Vol. 87

Compressed sensing (CS) relies on the sparse priorin posed on the signal to solve the ill-posed recovery problem in an under-determined linear system (ULS). Motivated by the theory, this paper proposes a new algorithm called regularized re-weighted inverse trigonometric smoothed function approximating L₀-norm minimization (RRITSL0) algorithm, where the inverse trigonometric (IT) function, iteratively re-weighted scheme and regularization mechanism constitute the core of the proposed RRITSL0 algorithm. Compared with other state-of-the-art functions, our proposed IT function cluster can better approximate the L₀-norm, thus improving the reconstruction accuracy. And the new re-weighted scheme we adopted can promote sparsity and speed up convergence. Moreover, the regularization mechanism makes the RRITSL0 algorithm more robust against noise. The performance of the proposed algorithm is verified via numerical experiments with additive noise. Furthermore, the experiments prove the superiority of the RRITSL0 algorithm in magnetic resonance (MR) image recovery.

In indoor scenario, radar echoes are interfered by clutter from walls, ceilings, floors, and other indoor objects. Therefore, clutter suppressing is one of the key problems for indoor radar. This paper focuses on the problem of clutter suppressing for a secondary radar system which can be used in indoor localization. A clutter suppressing method based on orthogonal polarization character is presented. The orthogonal polarization character here is achieved by a designed transceiver, which can transpond electromagnetic waves in vertical polarization if and only if the received signal is in horizontal polarization. Thus the newly introduced polarization character can be used to discriminate target from clutter. Clutter is suppressed after calculating scattering similarity parameters via Pauli decomposition. Simulations and an experiment are conducted to demonstrate the proposed method. Compared with previous methods, the proposed method can distinguish stationary target with both static and varying clutters. Therefore, it is more practical for applications.

The resonant frequency of an antenna plays a crucial role in the design of a reconfigurable antenna. In this article, we have developed a dual-band reconfigurable terahertz patch antenna by using graphene. The simulation results demonstrate that the designed structure can provide excellent properties in terms of dual wide-band performance, frequency-reconguration by applying different voltages on the graphene. These initial results are particularly promising for various applications in the THz regime. Furthermore, we have investigated the effect of the additional parameter such as temperature and relaxation time. The modeling is done by using a new equation of the Wave Concept Iterative Process (WCIP) method, and the validation is achieved by comparison with CST simulator. Here, we propose to develop a new efficient and flexible numerical tool for graphene modeling.

A coplanar waveguide (CPW) fed folded dipole with a 20% impedance bandwidth and 4-6 dBi endfire gain with stable patterns is proposed. Since the proposed element is electrically large (2.1λ x 2λ) conformal topology of this endfire radiator is designed and characterized. The input impedance is not altered significantly compared to the planar element. The radiation in the H plane indicates an increase in specific absorption rate when integrated with a typical mobile terminal. In order to mitigate this effect, a compact (0.8λ x 0.8λ) wideband reflector with periodic sinusoidal slots is proposed and mounted with the conformal element at an offset of 0.2λ from the radiator. The proposed antenna has an operating bandwidth from 24 to 30 GHz (20%) with an endfire gain of 6-7 dBi across the band. The front to back ratio is more than 12 dB across the band. Pattern diversity of the conformal antenna is also investigated. Simulated and measurement results are presented in detail.

This paper introduces an Ultra-Wideband (UWB) circle antenna array with beam forming techniques that combine Self-Adaptive Dynamic Differential Evolution (SADDE) which is capable to minimize the Bit Error Rate (BER) for Multi-User Multiple Input Multiple Output (MU-MIMO) in indoor communication system. By using the ray tracing techniques to compute any given indoor wireless environment, the impulse response of the system can be calculated, and the BER can be computed accordingly. Next, we analyze the BER performance of the UWB MU-MIMO system that applies beam forming for spatial division multiple accesses. Numerical results show that the SADDE can control the antenna feed length to reduce the BER and form the radiation beam pattern towards the direction of the desired signals while forming nulls to co-channel interferers for MU-MIMO system.

By taking into account the facts that thick dielectrics are required for low frequency absorbers, that thick dielectrics are not always flexible, and that targets are not always planar, an efficient tool for the systematic design of flexible broadband radar absorbers using the least-square method is presented in this paper. Two approaches for designing the physical model of the absorber are presented. The first one consists of resistive square loops deposited on top of a dielectric, and the second one consists of metallic square loops associated with lumped resistors. More than 90% of absorption rate is obtained in the required bandwidth for both transverse electric and transverse magnetic polarizations with the two approaches and achieving a performance of operational bandwidth to thickness ratio of 7.69. Finally, the required dimensions of flexible absorbers in some low frequency bands are given in order to show the versatility of the approach.

Cylindrical/ring-shaped permanent magnets with diametrical magnetization can be found in many applications, ranging from electrical motors to position sensory systems. In order to correctly calculate the magnetic field generated by a permanent magnet of this kind with low computational cost, several studies have been reported in literature providing analytical expressions. However, these analytical expressions are either limited for an infinite cylinder or for computing the magnetic field only on the central axis of a finite cylinder. The others are derived to calculate the magnetic field at any point in three-dimensional (3D) space but only with low accuracy. This paper presents an exact analytical model of the magnetic field, generated by a diametrically magnetized cylindrical/ring-shaped permanent magnet with a limited length, which can be used to calculate the magnetic field of any point in 3D space fast and with very high accuracy. The expressions were analytically derived, based on geometrical analysis without calculating the magnetic scalar potential. Also, there is no approximation in the derivation steps that yields the exact analytical model. Three components of the magnetic field are analytically represented using complete and incomplete elliptical integrals, which are robust and have low computational cost. The accuracy of the developed analytical model was validated using Finite Element Analysis and compared against existing models.

Non-contact vibration detection using microwave radar is becoming a popular research area. However, vibration sensing using Doppler radar based measurements suffers from the problem of `Null point'. In order to mitigate this, traditional designs incorporate phase measurements using Quadrature (I/Q) radar. Such Quadrature radars are not cost effective for large scale indoor deployment scenarios. In this paper, we propose a new configuration of `Indented Radar'; a system of two singlechannel radars offset in space by a path length, which is equivalent to 90 degree phase shift. However, such a system of two independent channels is prone to different imbalances such as amplitude, phase and DC. This work closely examines the imbalance effect on the two-radar system and reports a novel approach that can be used to tackle such imbalance in a two-radar configuration. Our approach yields superior results over other commonly used I/Q algorithms, while measuring vibrational frequencies. Thus, our work can find immense application in both vital sign detection and structural vibration detection use-cases where affordable solution is sought.

Partially dielectric-filled empty substrate integrated waveguide (PFESIW) is introduced for millimeter-wave application alongside partially dielectric-filled ESIW filter by using inverter and resonators technique. The new design presents good transition implementation in order to introduce a waveguide compatible with planar integrated circuits. The main goal of introducing the new transmission line PFESIW is to control characteristic impedance without changing the cutoff frequency. The presented transmission line is analyzed by calculating attenuation constant α, phase constant β and characteristic impedance Z. The PFESIW is used to build a resonator with high-quality factor. The filter based on the combination of of ESIW and PFESIW is proposed. Furthermore, the designed filter showed very good performance in terms of bandwidth and cost. Transmission line and filter prototypes are manufactured using standard printed circuit board fabrication. Partially dielectric-filled ESIW measurement displays very good results in terms of phase constant β, attenuation constant α, return loss (S₁₁) and insertion loss (S₁₂). Measured return loss (S₁₁) and insertion loss (S₁₂) for waveguide and filter agree very well with simulation.

Through-the-wall imaging (TWI) of human vital signs by bioradar is a hot research topic in recent years. Unknown wall parameters (mainly thickness and dielectric constant) are huge challenges for TWI. Ambiguities in wall parameters will degrade the image focusing quality, lower signal-to-noise-clutter ratio (SNCR) of vital signs, cause vital signs to be imaged away from their true positions and blur the close vital signs from multiple humans caused by the imaging resolution declination. A through-the-wall propagation model of vital signs for multiple-input and multiple-output (MIMO) bioradar is first built to analyze the influence of wall on imaging. In order to obtain focused image of vital signs quickly, an imaging model and a novel autofocusing imaging method of vital signs are proposed in this paper. Since vital signs of human are weak and sensitive to interferences, the SNCR-enhanced imagery of vital signs after change detection (CD) is applied to evaluate the focusing quality of image. Reflections of wall in the stationary targets imaging result are line structure approximately, so Hough transform is used to extract the positions of the front edge and rear edge of wall automatically. Propagation time in the wall of electromagnetic waves is estimated and used to build the constraint relationship of wall parameters. The number of unknown parameters is reduced to only one and the efficiency of autofocusing imaging improves. Several cases, including the case of single human, multiple human objects close to each other and the case of non-human objects, are simulated. The magnetic resonance imaging (MRI) image of human chest is put into simulation scene. And then the simulation data of human vital signs are calculated by the finite-difference time-domain (FDTD) method. The results show that the proposed method can effectively estimate the wall parameters and improve the focusing performance of human vital signs. And also the kurtosis of image can be used as a feature to efficiently decide the human vital signs are existed or not. Thus the SNCR of vital signs and resolution of imaging are improved, which are beneficial for detection of vital signs. The position errors of human vital signs are also corrected.

In this paper, a circularly polarized (CP), high gain and wide bandwidth metal plated microstrip antenna (MSA) using partially reflecting surface (PRS) and artificial magnetic conductor (AMC) layers is proposed. The bandwidth of MSA is increased primarily, using AMC layers and gain is increased by placing the antenna in a Fabry-Perot cavity (FPC) resonator. The two slotted AMCs are designed to resonate at two frequencies which electromagnetically couple to provide wide bandwidth. The FPC antenna with PRS and AMC layers provides higher gain, more impedance bandwidth, less gain variation and more miniaturization than the antenna without AMC layers. The proposed antenna provides S₁₁ < -10 dB, axial ratio (AR) < 3dB and 17.4 dBi peak gain with gain variation < 3 dB over 5.725 GHz to 6.4 GHz frequency band. Broadside radiation patterns have side lobe level (SLL) < -20 dB, cross polarization (CPL) < -16 dB and front to back (F/B) lobe ratio > 20 dB. The overall antenna dimensions are 2.83λ₀ × 3.23λ₀ × 0.49λ₀, where, λ₀ is the free space wavelength corresponding to the central frequency of 5.725-6.4 GHz. The proposed structure is fabricated, and the measured results agree with simulation ones.

In this paper, a novel and simple solution for generating vortex electromagnetic wave and reducing divergence simultaneously in a wideband is presented. Based on phase gradient metasurface, we design a metasurface that can convert an ordinary electromagnetic wave into a vortex one and focus the vortex wave in X-band. Double layer rectangular metal patch units of different sizes are arranged in a certain order to compose the metasurface. The phase introduced by the metasurface is superimposed by the vortex phase and focusing phase. Compared to a general vortex wave metasurface, the simulation results show that the divergence of the reflected vortex wave generated by our designed metasurface is dramatically reduced in the frequency range from 8 GHz to 12 GHz. It is indicated that the designed metasurface has a highly efficient focusing effect, and it is also in a good agreement with the theoretical analysis. The proposed reflective metasurface paves an effective way to reduce the divergence of vortex electromagnetic wave for OAM-based system in microwave and radio frequency.

In this paper a compact planar dual band-notched ultra-wideband (UWB) multiple input multiple output (MIMO) antenna is presented for universal synchronous bus (USB) dongle application with pattern diversity characteristic. The MIMO configuration has orthogonally placed elements with overall size of 16×37.6 mm², and the common ground plane of the MIMO antenna is further extended by 20 mm for its practical use. The measured and simulated reflection coefficients of the antenna show good impedance bandwidth matching over the range from 3 GHz to 12 GHz excluding the dual notched bands. Both elements show good band-reject property at the notched bands. The simulated results are verified through measurements and calculations. Moreover, absence of decoupling network makes circuit less complex and very compact. Radiation pattern of the MIMO antenna is almost omnidirectional. Furthermore, diversity performance of MIMO antenna is validated with its envelope correlation coefficient (ECC) and pattern diversity characteristic. These characteristics demonstrate its candidacy as a compact dual band-notched UWB MIMO antenna for USB dongle application.

Distributed antenna arrays are arbitrarily large groups of neighboring nodes which are controlled to form virtual antenna arrays for both transmission and reception. Distributed beamforming (DBF) is widely used in wireless sensor networks (WSNs) and distributed massive Multi-Input Multi-Output (MIMO) systems. The research in DBF has been divided into four major research trends: radiation pattern analysis, optimization of power and lifetime, nodes synchronization, and array design. In this paper, a new algorithm is introduced to synthesize the radiation pattern of an arbitrarily distributed array using reduced number of distributed nodes. In this context, the reduction in the number of nodes results in minimizing the synchronization complexity between the synthesized array nodes and in minimizing the number of RF front ends. Thus, the overall system cost is reduced. In this algorithm, the three antenna array parameters (number of nodes, nodes locations, and nodes excitations) are properly adjusted to construct a close copy of the original array pattern. Different nodes selection ways are utilized to select the nodes required to synthesize the array for a desired radiation pattern. Also, uniform feeding and non-uniform feeding scenarios are introduced. In simulations, the proposed algorithm is applied to the synthesis of pencil-beam patterns. The simulation results reveal that the synthesized radiation patterns highly agree with the ordinary distributed array pattern in the case of non-uniform feeding. Also, the proposed algorithm can be applied to the synthesis of shaped-beam patterns via controlling the three aforementioned antenna array parameters and taking the shaped-beam pattern as the desired pattern in the algorithm.

A novel frequency reconfigurable antenna is proposed for WiMAX and WLAN applications. It has a simple structure and compact size of 0.44λ_g×0.37λ_g. The proposed approach is based on combining of double planar U-shaped antenna. Furthermore, to achieve a reconfigurable function, a PIN diode switch is introduced across the slot between the two U-shaped patches. By controlling the PIN diode, the antenna resonates at two modes of single and dual band (WiMAX 3.2/3.5 GHz, and WLAN 5.2/5.8 GHz). The obtained gain ranges from 2.3 to 3.9 dBi within the whole operating bands. The simple configuration and low profile nature of the proposed antenna is suitable for Wireless communication systems.

This paper presents the design investigation and experimental testing of a flux-focusing magnetic gearbox with a fully laminated rotor structure. The unique feature of this flux-focusing magnetic gearbox design is that the three rotors are each made of a single lamination stack, and the central modulation structure is retained in place without the use of a resin filler such as epoxy. Ferromagnetic bridges are used to connect individual pole pieces together. It is shown that the use of the ferromagnetic bridges reduces the calculated torque density from 156 Nm/L to 139 Nm/L (a reduction of 11%). The experimentally measured torque density is, however, only 97 Nm/L. The reason for this discrepancy is associated with the demagnetization of the magnets.

A new compact topology of rectenna, which combines a miniaturized wideband printed antenna and a rectifier integrated on the radiating surface, is reported in this paper. The rectenna is designed for ISM 900 MHz band and applied to wireless power transmission and energy harvesting to supply Ultra-Wideband tags for 3D indoor localization. The rectenna allows activating a DC-DC boost converter that supplies power to the tags. It exhibits a minimum conversion efficiency of 25% for very low microwave power densities (>0.18 μW/cm²) on the non-optimal loading impedance (of about 10 kΩ) of a commercial DC-to-DC boost converter and power management unit. The harvested DC voltage obtained from this novel rectenna exceeds 330 mV for microwave power density of 0.22 μW/cm². This measured DC voltage is in the range of the cold turn-on/start-up voltage of nowadays commercial off-the-shelf DC-to-DC boost converters and power management units. The proposed rectenna is also very compact, as its surface (10.5x6 cm²) is of 0.05λ² at the operating frequency (860 MHz).

Hybrid beamforming systems are a cost and energy efficient architectural approach for large-scale antenna arrays operating at millimetre-wave frequencies. The separation of the beamforming process into an analogue beamforming network and a digital precoding part enables the reduction of digital channels, while preserving a precise beam steering capability. Especially subarray-based hybrid beamforming systems distinguish them due to a low complex analogue beamforming network. However, to determine the ideal analogue and digital precoding matrices the channel state information has to be estimated. This estimation process is hampered by the electrical interconnection of different antenna elements within the analogue beamforming network. Hence, a separation of the antenna elements of the subarrays in the digital domain is not possible. Furthermore, actual channel estimation methods for hybrid beamforming systems are based on beam training techniques, which suffer from long estimation times. To overcome these problems we developed a two-stage channel estimation method for subarraybased hybrid beamforming systems using sparse array estimations. In the first stage, only one antenna element of each subarray at the transmitter is active during the channel estimation, resulting in a sparse array estimation. To distinguish the transmitters at the receiver side the transmitters are separated in the frequency domain using different orthogonal frequency division multiplexing subcarriers. For recovering the full-dimensional channel matrix we present two algorithms. The first algorithm is based on a two-dimensional interpolation of the channel matrix, while the second algorithm uses multiple subsequent channel measurements. The presented estimation method enables thereby a direct determination of the channel matrix with only one or a few measurements.

In this paper, a novel ultrathin five-band polarization insensitive Metamaterial Absorber (MA) is proposed. The proposed structure consists of a periodic array of six arrows with two concentric hexagonal rings, having novel hexagonal 2D-bravais lattices on a grounded FR-4 dielectric substrate (ε_r = 4.25, loss-tangent tanδ = 0.02). The simulated result shows five discrete absorption peaks. The near unity absorption occurs at 2.7, 6.9, 7.3, 13.6 and 16.9 GHz with peak absorptivity of 88.99, 94.45, 87.58, 93.06 and 90.42% respectively. The proposed absorber is ultrathin with thickness of 0.056λ₀ corresponding to the highest frequency of absorption. In order to analyze the absorption mechanism of the structure electromagnetic parameters such as effective permittivity (ε_eff) and effective permeability μ_eff) are retrieved and plotted. Wave absorption phenomena are explained by comparative tabulation of real and imaginary parts of electromagnetic parameters. Absorption is further explained by the characteristics impedance and surface current distribution. The structure, being a six-fold symmetric, has been found to be polarization-insensitive under normal incidence. For the oblique incidence of waves, it also achieves high values of absorption for both TE and TM polarizations. The proposed absorber is fabricated, and scattering parameters are measured. Simulated and measured results are in close agreement. Performance of the proposed MA is further investigated by calculating Fractional Bandwidth (FBW). This absorber can find its applications in phase imaging, photo-detector, hyper-spectral imaging, micro-bolometer, spectroscopic detection, surveillance radar and other defence applications.

The results of experimental research and development of the diffraction radiation oscillator with a periodic structure in form of a reflective double comb and with frequency tuning on mutual coupled modes in its open resonant system were presented. As an operating mode we chose the mutual coupled modes TEM₀₀₂ → TEM₁₀₁, which arise in the open resonant system with the shift between mirrors symmetry planes. To analyse its features, a rigorous electrodynamical 2-D model of the open resonant system was used, and the optimal shift width was established. As a result, the operation on the mutual coupled modes allowed to extend the frequency tuning range without failures in output power and to exclude the influence of higher-order modes (TEM_20q, TEM_30q etc.) on the output characteristics of the oscillator. The research has been carried out in Ka band.