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 Poisson’s 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, Poisson’s 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 Poisson’s 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 “upper left and lower right” or “lower left and upper right”. 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 rigid–flexible 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 rigid–flexible 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 Tm³⁺/Gd³⁺-Doped Hexagonal NaYbF₄ Nanoparticles with Engineered Efficient Near Infrared-to-Near Infrared Upconversion for In Vivo Imaging

Hexagonal NaYbF₄:Tm³⁺ 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 NaYbF₄:Tm³⁺ nanoparticles with superior upconversion. We found that doping appropriate concentrations of trivalent gadolinium (Gd³⁺) can convert NaYbF₄:Tm³⁺ 0.5% nanoparticles with cubic phase and irregular shape into highly monodisperse NaYbF₄:Tm³⁺ 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 NaYF₄ shell layer of ∼2 nm on the ∼22 nm core of hexagonal NaYbF₄:Gd³⁺ 30%/Tm³⁺ 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