arXiv subjects

Lei Chen

Publications and source records attributed to Lei Chen.

659 records · Page 37Linked to original sources

arXiv2012

On Optimal Link Activation with Interference Cancellation in Wireless Networking

A fundamental aspect in performance engineering of wireless networks is optimizing the set of links that can be concurrently activated to meet given signal-to-interference-and-noise ratio (SINR) thresholds. The solution of this combinatorial problem is the key element in scheduling and cross-layer resource management. Previous works on link activation assume single-user decoding receivers, that treat interference in the same way as noise. In this paper, we assume multiuser decoding receivers, which can cancel strongly interfering signals. As a result, in contrast to classical spatial reuse, links being close to each other are more likely to be active simultaneously. Our goal here is to deliver a comprehensive theoretical and numerical study on optimal link activation under this novel setup, in order to provide insight into the gains from adopting interference cancellation. We therefore consider the optimal problem setting of successive interference cancellation (SIC), as well as the simpler, yet instructive, case of parallel interference cancellation (PIC). We prove that both problems are NP-hard and develop compact integer linear programming formulations that enable us to approach the global optimum solutions. We provide an extensive numerical performance evaluation, indicating that for low to medium SINR thresholds the improvement is quite substantial, especially with SIC, whereas for high SINR thresholds the improvement diminishes and both schemes perform equally well.

cs.IT↗

arXiv2011

First-principles study of pressure-induced magnetic phase transitions in ternary iron selenide K$_{0.8}$Fe$_{1.6}$Se$_2$

We have studied the pressure effect on electronic structures and magnetic orders of ternary iron selenide K$_{0.8}$Fe$_{1.6}$Se$_2$ by the first-principles electronic structure calculations. At low pressure, the compound is in the blocked checkerboard antiferromagnetic (AFM) semiconducting phase, as observed by the neutron scatting measurements. Applying pressure induces two phase transitions, first from the blocked checkerboard AFM semiconducting phase to a collinear AFM metallic phase around 12 GPa, and then to a non-magnetic metallic phase around 25 GPa, respectively. Our results help to clarify the recent experimental measurements under pressure.

cond-mat.supr-con↗

arXiv2011

Spectral Line-by-Line Pulse Shaping of an On-Chip Microresonator Frequency Comb

We report, for the first time to the best of our knowledge, spectral phase characterization and line-by-line pulse shaping of an optical frequency comb generated by nonlinear wave mixing in a microring resonator. Through programmable pulse shaping the comb is compressed into a train of near-transform-limited pulses of \approx 300 fs duration (intensity full width half maximum) at 595 GHz repetition rate. An additional, simple example of optical arbitrary waveform generation is presented. The ability to characterize and then stably compress the frequency comb provides new data on the stability of the spectral phase and suggests that random relative frequency shifts due to uncorrelated variations of frequency dependent phase are at or below the 100 microHertz level.

physics.optics↗

arXiv2009

Reprocessed emission from warped accretion discs induced by the Bardeen-Petterson effect

The broad Balmer emission-line profiles resulting from the reprocessing of UV/X-ray radiation from a warped accretion disc induced by the Bardeen-Petterson effect are studied. We adopt a thin warped disc geometry and a central ring-like illuminating source in our model. We compute the steady-state shape of the warped disc numerically, and then use it in the calculation of the line profile. We find that, from the outer radius to the inner radius of the disc, the warp is twisted by an angle of $\simπ$ before being flattened efficiently into the equatorial plane. The profiles obtained depend weakly on the illuminating source radius in the range from $3r_{g}$ to $10r_g$, but depend strongly on this radius when it approaches the marginally stable orbit of an extreme Kerr black hole. Double- or triplet-peaked line profiles are present in most cases when the illuminating source radius is low. The triplet-peaked line profiles observed from the Sloan Digital Sky Survey may be a {"}signature" of a warped disc.

astro-ph.HE↗

arXiv2009

Vertical absorption edge and universal onset conductance in semi-hydrogenated graphene

We show that for graphene with any finite difference in the on-site energy between the two sub-lattices ($Δ$), The optical absorption edge is determined by the $Δ$. The universal conductance will be broken and the conductance near the band edge varies with frequency as $1/ω^2$. Moreover, we have identified another universal conductance for such systems without inversion symmetry, i.e., the onset conductance at the band edge is $% σ_c=2σ_0=πe^2/2h$, independent of the size of the band gap. The total integrated optical response is nearly conserved despite of the opening of the band gap.

cond-mat.mes-hall↗

arXiv2009

Vertices in all minimum paired-dominating sets of block graphs

Let $G=(V,E)$ be a simple graph without isolated vertices. A set $S\subseteq V$ is a paired-dominating set if every vertex in $V-S$ has at least one neighbor in $S$ and the subgraph induced by $S$ contains a perfect matching. In this paper, we present a linear-time algorithm to determine whether a given vertex in a block graph is contained in all its minimum paired-dominating sets.

math.CO↗

arXiv2009

Discovering Matching Dependencies

The concept of matching dependencies (mds) is recently pro- posed for specifying matching rules for object identification. Similar to the functional dependencies (with conditions), mds can also be applied to various data quality applications such as violation detection. In this paper, we study the problem of discovering matching dependencies from a given database instance. First, we formally define the measures, support and confidence, for evaluating utility of mds in the given database instance. Then, we study the discovery of mds with certain utility requirements of support and confidence. Exact algorithms are developed, together with pruning strategies to improve the time performance. Since the exact algorithm has to traverse all the data during the computation, we propose an approximate solution which only use some of the data. A bound of relative errors introduced by the approximation is also developed. Finally, our experimental evaluation demonstrates the efficiency of the proposed methods.

cs.DB↗

arXiv2009

Steady state solution of warped accretion discs

We consider a thin accretion disc warped due to the Bardeen-Petterson effect, presenting both analytical and numerical solutions for the situation that the two viscosity coefficients vary with radius as power law, with the two power law indices not necessarily equal. The analytical solutions are compared with numerical ones, showing that our new analytical solution is more accurate than previous one, which overestimates the inclination changing in the outer disc. Our new analytical solution is appropriate for moderately warped discs, while for extremely misaligned disc, only numerical solution is appropriate.

astro-ph.CO↗

arXiv2008

Measurement Induced Quantum Coherence Recovery

We show that measurement can recover the quantum coherence of a qubit in a non-Markovian environment. The experimental demonstration in an optical system is provided by comparing the visibilities (and fidelities) of the final states with and without measurement. This method can be extended to other two-level quantum systems and entangled states in a non-Markovian evolution environment. It may also be used to implement other quantum information processing.

quant-ph↗

arXiv2007

Orientation dependence of the optical spectra in graphene at high frequencies

On the basis of the Kubo formula we evaluated the optical conductivity of a graphene sheet. The full behavior of frequency as well as temperature dependence of the optical conductivity is presented. We show that the anisotropy of conductivity can be significantly enhanced at high frequencies. The photon absorption depends on the field polarization direction. At the frequency comparable to the maximum separation of upper and lower bands the photon-induced conduction of electrons is strongly suppressed if the polarization of field is along the zigzag direction. The corresponding optical conductivity is several orders of magnitude weaker than that when the light is polarizing along the armchair direction. We propose that the property of orientation selection of absorption in the graphene can be used as a basis for a high-frequency partial polarizer.

cond-mat.mes-hall↗

arXiv2007

Enhancement of polarization in a spin-orbit coupling quantum wire with a constriction

We investigate the enhancement of spin polarization in a quantum wire in the presence of a constriction and a spin-orbit coupling segment. It is shown that the spin-filtering effect is significantly heightened in comparison with the configuration without the constriction. It is understood in the studies that the constriction structure plays a critical role in enhancing the spin filtering by means of confining the incident electrons to occupy one channel only while the outgoing electrons occupy two channels. The enhancement of spin-filtering has also been analyzed within the perturbation theory. Because the spin polarization arises mainly from the scattering between the constriction and the segment with spin-orbit coupling, the sub-band mixing induced by spin-orbit interaction in the scattering process and the interferences result in higher spin-filtering effect.

cond-mat.mes-hall↗