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A 4-channel code-multiplexed digital receiver is presented for multiple-input-multiple-output (MIMO) applications targeting 5G millimeter-wave (mm-Wave) communications. The receiver employs a code-multiplexing (CM) topology where multiple channels are encoded with unique orthogonal Walsh-Hadamard codes and multiplexed into a single-channel for digitization. This approach overcomes the bottleneck of hardware complexity, cost, and power consumption in traditional multiplexing topologies by employing a single wideband analog-to-digital converter (ADC) to serve several channels. The article presents an end-to-end testbed to demonstrate the effectiveness of the proposed Code-Multiplexed Digital Receiver (CMDR) that consists of 1) ultrawideband (UWB) tightly-coupled dipole array (TCDA), 2) a custom-designed encoder circuit board (ECB), and 3) a Radio-Frequency System-on-Chip (RFSoC) field-programmable gate array (FPGA) for encoding and decoding. The code sequences were generated at a maximum clock frequency of 400 MHz. Extensive experimental measurements were performed and test results were validated using performance metrics such as normalized mean square error (NMSE) and adjacent channel interference (ACI). Test results showed ACI of $>$20 dB, NMSE = -24.592 dB and little or no degradation in signal-to-noise ratio (SNR). To the best of our knowledge, this is the highest clock frequency and ACI value for hardware validation of channel multiplexing scheme reported in the literature.

Spectrum scarcity for wireless communication and data transfers has prompted interest to analyze interference among wireless links. As more bandwidth is utilized for communication links, there is a growing need to mitigate interference. By exploiting code division multiplexing (CDM) across an ultra-wideband (UWB) spectrum, it is possible to combat dynamic interference scenarios, including spot, sweep, barrage, or base interference. With this in mind, this paper presents a more generalized theoretical analysis of bit error rates (BER) in presence of dynamic interference scenarios. The presented approach is validated using an ultra-wideband secure communication link over a large contiguous 1.3 GHz bandwidth with direct digital conversion. To our knowledge, this is the first-ever validation for such UWB channel. As part of our assessment, this UWB system uses direct RF sampling of the received and transmitted signals. Overall, for the first time, code spreading across a 1.3 GHz band demonstrated up to 1000-fold interference mitigation. This new capability was evaluated experimentally using a custom wireless BER test bench.

A novel approach is presented and demonstrated for integrating a wideband vertically fed antenna array at Millimeter-Wave (mm-Wave) frequencies. Specifically, a novel cost-effective Antenna-In-Package (AiP) fabrication approach is presented, then a prototype operating from 55 GHz to 64 GHz is built while predicted performance is verified via measurements. The presented approach relies on separating fabrication into 1) the array and 2) the back-end feeding board. Assembly of the various components is achieved through several precision microfabrication steps utilizing a Ball-Grid-Array (BGA) technology with thermally stable conductive solder paste. The paper describes the realization of the final AiP aperture in terms of design tolerances, laboratory equipment, component placement and alignment, curing time duration and temperature ranges, and prototype on-the-go testing.

An ultra-wideband (UWB) phased array is presented operating from UHF to C-bands (0.14 to 5.85 GHz). This new design employs an integrated, compact tapered spline balun in conjunction with a triple-layer semi-resistive frequency selective surface (FSS) network. Notably, the integration of the meandered balun within a dual-polarized tightly-coupled dipole array (TCDA) allowed for a contiguous bandwidth of 40:1 with VSWR < 2.5 at broadside. Moreover, the use of the semi-resistive FSS superstrate enabled scanning down to ±60° with a VSWR < 3.5 in the E-, H-, and D-planes. The overall array thickness is $\lambda _{Low} / 17.93$ (where $\lambda _{Low}$ is the wavelength at the lowest frequency of operation, 140 MHz), and the array’s radiation efficiency is 75% on average across the operational bandwidth. That is, compared to previous UWB TCDAs with contiguous operational bandwidths of 40:1 or greater, the proposed array is 30% thinner and has equally good or better efficiency. This paper presents the analysis and design of the dipole array, with emphasis on the spline balun. An $8\times8$ prototype ( $12\times12$ element FSS network footprint) was fabricated and measured to validate the performance of a finite array.

Abstract A novel antenna design technique is proposed, which offers bandwidth enhancement up to the limits defined by element radiation efficiency. The employed technique is referred as frequency pulling (FP) as it mimics the ‘insertion loss design methodology of band‐pass filters’. This is essentially a wideband matching approach pushing the antenna efficiency to the limits set up by radiation efficiency. There are three options towards this trend: (i) first to enhance a single element bandwidth (compact element) exploiting its possibly multiple symmetrical feeding points as distinct resonator ports, (ii) frequency pulled array as to design a small antenna array (less than about 10 elements) where each element acts as a resonator and (iii) second order frequency‐pulled array as to build a small array using compact elements of category (i). Similar to the band‐pass filter design, all antennas or distinct‐port circuits resonate at the same resonant frequency when isolated, cascading two or more of them; FP yields to multiple‐overlapping successive resonances in their overall response. Although the proposed technique is general within this first effort, it is applied to simple patch antenna elements exhibiting multiple symmetrical feeding points, namely two—for rectangular, four—for square and five—for pentagonal. The third option is applied to an array of three compact 4‐feeding point square elements offering triple bandwidth with respect to the already wideband single element. However, this is achieved at the expense of a significant beam squint. Thus, in general, these wideband compact elements should be used within a classical array design. Further bandwidth enhancement using FP to antenna elements with inherent multiple resonances as patches with slots or truncated edges constitutes our next task. Their inherent wider bandwidth in radiation efficiency is expected to allow multiply higher bandwidths when exploited with our FP technique.

To overcome the limited payload dedicated to onboard antennas for CubeSat/SmallSat applications, this paper presents a novel, foldable, dual-polarized Tightly Coupled Dipole Array (TCDA). In comparison to previous vertical feeds used for TCDAs, this array integrates a planar microstrip balun feed to enable foldability. The proposed array attains 5.4:1 (0.6-3.20 GHz) impedance bandwidth with VSWR < 3 at broadside and scans down to 45° at all azimuth planes. Notably, a substrate-integrated in-plane folding mechanism based on Lamina Emergent Torsion (LET) joints is employed to achieve foldability coupled with a simple fabrication process. The average simulated radiation efficiency was 95% across the band. A $5\,\, \times8$ prototype array was fabricated and tested to verify the finite array’s foldability, bandwidth, and gain performance. This prototype achieves 80% one-dimensional size reduction, yielding 60% overall volume reduction. Consequently, the array prototype can be folded and stowed in a compact volume of 1.4U (25 cm $ \times7.2$ cm $ \times9$ cm).

Wireless passive neural recording systems integrate sensory electrophysiological interfaces with a backscattering-based telemetry system. Despite the circuit simplicity and miniaturization with this topology, the high electrode–tissue impedance creates a major barrier to achieving high signal sensitivity and low telemetry power. In this paper, buffered impedance is utilized to address this limitation. The resulting passive telemetry-based wireless neural recording is implemented with thin flexible packages. Thus, the paper reports neural recording implants and integrator systems with three improved features: (1) passive high impedance matching with a simple buffer circuit, (2) a bypass capacitor to route the high frequency and improve mixer performance, and (3) system packaging with an integrated, flexible, biocompatible patch to capture the neural signal. The patch consists of a U-slot dual-band patch antenna that receives the transmitted power from the interrogator and backscatters the modulated carrier power at a different frequency. When the incoming power was 5–10 dBm, the neurosensor could communicate with the interrogator at a maximum distance of 5 cm. A biosignal as low as 80 µV peak was detected at the receiver.

Within the context of modern digital phased array (DPA) radios, differential Radio Frequency (RF) front-ends provide much greater immunity to noise and distortion by suppressing second order harmonics. Recent advancements in differential RF front-ends offer high dynamic range, high linearity, and low noise in the transceiver chain. However, a major roadblock to fully differential systems is the presence of common-mode resonances. In this article, we present a novel wideband differential feed for dipole arrays that overcomes this bottleneck. Our dipole array is developed for the L-S band (viz. 1.05 GHz to 3.2 GHz) with emphasis on dual-linear polarization and resonance-free scanning to low angles. The novelty of this article is the Balanced Wideband Impedance Transformer (BWIT) feed that mitigates common-mode currents across the entire band while scanning to low angles. Array simulations are verified with measurements of an $8\times 8$ prototype in a volume of 384mmx384mmx57 mm. The array achieves resonance-free scanning across a 3:1 impedance bandwidth with VSWR

An origami-based Tightly Coupled Dipole Array (TCDA) is proposed for small satellite applications. The array is formed by a two-layered structure using rigid and flexible substrates to enable accordion-like folding. The proposed TCDA operates across 0.4-2.4 GHz with VSWR < 3 at broadside and across 0.6-2.4 GHz with VSWR < 3 when scanning down to 45° in the E-, D-, and H-plane. An 8 $\times $ 8 prototype was fabricated using Kapton Polyimide and FR4 and tested to verify the bandwidth and gain of the origami array. The fabricated prototype was demonstrated to be packable, low-profile, and lightweight (only 1.1kg). Notably, when packed, the array has a one-dimensional size reduction of 75%. As will be discussed, the packing compression is made possible by eliminating vertical PCB boards and incorporating the balun feeds within the dipole layer. To our knowledge, this is one of the first foldable, low profile, and low-scanning ultra-wideband arrays in the literature.

To date, CubeSat radars and imagers have been limited to operations beyond S-band due to the challenges associated with the design of wideband, compact, low-frequency antennas. Concurrently, the frequencies at VHF/UHF bands can image through clouds and foliage, and are very attractive for ice, water and biomass sensing. There is thus a need to develop wideband antennas that can operate at VHF/UHF bands which are low cost, light-weight, and packable. In this paper, we present a CubeSat deployable Tightly Coupled Dipole Array (TCDA) that achieves VSWR

Reconfigurable and deployable antenna arrays are required communications and remote sensing onboard small satellites/CubeSats. There is also a need for packing before launch due to the limited payload space for exo-atmospheric deployments. In this context, origami-based aperture packing and unpacking is relevant and attractive for low-volume storage. However, after several physical morphing cycles, origami apertures may not maintain the performance exhibited by fixed apertures. That is, origami-based antenna arrays are likely to suffer from reduced gain and polarization purity. Therefore, physical effects caused by origami folding should be analytically incorporated into the design. This paper proposes a statistical analysis to study the gain performance of deployable ultra-wideband Miura-ori apertures by including geometrical errors inherent to a partially deployed state. A closed-form expression of the gain degradation is derived and verified using full-wave simulations.

Modern phased arrays require large instantaneous bandwidths, wide fields of view, and low profiles to conduct multiple functions. Many of these phased arrays rely on emerging high speed ADCs and advanced balanced transceivers. The benefits of balanced front-ends include improved linearity, dynamic range, isolation, and noise resilience. The application of a differential phased array in such a system removes extraneous losses caused by baluns, though the issue of feed-borne E-plane scan resonances must be considered. We address the E-plane scan resonance issue through an improved Balanced Wideband Impedance Transformer (BWIT) feed for the ultra-wideband (UWB) Tightly Coupled Dipole Array (TCDA). This BWIT feed has already demonstrated mitigated common-modes over a 3:1 bandwidth ratio while scanning to low angles. Here, our differential TCDA (D-TCDA) is developed for the L-C band (viz. 1.0 GHz to 6.1 GHz) with emphasis on resonance-free wide-angle scanning. Rigorous EM model and circuit analysis is included to verify the BWIT performance. Under a VSWR < 3 definition, the improved array achieves a 6:1 impedance bandwidth ratio (BWR) with resonance-free scanning in all planes. An exception is the H-plane scanning at 60° where the VSWR < 4. Array simulations are verified with measurements for an $8\times 8$ single-polarized prototype.

Existing beamforming arrays suffer from the size and cost of the RF front-end and digital back-end components. In this paper, a novel hybrid beamforming configuration for wideband receivers is introduced. The design replaces phase shifters and local oscillators (LO) with cross mixing antenna elements to maximize diversity gain. In this paper, an analytical model of the cross mixing beamformer (CMB) is first presented. Simulations are carried out showing that a maximum diversity gain can be achieved with the CMB approach. Two prototypes were implemented using $2\times 1$ and $4\times 2$ element arrays and tested at 2.31 GHz. Measured results show that CMB achieves coherent signal combining and can preserve the phase delay information needed for hybrid beamforming setups.

A new approach to enhance the bandwidth of Tightly Coupled Dipole Arrays (TCDA) is presented. The new design achieves the integration of a semi-resistive Frequency Selective Surface network (FSS) composed of a non-resistive low-pass FSS and two resistive band-stop FSSs. The integration of this FSS network within a dual-polarized Tightly Coupled Dipole Array (TCDA) led to an increased impedance bandwidth of 28:1 from 0.20GHz to 5.6GHz. Notably, the use of an FSS superstrate allowed for scanning down to 60° at VSWR

With the growth of wireless data traffic, additional spectrum is required to meet consumer demands. Consequently, innovative approaches are needed for efficient management of the available limited spectrum. To double the achievable spectral efficiency, a transceiver can be designed to receive and transmit signals simultaneously (STAR) across the same frequency band. However, due to the coupling of the high power transmitted signal into the collocated receiver, the receiver's performance is degraded. For successful STAR realization, the coupled high-power transmit (Tx) signal should be suppressed by 100-120 dB over the entire operational bandwidth. So far, most STAR implementations are narrowband, and not useful for ultra wideband (UWB) communications. In this paper, we present a review of novel approaches employed to achieve improved cancellation across wide bandwidths in RF and propagation domains. Both single and multi-antenna systems are considered. Measurements show an average cancellation of 50 dB using two stages of RF signal cancellation.

The development of reconfigurable antennas compatible with a CubeSat form factor can aid several space missions. Often, satellite missions require multiple wireless links with the same radio, but the design of such antennas is challenging due to the mechanical constraints and the limited power aboard a CubeSat. In this article, we present a unique reconfigurable antenna concept enabled by adhesive polyimide tapes. The presented antenna can switch from a conventional patch to a monopole-like antenna with minimal actuation complexity. This reconfiguration provides choices for polarization, pattern, and gain without use of active components for size, cost and power consumption reductions. The frequency of operation is S-band (2.4 GHz), and the antenna achieves $S_{11}

Obtaining the exact position of accumulated calcium on the inner walls of coronary arteries is critical for successful angioplasty procedures. For the first time to our knowledge, in this work, we present a high accuracy imaging of the inner coronary artery using microwaves for precise calcium identification. Specifically, a cylindrical catheter radiating microwave signals is designed. The catheter has multiple dipole-like antennas placed around it to enable a 360° field-of-view around the catheter. In addition, to resolve image ambiguity, a metallic rod is inserted along the axis of the plastic catheter. The reconstructed images using data obtained from simulations show successful detection and 3D localization of the accumulated calcium on the inner walls of the coronary artery in the presence of blood flow. Considering the space and shape limitations, and the highly lossy biological tissue environment, the presented imaging approach is promising and offers a potential solution for accurate localization of coronary atherosclerosis during angioplasty or other related procedures.

We present a novel ultra-wideband low-cost transmit/receive architecture with reduced power and hardware requirements, which includes interference mitigation capabilities and accommodates multiple users. Specifically, a novel encoding approach is introduced to multiplex signals across large bandwidths and achieve secure communication and interference mitigation through spread signals. In this paper, a detailed description of the proposed high data rate system architecture is presented. Simulations are also provided in the presence of high-power interferers with eight concurrent users. To assess the performance and impact of the encoding/decoding process, a noise analysis was conducted for the transmitter and receiver chains. Specifically, bit error rate curves are generated at different stages of the system. The measurements show interference margin ratios improvement by at least 16 dB.

With the growing demand for spectrum utilization, the concept of full duplex transceivers has become attractive. These simultaneous transmit and receive systems (STAR) are attractive as they double bandwidth utilization. To realize STAR, we must suppress self-interference (SI) between transmit and receive antennas. Doing this across a wide bandwidth presents an even greater challenge. Further, for antenna arrays, SI can be higher due to mutual coupling among neighboring elements. In this paper, we achieve 100 dB isolation for practical STAR, across a large 500 MHz bandwidth. Specifically, four stages of the self-interference cancellation are proposed. In addition to high isolation cross-polarized array elements, we employ multi-tap filters at the transceiver RF front-end for interference cancellation. These filters emulate direct SI coupling using frequency-domain optimization techniques. Measured results show an average of 25 dB filter cancellation is possible across 500 MHz.

Recently, a novel class of on-site coding receivers was proposed. The architecture is suitable for digital beamforming in addition to offering multiple-input multiple-output capabilities. Essential to its realization is a code division multiplexing technique aggregating multiple signal paths at the analog front end into a single analog-to-digital converter. As a result, a significant hardware reduction and a higher power efficiency are achieved when compared with the conventional digital beamforming techniques. In this paper, we examine the system's performance with different types of spreading codes, both orthogonal and nonorthogonal, namely Walsh-Hadamard and Gold codes. Bit error rate calculations show that Walsh-Hadamard codes outperform Gold codes in achieving higher dynamic range with less signal-to-noise ratio degradation, assuming a perfectly synchronous system.

Recently, we proposed a code-modulated multipath receiver front-end to reduce the number of analog-to-digital converters (ADCs) behind antenna elements and realize significant area, cost, and power reduction. More specifically, code division multiplexing was implemented at the analog front-end for path combining into a single ADC. At the digital baseband, the reverse process was applied to recover signals pertaining to each path. Such front-ends are suitable for spatial diversity and multiplexing and for beamforming. For the latter, it is important to accurately predict the angle of arrival. That is, it is important to faithfully recover the phase difference between adjacent signal paths at the digital baseband. In this paper, the impact of on-site coding on phase error is examined for a two-path receiver using orthogonal Walsh-Hadamard codes of length 8. Simulations show that the relative phase difference, Δφ, between signal paths can be faithfully recovered at the digital baseband. Hardware implementation of a two-path receiver with on-site coding was realized and three different phase measurements were conducted, namely, Δφ = 27°, 40°, and 45°. These measurements confirmed that upon signal and code synchronization, the phases were faithfully recovered with minimal degradation.

Indium tin oxide (ITO) is one of the most commonly used optically transparent conductors in applications, such as electro-optic antennas, displays, and optical coatings. However, their RF frequency-dependent electrical properties have not been reported in the literature. In this paper, we present measurements of the electrical properties (permittivity and conductivity) of ITO films in the 0.1-20-GHz frequency range. Measurements were carried out using an in-house open-ended coaxial probe technique employing a one-port reflection coefficient. As usual, calibration and numerical post-processing is needed to extract the electrical properties of the ITO film placed on a 0.5-mm-thick Eagle glass. The measured conductivity was on the order of 105 throughout the frequency range, and the real and imaginary parts of the permittivity were on the order of 106 at lower frequencies and 105 at higher frequencies.

Typical radio frequency (RF) digital beamformers can be highly complex. In addition to a suitable antenna array, they require numerous receiver chains, demodulators, data converter arrays, and digital signal processors. To recover and reconstruct the received signal, synchronization is required since the analog-to-digital converters (ADCs), digital-to-analog converters (DACs), field programmable gate arrays (FPGAs), and local oscillators are all clocked at different frequencies. In this article, we present a clock synchronization topology for a multichannel on-site coding receiver (OSCR) using the FPGA as a master clock to drive all RF blocks. This approach reduces synchronization errors by a factor of 8, when compared to conventional digital beamformer.

Metamaterial structures provide innovative ways to manipulate electromagnetic wave responses to realize new applications. This paper presents a conformal wideband metamaterial array that achieves as much as 10 : 1 continuous bandwidth. This was done by using interelement coupling to concurrently achieve significant wave slow-down and cancel the inductance stemming from the ground plane. The corresponding equivalent circuit of the resulting array is the same as that of classic metamaterial structures. In this paper, we present a wideband Marchand-type balun with validation measurements demonstrating the metamaterial (MTM) array’s bandwidth from 280 MHz to 2800 MHz. Bandwidth reconfiguration of this class of array is then demonstrated achieving a variety of band-pass or band-rejection responses within its original bandwidth. In contrast with previous bandwidth and frequency response reconfigurations, our approach does not change the aperture’s or ground plane’s geometry, nor does it introduce external filtering structures. Instead, the new responses are realized by making simple circuit changes into the balanced feed integrated with the wideband MTM array. A variety of circuit changes can be employed using MEMS switches or variable lumped loads within the feed and 5 example band-pass and band-rejection responses are presented. These demonstrate the potential of the MTM array’s reconfiguration to address a variety of responses.

A model-based imaging framework is applied to correct the target distortion seen in microwave imaging through a periodic wall structure. In addition to propagation delays caused by the wall, it is shown that the structural periodicity induces high-order space harmonics leading to other ghost artifacts in the through-wall image. To overcome these distortions, the periodic layer Green’s function is incorporated into the forward model. A linear back-projection solution and a nonlinear minimization solution are applied to solve the inverse problem. The model-based back-projection image corrects the distortion and has higher resolution compared with free space due to the inclusion of multipath propagation through the periodic wall, but considerable sidelobe clutter is present. The nonlinear solution not only corrects target distortion without clutter but also reduces the solution to a sparse form.