11,995 research outputs found
Spectra of cosmic ray electrons and diffuse gamma rays with the constraints of AMS-02 and HESS data
Recently, AMS-02 reported their observed results of cosmic rays(CRs). In
addition to the AMS-02 data, we add HESS data to estimate the spectra of CR
electrons and the diffuse gamma rays above TeV. In the conventional diffusion
model, a global analysis is performed on the spectral features of CR electrons
and the diffuse gamma rays by GALRPOP package. The results show that the
spectrum structure of the primary component of CR electrons can not be fully
reproduced by a simple power law and the relevant break is around hundred GeV.
At 99\% C.L., the injection indices above the break decrease from 2.54 to 2.35,
but the ones below the break are only in the range 2.746 - 2.751. The spectrum
of CR electrons does not need to add TeV cutoff to match the features of HESS
data too. Based on the difference between the fluxes of CR electrons and the
primary component of them, the predicted excess of CR positrons is consistent
with the interpretations as pulsar or dark matter. In the analysis of the
Galactic diffuse gamma rays with the indirect constraint of AMS-02 and HESS
data, it is found that the fluxes of Galactic diffuse gamma rays are consistent
with GeV data of Fermi-LAT in the high latitude regions. The results indicate
that the inverse Compton scattering(IC) is the dominant component in the range
of the hundred GeV to tens of TeV respectively from the high latitude regions
to the low ones, and in the all regions of Galaxy the flux of diffuse gamma
rays is less than CR electrons at the energy scale of 20 TeV.Comment: 25 pages, 6 figures and 5 tables, revised version accepted for
publication in AP
Network-Connected UAV Communications: Potentials and Challenges
This article explores the use of network-connected unmanned aerial vehicle
(UAV) communications as a compelling solution to achieve high-rate information
transmission and support ultra-reliable UAV remote command and control. We
first discuss the use cases of UAVs and the resulting communication
requirements, accompanied with a flexible architecture for network-connected
UAV communications. Then, the signal transmission and interference
characteristics are theoretically analyzed, and subsequently we highlight the
design and optimization considerations, including antenna design,
non-orthogonal multiple access communications, as well as network selection and
association optimization. Finally, case studies are provided to show the
feasibility of network-connected UAV communications
Power Control in UAV-Supported Ultra Dense Networks: Communications, Caching, and Energy Transfer
By means of network densification, ultra dense networks (UDNs) can
efficiently broaden the network coverage and enhance the system throughput. In
parallel, unmanned aerial vehicles (UAVs) communications and networking have
attracted increasing attention recently due to their high agility and numerous
applications. In this article, we present a vision of UAV-supported UDNs.
Firstly, we present four representative scenarios to show the broad
applications of UAV-supported UDNs in communications, caching and energy
transfer. Then, we highlight the efficient power control in UAV-supported UDNs
by discussing the main design considerations and methods in a comprehensive
manner. Furthermore, we demonstrate the performance superiority of
UAV-supported UDNs via case study simulations, compared to traditional fixed
infrastructure based networks. In addition, we discuss the dominating technical
challenges and open issues ahead
A Novel Low Power UWB Cascode SiGe BiCMOS LNA with Current Reuse and Zero-Pole Cancellation
A low power cascode SiGe BiCMOS low noise amplifier (LNA) with current reuse
and zero-pole cancellation is presented for ultra-wideband (UWB) application.
The LNA is composed of cascode input stage and common emitter (CE) output stage
with dual loop feedbacks. The novel cascode-CE current reuse topology replaces
the traditional two stages topology so as to obtain low power consumption. The
emitter degenerative inductor in input stage is adopted to achieve good input
impedance matching and noise performance. The two poles are introduced by the
emitter inductor, which will degrade the gain performance, are cancelled by the
dual loop feedbacks of the resistance-inductor (RL) shunt-shunt feedback and
resistance-capacitor (RC) series-series feedback in the output stage.
Meanwhile, output impedance matching is also achieved. Based on TSMC 0.35{\mu}m
SiGe BiCMOS process, the topology and chip layout of the proposed LNA are
designed and post-simulated. The LNA achieves the noise figure of 2.3~4.1dB,
gain of 18.9~20.2dB, gain flatness of \pm0.65dB, input third order intercept
point (IIP3) of -7dBm at 6GHz, exhibits less than 16ps of group delay
variation, good input and output impedances matching, and unconditionally
stable over the whole band. The power consuming is only 18mW.Comment: 7 pages, 13 figure
DR^2Track: Towards Real-Time Visual Tracking for UAV via Distractor Repressed Dynamic Regression
Visual tracking has yielded promising applications with unmanned aerial
vehicle (UAV). In literature, the advanced discriminative correlation filter
(DCF) type trackers generally distinguish the foreground from the background
with a learned regressor which regresses the implicit circulated samples into a
fixed target label. However, the predefined and unchanged regression target
results in low robustness and adaptivity to uncertain aerial tracking
scenarios. In this work, we exploit the local maximum points of the response
map generated in the detection phase to automatically locate current
distractors. By repressing the response of distractors in the regressor
learning, we can dynamically and adaptively alter our regression target to
leverage the tracking robustness as well as adaptivity. Substantial experiments
conducted on three challenging UAV benchmarks demonstrate both excellent
performance and extraordinary speed (~50fps on a cheap CPU) of our tracker.Comment: 8pages, 7 figures, accepted by 2020 IEEE/RJS International Conference
on Intelligent Robots and Systems(IROS
Automatic Failure Recovery and Re-Initialization for Online UAV Tracking with Joint Scale and Aspect Ratio Optimization
Current unmanned aerial vehicle (UAV) visual tracking algorithms are
primarily limited with respect to: (i) the kind of size variation they can deal
with, (ii) the implementation speed which hardly meets the real-time
requirement. In this work, a real-time UAV tracking algorithm with powerful
size estimation ability is proposed. Specifically, the overall tracking task is
allocated to two 2D filters: (i) translation filter for location prediction in
the space domain, (ii) size filter for scale and aspect ratio optimization in
the size domain. Besides, an efficient two-stage re-detection strategy is
introduced for long-term UAV tracking tasks. Large-scale experiments on four
UAV benchmarks demonstrate the superiority of the presented method which has
computation feasibility on a low-cost CPU.Comment: 8pages, 8 figures, accepted by 2020 IEEE/RSJ International Conference
on Intelligent Robots and Systems(IROS
Raman study of electron-phonon coupling in thin films of LiTiO spinel oxide superconductor
We performed a Raman scattering study of thin films of LiTiO spinel
oxide superconductor. We detected four out of five Raman active modes, with
frequencies in good accordance with our first-principles calculations. Three
T modes show a Fano lineshape from 5 K to 295 K, which suggests an
electron-phonon coupling in LiTiO. Interestingly, the electron-phonon
coupling shows an anomaly across the negative to positive magnetoresistance
transition at 50 K, which may be due to the unset of other competing orders.
The strength of the electron-phonon interaction estimated from the Allen's
formula and the observed lineshape parameters suggests that the three T
modes contribute little to superconductivity.Comment: 5 pages, 3 figures, 1 tabl
Edge states in self-complementary checkerboard photonic crystals: Zak phase, surface impedance and experimental verification
Edge states of photonic crystals have attracted much attention for the
potential applications such as high transmission waveguide bends, spin
dependent splitters and one-way photonic circuits. Here, we theoretically
discuss and experimentally observe the deterministic edge states in
checkerboard photonic crystals. Due to the self-complementarity of checkerboard
photonic crystals, a common band gap is structurally protected between two
photonic crystals with different unit cells. Deterministic edge states are
found inside the common band gap by exploiting the Zak phase analysis and
surface impedance calculation. These edge states are also confirmed by a
microwave experiment.Comment: 13 pages, 4 figure
Performance Optimization of Region-Based Group Key Management in Mobile Ad Hoc Networks
We propose and analyze a scalable and efficient region-based group key management protocol for secure group communications in mobile ad hoc networks. For scalability and dynamic reconfigurability, we take a region-based approach by which group members are broken into region-based subgroups. Leaders in subgroups securely communicate with each other to agree on a group key in response to membership change and member mobility-induced events. We propose a novel approach to identify the optimal setting of the region-based key management protocol to maximize the performance of the system. We show that secrecy requirements for secure group communication are satisfied, and that there exists an optimal region size that minimizes the network traffic as a result of efficiently trading inter-regional vs. intra-regional group key management overheads. We compare the proposed region-based key management protocol with non-region-based key management to demonstrate the effectiveness. Analytical results are validated by extensive simulation.
Unmanned Aerial Vehicle-Aided Communications: Joint Transmit Power and Trajectory Optimization
This letter investigates the transmit power and trajectory optimization
problem for unmanned aerial vehicle (UAV)-aided networks. Different from
majority of the existing studies with fixed communication infrastructure, a
dynamic scenario is considered where a flying UAV provides wireless services
for multiple ground nodes simultaneously. To fully exploit the controllable
channel variations provided by the UAV's mobility, the UAV's transmit power and
trajectory are jointly optimized to maximize the minimum average throughput
within a given time length. For the formulated non-convex optimization with
power budget and trajectory constraints, this letter presents an efficient
joint transmit power and trajectory optimization algorithm. Simulation results
validate the effectiveness of the proposed algorithm and reveal that the
optimized transmit power shows a water-filling characteristic in spatial
domain.Comment: IEEE Wireless Communications Letters, 201
- …