727 research outputs found
Non-Universality of Density and Disorder in Jammed Sphere Packings
We show for the first time that collectively jammed disordered packings of
three-dimensional monodisperse frictionless hard spheres can be produced and
tuned using a novel numerical protocol with packing density as low as
0.6. This is well below the value of 0.64 associated with the maximally random
jammed state and entirely unrelated to the ill-defined ``random loose packing''
state density. Specifically, collectively jammed packings are generated with a
very narrow distribution centered at any density over a wide density
range with variable disorder. Our results
support the view that there is no universal jamming point that is
distinguishable based on the packing density and frequency of occurence. Our
jammed packings are mapped onto a density-order-metric plane, which provides a
broader characterization of packings than density alone. Other packing
characteristics, such as the pair correlation function, average contact number
and fraction of rattlers are quantified and discussed.Comment: 19 pages, 4 figure
Emergence of an incipient ordering mode in FeSe
The structurally simplest Fe-based superconductor FeSe with a critical
temperature 8.5 K displays a breaking of the four-fold
rotational symmetry at a temperature K. We investigated the
electronic properties of FeSe using scanning tunneling microscopy/spectroscopy
(STM/S), magnetization, and electrical transport measurements. The results
indicated two new energy scales (i) 75 K denoted by an onset of
electron-hole asymmetry in STS, enhanced spin fluctuations, and increased
positive magnetoresistance; (ii) 22 - 30 K, marked by opening
up of a partial gap of about 8 meV in STS and a recovery of Kohler's rule. Our
results reveal onset of an incipient ordering mode at and its
nucleation below . The ordering mode observed here, both in spin as
well as charge channels, suggests a coupling between the spins with charge,
orbital or pocket degrees of freedom.Comment: 5 pages, 4 figure
LCRT: A ToA Based Mobile Terminal Localization Algorithm in NLOS Environment
Non line-of-sight (NLOS) propagation in range measurement is a key problem for mobile terminal localization. This paper proposes a low computational residual test (LCRT) algorithm that can identify the number of line-of-sight (LOS) transmissions and reduce the computational complexity compared with the residual test (RT) algorithm. LCRT is based on the assumption that when all range measurements are from LOS propagations, the normalized residual distribution follows the central chi-square distribution while for NLOS cases it is non-central. An optimized procedure to generate the sets of range measurements is adopted and least square (LS) instead of approximate maximum likelihood (AML) is used during the identification of LOS propagations, resulting in reduced computation complexity. Simulation results show that the LCRT can efficiently identify the set of LOS. The correct decision rate is higher than 92% and the variances of results are approaching to the Cramer-Rao lower bound (CRLB) when there are more than 3 LOS propagations
Modeling and Performance Analysis of Channel Assembling in Multi-channel Cognitive Radio Networks with Spectrum Adaptation
[EN] To accommodate spectrum access in multichannel
cognitive radio networks (CRNs), the channel-assembling technique,
which combines several channels together as one channel,
has been proposed in many medium access control (MAC) protocols.
However, analytical models for CRNs enabled with this
technique have not been thoroughly investigated. In this paper,
two representative channel-assembling strategies that consider
spectrum adaptation and heterogeneous traffic are proposed, and
the performance of these strategies is evaluated based on the proposed
continuous-time Markov chain (CTMC) models. Moreover,
approximations of these models in the quasistationary regime
are analyzed, and closed-form capacity expressions are derived
in different conditions. The performance of different strategies,
including the strategy without assembling, is compared with one
another based on the numerical results obtained from these
models and validated by extensive simulations. Furthermore,
simulation studies are also performed for other types of traffic
distributions to evaluate the validity and the preciseness of the
mathematical models. Through both analyses and simulations,
we demonstrate that channel assembling represented by the investigated
strategies can improve the system performance if a
proper strategy is selected with appropriate system parameter
configurations.This work was supported in part by the European Union Seventh Framework Programme Marie Curie Actions International Research Staff Exchange Scheme (EU FP7-PEOPLE-IRSES) through the Security, Services, Networking, and Performance of Next Generation IP-Based Multimedia Wireless Networks (S2EuNet) Project under Agreement 247083 and by the Spanish Government through Project TIN2010-21378-C02-02. The review of this paper was coordinated by Prof. Y. Ma.Jiao, L.; Li, FY.; Pla, V. (2012). Modeling and Performance Analysis of Channel Assembling in Multi-channel Cognitive Radio Networks with Spectrum Adaptation. IEEE Transactions on Vehicular Technology. 61:2686-2697. https://doi.org/10.1109/TVT.2012.2196300S268626976
Capacity Upper Bound of Channel Assembling in Cognitive Radio Networks with Quasistationary Primary User Activities
In cognitive radio networks (CRNs) with multiple channels, various channel-assembling (ChA) strategies may be applied to secondary users (SUs), resulting in different achieved capacity. However, there is no previous work on determining the capacity upper bound (UB) of ChA for SUs under given system configurations. In this paper, we derive the maximum capacity for CRNs with ChA through Markov chain modeling, considering that primary user (PU) activities are relatively static, compared with SU services. We first deduce a closed-form expression for the maximum capacity in a dynamic ChA strategy and then demonstrate that no other ChA strategy can provide higher capacity than that achieved by this dynamic strategy. © 2012 IEEE.The work of L. Jiao and F. Y. Li was supported in part by the Research Council of Norway through the ECO-boat MOL project under Grant 210426. The work of E. Song was supported in part by the National Natural Science Foundation of China under Grant 60901037 and in part by the Foundation for Basic Research of Sichuan University for Distinguished Young Scholars under Grant 0082604132188. The work of V. Pla was supported by the Spanish Ministerio de Ciencia e Innovacion through Project TIN2010-21378-C02-02. The review of this paper was coordinated by Prof. B. Hamdaoui.Jiao, L.; Song, E.; Pla, V.; Li, FY. (2013). Capacity Upper Bound of Channel Assembling in Cognitive Radio Networks with Quasistationary Primary User Activities. IEEE Transactions on Vehicular Technology. 62(4):1849-1855. https://doi.org/10.1109/TVT.2012.2236115S1849185562
Global Slim Accretion Disk Solutions Revisited
We show that there exists a maximal possible accretion rate, beyond which
global slim disk solutions cannot be constructed because in the vertical
direction the gravitational force would be unable to balance the pressure force
to gather the accreted matter. The principle for this restriction is the same
as that for the Eddington luminosity and the corresponding critical accretion
rate, which were derived for spherical accretion by considering the same force
balance in the radial direction. If the assumption of hydrostatic equilibrium
is waived and vertical motion is included, this restriction may become even
more serious as the value of the maximal possible accretion rate becomes
smaller. Previous understanding in the literature that global slim disk
solutions could stand for any large accretion rates is due to the
overestimation of the vertical gravitational force by using an approximate
potential. For accretion flows with large accretion rates at large radii,
outflows seem unavoidable in order for the accretion flow to reduce the
accretion rate and follow a global solution till the central black hole.Comment: Accepted by Ap
Magnetic and defect probes of the SmB surface state
The impact of non-magnetic and magnetic impurities on topological insulators
is a central problem concerning their fundamental physics and possible novel
spintronics and quantum computing applications. SmB, predicted to be a
topological Kondo insulator, is considered a benchmark material. Using a
spin-polarized tip in scanning tunneling spectroscopy destroys the signature
peak of the topological surface state, revealing its spin texture. Further,
combining local STS with macroscopic transport measurements on SmB
containing different substitutions enables us to investigate the effect of
impurities. The surface states around impurities are locally suppressed with
different length scales depending on their magnetic properties and, for
sufficiently high impurity level, globally destroyed. Our study points directly
to the topological nature of SmB, and unveils, microscopically and
macroscopically, how impurities -- magnetic or non-magnetic -- affect
topological surface states
On the Performance of Channel Assembling and Fragmentation in Cognitive Radio Networks
[EN] Flexible channel allocation may be applied to multi-channel cognitive radio networks (CRNs) through either channel assembling (CA) or channel fragmentation (CF). While CA allows one secondary user (SU) occupy multiple channels when primary users (PUs) are absent, CF provides finer granularity for channel occupancy by allocating a portion of one channel to an SU flow. In this paper, we investigate the impact of CF together with CA for SU flows by proposing a channel access strategy which activates both CF and CA and correspondingly evaluating its performance. In addition, we also consider a novel scenario where CA is enabled for PU flows. The performance evaluation is conducted based on continuous time Markov chain (CTMC) modeling and simulations. Through mathematical analyses and simulation results, we demonstrate that higher system capacity can be achieved indeed by jointly employing both CA and CF, in comparison with the CA-only strategies. However, this benefit is obtained only under certain conditions which are pointed out in this paper. Furthermore, the theoretical capacity upper bound for SU flows with both CF and CA enabled is derived when PU activities are relatively static compared with SU flows.This work was supported by the EU Seventh Framework Programme FP7-PEOPLE-IRSES under Grant agreement 247083, project acronym S2EuNet. The work of L. Jiao was supported by the Research Council of Norway through the ECO-boat MOL project under Grant 210426. The work of V. Pla was supported in part by the Ministry of Economy and Competitiveness of Spain under Grant TIN2010-21378-C02-02. The associate editor coordinating the review of this paper and approving it for publication was H. Wymeersch.Jiao, L.; Balapuwaduge, IAM.; Li, FY.; Pla, V. (2014). On the Performance of Channel Assembling and Fragmentation in Cognitive Radio Networks. IEEE Transactions on Wireless Communications. 13(10):5661-5675. https://doi.org/10.1109/TWC.2014.2322057S56615675131
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