Volume 79, Issue 5-6

Channel estimation of sparse signal is a challenging task for millimeter-wave massive multi-input multi-output systems. Due to the hardware imperfections and use of large carrier frequency oscillator in hybrid precoding architectures leads to random phase drifts in the received pilots. Channel estimation with random phase drifts exploits partial coherence, where pilots share phase distortion within a frame but differ across different frames. To achieve the reliable and efficient communication, the support vector initialization needs to be optimized effectively. In this article, the singular value decomposition-based on-grid partially coherent compressive phase retrieval (SVD-PC-CPR) algorithm is proposed. This algorithm optimizes the initial support vector by finding the best dimensional subspace of the k sparsity elements by forming the singular vectors. The channel vectors are estimated by applying the gradient descent method through iterative refinements. This algorithm utilizes SVD for identifying the dominant paths from the phases of the measurements by selecting basis vectors. Based on the simulation results, the proposed algorithm achieves better normalized mean squared error, a multi-user sum-rate gain of 19 bits/s/Hz, and a 48 % improvement in estimation accuracy over the conventional PC-CPR algorithm at 20 dB signal-to-noise ratio.

In this paper, a resonating absorber is proposed for radar cross section reduction in the X-band, with the inclusion of a high loss dielectric layer. The absorber consists of a pair of co-centered resonating Jerusalem cross printed on a FR4 substrate cascaded with a high-loss dielectric layer and a metallic back plane. The designed absorber offers 90 % absorption from 7.8–12 GHz with a fractional bandwidth of 42.42 %. The designed absorber is 0.106 λ L thick with a compact square unit cell of size 0.076 λ L (wavelength at the lowest frequency). An equivalent circuit model is developed to analyze the proposed absorber. The absorber is angularly stable for varying angle of incidences up to 40°–60° for TE and TM polarizations respectively. The planar as well as the conformal structure of the proposed absorber offer a minimum 10 dB radar cross section reduction in the X-band. The conventional approach which involves complex fabrication process of soldering thousands of lumped resistors to increase the bandwidth, whereas the proposed absorber is realized to offer wide absorption using simple PCB etching technique. To validate the design prototype is fabricated and the measurement is carried out.

This paper proposes a tunable dual-band polarization-sensitive absorber that displays two distinct absorptance peaks for two different polarized incident lights. The proposed absorber uses graphene as the material for the metasurface to utilize its tunable surface conductivity for achieving a tunable resonant frequency. The tunable absorber displayed two distinct absorption peaks for two separately polarized incident waves. For x -polarized incident light, the proposed absorber displayed perfect absorptance for a tunable bandwidth of 3 THz between 2.85 THz and 5.85 THz, and for y -polarized incident light, the proposed absorber displayed perfect absorptance for a tunable bandwidth of 1.5 THz between 11 THz and 12.5 THz. Numerical computations were performed with extremely fine mesh quality to study the absorptance spectrum of the proposed absorber for different Fermi energy levels, incident angles, and azimuthal angle of incidence. The proposed absorber displays appreciative characteristics which makes it a suitable candidate for energy harvesting, polarization detection, and polarization conversion of unpolarized light.

This paper presents a compact broadband absorber designed for sensing applications at terahertz frequencies. The proposed design includes a dielectric material, a metallic ground plane made of copper, and a graphene-based elliptical slot-loading rectangular resonator. The unit cell of the proposed absorber measures dimensions of 6 μm × 6 μm. The proposed absorber demonstrates remarkable absorption characteristics, with over 90 % absorption across a wide frequency spectrum of 0–18 THz. At 12.2 THz, the absorption performance is nearly perfect, with an absorption rate of 99.9 %. Further, the proposed absorber has a symmetric design, facilitating it to be insensitive to polarization. It possesses stable characteristics for both Transverse-Electric (TE) and Transverse-Magnetic (TM) waves for the incidence angle ( θ ) ≥ 40°. Additionally, the performance of the suggested absorber is analysed with regard to the effects of various analyte materials as well as modifications in the graphene characteristics. This graphene-based absorber outperforms previously reported THz absorbers in terms of relative bandwidth, absorption peak, oblique stability, and overall volume.

This paper investigates a metasurface-based broadband polarization converter (BPC). Notably, to achieve a broadband working frequency region (4.22–10.73 GHz), a single substrate layer is used instead of several substrate layers. The design device converts an incidence wave that is linearly polarized (LP) into a reflected wave that is cross-polarized (CrP) over the frequency range of 4.22–10.73 GHz up to an oblique incident angle of 20°. In addition, the design is fabricated, and there is a strong agreement between the modeling and measurement results.

The issue of synthesizing a two-dimensional thinned virtual aperture multiple input-multiple output (MIMO) antenna array based on one-dimensional thinned linear antenna arrays is considered. The receiving and transmitting elements of the MIMO antenna array are located on two orthogonal lines. They are corresponding to a linear antenna array of appropriate length. These linear antenna arrays are thinned out by a genetic algorithm. To form a thinned virtual aperture of a MIMO antenna array, the location of its receiving and transmitting elements corresponds to the location of the elements of the thinned antenna arrays. It is shown that the directivity multiplier of the thinned virtual aperture of a MIMO antenna array has lower side lobes than the full aperture. The dependence of the level of the maximum side lobe on the thinning coefficient of the virtual aperture of the MIMO antenna array is given.

This communication introduces a Multiple Input Multiple Output (MIMO) antenna with dual resonance features, achieved through four dielectric resonators (DR). The design includes two larger (DR-1 and DR-2) and two smaller (DR-3 and DR-4) ceramic resonators ( ɛ r  = 9.8, tan δ  = 0.002) positioned on a fan-shaped FR-4 substrate, fed with a circular ring-shaped aperture for generation of lower and upper-frequency bands respectively. The fan-shaped substrate, combined with an arc-shaped defective ground structure (DGS), is used to intensify the bandwidth of the lower band and improve the isolation between radiating DRs. The measured operating bands for larger-sized DRs are found to be 2.7 GHz–4.12 GHz (41.6 % fractional bandwidth (FBW)) and 2.7 GHz–4.2 GHz (43.47 % FBW), while for smaller-sized DRs are depicted to be 5.21 GHz–5.96 GHz (13.29 % FBW) and 5.21 GHz–6.12 GHz (15.93 % FBW) respectively. The measured isolations in the lower and upper bands at resonant frequencies 3.6 GHz and 5.5 GHz are determined to be better than −31.2 dB and −24.5 dB, respectively. The closeness between simulated and measured results makes the proposed antenna appropriate for Sub-6 GHz wireless applications like WLAN (5.25–5.95 GHz), WiMAX (3.3–3.8 GHz), 5G (n48, n77, n78 bands), and Wi-Fi (n46) enabled IoT.

The on-body and implanted antenna’s communication is examined in this work. Short-range communication occurs within the human body during this kind of communication, which is sometimes referred to as intra-body communication (IBC). With on-body antenna compatibility, an implantable F-shaped antenna is made for intra-body communication. The effectiveness of the antenna was assessed using SAR analysis, radiation properties, reflection coefficient, and biocompatibility. A correlation between the transmitted signal from the implanted antenna and the received signal from the on-body antenna is determined, together with the intra-body communication fidelity factor and group latency. Furthermore, the link margin is established, and the implantable antenna allows high-speed data transfer at 93.32 Mbps with a 40 cm distance and a link margin of 20 dB between the implantable antenna and wearable antenna. With a gain of −24.8 dBi, the muscle layer’s impedance bandwidth is 24.7 % (2.31–2.79 GHz). In order to account for health concerns, the specific absorption rate (SAR) was determined. The outcome fell between the IEEE and the International Commission on Non-ionizing Radiation Protection’s standard bounds (ICNRP). A human body phantom made with skin-mimicking gel was used to test the suggested antenna. There was a strong correlation between the simulated and measured data.

The proposed work presents the design of a frequency and pattern reconfigurable rectangular patch antenna. The antenna consists of a simple arrow shape patch, incorporating slits connected to the upper patch through a single PIN diode switch to enable frequency and pattern reconfiguration. The proposed antenna operates at multiple frequencies: 3.21 GHz, 4.0 GHz and 5.32 GHz with the capability to change the radiation pattern, including pattern tilts of +30° to −30°. The prototype antenna is fabricated, and its performance is validated through measurements, which show good agreement with simulated results. These design improvements and validations highlight the efficiency and versatility of the proposed antenna compared to existing designs.

Radiofrequency (RF) power source during high power operation of a particle accelerator often exhibit saturation nonlinearity in amplitude and phase for sinusoidal input signal. This leads to an undesirable distorted RF signal as input to the RF cavity, which is the basic building block of an accelerator. Linearization of RF power source is an important necessity for an operational accelerator to improve and simplify RF cavity control performance, in addition to improving efficiency. The RF power source being a final control element of the RF cavity electric field control loop, its linear operation over a wider RF input amplitude range is often sought. Digital predistortion has been one of the preferred approaches for linearizing RF power source. This approach needs the determination of the input-output characteristics of the predistorter from those of an RF power source. Among many approaches, least squares estimation techniques are suitable for the design of digital predistorter. Among them, a recursive least squares estimation is most suitable and is elaborated in this paper. Amplifier nonlinearity and its handling using adaptive least squares estimation is explained, followed by modeling and simulation results. Input-output data acquired from an experimental 200 W, 30 MHz RF amplifier is used as input for simulations to demonstrate linearization capability by predistortion. After building up the basics, a technique for amplitude and phase linearization for data generated from the Saleh model of an RF power amplifier is presented. For an assessment of linearization capability, one needs to measure it. The effect of nonlinearity extent on linearity as a function of the number of model coefficients is analyzed. Experimental data is used to analyze the effect of nonlinearity extent and measurement noise. This paper is a comprehensive coverage of the design and analysis of a linearization scheme and is applicable for any general engineering application. A digital predistortion algorithm needs to be implemented in a control system for the RF cavity. This problem forms an important inverse problem of general nature under the purview of the modern control system community.