Multiprobe Anechoic Chamber Setup Research Articles

Over-the-air testing for dual-polarized MIMO devices: Setup design and validation

Over-the-air (OTA) testing is considered as the only feasible solution to evaluate radio performances of the fifth-generation (5G) wireless devices which feature two important technologies, i.e., massive multiple-input multiple-output (MIMO) and millimeter-wave (mmWave). The multi-probe anechoic chamber (MPAC) based OTA setup is able to emulate realistic multipath propagation conditions in a controlled manner. This paper investigates an MPAC OTA setup which is capable of evaluating the performances of 5G base stations as the devices-under-test (DUTs) which are equipped with dual-polarized antennas. Both end-to-end setup and probe configuration for the considered MPAC setup will be elaborated. Furthermore, since building a practical MPAC setup is expensive, time-consuming, and error-prone, an end-to-end software testbed is established for validation purpose to avoid technical risks before finalizing an MPAC setup. The architecture of the testbed is presented, which can emulate both the channel profiles perceived by the DUT and the physical-layer behaviors of the considered link conforming to 5G new radio (NR) specifications. Results show that the performances under the emulated channel agree well with those under the target channel, validating the accuracy and effectiveness of the MPAC method.

On Efficient Non-Stationary Channel Emulation in Conductive Phase Matrix Setup for Massive MIMO Performance Testing

Massive multiple-input multiple-output (MIMO) is considered as a critical technique in 5 G New Radio (NR), due to its capability to significantly improve data rate and link reliability for the radio link. To make it a reality, it is essential to evaluate massive MIMO performance in realistic spatial fading channel conditions. Specifically, dynamic spatial channel emulation is crucial to evaluate the massive MIMO beam operations. Due to the high cost associated with both the traditional conduced testing method and the standardized multi-probe anechoic chamber (MPAC) based Over-the-Air (OTA) testing setup for massive MIMO performance testing, fading channel emulation for massive MIMO testing using a conductive phase matrix setup has been proposed in the literature. However, it is still a challenging problem to emulate dynamic spatial channels in a cost-effective and efficient manner for the conductive phase matrix setup, since traditional algorithms suffer from high computational complexity and system cost. This paper proposes a fast and accurate framework to emulate dynamic spatial channels in the phase matrix setup, where the basic idea is to mimic a virtual MPAC setup. By doing so, we can mimic MPAC setup with flexible probe configurations. Furthermore, a novel successive cancellation strategy and a closed-form algorithm are proposed to determine the probe locations and probe weights, respectively, which can significantly reduce the computational complexity associated with dynamic channel emulation. Simulation results show that the proposed algorithm is highly accurate and computationally efficient, compared to state-of-art solutions. Further, we apply our developed algorithm to emulate measured non-stationary channels and excellent emulation accuracy can be achieved. The proposed method can be employed for emulating dynamic spatial channels, which is of great importance for massive MIMO performance testing.

Key issues and algorithms of multiple-input-multiple-output over-the-air testing in the multi-probe anechoic chamber setup

Key issues and algorithms of multiple-input-multiple-output over-the-air testing in the multi-probe anechoic chamber setup

Over-the-Air Testing of 5G Communication Systems: Validation of the Test Environment in Simple-Sectored Multiprobe Anechoic Chamber Setups

Over-the-air (OTA) radiated testing is seen as inevitable for 5G antenna systems. The simple-sectored multiprobe anechoic chamber (SS-MPAC) setup is a potential testing candidate for the evaluation of 5G antenna systems under real-world propagation conditions. Validation of emulated channel models in the practical SS-MPAC setup is essential, since it is important to ensure that the target channel models are correctly emulated. Our objective is to detect the joint-angle delay power profile of the reproduced channels in the SS-MPAC setup. In this article, we discuss two joint-angle delay estimation (JADE) algorithms: 1) an existing algorithm with high computation complexity as the reference method and 2) a novel, low-cost, sequential, 1D algorithm. Both numerical simulations and experimental verification measurements in a preliminary SS-MPAC setup are provided to validate the two discussed algorithms. The proposed sequential 1D search method is demonstrated to be highly accurate and effective and, therefore, is recommended for validation measurements in SS-MPAC setups. Measurement setup and procedures for test-environment validation are considered as well.

Single-cut Far-Field Antenna Radiation Pattern Reconstruction Accuracy Analysis in Compact Anechoic Chamber Setup

In production testing, it is of importance to measure the key radiation parameters of an antenna under test (AUT), e.g., main beam peak and direction, sidelobes, and null depth and direction in a cost-effective setup with a short measurement time. As a result, practical measurement setups are often compact and equipped with only a few probes (or probe locations). However, these system limitations would introduce errors for antenna testing. This problem has become even more pronounced for 5G radios due to utilization of large-scale antenna configurations and high frequency bands. Spherical near-field measurements are nowadays an accurate and mature technique for characterizing AUTs, which however, necessitates a full spherical acquisition, leading to a long measurement time. Singlecut near-to-far-field transformation is a promising strategy since most of the key AUT parameters are available in the single-cut pattern and it requires much reduced measurement time. In this work, a simple and flexible scheme is proposed to evaluate errors introduced by limitations in practical setups for single-cut far-field (FF) antenna radiation pattern reconstruction, where the near-field data can be easily generated and modified according to the limitations introduced in practical multi-probe anechoic chamber setups, e.g., measurement distance, truncation range, and sampling interval. The reconstructed FF pattern is obtained using a commercial near-field to far-field transformation tool, SNIFT. The proposed scheme is numerically validated via comparing the reference FF pattern of a 4 × 8 uniform planar array composed of ideal Hertzian dipoles and reconstructed FF pattern. With the proposed scheme, the impact of practical system limitations on single-cut reconstruction accuracy can be easily analyzed.

On Over-the-Air Testing for Devices With Directional Antennas

This paper investigates the impact of devices under test (DUTs) with directional antennas on the recreated channel in the over-the-air (OTA) testing for massive multiple-input multiple-output (MIMO) devices using prefaded signal synthesis (PFS) technique. Assuming the uplink in real radio environments, the directional receiver (Rx) antenna patterns would affect the gains in different incoming directions, thereby affecting the spatial characteristics of the target channel. A typical multi-probe anechoic chamber (MPAC) setup for massive MIMO base stations (BSs) is studied, where a limited number of probe antennas are distributed over the probe wall to emulate the target channel by proper probe weighting. The directional antenna patterns would have nonnegligible effects on the emulated spatial characteristics which may become inaccurate for DUTs with directional antennas and thus need to be re-examined. Therefore, the reconstructed channel with the PFS technique will be investigated under the MPAC setup in this paper, with particular focus on the DUTs with directional antennas. The deviations between the covariance matrix of the Rx antennas under the target channel and the covariance matrix under the reconstructed channel are used as the metric to evaluate the performance of the reconstructed channel, and the optimal probe weights are also re-determined. Furthermore, given beamforming capability of massive MIMO BSs, the impact of the BS antenna patterns on angular spectrum is investigated as well. In addition, since the reconstructed channel needs to not only reflect the spatial characteristics of Rx antennas but also reflect the spatial characteristics of transmitter antennas and propagation environment, the end-to-end channel capacities are evaluated. Simulation results under various scenarios show that the accuracies of the reconstructed channel in terms of spatial correlation, angular spectrum and channel capacity can be improved by using the re-optimized probe weights when directional antenna patterns of the DUTs are taken into account.

Virtual Drive Testing Over-the-Air for Vehicular Communications

Multiple-input multiple-output (MIMO) over-the-air (OTA) testing is a standardized procedure to evaluate the performance of MIMO-capable devices such as mobile phones and laptops. With the growth of the vehicle-to-everything (V2X) service, the need for vehicular communication testing is expected to increase significantly. The so-called multi-probe anechoic chamber (MPAC) setup is standardized for MIMO OTA testing. Typically, a test zone of 0.85 wavelength in diameter can be achieved with an 8-probe MPAC setup, which can encompass device-under-test (DUT) of small form factors. However, a test zone of this size may not be large enough to encompass DUTs such as cars. In this article, the sufficient number of OTA probes for the MPAC setup for car testing is investigated with respect to the emulation accuracy. Our investigation shows that the effective antenna distance of the DUT is more critical than its physical dimensions to determine the required number of OTA probes. In addition, throughput measurements are performed under the standard SCME UMa and UMi channel models with the 8-probe MPAC setup and the wireless cable setup, i.e. another standardized testing setup. The results show reasonably good agreement between the two setups for MIMO OTA testing with cars under the standard channel models.

Comparing Channel Emulation Algorithms by Using Plane Waves and Spherical Vector Waves in Multiprobe Anechoic Chamber Setups

This paper evaluates the performances of channel emulation algorithms in the multiprobe anechoic chamber (MPAC) by using plane wave (PW) and spherical vector wave (SVW) theories. Channel emulation in MPAC enables the over-the-air (OTA) testing of performances of wireless devices under realistic propagation scenarios, through setting excitation voltages of probes and utilizing the polarized radiation patterns, locations, and orientations of probe antennas to emulate desired fields in test zone. Accurate emulation of radio wave propagation in target scenario guarantees that the device under test (DUT) be assessed fairly in the laboratory. Dynamic multipath scenario and orthogonal polarization can be emulated by exciting the multiple probes in such a way that the total fields from probes resemble the target impinging field in the test zone. The excitation voltages can be either calculated by PW or SVW theories. Despite the fact that PW and SVW are mathematically equal in the far field, different treatments on rotation and translation of waves as well as different linear equations used in two methods result in different computed voltages, hence, different emulated fields. The emulation performances of the two methods with different MPAC setups (e.g., test zone size, probe number, probe sphere radius, and probe directivity) are investigated. Both scenarios of the 2-D field emulation with the 2-D probe configuration and the 3-D (or 2.5-D) field emulation with the 3-D probe configuration are discussed, and instructions on how to wisely use the emulation algorithm are provided.

Implementation and Analysis of 3D Channel Emulation Method in Multi-Probe Anechoic Chamber Setups

The multi-probe anechoic chamber (MPAC) based method is a candidate to assess the performance of multiple input multiple output (MIMO) devices over the air. This paper investigates the core problem, channel emulation method (CEM), in MPAC setups. The CEM is utilized to map the desired channel onto the limited number of probes to synthesize the target channel environment within a certain test area. In addition, the channel in realistic propagation environment is a three-dimensional (3D) channel, which can achieve higher data transmission rate by containing both azimuth and elevation dimension. Hence, it is urgent to investigate the CEM for 3D channels in MPAC setups. In this paper, the implementation of a novel CEM for 3D channels is presented, which allocates the subpaths in a simple way and complements the drawbacks of prefaded synthesis method and spatial fading emulator. Furthermore, the impact of the target channel and probe configuration on the emulation accuracy in terms of temporal correlation function (TCF) and spatial correlation function (SCF) is discussed. Simulation results demonstrate that the emulation accuracy of TCF depends more on the motion direction of MIMO devices while that of SCF depends more on the probes distribution. Moreover, probes number decides the emulation accuracy first. If the same number of probes are applied, the coincidence of the probe configuration and the target channel determines the emulation accuracy.

A Virtual Over-the-Air Method for 5G Massive MIMO Base Station Testing With Flexible Virtual Probes

Massive multiple-input-multiple-output (MIMO) is known as a large-scale antenna technology and has been enabled for the fifth generation (5G) mobile communication systems. The over-the-air (OTA) testing of massive MIMO antenna system has attracted a lot of attention in terms of evaluating the radio performance of a 5G base station (BS). As a promising OTA test methodology, the sectored multi-probe anechoic chamber (MPAC) setup is employed for the performance testing of massive MIMO BS. However, the system cost and installation are still an issue to be considered in practice (e.g., the anechoic chamber and multiple probes), though it can accurately reconstruct the flexible propagation channels by channel emulation methods. This paper proposes a virtual OTA test framework for the performance testing of massive MIMO BS on the basis of MPAC setups. As an alternative, the proposed virtual OTA test system adopting the concept of virtual probes solves the inconveniences and limitations of the actual probe installation in MPAC setups. The BS array responses to the actual probes of MPAC setups are generated in a phase matrix for the virtual probes. First, in this paper, the system description of virtual OTA testing is given. Then, the emulation method of spatial clusters is presented for the virtual OTA testing. Finally, simulations are performed with good channel emulation accuracy.

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.

Test Zone Size Characterization With Measured MIMO Throughput for Simulated MPAC Configurations in Conductive Setups

This correspondence discusses over-the-air (OTA) testing for multiple-input multiple-output (MIMO) capable terminals, with an emphasis on test zone size characterization for multi-probe anechoic chamber (MPAC) OTA configurations. For an MPAC setup, it is important to understand the test zone size that can be supported by its design. Further, it is desirable that the test zone size should be determined in terms of measured throughput deviation. However, such works have not been reported in the literature yet, due to challenges in practical MPAC setups. In this correspondence, we propose to simulate the MPAC OTA configuration and MS design in a channel emulator and perform MIMO throughput measurements in a conductive setup. With the proposed scheme, we can investigate how large a test zone can be supported for the synthetic MPAC configuration in terms of throughput deviation. This scheme is attractive, since various MS antenna designs and MPAC configurations can be flexibly selected and test zone size in terms of measured MIMO throughput deviation can be achieved. Measurement results show that with a high antenna correlation at the base station (BS) side, measured throughput results would be low, irrelevant to MPAC OTA designs and antenna designs at the MS side. Therefore, the antenna correlation at BS side should be set uncorrelated to ensure that true MS performance can be measured. Further, spatial correlation accuracy at the MS side becomes critical for throughput accuracy only when correlation values are in the high region (e.g., $\rho > 0.5$ ).

Experimental Evaluation of MIMO Terminals With User Influence in OTA Setups

User influence, together with propagation environments and antenna designs, determines how well multiple-input–multiple-output (MIMO) terminals operate in true usage conditions. In this letter, we investigate to what extent the user influence affects the performance of a realistic MIMO terminal under various reproduced spatial channel models in a practical 3-D multiprobe anechoic chamber setup. A terminal mock-up, operating at 2.55 GHz, together with a realistic user phantom was used, and channel models with different spatial and polarization profiles, device under test with different operation modes and orientation angles were tested.

Experimental Evaluation of User Influence on Test Zone Size in Multi-Probe Anechoic Chamber Setups

Over-the-air (OTA) radiated testing for multiple-input multiple-output (MIMO) capable mobile terminals has been actively discussed in the standardization in recent years, where multi-probe anechoic chamber (MPAC) method has been selected, together with the radiated two-stage method. The supported test zone size is a key parameter to determine for an MPAC design, and the test zone size is restricted by the number of OTA antennas. A larger test zone would necessitate more OTA antennas, each port of which is driven by an expensive channel emulator radio frequency interface. Results available in the literature are typically limited to free space scenarios, where no user effect in the vicinity of MIMO terminal is present. There is a concern whether or not the test zone size should encompass the user phantom, together with the mobile terminal in the MPAC setup. To address this issue, an extensive measurement campaign was carried out in this paper. Two realistic long term evolution mockups were designed and their performance were evaluated under standard spatial channel models with and without the presence of user phantom. The measurement results have shown that the nearby user phantom can significantly affect the MIMO performance. However, its impact on the test zone size of the MPAC system is negligible, since emulation accuracy in terms of received power, branch power ratio, antenna correlation, and measured throughput under the target and the emulated channels is not affected by the presence of user phantom. Moreover, results measured with the synthetic MPAC method generally match those obtained with the reference two-stage method. These findings are valuable inputs for the ongoing MIMO OTA harmonization work in the standardization.

MIMO OTA Testing in Small Multiprobe Anechoic Chamber Setups

Over-the-air (OTA) testing of multiple-input-multiple-output (MIMO)-capable terminals is often performed in large anechoic chambers, where planar waves impinging the test area are assumed. Furthermore, reflections from the chamber and probe coupling are often considered negligible due to the large dimensions of the chamber. This letter investigates the feasibility of reducing the physical dimension of 2-D multiprobe anechoic chamber setups for MIMO OTA testing, with the purpose of reducing the cost and space of the setup. In the letter, a channel emulation algorithm and chamber compensation technique are proposed for MIMO OTA testing in small anechoic chambers. The performance deterioration in a small anechoic chamber, i.e., with a ring radius of 0.5 m, is demonstrated via simulations.

Emulating Ray-Tracing Channels in Multiprobe Anechoic Chamber Setups for Virtual Drive Testing

This paper discusses virtual drive testing (VDT) for multiple-input multiple-output (MIMO)-capable terminals in multiprobe anechoic chamber (MPAC) setups. We propose to perform VDT, via reproducing ray-tracing (RT)-simulated channels with the field synthesis technique. Simulation results demonstrate that realistic RT channels can be accurately reproduced within the test zone with a limited number of probes in MPAC setups. The feasibility of performing VDT via reproducing RT-simulated channels is supported by measurement results in a practical three-dimensional (3-D) MPAC setup. The amplitude and phase of the electric field have been measured throughout the test zone with a calibration dipole, and excellent match between simulation and measurement was achieved.

Wideband MIMO Channel Capacity Analysis in Multiprobe Anechoic Chamber Setups

This paper discusses over-the-air (OTA) testing for multiple-input–multiple-output (MIMO) capable terminals with emphasis on wideband MIMO channel capacity analysis in a multiprobe anechoic chamber setup. In the literature, the spatial correlation simulation accuracy at the receiver (Rx) side has been used to determine the test area size for a limited number of probes. However, it is desirable that the test area size is defined in terms of data rate deviation of the simulated channel in the laboratory from that of the target channel model. This paper reports MIMO capacity analysis results for wideband spatiotemporal channel models, with emphasis on the impact of the spatial correlation at the transmit (Tx) side, the channel model, and the spatial correlation at the Rx side on the capacity simulation accuracy. Simulation results show that the number of probes is irrelevant to capacity simulation accuracy when the spatial correlation at the Tx side is in the high region (e.g., $\rho > \mbox{0.7}$ ). Furthermore, when correlation at the Tx side is low, the spatial correlation accuracy is less critical with small correlation at the Rx side. The simulation results are further supported by measurements in a practical multiprobe anechoic chamber setup. The capacity simulation accuracy is shown to be a valid measure to determine the test area size.

Emulating Realistic Bidirectional Spatial Channels for MIMO OTA Testing

This paper discusses over the air (OTA) testing for multiple input multiple output (MIMO) capable terminals with emphasis on modeling bidirectional spatial channel models in multiprobe anechoic chamber (MPAC) setups. In the literature, work on this topic has been mainly focused on how to emulate downlink channel models, whereas uplink channel is often modeled as free space line-of-sight channel without fading. Modeling realistic bidirectional (i.e., both uplink and downlink) propagation environments is essential to evaluate any bidirectional communication systems. There have been works stressing the importance of emulating full bidirectional channel and proposing possible directions to implement uplink channels in the literature. Nevertheless, there is no currently published work reporting an experimental validation of such concepts. In this paper, a general framework to emulate bidirectional channels for time division duplexing (TDD) and frequency division duplexing (FDD) communication systems is proposed. The proposed technique works for MPAC setups with arbitrary uplink and downlink probe configurations, that is, possibly different probe configurations (e.g., number of probes or their configurations) in the uplink and downlink. The simulation results are further supported by measurements in a practical MPAC setup. The proposed algorithm is shown to be a valid method to emulate bidirectional spatial channel models.

Rician Channel Modeling for Multiprobe Anechoic Chamber Setups

This letter discusses over-the-air (OTA) testing for multiple-input–multiple-output (MIMO)-capable terminals, with emphasis on modeling Rician channel models in the multiprobe anechoic chamber setups. A technique to model Rician channels is proposed. The line-of-sight (LOS) component, within arbitrary polarization and an arbitrary impinging direction, and non-LOS (NLOS) component, with any impinging power angular spectrum (PAS), can be created. Simulation results show that the emulated Rician channels approximate the target models accurately in terms of field envelope distribution, $K$ -factor, Doppler spectrum, and spatial correlation at the receiver (Rx) side.