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Digital coding and programmable metasurfaces stand out as promising candidates in future wireless communications due to their low costs and fast‐action reprogrammable capability. However, 2 significant obstacles, namely integration and power consumption, must be addressed before large‐scale engineering application of the programmable metasurface. This work proposes an easy‐integration and energy‐saving meta‐system and demonstrates its application in wireless communications. The meta‐system features a hybrid architecture and consists of a programmable feed array and a metasurface lens. The feed array comprises 3 subarrays, in which active elements are doped with passive elements and interconnected configurations are used to efficiently reduce the number of active elements by one‐third. The meta‐lens acts as a passive phase shifter to enhance the beamforming performance, further reducing the number of active elements. Moreover, the meta‐system can switch between different modes to achieve various functions. In particular, the maximum power consumption of the meta‐system is only 54 mW, which can be considered nearly passive. A wireless communication experiment is presented, where the meta‐system simultaneously serves as the direct digital modulator and the transmitting antenna. Owing to the low cost, high integration, and nearly passive and programmable features, the meta‐system has remarkable potential for future applications in wireless communications. [ABSTRACT FROM AUTHOR]

Micro‐Doppler effect is a vital feature of a target that reflects its oscillatory motions apart from bulk motion and provides an important evidence for target recognition with radars. However, establishing the micro‐Doppler database poses a great challenge, since plenty of experiments are required to get the micro‐Doppler signatures of different targets for the purpose of analyses and interpretations with radars, which are dramatically limited by high cost and time‐consuming. Aiming to overcome these limits, a low‐cost and powerful simulation platform of the micro‐Doppler effects is proposed based on time‐domain digital coding metasurface (TDCM). Owing to the outstanding capabilities of TDCM in generating and manipulating nonlinear harmonics during wave‐matter interactions, it enables to supply rich and high‐precision electromagnetic signals with multiple micro‐Doppler frequencies to describe the micro‐motions of different objects, which are especially favored for the training of artificial intelligence algorithms in automatic target recognition and benefit a host of applications like imaging and biosensing. [ABSTRACT FROM AUTHOR]

The present study aimed to evaluate the effects of delayed harvest and storage length on fermentation products, bacterial community, and metabolic shifts of elephant grass silage. The late-harvested elephant grass (LG) was naturally fermented (NLG) for 1, 3, 7, 15, 30, and 60 days, respectively. After 60-day ensiling, NLG displayed homolactic fermentation with low pH value, butyric acid, and ammonia nitrogen concentrations, and high lactic acid concentration, and ratio of lactic acid to acetic acid. Pseudomonas, Sphingomonas , and Pantoea dominated the bacterial community in LG, but Lactobacillus, Lactococcus , and Pediococcus were the advantageous genera in a 3-day and 60-day NLG. The correlation heatmap revealed that Acetobacter was positively related to acetic acid, ethanol, ammonia nitrogen, and butyric acid concentrations. There were distinct differences in the KEGG (Kyoto Encyclopedia of Genes and Genomes) metabolic profiles of fresh and ensiled LG. Ensiling suppressed the metabolism of amino acid, vitamins, and energy, while promoted the metabolism of carbohydrate. The LG can be well-fermented without additives, but its low crude protein content should not be ignored when applied in agricultural practice. The ensiling process remarkably affected the fermentation quality, bacterial community, and metabolic profiles of NLG. [ABSTRACT FROM AUTHOR]

This study investigated the effects of developmental stage and store time on the chemical composition, microbial community, co-occurrence networks and fermentation characteristics of sweet sorghum silage. Sweet sorghum harvested at two developmental stages (heading stage, S1; hard dough stage, S2) was treated as follows: (I) natural fermentation of S1 (NS1); (II) natural fermentation of S2 (NS2) and ensiled for 1, 3, 7, 15, 30 and 60 days. After 60 days of ensiling, NS2 silage had higher lactic acid concentration and the ratio of lactic to acetic acid, and lower pH value and acetic acid concentration than NS1 silage. Meanwhile, NS2 silage also had higher ammonia nitrogen (NH3-N) content than NS1 silage, but the NH3-N content of both treatments was lower than 100 g/kg TN. Leuconostoc and Lactococcus were respectively dominant in both 7-day NS1 and NS2 silages, while Lactobacillus was the most abundant genus in 30-day NS1 and NS2 silages. The developmental stage altered the bacterial co-occurrence networks of fresh and ensiled sweet sorghum. Spearman's correlation heatmap showed that the higher lactic acid content and ratio of lactic to acetic acid in NS2 silage could be associated with the higher water-soluble carbohydrate content in S2 material, the higher abundance of Lactococcus in the initial phase of ensiling and the higher abundance of Lactobacillus in the late phase of ensiling. These results concluded that the sweet sorghum harvested at the hard dough stage is preferred for silage production over the sweet sorghum harvested at the heading stage. Microbial succession pattern was studied on fresh and ensiled samples. Growth stage affected the chemical and microbial parameters of sweet sorghum. Epiphytic microbiota was correlated with the chemical composition of forage. Metagenomic sequencing showed Lactobacillus was dominated in all silages. Growth stage accelerated the fermentation of sweet sorghum silage. [ABSTRACT FROM AUTHOR]

Direction-of-arrival (DOA) estimation is one of the most critical technologies of radar, remote sensing, and wireless communications. The traditional DOA estimation is closely related to phased array antennas, which require complicated and expensive hardware and high-power consumption. Metasurface can manipulate electromagnetic waves without using massive transceivers, which makes it possible to realize antenna arrays in a cost-effective way. Here, we propose a strategy of DOA estimations by using a time-domain-coding digital metasurface and a single receiver. Specifically, the incident wave on the metasurface is modulated by the time-domain orthogonal codes impressed on the meta-atoms, and their amplitude and phase distributions are precisely retrieved from the signals detected by the receiver antenna. The effectiveness and accuracy of the proposed strategy are verified by experiments on a two-dimensional metasurface with individually addressable meta-atoms. The strategy features low cost and high flexibility and will facilitate various wireless applications. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) are of potential use in wireless communication systems due to their ability to perform beam steering, harmonic wave manipulation, and polarization conversion. However, the main challenge in designing RISs is to obtain their electromagnetic (EM) responses, which is time-consuming and computationally prohibitive. In this work, an efficient modeling method is proposed for fast evaluation of the reflection coefficient and far-field patterns of RISs. The proposed method utilizes the microwave network theory to compute the accurate reflection coefficient of RISs elements under the illumination of incident waves from arbitrary directions. Compared to the classical equivalent circuit model, it enables the separate design of meta-atoms and tunable devices at the network level. The proposed method is verified by the full-wave simulation results of two RIS elements and experimental results of a 3-bit RIS. This work forms a fast and feasible route to obtain the responses of RISs, which allows the RISs to find extensive applications in many scenarios. [ABSTRACT FROM AUTHOR]

Recent progress in space‐time‐coding digital metasurface (STCM) manifests itself a powerful tool to engineer the properties of electromagnetic (EM) waves in both space and time domains, and greatly expands its capabilities from the physical manipulation to information processing. However, the current studies on STCM are focused under the synchrony frame, namely, all meta‐atoms follow the same variation frequency. Here, an asynchronous STCM is proposed, where the meta‐atoms are modulated by different time‐coding periods. In the proposed asynchronous STCM, the phase discontinuities on traditional metasurface are replaced with the frequency discontinuities. It is shown that dynamic wavefronts can be automatically realized for both fundamental and high‐order harmonics by elaborately arranging the spatial distribution of meta‐atoms with various time‐coding periods. The physics insight is due to the accumulated rapidly changing phase difference with time, which offers an additional degree of freedom during the wave‐matter interactions. As a proof‐of‐principle example, an asynchronous STCM for automatic spatial scanning and dynamic scattering control is investigated. From the theory, numerical simulations, and experiments, it can be found that the proposed STCM exhibits significant potentials for applications in radars and wireless communications. [ABSTRACT FROM AUTHOR]