Indoor localization using Wi-Fi fingerprinting based on Received Signal Strength (RSS) has gained widespread attention due to its immunity to external factors and ability to penetrate obstacles. The localization process involves an offline phase for building a radio map and an online phase for matching location queries. Existing matching algorithms often prioritize enhancing online phase accuracy, overlooking the importance of offline data preprocessing, which can negatively impact overall performance. This study introduces a novel approach called Fingerprint Dictionary Preprocessing (FDP) that employs Convolutional Dictionary Learning (CDL) to process radio map data. CDL learns a set of kernels capturing site characteristics, representing RSS values from Access Points (APs) in a sparse manner. The proposed FDP system compresses data through feature learning, reducing storage requirements for data transmission. In the online phase, CDL is utilized for assisting matching fingerprints against the learned dictionary, accurately locating users. The contributions of the FDP system presenting a cost-effective and practical solution for indoor localization, addressing the challenges associated with large data collection and multi-dimensional data requirements, making it a promising approach for real-world applications. We conducted experiments in two real indoor environments, and the results indicated that the proposed FDP system, whether applied to the original radio map or the preprocessed fingerprint database, led to improved localization accuracy and reduced localization time.

Polyimides (PIs) are widely used in microelectronics and advanced optical industries due to their excellent overall properties. However, the efficient and mild synthesis of PIs from commercially available raw materials poses great challenges in polymer chemistry. Here, a rapid and versatile thiol-Michael click reaction between imide-containing bismaleimides (BMI) and aromatic dithiol is developed. The structural diversity of the imide-containing BMI allows for manipulation of the optical and thermal properties by a molecular design with different substituents or dianhydrides in the backbone. The polymerization is good enough so that it can be completed in only 1 min at room temperature (25 degrees C) in the presence of triethylamine, affording high molecular weight PIs with good thermal stability, high tensile strength of up to 126.7 MPa, and desirable solubility and processability. With the structural design of the PIs and the introduced sulfur elements, the refractive indices of PIs are as high as 1.6601, while ensuring a high optical transmittance of up to 85% at 400 nm. This thiol-maleimide click polymerization significantly speeds up the preparation of functional PIs that have potential applications in advanced optical devices.

Mobile crowdsensing (MCS), which is a grassroots sensing paradigm that utilizes the idea of crowdsourcing, has attracted the attention of academics. More and more researchers have devoted themselves to adopting MCS in space–air–ground–sea integrated networks (SAGSINs). Given the dynamics of the environmental conditions in SAGSINs and the uncertainty of the sensing capabilities of mobile people, the quality and coverage of the sensed data change periodically. To address this issue, we propose a novel UAV-assisted cluster-based task allocation (UCTA) algorithm for MCS in SAGSINs in a two-stage process. We first introduce the edge nodes and establish a three-layer hierarchical system with UAV-assistance, called “Platform–Edge Cluster–Participants”. Moreover, an edge-aided attribute-based cluster algorithm is designed, aiming at organizing tasks into clusters, which significantly diminishes both the communication overhead and computational complexity while enhancing the efficiency of task allocation. Subsequently, a greedy selection algorithm is proposed to select the final combination that performs the sensing task in each cluster. Extensive simulations are conducted comparing the developed algorithm with the other three benchmark algorithms, and the experimental results unequivocally endorse the superiority of our proposed UCTA algorithm.

The impacted-type piezoelectric energy harvesters (IPEHs) are widely used to harvest theenergy of low-frequency impact forces from raindrops, human motion, machines, and otherenvironmental sources. Owing to the typically low impact frequency, the output voltagebetween the two excitations undergoes damping. The attenuation coefficient lambda, which reflectsthe damping rate, directly affects the performance of the energy harvesting circuit. This studyanalyzes and compares the variations of three energy harvesting circuits: full-bridge rectifier(FBR), parallel synchronized switch harvesting on inductor (P-SSHI), and synchronouselectrical charge extraction (SECE) circuits under varying lambda. First, the ideal energies of thethree energy harvesting circuits during one impact event are summarized according to theirmaximum output power per half-cycle related to lambda. Derivations of the IPEH with self-poweredFBR, P-SSHI, and SECE circuits are provided, including the energy consumption and start-upvoltage. Furthermore, these self-powered circuits connected with an IPEH are simulated. Theoptimal load resistance and capacitance of these circuits are analyzed relative to the variation of lambda.The optimal load resistance of the SECE circuit varies significantly as lambda increases, whereasthe optimal loads of other circuits remain relatively constant. The output energies of the threeself-powered circuits are analyzed as the force intensity and lambda vary. The P-SSHI circuit yieldsthe highest energy when appropriate components are selected. Under large lambda or weak externalforces, the output energy of the FBR circuit surpasses that of the SECE circuit. Finally, the threeself-powered circuits are implemented in the IPEH. The experimental results show that theself-powered P-SSHI circuit generates the highest energy, with the figure of merit graduallyincreasing with the external force, which is consistent with the theoretical analysis. Theguidance provided in this study is a reference for designing IPEH circuits.

Mobile CrowdSensing (MCS) has become a convenient method for many Internet of Things (IoT) applications in urban scenarios due to the full utilization of the mobility of people and the powerful capabilities of their intelligent devices. Nowadays, edge computing has been introduced into MCS to reduce the time delays and computational complexity in cloud platforms. To improve task completion and coverage rates, how to design a reasonable user recruitment algorithm to find suitable users and take full advantage of edge nodes has raised huge challenges for Mobile CrowdSensing. In this study, we propose a Reputation-based Collaborative User Recruitment algorithm (RCUR) under a certain budget in an edge-aided Mobile CrowdSensing system. We first introduce edge computing into MCS and build an edge-aided MCS system in urban scenarios. Moreover, we analyze the influence of user reputation on user recruitment. Then we establish a user reputation module to deduce the user reputation equation by combining the user’s past reputation score with an instantaneous reputation score. Finally, we utilize the sensing ability of edge nodes and design a collaborative sensing method. We use the greedy method to help choose the appropriate users for the tasks. Simulation results compared with the other three algorithms prove that our RCUR approach can significantly achieve better performance in task completion rate and task coverage rate.

Membrane-based gas separation technique shows great potential in CO2 elimination from natural gas and N2 enrichment from compressed air. In this study, a double-decker-shaped phenyl-substituted silsesquioxane (DDSQ)-based diamine monomer, that is, DDSQ-diamine, was synthesized and subsequently copolymerized with 4,4 '-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) in combination with 4,4 '-oxydianiline (ODA) to fabricate a series of DDSQ-based polyimide membranes with intrinsic microporosity (PIM-PIs). The resulting membranes showcase desirable mechanical robustness, satisfactory heat resistance and good organo solubility. Owing to the inherent internal nanometer-sized cavity of DDSQ cage, DDSQ-based membranes are more nanoporous than DDSQ-free sample, accordingly exhibiting decreasing membrane density and increasing gas permeability. Among all membranes, DDSQ-25 and DDSQ-50 show more desirable overall gas separation per-formance, which almost achieve the 1991 Robeson upper bound. Additionally, DDSQ-25 demonstrates good plasticization resistance and CO2/CH4 mixed-gas separation properties when measured with 50:50 CO2/CH4 binary gas mixture at upstream pressure up to 20 bar. Despite physical aging at ambient conditions for 300 days, the resultant DDSQ-based polyimide membranes still have stable gas permeability and almost constant selec-tivity, showcasing satisfactory anti-aging characteristic.

Perovskite solar cells (PSCs) are popular light-to-electric energy converters thanks to their high power conversion efficiency and ease of manufacture. However, the hysteresis associated with the characteristics of PSCs has become a new challenge for energy harvesting technology. This paper presents a PSC model that adequately reflects the hysteresis and investigates the impact of the hysteresis on two maximum power point tracking (MPPT) methods. Oscillation caused by hysteresis occurs when the PSC is connected to an MPPT-controlled load. By overcoming this disadvantage, the incremental conductance algorithm performs better because it takes 78.6% less time to reach the maximum power point (MPP) than the perturbation and observation algorithm.

A rotational electromagnetic energy harvester based on a Halbach array (HC-EMEH) with a self-powered Maximum power point tracking (MPPT) circuit is presented in this paper. The magnetic field distribution model of the Halbach array was derived. Based on the law of electromagnetic induction, when the rotor rotates, the coils cut the magnetic induction lines of the Halbach array and produce the AC electric energy. However, the open circuit voltage at the harvester's output varies with rotational speed. Thus, a maximum power point tracking circuit is designed between the load and harvester to extract the maximum power from a harvester, no matter how the rotational speeds change. The designed MPPT circuit powered by a self-starting circuit can generate stable voltage for the power supply of the MPPT circuit. The experimental results show that the efficiency of the MPPT circuit reaches more than 80%, and the power consumption is only $\mathbf{15}\boldsymbol{\mu}\mathbf{W}$ when the harvester rotates at 1800rpm-3000rpm.

Developing efficient sulfur cathodes toward low conductivity of S8 and severe polysulfide shuttling is of great significance for lithium-sulfur batteries. Herein, a highly aligned and low tortuosity nanoarray engineering composed of N-doped carbon nanotube arrays uniformly coated with a thin layer of TiO2 (noted as aligned TiO2@CNT@CC) was facilely synthesized. As revealed by density functional theory calculations and COMSOL simulations, the vertically aligned CNTs provide fast electronic transport paths and the TiO2 layer catalyzes fast redox reaction kinetics of polysulfides. As further confirmed by electrochemical tests, the low tortuosity TiO2@CNT@CC shows better performance than both bare CNT@CC and disordered TiO2@CNT@CC electrodes. Typically, a specific energy density of 1298 mAh g-1 at 0.1C is obtained, which retains 715 mAh g-1 at a high rate of 3C. Remarkably, the aligned TiO2@CNT@CC cathode shows a high specific discharge capacity of 779 mAh g-1 with sulfur loading of 3 mg cm-2 after 100 cycles at a E/S ratio of 10 mu L mg-1. Our approach would provide a promising route for the facile structural & surface engineering of ordered array electrodes with high electrochemical performance.

A series of high-performance poly-(imide-imine) hybrid (PIIH) vitrimers derived from five diamines containing an internal bisimide unit and alicyclic structure have been constructed. The mechanical and thermal properties have been drastically improved to 97.0 MPa for tensile strength, 2.64 GPa for tensile modulus, 306 degree celsius for glass transition temperature, and 348 degree celsius for onset thermo-decomposition temperature, much higher than those of many reported polyimines. Owing to the inclusion of a hydrophobic alicyclic structure, the water uptake of the resulting PIIHs significantly decreases, and their polymer network remains almost intact albeit after soaking in water for 24 h. The resultant PIIHs can be healed and exhibit similar to 90% healing efficiency for tensile strength in the presence of primary amine and can be rapidly degraded in primary amine solution and acid solution containing DMF. The proposed hybridization strategy enables the development of rehealable polymers with robust and heat-resistant polymer networks, making them well-suited for potential use in some harsh environments requiring polymer rehealing and degradation.

This paper investigates the performance of the structural acoustic controlled active micro-perforated panel absorber (SAC-AMPPA), which can achieve wide-band perfect low frequency sound absorption (absorption coefficient is close to 1). The SAC-AMPPA applies point force-controlled backing panel to actively improve the low-frequency sound absorption of the MPPA with the purpose of saving space to suit it better for applications. The theoretical model of the SAC-AMPPA is firstly established using the modal analysis approach. Influence of structure size and point force position on sound absorption performance is explored. Then, the experimental tests were performed to validate the theoretical modeling and findings. Finally, the physical mechanisms of active control are analyzed in detail and some physical insights are summarized. Simplified error sensing strategy for small sized SAC-AMPPA is also constructed. Results obtained show that the preconditions of the point force locating at the center of the backing panel or relatively small sized SAC-AMPPA can guarantee less cavity modes being excited and achieving perfect sound absorption in a very wide controllable bandwidth. The main reason of this lies in the key findings, i.e., except for the (0,0,m) mode, other cavity modes excited by the backing panel cannot contribute to the improvement of low-frequency sound absorption below their resonant frequencies. They radiate sound energy towards the outside of SAC-AMPPA and play a negative role above their resonant frequencies. Provided a uniform cavity sound field is guaranteed in controlled condition, the sound pressure release (PR) and impedance matching (IM) strategies can be used to conveniently construct error sensing strategy of the SAC-AMPPA.

This paper presents a theoretical investigation on actively controlling the low frequency sound absorption of large-sized micro-perforated panel absorber (MPPA) by using point source placed in the cavity. The large sized active MPPA is an easily implemented and cost-effective scheme for achieving superior low frequency sound absorption in large area. The modal analysis approach is used to establish the theoretical model of such active MPPA. The physical mechanism of active control is analyzed and some physical insights are summarized. Based on these conclusions, the error sensing strategy of the large sized active MPPA is also constructed. Finally, experimental tests are carried out to validate the theoretical modeling and findings. Results show that the sound absorption performance can be significantly improved after control in a wide low frequency range below the cutoff frequency. The optimal position of the point source is at the central of the cavity section, in which case the cutoff frequency is the largest and depends on the length of the short side of the active MPPA. The control mechanism of sound absorption improvement is to enhance the Helmholtz type resonant absorption by suppressing the cavity sound field. The high order cavity mode (except for (0,0,0) mode) has no contribution to sound absorption improvement due to the symmetrical property of their mode shape. The pressure release (PR) and impedance matching (IM) strategies are still applicable for the large sized active MPPA. Relative small sized cavity or low frequency excitation can guarantee the uniformity of the cavity sound field after control, which is conducive to achieve remarkable improvement of sound absorption.

To improve the output power of impact‐type piezoelectric energy harvesters (IPEHs), a high‐efficiency energy management circuit combining a self‐powered synchronized switch harvesting on inductor (SP‐SSHI) rectifier and an alternative impedance matching circuit is proposed. The SP‐SSHI rectifier eliminates the effect of inherent capacitance of IPEH through LC oscillation and enhances the output power. The alternative impedance matching circuit provides two optimal load resistances for different periods of IPEHs to increase the harvested power. Furthermore, to decrease the power dissipation, the proposed circuit is activated only when the output power of IPEH is sufficiently high and the SSHI is active. A control chip of the proposed circuit is designed and fabricated in a 0.18 μm complementary metal‐oxide semiconductor process. The proposed circuit is applied for a microwind IPEH, and the peak figure of merit and efficiency are 3.68 and 90.4%, at the wind speed of 1.46 m s −1 , respectively. The results indicate a significant improvement in output power and harvesting efficiency of the proposed IPEH‐specialized circuit. When a flame sensor (USEQFSEA22L80) is powered by the microwind energy harvesting system, the proposed circuit increases operating time of the sensor as 4.6 times, as compared with the traditional impedance matching circuit.

Mobile edge caching can deliver contents directly without the backhaul link, which can effectively solve the problem of spectrum scarcity caused by huge mobile data traffic. In this paper, different from the existing user-centric clustering algorithms, a distributed caching algorithm is proposed based on content providers (CPs), which can form a CPs cluster as large as possible to satisfy the UE requirements. The cache capacity in the cluster formed by this algorithm is collectively used to provide higher content hit probability and diversity. Furthermore, considering the impact of social interests on the performance of caching strategies, a closed-form expression of the network hit ratio of the entire cache is derived on the basis of random geometry theory. Then, a network hit ratio maximization optimization problem is constructed and solved. The simulation results show that the proposed strategy has superior data offloading performance than other cooperative caching strategies.

In the past decade, single-atom and nanocluster catalysts have emerged as promising materials for high efficiency electrocatalysis, while geometric morphologies and electronic properties of the supporting substrates dramatically affect the overall electrocatalytic performances. We have constructed highly oriented nitrogen-doped carbon nanotubes on carbon cloth (V-CNTs/CC) and demonstrated it can serve as superior substrate for Pt nanoclusters that are typical electrocatalysts for hydrogen evolution reactions. Compared with Pt@CNTs/CC, the Pt@V-CNTs/CC has significantly improved electron transport and charge transfer efficiency, as well as gas diffusion capability, leading to excellent catalytic activity and stability. The V-CNTs/CC designed in this work can serve as general support for various single-atom or nanocluster catalysts for high-performance electrocatalysis.

Mobile edge caching can deliver contents directly without the backhaul link, which can effectively solve the problem of spectrum scarcity caused by huge mobile data traffic. The cooperative cache strategy is able to make full use of user equipments with limited cache capacity to improve the network hit rate and cache diversity. First, this paper proposes a clustering algorithm based on the content providers, which is different from the existing clustering algorithms. The cache capacity in the cluster formed by this algorithm is collectively used to provide higher content hit probability and diversity. We further consider the impact of social interest on the performance of caching strategies. On the basis of random geometry theory, we derive the closed-form expressions of the hit rate of intra-cluster cache, inter-cluster cache and base station cache respectively. Finally, we obtain the closed-form expression of the network hit rate of the entire cluster cache, and investigate the problem of maximizing the network hit rate from the content caching strategy. Through the simulation of the key indicators, the results show that the proposed caching strategy is better than other cooperative caching strategies in data offloading performance.

A near-field vector sensing (VS) strategy is developed for three-dimensional (3D) large-scale hybrid sound absorption based on a lightweight structure. By simultaneously detecting sound pressures and normal particle velocities at discrete positions on the absorbing surface, the reflected sound power is minimized to obtain the optimal secondary excitation. For the one-dimensional case, low-frequency quasi-perfect absorption could be realized by one-point VS. For the 3D case (at the incident angle of 20°), the optimized two-point VS is able to realize commendable broadband absorption from 50 to 800 Hz and extraordinary absorption between 50 and 300 Hz.

An ultra-thin hybrid sound absorber consisting of a micro-perforated panel and a planar actuator (MPP-PA) is proposed. With the optimization of excitations on the PA plate, the absorber, almost half the thickness of a conventional passive MPP structure, can realize broadband oblique-incidence sound absorption from 100 to 1000 Hz due to the strong plate-cavity coupling. Particularly at frequencies dominated by the zeroth cavity mode and lower-order plate modes (100 to 300 Hz), extraordinary absorption is achieved and barely changes with the incident angle. Consequently, the hybrid MPP-PA provides an ultra-thin solution for the broadband high absorption in low-and-mid frequency ranges.

In order to enhance the low frequency sound absorption performance and the applicability of the traditional micro-perforated absorbers under complicated noise environments, a composite sound absorption structure consisting of a front flexible micro-perforated panel (MPP), an air cavity and a back planar loudspeaker (PL) is proposed. In addition to the mid and high frequency sound absorption properties of the MPP, with a properly optimized exciting force on the planar loudspeaker, the composite absorber has the ability to absorb low frequency sound. The planar loudspeaker is treated as a plate driven by a controllable point force. The theoretic analytical model is set up by the modal expansion and vibro-acoustic coupling method. Then a Finite Element (FE)based acoustic simulation model of the proposed structure is established. Theoretical analysis and numerical simulations are carried out to validate the proposed model.

Received Signal Strength Indicator (RSSI) localization using fingerprint has become a prevailing approach for indoor localization. However, the fingerprint-collecting work is repetitive and time-consuming. After the original fingerprint radio map is built, it is laborious to upgrade the radio map. In this paper, we describe a Fingerprint Renovation System (FRS) based on crowdsourcing, which avoids the use of manual labour to obtain the up-to-date fingerprint status. Extended Kalman Filter (EKF) and Gaussian Process Regression (GPR) in FRS are combined to calculate the current state based on the original fingerprinting radio map. In this system, a method of subset acquisition also makes an immediate impression to reduce the huge computation caused by too many reference points (RPs). Meanwhile, adjusted cosine similarity (ACS) is employed in the online phase to solve the issue of outliers produced by cosine similarity. Both experiments and analytical simulation in a real Wireless Fidelity (Wi-Fi) environment indicate the usefulness of our system to significant performance improvements. The results show that FRS improves the accuracy by 19.6% in the surveyed area compared to the radio map un-renovated. Moreover, the proposed subset algorithm can bring less computation.