙΑΛΕΙΨΕΙΣ ΛΟΓΩ ΠΟΛΥΔΙΑΔΡΟΜΙΚΗΣ ΔΙΑΔΟΣΗΣ. ΣΤΗ ΣΥΝΕΧΕΙΑ ΚΑΤΑΣΚΕΥΑΖΕΤΑΙ ΕΝΑΣ ΠΡΟΣΟΜΟΙΩΤΗΣ ΣΕ ΥΠΟΛΟΓΙΣΤΗ, ΤΟΥ ΔΟΡΥΦΟΡΙΚΟΥ ΚΑΝΑΛΙΟΥ ΚΙΝΗΤΩΝ ΕΠΙΚΟΙΝΩΝΙΩΝ ΜΕ ΒΑΣΗ ΕΝΑ ΥΠΑΡΧΟΝ ΑΝΑΛΟΓΙΚΟ ΜΟΝΤΕΛΟ ΚΑΙ ΜΕ ΧΡΗΣΗ ΤΟΥ ΠΡΟΣΟΜΟΙΩΤΗ RAYLEIGH ΠΟΥ ΑΝΑΠΤΥΧΘΗΚΕ. ΤΕΛΟΣ ΓΙΝΕΤΑΙ ΜΙΑ ΛΕΠΤΟΜΕΡΗΣ ΠΑΡΟΥΣΙΑΣΗ ΤΗΣ ΜΕΘΟΔΟΛΟΓΙΑΣ ΤΩΝ ΜΕΤΡΗΣΕΩΝ ΠΟΥ ΔΙΕΞΗΧΘΗΣΑΝ ΣΤΟ ΕΜΠΟΡΙΚΟ ΚΕΝΤΡΟ ΤΗΣ ΑΘΗΝΑΣ ΚΑΙ ΤΩΝ ΑΠΟΤΕΛΕΣΜΑΤΩΝ ΤΗΣ ΣΤΑΤΙΣΤΙΚΗΣ ΑΝΑΛΥΣΗΣ ΤΩΝ ΔΕΔΟΜΕΝΩΝ ΠΟΥ ΣΥΛΛΕΧΘΗΚΑΝ.THE FOLLOWING THESIS AIMS AT THE CHARACTERIZATION OF THE LAND MOBILE SATELLITE CHANNEL AT URBAN ENVIRONMENTS, AND PARTICULARLY AT CITY CENTERS WITH NARROW STREETS, LARGE BUILDING BLOCKS AND HEAVY TRAFFIC, WHERE AT THE PRESENT TIME THERE IS A LIMITED AMOUNT OF INFORMATION FOR THE PROPAGATION CHARACTERISTICS. THE THESIS FOCUSES ON HIGH ELEVATION ANGLES (>60 DEGREES), AND ON L - BAND. A COMPARATIVE PRESENTATION OF ALL AVAILABLE EMPIRICAL MODELS IS GIVEN. A NEW EMPIRICAL MODEL EXTRACTED FROM A MEASUREMENT CAMPAIGN CONDUCTED IN ATHENS CITY CENTER, IS GIVEN AFTERWARDS, FOR THE LINK MARGIN CALCULATION AT URBAN NARROW STREETS, AT HIGH ELEVATION ANGLES (60 DEGREES, 70 DEGREES, 80 DEGREES), AND AT L -BAND. A NEW 3D THEORETICAL SCATTERING MODEL IS DEVELOPED, THAT TAKES INTO CONSIDERATION THE MOVEMENT OF THE SATELLITE WITH RESPECT TO THE MOBILE STATION,AND THE ADJACENT SCATTERS. FURTHERMORE, A METHOD IS PRESENTED, FOR CONSTRUCTING THE LAND MOBILE SATELLITE MULTIPATH FADING SIGNAL ON A COMPUTER. MOREOVER,A LAND MOBILE SATELLITE CHANNEL SIMULATOR IS DEVELOPED ON A PERSONAL COMPUTER, BASED ON AN ANALOG CHANNEL MODEL, AND THE RAYLEIGH SIMULATOR PREVIOUSLY DESCRIBED. FOLLOWING, A DETAILED ANALYSIS IS GIVEN, OF ALL EXPERIMENTAL ASPECTS AND THE RESULTS FROM THE MEASUREMENTS CONDUCTED IN ATHENS CITY CENTER
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