157 research outputs found
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Power Control and Resource Allocation for QoS-Constrained Wireless Networks
Developments such as machine-to-machine communications and multimedia services are placing growing demands on high-speed reliable transmissions and limited wireless spectrum resources. Although multiple-input multiple-output (MIMO) systems have shown the ability to provide reliable transmissions in fading channels, it is not practical for single-antenna devices to support MIMO system due to cost and hardware limitations. Cooperative communication allows single-antenna devices to share their spectrum resources and form a virtual MIMO system where their quality of service (QoS) may be improved via cooperation. Most cooperative communication solutions are based on fixed spectrum access schemes and thus cannot further improve spectrum efficiency. In order to support more users in the existing spectrum, we consider dynamic spectrum access schemes and cognitive radio techniques in this dissertation.
Our work includes the modelling, characterization and optimization of QoS-constrained cooperative networks and cognitive radio networks. QoS constraints such as delay and data rate are modelled. To solve power control and channel resource allocation problems, dynamic power control, matching theory and multi-armed bandit algorithms are employed in our investigations. In this dissertation, we first consider a cluster-based cooperative wireless network utilizing a centralized cooperation model. The dynamic power control and optimization problem is analyzed in this scenario. We then consider a cooperative cognitive radio network utilizing an opportunistic spectrum access model. Distributed spectrum access algorithms are proposed to help secondary users utilize vacant channels of primary users in order to optimize the total utility of the network. Finally, a noncooperative cognitive radio network utilizing the opportunistic spectrum access model is analyzed. In this model, primary users do not communicate with secondary users. Therefore, secondary users are required to find vacant channels on which to transmit. Multi-armed bandit algorithms are proposed to help secondary users predict the availability of licensed channels.
In summary, in this dissertation we consider both cooperative communication networks and cognitive radio networks with QoS constraints. Efficient power control and channel resource allocation schemes have been proposed for optimization problems in different scenarios.Cambridge Overseas Trust; China Scholarship Counci
Disorder and metal-insulator transitions in Weyl semimetals
The Weyl semimetal (WSM) is a newly proposed quantum state of matter. It has
Weyl nodes in bulk excitations and Fermi arcs surface states. We study the
effects of disorder and localization in WSMs and find three exotic phase
transitions. (I) Two Weyl nodes near the Brillouin zone boundary can be
annihilated pairwise by disorder scattering, resulting in the opening of a
topologically nontrivial gap and a transition from a WSM to a three-dimensional
(3D) quantum anomalous Hall state. (II) When the two Weyl nodes are well
separated in momentum space, the emergent bulk extended states can give rise to
a direct transition from a WSM to a 3D diffusive anomalous Hall metal. (III)
Two Weyl nodes can emerge near the zone center when an insulating gap closes
with increasing disorder, enabling a direct transition from a normal band
insulator to a WSM. We determine the phase diagram by numerically computing the
localization length and the Hall conductivity, and propose that the exotic
phase transitions can be realized on a photonic lattice.Comment: 7 pages with appendix, 6 figure
Theory Research on Evolution and Protection of River Ecosystem under the Influence of Human Activities
Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchive
Competitive distributed spectrum access in QoS-constrained cognitive radio networks
Dynamic spectrum access is an important issue in cognitive radio networks (CRNs) as secondary users (SUs) can benefit from accessing the vacant licensed channels of primary users (PUs). In this paper, we consider the problem of competitive distributed spectrum access in CRNs with quality of service (QoS) constraints. We first propose a distributed matching algorithm (DMA) to handle spectrum access in a QoS-constrained CRN. We then propose a fast distributed matching algorithm (FDMA) for competitive spectrum access in a large- scale CRN. The distributed algorithms for the PU and SU are given separately for practical implementation. The performance and complexity of both algorithms are analyzed and demonstrated via simulation results
Fully Automated Deep Learning-enabled Detection for Hepatic Steatosis on Computed Tomography: A Multicenter International Validation Study
Despite high global prevalence of hepatic steatosis, no automated diagnostics
demonstrated generalizability in detecting steatosis on multiple international
datasets. Traditionally, hepatic steatosis detection relies on clinicians
selecting the region of interest (ROI) on computed tomography (CT) to measure
liver attenuation. ROI selection demands time and expertise, and therefore is
not routinely performed in populations. To automate the process, we validated
an existing artificial intelligence (AI) system for 3D liver segmentation and
used it to purpose a novel method: AI-ROI, which could automatically select the
ROI for attenuation measurements. AI segmentation and AI-ROI method were
evaluated on 1,014 non-contrast enhanced chest CT images from eight
international datasets: LIDC-IDRI, NSCLC-Lung1, RIDER, VESSEL12, RICORD-1A,
RICORD-1B, COVID-19-Italy, and COVID-19-China. AI segmentation achieved a mean
dice coefficient of 0.957. Attenuations measured by AI-ROI showed no
significant differences (p = 0.545) and a reduction of 71% time compared to
expert measurements. The area under the curve (AUC) of the steatosis
classification of AI-ROI is 0.921 (95% CI: 0.883 - 0.959). If performed as a
routine screening method, our AI protocol could potentially allow early
non-invasive, non-pharmacological preventative interventions for hepatic
steatosis. 1,014 expert-annotated liver segmentations of patients with hepatic
steatosis annotations can be downloaded here:
https://drive.google.com/drive/folders/1-g_zJeAaZXYXGqL1OeF6pUjr6KB0igJX
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Patterning-Induced Ferromagnetism of Fe3GeTe2 van der Waals Materials beyond Room Temperature.
Magnetic van der Waals (vdW) materials have emerged as promising candidates for spintronics applications, especially after the recent discovery of intrinsic ferromagnetism in monolayer vdW materials. There has been a critical need for tunable ferromagnetic vdW materials beyond room temperature. Here, we report a real-space imaging study of itinerant ferromagnet Fe3GeTe2 and the enhancement of its Curie temperature well above ambient temperature. We find that the magnetic long-range order in Fe3GeTe2 is characterized by an unconventional out-of-plane stripe-domain phase. In Fe3GeTe2 microstructures patterned by a focused ion beam, the out-of-plane stripe domain phase undergoes a surprising transition at 230 K to an in-plane vortex phase that persists beyond room temperature. The discovery of tunable ferromagnetism in Fe3GeTe2 materials opens up vast opportunities for utilizing vdW magnets in room-temperature spintronics devices
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