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In this paper, a metamaterial electromagnetic energy harvester constructed via the capacitive loading of metal circular split rings is presented. Each energy-harvesting cell is loaded with a resistance that imitates the input impedance of a rectifier circuit. Specifically, the metamaterial energy harvester has high selective harvesting for left- and right-handed circularly polarized waves. Here, the energy absorption is mostly induced by the resistive load; thus, effective energy harvesting can be achieved. Moreover, the proposed energy harvester exhibits a high-efficiency harvesting for right-handed circularly polarized waves over a wide range of incident angles. Further, a transmission line model is adopted to interpret the energy harvesting mechanism, which shows that a good impedance matching and low dielectric loss can further enhance the harvesting efficiency. To demonstrate the design, a 15 × 15 unit-cell prototype is fabricated and measured, and the measured results reasonably agree with the simulated ones.

In this study, the design, realization, and characterization of an ultrathin triple-band polarization-insensitive wide-angle metamaterial absorber are reported. The metamaterial absorber comprises a periodic array of modified six-fold symmetric snowflake-shaped resonators with strip spiral line load, which is printed on a dielectric substrate backed by a metal ground plane. It is shown that the absorber exhibits three distinct near-unity absorption peaks, which are distributed across C, X, Ku bands, respectively. Owing to the six-fold symmetry, the absorber is insensitive to the polarization of the incident radiation. In addition, the absorber shows excellent absorption performance over wide oblique incident angles for both transverse electric and transverse magnetic polarizations. Simulated surface current and field distributions at the three absorption peaks are demonstrated to understand the absorption mechanism. Particularly, the absorption modes come from the fundamental and high-order dipole resonances. Furthermore, the experimental verification of the designed absorber is conducted, and the measured results are in reasonable agreement with the simulated ones. The proposed ultrathin (∼0.018λ0, λ0 corresponding to the lowest peak absorption frequency) compact (0.168λ0×0.168λ0 corresponding to the area of a unit cell) absorber enables potential applications such as stealth technology, electromagnetic interference and spectrum identification.

The problem of global finite-time convergent observer design for a class of nonlinear systems with time-varying coefficients is investigated in this paper. To solve this problem, a new time-varying finite-time convergent observer is first designed for the nominal system without nonlinearities. Using a recursive argument, an explicitly homogeneous Lyapunov function is constructed to prove the finite-time convergence of the error dynamic system. Then, using the homogeneous domination approach, we scale the finite-time observer with an appropriate choice of gain to obtain a global finite-time convergent observer for the nonlinear system.

This paper considers the problem of using a sampled-data controller to globally stabilize a class of feedforward nonlinear systems. Based on the continuous-time controller proposed in [3], a nested saturation sampled-data control law is first designed to drive states of the feedforward system into a small region around the origin in a finite time. Inside this small region, the nested saturation sampled-data control law is then reduced to a linear sampled-data control law. An explicit formula for the maximum allowable sampling period is computed to guarantee global stability of feedforward systems under the proposed sampled-data controller with appropriate gains.

The electronic and materials properties of carbon nanotubes (CNTs), like those of recent discovered graphene and earlier found fullerenes, attracted lots of research interests due to their appealing applications1 in the field of molecular electronics or for the refinement of materials, such as antistatic paints and shieldings, sensor, and catalytic functionality in fuel cells, etc. Like its two/three dimensional siblings graphene and graphite, CNTs consist sp2 hybridized carbon atoms and are either semiconducting or metallic depending on their helicity. CNTs can be considered in structure as rolling up a single sheet of graphene into a cylinder. Polymer electrolyte membrane fuel cells (PEMFCs) are the best candidates for automobile propulsion owing to their zero emissions, low temperature, and high efficiency. Precious platinum (Pt) catalyst is a key ingredient in fuel cells, which produce electricity and water as the only byproduct from hydrogen fuel.2 However, platinum is rare and expensive. Reducing the amount of Pt loading by identifying new catalysts is one of the major targets in the current research for the large-scale commercialization of fuel cells. Specifically, developing alternative catalysts to substitute platinum for the oxygen reduction reaction (ORR) in the fuel cell cathodes is essential because the slow kinetics of this reaction causes significant efficiency losses in the fuel cells. Recent intensive research efforts in reducing or replacing Pt-based electrode in fuel cells have led to the development of new ORR electrocatalysts, including carbon nanotube–supported metal particles.3,4 In 2006, Ozkan and coworkers reported that nitrogen-containing nanostructured carbons and nanotubes have promising catalytic activity towards ORR.5,6 In a 2008 report, Yang et al. at Argonne National Laboratory showed that the vertically-aligned CNT arrays, which are functionalized through nitrogen and iron doping by a chemical vapor deposition (CVD) process, can be electrocatalytically active toward ORR.7 The functionalized CNTs show promise properties as an alternative non-Pt electrocatalyst with a unique nano-architecture and advantageous material properties for the cathode of PEMFC. They further identified FeN4 sites, which are incorporated into the graphene layers of aligned carbon nanotubes, being electrocatalytic active towards ORR, by their X-ray absorption spectroscopy and other characterization techniques.

Considering the multi-variable, nonlinearity, time lag and time variation characters of air conditioning systems, it is difficult for traditional Proportional Integral Derivative (PID) control to get good control performance.A compound control strategy based on cerebellar model articulation controller (CMAC) and PID controller is used to the system. Obtained results show that the compound control algorithm can improve the precision of control, the speed of responding, and has better dynamic and steady-state characteristics.

For large cooperative wireless networks where direct links are limited in the neighbors of each node, we develop a diffusion automatic repeat request (ARQ) protocol based on selection cooperation. The number of potential relays for selection increases as the ARQ process continues and even the nodes that have no direct link with the source could cooperate with it. We analyze the diversity-multiplexing-delay (D-M-D) tradeoff performance of the protocol and demonstrate that the diffusion results in additional diversity benefit. The proposed protocol is suitable for delay-tolerant networks, such as wireless sensor networks and certain systems with relaxed delay constraints. It can also be used for highly reliable and loose latency services in the networks that support different quality of service (QoS) grade.

Through automatic repeat request (ARQ) mechanism, additional diversity benefit can be obtained for delay-tolerant wireless networks. In this paper, we combine the distributed selection cooperation protocols with ARQ mechanism to develop more powerful cooperative schemes and analyze their performance from the perspective of diversity-multiplexing-delay (D-M-D) tradeoff. We prove that directly extending the selection cooperation protocol with single round of feedback to multiple rounds does not lead to optimal D-M-D tradeoff. Then we develop an improved scheme achieving the upper bound of D-M-D tradeoff and also propose an efficient scheme that greatly reduces the number of feedbacks and almost achieves the optimal performance. The D-M-D tradeoff analysis and simulation results demonstrate the significant performance improvement of all the proposed schemes.

Temperature control in Variable Air Volume (VAV) air conditioning systems is a complicated process. Either the traditional Proportional Integral Derivative (PID) controller or the Fuzzy Logic Controller (FLC) is hard to get better control performance. The combination control system with fuzzy control and PID control can improve the control performance. Based on the conventional Fuzzy-PID (FPID) controller, this paper proposes a FPID controller with variable integral parameter for temperature control in air conditioning systems, which increases a fuzzy rule base to control the integral parameter of conventional Proportional Integral (PI) controller. Through online adjustment of integral coefficients, fuzzy control rule of conventional fuzzy controller is indirectly adjusted to improve its function. Simulation is carried to verify the presented theory. Obtained results show that the proposed controller not only is robust, but also gives excellent dynamic and steady-state characteristics compared with traditional controllers.

Considering the multi-variable, nonlinearity, time lag and time variation characters of Variable Air Volume (VAV) air conditioning systems, it is difficult for traditional Fuzzy Logic Controller (FLC) or Proportional Integral Derivative (PID) control to get good control performance. In order to overcome the shortcoming of conventional controller, the fuzzy PID composite control method is proposed. This paper builds the mathematical model of the controlled plant and proposes a self-tuning fuzzy PID (SFPID) controller in VAV air conditioning systems, which increases a fuzzy rule base to control the integral parameter of conventional Proportional Integral (PI) controller and applies the modifying factor in controller. Through online adjustment of the integral coefficient and the modifying factor, fuzzy control rule of conventional fuzzy controller is adjusted to improve its function. Simulation is carried to verify the presented theory. Obtained results show that the proposed controller not only is robust, but also gives excellent dynamic and steady-state characteristics compared with traditional controllers.

This paper analyzes the feasibility of designing nonlinear controllers to regulate the drum pressure and reheater pressure of a drum-boiler turbine system. In the case when all the parameters are precisely known, an exponential stabilizer is constructed using a Lyapunov-based design. In the case when some of the parameters are unknown, we first design a parameter estimator whose estimates are guaranteed to converge to the real values in a finite time. With the help of the estimates, we construct two controller dynamics which regulate the pressures of the drum and heaters to their respective desired values. The controllers are also evaluated through computer-based simulation, which illustrate that the adaptive controllers can achieve good performance while maintaining the controller values within the operational limit.

Based on the modified Chua's circuit, this paper made a deep study on a "master-slave" chaotic synchronization system. First, it built the mathematic model and made the simulation; then, based on the mathematic simulation, it constructed the circuits model of the system. Also it made the simulation; At last, this article made the real circuit board, through the observation of the oscillator's display, it could display the satisfied result. At the end of the paper, it made a summary and a prediction of the possibility of this system to be applied to the secure communication in the future.

We report the optimization of a $4\mathrm{Ar}∕\mathrm{Ag}(111)\text{\ensuremath{-}}(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.1\ifmmode^\circ\else\textdegree\fi{}$ monolayer using first-principles density functional theory. The adsorption energies resulting from our ground-state minimizations are similar for all three anchor sites. A commensurate adsorbate cell is comprised of four argon atoms which settle into three different configurations. We refer to these sites as top, threefold hcp hollow, and threefold fcc hollow sites. Each of these structures has one Ar atom located at one of the base sites of the Ag(111) surface. The remaining three Ar atoms are found near the bridge sites of the silver surface. In both of the hollow site structures, the bridge locations are measurably off of the exact geometric bridge positions while maintaining the $\sqrt{7}R19.1\ifmmode^\circ\else\textdegree\fi{}$ unit cell. Our results show that both the hcp and fcc hollow sites have slightly lower adsorption energies than the top site, by only 1.5 and $4.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, respectively (per cell of four Ar atoms). The vertical harmonic vibration frequencies were computed for the top, hcp, and fcc hollow sites. They measured 5.22, 5.01, and $5.21\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, respectively. We studied the possible cause for this high-order commensurate monolayer. We have concluded that neither steric repulsion nor hybridization of the electrons of the Ar atoms is the reason for the commensurability in our model. We propose that the causal mechanism is the Novaco-McTague [Phys. Rev. Lett. 38, 1286 (1977)] theory for orientational epitaxy.

We report a density functional calculation which optimizes a structure of supported Au on titania $\mathrm{Au}∕\mathrm{Ti}{\mathrm{O}}_{3}∕\mathrm{Mo}(112)$. Our computations include two different surface configurations $(1\ifmmode\times\else\texttimes\fi{}1)$ and $(1\ifmmode\times\else\texttimes\fi{}3)$. The results of our study detail the geometric and the electronic structure of these systems. The $(1\ifmmode\times\else\texttimes\fi{}1)$ structure has a ${\mathrm{Au}}^{\ensuremath{\delta}\ensuremath{-}}$ monolayer for its top surface. When the Au coverage is increased, the partial second layer of Au (1/3 ML) causes the titania layer to reconstruct, providing a surface made up of three forms of Au (${\mathrm{Au}}^{\ensuremath{\delta}\ensuremath{-}}$, ${\mathrm{Au}}^{0}$, and ${\mathrm{Au}}^{\ensuremath{\delta}+}$). We have added a vibrational component to our study which permits a possible experimental check on our proposed structures. We calculated the frequencies for a CO molecule when chemisorbed and optimized on these ${\mathrm{Au}}^{\ensuremath{\delta}\ensuremath{-}}$, ${\mathrm{Au}}^{0}$, and ${\mathrm{Au}}^{\ensuremath{\delta}+}∕{\mathrm{Au}}^{0}$ bridge sites. The ${\mathrm{Au}}^{\ensuremath{\delta}+}∕{\mathrm{Au}}^{0}$ bridge site has the largest charge transfer with the adsorbing CO and the CO vibrational frequency is redshifted by $321\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ $(40\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$. We observe that the $2{\ensuremath{\pi}}^{*}$ antibonding level of the CO is above the Fermi level and the electronic density of states shows that the electronic levels reach continuously down passed the Fermi level.

Methane adsorbed on graphite was studied using density-functional theory. The structure was fully optimized with strict energy and force convergence criteria. The methane converged to the preferred adsorption sites giving the vibrational frequencies, the energy of adsorption, charge distributions, and the electronic density of states. Under the two coverages studied ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ and $2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3}$) and a differing number of graphite layers (1--6), the methane molecules favored atop sites on the graphite surface with the hydrogen tripod down. We found the methane carbon $3.21\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ above the graphite carbon and the adsorption energy to be $118\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for the lower coverage. The independent harmonic oscillator vibrational frequency perpendicular to the surface for the $\mathrm{C}{\mathrm{H}}_{4}$ molecule was computed to be $87\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{-1}$. The graphite surface contracted 5.0% and 4.1% for the first and second layers, respectively, from the spacing relative to their bulk values. To benchmark our frequency calculations, we also used second-order Moller-Plesset theory and the local spin density approximation (LSDA) in GAUSSIAN 03 for the molecule. All of our LDA results are in good agreement with corresponding experiments, while under the generalized gradient approximation approximation we get only qualitatively results.

This paper presents a new approach for quality of service (QoS) support in optical burst switching (OBS) networks. In the approach, the data channels of an outgoing link at a core node are divided into multiple groups, with each group corresponding to a service class. The number of data channels in each group is mainly determined by data traffic. In general, a data burst (DB) can be sent on a data channel reserved by its burst head packet (BHP) only in its own group. Upon failing to reserve any bandwidth in its own group, the BHP tries to re-reserve even preempt bandwidth on a data channel in a lower-priority group. A lower-priority BHP can't reserve bandwidth on any data channel in a higherpriority group. In addition, this paper also investigates the reasonable relation between the preempting DB length and the preempted DB length.

The stream control transmission protocol (SCTP) will play an important role in the next generation of the Internet. Up to now, there are no literatures about differentiated services for SCTP associations. Regarding the throughput as SCTP criteria to support differentiated services, the differentiated services between SCTP associations can be obtained by introducing the weighted factor to a or b for AIMD(a,b)-based SCTP congestion control. Theoretic analysis and simulation results show that the proposed scheme can achieve well proportional differentiated services for SCTP associations with few additional overheads, and it is easy to regulate the fluctuation of data sending rate or SCTP response to networks.

To mitigate TCP global synchronization and improve network throughput, an improved TCP congestion control mechanism is proposed, namely P-TCP, which adopts the probability-based way to adjust congestion window independently when the network occurs congestion. Therefore, some P-TCP connections may decrease the congestion window greatly while other P-TCP connections may decrease the congestion window lightly. Simulation results show that TCP global synchronization can be effectively mitigated, which leads to efficient utilization of network resources as well as the effective mitigation for network congestion. Simulation results also give some valuable references for determining the related parameters in P-TCP.

UAV image transmission system based on satellite relay is very important in remote sensing applications. In this paper, a practical UAV image transmission system is brought forward. Some key specifications of this system are discussed and decided such as aircraft uplink transmission power, image transmission rate, satellite downlink transmission power and so on. The image compression sub-system with its circuit designing is analyzed thoroughly. The results of experiments show this image transmission system is feasible.

This paper put forward a new embedded zero tree algorithm RMEZW. It is based on the conception of multi-threshold and needs only one zigzag scan. It concentrates the high frequency coefficients energy of wavelet image through coefficients reorganization loop. Simulation results show that RMEZW algorithm has better reconstructed image quality and higher compression ratio meanwhile the time cost is reduced.

Turbo codes and low-density parity-check (LDPC) codes both have their advantages and can achieve the Shannon limited performance. The constituent recursive systematic convolutional (RSC) codes in turbo codes are more structural and this lends the encoding problem easier with a shift-register circuit. While the encoding of LDPC codes is performed via matrix multiplication, and this is more complex than it appears for capacity-approaching LDPC codes. On the other hand, the soft-input soft-output BCJR algorithm, or the sub-optimal version of it, used for turbo-decoding is rather complex while sum-product algorithm used for LDPC decoding lends itself to parallel implementation and is computationally simpler. Combining the turbo codes encoding and LDPC decoding, a new scheme based on factor graphs and sum-product algorithm is developed, it can reduce the decoding complexity of turbo codes greatly, and also has some guides in the designing of interleaver and the choosing of RSC constituent codes. Simulation shows the correctness of the scheme.

Turbo codes and low-density parity-check (LDPC) codes both have their advantages and can achieve the Shannon limited performance. Combining the turbo codes encoding and LDPC decoding, a new scheme based on factor graphs and sum-product algorithm is developed in this paper. It can reduce the decoding complexity of turbo codes greatly, and also has some guides in the designing of interleaver and the choosing of recursive systematic convolutional (RSC) constituent codes.

First principles density functional theory — based (GW) method — was used to simulate the electronic structure of the novel iron-based superconductor K 0.8 Fe 2 Se 2. The calculated band gap of K 0.8 Fe 2 Se 2 at the Γ point is 0.15 eV, which is significantly lower than the 0.61 eV of vacancy free crystal KFe 2 Se 2. The d-orbital of Fe atom is overlapped with the p-orbital of Se atom. Charge density analysis shows strong lattice distortion and vacancy related electric dipole and quadruple near the K vacancy. The reflectivity is anisotropic in three coordinate directions.

Rare earth element doping of chromium is much desired for various applications, but is technically difficult because of dopant segregation. Using a room temperature mechanical alloying method, dilute yttrium doping into nanosized chromium was achieved. Synchrotron-based high-pressure X-ray diffraction indicated that the Cr-Y alloy (Cr0.97Y0.03) was stable at up to 39 GPa, and the bulk modulus was 203 ± 2.6 GPa. The experimental results were consistent with first-principles density functional theory simulation. The diffraction line broadening profiles indicated the deformation anisotropy of the nanoalloy. This study suggests that Cr0.97Y0.03 alloy is promising for ultrahigh stress applications such as airplane engines and land-based turbines.

Pressed bulk samples of C60 doped with P, Co, Al, and Bi have been investigated for their thermoelectric properties. These samples show extremely low thermal conductivity, typically in the range of 0.1–0.3W∕Km at room temperature. The Seebeck coefficients of Co, Al, and Bi doped C60 solids are in the tens of μV∕K; however, for P doped C60 samples, a very large Seebeck coefficient in the order of 103μV∕K was observed. The value of the Seebeck coefficient seems to depend sensitively on the P concentration and changes sign upon annealing at 100°C. Ab initio density functional theory calculations show that the calculated electronic structures and the activation energies strongly depend on the dopants in C60 solids. The high Seebeck coefficient in studied P doped C60 is due to the system’s unique dopant and concentration.