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Wen Xu

Publications and source records attributed to Wen Xu.

At least 73 records · Page 4Linked to original sources
arXiv2019

Arithmetic Coding Based Multi-Composition Codes for Bit-Level Distribution Matching

A distribution matcher (DM) encodes a binary input data sequence into a sequence of symbols (codeword) with desired target probability distribution. The set of the output codewords constitutes a codebook (or code) of a DM. Constant-composition DM (CCDM) uses arithmetic coding to efficiently encode data into codewords from a constant-composition (CC) codebook. The CC constraint limits the size of the codebook, and hence the coding rate of the CCDM. The performance of CCDM degrades with decreasing output length. To improve the performance for short transmission blocks we present a class of multi-composition (MC) codes and an efficient arithmetic coding scheme for encoding and decoding. The resulting multi-composition DM (MCDM) is able to encode more data into distribution matched codewords than the CCDM and achieves lower KL divergence, especially for short block messages.

cs.IT↗
arXiv2018

Probabilistically Shaped Multi-Level Coding with Polar Codes for Fading Channels

A probabilistic shaping method for multi-level coding (MLC) is presented, where the transmitted symbols are forced to have a shaped non-uniform distribution. It is shown that shaping only a single bit-level suffices to compensate for most of the shaping loss on the fading channels. A polar code based implementation of the proposed scheme is presented, where shaping is performed by using a precoder at the transmitter without increasing the decoding complexity. Simulation results show that performance improvements can be obtained compared to BICM- and MLC-based polar coding without shaping on Rayleigh fading channels.

cs.IT↗
arXiv2018

libmpk: Software Abstraction for Intel Memory Protection Keys

Intel memory protection keys (MPK) is a new hardware feature to support thread-local permission control on groups of pages without requiring modification of page tables. Unfortunately, its current hardware implementation and software supports suffer from security, scalability, and semantic-gap problems: (1) MPK is vulnerable to protection-key-use-after-free and protection-key corruption; (2) MPK does not scale due to hardware limitations; and (3) MPK is not perfectly compatible with mprotect() because it does not support permission synchronization across threads. In this paper, we propose libmpk, a software abstraction for MPK. libmpk virtualizes protection keys to eliminate the protection-key-use-after-free and protection-key corruption problems while supporting a tremendous number of memory page groups. libmpk also prevents unauthorized writes to its metadata and supports inter-thread key synchronization. We apply libmpk to three real-world applications: OpenSSL, JavaScript JIT compiler, and Memcached for memory protection and isolation. An evaluation shows that libmpk introduces negligible performance overhead (<1%) compared with insecure versions, and improves their performance by 8.1x over secure equivalents using mprotect(). The source code of libmpk will be publicly available and maintained as an open source project.

cs.CR↗
arXiv2018

Three-Dimensional Fermi-Surface and Electron-Phonon Coupling in Semimetallic 1T-TiTe2 studied by Angle-Resolved Photoemission Spectroscopy

We present an investigation on electronic structure of 1T-TiTe2 material via high-resolution angle-resolved photoemission spectroscopy (ARPES), utilizing tunable photon energy excitations. The typical semimetal-like electronic structure is observed and examined, where multiple hole pockets related to Te 5p bands and one electron pockets related to Ti 3d band are populated. The obtained results reveals i) a pronounced three-dimensional (3D) electronic structure of 1T-TiTe2 with typical semi-metallic features, for both the Ti 3d and the Te 5p states; ii) multiple Fermi surface (FS) sheets and complex band structure; and iii) an obvious kink in dispersion at an energy of about 18 meV below the Fermi energy, the first experimental observation of a kink structure in 1T-TiTe2, which may originate from electron-phonon coupling. These important and significant findings can help us to gain an in-depth understanding of the 3D electronic structure of semimetallic 1T- TiTe2.

cond-mat.str-el↗
arXiv2018

Polar Codes with Integrated Probabilistic Shaping for 5G New Radio

A modification to 5G New Radio (NR) polar code is proposed, which improves the error correction performance with higher order modulation through probabilistic shaping. The presented scheme mainly re-uses existing hardware at the transmitter, and modifications at the receiver are small. Simulation results show that the presented approach can improve the performance by up to 1dB for 256-QAM on AWGN channels

cs.IT↗
arXiv2018

Polar Code Construction for List Decoding

A heuristic construction of polar codes for successive cancellation list (SCL) decoding with a given list size is proposed to balance the trade-off between performance measured in frame error rate (FER) and decoding complexity. Furthermore, a construction based on dynamically frozen bits with constraints among the "low weight bits" (LWB) is presented. Simulation results show that the LWB-polar codes outperform the CRC-polar codes and the eBCH-polar codes under SCL decoding.

cs.IT↗
arXiv2017

Use Cases, Requirements, and Design Considerations for 5G V2X

Ultimate goal of next generation Vehicle-to-everything (V2X) communication systems is enabling accident-free cooperative automated driving that uses the available roadway efficiently. To achieve this goal, the communication system will need to enable a diverse set of use cases, each with a specific set of requirements. We discuss the main use case categories, analyze their requirements, and compare them against the capabilities of currently available communication technologies. Based on the analysis, we identify a gap and point out towards possible system design for 5G V2X that could close the gap. Furthermore, we discuss an architecture of the 5G V2X radio access network that incorporates diverse communication technologies, including current and cellular systems in centimeter wave and millimeter wave, IEEE 802.11p and vehicular visible light communications. Finally, we discuss the role of future 5G V2X systems in enabling more efficient vehicular transportation: from improved traffic flow through reduced inter-vehicle spacing on highways and coordinated intersections in cities (the cheapest way to increasing the road capacity), to automated smart parking (no more visits to the parking!), ultimately enabling seamless end-to-end personal mobility.

cs.NI↗
arXiv2017

On the Beamformed Broadcast Signaling for Millimeter Wave Cell Discovery: Performance Analysis and Design Insight

Availability of abundant spectrum has enabled millimeter wave (mm-wave) as a prominent candidate solution for the next generation cellular networks. Highly directional transmissions are essential for exploitation of mm-wave bands to compensate high propagation loss and attenuation. The directional transmission, nevertheless, necessitates a specific design for mm-wave initial cell discovery, as conventional omni-directional broadcast signaling may fail in delivering the cell discovery information. To address this issue, this paper provides an analytical framework for mm-wave beamformed cell discovery based on an information theoretical approach. Design options are compared considering four fundamental and representative broadcast signaling schemes to evaluate discovery latency and signaling overhead. The schemes are then simulated under realistic system parameters. Analytical and simulation results reveals four key findings: (i) For cell discovery without knowledge of beacon timing, analog/hybrid beamforming performs as well as digital beamforming in terms of cell discovery latency; (ii) Single beam exhaustive scan optimize the latency, however leads to overhead penalty; (iii) Multi-beam simultaneous scan can significantly reduce the overhead, and provide the flexibility to achieve trade-off between the latency and the overhead; (iv) The latency and the overhead are relatively insensitive to extreme low block error rates.

cs.IT↗
arXiv2016

Modeling the Evolution of Line-of-Sight Blockage for V2V Channels

We investigate the evolution of line of sight (LOS) blockage over both time and space for vehicle-to-vehicle (V2V) channels. Using realistic vehicular mobility and building and foliage locations from maps, we first perform LOS blockage analysis to extract LOS probabilities in real cities and on highways for varying vehicular densities. Next, to model the time evolution of LOS blockage for V2V links, we employ a three-state discrete-time Markov chain comprised of the following states: i) LOS; ii) non-LOS due to static objects (e.g., buildings, trees, etc.); and iii) non-LOS due to mobile objects (vehicles). We obtain state transition probabilities based on the evolution of LOS blockage. Finally, we perform curve fitting and obtain a set of distance-dependent equations for both LOS and transition probabilities. These equations can be used to generate time-evolved V2V channel realizations for representative urban and highway environments. Our results can be used to perform highly efficient and accurate simulations without the need to employ complex geometry-based models for link evolution.

cs.NI↗
arXiv2015

Parametric cooling of a degenerate Fermi gas in an optical trap

We demonstrate a novel technique for cooling a degenerate Fermi gas in a crossed-beam optical dipole trap, where high-energy atoms can be selectively removed from the trap by modulating the stiffness of the trapping potential with anharmonic trapping frequencies. We measure the dependence of the cooling effect on the frequency and amplitude of the parametric modulations. It is found that the large anharmonicity along the axial trapping potential allows to generate a degenerate Fermi gas with anisotropic energy distribution, in which the cloud energy in the axial direction can be reduced to the ground state value.

cond-mat.quant-gas↗
arXiv2015

Essential norms and weak compactness of integral operators between weighted Bergman spaces

We consider Volterra-type integration operators $T_g$ between Bergman spaces induced by weights $ω$ satisfying a doubling property. We derive estimates for the operator norms, essential and weak essential norms of $T_g: A_ω^p \to A_ω^q$, $0<p\leq q<\infty$. In particular, the operator $T_g: A_ω^1\to A_ω^1$ is weakly compact if and only if it is compact.

math.CV↗
arXiv2015

Direction dependent thermal conductivity of monolayer phosphorene: parameterization of Stillinger-Weber potential and molecular dynamics study

A Stillinger-Weber interatomic potential is parameterized for phosphorene. It well reproduces the crystal structure, cohesive energy and phonon dispersion predicted by first-principles calculations. The thermal conductivity of phosphorene is further explored by equilibrium molecular dynamics simulations adopting the optimal set of potential parameters. At room temperature, the intrinsic thermal conductivities along zigzag and armchair directions are about 152.7 and 33.0 W/mK, respectively, with a large anisotropy ratio of five. The remarkably directional dependence of thermal conductivity in phosphorene, consistent with previous reports, is mainly due to the strong anisotropy of phonon group velocities, and weak anisotropy of phonon lifetimes as revealed by lattice dynamics calculations. Moreover, the effective phonon mean free paths at zigzag and armchair directions are about 141.4 and 43.4nm, respectively.

cond-mat.mes-hall↗
arXiv2011

Time-Dependent Behavior of Lyman$α$ Photon Transfer in High Redshift Optically Thick Medium

With Monte Carlo simulation method, we investigate the time dependent behavior of Ly$α$ photon transfer in optically thick medium of the concordance $Λ$CDM universe. At high redshift, the Ly$α$ photon escaping from optically thick medium has a time scale as long as the age of the luminous object, or even comparable to the age of the universe. In this case, time-independent, or stationary solutions of the Ly$α$ photon transfer with resonant scattering will overlook important features of the escaped Ly$α$ photons in physical and frequency spaces. More seriously, the expansion of the universe leads to that the time-independent solutions of the Ly$α$ photon transfer may not exist. We show that time-dependent solutions sometimes are essential for understanding the Ly$α$ emission and absorption at high redshifts. For Lyαphotons from sources at redshift 1+z=10 and being surrounded by neutral hydrogen IGM of the $Λ$CDM universe, the escape coefficient is found to be always less, or much less than one, regardless of the age or life time of the sources. Under such environment, we also find that even when the Ly$α$ photon luminosity of the sources is stable, the mean surface brightness is gradually increasing in the first 10^6 years, and then decreasing with a power law of time, but never approaches a stable, time-independent state. That is, all 1+z=10 sources in a neutral Hubble expanding IGM with Ly$α$ luminosity L have their maximum of mean surface brightness ~ 10^{-21}(L/(10^{43}erg/s)) erg s^{-1} cm^{-2} arcsec^{-2} at the age of about 10^6 years. The time-dependent effects on the red damping wing profile are also addressed.

astro-ph.CO↗
arXiv2011

Weighted Integral Means of Mixed Areas and Lengths under Holomorphic Mappings

This note addresses monotonic growths and logarithmic convexities of the weighted ($(1-t^2)^αdt^2$, $-\infty<α<\infty$, $0<t<1$) integral means $\mathsf{A}_{α,β}(f,\cdot)$ and $\mathsf{L}_{α,β}(f,\cdot)$ of the mixed area $(πr^2)^{-β}A(f,r)$ and the mixed length $(2πr)^{-β}L(f,r)$ ($0\leβ\le 1$ and $0<r<1$) of $f(r\mathbb D)$ and $\partial f(r\mathbb D)$ under a holomorphic map $f$ from the unit disk $\mathbb D$ into the finite complex plane $\mathbb C$.

math.CV↗
arXiv2010

On the formation of Lyman $α$ emission from resonantly scattered continuum photons of GRB's afterglow

We study the formation and evolution of the spectral imprint of resonantly scattered Lyman $α$ photons in the context of GRB's continuum optical afterglow. Based on an analytic model and a complete treatment of all the scatterings using Monte Carlo simulations, we found that the flux of the Ly$α$ emission is mainly contributed by photons which are scattered only once. The flux is of order $10^{-4}$ to $10^{-9}$ relative to the undecayed maximum flux of the transmitted continuum, making the feature negligible but potentially observable. If not obscured by host galaxy's DLA or intergalactic neutral hydrogen, the feature may appear sometime from one hour to several years when the directly transmitted light has faded away.This scattered emission feature can be distinguished from Ly$α$ photons of other origins by its luminosity evolution, and by its gradual narrowing of profile with time. The typical time scale for spectral variance is that of the light crossing time of a hydrogen clump close to the GRB.

astro-ph.CO↗
arXiv1999

Virialization of Galaxy Clusters and Beyond

Using samples of structures identified by a multi-scale decomposition from numerical simulation, we analyze the scale-dependence of the virialization of clusters. We find that beyond the scale of full virialization there exists a radius range over which clusters are quasi-virialized, i.e. while the internal structure of an {\it individual} cluster is at substantial departure from dynamical relaxation, some {\it statistical} properties of the multi-scale identified clusters are approximately the same as those for the virialized systems. The dynamical reason of the existence of quasi-virialization is that some of the scaling properties of dynamically relaxed systems of cosmic gravitational clustering approximately hold beyond the full virialization regime. The "individual-statistical" duality of the quasi-virialization provides an explanation of the observed puzzle that the total masses of clusters derived from virial theorem are statistically the same as the gravitational lensing determined masses, in spite of the presence of irregular configuration and substructures in individual clusters. It also explains the tight correlation between the velocity dispersion of optical galaxies and the temperature of X-ray emitting gas. Consequently, the virial mass estimators based on the assumptions of isothermal and hydrostatic model are statistically applicable to scales on which the clusters are quasi-virialized. In the quasi-virialization regime, the temperature functions of clusters also show scaling. This feature is a useful discriminator among cosmological models.

astro-ph↗
arXiv1999

The Flatness of Mass-to-Light Ratio on Large Scales

It has been suggested that the mass-to-light ($M/L$) ratio of gravitationally clustering objects is scale-independent on scales beyond galaxy clusters, and may also be independent of the mass of the objects. In this paper, we show that the scale behavior of $M/L$ ratio is closely related to the scaling of cosmic structures larger than clusters. The scale dependence of the $M/L$ ratio can be determined by comparing the observed scaling of richness function (RF) of multi-scale identified objects with the model-predicted scaling of mass function (MF) of large scale structures. Using the multi-scale identified clusters from IRAS 1.2 Jy galaxy survey, we have made comparisons of the observed RF scaling of IRAS $r_{cl}$-clusters with the MF scalings given by simulations of three popular models SCDM, LCDM and OCDM. We find that, the M/L ratio basically is scale-independent from the Abell radius up to about 24 $h^{-1}$Mpc, while it seems to show a slight, but systematical, increase over this scale range. This result is weakly dependent on the cosmological parameters.

astro-ph↗