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A solar heat pump heating system with an aluminum tubular collector is proposed in this paper. Mathematical models are established for solar energy absorption and air energy absorption of the aluminum tubular collector, as well as the heat-absorption coefficient of the working substance. The electronic expansion valve is controlled via the fuzzy PID method to adjust the working substance flow rate, as well as to control the evaporator overheating and set the indoor heating temperature. TRNSYS is used to analyze the effects of the wind speed, solar radiation amount, environment temperature, and working substance flow rate on the heat transfer performance of the aluminum tubular collector. The results indicate that the heat transfer performance of the aluminum tubular collector is significantly affected not only by the solar radiation but also the wind speed. For wind speed > 2 m/s, the absorbed power of the collector increases rapidly with an increase in the wind speed; when the working-medium flow rate reaches 4 m/s, the collector absorption power tends to be saturated. Subsequently, an experimental heating system with a heating area of 170 m2 is constructed. Experiments revealed that the maximum coefficient of performance (COP) of the heating system is 4.46, and the average COP value is 3.95, indicating good heating effect.

Field-induced metasurface is an applicable technique to prevent the high-intensity-radiated-field (HIRF). To improve the measurement accuracy, this article presents the design of a waveguide that forms high-intensity TEM waves within its closed transverse boundaries. The proposed quasi-TEM waveguide adopts two artificial magnetic conductors (AMCs) on the sidewalls of a standard rectangular waveguide. And the construction of the AMC is based on the theory of high impedance surface (HIS), which performs perfect magnetic conductor (PMC) properties at its resonant frequency. Then, a prototype is designed and fabricated to measure the electric field distributions and demonstrate the theoretical analysis and simulation. The measurement results indicate that the proposed waveguide could realize a 1 dB rippled electric-field distribution in more than 90% region, compared to that of 30% in the standard waveguide. Finally, high-intensity-field measurements of a field-induced sample, energy selective surface (ESS), are compared in the proposed waveguide and a standard waveguide. The comparison result has proved that the proposed waveguide could better evaluate the HIRF performance of the field-induced metasurface. [ABSTRACT FROM AUTHOR]

This article presents a detailed circuit analysis of energy selective surface (ESS), which adaptively protects electronic equipment from high-power microwaves (HPMs) thanks to its self-actuated response and nonlinear transmission feature. With the help of the equivalent circuit model (ECM), four types of ESS schemes are individuated and two of them are analyzed in detail to explore the influence of ECM parameters on the performance of ESS. Then, a high-performance ESS is designed through the proposed method. The proposed structure is realized by metal periodic structures loaded with diodes and lumped capacitors. A prototype is fabricated and measured. The results of simulations and experiments agree well, showing that the proposed ESS provides more than 30 dB shielding for high-power signals and an insertion loss lower than 1 dB for low-power signals. The excellent nonlinear transmission characteristic makes the proposed ESS practical for HPM protection. [ABSTRACT FROM AUTHOR]

This article presents an adaptive high-impedance surface (A-HIS) that can be applied in the prevention of high-intensity wave propagating in $X$ -band waveguide. Once the embedded diodes loaded on A-HIS are switched from OFF- to ON-states by the high-intensity incident wave, the boundary condition of the waveguide’s bottom wall would adaptively change from a perfect electric conductor (PEC) to a perfect magnetic conductor (PMC), and the propagation would be prevented. To demonstrate this nonlinear characteristic, eigenmodes of the periodic unit-cells in two states of diodes were calculated, and the dispersive performance was analyzed. The bandwidth was further derived through the bandgaps between its intrinsic modes. Finally, a prototype with $5\times 10$ elements was designed, fabricated, and measured in a WR90 waveguide. The results indicate that, on condition of incident wave with relatively low power intensity, signals can propagate through the A-HIS with a low insertion loss by no more than 1 dB; in the case of relatively high power intensity, this loaded A-HIS can attenuate the incident wave by more than 10 dB and absorb more than 80% power within its operation bandwidth. [ABSTRACT FROM AUTHOR]

In this article, a PIN-diode-based high-intensity radiation fields (HIRF) protection method was proposed and the sensitivity to the field intensity of incident wave has been verified by a printed dipole antenna loaded with three pairs of PIN diodes at S-band. To illustrate this method, an equivalent linear circuit was raised to analyze the basic operation principle of the proposed antenna. With a low-power incident wave, the PIN diodes are in off-state and the gain of the proposed antenna remains unaffected; while it drops considerably as the PIN diodes are triggered by HIRF. Furthermore, extensive parametric studies of the structure were analyzed by full-wave simulation to investigate the key factors affecting the performance of HIRF protection and antenna characteristics. Finally, a prototype was fabricated and experiments were implemented to verify the feasibility, which shows a high isolation of 19 dB. To the best of our knowledge, the proposed antenna is the first one to achieve HIRF protection through integrated antenna technology, which features with low cost and compact structure, providing a new perspective. [ABSTRACT FROM AUTHOR]

This correspondence paper aims at addressing the estimation of carrier frequency offset (CFO) for the uplink orthogonal frequency-division multiple access systems. Since the CFOs of the signals from different active users are sparsely distributed in the frequency domain, a sparse Bayesian learning (SBL) is tailored to determine the CFO in this paper, ending up with the SBL assisted CFO (SBL-CFO) estimator. In particular, the CFO estimation problem is first formulated as a sparse nonnegative least squares (S-NNLS) problem. Meanwhile, background noise and sampling errors are mitigated utilizing a selection matrix and a whitening filter, respectively. This enables us to exploit the SBL with nonnegative Laplace prior (SBL-NLP) to solve the S-NNLS problem. Furthermore, in order to make the convergence of the SBL-NLP algorithm faster and its estimation more accurate, the hyperprior inherent in the SBL-NLP algorithm is initialized by the traditional SBL with nonnegative Gaussian prior. Simulation results show that our proposed SBL-CFO estimator significantly outperforms the state-of-the-art estimators in terms of estimation accuracy, especially when the CFOs and the number of active users are large. [ABSTRACT FROM AUTHOR]

This paper presents an energy selective surface (ESS) for high-power microwave (HPM) protection in waveguide, which has a nonlinear transmission response and can adaptively react to the power intensity of an incident wave. A prototype of four unit cells loaded with p-i-n diodes is designed and fabricated in $L$ -band. The nonlinear and adaptive transmission characteristics are demonstrated by the waveguide measurements both in the time domain and frequency domain. On condition of signal power below 35 dBm, the wave in the interested band can propagate through waveguide with a low insertion loss down to 1 dB; while on condition of signal power above 47 dBm, high power would render the ESS into a high-impedance surface, and the incident wave would be reflected and dissipated with a transmission coefficient of less than −10 dB, thereby providing the HPM protection ability. Furthermore, the subsequent simulations illustrate that the inserted ESS will not worsen the radiation patterns of the following horn antenna and the induced high-order modes in waveguide keep negligible values below −65 dB. [ABSTRACT FROM AUTHOR]

A novel ultrawideband energy-selective surface (ESS) in S-band is presented for adaptive high-power microwave (HPM) protection. Periodic metallic textures are designed and loaded with pin diodes to obtain the nonlinear response, which is dependent on the power intensity of the incident wave. After the equivalent circuit analysis and full-wave simulation, a prototype is fabricated, and experiments are conducted under different power levels, which have a good coincident. The insertion loss is less than 1 dB for low-power signals in an operating band, whereas the shielding effectiveness is more than 20 dB when exposed to HPM. Both the measurement results of frequency and time domains have verified the nonlinear transmission characteristics, which demonstrated the HPM protection abilities of the proposed ESS. [ABSTRACT FROM AUTHOR]

A broadband switchable absorber from S-band to C-band is presented in this letter. The proposed structure is composed of three layers separated by air gap. From top to bottom, the layers of double split rings with lumped resistors, cut metal wires connected with p-i-n diodes and ground, are integrated to realize broadband absorption, switching feasibility, and independent polarization. For demonstration, the proposed absorber is investigated using a circuit analogy method, and the distributions of the surface current are also discussed in detail to analyze the mechanism of broadband absorption. Numerical calculations are carried out to investigate the specific performance and experiments are conducted to validate the design furthermore. [ABSTRACT FROM AUTHOR]

This paper proposes a novel blind carrier frequency offset (CFO) estimator, namely the sparse recovery assisted CFO (SR-CFO) estimator, for the uplink orthogonal frequency-division multiple access (OFDMA) systems. By exploiting the sparsity embedded in the OFDMA data, the CFO estimation is formulated as an optimization problem of sparse recovery with high-resolution. Meanwhile, in order to enhance the estimation accuracy of CFOs, background noise and sampling errors are mitigated by exploiting the structure of the noise covariances matrix in the transformed observation data, and the asymptotic distribution of the sampling errors. Furthermore, we propose an approach for deriving the regularization parameter used by the SR-CFO estimator, so as to control the tradeoff between the data fitting error and the sparsity of solution. The performance of the proposed SR-CFO estimator along with other four existing estimators is investigated and compared. Numerical results show that the proposed SR-CFO estimator is superior to the state-of-the-art estimators in terms of the estimation reliability. [ABSTRACT FROM PUBLISHER]

This paper proposed a novel algorithm to suppress the deception jamming against the single channel synthetic aperture radar (SAR), which provides an effective and economic way to improve the anti-deception-jamming ability of the conventional SAR system. The dynamic apertures are employed to provide multiple observations about the time-frequency distributions of the deception jamming and real echoes. The amplitude loss in different dynamic apertures is modeled to formulate the optimization problem for deception jamming suppressing. Super-resolution scheme is also involved to tighten the feasible region of the problem. The effectiveness of the proposed algorithm is verified by simulation. [ABSTRACT FROM PUBLISHER]

This letter considers a wireless powered communication network (WPCN) where users first harvest energy in downlink and then utilize the energy to transmit information signal in uplink simultaneously. Our goal is to maximize the energy efficiency (EE) of the network via joint time allocation and power control. However, directly solving this issue is of significant challenge due to its nonconvex property. By exploiting the nonlinear fractional programming, an iterative resource allocation method based on the Dinkelbach structure is proposed to solve the considered nonconvex optimization problem. In each iteration, we optimize power with fixed time allocation and then optimize time with a given power allocation. Simulation results are presented to show that the system EE is greatly improved by the proposed approach. [ABSTRACT FROM PUBLISHER]

This paper proposes a new approach to improve the error performance of the synergy netting deception jamming against synthetic aperture radar (SAR). The existing methods cannot tackle the issue of error amplification induced by the linear equations constructed for jamming coefficients calculation. To handle this problem, we develop an optimal layout for radar receivers to minimize the effect of the error by minimizing the condition number, which measures the sensitivity of the synergy deception jamming system to the perturbation. It is revealed that the devised approach relies little on the SAR kinematic parameters. To further improve the error performance of the jamming system, collaborative receivers, which provide more observations of time difference of arrivals, are introduced to perform a linear least-squares estimation. The error reduction level due to the cooperative receivers is analyzed. Simulation results are provided to illustrate the superiority of the proposed method. [ABSTRACT FROM PUBLISHER]

To efficiently trade off system sum rate and link fairness, power allocation in wireless spectrum-sharing networks often concentrates upon proportional fairness. The corresponding problem has been proved to be convex for a single tone but NP-hard for more than two tones. However, in the two-tone case, the complexity of the problem for achieving proportional fairness has not been addressed yet. In this paper, we prove that the issue of proportional-fairness optimization for the two-tone situation is NP-hard by reducing the problem of finding the maximum independent set in an undirected graph to it. Moreover, a computationally efficient algorithm is proposed to provide an efficient suboptimal solution. Simulation results are presented to illustrate the effectiveness of our proposal. [ABSTRACT FROM PUBLISHER]

The optimization of energy efficiency (EE) in spectrum-sharing networks has received increasing attention. While much effort has been devoted to the network-side overall EE, the fairness of EE among users has not been adequately concerned. Due to the complicated interference between different users, the global optimality of power allocation problems for achieving proportional, harmonic, and max–min fair EE with guarantees on users’ quality of service has not been fully addressed. By introducing auxiliary variables, changing variables, and transforming the objective and constraint functions, we convert the proportional, harmonic as well as max–min fairness-aware problems to convex ones. As a result, mature algorithms, such as interior-point methods, can be applied to achieve the global optimality with considerable efficiency. [ABSTRACT FROM PUBLISHER]