A broadband compact smooth horn with flat-top radiation pattern is presented in this Letter. Using a large flare angle, the whole length of the horn is reduced to 1.875λ_c (λ_c is the wavelength of 95 GHz) and the TE₂₁ mode and higher order electromagnetic modes can be excited to exhibit flat-top radiation performance. The measured radiation pattern of the presented horn can achieve <1 dB amplitude ripple in a beam width range of ±10° at 85–105 GHz (21%) where the reflection coefficient is below −22 dB. It indicates that the broadband compact smooth horn with flat-top radiation pattern can be applied in wireless communication and reflector systems in the millimetre-wave band.

Mathematical analysis of the memristive properties of a full-wave diode rectifier is presented in this Letter. Proofs of the rectifier's three memristive fingerprints are provided in the context of a pinched hysteretic current–voltage characteristic of the rectifier.

A new design method for a second-order bandstop filter is presented. The proposed filter is based on the source follower configuration with a partial positive feedback. The positive feedback is used to increase the quality factor of the filter, and the feed-forward paths are used to create complex zeros. The proposed bandstop filter has electronically tunable centre frequency, bandwidth and attenuation with low power consumption. The proposed design has been simulated in TSMC 180 nm CMOS technology. The filter achieves input-referred noise density. For a 2 GHz input signal, the filter achieves a signal-to-noise ratio of 60 dB for 238 input signal amplitude. The simulated results show that the centre frequency tuning range is between 1.6 and 3 GHz and the bandwidth is tunable from 0.6 to 1 GHz. The filter consumes about 2.5 mW for a supply voltage of 1.8 V at a centre frequency of 2.3 GHz.

On-orbit processors generally require reinforcement due to the harsh space radiation environment. Although the traditional full triple modular redundancy (TMR) method can effectively reinforce circuits, it requires excessive physical footprints and energy consumptions. To address this problem, this Letter proposes a general partial TMR method that can effectively reinforce a circuit. The method is based on the PageRank algorithm for sorting the importance of the triggers in a circuit, and the mean time between failure based on the dual logic cone model is used as the criterion to determine triggers that require redundancy under the partial TMR method. The arithmetic module of an optical image processing platform is tested to verify the effectiveness of the partial TMR method proposed in this Letter.

A promising approach integrating non-real-time (NRT) robot operating system (ROS) packages and RT tasks is proposed to enhance the development of RT robot control applications. Since ROS alone does not provide RT properties essential for achieving precise control period in manipulating multiple devices and complicated software, Xenomai, an RT extension of Linux is adapted. However, using NRT ROS packages inside RT tasks triggers mode switching that causes inability to satisfy critical temporal constraints. To address this issue, a message pipe mechanism termed cross-domain datagram protocol (XDDP) is applied. In comparison to traditional inter-task mechanisms, XDDP provides a communication interface between RT and NRT tasks. This greatly improves robot application development utilising ROS tools and packages with RT tasks on the Xenomai domain ensuring priority-based scheduling and deterministic response in a multitasking environment. Feasibility of the proposed method was validated for practical use by realisation on the open embedded controller for a telepresence robot. Experiments were conducted to actuate the mobile base of the robot using ROS navigation packages. The results indicate that the robot accomplishes its objectives while satisfying RT constraints.

The bilateral filter perhaps is the most fundamental smoothing tool in image processing. However, low-computational efficiency limits its applications. To the best of authors' knowledge, all existing fast bilateral filtering algorithms are handcrafted by talented authors. However, it is a painstaking and effortless work to design an acceleration algorithm. In this Letter, the authors propose a radial basis function network to automatically produce the fast bilateral filtering algorithm. This is done by establishing the one–one connection between the radial basis function network and the fast bilateral filtering algorithm. Experiments demonstrate that their method is able to produce state-of-the-art results.

Reusable glass containers have become extremely popular in recent years due to their cost effectiveness. Quality control such as inspecting and identifying container defects is an essential part of the reusable containers production systems. Many aspects of modern society already benefit from developments in machine learning (ML) technology. However, to the authors’ knowledge, the ML technology approaches have not been extensively applied in the practical inspection instrumentations for glass containers. In this Letter, a comprehensive inspection system for reusable containers based on a deep learning framework is proposed. The experimental results demonstrated that the developed system is capable of inspecting defects of glass containers with superior accuracy and speed. After the success of other practical applications with deep learning approaches, they wish that this Letter could inspire more and more research results in deep learning methods to be widely applied to comprehensive inspection tasks.

This Letter demonstrates design, simulation, fabrication and performance of a metamaterial (MTM) absorber having wide incident angle insensitivity for conformal applications in X-band region. The important feature of the proposed structure is achieving more than 90% absorptivity with very low thickness. In this design, an FR4 material having thickness 0.1 mm is used as a substrate. Hence, the overall thickness of the proposed MTM absorber is only 1/81 of the wavelength at the resonance frequency of 9.2 GHz. Also, the presented absorber is offering more than 80% absorptivity up to 50° incident angle which is very much suitable for conformal applications where curved surfaces are involved. Designed MTM absorber has been fabricated and simulation results are experimentally verified.

In this Letter, novel asymmetric coupled lines are proposed to suppress common-mode (CM) noise in bent differential lines. Based on the concept of the relative permittivity of a single microstrip line, the width of the outer line is made smaller than that of the inner line in bent differential lines to reduce the time skew and the other design parameters are adjusted to account for the differential impedance. To verify the improvement afforded by the proposed method, the simulated and measured results of the proposed structure were compared with two different designs; the conventional bent differential lines and the compensated differential lines using a serpentine inner line. The results of the proposed method show better signal integrity than the other methods.

Magnetic resonance (MR) signals captured by RF array coils should be delivered to analogue-to-digital converter (ADC) channels through an independent receiver (Rx) circuitry for precise MRI process. Since different types of RF coils should be utilised by a fixed port configuration on a patient table with a limited number of ADC channels, a highly flexible Rx front-end design is necessary. In this Letter, a compact distributed switch matrix structure for multi-channel RF coil Rx is proposed for supporting different types of array coils with small circuit size and sufficient reconfigurability. The proposed structure is configured by a multiple number of unit matrix cells consisting of a single double pole double throw and two single pole double throw RF switches to control the pathways of received MR signals. The proposed structure was implemented and verified at 9.4-Tesla (9.4 T) MR frequency (400 MHz) by comparing different RF path losses.

A novel dual-band bandpass filter based on substrate integrated waveguide (SIW) which is realised with mode distribution of the special cross-shaped structure is presented in this Letter. In the cross-shaped SIW, there are six symmetrically stimulated resonant modes in the first eight higher order resonant modes that are perturbed to achieve dual-band responses, and each passband contains triple-mode. By introducing 45° rotated cross via-holes at the centre position of the structure, the two lower symmetrical resonant modes are closer to the unaffected asymmetrical higher resonant mode in both passbands. Compared with existing multimode multi-band filters which are achieved by coupling or cascading mechanism, the presented filter design based on single cross-shaped SIW is simple and easy for implementation. A prototype dual-band filter operating at 8.23 and 11.48 GHz with 3 dB fractional bandwidths of 7.1 and 12.1% is designed and fabricated. Measured and simulated results are presented to validate the predicted performances of the proposed filter.

The authors proposed a waveguide coupler that extracts the radiation from a resonant-tunnelling diode (RTD) oscillator at 420 GHz as the fundamental mode of a rectangular waveguide compatible to the WR-2.2 standard. In the proposed structure, an RTD oscillator chip is embedded inside a hyper hemispherical silicon lens to reshape the radiation from the tiny chip into a collimated beam, which is subsequently converted into the waveguide mode using a linear horn. The authors estimate the transmission efficiency of the coupler to be −3 dB. Integrated in one package, this approach enables handy use of RTD oscillators as a platform for various terahertz applications.

The authors present an experimental demonstration of a fluidic-based plasmonic device for spectral analysis. The demonstrated device is based on the modification of surface plasmon resonance in periodically coupled metallic nanostructures using external refractive index perturbation for spectral analysis. The refractive index perturbation alters the resonance condition of the internal surface plasmon waves and introduces a spectral shift in the transmission band for spectral sampling. In this demonstration, precise control of the centre wavelength of the surface plasmon resonance was achieved by changing the local environment of the nanostructures via microfluidic channels. The fluidic-based approach has a great impact on existing resonance-based spectral analysis devices by offering an efficient way to acquire a very large number of spectral sampling.

Indium arsenide (InAs)-based quantum cascade lasers (QCLs) operating close to 25 µm are reported. Thick n-doped InAs cladding layers were used for optical confinement. Fabry–Perot lasers exhibited a threshold current density as low as 1.1 kA/cm² in pulsed mode at 80 K. The lasers operated in this regime up to 240 K, where the threshold current density increased up to 1.7 kA/cm². These are the lowest threshold current densities for QCLs operating above 20 µm.

A charge pump is presented by integrating two identical voltage doublers and a modified rat-race coupler to enable its dual-mode operations: radio-frequency (RF)-to-direct current (dc) power conversion and second-harmonic readout. Due to the property of the modified rat-race coupler, it is able to differentially divide incident RF signals for successive voltage doublers’ high-efficiency rectification, and couple second harmonics for readout in two highly isolated channels. Output dc voltages are combined in parallel on a single load resistance for more practicality. Experimental validations are demonstrated by both simulation and measurement with a maximum RF-to-dc power conversion efficiency of 71.3% and −14.2 dBm second-harmonic generation with an incident signal power of 19 dBm. This dual-mode charge pump brings a prospect for antenna alignment in the wireless power transfer system by further exploiting second harmonics.

A novel multiphase voltage regulator with small output ripple in both continuous conduction mode (CCM) and discontinuous conduction mode (DCM) is proposed. Mathematical analysis proves that the multiphase regulator has the smallest output ripple if phases are equally interleaved. The proposed regulator could minimise the output ripple through precisely interleaved phases by using a PLL-based synchronous clock in CCM and a self-adjusted staggered-time generator in DCM. The design is validated by fabrication in 0.18 µm CMOS process. Test results show that the peak–peak values of output ripple are 6 and 9 mV with 4 and 0.3 A loads, respectively.

Micro-Doppler (mD) characteristics of the ballistic targets have attracted great attention due to the potential in target discrimination. However, current investigations mainly focus on blunt-nosed cone and cone-cylinder-shaped targets. In practical applications, ballistic targets are always designed as the streamlined shape due to smaller aerodynamic resistance and lower detectability. In this Letter, the mD characteristics of streamlined ballistic targets are analysed. Owing to the unique mD signatures, the sliding scattering centre induced by the reflection from the streamlined curved surface is particularly focused. Finally, simulation results via the electromagnetic computation verify the correctness of the analysis.

Several types of vehicles including sedans, trucks, and buses simultaneously navigate roads. Information on the length and width of vehicles can be useful for tracking in scenarios such as autonomous driving. In this Letter, a vehicle-length estimation method using data from automotive frequency-modulated continuous-wave (FMCW) radars is proposed. Based on the similarity of the range profile between up- and down-chirp spectra, the authors can consistently determine the vehicle length. In addition, they consider the generation of different targets for long vehicles. Therefore, the proposed method can adaptively estimate the vehicle length. The authors verify the feasibility of the proposed method using on-road measurement data.

Orbital radar sounders are a useful tool to investigate planetary bodies. The sounding signal lies in the high frequency or very high frequency band, allowing a good ground penetration but limiting the system's resolution. Moreover, electromagnetic interference (EMI) may affect the system, increasing the noise in the radar products. In this Letter, a method to enhance the resolution and to suppress the EMI is presented. The method's performance and the parameters setting are investigated. Finally, some results for the Mars Advanced Radar for Subsurface and Ionosphere Sounding and for the Shallow Radar sounders are presented.

A carrier frequency estimation algorithm for the real signal based on the modulated wideband converter compressed sampling (CS) wideband digital receiver without signal reconstruction is proposed. First, the weighted coefficients matrix can be obtained by mixing function in the receiver. Then, between every two branches of the CS receiver, the results of CS signals multiplied by the inverse of their corresponding weighted coefficients sub-matrix are all equal. According to the equivalent relationship above, the weighted coefficients sub-matrix corresponding received signal can be searched in a weighted coefficients matrix. In addition, the weighted coefficients sub-matrix corresponding received signal includes the information of signal-band index which represents the frequency support. Finally, the carrier frequency estimation of the received signal can be obtained by signal-band index and frequency estimation of CS signals. The simulation results demonstrate the efficiency of the proposed algorithm.

In this Letter, the authors investigate the relationship between a number of parallel transmit symbols and the spectral efficiency of a millimetre wave (downlink) massive MIMO system with multiple antenna array sizes and a hybrid beamforming strategy. The proposed approach determines the optimal number of parallel data symbols that maximises both spectral efficiency and energy efficiency. A heuristic algorithm is proposed that adapts the number of symbols at the transmitter based on the instantaneous channel state information. The proposed algorithm outperforms non-adaptive strategies for multiple array sizes and achieves a significant improvement in spectral and energy efficiency.

A low complexity soft-decision decoding technique is presented for Reed–Solomon codes, over magnetic recording channels (MRC), by considering symbol and bit reliability, jointly. A symbol detection scheme for burst error scenarios has been put forward for the iterative error-and-erasure decoding algorithm (IEED), which is based on multiple trials, and is a combination of the generalised minimum distance and Chase-2 decoding schemes. In order to decrease complexity, the decoding test patterns are used in a certain order, while the stopping criteria is also discussed. Simulations show that the proposed algorithms can achieve almost the same performance as that of IEED for the additive white Gaussian noise channel and slightly better performance in MRC, but with significantly less complexity.

Energy conservation is a topic of great interest in wireless sensor networks (WSNs). Various techniques have been proposed to minimise the energy consumption. One approach is to design medium access control (MAC) protocols capable of adjusting the sensor node cycle according to the available energy in the battery. The state of charge (SOC) is an indicator of the available energy stored in the battery before discharging. This work proposes a simplified battery model to estimate the SOC and compares the accuracy and computational load of the algorithm as metrics for the implementation of the MAC protocol design.