Plane Wave Synthesis Research Articles

A Review of Over-the-Air Testing Methods for Performance Measurement of Antennas and Devices in Communication Systems

With the fast development of communication techniques, over-the-air testing has developed into the main testing method for current communication systems due to its advantages including no cable connection, high measurement efficiency, and independence from environment. OTA testing has become almost the only solution for 5G system testing especially after the popularity of 5G massive multiple-input-multiple-output. In this paper, we summarize the three main methods of OTA testing including the reverberation chamber-based method, the radiation two-step method, and the multiprobe anechoic chamber- (MPAC-) based method, among which we focus on the MPAC method systematically. In the MPAC method, there are two main channel synthesis techniques that have been heavily studied, namely, prefaded signal synthesis and plane wave synthesis. We summarize the current main research based on these two techniques including probe configurations, probe selection algorithms, and probe design based on the PFS technique and the design of plane wave generators based on PWS technique. The main comparisons are made for the performance of different probe selection algorithms and the performance of different plane wave generators as well as the latest advances.

Laser ultrasonics in a multilayer structure: Plane wave synthesis and inverse problem for nondestructive evaluation of adhesive bondings.

A laser ultrasonic method is proposed for the nondestructive evaluation of bonded assemblies based on the analysis of elastic plane waves reflected from the bonding interface. Plane waves are numerically synthesized from experimentally detected cylindrical waves. Several angles of incidence with respect to the bonding interface are achieved by varying the delay in the synthesis step. An inverse problem using these plane waves is then solved to identify the normal and transverse interfacial stiffnesses that model the mechanical coupling between two bonded media. The semi-analytic model developed and detailed in Hodé et al. [J. Acoust. Soc. Am. 150, 2065 (2021)] is used to create the database that contains simulated laser-generated ultrasounds required to solve the inverse problem. The developed method is first validated with semi-analytic simulated input data where Gaussian noise has been added. Next, the method is applied using signals acquired on an aluminum alloy plate and on assemblies (with and without adhesion defects) made of two aluminum alloy plates bonded by an aeronautical structural epoxy adhesive film. Differences between the identified values of interfacial stiffnesses distinguish the three samples and obtain quantitative values to characterize the adhesive bonding.

Plane-Wave Synthesis Employing Propagating Plane-Wave Expansion for 3-D and 2-D RCS Prediction Including the Multiple Scattering Effects

The plane-wave synthesis (PWS) methods are proposed to predict the 3-D and 2-D RCS accurately with multiple scattering effects. The 3-D method employs the propagating plane-wave expansion and can be implemented identically as the near-field far-field transformation (NFFFT). Either the single-level or multilevel implementation can be applied, and in the single-level implementation, a single pseudo-inverse matrix can be used to synthesize plane waves propagating various incident directions. In addition to the 3-D formulation, the single-cut variation of the method (SC-PWS) is also presented for the 2-D RCS prediction. This method is based on the 2-D propagating plane-wave expansion and formulated employing the approximations on the 3-D Green's function. The numerical and experimental results are presented for the PWS and the SC-PWS.

Accuracy Comparisons of Channel Emulation Methods for Two-dimensional Uniform and Three-dimensional Sectored Multi-probe Anechoic Chamber

Multiple-input-multiple-output (MIMO) over-the- air (OTA) testing has been seen as a promising solution for evaluation of the radio performance of MIMO devices. In this work, we compare the accuracy of two channel emulation methods, i.e., the prefaded signal synthesis (PFS) and the plane wave synthesis (PWS), in two-dimensional (2D) uniform and 3D sectored multi-probe anechoic chamber (MPAC), respectively. The PWS method is proven to be more accurate than the PFS for 2D uniform MPAC system. However, for 3D sectored MPAC system, the PFS method emerged to be better than the PWS method. To explain these seemingly contradicting findings, both the required number of active probes and the leakage of power spectrum are considered in this paper. It is found that the PWS method has higher emulation accuracy than the PFS method when the number of active probes becomes sufficiently large, whereas the PFS is more robust to the undersampling due to the limited number of active probes in practical 3D sectored MPAC system. Moreover, when the number of active probes is particularly small (less than the number of clusters in the probe panel), the emulation accuracy of the PWS method outperform its counterpart.

Design of an Array-based Plane Wave Generator for Compact Field Antenna Testing

A near-field planar array of wideband dual polarized antennas for plane-wave synthesis has been investigated. The paper presents a PWG with a concentric ring array of 84 Vivaldi antennas which can work at 0.6GHz-3GHz. The constraint least square (CLS) algorithm is used to optimize the weight coefficients of the array elements, which effectively reduce the influence of system uncertainty on quiet zone (QZ) ripple. To reduce the system cost and complexity, the array elements in each concentric ring form a sub-array and the elements of the same sub-array adopt a passive beam forming network. The simulation and initial test results of a circular QZ with a diameter of 600 mm are given. The results demonstrate the feasibility of our design.

Robust Plane Wave Generator Design in Small Anechoic Chamber Setup Using Parameterized Field Method

Plane Wave Generator (PWG) has attracted great research interest in the industry and academia for sub-6 GHz massive MIMO base station (BS) measurement, thanks to its low-cost and smaller system setup. One major concern in the small setup is its high reflectivity level inside the anechoic chamber, which might distort the quiet zone performance significantly. Meanwhile, it is critical that the quiet zone performance is insusceptible to system inaccuracies. However, these two aspects are largely overlooked in the plane wave synthesis (PWS) study in the literature. In this article, a new PWS strategy is proposed based on parametrized field distribution over an enlarged sampling area. The proposed method offers robust design and low reflectivity level. The effectiveness and robustness of the proposed PWS strategy is demonstrated in a numerical simulation.

3-D Monostatic RCS Determination From Multistatic Near-Field Measurements by Plane-Wave Field Synthesis

A fully polarimetric plane-wave field synthesis-based approach is presented to accurately determine the 3-D monostatic radar cross section (RCS) of an object under test from a set of bistatic near-field (NF) measurements. It allows the use of small scan surfaces around the transmitting antenna to acquire the scattered field, thus significantly reducing the required number of measurements compared to a full bistatic approach. At the same time, and unlike image-based approaches, it does not suffer from inaccuracies due to multiple interactions at the object. The method solves a series of inverse source problems for all NF illuminations and computes the monostatic RCS by a subsequent plane-wave synthesis of the incident field. Great flexibility and computational efficiency are achieved by working with a hierarchical multilevel propagating plane-wave expansion of the incident and scattered field. This allows almost arbitrarily shaped scan surfaces and arbitrary measurement probes to be processed very efficiently. The theory of the algorithm is outlined in detail and its performance is assessed by considering numerical studies as well as experimental data from measurements. It is shown that the overall accuracy mainly depends on the quality of the synthesized plane-wave field, which is controlled by adapting the angular range spanned by the locations of the transmitting antenna.

Model-based beamforming with plane wave synthesis in medical ultrasound.

We are interested in examining how our model-based beamforming algorithm, referred to as aperture-domain model image reconstruction (ADMIRE), performs on plane wave sequences in conjunction with synthetic aperture beamforming. We also aim to identify the impact of ADMIRE applied before and after synthetic focusing. We employed simulated phantoms using Field II and tissue-mimicking phantoms to evaluate ADMIRE as applied to synthetic sequencing. We generated plane wave images with and without synthetic aperture focusing (SAF) and measured contrast and contrast-to-noise ratio (CNR). For simulated cyst images formed from single plane waves, the contrast for delay-and-sum (DAS) and ADMIRE are 15.64 and 28.34dB, respectively, whereas the CNR are 1.76 and 3.90dB, respectively. We also applied ADMIRE to simulated resolution phantoms having a point target at 3cm depth on-axis. We simulated the point spread functions from data obtained from 1 plane wave and 75 steered plane waves, along with linear scans with 3 and 4cm- focal depths. We then compared the outcome of applying ADMIRE before and after SAF using 3 and 11 steered plane waves. Finally, we applied this to an in vivo carotid artery. Based on the findings in this study, ADMIRE can be adapted to full field insonification sequences to improve image quality in plane wave imaging. Additionally, we investigated how robustly ADMIRE performs in the presence of random noise. We then address identified limitations using a conventional envelope detection method with decluttered signals.

On Channel Emulation Methods in Multiprobe Anechoic Chamber Setups for Over-the-Air Testing

Multiple-input multiple-output (MIMO) over-the-air (OTA) testing gives a way to evaluate the radio performance of MIMO-capable devices under realistic propagation channels as an alternative to expensive and uncontrollable drive testing. In this paper, we review two major channel emulation methods for MIMO OTA testing under the multiprobe anechoic chamber setup, i.e., the prefaded signals synthesis (PFS) and the plane wave synthesis (PWS). The target channel model for emulation is the geometry-based stochastic channel model (GSCM). The signal models for both channel emulation methods for the whole link from the transmitter (Tx) side to the receiver (Rx) side are given. The comparison analysis gives some new insights into the two channel emulation methods. The analytic expression of the joint space–time correlation function is derived for both methods in comparison to that of the target channel. It shows the clusterwise channel emulated by the PFS method is Kronecker structured, which is different from the general definition of GSCMs. In contrast, the channel emulated with the PWS method is consistent with GSCMs. Moreover, the emulation accuracy for the two methods are compared under different target channel settings, i.e., different cluster angular spreads. The simulation results demonstrate the advantage of the PWS method over the PFS method, especially when cluster angular spreads are small.

Modelling broadband scatter from periodic sea surfaces

Surface waves are the primary source of short time-scale variation of the channel impulse response in underwater transmission scenarios when source and receiver positions are fixed. This study proposes an exact method for modeling the single surface bounce path of the channel impulse response, which contains most of short time-scale effect. This method is designed for one dimensional, periodic wave profiles, and solves the scattering problem as a series of motionless surfaces. To create a time series for each surface, the proposed method solves the Helmholtz equation at many frequencies, performing three steps at each frequency. First, the point source is decomposed into a series of plane waves, each of which scatter from the surface at a finite number of Bragg angles. Second, the amplitudes of these discrete scattered plane waves are found with an exact method. Finally, the surface response for a three-dimensional point source is recovered with a plane wave synthesis of these scattered plane waves. The results of this study are intended as a reference to inform the interpretation of surface scattering results from more common underwater acoustic models.

On the Field Emulation Techniques in Over-the-Air Testing: Experimental Throughput Comparison

In this letter, we compare two field emulation techniques, namely plane-wave synthesis (PWS) and prefading signal synthesis (PFS) for multiple-input-multiple-output over-the-air (MIMO OTA) testing of mobile terminals in an anechoic chamber equipped with multiple probes. The comparison is performed in terms of measured throughput. The measurements were performed under urban microcellular scenario of the spatial channel model extended and at Long Term Evolution lower band 20. A practical mobile terminal is used as a test device. The throughput of four different browsing positions of the mobile terminal utilizing both emulation techniques has been measured. Results shows that at a fixed median throughput, the differences of the downlink channel power between the PFS and PWS techniques are small and are in the range of 0.2-1.0 dB for different browsing positions of the mobile terminal. The PWS and PFS techniques are equally capable of emulating the target field inside the test zone and are suitable for the MIMO OTA antenna testing based on anechoic chambers equipped with multiple probes.

Traveling Internal Plane-wave Synthesis (TIPS) for uniform B1 in high field MRI

A new target-field approach to generating uniform radio frequency (RF) fields within the human body for high field MRI is described. The method involves producing a set of external fields which, after interaction with a dielectric object, superimpose to produce a traveling plane wave, exposing all spins to the same RF amplitude (B1) over a cycle of the harmonic field. Conceptually this is similar to conventional RF shimming, but uses a different RF source design, input data, and objective function. The method requires a detailed knowledge of the coupling between exterior field modes, produced by an array of RF sources, and field modes within the body. Given an estimate of the coupling matrix, the linear superposition of external modes that produces a desired internal target field can be determined. The new method is termed Traveling Internal Plane-wave Synthesis (TIPS). A simple design of a coil array is described that can, in principle, generate the required field modes. Simulations demonstrate that radio frequency magnetic fields of nearly uniform (<1% variation) magnitude can be produced within dielectric objects larger than a wavelength in size. If the dielectric medium has non-zero conductivity, traveling waves are attenuated as they traverse the object, but field uniformity within planar slices is preserved. For general 3D imaging, a superposition of plane waves can provide field focusing to balance conductive losses, thereby achieving nearly uniform-magnitude B1+ magnetic fields over a volume of interest.

On Analytic Methods for 2.5-D Local Sound Field Synthesis Using Circular Distributions of Secondary Sources

Local sound field synthesis allows for synthesizing a given desired sound field inside a limited target region such that the field is free of considerable spatial aliasing artifacts. Spatial aliasing artifacts are a consequence of overlaps due to unavoidable repetitions of the space-spectral coefficients of the secondary source driving function. We analyze various conceivable analytic ways of restricting the bandwidth of the spatial spectrum of the driving function such that considerable overlapping is prevented: local spatial bandlimitation (A), spectral windowing (B), and local spatial bandlimitation plus spectral windowing (C). While solution B is computationally significantly more efficient than A and C, it provides only limited control over the spatial location around which the aliasing-free region evolves. Solutions A and C provide more flexibility and higher accuracy whereby both achieve largely identical results so that the spectral windowing after the local spatial bandlimitation may be skipped. We present a detailed analysis of the properties of the spatial aliasing artifacts arising in the synthesis of a virtual plane wave. We establish a procedure for predicting the maximum possible size of the aliasing-free target region depending on its location and on the propagation direction of the desired sound field. The results can help reducing regularization in numerical solutions as they represent physical limitations that can be considered in the choice of parameters.

Plane-Wave Synthesis and RCS Extraction via 3-D Linear Array SAR

This letter presents an improved back-projection (BP) technique for far-field radar cross section (RCS) feature extraction from near-field measurement. The presented technique can synthesize a plane-wave response from a 2-D near-field planar scanner and obtain a 3-D scattering coefficient image. Due to the 3-D resolving power, it can spatially extract different parts of a complex object, or target’s scattering coefficient from the whole image which may include environment noise such as ground and support. From the 3-D scattering coefficient image, far-field RCS can be obtained as a function of direction and frequency. Simulations of RCS feature extraction for three squares and a 3-D complex-shaped electric-large car model are presented to validate the improved BP method.

Antenna Pattern Impact on MIMO OTA Testing

This paper investigates the impact of the DUT antenna pattern on the test area performance for multi-probe based MIMO OTA setup in terms of received voltage and spatial correlation. The plane wave synthesis (PWS) technique has been proposed for vertical polarization in the literature, where the goal is to approximate plane waves with arbitrary directions. The received voltage at the antenna terminal depends on the antenna radiation pattern and the impinging plane waves. A novel closed form technique to reproduce the received voltage with arbitrary incoming plane waves based on trigonometric interpolation is presented. The proposed technique provides a closed form solution for the PWS when the probe ring radius is infinite. The proposed technique shows that the impact of the antenna pattern on the induced received voltage accuracy is ruled by Nyquist sampling theory. Furthermore, the impact of the antenna pattern on spatial correlation accuracy for prefaded signal synthesis (PFS) technique is investigated as well. Simulation and measurement results show that the number of required probes depend directly on the DUT antenna pattern. To test realistic DUTs with higher variations in directivity, we need more probes to maintain the same received voltage and spatial correlation accuracy level.

A fully probe corrected near-field far-field transformation technique employing plane-wave synthesis

Abstract. The far-field behavior of an antenna under test (AUT) can be obtained by exciting the AUT with a plane wave. In a measurement, it is sufficient if the plane wave is artificially generated in the vicinity of the AUT. This can be achieved by using a virtual antenna array formed by a probe antenna which is sequentially sampling the radiating near-field of the AUT at different positions. For this purpose, an optimal filter for the virtual antenna array is computed in a preprocessing step. Applying this filter to the near-field measurements, the far-field of the AUT is obtained according to the propagation direction and polarization of the synthesized plane wave. This means that the near-field far-field transformation (NFFFT) is achieved simply by filtering the near-field measurement data. Taking the radiation characteristic of the probe antenna into account during the synthesis process, its influence on the NFFFT is compensated. The principle of the plane-wave synthesis and its application to the NFFFT is presented in detail in this paper. Furthermore, the method is verified by performing transformations of simulated near-field measurement data and of near-field data measured in an anechoic chamber.

Use of a Plane-Wave Synthesis Technique to Obtain Target RCS From Near-Field Measurements, With Selective Feature Extraction Capability

A novel 3-D radar cross section (RCS) feature extraction technique using a combination of plane-wave synthesis and 3-D imaging procedures is presented. The technique enables RCS as a function of frequency to be obtained for a target containing multiple scattering centers, where individual scattering elements can be evaluated in isolation from neighboring elements. The technique is validated against numerical simulations and measurements using a synthetic aperture radar (SAR) facility over the 8–12-GHz frequency range.

PLANE-WAVE SYNTHESIS FOR COMPACT ANTENNA TEST RANGE BY FEED SCANNING

The plane-wave synthesis is brought to the CATR. The feed scans in the focal plane and antenna pattern is measured several times in different feed positions. The corrected antenna pattern is obtained by weighting the measured patterns. The weight is obtained by the least squares method for the electric fields in the aperture plane of the test antenna. The fashion, number and spacing of feed scanning are investigated. By the plane-wave synthesis in the CATR, a larger antenna can be tested, and higher test accuracy can be obtained than that of the CATR. Lower test number and less test time are needed than the conventional plane-wave synthesis. The technique proposed in this paper is numerically investigated in an offset single paraboloid CATR and verified experimentally in a Cassegrain CATR at 100 GHz.

Channel Modelling for Multiprobe Over-the-Air MIMO Testing

This paper discusses over-the-air (OTA) test setup for multiple-input-multiple-output (MIMO) capable terminals with emphasis on channel modelling. The setup is composed of a fading emulator, an anechoic chamber, and multiple probes. Creation of a propagation environment inside an anechoic chamber requires unconventional radio channel modelling, namely, a specific mapping of the original models onto the probe antennas. We introduce two novel methods to generate fading emulator channel coefficients; the prefaded signals synthesis and the plane wave synthesis. To verify both methods we present a set of simulation results. We also show that the geometric description is a prerequisite for the original channel model.

Design optimization studies for large-scale contoured beam deployable satellite antennas

Satellite communications systems over the past two decades have become more sophisticated and evolved new applications that require much higher flux densities. These new requirements to provide high data rate services to very small user terminals have in turn led to the need for large aperture space antenna systems with higher gain. Conventional parabolic reflectors constructed of metal have become, over time, too massive to support these new missions in a cost effective manner and also have posed problems of fitting within the constrained volume of launch vehicles. Designers of new space antenna systems have thus begun to explore new design options. These design options for advanced space communications networks include such alternatives as inflatable antennas using polyimide materials, antennas constructed of piezo-electric materials, phased array antenna systems (especially in the EHF bands) and deployable antenna systems constructed of wire mesh or cabling systems. This article updates studies being conducted in Japan of such deployable space antenna systems [H. Tanaka, M.C. Natori, Shape control of space antennas consisting of cable networks, Acta Astronautica 55 (2004) 519–527]. In particular, this study shows how the design of such large-scale deployable antenna systems can be optimized based on various factors including the frequency bands to be employed with such innovative reflector design. In particular, this study investigates how contoured beam space antennas can be effective by constructed out of so-called cable networks or mesh-like reflectors. This design can be accomplished via “plane wave synthesis” and by the “force density method” and then to iterate the design to achieve the optimum solution. We have concluded that the best design is achieved by plane wave synthesis. Further, we demonstrate that the nodes on the reflector are best determined by a pseudo-inverse calculation of the matrix that can be interpolated so as to achieve the minimum deviation from the reflectors’ ideal shape. The RMS characteristics of the “cable network” in any deployable space antenna are constrained from exactly achieving the ideal contour because they ultimately contain very small plane facets. The effect of these plane facets on the characteristics of the contoured beams can be identified, however, by numerical simulations. Then these characteristics can be improved with the node locations being optimized as these simulations are undertaken. This process, however, must take into account a non-dimensional parameter. This parameter is based on the frequencies being employed, the plane facet size, and the angle of plane wave from the z-axis. When this optimization process is undertaken, the optimum location of the nodes in the “cable network” that compose the space antenna reflector can be precisely identified.