arXiv subjects

Yuan Zheng

Publications and source records attributed to Yuan Zheng.

33 records · Page 2Linked to original sources

arXiv2020

Joint Beamforming and Power Control for Throughput Maximization in IRS-assisted MISO WPCNs

Intelligent reflecting surface (IRS) is an emerging technology to enhance the energy- and spectrum-efficiency of wireless powered communication networks (WPCNs). In this paper, we investigate an IRS-assisted multiuser multiple-input single-output (MISO) WPCN, where the single-antenna wireless devices (WDs) harvest wireless energy in the downlink (DL) and transmit their information simultaneously in the uplink (UL) to a common hybrid access point (HAP) equipped with multiple antennas. Our goal is to maximize the weighted sum rate (WSR) of all the energy-harvesting users. To make full use of the beamforming gain provided by both the HAP and the IRS, we jointly optimize the active beamforming of the HAP and the reflecting coefficients (passive beamforming) of the IRS in both DL and UL transmissions, as well as the transmit power of the WDs to mitigate the inter-user interference at the HAP. To tackle the challenging optimization problem, we first consider fixing the passive beamforming, and converting the remaining joint active beamforming and user transmit power control problem into an equivalent weighted minimum mean square error (WMMSE) problem, where we solve it using an efficient block-coordinate descent (BCD) method. Then, we fix the active beamforming and user transmit power, and optimize the passive beamforming coefficients of the IRS in both the DL and UL using a semidefinite relaxation (SDR) method. Accordingly, we apply a block-structured optimization (BSO) method to update the two sets of variables alternately. Numerical results show that the proposed joint optimization achieves significant performance gain over other representative benchmark methods and effectively improves the throughput performance in multiuser MISO WPCNs.

cs.NI↗

arXiv2020

Time-Optimal Guidance to Intercept Moving Targets by Dubins Vehicles

This paper is concerned with a Minimum-Time Intercept Problem (MTIP), for which a Dubins vehicle is guided from a position with a prescribed initial orientation angle to intercept a moving target in minimum time. Some geometric properties for the solution of the MTIP are presented, showing that the solution path must lie in a sufficient family of 4 candidates. In addition, necessary and sufficient conditions for optimality of each candidate are established. When the target's velocity is constant, by employing the geometric properties, those 4 candidates are transformed to a class of sufficiently smooth real-valued functions. In order to compute all the 4 candidates, an efficient and robust algorithm to find all the zeros of sufficiently smooth real-valued functions is developed. Since the MTIP with a constant target's velocity is equivalent to the path planning problem of Dubins vehicle in a constant drift field, developing such an algorithm also enables efficiently finding the shortest Dubins path in a constant drift field. Finally, some numerical examples are presented, demonstrating and verifying the developments of the paper.

math.OC↗

arXiv2020

Real-Time Cardiac Cine MRI with Residual Convolutional Recurrent Neural Network

Real-time cardiac cine MRI does not require ECG gating in the data acquisition and is more useful for patients who can not hold their breaths or have abnormal heart rhythms. However, to achieve fast image acquisition, real-time cine commonly acquires highly undersampled data, which imposes a significant challenge for MRI image reconstruction. We propose a residual convolutional RNN for real-time cardiac cine reconstruction. To the best of our knowledge, this is the first work applying deep learning approach to Cartesian real-time cardiac cine reconstruction. Based on the evaluation from radiologists, our deep learning model shows superior performance than compressed sensing.

eess.IV↗

arXiv2020

Throughput Optimization of Intelligent Reflecting Surface Assisted User Cooperation in WPCNs

Intelligent reflecting surface (IRS) can effectively enhance the energy and spectral efficiency of wireless communication system through the use of a large number of lowcost passive reflecting elements. In this paper, we investigate throughput optimization of IRS-assisted user cooperation in a wireless powered communication network (WPCN), where the two WDs harvest wireless energy and transmit information to a common hybrid access point (HAP). In particular, the two WDs first exchange their independent information with each other and then form a virtual antenna array to transmit jointly to the HAP. We aim to maximize the common (minimum) throughput performance by jointly optimizing the transmit time and power allocations of the two WDs on wireless energy and information transmissions and the passive array coefficients on reflecting the wireless energy and information signals. By comparing with some existing benchmark schemes, our results show that the proposed IRS-assisted user cooperation method can effectively improve the throughput performance of cooperative transmission in WPCNs.

cs.NI↗

arXiv2020

Projective Quasiparticle Interference of a Single Scatterer to Analyze the Electronic Band Structure of ZrSiS

Quasiparticle interference (QPI) of the electronic states has been widely applied in scanning tunneling microscopy (STM) to analyze the electronic band structure of materials. Single-defect induced QPI reveals defect-dependent interaction between a single atomic defect and electronic states, which deserves special attention. Due to the weak signal of single-defect-induced QPI, the signal-to-noise ratio (SNR) is relatively low in a standard two-dimensional QPI measurement. In this paper, we introduce a projective quasiparticle interference (PQPI) method, in which a one-dimensional measurement is taken along high-symmetry directions centered on a specified defect. We apply the PQPI method to a topological nodal-line semimetal ZrSiS. We focus on two special types of atomic defects that scatter the surface and bulk electronic bands. With enhanced SNR in PQPI, the energy dispersions are clearly resolved along high symmetry directions. We discuss the defect-dependent scattering of bulk bands with the non-symmorphic symmetry-enforced selection rules. Furthermore, an energy shift of the surface floating band is observed and a new branch of energy dispersion (q6) is resolved. This PQPI method can be applied to other complex materials to explore defect-dependent interactions in the future.

cond-mat.mtrl-sci↗

arXiv2020

Possible strain induced Mott gap collapse in 1T-TaS$_2$

Tuning the electronic properties of a matter is of fundamental interest in scientific research as well as in applications. Recently, the Mott insulator-metal transition has been reported in a pristine layered transition metal dichalcogenides 1T-TaS$_2$, with the transition triggered by an optical excitation, a gate controlled intercalation, or a voltage pulse. However, the sudden insulator-metal transition hinders an exploration of how the transition evolves. Here, we report the strain as a possible new tuning parameter to induce Mott gap collapse in 1T-TaS$_2$. In a strain-rich area, we find a mosaic state with distinct electronic density of states within different domains. In a corrugated surface, we further observe and analyze a smooth evolution from a Mott gap state to a metallic state. Our results shed new lights on the understanding of the insulator-metal transition and promote a controllable strain engineering on the design of switching devices in the future.

cond-mat.str-el↗

arXiv2020

Reusing Wireless Power Transfer for Backscatter-assisted Relaying in WPCNs

User cooperation is an effective technique to tackle the severe near-far user unfairness problem in wireless powered communication networks (WPCNs). In this paper, we consider a WPCN where two collaborating wireless devices (WDs) first harvest wireless energy from a hybrid access point (HAP) and then transmit their information to the HAP. The WD with the stronger WD-to-HAP channel helps relay the message of the other weaker user. In particular, we exploit the use of ambient backscatter communication during the wireless energy transfer phase, where the weaker user backscatters the received energy signal to transmit its information to the relay user in a passive manner. By doing so, the relay user can reuse the energy signal for simultaneous energy harvesting and information decoding (e.g., using an energy detector). Compared to active information transmission in conventional WPCNs, the proposed method effectively saves the energy and time consumed by the weaker user on information transmission during cooperation. With the proposed backscatter-assisted relaying scheme, we jointly optimize the time and power allocations on wireless energy and information transmissions to maximize the common throughput. Specifically, we derive the semi-closed-form expressions of the optimal solution and propose a low-complexity optimal algorithm to solve the joint optimization problem. By comparing with some representative benchmark methods, we simulate under extensive network setups and demonstrate that the proposed cooperation method effectively improves the throughput performance in WPCNs.

cs.NI↗

arXiv2019

PTB-TIR: A Thermal Infrared Pedestrian Tracking Benchmark

Thermal infrared (TIR) pedestrian tracking is one of the important components among numerous applications of computer vision, which has a major advantage: it can track pedestrians in total darkness. The ability to evaluate the TIR pedestrian tracker fairly, on a benchmark dataset, is significant for the development of this field. However, there is not a benchmark dataset. In this paper, we develop a TIR pedestrian tracking dataset for the TIR pedestrian tracker evaluation. The dataset includes 60 thermal sequences with manual annotations. Each sequence has nine attribute labels for the attribute based evaluation. In addition to the dataset, we carry out the large-scale evaluation experiments on our benchmark dataset using nine publicly available trackers. The experimental results help us understand the strengths and weaknesses of these trackers.In addition, in order to gain more insight into the TIR pedestrian tracker, we divide its functions into three components: feature extractor, motion model, and observation model. Then, we conduct three comparison experiments on our benchmark dataset to validate how each component affects the tracker's performance. The findings of these experiments provide some guidelines for future research. The dataset and evaluation toolkit can be downloaded at {https://github.com/QiaoLiuHit/PTB-TIR_Evaluation_toolkit}.

cs.CV↗

arXiv2019

The study of intrinsic defect state of FeSe with scanning tunneling microscopy

We apply high resolution scanning tunneling microscopy to study intrinsic defect states of bulk FeSe. Four types of intrinsic defects including the type I dumbbell, type II dumbbell, top-layer Se vacancy and inner-layer Se-site defect are extensively analyzed by scanning tunneling spectroscopy. From characterized depression and enhancement of density of states measured in a large energy range, the type I dumbbell and type II dumbbell are determined to be the Fe vacancy and Se$_\mathrm{Fe}$ defect, respectively. The top-layer Se vacancy and possible inner-layer Se-site vacancy are also determined by spectroscopy analysis. The determination of defects are compared and largely confirmed in the annular dark-field scanning transmission electron microscopy measurement of the exfoliated FeSe. The detailed mapping of defect states in our experiment lays the foundation for a comparison with complex theoretical calculations in the future.

cond-mat.supr-con↗

arXiv2019

Visualizing the charge order and topological defects in an overdoped (Bi,Pb)$_2$Sr$_2$CuO$_{6+x}$ superconductor

Electronic charge order is a symmetry breaking state in high-$T_\mathrm{c}$ cuprate superconductors. In scanning tunneling microscopy, the detected charge-order-induced modulation is an electronic response of the charge order. For an overdoped (Bi,Pb)$_2$Sr$_2$CuO$_{6+x}$ sample, we apply scanning tunneling microscopy to explore local properties of the charge order. The ordering wavevector is non-dispersive with energy, which can be confirmed and determined. By extracting its order-parameter field, we identify dislocations in the stripe structure of the electronic modulation, which correspond to topological defects with an integer winding number of $\pm 1$. Through differential conductance maps over a series of reduced energies, the development of different response of the charge order is observed and a spatial evolution of topological defects is detected. The intensity of charge-order-induced modulation increases with energy and reaches its maximum when approaching the pseudogap energy. In this evolution, the topological defects decrease in density and migrate in space. Furthermore, we observe appearance and disappearance of closely spaced pairs of defects as energy changes. Our experimental results could inspire further studies of the charge order in both high-$T_\mathrm{c}$ cuprate superconductors and other charge density wave materials.

cond-mat.supr-con↗

arXiv2018

Backscatter-assisted Relaying in Wireless Powered Communications Network

This paper studies a novel cooperation method in a two-user wireless powered communication network (WPCN), in which one hybrid access point (HAP) broadcasts wireless energy to two distributed wireless devices (WDs), while the WDs use the harvested energy to transmit their independent information to the HAP. To tackle the user unfairness problem caused by the near-far effect in WPCN, we allow the WD with the stronger WD-to-HAP channel to use part of its harvested energy to help relay the other weaker user's information to the HAP. In particular, we exploit the use of backscatter communication during the wireless energy transfer phase such that the helping relay user can harvest energy and receive the information from the weaker user simultaneously. We derive the maximum common throughput performance by jointly optimizing the time duration and power allocations on wireless energy and information transmissions. Our simulation results demonstrate that the backscatter-assisted cooperation scheme can effectively improve the throughput fairness performance in WPCNs.

cs.NI↗

arXiv2018

Electronic effect of doped oxygen atoms in Bi2201 superconductors determined by scanning tunneling microscopy

The oxygen dopants are essential in tuning electronic properties of Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4+δ}$ superconductors. Here we apply the technique of scanning tunneling microscopy and spectroscopy to study the influence of oxygen dopants in an optimally doped Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ and an overdoped Bi$_{2-y}$Pb$_y$Sr$_2$CuO$_{6+δ}$. In both samples, we find that interstitial oxygen atoms on the SrO layers dominate over the other two forms of oxygen dopants, oxygen vacancies on the SrO layers and interstitial oxygen atoms on the BiO layers. The hole doping is estimated from the oxygen concentration, as compared to the result extracted from the measured Fermi surface. The precise spatial location is employed to obtain a negative correlation between the oxygen dopants and the inhomogeneous pseudogap.

cond-mat.supr-con↗

arXiv2018

The study of electronic nematicity in an overdoped (Bi, Pb)$_2$Sr$_2$CuO$_{6+δ}$ superconductor using scanning tunneling spectroscopy

The pseudogap (PG) state and its related intra-unit-cell symmetry breaking remain the focus in the research of cuprate superconductors. Although the nematicity has been studied in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, especially underdoped samples, its behavior in other cuprates and different doping regions is still unclear. Here we apply a scanning tunneling microscope to explore an overdoped (Bi, Pb)$_2$Sr$_2$CuO$_{6+δ}$ with a large Fermi surface (FS). The establishment of a nematic order and its real-space distribution is visualized as the energy scale approaches the PG.

cond-mat.supr-con↗

arXiv2018

Visualization of electronic topology in ZrSiSe by scanning tunneling microscopy

As emerging topological nodal-line semimetals, the family of ZrSiX (X = O, S, Se, Te) has attracted broad interests in condensed matter physics due to their future applications in spintonics. Here, we apply a scanning tunneling microscopy (STM) to study the structural symmetry and electronic topology of ZrSiSe. The glide mirror symmetry is verified by quantifying the lattice structure of the ZrSe bilayer based on bias selective topographies. The quasiparticle interference analysis is used to identify the band structure of ZrSiSe. The nodal line is experimentally determined at $\sim$ 250 meV above the Fermi level. An extra surface state Dirac point at $\sim$ 400 meV below the Fermi level is also determined. Our STM measurement provides a direct experimental evidence of the nodal-line state in the family of ZrSiX.

cond-mat.mtrl-sci↗

arXiv2015

The Development of High-Performance Alkali-Hybrid Polarized $^3\mathrm{He}$ Targets for Electron Scattering

We present the development of high-performance polarized $^3\mathrm{He}$ targets for use in electron scattering experiments that utilize the technique of alkali-hybrid spin-exchange optical pumping. We include data obtained during the characterization of 24 separate target cells, each of which was constructed while preparing for one of four experiments at Jefferson Laboratory in Newport News, Virginia. The results presented here document dramatic improvement in the performance of polarized $^3\mathrm{He}$ targets, as well as the target properties and operating parameters that made those improvements possible. Included in our measurements were determinations of the so-called $X$-factors that quantify a temperature-dependent and as-yet poorly understood spin-relaxation mechanism that limits the maximum achievable $^3\mathrm{He}$ polarization to well under 100%. The presence of this spin-relaxation mechanism was clearly evident in our data. We also present results from a simulation of the alkali-hydrid spin-exchange optical pumping process that was developed to provide guidance in the design of these targets. Good agreement with actual performance was obtained by including details such as off-resonant optical pumping. Now benchmarked against experimental data, the simulation is useful for the design of future targets. Included in our results is a measurement of the $\mathrm{K}$-$^3\mathrm{He}$ spin-exchange rate coefficient $k^\mathrm{K}_\mathrm{se} = \left ( 7.46 \pm 0.62 \right )\!\times\!10^{-20}\ \mathrm{cm^3/s}$ over the temperature range 503 K to 563 K.

physics.atom-ph↗