13 research outputs found
Channel Cycle Time: A New Measure of Short-term Fairness
This paper puts forth a new metric, dubbed channel cycle time (CCT), to
measure the short-term fairness of communication networks. CCT characterizes
the average duration between two consecutive successful transmissions of a
user, during which all other users successfully accessed the channel at least
once. In contrast to existing short-term fairness measures, CCT provides more
comprehensive insight into the transient dynamics of communication networks,
with a particular focus on users' delays and jitter. To validate the efficacy
of our approach, we analytically characterize the CCTs for two classical
communication protocols: slotted Aloha and CSMA/CA. The analysis demonstrates
that CSMA/CA exhibits superior short-term fairness over slotted Aloha. Beyond
its role as a measurement metric, CCT has broader implications as a guiding
principle for the design of future communication networks by emphasizing
factors like fairness, delay, and jitter in short-term behaviors
Static Compressive Properties of Polypropylene Fiber Foam Concrete with Concave Hexagonal Unit Cell
For the purpose of studying the influence of fiber on the negative Poissonâs ratio effect of foam concrete, a concave hexagonal unit cell structure of polypropylene fiber foam concrete was proposed. The effects of different fiber volume contents on the structural mechanical parameters, Poissonâs ratio, and energy absorption capacity of the unit cells were studied by static compression of concave hexagonal unit cells and cube specimens. The results show that the compressive strength of foam concrete is reduced by adding polypropylene fiber, and the peak stress of concave hexagonal unit cells decreases less rapidly than that of cube specimens. The proper amount of polypropylene fiber can enhance the deformation ability of the unit cells in foam concrete, and the Poissonâs ratio of the unit cells in foam concrete with 1.5% fiber content is the lowest. In the process of failure of concave hexagonal unit cells, the failure phenomenon is mainly concentrated on the concave surfaces on both sides, and the cracks are distributed in the form of âupper left and lower rightâ or âlower left and upper rightâ. When the content of polypropylene fiber is 0.5%, the total energy absorbed by the concave hexagonal cells in the compression deformation process increases by 12.98%
Dynamic Analysis and Numerical Simulation of Arresting Hook Engaging Cable in Carried-Based UAV Landing Process
Carrier-based unmanned aerial vehicles (UAVs) require precise evaluation methods for their landing and arresting safety due to their high autonomy and demanding reliability requirements. In this paper, an efficient and accurate simulation method is presented for studying the arresting hook engaging arresting cable process. The finite element method and multibody dynamics (FEM-MBD) approach is employed. By establishing a rigidâflexible coupling model encompassing the UAV and arresting gear system, the simulation model for the engagement process is obtained. The model incorporates multiple coordinate systems to effectively capture the relative motion between the rigid and flexible components. The model considers the material properties, arresting gear system characteristics, and UAV state during engagement. Verification is conducted by comparing simulation results with experimental data from a referenced arresting hook rebound. Finally, simulations are performed under different touchdown points and roll angles of the UAV to analyze the stress distribution of the hook, center of gravity variations, and the tire touch and rollover cable response. The proposed rigidâflexible coupling arresting dynamics model in this paper enables the effective analysis of the dynamic behavior during the arresting hook engaging arresting cable process
Size-Tunable and Monodisperse Tm<sup>3+</sup>/Gd<sup>3+</sup>-Doped Hexagonal NaYbF<sub>4</sub> Nanoparticles with Engineered Efficient Near Infrared-to-Near Infrared Upconversion for In Vivo Imaging
Hexagonal NaYbF<sub>4</sub>:Tm<sup>3+</sup> upconversion nanoparticles hold promise for use in high
contrast near-infrared-to-near-infrared (NIR-to-NIR) in vitro and
in vivo bioimaging. However, significant hurdles remain in their preparation
and control of their morphology and size, as well as in enhancement
of their upconversion efficiency. Here, we describe a systematic approach
to produce highly controlled hexagonal NaYbF<sub>4</sub>:Tm<sup>3+</sup> nanoparticles with superior upconversion. We found that doping appropriate
concentrations of trivalent gadolinium (Gd<sup>3+</sup>) can convert
NaYbF<sub>4</sub>:Tm<sup>3+</sup> 0.5% nanoparticles with cubic phase
and irregular shape into highly monodisperse NaYbF<sub>4</sub>:Tm<sup>3+</sup> 0.5% nanoplates or nanospheres in a pure hexagonal-phase
and of tunable size. The intensity and the lifetime of the upconverted
NIR luminescence at 800 nm exhibit a direct dependence on the size
distribution of the resulting nanoparticles, being ascribed to the
varied surface-to-volume ratios determined by the different nanoparticle
size. Epitaxial growth of a thin NaYF<sub>4</sub> shell layer of âŒ2
nm on the âŒ22 nm core of hexagonal NaYbF<sub>4</sub>:Gd<sup>3+</sup> 30%/Tm<sup>3+</sup> 0.5% nanoparticles resulted in a dramatic
350 fold NIR upconversion efficiency enhancement, because of effective
suppression of surface-related quenching mechanisms. In vivo NIR-to-NIR
upconversion imaging was demonstrated using a dispersion of phospholipid-polyethylene
glycol (DSPE-PEG)-coated core/shell nanoparticles in phosphate buffered
saline