Semi-Anchored Multi-Step Gradient Descent Ascent Method for Structured Nonconvex-Nonconcave Composite Minimax Problems

Minimax problems, such as generative adversarial network, adversarial training, and fair training, are widely solved by a multi-step gradient descent ascent (MGDA) method in practice. However, its convergence guarantee is limited. In this paper, inspired by the primal-dual hybrid gradient method, we propose a new semi-anchoring (SA) technique for the MGDA method. This makes the MGDA method find a stationary point of a structured nonconvex-nonconcave composite minimax problem; its saddle-subdifferential operator satisfies the weak Minty variational inequality condition. The resulting method, named SA-MGDA, is built upon a Bregman proximal point method. We further develop its backtracking line-search version, and its non-Euclidean version for smooth adaptable functions. Numerical experiments, including a fair classification training, are provided

Massive MIMO Channel Prediction: Kalman Filtering vs. Machine Learning

This paper focuses on channel prediction techniques for massive multiple-input multiple-output (MIMO) systems. Previous channel predictors are based on theoretical channel models, which would be deviated from realistic channels. In this paper, we develop and compare a vector Kalman filter (VKF)-based channel predictor and a machine learning (ML)-based channel predictor using the realistic channels from the spatial channel model (SCM), which has been adopted in the 3GPP standard for years. First, we propose a low-complexity mobility estimator based on the spatial average using a large number of antennas in massive MIMO. The mobility estimate can be used to determine the complexity order of developed predictors. The VKF-based channel predictor developed in this paper exploits the autoregressive (AR) parameters estimated from the SCM channels based on the Yule-Walker equations. Then, the ML-based channel predictor using the linear minimum mean square error (LMMSE)-based noise pre-processed data is developed. Numerical results reveal that both channel predictors have substantial gain over the outdated channel in terms of the channel prediction accuracy and data rate. The ML-based predictor has larger overall computational complexity than the VKF-based predictor, but once trained, the operational complexity of ML-based predictor becomes smaller than that of VKF-based predictor.Comment: Accepted to IEEE Transactions on Communication

Hierarchical Co–Pi Clusters/Fe2O3 Nanorods/FTO Micropillars 3D Branched Photoanode for High-Performance Photoelectrochemical Water Splitting

In this study, an efficient hierarchical Co–Pi cluster/Fe2O3 nanorod/fluorine-doped tin oxide (FTO) micropillar three-dimensional (3D) branched photoanode was designed for enhanced photoelectrochemical performance. A periodic array of FTO micropillars, which acts as a highly conductive “host” framework for uniform light scattering and provides an extremely enlarged active area, was fabricated by direct printing and mist-chemical vapor deposition (CVD). Fe2O3 nanorods that act as light absorber “guest” materials and Co–Pi clusters that give rise to random light scattering were synthesized via a hydrothermal reaction and photoassisted electrodeposition, respectively. The hierarchical 3D branched photoanode exhibited enhanced light absorption efficiency because of multiple light scattering, which was a combination of uniform light scattering from the periodic FTO micropillars and random light scattering from the Fe2O3 nanorods. Additionally, the large surface area of the 3D FTO micropillar, together with the surface area provided by the one-dimensional Fe2O3 nanorods, contributed to a remarkable increase in the specific area of the photoanode. Because of these enhancements and further improvements facilitated by decoration with a Co–Pi catalyst that enhanced water oxidation, the 3D branched Fe2O3 photoanode achieved a photocurrent density of 1.51 mA cm−2 at 1.23 VRHE, which was 5.2 times higher than that generated by the non-decorated flat Fe2O3 photoanode

A large-area fabrication of moth-eye patterned Au/TiO2 gap-plasmon structure and its application to plasmonic solar water splitting

TiO2 is promising candidate for solar water splitting material. To enhance the insufficient absorption range of TiO2, TiO2 decorated with Au nanoparticles(AuNPs) structure have been studied intensively. This plasmonic approach can extend absorption range to visible light, but its incident photon-to-current conversion efficiency is still insufficient. Therefore, it is important to amplify surface plasmon resonance, obtained by using AuNPs. At the same time, for application to next-generation energy device, the way to improve this plasmonic effect needs to be available to large area fabrication, high reproducibility, low cost and so on. In this study, we successfully fabricated a three-dimensional (3D) moth-eye AuNP/TiO2/Au hierarchical structure for water splitting via direct printing method and deposition process. The proposed structure can effectively intensify the light???matter interaction owing to two mechanisms: Photonic mode light trapping attributed by moth-eye structure and enhanced surface plasmon resonance by gap-plasmon structure. Moth-eye structure, densely packed subwavelength-nanocone array, was easily fabricated by direct printing method. Using this moth-eye structure as a template, we effectively fabricated 3D moth-eye patterned gap-plasmon structure on 5 ?? 5 cm2. Compared with the two-dimensional (2D) AuNP/TiO2/Au absorber, the 3D moth-eye type absorber has higher absorption in entire 300???800 nm range. In accordance with this result, the 3D moth-eye absorber provides a photocurrent density of approximately 52.82 ??A cm???2, which is approximately 2.3 times higher than that of the flat 2D TiO2/Au thin film (22.96 ??A cm???2). Notably, it exhibits extraordinary enhancement of the photocurrent density???from 1.5 to 22.51 ??A cm???2???in the visible range (???420 nm)

Fabrication of perovskite solar cell with high short-circuit current density (J(SC)) using moth-eye structure of SiOX

The performance of solar cells is determined by three factors: the open-circuit voltage (V-OC), short-circuit current density (J(SC)), and fill factor (FF). The V-OC and FF are determined by the material bandgap and the series/shunt resistance, respectively. However, J(SC) is determined by the amount of incident light in addition to the bandgap of the material. In this study, a moth-eye pattern was formed on a glass surface via direct printing to increase the amount of incident light and thus increase J(SC). The moth-eye pattern is a typical antireflection pattern that reduces the reflection by gradually increasing the refractive index. A flat perovskite solar cell (F-PSC) and a moth-eye patterned perovskite solar cell (M-PSC) had J(SC) values of 23.70 and 25.50 mA/cm(2), respectively. The power-conversion efficiencies of the F-PSC and M-PSC were 19.81% and 21.77%, respectively

High thermoelectric figure of merit of porous Si nanowires from 300 to 700 K.

Thermoelectrics operating at high temperature can cost-effectively convert waste heat and compete with other zero-carbon technologies. Among different high-temperature thermoelectrics materials, silicon nanowires possess the combined attributes of cost effectiveness and mature manufacturing infrastructures. Despite significant breakthroughs in silicon nanowires based thermoelectrics for waste heat conversion, the figure of merit (ZT) or operating temperature has remained low. Here, we report the synthesis of large-area, wafer-scale arrays of porous silicon nanowires with ultra-thin Si crystallite size of ~4 nm. Concurrent measurements of thermal conductivity (κ), electrical conductivity (σ), and Seebeck coefficient (S) on the same nanowire show a ZT of 0.71 at 700 K, which is more than ~18 times higher than bulk Si. This ZT value is more than two times higher than any nanostructured Si-based thermoelectrics reported in the literature at 700 K. Experimental data and theoretical modeling demonstrate that this work has the potential to achieve a ZT of ~1 at 1000 K