Subwavelength perfect optical absorption structures based on monolayer-graphene are analyzed and demonstrated experimentally. The perfect absorption mechanism is a result of critical coupling ...relating to a guided mode resonance of a low index two-dimensional periodic structure. Peak absorption over 99% at wavelength of 1526.5 nm with full-width at half maximum (FWHM) about 18 nm is demonstrated from a fabricated structure with period of 1230 nm, and the measured results agree well with the simulation results. In addition, the influence of geometrical parameters of the structure and the angular response for oblique incidence are analyzed in detail in the simulation. The demonstrated absorption structure in the presented work has great potential in the design of advanced photo-detectors and modulators.
Cell-free massive multiple-input-multiple-out (CF-mMIMO) systems provide limited coverage because of expensive wired fronthaul between access points (APs) and central processing unit (CPU). To ...address this challenge, we propose a novel framework where an unmanned aerial vehicle (UAV), acting as an aerial AP, works coherently with the ground APs to expand the coverage of conventional CF-mMIMO system. To fully utilize the spectrum resource, the wireless fronthaul between the CPU and UAV shares the total bandwidth with the radio access networks. Considering limited power supply of the UAV and for the goal of green communications, we formulate a weighted sum power minimization problem to jointly optimize downlink beamforming and fronthaul compression, as well as UAV placement. The formulated problem is a mixed timescale problem, thus we propose a two-timescale optimization framework in which the UAV placement is optimized in each long timescale based on statistical channel state information (CSI), then the downlink beamforming and fronthaul compression are optimized in each short timescale based on instantaneous CSI. Specifically, uplink-downlink duality and semidefinite relaxation (SDR) based alternating optimization techniques are introduced to find solutions to the short timescale issue, while successive convex approximation and SDR methods are invoked to find solutions to the long timescale issue. Finally, simulation results corroborate the performance of the proposed algorithm.
We exploit a behavior-shaping proactive mechanism, namely, recommendation, in cache-assisted non-orthogonal multiple access (NOMA) networks, aiming at minimizing the average system's latency. ...Thereof, the considered latency consists of two parts, i.e., the backhaul link transmission delay and the content delivery latency. Towards this end, we first examine the expression of system latency, demonstrating how it is critically determined by content cache placement, personalized recommendation, and delivery associated NOMA user pairing and power control strategies. Thereafter, we formulate the minimization problem mathematically taking into account the cache capacity budget, the recommendation-oriented requirements, and the total transmit power constraint, which is a non-convex, multi-timescale, and mixed-integer programming problem. To facilitate the process, we put forth an entirely new paradigm named divide-and-rule . Specifically, we first solve the short-term optimization problem regarding user pairing as well as power allocation and the long-term decision-making problem with respect to recommendation and caching, respectively. On this basis, an iterative algorithm is developed to optimize all the optimization variables alternately. Particularly, for solving the short-timescale problem, graph theory enabled NOMA user grouping and efficient inter-group power control manners are invoked. Meanwhile, a dynamic programming approach and a complexity-controllable swap-then-compare method with convergence insurance are designed to derive the caching and recommendation policies, respectively. From Monte-Carlo simulation, we show the superiority of the proposed joint optimization method in terms of both system latency and cache hit ratio when compared to extensive benchmark strategies.
Hybrid systems consisting of graphene and various two-dimensional materials would provide more opportunities for achieving desired electronic and optoelectronic properties. Here, we focus on a ...superlattice composed of graphene and monolayer MoS2. The geometric and electronic structures of the superlattice have been studied by using density functional theory. The possible stacking models, which are classified into four types, are considered. Our results revealed that all the models of graphene/MoS2 superlattices exhibit metallic electronic properties. Small band gaps are opened up at the K-point of the Brillouin zone for all the four structural models. Furthermore, a small amount of charge transfer from the graphene layer to the intermediate region of C–S layers is found. The band structure and the charge transfer together with the buckling distortion of the graphene layer in the superlattice indicate that the interaction between the stacking sheets in the superlattice is more than just the van der Waals interaction.
Under the organization of the base station (BS), wireless federated learning (FL) enables collaborative model training among multiple devices. However, the BS is merely responsible for aggregating ...local updates during the training process, which incurs a waste of the computational resource at the BS. To tackle this issue, we propose a semi-federated learning (SemiFL) paradigm to leverage the computing capabilities of both the BS and devices for a hybrid implementation of centralized learning (CL) and FL. Specifically, each device sends both local gradients and data samples to the BS for training a shared global model. To improve communication efficiency over the same time-frequency resources, we integrate over-the-air computation for aggregation and non-orthogonal multiple access for transmission by designing a novel transceiver structure. To gain deep insights, we conduct convergence analysis by deriving a closed-form optimality gap for SemiFL and extend the result to two extra cases. In the first case, the BS uses all accumulated data samples to calculate the CL gradient, while a decreasing learning rate is adopted in the second case. Our analytical results capture the destructive effect of wireless communication and show that both FL and CL are special cases of SemiFL. Then, we formulate a non-convex problem to reduce the optimality gap by jointly optimizing the transmit power and receive beamformers. Accordingly, we propose a two-stage algorithm to solve this intractable problem, in which we provide the closed-form solutions to the beamformers. Extensive simulation results on two real-world datasets corroborate our theoretical analysis, and show that the proposed SemiFL outperforms conventional FL and achieves 3.2% accuracy gain on the MNIST dataset compared to state-of-the-art benchmarks.
Abstract We report two phenomena detected in PSR J0344−0901 from two observations conducted at frequencies centered at 1.25 GHz using the Five-hundred-meter Aperture Spherical Radio Telescope. The ...first phenomenon manifests as the pulse emission shifting to later longitudinal phases and then gradually returning to its original location. The event lasts for about 216 pulse periods, with an average shift of about 0.°7 measured at the peak of the integrated profile. Changes in the polarization position angle (PPA) are detected around the trailing edge of the profile, together with an increase in the profile width. The second phenomenon is characterized by the apparent movement of subpulses, which results in different subpulse track patterns across the profile window. For the first time in this pulsar, we identify four emission modes, each with unique subpulse movement, and determine the pattern periods for three of them. Pulse nulling was not detected. Modeling of the changes in the PPA using the rotating vector model gives an inclination angle of 75.°12 ± 3.°80 and an impact parameter of −3.°17 ± 5.°32 for this pulsar. We speculate that the subpulse movement may be related to the shifting of the pulse emission.
Motivated by the ever-increasing demands for massive data processing and intelligent data analysis at the network edge, federated learning (FL), a distributed architecture for machine learning, has ...been introduced to enhance edge intelligence without compromising data privacy. Nonetheless, due to the large number of edge devices (referred to as clients in FL) with only limited wireless resources, client scheduling, which chooses only a subset of devices to participate in each round of FL, becomes a more feasible option. Unfortunately, the training latency can be intolerable in the iterative process of FL. To tackle the challenge, this article introduces update-importance-based client scheduling schemes to reduce the required number of rounds. Then latency-based client scheduling schemes are proposed to shorten the time interval for each round. We consider the scenario where no prior information regarding the channel state and the resource usage of the devices is available, and propose a scheme based on the multi-armed bandit theory to strike a balance between exploration and exploitation. Finally, we propose a latency-based technique that exploits update importance to reduce the training time. Computer simulation results are presented to evaluate the convergence rate with respect to the rounds and wall-clock time consumption.
Aims
Secoisolariciresinol (SECO) is increasingly recognized for potential clinical application because of its preventive effects against breast and colon cancers, atherosclerosis and diabetes, and ...its production through biotransformation has been attempted. However, previously reported bacteria all required stringent anaerobic culture conditions, precluding large‐scale production. Here, we report the isolation and characterization of bacteria that produce SECO under less stringent anaerobic culture conditions.
Methods and Results
Using defatted flaxseed as raw material, we isolated a facultative anaerobic bacterium from human faeces that hydrolysed secoisolariciresinol diglucoside‐3‐hydroxy‐3‐methyl glutaric acid (SDG‐HMGA) oligomers in flaxseed to produce SECO. Both conventional assays and 16S rRNA gene sequence analysis demonstrated its close relatedness with Bacteroides uniformis. The transformation efficiency of SDG in defatted flaxseed to SECO was more than 80% by this bacterial strain. We investigated factors that might influence fermentation, such as redox potential and pH, for large‐scale fermentation of defatted flaxseed to produce SECO.
Conclusions
The method to produce SECO through biotransformation of defatted flaxseed with this bacterial strain is highly efficient and economic.
Significance and Impact of the Study
This bacterial strain can transform SDG to SECO under less stringent anaerobic culture conditions, which will greatly facilitate industry‐scale production of SECO.
Strongly textured Mg–3Al–1Zn (wt%) alloy samples with a high density of twins cut from three directions were used to investigate the effect of crystallographic orientation on dynamic ...recrystallization (DRX). The samples were deformed under a high strain rate at 300°C. The high strain rate allows rapid dislocation accumulation, making DRX possible at relatively low strain values. We demonstrate that crystallographic orientation and the initial twins have significant effects on DRX because of the different activities of dislocations and their interactions with twin boundaries. This research provides a potential pathway for improving the formability of Mg alloys.
We have recently reported that a high strain-rate rolling process is effective for producing strong and ductile Mg alloy sheets. Here we elucidate the fundamental mechanisms that are responsible for ...plastic deformation in this process via systematic physical thermomechanical plane-strain rolling simulations on a Mg–Zn–Zr alloy. The strain-rate sensitivities of the alloy’s microstructure and flow curves were closely correlated to the onset of deformation twinning and dynamic recrystallization (DRX). Unlike deformation at low strain rates, deformation at the high strain rates applied here induced a high number density of twins, including a predominance of {101¯1}–{101¯2} double twins in coarse grains, and a predominance of {101¯2} twins in fine DRX grains. We also report a new observation of {101¯2} nanotwins in ultrafine grains with high density in bulk Mg alloy when processed at high strain rates. We propose that the high propensity for twinning at high strain rates provides nucleation sites for DRX, resulting in a uniform, ultrafine-grained microstructure that exhibits a weakened basal texture and thus excellent mechanical properties.
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