Over-the-air Testing Research Articles

Switchable metasurface absorber used for enabling reconfigurable power angular spectrum in reverberation chamber

The reverberation chamber (RC) provides a fast and repeatable method for over-the-air (OTA) testing of wireless devices. Moreover, the RC-based method can reduce the OTA testing cost to a great extent. But the defect of the RC is also obvious. Compared to a multi-probe anechoic chamber, the channel spatial characteristics in RC are uncontrollable. The device under test in an RC usually sees a statistically isotropic channel and there is a strong impediment to control the channel in RC, which constrains the RC’s OTA applications. In this paper, we propose a method to realize reconfigurable RC enabling arbitrary channel power angular spectrum (PAS) by using a switchable metasurface absorber. Specifically, the unit cell of the metasurface can be switched between the reflection and absorption states by providing different bias voltages. By mounting the switchable metasurface absorber on the RC’s inside walls, the boundary conditions of the RC in the covered area could be switched electronically. Consequently, the channel’s PAS can be controlled by changing the local states of the switchable metasurface absorber. As a proof of concept, a prototype of the switchable metasurface absorber is made and comprehensive experiments are conducted to verify the effectiveness of the switchable metasurface applied to enable reconfigurable PAS in RC.

Resistance frequency selection surface framed reflector antenna for 5G OTA measurement

AbstractIndirect far field (IFF) over‐the‐air (OTA) test could be one of the most applicable test methods for 5G mmWave devices. The compact antenna test range (CATR) method has been introduced to the OTA test by 3GPP. Conventional metallic parabolic reflectors without edge treatment can lead to serious edge diffraction which results in high noise and low accuracy. The serrated reflector improves edge diffraction but performs poorly at low frequencies. Reflector antenna with a resistance frequency selection surface (RFSS) frame is proposed for 5G OTA CATR measurement at millimeter frequency, which collimates spherical incident waves into quasi‐plane waves in a relatively short distance, thus forming a quiet zone (QZ) in which large devices under test can be measured in small chamber. In this paper, the reflector with RFSS is investigated through the numerical and experimental methods, it is found that serrated RFSS frame with low edge diffraction could achieve the excellent QZ, which could be helpful in miniaturing CATR chambers for 5G OTA measurement.

A Trigger-Free Multi-Active-Probe Anechoic Chamber System for 5G/6G Millimeter Wave OTA Test

The multi-probe anechoic chamber (MPAC) approach is one of the most promising candidates for fifth-generation (5G)-advanced/sixth-generation (6G) massive multiple-input-multiple-output (MIMO) over-the-air (OTA) performance testing. In this article, a trigger-free multi-active-probe anechoic chamber system (MAPAC) is proposed for 5G/6G millimeter wave (mmWave) OTA test, which allocates the transmitting (Tx) and receiving (Rx) probes separately and then avoids the requirement for uplink (UL)/downlink (DL) switching signals. The system architecture is proposed, and a figure of merit for the UL/DL channel reciprocity is provided. For system design, simulations are conducted on essential items, such as the measurement distance, probe density, Tx/Rx probe spacing, etc. A novel dual-polarization active probe (AP) with independent Tx and Rx antennas is developed for the test system, which has the advantages of low loss, low cost, and high integration. The accuracy of the trigger-free mmWave MAPAC is experimentally verified by 3rd Generation Partnership Project (3GPP) clustered delay line (CDL)-A and CDL-C channels with promising application potential.

A Reconfigurable Over-the-Air Chamber for Testing Multi-Antenna Wireless Devices

The reconfigurable over-the-air (OTA) chamber (ROTAC) is a reverberation chamber with antennas positioned along the chamber walls. By controlling the signals driving a small number of these antennas and tunable impedances terminating the ports of the remaining antennas, the ROTAC offers the potential to dynamically control the field incident on a mobile device during OTA testing. This paper uses finite-difference time-domain electromagnetic simulations and measurements from a ROTAC prototype in combination with a network model to characterize the fields incident on the device as a function of the excitations and terminations. A gradient search optimization is formulated to determine the excitations and terminations required to approximate specified field characteristics in the chamber test zone, including multipath direction of arrival, signal correlation, and fading statistics. The paper further explores the relationship between the field spatial sampling within the test zone and the ability to synthesize a field with the specified characteristics.

A Non-Kronecker Structured PFS Method for Channel Emulation in Multiprobe OTA Setups

The realistic channel environment can be characterized by a geometry-based stochastic channel (GBSC) model composed of multiple clusters, each of which consists of many rays. For the GBSC model, the power angle of arrival (AoA) spectrum of a cluster depends on the power angle of departure-Doppler (AoD-Doppler) spectrum. Hence, this model is non-Kronecker structured. In over-the-air (OTA) testing, the prefaded signal synthesis (PFS) method is valid for emulating the GBSC to measure devices under test (DUTs). However, the emulated channel under the conventional PFS method is a Kronecker case due to the independently and identically distributed fading sequences between probes when synthesizing clusters. It will lead to inaccurate link characteristics for emulation. Therefore, a non-Kronecker structured PFS (NKSPFS) method is proposed. Specifically, the rays mapped to probes are individually weighted to ensure that fading sequences between probes are independent but not identically distributed. To determine the ray weights without degrading marginal correlations for emulation, the constraints are introduced on the deviation between the target spatial-temporal correlation and the emulated spatial-temporal correlation according to the emulated spatial correlation and temporal correlation. Simulation results show that the proposed NKSPFS method outperforms the conventional PFS method in OTA testing of multi-antenna devices.

Research on OTA Testing Optimization of 5G IoT Devices

Over-the-air (OTA) testing for wireless devices is crucial to guarantee actual network performance. Current OTA testing requires respectively 266 and 62 grid points for Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) measurement. For 5G Internet of Things (IoT) devices, especially for low transmit duty-cycle devices, this number of grid points could lead to an unacceptably large amount of test time. Therefore, test time reduction is significant. The objective of this paper is to determine a suitable measurement grid for OTA testing of 5G IoT devices which balances measurement grid uncertainty/errors with test time. Two reference patterns representing a reasonable worst-case scenario of IoT devices are proposed for measurement uncertainty (MU) analysis of different grid configurations. The effects of different grid configurations on OTA testing accuracy are evaluated. The determination of associated MU term is proposed and determined based on statistical analysis. Finally, the reduction of grid points from currently required 62 (30/30 in theta/phi) to 32 (45/36) could save roughly 50% test time while requiring an uncertainty increase of less than 0.2 dB, which is acceptable for OTA testing of 5G IoT devices.

Correcting Deviations of Emulated Spatial Correlation for Prefaded Signal Synthesis Method With Limited Number of Random Drops in MIMO OTA Testing

The prefaded signals synthesis (PFS) is a well-established over-the-air (OTA) testing solution, which can reconstruct the desired spatial profiles of the target channel by adjusting the power weights of the multiple probes. However, practical channel emulation campaigns tend to use a limited number of random drops due to limited time budget and hardware capacity constraint, which may cause the emulated spatial correlation to be deviated from the expected one due to the loss of ergodicity. In this article, the properties of the emulated channel under the PFS method are reexamined for a given set of initial subpath phases that are randomly configured at different probes. The analytical expressions of the spatial–temporal correlation function (STCF) are thus derived by averaging over time, compared with the general manipulations of ensemble averaging in the literature. Our derivations indicate that the spatial correlation function (SCF) of the emulated channel may deviate from the expected form for a given set of initial subpath phases in the classic PFS method. An improved configuration method for initial subpath phases is proposed. Simulation results show that the proposed method can reproduce the exact SCF in any random drop while unaffecting other important channel properties, for example, the temporal correlation function (TCF).

Measurement and Estimation Methodology for EMC and OTA Testing in the VIRC

In recent years, promising results have been reported with the vibrating intrinsic reverberation chamber (VIRC) combining performance and cost-efficiency for electromagnetic compatibility (EMC) measurements. This also makes it a potentially attractive solution for over-the-air (OTA) testing, which is yet to be investigated. Therefore, this article proposes the first systematic and thorough methodology to characterize the VIRC for use in EMC and OTA testing of wireless baseband algorithms of narrow-band single-input single-output channels. This methodology has been developed to measure and estimate the channel first- and second-order temporal and spectral characteristics taking into consideration the effect of different carrier frequencies, rotational speeds of VIRC motors, and loading conditions. It is then applied to a channel setup inside a VIRC for a preliminary investigation before the VIRC itself is fully characterized. It is shown that mounting an absorber in a specific location on the hatch significantly improves the rejection rate of the chi-squared goodness-of-fit test for Rician distribution without increasing the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K$</tex-math></inline-formula> -factor above <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 10 dB over the frequency range 755–2740 MHz in the VIRC under investigation. However, the proposed methodology has been devised to be universal to any reverberation chamber, and the obtained results can be used to improve EMC testing due to the better understanding of the unique VIRC environment.

Multifunctional Frequency Selective Absorber Enabling FR1 and FR2 5G OTA Tests in a Hybrid Reverberation Chamber

Due to the special advantages of the reverberation chamber (RC), it has been recognized as a standardized over-the-air (OTA) testing methodology by the cellular telecommunications industry association (CTIA). Currently, OTA tests of the fifth-generation (5G) wireless terminals are usually performed in different testing environments, e.g., in a reverberation chamber for fast isotropic tests in the sub-6 GHz frequency band, and in an anechoic chamber for directional tests in the millimeter-wave (mm-wave) frequency band, which are inconvenient and costly. In this work, two multi-functional frequency selective absorbers (FSAs) are designed to realize absorption in the 5G mm-wave band and reflection/transmission in the sub-6 GHz band. By loading the FSAs to the RC, an integrated testing environment for 5G terminals can be established. Namely, a quasi-anechoic environment in the mm-wave band and a reverberation environment in the sub-6 GHz band can be simultaneously achieved in the RC loaded with the proposed FSAs. Experimental and simulated results are presented to demonstrate the effectiveness of the proposed scheme.

Active 5G radio resource management measurements using a multiple CATR reflector system

AbstractThis paper presents results for active and passive measurements using a novel method based on multiple compact antenna test range (CATR) reflectors to perform simultaneous multiple angle measurements in order to characterize the beam-forming characteristics in a real environment of the 5G devices operating in the millimeter wave frequency band: 24–44 GHz. The over-the-air (OTA) system generates four planar wavefronts with different incidence angles, realizing up to five pairs of angular spreads or four switched/simultaneous angles of arrival. The initial target application is radio resource management (RRM) testing, where the execution of mobility procedures and radio link monitoring of a 5G millimeter wave device are evaluated. The applicability of the multi-reflector approach to RRM testing is measured with commercial 5G handsets, through three test scenarios. The paper demonstrates that baseband (non OTA) testing is not sufficient for RRM FR2, as the results are influenced by the direction of arrival of the signal. It is further shown that OTA testing in a multi-reflector CATR system and a careful selection of a representative set of test directions is critical for full characterization of the performance of a wireless device operating in the millimeter wave bands.

Broadband absorption of macro pyramid structure based flame retardant absorbers

• A flame retardant and microwave absorbing carbon nanotube/ammonium polyphosphate composite is prepared by a simple process. • The wave transparent flame retardant coating ensure the flame retardant and absorbing material not be ignited under open fire. • The the maximum absorption bandwidth can reach 14.36 GHz by the pyramid structure. • The designed pyramid structure can ensure the absorption bandwidth of 14 GHz at the incident angle within 60°. Flame-retardant materials with good electromagnetic wave absorption play an increasingly important role in Over-the-Air (OTA) tests in microwave anechoic chambers. In this work, carbon nanotubes and ammonium polyphosphate (CNTs@APP) composite were prepared by crosslinking with the silane coupling agent (KH550). The flame retardancy is improved with the increase of APP. However, the microwave absorption performance (MAP) and processing difficulty changed greatly as well. Therefore, magnesium aluminum double-layer metal hydroxide (LDH) was obtained by coprecipitation method and processing into the coating, so as to realize flame retardancy which reaches the limiting oxygen index (LOI) of 24.8% and good MAP at the same time. Then, a high-frequency structure simulator (HFSS™) was used to simulate the pyramid model to realize the broadband absorption, and the optimal parameter range was obtained, that is, when the total height of the material is 40 mm, the height of the base accounts for 1/5–1/3, and the tangent of the half top angle is near 1/5, which result in the best MAP. Finally, the broadband absorption of 14.36 GHz (3.64–18 GHz) was realized when the base height is 1/3, and the broadband absorption of 14 GHz (4–18 GHz) can be realized under the condition of large-angle oblique incidence of 0°–60°. Based on the pyramid structure of flame retardant absorbers, the obtained samples possess both good flame retardancy and microwave absorption, which show great application prospects in the field of OTA test, broadband microwave absorption performance, and brodaband oblique incidence absorption.

IO-Link Wireless Sensitivity Testing Methods in Reverberation Chambers

Communication reliability is a challenging requirement, which implies the need for over-the-air (OTA) testing. Reverberation chambers (RCs) are widely used for OTA tests in various fields. Due to their properties, such as inherent radio channel emulation or the arbitrary orientation of the equipment under test (EUT) in the test volume, they can be used as advantageous test environments for wireless products in the field of industrial manufacturing automation, such as for the IO-Link Wireless (IOLW) standard. In this paper, the different OTA sensitivity test procedures total isotropic sensitivity (TIS), average fading sensitivity (AFS) and mean channel packet error (MCPE) method, which is based on the fundamental channel model of the wireless standard, are described and evaluated in various variants. A core aspect of the proposal is the impact of the possible use of frequency hopping of the wireless equipment under test. The respective advantages and disadvantages are shown. Overall, TIS proves to be a suitable alternative for IOLW OTA sensitivity testing.

Research on Accurate OTA Testing for Large-Form-Factor 5G IoT Devices in Medium-Sized Anechoic Chamber

This letter focuses on whether accurate over-the-air (OTA) performance of large-form-factor (LFF) Internet-of-Things (IoT) devices (especially for vehicles) can be obtained from medium-sized anechoic chambers (ACs). For LFF IoT devices under test (DUTs), antenna locations are usually deviated significantly from the center of the OTA testing system due to mechanical positioner or chamber constraints in medium-sized ACs, which will lead to distortion for assessed OTA performance. This work evaluates the effect of offsetting the antenna under test integrated in LFF IoT DUTs from the center of the quiet zone on OTA integrated-type quantities. A new figure of merit <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_OFF</i> has been defined to characterize the off-center extent, while the influence of which on OTA performance as well as associated simulations is presented based on statistical analysis. It can be found that the performance deviation is determined by <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_OFF</i> rather than absolute off-center distance. The relationship between <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_OFF</i> and performance deviation can be derived by using the least-squares fitting method. Additionally, it is acquired that for off-center scenario, performance deviation cannot be eliminated by increasing measurement grid density, yet can be reduced significantly by introducing off-center correction.

Over-the-Air Performance Testing of 5G New Radio User Equipment: Standardization and Challenges

The Third Generation Partnership Project (3GPP) is accelerating 5G new radio (NR) global standards aimed at significant enhancement of wireless system performance for higher date rate, better energy efficiency, and higher reliability than the current 4G cellular systems. The operators, manufacturers, and test equipment vendors have worked together to develop standardized over-the-air (OTA) test methodologies for the overall performance evaluation of 5G NR devices. 3GPP is taking the lead in standardizing the OTA testing of 5G NR under fading channel conditions. In 3GPP specifications, test methods have been studied to verify the multiple-input multiple-output (MIMO) performance of 5G NR user equipments (UEs) in OTA mode. This article follows the 3GPP standardization work and discusses the MIMO OTA test methodologies for 5G NR UEs working at frequency range 1 (FR1) and FR2, with a focus on its new challenges and solutions compared to 4G MIMO OTA testing methods. Then the OTA throughput testing results of real 5G NRUEs are demonstrated under the standard channel models. Finally, the challenges and limitations of standard 5G MIMO OTA test solutions are also highlighted.

A Switchable Metamaterial Absorber for Fine-Tuning of the Coherence Bandwidth in a Reverberation Chamber

Since wireless communication technology has been widely applied in our daily lives, wireless devices exhibit explosive growth. A reliable, efficient, and repeatable over-the-air (OTA) testing method is vital for the research and development of wireless devices. Due to the particular advantages of the reverberation chamber ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> ), it has become a popular facility to perform OTA tests nowadays. In the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> -based OTA tests, loading the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> with radio frequency (RF) absorbers is a standardized procedure to control the frequency selectivity of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> channel according to the cellular telecommunications industry association (CTIA). However, this procedure involves iterative manual calibrations and, therefore, is tedious and time-consuming. Moreover, it may be difficult to tune the coherence bandwidth (CBW) to a desired value accurately given fixed RF absorbers. In this work, a locally switchable metamaterial absorber (MA) is proposed for fine-tuning of the CBW of the channel in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> . It is experimentally verified that the CBW can be easily controlled by switching on and off of the absorbing/reflection units of the MA. The proposed MA can provide fine-tuning of the CBW in an efficient and convenient manner.

An Improved Method for Total Radiated Power Tests in Anechoic Chambers

This paper proposes an improved method for Total Radiated Power (TRP) tests based on a numerical integration method called Clenshaw-Curtis quadrature. It is known that TRP tests in an anechoic chamber need integral calculation of the measured samples. And the samples are discretized in the angular domain in practical over-the-air (OTA) testing. By adopting the Clenshaw-Curtis quadrature method to the TRP tests, we can achieve higher accuracy without increasing the number of sampling points or measurement time. Note that quadrature method must be selected judiciously by considering practical OTA setting. A numerically accurate quadrature method may not necessarily ensure accurate OTA tests. To illustrate this point, another measurement approach based on a modified Clenshaw-Curtis quadrature method (with even better numerical accuracy) is used for comparison. The proposed method based on the Clenshaw-Curtis quadrature shows superior performance to the conventional and modified approaches. The above findings are verified by both simulation and measurement experiments.

UE Reporting Uncertainty Analysis in Radiated Two-Stage MIMO Measurements

Over-the-air (OTA) performance evaluations of wireless terminals need to be accurate and consistent. The radiated two-stage (RTS) method is a promising solution for the 5G multiple-input and multiple-output (MIMO) OTA test because of its advantages in cost and efficiency. However, the accuracy of the RTS method has not been theoretically investigated. In this article, an uncertainty analysis on the throughput performance using the RTS method with regard to the reporting errors in the device under test (DUT) reporting system is presented. A mathematical model of the RTS method is constructed and error factors are introduced. Based on the analysis, the reporting errors in the DUT antenna pattern measurement are eliminated using the proposed RTS implementation in the final throughput evaluations. In other words, the RTS method is free of the effects of the DUT reporting errors, contributing to the accuracy and consistency of the test. An example DUT was tested to demonstrate the impact of the reporting errors and validate that the reporting errors are eliminated in the proposed RTS implementation. The theoretical proof of the accuracy of the RTS method helps further develop the method for 5G MIMO OTA testing.

AI-Aided Online Adaptive OFDM Receiver: Design and Experimental Results

Orthogonal frequency division multiplexing (OFDM) has been widely applied in many wireless communi- cation systems. The artificial intelligence (AI)-aided OFDM receivers are currently brought to the forefront to replace and improve the traditional OFDM receivers. In this paper, we first compare two AI-aided OFDM receivers, namely, data-driven fully connected deep neural network and model-driven ComNet, through extensive simulation and real-time video transmission using a 5G rapid prototyping system for an over-the-air (OTA) test. We find a performance gap between the simulation and the OTA test caused by the discrepancy between the channel model for offline training and the real environment. We develop a novel online training system, which is called SwitchNet receiver, to address this issue. This receiver has a flexible and extendable architecture and can adapt to real channels by training only several parameters online. From the OTA test, the AI-aided OFDM receivers, especially the SwitchNet receiver, are robust to OTA environments and promising for future communication systems. At the end of this paper, we discuss potential challenges and future research inspired by our initial study in this paper.

Predicting Over-the-Air Performance Using a Hand Phantom With Large Dielectric Flexibility

This article developed a hand phantom with a large dielectric deviation from the target values, which can be used to estimate over-the-air (OTA) performance just like the Cellular Telecommunications and Internet Association hand phantom. In this article, the dielectric properties were adjusted by varying the graphite composition in the carbon-loaded silicone. Numerical simulations were used to determine the appropriate material recipe with the aid of the numerical models of four mobile devices operating from 870 to 2700 MHz, using both top and bottom antenna configurations. The results estimated that the material with a difference of 44.65% ± 20.15% (mean ± standard deviation, the maximum value is 77.97%) and 6.79% ± 3.35% (mean ± standard deviation, the maximum value is 9.43%) from the target conductivity and relative permittivity, respectively, provided the maximum deviation of 1.82 dB in the case of total radiated power (TRP) and 1.98 dB for total isotropic sensitivity (TIS). Accordingly, the molded hand phantom was validated with mechanical and OTA tests (maximum error below 1.30 and 1.80 dB for TRP and TIS, respectively). The results demonstrated that using the material with a larger dielectric deviation than the prescribed standard can be used to fabricate hand phantom for predicting the OTA performance. The findings shed light on designing and manufacturing of the hand phantom for in-house research and development purposes.

Removing Phase Misalignment in the Validation of a Compact Antenna Test Range for 5G mm-Wave UE OTA Testing

Over-the-air (OTA) measurement is mandatory for 5G mm-wave wireless devices. The compact antenna test range (CATR) is the baseline method in the current version of TR 38.810 in 3rd generation partnership project (3GPP) and cellular telecommunications industry association (CTIA) millimeter-wave (mm-Wave) test plan. For CATR, both amplitude and phase variations inside the quiet zone (QZ) could affect the final OTA results significantly and hence need to be characterized carefully. Especially in the phase validation of QZ, even slight misalignments between sampling plane and incident plane wavefront due to the limited precision of the positioner or tolerance in antenna fixture could result in large phase variations, which is not the true phase property of QZ. This letter presents a method to remove the effect of the angular error due to misalignment when measuring the phase variation inside QZ. By adopting the proposed method to the measured phase data, any angular error due to misalignment with respect to the incident plane wavefront will be eliminated mathematically, leaving the true phase variations of QZ only. In the meanwhile, the proposed method can alleviate the requirements of fixture manufacture and alignment precision and reduce the test cost.