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The occurrence of passive intermodulation (PIM) is inevitable in high-power microwave devices. It interferes with wireless communication and radar systems, resulting in severe deterioration of receiving performance. So far, narrowband PIM analyzers have been realized for different business bands. However, they are incapable of testing wideband systems. To solve this problem, rack-mounted analyzers piled with multiple narrowband analyzers were implemented. While such solutions significantly increase the volume, cost, and power consumption, they cannot provide a wide, continuous bandwidth for PIM localizations. Aiming to solve this problem, we propose a novel solution for realizing ultrawideband PIM analyzers. Based on the principle of cooperative radio frequency and baseband (RF-BB) nulling, we demonstrate an experimental prototype working within a continuous frequency band covering three octaves, exhibiting promising potential in PIM testing applications. [ABSTRACT FROM AUTHOR]

Capable of remote presentations of human heart activities, the Doppler cardiogram (DCG) can be detected by an agile Doppler radar sensor. However, as a recently developed technology, its performance needs sustained improvements for the future practical applications. In this work, a continuous-wave, ${V}$ -band Doppler radar sensor was implemented to improve the discrimination of the receiver, for detecting micrometer-level details of heartbeat motions conducted to the chest skin. Besides, new algorithms were derived to achieve high linearity, to demodulate the Doppler phases and retrieve the fine details of DCG waveforms. Experimental measurements to different subjects showed significantly enhanced robustness and accuracy. The obtained results imply a promising potential of DCGs in wide application scenarios such as medical wisdom and personal healthcare. [ABSTRACT FROM AUTHOR]

Microwave imaging based on the inverse scattering problem has attracted many interests in microwave community. To obtain a satisfactory imaging quality, in most cases, a scattering-free environment is desired. In this work, to provide such an environment, we experimentally demonstrate a compact microwave imager integrated with a board-integrated, multiport vector network analyzer and a miniaturized, cylindrical anechoic chamber dedicatedly designed for inverse scattering-based microwave imaging. Based on the principle of electrically induced transparency and the effective medium equivalence, an inhomogeneous, ultrathin, dual-angle absorbing surface was designed, simulated, and implemented, to simultaneously absorb incident and scattered fields. Experimental imaging verified its effectiveness. The proposed imager would help the promotion of practical imaging, showing the potential in on-side imaging. [ABSTRACT FROM AUTHOR]

Recently, microwave imaging based on randomized illuminations has demonstrated promising potentials. The key to realize such imaging is to implement an imager with a reconfigurable, randomized aperture. In this work, we discuss the theory and the guidelines for implementing such an imager and demonstrate an experimental prototype working in the 24-GHz ISM band. It is realized with a board-integrated, space-fed, 400-element phased array capable of realizing independent 1-bit phase modulations for each element. The fields transmitting through this aperture can be deterministically randomized by simply controlling direct current bias voltages. Assisted by optimized system calibration and standard $\ell _{1}$ -norm regularization, experimental imaging with a notably improved quality can be obtained. The proposed solution can be used in wide applications, especially suitable for the massive personal security screenings in common public, government, and military areas. [ABSTRACT FROM AUTHOR]

Personal devices such as smartphones and tablets are rapidly becoming personal communication, information, and control centers. Apart from multitouch screens, human gestures are considered as a new interactive human–smart device interface. In this work, we propose a noncontact solution to implement hand gesture recognitions for smart devices. It is based on a continuous wave, time-division-multiplexing (TDM), single-input multiple-output (SIMO) Doppler radar sensor that can be realized by slightly modifying existing RF front ends of smart devices, and a machine-learning algorithm to recognize predefined gestures by classifying deterministic Doppler signals. An experimental setup emulating a smartphone-based radar sensor was implemented, and the experimental results verified the robustness and the accuracy of the proposed approach. [ABSTRACT FROM AUTHOR]

To date, 3-D radar imaging of moving objects in a multipath coordinate space close to antennas remains a technical challenge. In this work, we demonstrate the implementation of such 3-D motion imaging based on a continuous-wave, multiple-input multiple-output (MIMO) Doppler radar sensor (DRS). Working with a time-division-multiplex (TDM) scheme, this portable, cost-effective DRS is a bistatic system with one transmitting and one receiving channels, and a radio frequency (RF) switch in each channel toggling between two antennas. Assisted by algorithms specifically derived for the TDM-MIMO architecture, highly sensitive 3-D imaging performed in a multipath indoor environment was experimentally implemented. Numerical simulations and experimental demonstrations verified the effectiveness and accuracy of the proposed approach. [ABSTRACT FROM AUTHOR]

Besides the prohibitive cost, complexity, and baseband scale, the development of millimeter-wave (mmWave) digital-beamforming fifth-generation (5G) frontends is facing a physical-layer challenge of thermal dissipation. This is caused by the massively integrated power amplifiers (PAs) and the extremely compact volume due to millimeter-scale wavelengths. In this work, we theoretically analyze and experimentally demonstrate a reconfigurable planar Fresnel lens, based on which a new-concept mmWave beamforming frontend can be implemented with significantly reduced number of PAs. It overcomes the inherent shortcomings of analog beamforming solutions relying on massive use of power dividers and phase shifters and can replace the convex lens used in the recently proposed hybrid beamforming solution. The implemented Ka-band lens is simple-structured, cost-effective, and easy to control, providing multiple modes for beamspace massive-multiple-input multiple-output (MIMO) communications. Experimental results verified its potential in different mmWave 5G systems such as base stations and point-to-point links. [ABSTRACT FROM AUTHOR]

Most medical instruments invented to measure human heart activities, such as electrocardiograms (ECGs), rely on contact electrodes. This causes discomfort and limits application scenarios. Remote acquisition of $\mu \text{V}$ -level bioelectrical cardiac signals through ECG measurement is theoretically challenging. Based on the analysis of magnetic resonance imaging to the volume change of human hearts, we found that a single radar sensor can be used to remotely detect a Doppler cardiogram (DCG) at a distance up to 1 m, by retrieving Doppler signals induced by combined atrial and ventricular motions conducted to the skin of the back chest. This DCG can provide all the timing information of the P-wave, QRS-waves, and T-wave carried in ECGs. The implemented miniature remote sensor could be an ideal portable instrument for patients, such as burn victims, extending application scenarios to personal healthcare, battlefield rescue, and others. [ABSTRACT FROM AUTHOR]

In electromagnetics, anechoic chambers are used to obtain artificial analogs of free space. Such chambers are electrically large due to the inherent angle-dependent nature of absorbing materials. Existing models of perfectly matched layer (PML) imply that angle-independent reflectionless absorptions can hardly be achieved without involving gain media. In this paper, we mathematically derived an unconventional PML model, based on which the angle-dependent nature of absorbing materials can be manipulated to construct electrically small anechoic chambers. As an example, a semiclosed cylindrical chamber was experimentally implemented with an inhomogeneous artificial surface as thin as 0.012 wavelength. The inner diameter of this chamber is six wavelengths. The spatial voltage standing wave ratio (VSWR) experimentally measured inside this chamber complies with the theoretical analysis and full-wave simulation. With the aid of the dielectric 3-D printing technology, high-performance, microsized anechoic chambers can be constructed for scientific research and technology development in the future. [ABSTRACT FROM AUTHOR]

Recently, microwave imaging under random field illuminations has attracted many interests. This paper theoretically analyzes and experimentally implements a qualitative microwave radar imaging system suitable for 2-D object. It can be customized on the demands of the desired object size and imaging resolution. Based on random illuminations generated by a minimum number of antenna elements, an analytical equation can be used to reconstruct the image of the object in near real time. The simple-structured and cost-effective imaging system can be implemented in microwave, millimeter-wave, and even sub-THz frequency bands, having promising potentials for various microwave imaging applications. [ABSTRACT FROM AUTHOR]

Noncontact detection of human vital signs based on miniaturized Doppler radar systems (DRSs) can be widely used in healthcare and biomedical applications. Although significant progresses have been achieved, a reliable wireless vital signs detection in the presence of large-scale random human body movements remains a technical challenge. In this paper, based on a comprehensive use of the high-dynamic-range radar architecture and linearized Doppler phase demodulation algorithms, we further introduce matched filters to retrieve the respiration and heartbeat spectra completely concealed under the wideband noise floor caused by large-scale body movements. Along with an existing 5.8-GHz system, a single-board integrated DRS operating at 24 GHz was designed to verify the effectiveness of the used architecture and algorithms. Experimental results comply with the theoretical expectation. The obtained results imply the potential to implement practical bioradar systems for human noncontact vital signs detections. [ABSTRACT FROM AUTHOR]

Passive intermodulation (PIM) potentially occurs in all high-power passive microwave devices. It can severely deteriorate the receiving performance of wireless communication and radar systems. To date, the simultaneous localization of multiple PIM sources in practical microwave devices is still a technical challenge. In this paper, we indicate that if a reference PIM source can be introduced into the standard PIM testing system, and the amplitude and frequency-dependent phase difference between the reference and the real PIM products can be used to construct a k-space multicarrier signal, PIM localization algorithms for both wideband and narrowband devices can be proposed based on the inverse k-space Fourier transform and inverse optimization of the partial multicarrier signal. Simulation and experimental results validate the effectiveness of the proposed approaches. These methods can solve the difficulty of multipoint PIM localization, and can be widely used in the design, production, testing, and troubleshooting of high-power microwave systems. [ABSTRACT FROM AUTHOR]

This paper presents a printed circuit board realization of a $K$ -band portable frequency-modulated continuous-wave radar transceiver with beamforming array for short-range localization. The transmitter channel of the proposed radar consists of a free-running voltage-controlled oscillator (VCO) and a single patch antenna. A linear frequency-modulated chirp signal is generated by the VCO, which is controlled by an analog “sawtooth” voltage generator. The receiver channel has a four-element linear beamforming array, a six-port circuit, and a baseband circuit. The beam of the array can be continuously steered in a range of ±45° on the $H$ -plane through an array of vector controllers. Each vector controller is capable of simultaneously controlling the phase and the amplitude of the corresponding array element. The design principle of the binary-phase-shift attenuator, the vector controller, and the radar system are discussed. Calibration method of the array is introduced to minimize the errors caused by component variation and fabrication. The radiation patterns of the array with phase-only beam steering and phase-amplitude beamforming are measured and compared, demonstrating the advantage of the beamforming design. System-level experiments showed that the proposed solution is suitable for short-range localization. In addition, experiments with a human subject revealed the capability of the proposed radar system to discriminate a human target from other objects based on the vital-Doppler effect. [ABSTRACT FROM AUTHOR]

The newly discovered “long-term” multicarrier multipactors that can be induced by low-energy electric fields potentially threatens the reliability of spaceborne microwave systems. It also causes a new technical challenge to the prediction of the global occurrence threshold of multicarrier multipactors. In this paper, we propose efficient approaches to rapidly compute the fluctuation of space electrons in the evolving process of a multicarrier multipactor, based on which Monte Carlo approaches are developed to efficiently find the global “worst case” waveforms and the occurrence thresholds for both “single-event” and “long-term” multipactors. The proposed approaches can be used for wideband devices, providing an efficient integrated solution for the multicarrier multipactor analysis for practical high-power microwave devices with inhomogeneous field distribution. [ABSTRACT FROM PUBLISHER]

Wireless localization has become a hot topic in the area of microwave engineering. In this paper, we propose a new approach for short-range wireless localization based on meta-aperture and compressed sensing (CS). Utilizing the plasmonic dispersion of a magnetically uniaxial metamaterial, a pushpin-shaped autocorrelation characteristic of the localization system can be obtained to ensure high localization precision. Compared with existing realizations based on traveling wave excited, randomly arranged subwavelength resonant elements, our method uses a homogeneous aperture for the generation of randomized illumination. Based on the CS, localization can be obtained with much less data and much faster data processing than conventional methods. Simulation and experimental investigation are conducted, and the results demonstrate the effectiveness of the proposed approach. With the simple implementation for fast localization, our method has promising potential in practical applications. [ABSTRACT FROM PUBLISHER]

This paper reviews recent progress of portable short-range noncontact microwave radar systems for motion detection, positioning, and imaging applications. With the continuous advancements of modern semiconductor technologies and embedded computing, many functionalities that could only be achieved by bulky radar systems in the past are now integrated into portable devices with integrated circuit chips and printed circuits boards. These portable solutions are able to provide high motion detection sensitivity, excellent signal-to-noise ratio, and satisfactory range detection capability. Assisted by on-board signal processing algorithms, they can play important roles in various areas, such as health and elderly care, veterinary monitoring, human-computer interaction, structural monitoring, indoor tracking, and wind engineering. This paper reviews some system architectures and practical implementations for typical wireless sensing applications. It also discusses potential future developments for the next-generation portable smart radar systems. [ABSTRACT FROM AUTHOR]

This paper presents a portable radar system for short-range localization, inverse synthetic aperture radar imaging, and vital sign tracking. The proposed sensor incorporates frequency-modulated continuous-wave (FMCW) and interferometry (Doppler) modes, which enable this radar system to obtain both absolute range information and tiny vital signs (i.e., respiration and heartbeat) of human targets. These two different operation modes can be switched through an on-board microcontroller. To simplify the system, the proposed radar utilizes the audio card of a laptop to sample the baseband signal. The FMCW mode of the radar uses operational-amplifier-based circuits to generate an analog sawtooth signal and a reference pulse sequence (RPS). The RPS is locked to the sawtooth signal to obtain coherence for the radar system. For the interferometry mode, a low-intermediate-frequency modulation method is implemented to avoid the slow vital signs from being distorted by the high-pass filter of the audio card. Several experiments were carried out to reveal the capability and distinct operational features of the proposed portable hybrid radar. The experiments also showed that the system can easily detect glass, which is usually difficult to identify for optical-based sensors. In addition, 2-D scanning in a complex environment revealed that the proposed radar was able to differentiate human targets from other objects. Moreover, ISAR images were used to isolate moving human targets from surrounding clutter. Finally, the proposed radar also demonstrated its ability to accurately measure vital signs when a human subject sits still. [ABSTRACT FROM PUBLISHER]

Short-range continuous-wave Doppler radar sensors have been mainly used for noncontact detection of various motions. In this paper, we investigate the feasibility to implement the function of a remote mouse, an input device of a computer, by recognizing human gestures based on a dual-channel Doppler radar sensor. Direct conversion architecture, symmetric subcarrier modulation, and bandpass sampling techniques are used to obtain a cost-effective solution. An arcsine algorithm and a motion imaging algorithm are proposed to linearly reconstruct the hand and finger motions in a 2-D plane from the demodulated Doppler phase shifts. Experimental results verified the effectiveness of the proposed architecture and algorithms. Different from the frequency-domain “micro-Doppler” approach, the proposed remote gesture recognition based on linear motion reconstruction is able to recognize definitive signatures for the corresponding motions, exhibiting promising potential in practical applications of human–computer interaction. [ABSTRACT FROM PUBLISHER]

Noncontact measurements of material parameters have important applications in various fields. In this paper, we propose a noncontact measurement of complex permittivity for electrically small samples at microwave frequencies. Based on Rayleigh approximation, we propose a noncontact measurement approach that can linearly retrieve complex permittivity from the measured impedance change. Using a subwavelength resonance, far-field radiation can be effectively suppressed, and thus the measurement is not notably influenced by unwanted environmental reflection. With the features of simple calibration and noncontact measurement, our method can be widely used in repeated on-site measurements of complex permittivity for small-sized samples at microwave frequencies. [ABSTRACT FROM AUTHOR]

The state-of-the-art phased array-based digital beamforming systems suffer from disadvantages of high power consumption, high cost, and system complexity due to the massive use of transmitter/receiver (T/R) modules and high-speed digital and digital–analog mixed devices. In this paper, we point out that by proposing a new concept “complex domain” radio frequency (RF) frontend, the relatively slowly changed waveform delay information required to accomplish adaptive beamforming can be separated from wideband RF signals, based on which a self-contained beamforming system can be implemented with a low-speed baseband. By introducing vector RF multipliers in the proposed frontend, the amplitude and phase of RF signals can be simultaneously controlled by the real and imaginary parts of complex numbers, such that beamforming algorithms derived in complex domain can be directly applied without any form of transformation. By doing so, the massive use of conventional T/R modules and high-speed baseband devices can be avoided. Theoretical analysis and experimental demonstration based on commercial components have validated the proposed approach. Our method is able to significantly simplify the realization and decrease the cost of wideband digital beamforming systems, and can be widely used in low cost, power efficient beamforming applications. [ABSTRACT FROM AUTHOR]

Terahertz technology is one of the research fronts in the microwave society. Among many technical challenges, achieving high-power terahertz radiation has been attracting many efforts. In this paper, we investigate the possibility of power synthesis at low-end frequencies of the terahertz gap based on discrete sources. We show that by applying precision digital phase control, such a power synthesis can be achieved, overcoming the difficulty of phase alignment at these frequencies. For demonstration, we implement a 110-GHz prototype system employing solid-state impact avalanche and transit time diodes. Using a simulation- and measurement-based design methodology, the impedance matching of the designed cavity is able to be simultaneously obtained at both the RF bias and the 110-GHz frequency without using any absorbing material. Detailed design, simulation, and measurement of the prototype are introduced and the experimental results comply well with analytical expectations. Analysis shows that with increased wide digital bit width, the proposed approach is able to provide sufficient phase control precision, making it possible to be used in the power synthesis applications at low terahertz frequencies. [ABSTRACT FROM AUTHOR]

Coupled electromagnetic resonances have various important applications in microwave engineering. Among them, the newly discovered electromagnetically induced transparency (EIT) has attracted much interest. In this paper, we explore the use of configurable electrically induced transparency to construct a microwave “diffractive medium” whose air-like optimally matched transparency and metal-like optimally reflective opacity can be conveniently switched by a binary dc voltage. Different from EITs based on closely coupled resonances, we use decoupled electrically induced resonances to achieve a low-loss transparency along with an optimal impedance matching with air. Satisfying the condition of a lossless perfectly matched layer, such transparency and opacity can be obtained in a wide range of incident angles. We validate our approach by experimentally demonstrating two electrically controlled steering antennas based on diffractive grating effect, both with thin planar apertures and binary digital control circuits. The proposed approach can be extended to higher frequencies, achieving configurable diffractive devices at millimeter-wave and terahertz frequencies. [ABSTRACT FROM PUBLISHER]

Miniaturized Doppler radar sensor (DRS) for noncontact motion detection is a hot topic in the microwave community. Previously, small-scale physiological signals such as human respiration and heartbeat rates are the primary interest of study. In this paper, we propose a comprehensive approach that can be used to improve the demodulation linearity of microwave DRSs, such that detailed time-domain motion information ranging from micro-scale to large scale can be accurately reconstructed. Experiments show that based on a digital-IF receiver architecture, dynamic dc offset tracking, and the extended differentiate and cross-multiply arctangent algorithm, the displacement and velocity of both micrometer-scale vibration of a tuning fork and meter-scale human walking can be accurately recovered. Our work confirms that substantial time-domain motion information is carried by the signals backscattered from moving objects. Retrieval of such information using DRSs can be potentially used in a wide range of healthcare and biomedical applications, such as motion pattern recognition and bio-signal measurements. [ABSTRACT FROM AUTHOR]

In this paper, we propose a microwave energy reception approach for space solar power satellite applications based on the concept of artificial perfectly matched layer. By embedding rectifying diodes into well-designed metamaterial cells, the obtained rectifying surface simultaneously exhibits a nearly perfect impedance matching to the air and the rectifying circuits, and a strong impedance mismatching to the air at harmonic frequencies, leading to a simple structure that can be implemented using commercial multi-layer printed circuit board technology. The fabricated sample shows that, with a thickness of nearly 1/40 of the free-space wavelength, this rectifying surface has an experimental power absorption rate of 99.92% under normal incident power density of 0.1 \mW/cm^2, and a 56.16-dB suppression to the second-order harmonic. The measured rectifying efficiency complies well with the theoretical expectation. By introducing an additional layer of passive power combiner network, the proposed approach can also be used in applications of harvesting weak ambient RF and microwave energy. We expect a wide range of applications to emerge from this novel concept in the future. [ABSTRACT FROM AUTHOR]

Flicker noise and dc offset have strong impacts on the performance of zero-IF receivers. In this paper, we propose a generalized approach to eliminate such impacts on zero-IF architecture pulse compression radars (PCRs). The conventional low-pass filter in a zero-IF receiver is replaced with a predefined filter (PDF) to filter out the dc and low-frequency components from the demodulated radar signal. Simulation and experimental results demonstrate that, based on the prior knowledge of the radar signal and the PDF, the performance degradation, due to the complete filtering of the low-frequency components, can be recovered through a newly deduced compensation equation. Our approach can be widely used in the implementation of highly integrated high-performance PCR systems in the future. [ABSTRACT FROM AUTHOR]

The measurement of complex permittivity of materials is of significant importance in scientific research and industrial applications. In this paper, we introduce the strongly coupled magnetic resonance previously used in high-efficiency midrange wireless power transfer to the noncontact measurement of the complex permittivity of materials. Theoretical analysis, full-wave simulation, and experimental measurements will show that the introduced high- Q-factor resonance can be used to effectively improve both the measurement sensitivity and the impedance matching to the instrument, avoiding the major weaknesses of the traditional magnetic induction method. The measurement setup can be easily calibrated for both solid and liquid materials and can provide the flexibility in noncontact measurement at a variable distance. We expect a wide range of applications to emerge from this novel approach. [ABSTRACT FROM PUBLISHER]

This paper presents an experimental investigation that demonstrates the possibility of 1-D imaging of human cardiac motion using a microwave Doppler sensor. Compared with the previous works that primarily monitored human respiration and heartbeat rates, the reconstruction of cardiac motion in this work will provide more information for time-domain clinical diagnosis. To fully recover the motion information from the backscattered microwave signal, an instrument-based digital-IF Doppler radar sensor employing a dc-offset removal and an extended differentiate and cross-multiply algorithm was used. A series of experiments were performed to investigate the effectiveness of the cardiac imaging from different orientations of a subject. Analysis on the experimental results indicates that in addition to the respiration and heartbeat rates, a 1-D time-domain cardiac motion that fits well with the known physiological description can be obtained. Our work reveals that substantial cardiac activity information is carried by the Doppler shifts of the backscattered microwave reflected from a human chest. The information can be reconstructed by properly designed hardware and algorithms. The possibility of noncontact cardiac imaging would have a great potential in clinical diagnosis and treatment of human heart diseases. [ABSTRACT FROM PUBLISHER]

In this paper, we experimentally realized a steering antenna using a type of active metamaterial with tunable refractive index. The metamaterial is realized by periodically printed subwavelength metallic resonant patterns with embedded microwave varactors. The effective refractive index can be controlled by low direct-current (dc) bias voltage applied to the varactors. In-phase electromagnetic waves transmitting in different zones of such metamaterial slab experience different phase delays, and, consequently, the output direction of the transmitted wave can be steered with progressive phase shift along the interface. This antenna has a simple structure, is very easy to configure the beam direction, and has a low cost. Compared with conventional phased-array antennas, the radome approach has more flexibility to operate with different feeding antennas for various applications. [ABSTRACT FROM PUBLISHER]

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