Showing papers in "IEEE Transactions on Microwave Theory and Techniques in 2008"

Design of a Dual-Band Implantable Antenna and Development of Skin Mimicking Gels for Continuous Glucose Monitoring

Abstract: In this study, we present a small-size dual medical implant communications service (MICS) (402-405 MHz) and industrial, scientific, and medical (ISM) (2.4-2.48 GHz) band implantable antenna for continuous glucose-monitoring applications. The antenna is optimized for dual-band operation by combining an in-house finite-element boundary integral electromagnetic simulation code and particle swarm optimization algorithm. In order to test the designed antenna in vitro, gels mimicking the electrical properties of human skin are also developed. The optimized antenna is fabricated and measured in the gel. The simulated and measured bandwidths are found to be 20.4% MICS, 4.2% ISM, and 35.3% MICS, and 7.1% ISM, respectively. Although we have emphasized continuous glucose monitoring throughout this paper, the antenna and skin mimicking gels presented here can be used for many other wireless telemetry applications.

Random Body Movement Cancellation in Doppler Radar Vital Sign Detection

Abstract: The complex signal demodulation and the arctangent demodulation are studied for random body movement cancellation in quadrature Doppler radar noncontact vital sign detection. This technique can be used in sleep apnea monitor, lie detector, and baby monitor to eliminate the false alarm caused by random body movement. It is shown that if the dc offset of the baseband signal is accurately calibrated, both demodulation techniques can be used for random body movement cancellation. While the complex signal demodulation is less likely to be affected by a dc offset, the arctangent demodulation has the advantage of eliminating harmonic and intermodulation interference at high carrier frequencies. When the dc offset cannot be accurately calibrated, the complex signal demodulation is more favorable. Ray-tracing model is used to show the effects of constellation deformation and optimum/null detection ambiguity caused by the phase offset due to finite antenna directivity. Experiments have been performed using 4-7 GHz radar to verify the theory.

Penetrating 3-D Imaging at 4- and 25-m Range Using a Submillimeter-Wave Radar

Abstract: We show experimentally that a high-resolution imaging radar operating at 576-605 GHz is capable of detecting weapons concealed by clothing at standoff ranges of 4-25 m. We also demonstrate the critical advantage of 3-D image reconstruction for visualizing hidden objects using active-illumination coherent terahertz imaging. The present system can image a torso with <1 cm resolution at 4 m standoff in about five minutes. Greater standoff distances and much higher frame rates should be achievable by capitalizing on the bandwidth, output power, and compactness of solid state Schottky-diode based terahertz mixers and multiplied sources.

Substrate Integrated Waveguide Cross-Coupled Filter With Negative Coupling Structure

Abstract: Substrate integrated waveguide (SIW) technology provides an attractive solution to the integration of planar and nonplanar circuits by using a planar circuit fabrication process. However, it is usually difficult to implement the negative coupling structure required for the design of compact canonical folded elliptic or quasi-elliptic cross-coupled bandpass filter on the basis of a single-layer SIW. In this paper, a special planar negative coupling scheme including a magnetic coupling post-wall iris and a balanced microstrip line with a pair of metallic via-holes is studied in detail. Two -band fourth-degree cross-coupled bandpass filters without and with source-load coupling using the negative coupling structures are then proposed and designed. The two novel SIW filters having the same center frequency of 20.5 GHz and respective passband width of 700 and 800 MHz are implemented on a single-layer Rogers RT/Duroid 5880 substrate with thickness of 0.508 mm. Measured results of those filters, which exhibit a high selectivity, and a minimum in-band insertion loss of approximately 0.9 and 1.0 dB, respectively, agree well with simulated results.

A Compact Open-Loop Filter With Mixed Electric and Magnetic Coupling

Abstract: A novel cascaded microstrip open-loop resonator filter with controllable electric and magnetic mixed coupling is presented to have one or more transmission zeros. For two coupled resonators, the coupled sides with the maximum magnetic field are combined with their open gaps with the maximum electric field, creating the electric and magnetic mixed coupling. By adjusting the distances between resonators and the positions of open gaps in a mixed coupled filter, the coupling coefficients can be changed, and controllable transmission zeros can be produced in the lower stopband, upper stopband, or both. Design and fabrication of the proposed second-and fourth-order filters show the advantages: such as a smaller number of resonators, higher rejection level, as well as asymmetrical response; moreover, the location of the transmission zeros can be precisely controlled.

Neural Network Inverse Modeling and Applications to Microwave Filter Design

Abstract: In this paper, systematic neural network modeling techniques are presented for microwave modeling and design using the concept of inverse modeling where the inputs to the inverse model are electrical parameters and outputs are geometrical parameters. Training the neural network inverse model directly may become difficult due to the nonuniqueness of the input-output relationship in the inverse model. We propose a new method to solve such a problem by detecting multivalued solutions in training data. The data containing multivalued solutions are divided into groups according to derivative information using a neural network forward model such that individual groups do not have the problem of multivalued solutions. Multiple inverse models are built based on divided data groups, and are then combined to form a complete model. A comprehensive modeling methodology is proposed, which includes direct inverse modeling, segmentation, derivative division, and model combining techniques. The methodology is applied to waveguide filter modeling and more accurate results are achieved compared to the direct neural network inverse modeling method. Full electromagnetic simulation and measurement results of Ku-band circular waveguide dual-mode pseudoelliptic bandpass filters are presented to demonstrate the efficiency of the proposed neural network inverse modeling methodology.

Open-Loop Digital Predistorter for RF Power Amplifiers Using Dynamic Deviation Reduction-Based Volterra Series

Abstract: In this paper, we propose an efficient open-loop digital predistorter (DPD) derived from the dynamic deviation reduction-based Volterra series that allows compensation for both nonlinear distortion and memory effects induced by RF power amplifiers in wireless transmitters. In this approach, the parameters of the predistorter can be directly extracted from an offline system identification process. This eliminates the usual requirement for a closed-loop real-time parameter adaptation, which dramatically reduces the implementation complexity of the system. It is shown that a further reduction in system complexity can be achieved by applying under-sampling theory in the model extraction and utilizing parameter interpolation in the DPD implementation. Experimental results show that by utilizing this technique with only a small number of parameters, nonlinear distortion induced by the PA can be significantly reduced, as evaluated by both adjacent channel power ratio reduction and normalized root mean square error improvement. A comparison with a memoryless polynomial function based predistorter and an analysis of the impact of decresting are also presented.

Design of Dual-Band Bandpass Filters Using Stub-Loaded Open-Loop Resonators

Abstract: In this paper, open-loop resonators loaded by shunt open stubs are proposed to design compact dual-band bandpass filters with improved out-of-band rejection characteristics. The second passband of the dual-band filter is obtained by tuning higher resonant modes of the open-loop resonator by the stub length and position. A tapped-line input/output feed structure is used for external coupling. Required external coupling is obtained by adjusting the tapping position and dimension of the stub-loaded resonator. A lossless transmission line model is used to determine the resonance properties of the resonator and the external quality factor. Theoretical predictions are verified by the experimental results of three dual-band filters.

Extraordinary Transmission Through Arrays of Electrically Small Holes From a Circuit Theory Perspective

Abstract: Extraordinary optical transmission of light or electromagnetic waves through metal plates periodically perforated with subwavelength holes has been exhaustively analyzed in the last ten years. The study of this phenomenon has attracted the attention of many scientists working in the fields of optics and condensed matter physics. This confluence of scientists has given rise to different theories, some of them controversial. The first theoretical explanation was based on the excitation of surface plasmons along the metal-air interfaces. However, since periodically perforated dielectric (and perfect conductor) slabs also exhibit extraordinary transmission, diffraction by a periodic array of scatterers was later considered as the underlying physical phenomenon. From a microwave engineering point of view, periodic structures exhibiting extraordinary optical transmission are very closely related to frequency-selective surfaces. In this paper, we use simple concepts from the theory of frequency-selective surfaces, waveguides, and transmission lines to explain extraordinary transmission for both thin and thick periodically perforated perfect conductor screens. It will be shown that a simple transmission-line equivalent circuit satisfactorily accounts for extraordinary transmission, explaining all of the details of the observed transmission spectra, and easily gives predictions on many features of the phenomenon. Although the equivalent circuit is developed for perfect conductor screens, its extension to dielectric perforated slabs and/or penetrable conductors at optical frequencies is almost straightforward. Our circuit model also predicts extraordinary transmission in nonperiodic systems for which this phenomenon has not yet been reported.

Investigation of High-Accuracy Indoor 3-D Positioning Using UWB Technology

Abstract: There are many challenges in building an ultra-wideband (UWB) indoor local positioning system for high-accuracy applications. These challenges include reduced accuracy due to multipath interference, sampling rate limitations, tag synchronization, and antenna phase-center variation. Each of these factors must be addressed to achieve millimeter or sub-millimeter accuracy. The developed system architecture is presented where a 300-ps Gaussian pulse modulates an 8-GHz carrier signal and is transmitted through an omni-directional UWB antenna. Receiver-side peak detection, a low-cost subsequential-sampling mixer utilizing a direct digital synthesizer, high fidelity 10-MHz crystals, and Vivaldi phase-center calibration are utilized to mitigate these challenging problems. Synchronized and unsynchronized experimental results validated with a sub-millimeter accurate optical tracking system are presented with a detailed discussion of various system errors.

A High-Efficiency 100-W GaN Three-Way Doherty Amplifier for Base-Station Applications

Abstract: A three-way Doherty 100-W GaN base-station power amplifier at 2.14 GHz is presented. Simple, but accurate design equations for the output power combiner of the amplifier are introduced. Mixed-signal techniques are utilized for uncompromised control of the amplifier stages to optimize efficiency, as well as linearity. The combination of the above techniques resulted in an unprecedented high efficiency over a 12-dB power backoff range, facilitating a record high power-added efficiency for a wideband code division multiple access test signal with high crest factor, while meeting all the spectral requirements for Universal Mobile Telecommunications System base stations.

Full-Wave Modal Dispersion Analysis and Broadside Optimization for a Class of Microstrip CRLH Leaky-Wave Antennas

Abstract: In this paper, a planar microstrip composite right/left-handed leaky-wave antenna is analyzed and designed as an infinite 1-D periodic microstrip leaky-wave antenna. A parametric study, based on a full-wave numerical modal approach that analyzes a unit cell using a periodic layered-medium Green's function, is shown to be an efficient approach to accurately design the structure, completely eliminating open-stopband effects and achieving an almost constant radiation efficiency when the beam is scanned through broadside. Results obtained by the proposed approach are compared with those obtained by means of both an artificial transmission-line analysis and a Bloch-wave analysis, which use the full-wave simulation of a finite-length structure. The balanced condition is interpreted in terms of the behavior of the phase and attenuation constants relevant to the radiating harmonic. Furthermore, it is shown how radiation at broadside is guaranteed by the presence of two radiating elements (one series and one shunt) within the equivalent circuit of the unit cell. The effectiveness of the analysis is demonstrated through the design of a finite-length antenna excited by a source at one end.

Low-Loss Two-Pole Tunable Filters With Three Different Predefined Bandwidth Characteristics

Abstract: Low-loss tunable filters with three different fractional-bandwidth variations were designed and fabricated on epsivr = 2.2, 0.787 mm Duroid substrates for 850-1400-MHz applications. A detailed analysis for realizing predefined bandwidth characteristics is presented, and a design technique to take into account the source and load impedance loading is discussed. It is found that independent electric and magnetic coupling makes it possible to realize three different coupling coefficient variations with the same filter structure. The proposed topology is different from the comb-line design in that all three filters have identical electrical lengths, the same varactors, and the same filter Q values. Three different filters are built using Schottky varactor diodes with a tuning range from ~ 850 to ~ 1400 MHz. The constant fraction-bandwidth filter has a 1-dB bandwidth of 5.4%plusmn0.3% and an insertion loss of 2.88-1.04 dB. The decreasing fractional-bandwidth filter shows a 1-dB bandwidth decrease from 5.2% to 2.9% with an insertion loss of 2.89-1.93 dB (this is effectively a 40-45-MHz constant absolute-bandwidth filter). The increasing fractional-bandwidth filter shows a 1-dB bandwidth increase from 4.3% to 6.5% with an insertion loss of 3.47-1.18 dB. The measured Q of the filters are between 53-152 from ~ 850 to ~ 1400 MHz. The measured third-order intermodulation intercept point ranges from 11.3 to 20.1 dBm depending on the bias voltage. To our knowledge, these planar tunable filters represent state-of-the art insertion-loss performance at this frequency range.

Digital Predistortion for Envelope-Tracking Power Amplifiers Using Decomposed Piecewise Volterra Series

Abstract: Due to dynamic changes of supply voltage, envelope-tracking (ET) power amplifiers (PAs) exhibit very distinct characteristics in different power regions. It is very difficult to compensate the distortion induced by these amplifiers by employing conventional digital predistortion techniques. In this paper, by introducing a new piecewise Volterra model based on a vector threshold decomposition technique, we first set several thresholds in the input power level according to the PA characteristics, and decompose the input complex envelope signal into several sub-signals by using these thresholds. We then process each sub-signal separately by employing the dynamic deviation reduction-based Volterra series, and finally recombine them together to produce the predistorted output. Experimental results show that by using this new decomposed piecewise digital predistorter model, the distinct characteristics of the ET system at different signal power levels can be accurately modeled, and thus, the distortion, including both static nonlinearities and memory effects, caused by the amplifier nonlinear behavior can be effectively compensated.

A Novel Approach to the Design and Implementation of Dual-Band Power Divider

Abstract: This paper presents the design of a new Wilkinson power divider for dual-band application. The proposed circuit also features a simple structure with realistic impedance values, distributed design with reduced parasitic effect (in compared to lumped component), and exact solution (ideal characteristics). Closed-form design equations are derived using the even-and odd-mode formulation. For verification, the measured results of a microstrip power divider operating at 1 and 2 GHz is shown.

Modeling of Conductor, Dielectric, and Radiation Losses in Substrate Integrated Waveguide by the Boundary Integral-Resonant Mode Expansion Method

Abstract: This paper presents the modeling of lossy substrate integrated waveguide interconnects and components by using the boundary integral-resonant mode expansion method. The extension of the numerical technique to account for conductor, dielectric and radiation losses is discussed. Moreover, a systematic investigation of the different contributions of loss and their dependence on some geometrical parameters is performed in the case of interconnects and components, aiming at minimizing the losses. The physical explanation of the different effects is also provided. Finally, the validity of the equivalent waveguide concept is extended to the case of lossy interconnects and components.

RF Amplitude and Phase-Noise Reduction of an Optical Link and an Opto-Electronic Oscillator

Abstract: In this paper we examine the optical sources of noise that degrade high-performance microwave photonic links. In particular, we study the residual phase noise due to laser frequency fluctuations and the detector nonlinearity on microwave signals transmitted on an optical fiber, or generated in the opto-electronic oscillator (OEO). Based on experimental findings, we identify a significant reduction of the relative intensity noise of the laser if the received optical power saturates the photodiode. Furthermore, we suggest the use of a semiconductor optical amplifier in saturation as yet another means to reduce the phase noise induced by laser intensity fluctuations. We also identify the use of multiple photodetectors to reduce the influence of associated 1/f noise, which adds to the phase noise of a transmitted microwave signal, and is the ultimate limitation to the phase noise of the high-performance OEO. Reduction of noise that is due to optical interferences is also addressed.

Single-Layer High-Order Miniaturized-Element Frequency-Selective Surfaces

Abstract: A new miniaturized-element frequency-selective surface is presented in this paper. This frequency-selective surface is made up of a 2-D periodic array of metallic loop and a wire grid of the same period printed on either side of a very thin substrate. Unique features of the new design include localized frequency-selective properties, high-order frequency response achieved by a single substrate, lack of passband harmonics in the frequency response, and very low frequency response sensitivity to the incidence angle. High-order frequency response is accomplished through the application of a thin substrate that allows considerable couplings between the elements on the two sides of the substrate. The layers' couplings in conjunction with each layer characteristics are designed to produce a high-Q bandpass frequency response, in addition to a transmission zero. It is shown that by inserting variable capacitors in the gap between the metallic loops, the center frequency of the passband can be tuned over nearly an octave. In addition, using a cluster of loops as the unit cell and modifying the parameters of the loops within the cluster, a dual-band characteristic from a single-layer miniaturized-element frequency-selective surface can be achieved. A prototype sample of the miniaturized-element frequency-selective surface, whose unit cell can be as small as lambda0 /12, is fabricated to verify the design performance through a standard free-space measurement setup. The transmission characteristic of the structure is measured and compared with numerical simulation results.

Novel Centrally Loaded Resonators and Their Applications to Bandpass Filters

Abstract: This paper presents several novel centrally loaded resonators and their applications to various types of bandpass filters. Based on the theoretical analysis, it is found that the even- mode resonance of the proposed resonators can be conveniently controlled without affecting the responses at the odd-mode resonant frequencies. Benefiting from this feature, the resonator can be utilized to design not only harmonic-suppressed bandpass filters and dual-band bandpass filters but also tunable bandpass filters. The tunable filter offers a fixed passband and a tunable one. The center frequency of the upper passband can be electrically tuned, whereas the performance of the lower one is maintained constant. To demonstrate these applications, four filters using the proposed resonators are implemented. The experiments verify the theoretical predictions and simulations.

A Design Technique for Concurrent Dual-Band Harmonic Tuned Power Amplifier

Abstract: In this paper, a novel technique to design concurrent dual-band high-efficiency harmonic tuned (HT) power amplifiers (PAs) is presented. The proposed approach is based on a methodology developed to design multifrequency passive matching networks, which allows concurrent operability. The network design criterion is heavily investigated and later generalized both from the theoretical and practical point of view. The design, realization, and the complete characterization of a concurrent dual-band high-efficiency HT PA is finally described. A 1-mm gate periphery GaN HEMT device was used for the design and realization of the PA operating concurrently at 2.45 and 3.3 GHz. The measurement results have shown 53% and 46% drain efficiency at 33- and 32.5-dBm output power in the two targeted bands if operated in continuous wave single mode. In concurrent mode, 35% average efficiency was achieved with two simultaneously applied orthogonal frequency-division multiplexing signals.

Eight-Way Substrate Integrated Waveguide Power Divider With Low Insertion Loss

Abstract: We describe a compact radial cavity power divider based on the substrate integrated waveguide (SIW) technology in this paper. The equivalent-circuit model is used to analyze the multiport structure, and a design procedure is also established for the structure. An eight-way C-band SIW power divider with low insertion loss is designed, fabricated, and measured. Good agreement between simulated and measured results is found for the pro posed power divider. The measured minimum insertion loss of the eight-way power divider is approximately 0.2 dB and return loss is approximately 30 dB at 5.25 GHz. The measured 15-dB return-loss bandwidth is found to be approximately 500 MHz, and its 1-dB insertion-loss bandwidth is approximately 1.2 GHz. Furthermore, the isolations between the output ports of the eight-way power divider are also discussed.

An Improved Small-Signal Parameter-Extraction Algorithm for GaN HEMT Devices

Abstract: A highly efficient and accurate extraction algorithm for the small-signal equivalent-circuit parameters of a GaN high electron-mobility transistor device is presented. Elements of the extrinsic equivalent-circuit topology are evaluated using a modified "cold field-effect transistor" approach whereby the undesirable need to forward bias the device's gate terminal is avoided. Intrinsic elements are determined based on a circuit topology, which identifies, for the first time, a time delay in the output conductance of GaN-based devices. The validity of the proposed algorithm has been thoroughly verified with excellent correlation between the measured and modeled S-parameters up to 50 GHz.

Quarter-Wavelength Side-Coupled Ring Resonator for Bandpass Filters

Abstract: This paper deals with a dual-mode ring resonator fed by quarter-wavelength side-coupled lines. The resonator synthesis was developed so as to fix the central frequency, bandwidth, and transmission zeros frequencies, as well as the insertion loss in the passband. Based on this resonator, several bandpass filters were designed, which include the cascaded rings and the combination of such ring resonator with coupled line sections. Simulations are proposed throughout the paper to illustrate the various possibilities offered by the concept. The filters' experimental results in microstrip technology are also presented to validate the idea.

Analytical Evaluation of Via-Plate Capacitance for Multilayer Printed Circuit Boards and Packages

Abstract: The via-plate capacitance for a via transition to a multilayer printed circuit board is evaluated analytically in terms of higher order parallel-plate modes. The Green's function in a bounded coaxial cavity for a concentric magnetic ring current is first derived by introducing reflection coefficients for cylindrical waves at the inner and outer cavity walls. These walls can be perfect electric conductor (PEC)/perfect magnetic conductor(PMC) or a nonreflective perfectly matched layer. By further assuming a magnetic frill current on the via-hole in the metal plate, an analytical formula is derived for the via barrel-plate capacitance by summing the higher order modes in the bounded coaxial cavity. The convergence of the formula with the number of modes, as well as with the radius of the outer PEC/PMC wall is discussed. The analytical formula is validated by both quasi-static numerical methods and measurements. Furthermore, the formula allows the investigation of the frequency dependence of the via-plate capacitance, which is not possible with quasi-static methods.

Characterization of Implantable Antennas for Intracranial Pressure Monitoring: Reflection by and Transmission Through a Scalp Phantom

Abstract: Characterization of implantable planar inverted-F antennas, designed for intracranial pressure (ICP) monitoring at 2.45 GHz, is presented. A setup, incorporating a scalp phantom emulating the implant environment and an absorbing chamber, was implemented for characterizing the antennas, in terms of their reflection coefficient (S 11), resonance frequency (fr), and transmission coefficient through the phantom (S 21) , and is reported for the first time. As a result of our observations that even a very slight change of the biocompatible (silicone) thickness can drastically change the characteristics of such antennas, several antenna prototypes with various silicone thicknesses were tested for a better understanding of the change in their performance with thickness. The main contributions of this paper rest in the evaluation of the antenna characteristics with respect to time, temperature, and far-field radiation, in an emulated biological environment. In this regard, the impact of the coating thickness on fr, drift of fr, S 11, and S 21 over time, and the effective radiated power (ERP) from the transmission (S 21) measurements were evaluated through careful measurements. A decrease in S 11 of 1.2-2.3 dB and an increase in S 21 of 2.2-2.4 dB, over a period of two days, were observed at 2.45 GHz. A decrease of 8-18 MHz for fr was also observed over the same period of time. This drift was due to the absorption of saline by the silicone, leading to a change in its effective dielectric property. An fr increase of approximately 14.5 MHz was also observed by raising the temperature from 20 degC to 37 degC, mainly because of the negative temperature coefficient of the phantom permittivity. Transmission measurements performed using both S 21 and the received power measurement (for an ICP device mimic) yielded a maximum ERP of approximately 2 mW per 1 W of power delivered to the antennas at 2.45 GHz.

Substrate Integrated Waveguide Cavity Resonators for Complex Permittivity Characterization of Materials

Abstract: A novel planar substrate integrated waveguide cavity resonator technique for measurement of complex permittivity is described, which has applications for dielectric measurement systems in the pharmaceutical industry. The high-Q resonant structure is a modernization of well-known measurement cells where the dielectric constant is deduced by cavity perturbation from the shift in resonant frequency and the change in the Q factor. The method uses extremely small amounts of a broad range of materials for accurate characterization. The ease of fabrication, low cost, and potential for integration with many other components on the same substrate allows it to be used in a disposable manner.

Passive Millimeter-Wave Imaging Module With Preamplified Zero-Bias Detection

Abstract: An analytical model and supporting measured data are presented for a preamplified W-band radiometer with a zero-bias detector appropriate for commercial millimeter-wave imaging cameras. Basic radiometer parameters, including RF bandwidth, are computed directly from simple low-frequency measurements and compare well with those obtained from RF measurements. A detailed analytical model shows how radiometer performance depends on internal component parameters, such as low-noise amplifier gain, noise factor, reflection coefficient, detector responsivity, etc. The measurements suggest that performance is sufficient for operation without a Dicke switch or mechanical chopping. A measured noise equivalent temperature difference of 0.45 K was obtained, assuming a single sensor is scanned across a focal plane, forming 32 pixels with 3.125-ms integration time per pixel. This sensitivity is considered sufficient by commercial manufacturers to obtain quality images in low-contrast (e.g., indoor) environments.

Active Microwave Imaging II: 3-D System Prototype and Image Reconstruction From Experimental Data

Abstract: A 3-D microwave imaging system prototype and an inverse scattering algorithm are developed to demonstrate the feasibility of 3-D microwave imaging for medical applications such as breast cancer detection with measured data. In this experimental prototype, the transmitting and receiving antennas are placed in a rectangular tub containing a fluid. The microwave scattering data are acquired by mechanically scanning a single transmit antenna and a single receive antenna, thus avoiding the mutual coupling that occurs when an array is used. Careful design and construction of the system has yielded accurate measurements of scattered fields so that even the weak scattered signals at S21 = -90 dB (or 30 dB below the background fields) can be measured accurately. Measurements are performed in the frequency domain at several discrete frequencies. The collected 3-D experimental data in fluid are processed by a 3-D nonlinear inverse scattering algorithm to unravel the complicated multiple scattering effects and produce high-resolution 3-D digital images of the dielectric constant and conductivity of the imaging domain. Dielectric objects as small as 5 mm in size have been imaged effectively at 1.74 GHz.

Multi-Lookup Table FPGA Implementation of an Adaptive Digital Predistorter for Linearizing RF Power Amplifiers With Memory Effects

Abstract: This paper presents a hardware implementation of a digital predistorter (DPD) for linearizing RF power amplifiers (PAs) for wideband applications. The proposed predistortion linearizer is based on a nonlinear auto-regressive moving average (NARMA) structure, which can be derived from the NARMA PA behavioral model and then mapped into a set of scalable lookup tables (LUTs). The linearizer takes advantage of its recursive nature to relax the LUT count needed to compensate memory effects in PAs. Experimental support is provided by the implementation of the proposed NARMA DPD in a field-programmable gate-array device to linearize a 170-W peak power PA, validating the recursive DPD NARMA structure for W-CDMA signals and flexible transmission bandwidth scenarios. To the best of the authors' knowledge, it is the first time that a recursive structure is experimentally validated for DPD purposes. In addition to the results on PA efficiency and linearity, this paper addresses many practical implementation issues related to the use of FPGA in DPD applications, giving an original insight on actual prototyping scenarios. Finally, this study discusses the possibility of further enhancing the overall efficiency by degrading the PA operation mode, provided that DPD may be unavoidable due to the impact of memory effects.

Precise Distance and Velocity Measurement for Real Time Locating in Multipath Environments Using a Frequency-Modulated Continuous-Wave Secondary Radar Approach

Abstract: We present a frequency-modulated continuous-wave secondary radar concept to estimate the offset in time and in frequency of two wireless units and to measure their distance relative to each other. By evaluating the Doppler frequency shift of the radar signals, the relative velocity of the mobile units is measured as well. The distance can be measured with a standard deviation as low as 1 cm. However, especially in indoor environments, the accuracy of the system can be degraded by multipath transmissions. Therefore, we show an extension of the algorithm to cope with multipath propagation. We also present the hardware setup of the measurement system. The system is tested in various environments. The results prove the excellent performance and outstanding reliability of the algorithms presented.