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Sökning: hsv:(TEKNIK OCH TEKNOLOGIER) hsv:(Elektroteknik och elektronik)

  • Resultat 41-50 av 68921
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41.
  • Wang, Chao, et al. (författare)
  • On the Degrees of Freedom of Parallel Relay Networks
  • 2010
  • Ingår i: 2010 IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE GLOBECOM 2010. - 1930-529X. - 9781424456383 - 9781424456369
  • Konferensbidrag (refereegranskat)abstract
    • We study the degrees of freedom (DOF) of a single-antenna M-user time-varying parallel relay network, where the communications between M pairs of unconnected sources and destinations are provided by a large number of half-duplex decode-and-forward (DF) relays. Unlike the conventional relaying strategy which demands all the relays to simultaneously assist the sources, we divide the relays into two clusters and permit them to take turns forwarding the source messages. With appropriate interference alignment design, it is proved that the M-user time-varying relay network has M DOF, provided that the number of relays is infinitely large.
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42.
  • Yanggratoke, Rerngvit, 1983- (författare)
  • Data-driven Performance Prediction and Resource Allocation for Cloud Services
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Cloud services, which provide online entertainment, enterprise resource management, tax filing, etc., are becoming essential for consumers, businesses, and governments. The key functionalities of such services are provided by backend systems in data centers. This thesis focuses on three fundamental problems related to management of backend systems. We address these problems using data-driven approaches: triggering dynamic allocation by changes in the environment, obtaining configuration parameters from measurements, and learning from observations. The first problem relates to resource allocation for large clouds with potentially hundreds of thousands of machines and services. We developed and evaluated a generic gossip protocol for distributed resource allocation. Extensive simulation studies suggest that the quality of the allocation is independent of the system size for the management objectives considered.The second problem focuses on performance modeling of a distributed key-value store, and we study specifically the Spotify backend for streaming music. We developed analytical models for system capacity under different data allocation policies and for response time distribution. We evaluated the models by comparing model predictions with measurements from our lab testbed and from the Spotify operational environment. We found the prediction error to be below 12% for all investigated scenarios.The third problem relates to real-time prediction of service metrics, which we address through statistical learning. Service metrics are learned from observing device and network statistics. We performed experiments on a server cluster running video streaming and key-value store services. We showed that feature set reduction significantly improves the prediction accuracy, while simultaneously reducing model computation time. Finally, we designed and implemented a real-time analytics engine, which produces model predictions through online learning.
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43.
  • Zarifi, D., et al. (författare)
  • A Gap Waveguide-Based D-Band Slot Array Antenna with Interdigital Feed Network
  • 2023
  • Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X .- 1558-2221. ; 71:9, s. 7124-7131
  • Tidskriftsartikel (refereegranskat)abstract
    • This work presents a high gain D-band slot array antenna with a corporate-fed distribution network designed by the gap waveguide. The gap waveguide technology is employed for overcoming the challenge of appropriate electrical contact related to the mechanical assemblage of the antenna feed network and the radiation layer. The antenna includes 24×16 slots and the basis of the antenna interdigital feeding network includes the ridge and groove gap waveguides. A standard WR-6 waveguide flange at the bottom is used to excite the presented antenna. To understand the fabrication challenges and tolerance issues around 140GHz, an antenna prototype was made by standard CNC milling techniques. The results show that an impedance bandwidth of 5.5% (141.8-149.8 GHz), efficiency higher than 78%, and a peak gain of 33.8 dBi are achieved for the proposed 24×16-element array antenna. According to the results, the presented planar array is a good option for D-band applications and can be used as the building block to develop high efficiency and high gain upper mm-wave antennas.
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44.
  • Zarifi, Davoud, et al. (författare)
  • Design of a dual-CP gap waveguide fed aperture array antenna
  • 2023
  • Ingår i: IET Microwaves, Antennas and Propagation. - 1751-8725 .- 1751-8733. ; 17:9, s. 723-730
  • Tidskriftsartikel (refereegranskat)abstract
    • A slot array antenna with dual-circular polarisation (CP) is presented in this paper. To overcome the fabrication and loss challenges in mm-wave frequencies, the gap waveguide technique is utilised. The right and left hand circular polarisation (RHCP and LHCP) radiations are achieved using slots-fed stepped septum polariser on the longitudinal slot arrays. The waveguides and feeding layer are based on ridge gap waveguide. The experimental results demonstrate the |S-11| and axial ratio of proposed antenna array are lower than -10 and 2 dB, respectively over the frequency range of 29.1-31.7 GHz. The proposed antenna exhibits the measured peak gain of 27.2 dBic at 30.2 GHz. The results prove that the proposed array antenna is a brilliant choice for 30 GHz-band applications and could be developed to realise larger dual-polarised planar arrays.
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45.
  • Zhang, Kewei (författare)
  • Secure GNSS-based Positioning and Timing : Distance-Decreasing attacks, fault detection and exclusion, and attack detection with the help of opportunistic signals
  • 2021
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • With trillions of devices connected in large scale systems in a wired or wireless manner, positioning and synchronization become vital. Global Navigation Satellite System (GNSS) is the first choice to provide global coverage for positioning and synchronization services. From small mobile devices to aircraft, from intelligent transportation systems to cellular networks, and from cargo tracking to smart grids, GNSS plays an important role, thus, requiring high reliability and security protection.       However, as GNSS signals propagate from satellites to receivers at distance of around 20 000 km, the signal power arriving at the receivers is very low, making the signals easily jammed or overpowered. Another vulnerability stems from that civilian GNSS signals and their specifications are publicly open, so that anyone can craft own signals to spoof GNSS receivers: an adversary forges own GNSS signals and broadcasts them to the victim receiver, to mislead the victim into believing that it is at an adversary desired location or follows a false trajectory, or adjusts its clock to a time dictated by the adversary. Another type of attack is replaying GNSS signals: an adversary transmits a pre-recorded GNSS signal stream to the victim receiver, so that the receiver calculates an erroneous position and time. Recent incidents reported in press show that the GNSS functionalities in a certain area, e.g., Black Sea, have been affected by cyberattacks composed of the above-mentioned attack types.        This thesis, thus, studies GNSS vulnerabilities and proposes detection and mitigation methods for GNSS attacks, notably spoofing and replay attacks. We analyze the effectiveness of one important and powerful replay attack, the so-called Distance-decreasing (DD) attacks that were previously investigated for wireless communication systems, on GNSS signals. DD attacks are physical layer attacks, targeting time-of-flight ranging protocols, to shorten the perceived as measured distance between the transmitter and receiver. The attacker first transmits an adversary-chosen data bit to the victim receiver before the signal arrives at the attacker; upon receipt of the GNSS signal, the attacker estimates the data bit based on the early fraction of the bit period, and then switches to transmitting the estimate to the victim receiver. Consequently, the DD signal arrives at the victim receiver earlier than the genuine GNSS signals would have, which in effect shortens the pseudorange measurement between the sender (satellite) and the victim receiver, consequently, affecting the calculated position and time of the receiver. We study how the DD attacks affect the bit error rate (BER) of the received signals at the victim, and analyze its effectiveness, that is, the ability to shorten pseudorange measurements, on different GNSS signals. Several approaches are considered for the attacker to mount a DD attack with high probability of success (without being detected) against a victim receiver, for cryptographically unprotected and protected signals. We analyze the tracking output of the DD signals at the victim receiver and propose a Goodness of Fit (GoF) test and a Generalized Likelihood Ratio Test (GLRT) to detect the attacks. The evaluation of the two tests shows that they are effective, with the result being perhaps more interesting when considering DD attacks against Galileo signals that can be cryptographically protected.       Moreover, this thesis investigates the feasibility of validating the authenticity of the GNSS signals with the help of opportunistic signals, which is information readily available in modern communication environments, e.g., 3G, 4G and WiFi. We analyze the time synchronization accuracy of different technologies, e.g., Network Time Protocol (NTP), WiFi and local oscillator, as the basis for detecting a discrepancy with the GNSS-obtained time. Two detection approaches are proposed and one testbench is designed for the evaluation. A synthesized spoofing attack is used to verify the effectiveness of the approaches.       Beyond attack detection, we develop algorithms to detect and exclude faulty signals, namely the Clustering-based Solution Separation Algorithm (CSSA) and the Fast Multiple Fault Detection and Exclusion (FM-FDE). They both utilize the redundant available satellites, more than the minimum a GNSS receiver needs for position and time offset calculation. CSSA adopts data clustering to group subsets of positions calculated with different subsets of available satellites. Basically, these positions, calculated with subsets not containing any faulty satellites, should be close to each other, i.e., in a dense area; otherwise they should be scattered. FM-FDE is a more efficient algorithm that uses distances between positions, calculated with fixed-size subsets, as test statistics to detect and exclude faulty satellite signals. As the results show, FM-FDE runs faster than CSSA and other solution-separation fault detection and exclusion algorithms while remaining equally effective.
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46.
  • Zhang, Lu, et al. (författare)
  • Lattice pilot aided DMT transmission for optical interconnects achieving 5.82-bits/Hz per lane
  • 2019
  • Ingår i: IET Conference Publications. - : Institution of Engineering and Technology. ; 2019:CP765
  • Konferensbidrag (refereegranskat)abstract
    • We experimentally demonstrate 276-Gbps net rate 128QAM-DMT transmission at soft-FEC limit in an IM/DD system with a single C-band packaged EML, a DAC and a photo-detector, proposing a lattice pilot algorithm for channel equalization with only 1% overhead reaching spectral efficiency of 5.82-bits/Hz per lane. 
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47.
  • Aydin, Osman, et al. (författare)
  • EU FP7 INFSO-ICT-317669 METIS, D 4.1 Summary on preliminary trade-off investigations and first set of potential network-level solutions
  • 2013
  • Rapport (övrigt vetenskapligt/konstnärligt)abstract
    • METIS WP4 covers research activities in network-level aspects of the advancement of wireless network technologies towards the year 2020 and beyond. The aim is to develop novel network-level technology concepts to address the challenges foreseen in future scenarios with regard to interference, traffic and mobility management issues. Moreover, another task of this work package is to propose functional enablers which can support the above potential solutions.This document provides* a report of the ongoing progress in WP4 regarding the research topics agreed upon in IR 4.1,* a high level description of the proposed concepts and approaches adopted by different partners.More specifically, the document describes, first set of potential network-level solutions and presents some first research results in order to position them with regards to the state of the art approaches. It also gives an overview of research activities to be considered later in WP4.
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48.
  • Beygi, Lotfollah, 1977 (författare)
  • Channel-Aware Multilevel Coded Modulation for Coherent Fiber-Optic Communications
  • 2013
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The past decades have shown an ever-increasing demand for high-rate Internet services, motivating a great effort to increase the spectral efficiency of optical networks. In general, fiber-optic links are non-Gaussian, and in contrast to additive white Gaussian noise (AWGN) channels, there is no standard framework for quantifying fundamental limits or designing capacity-approaching coding schemes for such channels. In this thesis, some steps are taken toward this challenging goal by first developing a channel model for fiber-optic links and, second, using an information-theoretic design framework to investigate joint design of forward error correction and multilevel modulation, so-called coded modulation (CM), techniques for these channels.We extend the signal statistics of highly nonlinear single-polarization fiber-optic links with negligible dispersion to the polarization-multiplexed case. Taking chromatic dispersion into account, we derive an analytical discrete-time model for single- wavelength, polarization-multiplexed, non-dispersion-managed (non-DM) links. According to this model, for high enough symbol rates, a fiber-optic link can be described as a linear dispersive channel with AWGN and a complex constant scaling.We exploit the proposed channel model for highly nonlinear fiber-optic links to devise a new channel-aware multilevel CM scheme based on the minimization of the total block error rate. We introduce a CM system with an N-dimensional constellation constructed from the Cartesian product of N identical one-dimensional constellations. The multidimensional scheme shows better trade-off between complexity and performance than a one-dimensional multilevel CM scheme.By invoking the introduced channel model for the dispersive non-DM links, we present a four-dimensional CM scheme, which shows a better trade-off between digital signal processing complexity and transparent reach than existing methods. This CM scheme together with a probabilistic signal shaping method is used to devise a rate-adaptive scheme with a single low-density parity-check code. The performance evaluation of the proposed CM scheme for a single-channel transmission fiber-optic system justifies the improvement of the system spectral efficiency for a wide range of transparent reaches, observing more than 1 dB performance gain compared to existing methods.
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49.
  • Björnson, Emil, et al. (författare)
  • DISTRIBUTED MASSIVE MIMO IN CELLULAR NETWORKS: IMPACT OF IMPERFECT HARDWARE AND NUMBER OF OSCILLATORS
  • 2015
  • Ingår i: 2015 23rd European Signal Processing Conference. - : Institute of Electrical and Electronics Engineers (IEEE). - 2076-1465. - 9780992862633 ; , s. 2436-2440
  • Konferensbidrag (refereegranskat)abstract
    • Distributed massive multiple-input multiple-output (MIMO) combines the array gain of coherent MIMO processing with the proximity gains of distributed antenna setups. In this paper, we analyze how transceiver hardware impairments affect the downlink with maximum ratio transmission. We derive closed-form spectral efficiencies expressions and study their asymptotic behavior as the number of the antennas increases. We prove a scaling law on the hardware quality, which reveals that massive MIMO is resilient to additive distortions, while multiplicative phase noise is a limiting factor. It is also better to have separate oscillators at each antenna than one per BS.
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50.
  • Björnson, Emil, et al. (författare)
  • Massive MIMO Systems With Non-Ideal Hardware : Energy Efficiency, Estimation, and Capacity Limits
  • 2014
  • Ingår i: IEEE Transactions on Information Theory. - : IEEE Press. - 0018-9448 .- 1557-9654. ; 60:11, s. 7112-7139
  • Tidskriftsartikel (refereegranskat)abstract
    • The use of large-scale antenna arrays can bring substantial improvements in energy and/or spectral efficiency to wireless systems due to the greatly improved spatial resolution and array gain. Recent works in the field of massive multiple-input multiple-output (MIMO) show that the user channels decorrelate when the number of antennas at the base stations (BSs) increases, thus strong signal gains are achievable with little interuser interference. Since these results rely on asymptotics, it is important to investigate whether the conventional system models are reasonable in this asymptotic regime. This paper considers a new system model that incorporates general transceiver hardware impairments at both the BSs (equipped with large antenna arrays) and the single-antenna user equipments (UEs). As opposed to the conventional case of ideal hardware, we show that hardware impairments create finite ceilings on the channel estimation accuracy and on the downlink/uplink capacity of each UE. Surprisingly, the capacity is mainly limited by the hardware at the UE, while the impact of impairments in the large-scale arrays vanishes asymptotically and interuser interference (in particular, pilot contamination) becomes negligible. Furthermore, we prove that the huge degrees of freedom offered by massive MIMO can be used to reduce the transmit power and/or to tolerate larger hardware impairments, which allows for the use of inexpensive and energy-efficient antenna elements.
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