Channel Acquisition for HF Skywave Massive MIMO-OFDM Communications

In this paper, we investigate channel acquisition for high frequency (HF) skywave massive multiple-input multiple-output (MIMO) communications with orthogonal frequency division multiplexing (OFDM) modulation. We first introduce the concept of triple beams (TBs) in the space-frequency-time (SFT) domain and establish a TB based channel model using sampled triple steering vectors. With the established channel model, we then investigate the optimal channel estimation and pilot design for pilot segments. Specifically, we find the conditions that allow pilot reuse among multiple user terminals (UTs), which significantly reduces pilot overhead. Moreover, we propose a channel prediction method for data segments based on the estimated TB domain channel. To reduce the complexity, we are able to formulate the channel estimation as a sparse signal recovery problem due to the channel sparsity in the TB domain and then obtain the channel by the proposed constrained Bethe free energy minimization (CBFEM) based channel estimation algorithm, which can be implemented with low complexity by exploiting the structure of the TB matrix together with the chirp z-transform (CZT). Simulation results demonstrate the superior performance of the proposed channel acquisition approach.Comment: 30 pages, 4 figure

Single-image based deep learning for precise atomic defects identification

Defect engineering has been profoundly employed to confer desirable functionality to materials that pristine lattices inherently lack. Although single atomic-resolution scanning transmission electron microscopy (STEM) images are widely accessible for defect engineering, harnessing atomic-scale images containing various defects through traditional image analysis methods is hindered by random noise and human bias. Yet the rise of deep learning (DL) offering an alternative approach, its widespread application is primarily restricted by the need for large amounts of training data with labeled ground truth. In this study, we propose a two-stage method to address the problems of high annotation cost and image noise in the detection of atomic defects in monolayer 2D materials. In the first stage, to tackle the issue of data scarcity, we employ a two-state transformation network based on U-GAT-IT for adding realistic noise to simulated images with pre-located ground truth labels, thereby infinitely expanding the training dataset. In the second stage, atomic defects in monolayer 2D materials are effectively detected with high accuracy using U-Net models trained with the data generated in the first stage, avoiding random noise and human bias issues. In both stages, we utilize segmented unit-cell-level images to simplify the model's task and enhance its accuracy. Our results demonstrate that not only sulfur vacancies, we are also able to visualize oxygen dopants in monolayer MoS2, which are usually overwhelmed by random background noise. As the training was based on a few segmented unit-cell-level realistic images, this method can be readily extended to other 2D materials. Therefore, our results outline novel ways to train the model with minimized datasets, offering great opportunities to fully exploit the power of machine learning (ML) applicable to a broad materials science community

Mesoporous nitrogen-doped TiO2 sphere applied for quasi-solid-state dye-sensitized solar cell

A mesoscopic nitrogen-doped TiO2 sphere has been developed for a quasi-solid-state dye-sensitized solar cell [DSSC]. Compared with the undoped TiO2 sphere, the quasi-solid-state DSSC based on the nitrogen-doped TiO2 sphere shows more excellent photovoltaic performance. The photoelectrochemistry of electrodes based on nitrogen-doped and undoped TiO2 spheres was characterized with Mott-Schottky analysis, intensity modulated photocurrent spectroscopy, and electrochemical impedance spectroscopy, which indicated that both the quasi-Fermi level and the charge transport of the photoelectrode were improved after being doped with nitrogen. As a result, a photoelectric conversion efficiency of 6.01% was obtained for the quasi-solid-state DSSC

Free vibration and damage identification of cracked functionally graded plates

This paper investigates the free vibration and crack identification of functionally graded material (FGM) plates with a through-width edge crack. The material properties of the FGM plates change continuously with the power law distribution along the plate thickness direction. The crack in an FGM plate is simulated as a massless rotational spring and the plate is separated into two sub-plates at the crack location connected by the line spring. The stress intensity factor (SIF) in the FGM strip is calculated to determine the stiffness of the spring. The governing equations of cracked FGM plates are derived from the Mindlin plate theory and solved by the differential quadrature (DQ) method to obtain modal parameters. The vibrational mode of a cracked FGM plate is analyzed by utilizing continuous wavelet transform (CWT). A novel damage index (DI) is developed based on calculated wavelet coefficients to localize the crack in FGM plates. This method can localize the crack accurately and reduce the edge effect even with the measurement noise

Magnetic Properties of the Mn55Bi45/Nd2Fe14B Hybrid Magnetic Alloys

The (Mn55Bi45)100−x/(Nd2Fe14B)x hybrid magnetic alloys were prepared by the ball milling of the combined annealed Mn55Bi45 powders and Nd2Fe14B powders. The magnetic properties at room temperature and elevated temperature were investigated. It was found that the saturation magnetization and the coercivity at room temperature increased significantly with the increasing Nd2Fe14B content. The enhanced energy product of 10.8 MGOe and 11.5 MGOe were obtained in (Mn55Bi45)40/(Nd2Fe14B)60 and (Mn55Bi45)20/(Nd2Fe14B)80. At elevated temperatures (350 K), the coercivities of 16.6 kOe and 16.1 kOe were obtained with Nd2Fe14B content of 20 wt.% and 40 wt.%, which were higher than those at room temperature. The temperature coefficients of coercivity of (Mn55Bi45)80/(Nd2Fe14B)20 and (Mn55Bi45)60/(Nd2Fe14B)40 were calculated to be positive, owing to the coercivity temperature characteristics of MnBi alloy. Finally, the energy products remained 10.5 MGOe and 10.1 MGOe in (Mn55Bi45)40/(Nd2Fe14B)60 and (Mn55Bi45)20/(Nd2Fe14B)80 at 350 K, which exhibited potential for high temperature applications

Magnetic Properties of the Mn₅₅Bi₄₅/Nd₂Fe₁₄B Hybrid Magnetic Alloys

The (Mn55Bi45)100−x/(Nd2Fe14B)x hybrid magnetic alloys were prepared by the ball milling of the combined annealed Mn55Bi45 powders and Nd2Fe14B powders. The magnetic properties at room temperature and elevated temperature were investigated. It was found that the saturation magnetization and the coercivity at room temperature increased significantly with the increasing Nd2Fe14B content. The enhanced energy product of 10.8 MGOe and 11.5 MGOe were obtained in (Mn55Bi45)40/(Nd2Fe14B)60 and (Mn55Bi45)20/(Nd2Fe14B)80. At elevated temperatures (350 K), the coercivities of 16.6 kOe and 16.1 kOe were obtained with Nd2Fe14B content of 20 wt.% and 40 wt.%, which were higher than those at room temperature. The temperature coefficients of coercivity of (Mn55Bi45)80/(Nd2Fe14B)20 and (Mn55Bi45)60/(Nd2Fe14B)40 were calculated to be positive, owing to the coercivity temperature characteristics of MnBi alloy. Finally, the energy products remained 10.5 MGOe and 10.1 MGOe in (Mn55Bi45)40/(Nd2Fe14B)60 and (Mn55Bi45)20/(Nd2Fe14B)80 at 350 K, which exhibited potential for high temperature applications

Crack identification of functionally graded beams using continuous wavelet transform

This paper proposes a new damage index for the crack identification of beams made of functionally graded materials (FGMs) by using the wavelet analysis. The damage index is defined based on the position of the wavelet coefficient modulus maxima in the scale space. The crack is assumed to be an open edge crack and is modeled by a massless rotational spring. It is assumed that the material properties follow exponential distributions along the beam thickness direction. The Timoshenko beam theory is employed to derive the governing equations which are solved analytically to obtain the frequency and mode shape of cracked FGM beams. Then, we apply the continuous wavelet transform (CWT) to the mode shapes of the cracked FGM beams. The locations of the cracks are determined from the sudden changes in the spatial variation of the damage index. An intensity factor, which relates to the size of the crack and the coefficient of the wavelet transform, is employed to estimate the crack depth. The effects of the crack size, the crack location and the Young's modulus ratio on the crack depth detection are investigated