512 research outputs found
Study on Force Schemes in Pseudopotential Lattice Boltzmann Model for Two-Phase Flows
Multiphase flows are very important in industrial application. In present study, the force schemes in the pseudopotential LBM for two-phase flows have been compared in detail and the force schemes include Shan-Chen, EDM, MED, and Guo’s schemes. Numerical simulations confirm that all four schemes are consistent with the Laplace law. For Shan-Chen scheme, the smaller τ is, the smaller the surface tension is. However, for other schemes, τ has no effect on surface tension. When 0.6<τ≤1, the achieved density ratio will reduce as τ reduces. During this range of τ, the maximum density ratio of EDM scheme will be greater than that of other schemes. For a constant T, the curves of the maximum spurious currents (u′) has a minimum value which is corresponding to τ′ except for EDM schemes. In the region of τ′<τ≤1, u′ will reduce as τ decreases. On the other hand, in the area of τ≤τ′, u′ will increase as τ decreases. However, for EDM scheme, u′ will increase as τ increases
Cross-Layer Software-Defined 5G Network
In the past few decades, the world has witnessed a rapid growth in mobile
communication and reaped great benefits from it. Even though the fourth
generation (4G) mobile communication system is just being deployed worldwide,
proliferating mobile demands call for newer wireless communication technologies
with even better performance. Consequently, the fifth generation (5G) system is
already emerging in the research field. However, simply evolving the current
mobile networks can hardly meet such great expectations, because over the years
the infrastructures have generally become ossified, closed, and vertically
constructed. Aiming to establish a new paradigm for 5G mobile networks, in this
article, we propose a cross-layer software-defined 5G network architecture. By
jointly considering both the network layer and the physical layer together, we
establish the two software-defined programmable components, the control plane
and the cloud computing pool, which enable an effective control of the mobile
network from the global perspective and benefit technological innovations.
Specifically, by the cross-layer design for software-defining, the logically
centralized and programmable control plane abstracts the control functions from
the network layer down to the physical layer, through which we achieve the
fine-grained controlling of mobile network, while the cloud computing pool
provides powerful computing capability to implement the baseband data
processing of multiple heterogeneous networks. We discuss the main challenges
of our architecture, including the fine-grained control strategies, network
virtualization, and programmability. The architecture significantly benefits
the convergence towards heterogeneous networks and it enables much more
controllable, programmable and evolvable mobile networks.Comment: 9 pages, 5 figures, submitted to Mobile Networks & Application
Joint Design of Access and Backhaul in Densely Deployed MmWave Small Cells
With the rapid growth of mobile data traffic, the shortage of radio spectrum
resource has become increasingly prominent. Millimeter wave (mmWave) small
cells can be densely deployed in macro cells to improve network capacity and
spectrum utilization. Such a network architecture is referred to as mmWave
heterogeneous cellular networks (HetNets). Compared with the traditional wired
backhaul, The integrated access and backhaul (IAB) architecture with wireless
backhaul is more flexible and cost-effective for mmWave HetNets. However, the
imbalance of throughput between the access and backhaul links will constrain
the total system throughput. Consequently, it is necessary to jointly design of
radio access and backhaul link. In this paper, we study the joint optimization
of user association and backhaul resource allocation in mmWave HetNets, where
different mmWave bands are adopted by the access and backhaul links.
Considering the non-convex and combinatorial characteristics of the
optimization problem and the dynamic nature of the mmWave link, we propose a
multi-agent deep reinforcement learning (MADRL) based scheme to maximize the
long-term total link throughput of the network. The simulation results show
that the scheme can not only adjust user association and backhaul resource
allocation strategy according to the dynamics in the access link state, but
also effectively improve the link throughput under different system
configurations.Comment: 15 page
Higher-order Topological Insulators and Semimetals in Three Dimensions without Crystalline Counterparts
Quasicrystals allow for symmetries that are impossible in crystalline
materials, such as eight-fold rotational symmetry, enabling the existence of
novel higher-order topological insulators in two dimensions without crystalline
counterparts. However, it remains an open question whether three-dimensional
higher-order topological insulators and Weyl-like semimetals without
crystalline counterparts can exist. Here, we demonstrate the existence of a
second-order topological insulator by constructing and exploring a
three-dimensional model Hamiltonian in a stack of Ammann-Beenker tiling
quasicrystalline lattices. The topological phase has eight chiral hinge modes
that lead to quantized longitudinal conductances of . We show that the
topological phase is characterized by the winding number of the quadrupole
moment. We further establish the existence of a second-order topological
insulator with time-reversal symmetry, characterized by a
topological invariant. Finally, we propose a model that exhibits a higher-order
Weyl-like semimetal phase, demonstrating both hinge and surface Fermi arcs. Our
findings highlight that quasicrystals in three dimensions can give rise to
higher-order topological insulators and semimetal phases that are unattainable
in crystals.Comment: 7 pages, 6 figure
Sum Rate Maximization under AoI Constraints for RIS-Assisted mmWave Communications
The concept of age of information (AoI) has been proposed to quantify
information freshness, which is crucial for time-sensitive applications.
However, in millimeter wave (mmWave) communication systems, the link blockage
caused by obstacles and the severe path loss greatly impair the freshness of
information received by the user equipments (UEs). In this paper, we focus on
reconfigurable intelligent surface (RIS)-assisted mmWave communications, where
beamforming is performed at transceivers to provide directional beam gain and a
RIS is deployed to combat link blockage. We aim to maximize the system sum rate
while satisfying the information freshness requirements of UEs by jointly
optimizing the beamforming at transceivers, the discrete RIS reflection
coefficients, and the UE scheduling strategy. To facilitate a practical
solution, we decompose the problem into two subproblems. For the first per-UE
data rate maximization problem, we further decompose it into a beamforming
optimization subproblem and a RIS reflection coefficient optimization
subproblem. Considering the difficulty of channel estimation, we utilize the
hierarchical search method for the former and the local search method for the
latter, and then adopt the block coordinate descent (BCD) method to alternately
solve them. For the second scheduling strategy design problem, a low-complexity
heuristic scheduling algorithm is designed. Simulation results show that the
proposed algorithm can effectively improve the system sum rate while satisfying
the information freshness requirements of all UEs
Galactic Disk Bulk Motions as Revealed by the LSS-GAC DR2
We report a detailed investigation of the bulk motions of the nearby Galactic
stellar disk, based on three samples selected from the LSS-GAC DR2: a global
sample containing 0.57 million FGK dwarfs out to 2 kpc, a local subset
of the global sample consisting 5,400 stars within 150 pc, and an
anti-center sample containing 4,400 AFGK dwarfs and red clump stars
within windows of a few degree wide centered on the Galactic anti-center. The
global sample is used to construct a three-dimensional map of bulk motions of
the Galactic disk from the solar vicinity out to 2 kpc with a spatial
resolution of 250 pc. Typical values of the radial and vertical
components of bulk motion range from 15 km s to 15 km s, while
the lag behind the circular speed dominates the azimuthal component by up to
15 km s. The map reveals spatially coherent, kpc-scale stellar
flows in the disk, with typical velocities of a few tens km s. Bending-
and breathing-mode perturbations are clearly visible, and vary smoothly across
the disk plane. Our data also reveal higher-order perturbations, such as breaks
and ripples, in the profiles of vertical motion versus height. From the local
sample, we find that stars of different populations exhibit very different
patterns of bulk motion. Finally, the anti-center sample reveals a number of
peaks in stellar number density in the line-of-sight velocity versus distance
distribution, with the nearer ones apparently related to the known moving
groups. The "velocity bifurcation" reported by Liu et al. (2012) at
Galactocentric radii 10--11 kpc is confirmed. However, just beyond this
distance, our data also reveal a new triple-peaked structure.Comment: 27 pages, 17 figures, Accepted for publication in a special issue of
Research in Astronomy and Astrophysics on LAMOST science
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