1,721,019 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
Downlink MIMO HCNs with Residual Transceiver Hardware Impairments
Full text linkA major limitation of heterogeneous cellular networks (HCNs) is the neglect of the additive residual transceiver hardware impairments (ARTHIs). The assumption of perfect hardware is quite strong and results in misleading conclusions. This letter models a general multiple-input multiple-output HCN with cell association by incorporating the RTHIs. We derive the coverage probability and shed light on the impact of the ARTHIs, when various transmission methods are applied. As the hardware quality decreases, the coverage probability worsens. Especially, this effect is more severe as the transmit power increases. Furthermore, we verify that in an HCN, it is better to employ at each base station as few transmit antennas as possible
Hybrid Precoding for Multiuser Millimeter Wave Massive MIMO Systems: A Deep Learning Approach
Full text linkIn multi-user millimeter wave (mmWave) multiple-input-multiple-output (MIMO) systems, hybrid precoding is a crucial task to lower the complexity and cost while achieving a sufficient sum-rate. Previous works on hybrid precoding were usually based on optimization or greedy approaches. These methods either provide higher complexity or have sub-optimum performance. Moreover, the performance of these methods mostly relies on the quality of the channel data. In this work, we propose a deep learning (DL) framework to improve the performance and provide less computation time as compared to conventional techniques. In fact, we design a convolutional neural network for MIMO (CNN-MIMO) that accepts as input an imperfect channel matrix and gives the analog precoder and combiners at the output. The procedure includes two main stages. First, we develop an exhaustive search algorithm to select the analog precoder and combiners from a predefined codebook maximizing the achievable sum-rate. Then, the selected precoder and combiners are used as output labels in the training stage of CNN-MIMO where the input-output pairs are obtained. We evaluate the performance of the proposed method through numerous and extensive simulations and show that the proposed DL framework outperforms conventional techniques. Overall, CNN-MIMO provides a robust hybrid precoding scheme in the presence of imperfections regarding the channel matrix. On top of this, the proposed approach exhibits less computation time with comparison to the optimization and codebook based approaches
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
Impact of General Channel Aging Conditions on the Downlink Performance of Massive MIMO
Full text linkRecent works have identified massive multiple-input multiple-output (MIMO) as a key technology for achieving substantial gains in spectral and energy efficiency. Additionally, the turn to low-cost transceivers, being prone to hardware impairments, is the most effective and attractive way for cost-efficient applications concerning massive MIMO systems. In this context, the impact of channel aging, which severely affects performance, is investigated herein by considering a generalized model. Specifically, we show that both Doppler shift due to the users' relative movement, as well as phase noise due to noisy local oscillators, contribute to channel aging. To this end, we first propose a joint model, encompassing both effects, to investigate the performance of a massive MIMO system based on the inevitable time-varying nature of realistic mobile communications. Then, we derive the deterministic equivalents for the signal-to-noise-and-interference ratios (SINRs) with maximum ratio transmission (MRT) and regularized zero-forcing (RZF) precoding. Our analysis not only demonstrates a performance comparison between MRT and RZF under these conditions but, most importantly, also reveals interesting properties regarding the effects of user mobility and phase noise. In particular, the large antenna limit behavior profoundly depends on both effects, but the burden due to user mobility is much more detrimental than phase noise even for moderate user velocities (≈ 30 km/h), whereas the negative impact of phase noise is noteworthy at lower mobility conditions. Moreover, massive MIMO systems are favorable even in general channel aging conditions. Nevertheless, we demonstrate that the transmit power of each user to maintain a certain quality of service can be scaled down, at most, by 1√M (M is the number of base station antennas), which indicates that the joint effects of phase noise and user mobility do not degrade the power scaling law but only the achievable sum-rate
Rate-Splitting Robustness in Multi-Pair Massive MIMO Relay Systems
Full text linkRelay systems improve both coverage and system capacity. Toward this direction, a full-duplex (FD) technology, being able to boost the spectral efficiency by transmitting and receiving simultaneously on the same frequency and time resources, is envisaged to play a key role in future networks. However, its benefits come at the expense of self-interference (SI) from their own transmit signal. At the same time, massive multiple-input massive multiple-output systems, bringing unconventionally many antennas, emerge as a promising technology with huge degrees-of-freedom. To this end, this paper considers a multi-pair decode-and-forward FD relay channel, where the relay station is deployed with a large number of antennas. Moreover, the rate-splitting (RS) transmission has recently been shown to provide significant performance benefits in various multi-user scenarios with imperfect channel state information at the transmitter (CSIT). Engaging the RS approach, we employ the deterministic equivalent analysis to derive the corresponding sum-rates in the presence of interferences. Initially, numerical results demonstrate the robustness of RS in half-duplex (HD) systems, since the achievable sum-rate increases without bound, i.e., it does not saturate at high signal-to-noise ratio. Next, we tackle the detrimental effect of SI in FD. In particular, and most importantly, not only FD outperforms HD, but also RS enables increasing the range of SI over which FD outperforms HD. Furthermore, increasing the number of relay station antennas, RS appears to be more efficacious due to imperfect CSIT, since SI decreases. Interestingly, increasing the number of users, the efficiency of RS worsens and its implementation becomes less favorable under these conditions. Finally, we verify that the proposed DEs, being accurate for a large number of relay station antennas, are tight approximations even for realistic system dimensions
Statistical modelling of the physical channel in wireless digital telecommunication systems with generalized fading channels
No full textThe effective design, assessment, and installation of a wireless radio network require an accurate characterization of the propagation channel and, in particular, the small and large scale fading. By taking this into consideration, the subject of this Philosophy Diploma (PhD) dissertation is summarized in the characterization of fading with new more general statistical models and it is composed from three thematic units that concern: a) models under LOS conditions, b) models that are characterized by lack of a LOS component as well as c) useful statistical expressions for the description of random noise FM and average channel capacity for some of the new channels. An important amount of scientific work shows that the existing distributions are not enough flexible and many times are insufficient for the adaptation in data coming from experimental measurements of intensity per unit of surface of the electromagnetic field for both indoor and outdoor channels. Based on this fact, the objective of the Thesis was the production of new more general models with always natural background. Thus, initially, the natural mechanism is given for the interpretation of each new presented model. Using the mathematic formalism that results from the modelling of wireless channel via various complex Gaussian processes with different attributes, useful expressions are provided for the description and evaluation of digital communication systems that operate in fading environments. Consequently, initially, the basic theoretical background is presented that is rendered useful and essential for the study of fading channels. Next, a study is presented that concerns models that assume that the multipath part coexists with a LOS component. Then, a study of models, which assume NLOS conditions, takes place. In the next part of the PhD, in regard to some of the presented models, a study of random noise FM and ACC takes place.Λαμβάνοντας υπόψη, ότι η αποδοτική σχεδίαση, αξιολόγηση και εγκατάσταση ενός ασύρματου δικτύου επικοινωνιών, απαιτούν τον ακριβή χαρακτηρισμό του καναλιού διάδοσης και ειδικότερα των διαλείψεων μικρής και μεγάλης κλίμακας, το αντικείμενο μελέτης της παρούσας Διδακτορικής Διατριβής (ΔΔ) εστιάζεται στη μοντελοποίηση των διαλείψεων με νέα γενικότερα στατιστικά μοντέλα και απαρτίζεται από τρεις θεματικές ενότητες που αφορούν: α) μοντέλα παρουσία συνιστώσας Οπτικής Επαφής (ΟΕ), β) μοντέλα που χαρακτηρίζονται από απουσία συνιστώσας ΟΕ καθώς και γ) χρήσιμες στατιστικές εκφράσεις για τη περιγραφή του τυχαίου θορύβου FM και της μέσης χωρητικότητας του καναλιού για μερικά από τα νέα κανάλια (α-μ, α-η-μ, α-κ-μ και α-λ-η-μ). Με βάση την υφιστάμενη διεθνή βιβλιογραφία, οι κατανομές που περιγράφουν κανάλια διαλείψεων δεν είναι αρκετά ευέλικτες και πολλές φορές είναι ανεπαρκείς για τη προσαρμογή αυτών σε δεδομένα προερχόμενα από πειραματικές μετρήσεις της έντασης ανά μονάδα επιφάνειας του ηλεκτρομαγνητικού πεδίου, τόσο για κανάλια εσωτερικών, όσο και για κανάλια υπαίθριων (εξωτερικών) χώρων. Έτσι, αρχικά δίδεται ο φυσικός μηχανισμός για την ερμηνεία κάθε νέου παρουσιαζόμενου μοντέλου. Χρησιμοποιώντας το μαθηματικό φορμαλισμό που προκύπτει από την μοντελοποίηση του ασύρματου καναλιού μέσω διαφόρων μιγαδικών Gaussian διεργασιών με διαφορετικές ιδιότητες, παρέχονται χρήσιμες εκφράσεις για την περιγραφή και αξιολόγηση των ψηφιακών συστημάτων επικοινωνιών που λειτουργούν σε περιβάλλοντα διαλείψεων. Στη συνέχεια, παρουσιάζεται η μελέτη που αφορά μοντέλα που υποθέτουν ότι η συνιστώσα πολυδιόδευσης συνυπάρχει με συνιστώσα. Έπειτα, πραγματοποιείται μελέτη μοντέλων που λειτουργούν σε συνθήκες διαλείψεων, όπου δεν υφίσταται συνιστώσα ΟΕ. Στη συνέχεια της ΔΔ, αναφορικά με κάποια από τα μοντέλα που παρουσιάστηκαν, πραγματοποιείται μελέτη και διερεύνηση του τυχαίου θορύβου FM και της Μέσης Χωρητικότητας του Καναλιού (ΜΧΚ)
Ανάπτυξη μοντέλων μηχανικής μάθησης για την ανίχνευση νευροεκφυλιστικών παθήσεων σε νευροαπεικονιστικά δεδομένα
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