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Hybrid beamforming for multiple-input/multiple-output (MIMO) communications is an attractive technology for realizing extremely massive MIMO systems envisioned for future wireless communications in a scalable and power-efficient manner. However, the fact that hybrid MIMO systems implement part of their beamforming in analog and part in digital makes the optimization of their beampattern notably more challenging compared with conventional fully digital MIMO. Consequently, recent years have witnessed growing interest in using data-aided artificial intelligence (AI) tools for hybrid beamforming design. This article reviews candidate strategies to leverage data to improve real-time hybrid beamforming design. We discuss the architectural constraints and characterize the core challenges associated with hybrid beamforming optimization. We then present how these challenges are treated via conventional optimization, and identify different AI-aided design approaches. These can be roughly divided into purely data-driven deep learning models and different forms of deep unfolding techniques for combining AI with classical optimization. We provide a systematic comparative study between existing approaches, including both numerical evaluations and qualitative measures. We conclude by presenting future research opportunities associated with the incorporation of AI in hybrid MIMO systems. [ABSTRACT FROM AUTHOR]

The increasingly demanding objectives for next-generation wireless communications have spurred recent research activities on multiantenna transceiver hardware architectures and relevant intelligent communication schemes. Among them belong full-duplex (FD) multiple-input, multiple-output (MIMO) architectures, which offer the potential for simultaneous uplink (UL) and downlink (DL) operations in the entire frequency band. However, as the number of antenna elements increases, the interference signal leaking from the transmitter (Tx) of the FD radio to its receiver (Rx) becomes more severe. In this article, we present a unified FD massive MIMO architecture comprising analog and digital transmit and receive beamforming (BF) as well as analog and digital self-interference (SI) cancellation, which can be jointly optimized for various performance objectives and complexity requirements. The performance evaluation results for applications of the proposed architecture to fully digital and hybrid analog and digital-BF operations using recent algorithmic designs as well as simultaneous communication of data and control (SCDC) signals are presented. It is shown that the proposed architecture, for both small and large numbers of antennas, enables improved spectral-efficient FD communications with fewer analog-cancellation elements compared to various benchmark schemes. The article concludes with a list of open challenges and research directions for future FD massive MIMO communication systems and their promising applications. [ABSTRACT FROM AUTHOR]

This article introduces a novel near-field focus (NFF) lens antenna with a three-dimensional (3-D) focus-steering capability operating at terahertz (THz) frequencies. The antenna consists of a pair of thin discrete dielectric lenses (DDLs) placed above a stationary feed horn. First, we demonstrate that the DDL doublet’s in- plane synchronous counter-rotation and co- rotation could scan its focus radially and azimuthally, respectively. A multi-focus synthesis method that compensates the required transmission phases at multi-focus points is developed to further improve its NFF-steering performance. The DDL-based antenna can realize 3-D focus-scanning using a combination of rotation and linear translation mechanisms. The 3-D printing is adopted to fabricate the THz antenna to simplify the manufacturing process and reduce the cost. Both numerical and experimental results demonstrate that the 15 mm-diameter antenna prototype can scan its focus in a 3-D cylindrical space with a diameter of 29 mm and a length of 8 mm at 300 GHz. The proposed 3-D focus-scanning antenna with a large field of view (FoV) of 72° will open new avenues for fast 3-D THz imaging applications. [ABSTRACT FROM AUTHOR]

Direction-of-arrival (DOA) information is vital for multiple-input-multiple-output (MIMO) systems to complete localization and beamforming tasks. Switched antenna arrays have recently emerged as an effective solution to reduce the cost and power consumption of MIMO systems. Switch-based array architectures connect a limited number of radio frequency chains to a subset of the antenna elements forming a subarray. This paper addresses the problem of antenna selection to optimize DOA estimation performance. We first perform a subarray layout alignment process to remove subarrays with identical beampatterns and create a unique subarray set. By using this set, and based on a DOA threshold region performance approximation, we propose two antenna selection algorithms; a greedy algorithm and a deep-learning-based algorithm. The performance of the proposed algorithms is evaluated numerically. The results show a significant performance improvement over selected benchmark approaches in terms of DOA estimation in the threshold region and computational complexity. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate an approach for local reduction of the electric field amplitude of the transmitted radio-frequency signal in ultrahigh-field magnetic resonance imaging (MRI). We excite a suitable combination of three coupled dipole hybrid resonances composing a single transmit antenna array element. Using numerical optimization, we designed a feeding network for three coupled dipoles placed over an electromagnetic phantom mimicking a human body. This network of discrete elements provides the appropriate amplitudes and phases of three dipole currents excited by a single input port. It allows controlling the electric field distribution in the vicinity of the antenna. Our goal was to obtain a minimum of the electric field at the given relatively small depth inside the phantom, where body implants are typically located while keeping a tolerable level of the magnetic field toward the phantom’s center. We designed and manufactured a three-dipole antenna prototype optimized for MRI of the human body at 7 T (proton Larmor frequency of 298 MHz). The experimental validation showed a 40 dB reduction of the electric field amplitude at a depth of 4 cm compared to a conventional single-dipole antenna. The coupling network can be rearranged to target different depths. Therefore, a principle of electric field minimization at a controllable position inside the body has been shown, which may be useful for designing transmit MRI antennas with improved safety of implants. [ABSTRACT FROM AUTHOR]

A novel scheme for designing electromagnetic functional surfaces (EMFSs) is proposed in this article. Different from traditional frequency selective surface (FSS), this scheme uses the antenna array and the back-loaded radio frequency circuit (BLRFC) to control the frequency response of EMFSs. The antenna array and the BLRFC can be designed independently resulting in great design flexibility and functional diversity. The sharp skirt out-of-band suppression and tunable frequency response can be achieved by using this scheme. Three design levels (element level, subarray level, and array level) are introduced to illustrate the design philosophy. Several EMFSs are designed, and one of them has been fabricated and measured. Actually, the prototype is a dual-polarized absorptive frequency selective reflector (AFSR). For both TE and TM polarizations with normal incidence, the AFSR exhibits one reflection band with 3 dB bandwidth from 10.6 to 16.3 GHz, two absorption bands with the absorptivity greater than 90% from 6 to 10 GHz, and from 16.8 to 19 GHz. The AFSR has a stable angular performance up to 50°. All the design examples and fabricated prototype verify the flexibility and the superiority of the novel scheme, and a multiplicity of EMFSs can be realized by using the BLRFC technique. [ABSTRACT FROM AUTHOR]

In this article, a complete and efficient wireless power transfer (WPT) system, consisting of a full-duplex antenna, a half-wave rectifier, and a WPT transmitter (Tx), is designed and validated for deep-body ingestible biomedical capsules. A two-port full-duplex antenna is adopted to perform seamless telemetry and to act as a receiver for WPT Tx without using a multiplexer circuit. The full-duplex implantable antenna operates at 915 MHz (Port 1) and 1300 MHz (Port 2) and occupies compact dimensions of 8 $\times $ 8 $\times $ 0.13 mm 3. It maintains high isolation of >30 dB and peak realized gains of −19.3 dBi at 915 MHz and −17.7 dBi at 1300 MHz. The miniaturization of this antenna is achieved using semicircular slots on the patch and circular slots on the ground plane while mutual coupling between the ports is reduced by a series of metallic vias. Moreover, a low-profile and efficient rectifier is designed at 1300 MHz and integrated with port 2 of the antenna. The suggested rectifier achieves a radio frequency (RF) to direct current (dc) conversion efficiency of 80.4% at an input power of 1 dBm. Furthermore, a patterned patch antenna is designed that acts as a WPT Tx for the biomedical capsule. After examining the individual performance of the antenna, rectifier, and WPT Tx, the overall WPT system is then validated by placing the capsule (rectifier integrated antenna) inside minced pork meat and powered by the WPT Tx. This WPT system is suitable for modern capsule devices due to its features, such as compact size, high gains, high RF-dc conversion efficiency, simultaneous wireless powering and telemetry, and high-power transfer efficiency (PTE). [ABSTRACT FROM AUTHOR]

Dynamic metasurface antennas (DMAs) arise as a promising technology in the field of massive multiple-input multiple-output (mMIMO) systems, offering the possibility of integrating a large number of antennas in a limited —and potentially large— aperture while keeping the required number of radio-frequency (RF) chains under control. Although envisioned as practical realizations of mMIMO systems, DMAs represent a new paradigm in the design of signal processing techniques (such as beamforming) due to the constraints inherent to their physical implementation, for which no complete models are available yet. In this work, we propose a complete and electromagnetic-compliant narrowband communication model for a generic DMA based system. Specifically, the model accounts for: i) the wave propagation and reflections throughout the waveguides that feed the antenna elements, ii) the mutual coupling both through the air and the waveguides, and iii) the insertion losses. Also, we integrate the electromagnetic model in the conventional digital communication model, providing a complete and useful framework to design and characterize the performance of these systems. Finally, the accuracy of the model is verified through full-wave simulations. [ABSTRACT FROM AUTHOR]

In this article, a comprehensive review specifically on the applications of rectennas for different wireless systems has been presented. Rectenna systems are used to convert radio-frequency (RF) energy into dc signals. Attracting ever-increasing interest among researchers, the rectenna has occupied a unique position in RF and microwave engineering systems. During the last four decades, the research on rectenna design has been focused on improving the power conversion efficiency (PCE) and compatibility while reducing the design complexity as well. This review article summarizes the overall rectenna research carried out for different applications, viz., wireless power transmission (WPT), wireless sensor networks (WSNs), solar power transmission (SPT), medical implants, and wireless energy-harvesting (WEH) systems. Such a review article has rarely been proposed in the open literature to the best of the authors’ knowledge. The main objectives of the article are to 1) accommodate the key design requirements of the rectenna, 2) study the potential application of rectennas and their possible configurations to achieve desired performance characteristics, and 3) to present a comparative analysis of the performance parameters of the different reported works. [ABSTRACT FROM AUTHOR]

An antenna-in-module (AiM) architecture at millimeter-wave frequencies for a base transceiver system (BTS) is developed for electromagnetic (EM) radiation with an agile polarization tunable capability. This may reduce the polarization mismatch to a user equipment (UE) antenna, where the mismatch becomes severe in wide-angle scans. The AiM first implements a dual-polarization feature on its antenna elements and then integrates active radio frequency (RF) phase shifters and power amplifiers/attenuators to provide dynamically adjustable weightings. Considering the received fields of a UE antenna, the weightings of the AiM antennas are appropriately tuned to obtain the maximum signal strength from the UE antenna. In this article, the theoretical concept and the antenna architecture are presented. Full-wave simulations using HFSS and the measurement results of the AiM prototype are presented to validate its feasibility. [ABSTRACT FROM AUTHOR]

A novel hybrid antenna for simultaneous RF energy harvesting (RF-EH) and dedicated wireless power transfer (WPT) is proposed. The hybrid antenna consists of a multiband dual-polarized omnidirectional antenna element (covering GSM-1800, UMTS-2100, ISM-2400/5800, 5G-3500, and wireless local area network (WLAN) bands) and a 2.4 GHz dual-polarized high-gain directional patch array element. In the WPT band (ISM-2400), directional beams locate at the null of omnidirectional beams, and a pair of spatially complementary beams is obtained. The omnidirectional beams are capable of capturing the ambient RF energy from low-elevation space for low-power circuits, while the directional beams receive the power from a dedicated source to activate high-power modules. Also, the spatially complementary feature contributes low mutual couplings between ports that support the independent operation of WPT and RF-EH. For validation, an outdoor RF-EH test and an indoor WPT test are conducted. The measured results agree with the simulated ones. [ABSTRACT FROM AUTHOR]

In this article, an efficient over-the-air (OTA) calibration and pattern estimation method under the midfield (MF) condition for the fifth-generation (5G) millimeter-wave (mmWave) large-scale arrays is proposed. A new transformation and revision algorithm is developed to calibrate the antenna array fast and accurate. Meanwhile, the time consumed in the accurate beamforming pattern estimation is reduced significantly. The proposed method requires a much smaller size anechoic chamber, a fixed probe antenna, and a vector network analyzer (VNA). The effectiveness is proven with the detailed theoretical analysis and experimented with a 64-element phased array in the 26 GHz band. The calibrated root-mean-square (rms) errors of the phase and magnitude are about 1.8°/0.4 dB. In addition, the proposed calibration and pattern estimation method is tremendously cost-effective and time-saving with similar accuracy as the far-field test method. [ABSTRACT FROM AUTHOR]

This article presents a cost-effective miniature surface-mount magnetoelectric (ME) dipole antenna. The antenna is realized by a single-layer FR4 substrate and combined with ball grid array (BGA) packaging technology to achieve miniaturization in size. Due to BGA packaging technology, the surface-mount capability allows interconnection between the proposed antenna and other surface-mount devices using a flip-chip interconnect technology, replacing bulky connectors. The proposed antenna element is very compact. Specifically, the electric dipole of the proposed antenna consists of four patches, whereas the magnetic dipole consists of two rows of plated through-holes and a metal ground between them. In addition, the electric and magnetic dipoles are simultaneously excited by an L-shaped metal probe. The prototype has been manufactured and carefully verified. Experimental results show that the proposed antenna element has a −10-dB bandwidth of 23% in the range of 25–31.5 GHz and achieves a peak realized gain of 6.98 dBi at 26 GHz. The dimensions of the proposed antenna element are only 5 mm $\times $ 5 mm $\times1.8$ mm. The proposed antenna elements can be widely used in the 5G millimeter-wave N257 (26.5–29.5 GHz) and N261 (27.5–28.35 GHz) bands. [ABSTRACT FROM AUTHOR]

The dc power collected by radio frequency energy harvesting (RFEH) absorbing surfaces is limited by their physical aperture. Here, a compact, conformable, and ultrathin sub-1 GHz large-area RFEH surface is proposed based on electrically small ($ka=0.58$) “wire-type” rectenna elements, with an effective area exceeding its physical aperture size. Using optimized large-signal complex source tuning, the rectifiers achieve up to 36% measured power conversion efficiency (PCE) at =20 dBm. The proposed 15 cm-diameter six-element array generates a 27.5 $\mu \text{W}$ at 1 V output from a $0.17~\mu \text{W}$ /cm2 incident power density across a 40 $\text{k}\Omega $ load. Owing to its series connection, the voltage output is boosted, and the load resistance dependence is suppressed enabling over 60% of the maximum RF-to-dc PCE to be preserved for loads between 7 and 100 $\text{k}\Omega $ , with an area-normalized figure-of-merit nearly 100% higher than previous arrays. The proposed array is integrated with a commercial DC-DC converter and demonstrated powering a Bluetooth low energy (BLE) wireless sensor node (WSN) from an unprecedented incident power density of 0.25 $\mu \text{W}$ /cm2. A practical demonstration using a commercial 3 W, 915 MHz Powercast source is presented, showing an 11 m operation range with a 1 V output and illustrating the impact of polarization mismatch on the proposed array. [ABSTRACT FROM AUTHOR]

In a backscatter communication system, the backscatter device (BD) transmits its messages to the backscatter receiver (BR) by reflecting the incident signal from an external radio frequency (RF) emitter, instead of using power-hungry active RF components themselves. Thus, backscatter communication has shown great potential for achieving low-power communication. The double-fading effect associated with the backscatter link, however, is a major limiting factor to achieve efficient backscatter communication. Reconfigurable intelligent surfaces (RISs), a recently developed technology, can be applied at the BD to enhance the backscatter link thanks to the fact that both RIS and backscatter communication share the same reflective principle. Such a design can also allow the backscatter communication system to capture the desired RF signal as a reflective carrier in a complex radio environment. In this article, a comprehensive overview of backscatter communication assisted by RIS is given. We first introduce the basics of backscatter communication, which covers the antenna scattering principle, backscatter modulation, and link budget calculation. Then, the details of RIS are discussed, which include antenna-based RIS and metamaterial-based RIS, followed by the discussion of the roles of RIS in backscatter communication. After that, we provide an overview of three types of backscatter communication systems assisted by RIS, including RIS-assisted unmodulated backscatter communication, RIS-assisted ambient backscatter communication, and RIS-assisted symbiotic radio. Emerging applications of these systems, technical challenges, and future opportunities in this emerging field are also presented. [ABSTRACT FROM AUTHOR]

In this article, a two-hop simultaneous wireless information and power transfer relay network using a decode-and-forward strategy is considered. In order to improve the spectral efficiency, the relay node performs in full-duplex (FD) mode, and each node is equipped with multiple antennas. First, the end-to-end achievable rate is formulated for FD and half-duplex (HD) modes under imperfect channel state information (ICSI). Then, the optimal closed-form expressions for power allocation at the source and relay, as well as the energy harvesting ratios, are derived for primal variables associated with the dual variables through analytical solutions. We propose an iterative primal-dual algorithm based on the ellipsoid method so as to obtain final answers. Numerical results demonstrate the FD achievable rate superiority over the HD mode under proper self-interference cancellation (SIC). According to the existing results, there is an immense discrepancy between the achievable rates in perfect CSI and ICSI cases. Also, the impact of the relay distance from the source node, the number of antennas, and different SIC modes are investigated on the end-to-end achievable rate. [ABSTRACT FROM AUTHOR]

It is essential to perform end-to-end performance testing of multiple-input–multiple-output (MIMO) systems under realistic propagation channel conditions. However, due to the highly integrated and compact radio frequency (RF) system design, the traditional conducted cable testing method is getting more and more problematic. The wireless cable solution, which can achieve the equivalent functionality of the conducted cable testing without actual RF coaxial cables, has attracted huge attention from the industry and standardization for over-the-air (OTA) testing in recent years. However, the state-of-the-art solutions to achieve wireless cable testing necessitate at least reference signal received power (RSRP) reporting per device under test (DUT) antenna port for MIMO capable terminals, which is demanding and might not be accessible for commercial DUTs. In this work, a novel wireless cable solution based only on the maximum RSRP measurement of all DUT antenna ports is proposed, which can significantly alleviate the requirement of DUT RSRP reporting. To achieve the wireless cable testing, a novel calibration procedure is proposed to determine the transfer matrix between the probe antenna ports and the DUT antenna ports based on the DUT maximum reported RSRP measurement. The proposed algorithm is theoretically derived and experimentally validated for a $2\times 2$ MIMO system, where the isolation of above 25 dB is achieved for the measurement setup. The numerical simulation and experimental validation demonstrate the efficiency and robustness of the proposed algorithm. [ABSTRACT FROM AUTHOR]

In this communication, a circularly polarized (CP) dielectric resonator antenna (DRA) based on quasi-self-complementary metasurface (MS) has been proposed. Due to the quasi-self-complementary characteristic of the proposed MS, the phase difference of the transmission coefficients between two orthogonal linear polarizations is 90° in the frequency band from 25.5 to 26.5 GHz. The DRA with the proposed MS can realize the transformation from linear polarization to circular polarization at 25.6–25.99 GHz. The proposed MS significantly increases the gain and CP bandwidth of the DRA. The working bandwidth of the proposed antenna is included in the New Radio FR2 frequency band totally, which is considered as the follow-up expansion frequency of fifth-generation (5G) communication. The proposed MS-based antenna has realized the linear-to-circular polarization transformation in millimeter frequency band without additional substrate, which has not been researched yet. [ABSTRACT FROM AUTHOR]

A novel integrated silicon micromachined diplexer-antenna module for compact terahertz (THz) heterodyne receiver is proposed. The module is composed of a radio frequency (RF) antenna, a local oscillator (LO) antenna, and a waveguide diplexer which are realized by using the silicon micromachining technology and stacking nine equal-thickness layers. The corrugated conical horn antenna and quasi-diagonal horn antenna are applied as the RF- and LO-antenna, respectively. The diplexer is composed of two third-order waveguide bandpass filters and an H-plane T-junction, providing the isolated RF and LO channels. An experimental study is carried out in the 340 GHz band, demonstrating its application potential in THz heterodyne receiver element and array. [ABSTRACT FROM AUTHOR]

In this article, we explore the relationship between an equalizer and a combiner operating on the available polarization states of a received signal in aeronautical mobile telemetry. The maximum likelihood combiner is derived and shown to be equivalent to summing the outputs of two filters matched to the channels in the horizontal and vertical polarization states. For historical reasons, current systems combine right-hand and left-hand circularly polarized antenna feed outputs using a maximum ratio combiner. To compare the two combining approaches, the aeronautical telemetry multipath channel is extended to include polarization state information. The channel extension is used in computer simulations. The simulations results for SOQPSK-TG with a constant modulus algorithm equalizer show that the postequalizer bit error rate for the two approaches is the same. [ABSTRACT FROM AUTHOR]

This article proposeda SPICE-compatible model to fast simulate the buzz noise problem in a camera device. Based on the reciprocity theorem, the proposed SPICE-compatible model consists of cascaded scattering (S) parameters extracted from the field coupling. It can provide an accurate estimation of the buzz noise transfer function (TFtotal) between the radio-frequency antenna and the audio system within an average error of 1 dB. Besides, the proposed model allows for coupling decomposition by separating the audio system into different regions, fast buzz noise mitigation by adding filtering circuits, and fast antenna evaluation/selection by characterizing the TFtotal with different antenna structures. The advantage of the proposed model is that, instead of multiple 3-D simulations during the predesign-stage troubleshooting, it is a fast SPICE-compatible method with SPICE simulation time of 4.5 min and one-time 3-D full-wave simulation on the camera system studied. It provides quick estimation on the potential buzz noise problems, coupling decomposition,buzz noise mitigation methods, and selection of antenna structures. [ABSTRACT FROM AUTHOR]

This article investigates the impact of precoding on the performance of massive multiple-input multiple-output (mMIMO) transmitters exhibiting nonnegligible antenna crosstalk. For this, a new metric, called the average active impedance, is introduced to quantify the extent of load modulation as a function of the antenna S-parameters and the employed precoding. The new metric is then used to investigate the load modulation under conventional precoding schemes. It is shown that these precoders yield large disparities in average-power levels across the radio frequency (RF) chains, which results in substantial performance variations with channel conditions. Based on this investigation, we propose three new precoding schemes that yield equal average-power (EP) levels across all RF chains, independent of the channel conditions. Numerical simulations and experiments conducted on an mMIMO transmitter prototype confirmed that the proposed schemes improve the transmitter’s resilience to the variation in channel conditions. [ABSTRACT FROM AUTHOR]

In this work, we investigate hybrid analog–digital beamforming (HBF) architectures for uplink cell-free (CF) millimeter-wave (mmWave) massive multiple-input multiple-output (MIMO) systems. We first propose two HBF schemes, namely, decentralized HBF (D-HBF) and semi-centralized HBF (SC-HBF). In the former, both the digital and analog beamformers are generated independently at each AP based on the local channel state information (CSI). In contrast, in the latter, only the digital beamformer is obtained locally at the access point (AP), whereas the analog beamforming matrix is generated at the central processing unit (CPU) based on the global CSI received from all APs. We show that the analog beamformers generated in these two HBF schemes provide approximately the same achievable rates despite the lower complexity of D-HBF and its lack of CSI requirement. Furthermore, to reduce the power consumption, we propose a novel adaptive radio frequency (RF) chain-activation (ARFA) scheme, which dynamically activates/deactivates RF chains and their connected analog-to-digital converters (ADCs) and phase shifters (PSs) at the APs based on the CSI. For the activation of RF chains, low-complexity algorithms are proposed, which can achieve significant improvement in energy efficiency (EE) with only a marginal loss in the total achievable rate. [ABSTRACT FROM AUTHOR]

We elucidate feasibility of wireless energy transfer (WET) with the help of an intelligent reflecting surface (IRS). We consider a source equipped with multiple antennas and a single radio-frequency (RF) chain. We propose a low complexity rule that does joint antenna selection (AS) at source and passive beamforming at IRS. We derive new expressions for probability of outage in WET under perfect and estimated channel knowledge and for both single and multiple users. We derive intuitive expressions for outage probability with large number of IRS elements and for line-of-sight scenarios. For a system with $M$ antennas at source and $N$ passive elements at IRS, we show that diversity order equals $M+N$. Extensions to subset AS, discrete phase-shift design, and performance under limited scattering are also presented. Our numerical results show that the proposed AS rule yields near-optimal performance while requiring only $M+N$ pilot transmissions compared to the $M+MN$ pilot transmissions required by the optimal AS rule in literature. We elucidate that we can trade-off active RF chains at source with passive elements at IRS to obtain improved performance both in terms of outage probability and power transfer efficiency. And 3-bit IRS is sufficient to obtain good performance at lower complexity. [ABSTRACT FROM AUTHOR]

We consider a generalized space shift keying (GSSK)-enabled multiple-input multiple-output (MIMO) ambient backscatter communication (ABC) system. We propose a scheme to exploit the multiple antenna structure of the system to achieve a lower error-rate performance than conventional ABC systems. Furthermore, we present a novel low complexity energy-based maximum likelihood (EML) GSSK detector, which does not require the perfect knowledge of the ambient source’s signal, unlike the conventional ABC receivers. To gain insights into the performance of the proposed scheme, we derive the exact pairwise error probability (PEP) of the EML detector and further obtain an upper bound on the probability of error. We also derive a simple asymptotic PEP expression as the number of antennas of the reader becomes large. Finally, we derive a simple, asymptotic PEP at the reader when the noise variance approaches zero, i.e., under the large signal-to-interference ratio regime. We validate our analysis through Monte Carlo simulations and show a small performance loss due to the approximations. [ABSTRACT FROM AUTHOR]

Future mobile networks are envisioned to become multi-service systems, enabling the dynamic deployment of services with vastly different performance requirements, accommodating the needs of diverse service providers. Virtualizing the mobile network infrastructure is of fundamental importance for realizing this vision in a cost-effective manner. While there have been extensive research efforts in virtualization for the mobile core network, virtualization in the radio access network (RAN) is still at an early stage. In this article, we present AIRTIME, a new RAN slicing system that enables the dynamic on-the-fly virtualization of RANs, with the programmability required by service providers to customize any aspect of their virtual RAN to meet their service needs. We present a prototype implementation of AIRTIME and evaluate the: (i) capacity to create virtual RANs on-the-fly, (ii) performance experienced by slice owners, (iii) isolation among multiple virtual RANs sharing the same physical infrastructure, and (iv) scalability to accommodate a large number of virtual RANs. [ABSTRACT FROM AUTHOR]

Here, a compact design of antenna-multiplexer is engineered specifically to meet the stringent requirement imposed by intricate subsystems operating at 5.2, 5.5, and 5.8 GHz frequency bands for Internet of Medical Things (IoMT) applications. The proposed design includes a hexagonal-shaped substrate integrated waveguide (HSIW) cavity, tripole-shaped radiating slot, tuning vias, and three inset microstrip feedlines. A tripole-shaped slot is imprinted on the top of the SIW. This slot subdivides the cavity into trio-radiating segments and each segment offers a single frequency band. Further, the frequency bands are tuned at 5.2/5.5/5.8 GHz. The design maintains mutual port isolation better than 23.9 dB. Compared with the conventional tri-frequency antennas, the proposed design is highly compact and doesn’t need any additional circuitry to improve the port isolations. The measured results confirm the expected performance of the design. Furthermore, the proposed antenna is optimized within an implantable medical device (IMD) and simulated inside a realistic Human Head model at a depth of 3 mm and the Specific Absorption Rate (SAR) value is estimated. The SAR values are well below 0.362 W/Kg at the functioning bands due to the unidirectional radiation pattern from the antenna. [ABSTRACT FROM AUTHOR]

This article presents the analysis, design, and characterization of a 61.25 GHz radio frequency identification (RFID) interrogator. Monostatic RFID systems have to solve the challenge of the transmit (TX)-to-receive (RX) leakage, which could desensitize the interrogator. Instead of suppressing or canceling this leakage, the interference of the transmitted and incident waves is exploited to demodulate the incident signal. The structure can be inserted between the power amplifier (PA) and the antenna pad of existing transmitters. It has a lower insertion loss (IL) than other ON-chip isolation structures used in monostatic interrogators reported to date. The transceiver front-end was fabricated on a 22 nm fully depleted silicon on insulator (FD-SOI) technology, and has a simple structure, where the low-power receiver requires 0.022 mm2 active area. The die was mounted on a radio frequency (RF) printed circuit board (PCB) and a general circuit board was used for baseband processing in the measurements. [ABSTRACT FROM AUTHOR]

This paper proposes a method to localize multiple partial discharge (PD) sources in air-insulated substation by using radio frequency (RF) antenna sensors combining with clustering algorithm, which solves the problem that multiple PD or interference sources are difficult to localize in air-insulated substation (AIS). Our research group made four omnidirectional RF antenna sensors, the detection frequency band was 250 MHz-3 GHz, the voltage standing wave ratio (VSWR) of the RF antenna in the detection frequency band was less than 2, which meets the detection requirements. The scenes of multiple PD sources in AIS were simulated in the laboratory, the time difference of arrival (TDOA) was collected through four RF antennas, and the clustering effects of K-means and Two Step algorithms were compared. Based on the normal distribution characteristics of TDOAs and the better Two Step algorithm, a $3\sigma $ -Two Step algorithm was proposed to process TDOAs and output the centers of TDOAs. Newton iterative algorithm was used to solve the localization equations. The results show that the average localization error of two PD sources was 0.21 m and that of three PD sources was 0.45 m. The PD localization tests were carried out in an actual AIS in China, the proposed method localized a corona discharge on a post insulator and a corona discharge on a disconnector. Through the verifications with an ultraviolet camera, it was obtained that the localization errors at each position were within the range of 0.6-2 m. [ABSTRACT FROM AUTHOR]

In this article, the design and implementation of a low-loss bidirectional microelectromechanical system (MEMS)-like passive beamformer for emerging millimeter-wave (mm-Wave) applications are illustrated. A 4 $\times $ 4 phased-array antenna (PAA) is fabricated employing the proposed beamformer for demonstration. The phase shifter (PS) embedded in the beamformer operates based on the principle of loaded-transmission line (TL) PSs. Slow wave mechanism is employed to shrink the size of the PS. The PS measurement results show the average insertion loss (IL) of 1.3 dB in all the tuning states and the IL variation is 0.9 dB. The PS provides 380° of the phase tuning range in a compact footprint area of 2.4 mm $\times $ 3 mm. The antenna array incorporates slot-coupled patch antennas with left-handed circularly polarized (LHCP) radiation. The operating frequency bandwidth of the antenna system ranges from 28 to 30 GHz to provide co-pol/X-pol discrimination of more than 12 dB for all the steering angles. Measurement results show the beam steering angular range of ± 30° in both elevation and azimuth planes. The measurement results also show the peak directivity of 18.48 dBic and the radiation efficiency of 58% at boresight at the frequency of 29 GHz. The efficiency drops 16% when the steering angle reaches the maximum. [ABSTRACT FROM AUTHOR]

Accurate and efficient estimation of the high dimensional channels is one of the critical challenges for practical applications of massive multiple-input multiple-output (MIMO). In the context of hybrid analog-digital (HAD) transceivers, channel estimation becomes even more complicated due to information loss caused by limited radio-frequency chains. The conventional compressive sensing (CS) algorithms usually suffer from unsatisfactory performance and high computational complexity. In this paper, we propose a novel deep learning (DL) based framework for uplink channel estimation in HAD massive MIMO systems. To better exploit the sparsity structure of channels in the angular domain, a novel angular space segmentation method is proposed, where the entire angular space is segmented into many small regions and a dedicated neural network is trained offline for each region. During online testing, the most suitable network is selected based on the information from the global positioning system. Inside each neural network, the region-specific measurement matrix and channel estimator are jointly optimized, which not only improves the signal measurement efficiency, but also enhances the channel estimation capability. Simulation results show that the proposed approach significantly outperforms the state-of-the-art CS algorithms in terms of estimation performance and computational complexity. [ABSTRACT FROM AUTHOR]

This study presented an implementation of weighted-polarization multiple-input–multiple-output (MIMO) antenna for wireless body area networks at 2.45 GHz. The weight function determined by considering the variation in the cross-polarization power ratio (XPR) and the antenna tilt angle was realized by a two-way power divider (PD), where tunable power ratio range of proposed PD could not only cross the boundary of equal power ratio (k2 = 1), but also maintain a wide tunable range at the same time. The patch array antenna fed by the proposed PD consists of microstrip lines, capacitors, resistors, and varactor diodes can overcome the polarization mismatch in dynamic off-body channel due to the variation in radio-propagation environment and human movement by adjusting the bias voltages. The experiment of 2 × 2 MIMO channel capacity was conducted using a three-dimensional spatial fading emulator. The measured and simulated results are in good agreement. Under the considered condition of arm-swinging angle and the XPR, the tunable power ratio range of proposed PD can be realized from −1.25 dB to 20 dB; therefore, it is suitable for radio-frequency control antenna to obtaining high data transmission in future wearable MIMO applications. [ABSTRACT FROM AUTHOR]

In this article, a meander line internal antenna used for wireless terminal is proposed. The current of this antenna is mostly distributed on the antenna radiator itself, rather than on the main board of the wireless device. As a result, the chance of having radio-frequency (RF) interference issues, which usually result in receiver desensitization in wireless radios, can be significantly reduced. The antenna has good radiation performance in the vertical polarization with a low physical profile, compared with the existing antenna designs for typical wireless terminals. The antenna has efficiency similar to the monopole antenna with much less reference/ground plane dependence, achieving lower RF interference, which is demonstrated by the noise coupling measurements in a predefined digital clock - antenna configuration. Furthermore, the mutual coupling (i.e., isolation) between two such antennas is studied and the envelope correlation coefficient between the two antennas is found to be low. A router assembled with the two proposed antennas is tested, and the total isotropic sensitivity is found lower compared with monopole antennas, due to the characteristics of low RF interference and high isolation of the proposed antenna. [ABSTRACT FROM AUTHOR]

A variable-phase-shifter (VPS) architecture with hybrid precoding for mixture use of phase shifters and switches, is proposed for millimeter wave massive multiple-input multiple-output communications. For the VPS architecture, a hybrid precoding design (HPD) scheme, called VPS-HPD, is proposed to optimize the phases according to the channel state information by alternately optimizing the analog precoder and digital precoder. To reduce the computational complexity of the VPS-HPD scheme, a low-complexity HPD scheme for the VPS architecture (VPS-LC-HPD) including alternating optimization in three stages is then proposed, where each stage has a closed-form solution and can be efficiently implemented. To reduce the hardware complexity introduced by the large number of switches, we consider a group-connected VPS architecture and propose a HPD scheme, where the HPD problem is divided into multiple independent subproblems with each subproblem flexibly solved by the VPS-HPD or VPS-LC-HPD scheme. Simulation results verify the effectiveness of the propose schemes and show that the proposed schemes can achieve satisfactory spectral efficiency performance with reduced computational complexity or hardware complexity. [ABSTRACT FROM AUTHOR]

Using a license-free industrial, scientific, and medical (ISM) frequency band as 2.45 GHz, a miniaturized incident angle-insensitive radio frequency (RF) energy harvester design for the Internet-of-Things (IoT) applications is proposed. For incident angle insensitivity of ambient RF energy and a low-profile compact design, the proposed harvester is used to a two-layer printed circuit board (PCB) substrate, which consists of orthogonally deployed antennas with an $LC$ balun, impedance matching, and Dickson charge pump circuits at each layer. The dual-polarized antenna for miniaturized design uses a bowtie-shaped meander dipole antenna having orthogonal arrangement. By measuring the efficiency of each diode stage of the rectifier circuit, a two-stage Dickson charge pump circuit with optimal efficiency was selected, and the dc outputs of the two rectifier circuits are connected in parallel. This rectifier circuit has a conversion efficiency of up to 71.4% and harvests approximately 1.6 times more power than a single-polarized circuit from any incident ambient RF signal. Due to its compact size and improved harvesting power at various incident angles, the proposed energy harvester can act as a power source, such as many kinds of ultralow power consumption systems, such as IoT sensors and medical applications. [ABSTRACT FROM AUTHOR]

Lower efficiencies of power amplifiers and increased path losses at higher frequencies are two reasons why utilizing millimeter-wave frequencies for future wireless communications is challenging. In this article, a high-gain multilens-horn antenna system is presented. The antenna system provides highly effective isotropic radiated power by using only a few high-gain antenna elements, where each element only needs a low transmit power of 26 dBm to reach users at far distances. Moreover, due to channel reciprocity, both down- and uplink benefit from an increased antenna gain. It is shown that a reasonable number of high-gain antennas can provide coverage in a 60° sector with an inter-site distance of 300 m. Based on radio-planning simulations, three horn shapes are identified to provide sufficient coverage. The final optimized dual-polarized multilens-horn design is fabricated and experimentally verified. With a low transmit power at the base station (BS) of 26 dBm, a signal strength of at least −65 dBm is achieved in 98.9% of the sector at 28 GHz under non-line-of-sight conditions for the vertical polarization. For the horizontal polarization, an area of 96.2% is covered. [ABSTRACT FROM AUTHOR]

A millimeter-wave (mm-wave) system architecture with enhanced-gain antenna solution is proposed for fifth-generation (5G) wireless communications. To enhance the antenna gain, the surface wave currents are converted to constructive far-field radiations through a novel two-stage metallic and dielectric rings. By applying low-profile dielectric ring, the electric near-field distributions of the grounded quarter-wavelength metallic ring are modified to create radiation apertures with appropriate polarization and orientation. The mutual coupling between transmit/receive antennas and cross polarizations is reduced by 7 and 5 dB, respectively, and the gain is enhanced over 4 dB. Low-loss mm-wave transitions are implemented in a standard FR-4 printed circuit board and in an interposer substrate to integrate the active antenna with the rest of the wireless system on a compact universal serial bus (USB)-interface platform. To evaluate the interposer package and define the output power of the embedded 60-GHz RF IC, an evaluation module with a standard WR-15 waveguide interface is designed and implemented. A prototype of the architecture, which is entirely realized using low-cost organic substrates, is presented and evaluated by experiments. Excellent correlations are achieved between simulations and measurements, demonstrating more than 25 dBm of equivalent isotropically radiated power (EIRP) over the unlicensed 60-GHz frequency band. [ABSTRACT FROM AUTHOR]

This article presents a cost-effective millimeter-wave (W-band) cavity-backed slot antenna-in-package (AiP) built using commercially available integrated passive device (IPD), printed circuit board (PCB), and flip-chip assembly technologies. An IPD-PCB resonant cavity is formed by flipping an IPD chip on a PCB substrate with surrounding solder bumps to provide radiation through a square slot, which is patterned on one of the IPD metal layers. Moreover, a substrate-integrated waveguide (SIW) cavity under the IPD-PCB cavity and a wideband-grounded coplanar waveguide (GCPW) are built in the PCB substrate for the transition between a radio frequency front-end (RFFE) and the IPD-PCB cavity. The fabricated AiP achieves a 3.75-dBi gain at 94.5 GHz (TE210 mode) and a 10-dB impedance bandwidth of 1.9 GHz with a 3.61 mm2 IPD chip area. [ABSTRACT FROM AUTHOR]

Load mismatch often occurs in radio frequency (RF) power amplifiers (PAs) in handset, which can complicate the nonlinear behavior of the transmitter, particularly when the voltage standing wave ratio (VSWR) is high. To effectively linearize handset PAs with load mismatch, in this article, we propose a low-complexity digital predistortion (DPD) model. After carefully analyzing the characteristics of handset transmitters with standing waves, we extend the magnitude-selective affine (MSA) function to construct nonlinear modeling terms in the DPD model. The new terms can effectively compensate for the distortion caused by load mismatch. Experimental results demonstrate that the proposed extended MSA (EMSA) model not only improves the modeling performance but also lowers the hardware complexity considerably, compared with the conventional models. The new model can thus serve as a good candidate for DPD deployment in 5G handset transmitters, where nonlinear behavior is more complicated and hardware resource is highly constrained. [ABSTRACT FROM AUTHOR]

In this paper, we investigate an intelligent reflecting surface (IRS) aided millimeter wave (mmWave) communication system using hybrid precoding of subarray-connected structure at the base station. To maximize the received signal power of the user, we jointly optimize the active beamforming at the BS and passive beamforming at the IRS, respectively. Exploiting the rank-one property of mmWave channels, we transform the original problem into an approximate optimization problem and give the closed-form solution of the approximated problem. Simulation results show that our proposed scheme can effectively improve the received signal quality of the user with a high energy efficiency. [ABSTRACT FROM AUTHOR]

An electrical balance duplexer (EBD) supports dual-band TX-RX isolation enabling frequency-division duplexing (FDD) operation at 5-7 GHz for Wi-Fi 6/6E. A programmable balance network in the EBD can balance the antenna impedance ($Z_{\text {ANT}}$) in the TX channel and RX channel independently and simultaneously. An on-chip passive bandstop filter as a part of the balance network is implemented, achieving sub-2-dB passband insertion loss (IL) and >20-dB stopband rejection with 10% frequency spacing. This filter separates two impedance tuners in the balance network and enables the independent tunability at two bands. A comprehensive and rigorous analysis of general LCR two-ports shows the limit of the $s_{21}$ frequency selectivity when built with finite- $Q$ elements. The analysis guides the filter design, which guarantees the maximum frequency selectivity. The analysis is then extended to LCR ${N}$ -ports as the complete analysis. The EBD provides >40-dB isolation in an 80-MHz channel bandwidth in the TX band (5-6 GHz), for any $Z_{\text {ANT}}(f_{\text {TX}})$ within VSWR = 2, and independently in the RX band (6-7 GHz) when $Q_{\text {ANT}}< 4.3$. The EBD is designed for ≤4-dB RX IL and ≤3.8-dB TX IL, and it supports +29-dBm TX output. The EBD is implemented in Towerjazz 65-nm RF silicon-on-insulator (SOI) CMOS technology and occupies an area of 2.3 mm2. [ABSTRACT FROM AUTHOR]

To study the characteristics of radio frequency (RF) power coupling, RF plasma diagnostics, actively cooled tube, and negative ion production, a plasma source was designed as a small-scale test facility. The matching network was carefully designed and successfully applied to the 2-MHz RF plasma source. In this article, the design method of the matching unit will be presented; with the assistance of this matching unit, the RF plasma source was successfully lit and the reflected power was reduced to a very low level. A $V\!\!-\!\!I$ probe was installed in front of or in the rear of the matching network, respectively, to in situ measure the RF voltage, current, and phase angle. The equivalent resistance of the RF plasma source was analyzed in detail. The RF power coupling efficiency was derived in two different methods, and the relationship between the coupling efficiency and the equivalent resistance of the plasma source is verified. At the same time, the equivalent resistance of the plasma source was also collected to optimize the matching unit. [ABSTRACT FROM AUTHOR]

Extremely large-scale multiple-input-multiple-output (XL-MIMO) is promising to meet the high rate requirements for future 6G. To realize efficient precoding, accurate channel state information is essential. Existing channel estimation algorithms with low pilot overhead heavily rely on the channel sparsity in the angular domain, which is achieved by the classical far-field planar-wavefront assumption. However, due to the non-negligible near-field spherical-wavefront property in XL-MIMO, this channel sparsity in the angular domain is not achievable. Therefore, existing far-field channel estimation schemes will suffer from severe performance loss. To address this problem, in this paper, we study the near-field channel estimation by exploiting the polar-domain sparsity. Specifically, unlike the classical angular-domain representation that only considers the angular information, we propose a polar-domain representation, which simultaneously accounts for both the angular and distance information. In this way, the near-field channel also exhibits sparsity in the polar domain, based on which, we propose on-grid and off-grid near-field XL-MIMO channel estimation schemes. Firstly, an on-grid polar-domain simultaneous orthogonal matching pursuit (P-SOMP) algorithm is proposed to efficiently estimate the near-field channel. Furthermore, an off-grid polar-domain simultaneous iterative gridless weighted (P-SIGW) algorithm is proposed to improve the estimation accuracy. Finally, simulations are provided to verify the effectiveness of our schemes. [ABSTRACT FROM AUTHOR]

We present a new low-complexity interference management algorithm that exploits reconfigurable antennas, removes most of the interference signal power in a $K$ -user interference channel, and achieves the promised but unrealized gain of interference alignment in low-to-medium signal-to-noise ratio (SNR) range. The reconfigurable antenna includes several elements, each controlled individually using a radio frequency (RF) switch. The antenna has only one RF chain, resulting in simple hardware design and enabling low-complexity algorithms. We exploit space-time diversity to find two separate combinations of activating or deactivating antenna elements to allow for interference neutralization/elimination. Finding such states is a cumbersome task for which we develop novel, efficient algorithms. We further devise simple encoding and decoding with short precoder length to exploit the benefits of our designs. Our implementation does not rely on channel state information at the transmitters; works at finite signal-to-noise ratios, unlike the typical degrees-of-freedom results; and works in slow-fading environments. We calculate the average achievable rates, the minimum number of elements to reach a specific level of interference suppression, and we provide outage analysis of our technique. [ABSTRACT FROM AUTHOR]

Phased array calibration is typically carried out in plane-wave conditions, which requires expensive far-field setups. This letter presents a novel phased array calibration method based on measured complex array signal, which works in the cost-effective compact testing setup. The basic principle is to employ multiprobe antennas in a short measurement distance such that we can effectively retrieve the array element patterns in the boresight direction based on the recorded complex signals at the multiple probes using the interpolation technique. Both theoretical derivations and measurement validations are provided to demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Substrate-integrated waveguide (SIW) is a modern day (21st century) transmission line that has recently been developed. This technology has introduced new possibilities to the design of efficient circuits and components operating in the radio frequency (RF) and microwave frequency spectrum. Microstrip components are very good for low frequency applications but are ineffective at extreme frequencies, and involve rigorous fabrication concessions in the implementation of RF, microwave, and millimeter-wave components. This is due to wavelengths being short at higher frequencies. Waveguide devices, on the other hand, are ideal for higher frequency systems, but are very costly, hard to fabricate, and challenging to integrate with planar components in the neighborhood. SIW connects the gap that existed between conventional air-filled rectangular waveguide and planar transmission line technologies including the microstrip. This study explores the current advancements and new opportunities in SIW implementation of RF and microwave devices including filters, multiplexers (diplexers and triplexers), power dividers/combiners, antennas, and sensors for modern communication systems. [ABSTRACT FROM AUTHOR]

A number of design challenges are associated with in-body devices, especially ingestible capsules, including selection of operation frequency and antenna design. Operation frequency, miniaturization, gain, and interference with the environment and the internal components of ingestible capsules are all challenging factors. In this work, we design and measure the performance of miniature antennas that can be included in ingestible capsules. The meandering pattern designs are implemented with a 433 MHz center frequency which is within one of the industrial, scientific and medical (ISM) bands. The antenna patterns are rolled into cylinders to reflect their configuration inside a capsule. The effects of different antenna design features, environmental dielectric changes, and the battery locations relative to the antenna traces are explored. We show that the optimized antenna can offer acceptable performance even when the center frequency shifts due to the modulation of the dielectric constant of the media and by the insertion of batteries. Both simulations and measurements provide insight into how the meandering antenna should be designed for the desired frequency that can be expanded to other ingestible and implantable systems.

Most collaborative analog-digital beamforming precoder architectures are commonly deployed for ground-to-underground (G2U) layer system sensing. The architectures are designed based on the target system’s signal channel links in order to leverage its array and multiplexing millimeter wave (mmWave) gains. Such gains includes link quality, energy consumption, delay and packet accuracy. But recent designs have mostly targeted narrowband-based channels and fewer wideband mmWave domains. In this paper, novel sparse-formulated in time-frequency compressed process resource block (PRB) beamforming, to decode-and-forward (DF) packets by joint-relaying it over radio frequency (RF) mmWave is proposed. The model is deployed into an indoor jobshop with remote sensing capability. The routing path’s transmission energy is minimized using an optimized three-way next-hop node selection over empirically characterized MIMO-ultrawideband RFs of dissimilar soil’s volumetric water content (VWC) and burial depths. The novel collaborative time-frequency PRB-based approach is further exploited for estimating the wideband mmWave channels. Results are corroborated by calibrating the system’s minimum transmission energy (MTE), packet-reception-ratio (PRR), link quality indicator (LQI), and end-to-end delay profiles using the latest vector network analyzer (VNA) 8722ES device. [ABSTRACT FROM AUTHOR]