A two-dimensional (2D) flat Luneburg lens antenna (LLA) operating at Ka-band is presented in this Letter. Based on the transformation optics, the 2D conventional Luneburg lens is compressed to the flat form. The proposed flat LLA is composed of six discretised dielectric layers with different permittivity, two metallic parallel plates and an E-shaped microstrip patch antenna array. By switching on/off the array elements, the lens antenna is able to achieve 1D beam-scanning with a scan coverage of ±30° at 28 GHz. The proposed lens antenna has a low profile of 7 mm, a focus-to-diameter ratio of 0.42, and the simulated/measured aperture efficiency for the on-axis beam is 56.8/36% at 28 GHz. The proposed 2D flat LLA is highly suitable for 5G mm-wave wireless communication with the advantages of low profile, wide beam-scanning range, low cost, and easy fabrication.

In this Letter, the design of a 3D printed fully dielectric Gutman lens is presented. The authors demonstrate the feasibility of using highly accessible and cheap additive manufacturing technology to produce a compact and high performing antenna lens. The lens is designed to operate at band and utilises a flat feed surface that approximates the focal sphere. The flat feed surface allows for beam steering that requires only translational movement of the feed. The lens has a measured realised gain of 20 dBi with 3 dB scan loss at . The lens finds applications in systems that require high gain antennas, such as the new generation of satellite and 5G communications and radar technology.

To obtain high direction-finding (DF) accuracy in electronic warfare applications, it is necessary to maximise the number of effective data points with a small angle-of-arrival error. Thus, a method to obtain the DF ambiguity probability for the selected set of array spacing is required. In this Letter, a novel method to obtain DF ambiguity probabilities for four- and five-element arrays by using gamma distributions is presented. The DF ambiguity probability obtained by the proposed method for the five-element array was in good agreement with the results of simulations. For the four-element array, the proposed method produced data that showed approximate agreement with the results of simulations. Thus, the proposed method should be useful for future applications.

A focused transmitarray antenna is presented in this Letter. To appraise the antenna, a quad-metallic-layer focused transmitarray antenna using I-shaped all-metal slot elements with an air gap between every two layers has been analysed, simulated and measured. It is illustrated that the electric field strength value at a distance of 3.1λ₀ from the transmitarray antenna reaches a maximum at 9.3 GHz operating frequency. It is the first time that a focal plane of approximately one wavelength has been achieved with a focused transmitarray antenna consisting of all-metal elements. Simulated and measured results show that the proposed antenna has good concordance, making it possible for microwave measurements.

In this Letter, a dual-band antenna with improved performance is proposed by generating a stronger coupling with the ground plane using two closed loops. The antenna is designed in such a way that it is coupled with two orthogonal ground modes in the lower and higher bands. Two closed loops are attached at the end of the ground plane to control the frequencies of the two ground characteristic modes. Through simulation and experimental verification, the proposed antenna has obtained greatly improved performance in both bands compared to the reference antenna without closed loops.

In this Letter, a differential-fed planar antenna system is presented with large isolation between transmission and reception ports for full-duplex applications. The proposed system contains two antenna pairs, each of which is composed of two identical single-ended microstrip patch antennas operating at 12 GHz. Each element in the array is fed 180° out-of-phase by utilising rat-race hybrid couplers making the whole system differential-fed. The patch antenna array and the hybrid ring couplers are designed on two separate substrate materials with the ground plane in between. Simulation results show high isolation through the entire band of interest while measured results approach in a reflective environment. The design also exhibits good impedance matching, reasonable gain values, and high efficiency.

Steady-state visual evoked potential, type of electroencephalography (EEG) signal, that is used for brain–computer interface systems are considered in this Letter. Steady-state visual evoked potential stimulator is needed for realising the signal on the scalp. Besides, information transfer rate is the most significant parameter to evaluate overall performance of a brain–computer interface. EEG signal classification methods, task completion time, and signal stimulator structure affect information transfer rate values. In this Letter, the authors aimed to reach a high information transfer rate by using the proposed signal stimulator and classification method that has new architectures. Eight flickering objects that provide 36 different characters to spell were used. This stimuli optimisation prevented the effect of eye fatigue on signal. Therefore, steady-state visual evoked potential was elicited dominantly. Moreover, 1D convolutional neural network for signal classification was proposed in this Letter. Online experimental data was also classified with canonical correlation analysis that is most commonly used in brain–computer interface systems. The authors compared results according to both of the classification methods. They have reached average value of information transfer rate as 50.67 bit/min with the proposed classification method. This result is significantly higher than similar studies.

A difference-equalisation based method is presented to estimate timing skew adaptively in time-interleaved analogue-to-digital converters (ADCs). By digital zero-crossing detection and calculating the mean of the absolute difference between adjacent sub-ADC's outputs, the timing mismatch information for each channel can be identified easily. Simulation results demonstrate that the proposed method can estimate timing skew accurately. Furthermore, the proposed method requires far fewer samples to estimation than the previous methods.

Fault attack is a type of active attack which retrieves the secret key by injecting computational faults. Laser is one of the most common fault injection methods. When attacking substitution-permutation networks-ciphers, S-box is often chosen as the laser injection target. However, the adversary has to know whether the S-box is corrupted or not after each injection, which was difficult in real-world attacks. Two analysing methods to distinguish the S-box faults for different ciphers is proposed in this Letter. A laser-based physical experiment is carried out to verify the authors' methods on Advanced Encryption Standard and PRESENT ciphers on an ATmega163L microcontroller. Experimental results show that their methods can precisely distinguish S-box faults merely using dozens of ciphertexts.

In this Letter, the authors put forward a loose classifier to determine whether the content of a given image belongs to screen content or natural scene. In the proposed classifier, they first extract 14 features by means of three typical histograms, the greyscale image histogram, local binary pattern histograms, and histograms of the oriented gradient, respectively. Then, in the second step, they try to build a non-linear mapping, with stacking-based integrated learners, from extracted features to the classification label. Experimental results prove that the effectiveness of their proposed method as compared with popular algorithms.

The common problem of visual simultaneous localisation and mapping systems is to suffer from featureless environments. It is possible for all environments to have such featureless situations; even though most of the mapped areas contain sufficient textures. This Letter brings a new approach using not only RGB values of the objects but also their positions in the map for feature extraction in order to decrease odometry loss in such situations. DepthTiling recolours RGB image using associated depth data. The experiments give promising results to increase the capability of visual odometry tracking. This study shows that it is possible to increase number of features using related depth data when RGB images are insufficient.

A dynamic power splitting (DPS) scheme for decode-and-forward (DF) relaying based simultaneous wireless information and power transfer (SWIPT) networks with a direct link (DL) is considered. The optimal DPS factor can maximise the received signal-to-noise ratio at the destination node. If the DPS factor is equal to zero, the proposed model will degenerate into a non-cooperative network which only utilises the DL to accomplish the transmission. The authors first derive the optimal DPS factor, and then derive analytical expressions for outage probability and ergodic capacity. Moreover, to obtain the diversity order, the asymptotic expression for outage probability is also derived. Numerical results show that the performance of the proposed scheme is the best compared to the other competitive schemes, especially in terms of the ergodic capacity.

Multiprocessors have become prevalent in real-time systems owing to their higher throughput. Various types of scheduling algorithms have been proposed for parallel real-time tasks, which differ from traditional tasks in that their subtasks execute in parallel. A parallel task is frequently modelled as a directed acyclic graph (DAG) that expresses the precedence constraints between its subtasks. In this Letter, the authors propose a decomposition algorithm to improve the Earliest Deadline First schedulability for DAG tasks, based on convex optimisation theory. Their experimental results demonstrate that their algorithm outperforms the two most recently published algorithms.

This Letter presents a third-order multilayer substrate integrated waveguide (SIW) filter with broadband stopband. The filter is composed of three vertically stacked square SIW cavities. The input/output feeding structure and the coupling slot in the middle metal layer of two cavities are properly designed to just allow the fundamental resonant mode to a couple while other higher spurious resonant modes are isolated. The fabricated filter is at a centre frequency of 6.96 GHz with a stopband suppression of 24 dB up to 2.77f ₀.

A compact filtering power divider is proposed with very wide stopband suppression in this letter. First, the dual-composite right- and left-handed (D-CRLH) resonator is employed, while one extended stopband can thus be achieved due to its inherently slow-wave response. Secondly, one out-of-phase feeding scheme is introduced to realise D-CRLH filtering power divider. By putting the input and output ports on the different sides of D-CRLH resonators, the mismatching can be realised for the harmonics below 10 GHz and wide stopband suppression is obtained accordingly. For experimental verification, a compact filtering power divider is designed, fabricated, and measured. Wide stopband suppression is observed until 5f ₀ without using bandstop structures or complex coupling topology.

A chaos shift parameter synchronisation (CSPS) system based on two different mutually coupled lasers synchronising with another laser is studied while a novel chaos shift orbit (CSO) encoding technique is presented for secret communication. CSPS between the emitter and receiver can still be obtained by shifting the emitter's parameter in real time, where the emitter operates its chaos to vary its orbit or jump across its orbit by using real-time current changes of one laser. Then, a CSO encoding scheme is implemented successfully. The emitter has numerous real-time shifting secret keys, so it is highly secure, and the signal is difficult to decipher.

Low phase noise RF oscillator is very important in modern communication systems and opto-electronic oscillator techniques have achieved superior phase noise. Development and characterisation of coupled symmetric multimode multi-section semiconductor laser pairs are reported for building a chip level RF oscillator. The symmetric laser structure operation is adjusted to achieve a self-mode locking of five modes at 26.4 GHz inter-modal oscillation frequency with the phase noise performance of −92 dBc/Hz at 10 kHz offset frequency.

A carrier-storage-enhanced superjunction (SJ) insulated gate bipolar transistor (IGBT) with n-Si and p-3C-SiC pillars (Si/SiC SJ IGBT) is studied. At the on-state, the n-Si/p-SiC heterojunction acts as a barrier for holes in the n-Si pillar, which helps to enhance the carrier-storage effect in the n-Si pillar and improves the tradeoff between turn-off loss (E _off) and on-state voltage drop (V _CE(sat)). It is found by simulations that V _CE(sat) of the Si/SiC SJ IGBT can be 0.35 V lower than that of the conventional SJ IGBT with the same breakdown voltage (V _B = 1450 V). Under E _off = 5 mJ/cm², V _CE(sat) of the Si/SiC SJ IGBT is 1.22 V, which is 0.30 and 0.81 V lower than V _CE(sat) of the conventional SJ IGBT and the field stop IGBT, respectively.

Highly c-axis-oriented scandium-doped aluminium nitride films (ScAlN) were prepared on platinum substrates by DC magnetron sputtering. The effect of sputtering power on the crystal quality and piezoelectric properties of ScAlN thin film was investigated. The test results show that the piezoelectric properties of the ScAlN film first strengthen and then weaken with the increase of sputtering power, and the optimal piezoelectric properties are obtained under the condition of 180 W. Furthermore, a surface acoustic wave (SAW) resonator with an interdigital transducer width of 500 nm was fabricated by electron beam lithography. The SAW device exhibits a resonant frequency of 1.887 GHz and a quality factor (Q) of 170.8.

Terahertz (THz) detectors imaging is an attractive technology that has been widely adopted in various imaging applications, but the low response has always been a critical problem for THz detectors. Here, the authors present some antenna-coupled 2.58 THz detectors fabricated using 55 nm CMOS process technology for the terahertz wave detection well beyond the device cut-off frequency, each detector consists of a patch antenna and one or few short-channel metal–oxide–semiconductor field-effect-transistors (MOSFETs). The authors demonstrated two drain-driven detectors different from the commonly used configurations, and proved that this structure can effectively improve the response to the quantum cascade laser. The output signal is extracted with a lock-in amplifier under room temperature.

The authors report, for the first time, a novel SiGe/Si n-type hetero nanotube (HNT) junctionless field-effect transistor (JLFET), which leads to a remarkably enhanced performance below the sub-10 nm gate length. It is shown that valence band discontinuity in the HNT JLFET causes tunnelling width of the channel–drain interface to increase and therefore diminishes the lateral tunnelling of electrons in the OFF state. The impact of high-κ spacers and core gate diameter on the dynamic performance of HNT JLFET has also been studied. They show that the inclusion of high-κ spacers does not affect the dynamic performance of the HNT JLFET. It is demonstrated using calibrated 2D simulations that the proposed device exhibits a smaller value of DIBL 4 mV/V, sub-threshold slope almost 60 mV/decade, and an improved I _ON/I _OFF ratio of ∼10¹² even for sub-10 nm gate lengths.

A new power allocation algorithm is proposed based on the Glicksberg game for cellular downlink non-orthogonal multiple access (NOMA) networks. First, a price-based user's utility function is proposed, and shown that it is effective and restrictive. Secondly, the Hessian matrix is used to derive an expression for power price based on the restricted transmission power and number of the served users in the cell. Then, the existence of a unique Nash equilibrium is proven and the optimum solution that maximises the utility function is presented. Finally, simulation results show that the proposed power allocation mechanism outperforms existing algorithms in terms of sum data rate and average data rate of the users.

In this Letter, the authors present a comprehensive performance analysis of a non-orthogonal multiple access (NOMA) system with generalised selection combining (GSC), which bridges the gap between the performance of NOMA with selection combining (SC) and NOMA with maximal-ratio combining (MRC). In particular, the authors derive a closed-form expression for an upper bound on the average achievable sum rate and an exact closed-form expression for the outage probability. The analysis reveals a tradeoff between the number of implemented receiver radio-frequency (RF) chains and the achieved performance and can be used to determine the appropriate number of paths to combine into a practical receiver design. A demonstrated agreement between the analytical and simulation results confirms the correctness of the analysis.

Spectrum handoff plays an important role in cognitive radio networks (CRNs). Secondary users (SUs) use spectrum handoff to hold on the idle channel or to free the channel for primary users (PUs). Spectrum handoff scheme greatly affects the transmission quality and the success rate of SUs connection. In this Letter, a reinforcement learning-based spectrum handoff scheme with the measured packet drop rate (PDR) for multimedia transmissions over CRNs is proposed. In a system model with multiple PUs and SUs, a new state space description method is designed and an observed state includes not only the status whether PUs arrive on each channel but also several other important factors. Also, the measured PDR, instead of the calculated one, is presented to update the mean opinion score, the Q-table and the handoff policy. Compared with the existing schemes with the calculated PDR from the Quality-of-Experience model, the authors' proposed scheme can converge more rapidly in the dynamic radio environment, and reduce the PDR of SUs more significantly.

Wireless communications experiments at high user velocities can be performed with much less effort at lower velocities by using time-stretching techniques. In OFDM, state-of-the-art time-stretching techniques do not consider the case of channel impulse responses exceeding the length of the cyclic prefix (CP). This is the case in any real-world OFDM system. In particular, multipath scenarios caused by relay nodes commonly found in public railways have a late, strong channel tap. In this work, the authors propose a time-stretching technique that preserves the effects of the insufficient CP with negligible error and without channel knowledge.