Ryan J. Pirkl

Reverberation Chamber Measurement Correlation

This contribution evaluates the utility of several different metrics for studying correlation between reverberation chamber measurements collected at different stirrer positions. Metrics considered are the autocovariance, the correlation matrix, and two metrics based upon the entropy of the data correlation matrix: 1) the effective number of uncorrelated measurements and 2) the measurement efficiency. The different metrics are shown to be useful for different correlation analyses. Application of these metrics reveals that the correlation between reverberation chamber measurements is strongly affected by stirring methodology, loading configuration, and measurement frequency.

Modeling the Effects of Nearby Buildings on Inter-Floor Radio-Wave Propagation

Two buildings (A and B) have been modeled and analyzed with a 2D TEz implementation of the finite-difference time-domain (FDTD) algorithm in order to identify and characterize the mechanisms allowing signals to propagate between floors, specifically reflection and scattering from nearby buildings. Results have been extended to 2.5D by assuming isotropic spreading in the third dimension. In both scenarios considered, reflections from surrounding buildings are found to increase the average received power on adjacent floors-up to 9.7 dB and 32 dB for buildings A and B respectively. Measurements of the impulse response in Building A, made with a sliding correlator channel sounder, show a number of long-delay pulses, which can be attributed to specific reflection paths. Based on these findings, a simple two-component propagation model to predict the sector-average signal strengths is proposed and validated against measurements of the received power. The direct component is modeled as free space with a 22 dB/floor attenuation factor, and the reflected component is modeled as free space with reflection/transmission coefficients of 0.5. The RMS prediction error for this model is 3.2 dB.

MIMO over-the-air research, development, and testing

Multiple-input multiple-output (MIMO) over-the-air (OTA) measurements and simulations for network and terminal performance evaluation and prediction have become very important research topics in recent years. Research into MIMO OTA for standardisation purposes has been ongoing in The Wireless Association (CTIA), the Third Generation Partnership Project (3GPP), and the European Cooperation in Science and Technology (COST) for three years. This is motivated by the urgent need to develop accurate, realistic, and cost-effective test standards for UMTS and LTE systems. Although many MIMO-capable networks are already deployed, there is pressure to finish the test standards by the end of 2012. While the first MIMO devices appeared some years ago and were commercially deployed two years ago, there are not yet any standards for testing MIMO performance OTA. The development of MIMO OTA test standards has proven to be particularly complex compared to single-input single-output (SISO) OTA, and developing a test standard is taking considerable time. Unlike SISO OTA, which was relatively straightforward and purely a function of the device, MIMO OTA is highly dependent on the interaction between the propagation characteristics of the radio channel and the receive antennas of the UE. Consequently, the existing SISO measurement techniques are unable to test the UE’s MIMO properties. Many different MIMO test methods have been proposed, which vary widely in their propagation channel characteristics, size, and cost. Many challenges remain in the areas of identifying the optimal channel models and test method(s), and it is possible that the outcome could be that more than one test methodology will be standardized. Current standards activities are concentrated on showing if the proposed test methodologies provide the same results, with the ultimate goal being to clearly differentiate good from bad MIMO devices. The aim of this special issue, guest edited by a balanced representation from across academia and industry is to provide a valuable source of information for the state of this important research area. Section 2 of this introductory paper provides an introduction to MIMO OTA standardization activities, and Section 3 describes the different test methodologies under consideration by 3GPP/CTIA. A comparison between test methodologies is made in Section 4. A summary of the papers accepted for publication in this special issue is presented in Section 5. These articles discuss important aspects of MIMO OTA testing and the latest advances of all test methodologies. The research represents the latest thinking of well-known experts in industry and academia and will undoubtedly influence future decisions on testing standardization. Some conclusions and future work are provided in Section 6.

Uncertainty From Choice of Mode-Stirring Technique in Reverberation-Chamber Measurements

We develop methods for assessing the component of measurement uncertainty arising from various combinations of mode-stirring techniques in reverberation-chamber measurements. We first develop a components-of-variance model that describes this component of uncertainty in terms of physical mechanisms related to the chamber. We illustrate the use of the model in conjunction with measurements to identify the optimal mode-stirring sequence for a measurement of received power.

Parameter Estimation and Uncertainty Evaluation in a Low Rician K -Factor Reverberation-Chamber Environment

In this paper, we study statistical methods for estimating the Rician K-factor when this parameter is small. A fiducial approach for making statistical inference on the K-factor is discussed. The approach requires a Monte Carlo method to compute the uncertainty of a K-factor estimate. Simulation and measurement studies are carried out to evaluate the performance of the proposed procedure and compare it with some of the existing procedures. A procedure for assessing the adequacy of the estimation of the K-factor via quantile plots is presented.

Spatial Autocovariances of Scattering Parameters Measured in a Lossy Reverberation Chamber

We derive a statistical model for the spatial autocovariance of two-port scattering parameters measured in a high-loss reverberation chamber for single-mode antennas. The theoretical model accounts for first-order antenna-antenna and antenna- environment interactions arising from scattering off the measurement antennas. The model is shown to provide a good description of the empirical spatial autocovariances calculated from measurements of scattering parameters in a reverberation chamber for different loading configurations. This provides new insight into the complex antenna-antenna and antenna-environment interactions that occur in reverberation chamber measurements. Additional applications include reducing the correlation between reverberation chamber measurements obtained with position stirring and measuring the orientation-averaged total scattering cross section of arbitrary structures.

The reverberation chamber's unstirred field: A validation of the image theory interpretation

Synthetic aperture measurements of a reverberation chambers unstirred wireless channel are used to compare the observed power, time-of-arrival, and angle-of-arrival of unstirred multipath components to that predicted by ray/image theory for a rectangular cavity. An examination of the ray paths corresponding to erroneously predicted unstirred multipath components revealed that these ray paths intersect the reverberation chambers mode-stirring paddles, absorber blocks, and various other objects in the chamber. This inspired a simple image-blocking model for the reverberation chambers unstirred wireless channel, whereby contributions from ray paths intersecting the chambers mode-stirring paddles and absorbers are neglected. This model elucidates the unstirred wireless channels geometry-based multipath structure, justifies established best practices for reverberation chamber measurements, and enables the development of more effective techniques for mitigating the reverberation chambers unstirred field components.

Characterizing reverberation chambers by measurements of the enhanced backscatter coefficient

Reverberation chambers (RC) are becoming a popular alternative testing facility for a wide range of electromagnetic applications. In order for these tests to have any meaning, we need to ensure that the RC is performing in a desired manner. One common approach for characterizing the RC is based on a measurement of field uniformity in the chamber. In this paper, we present an alternative technique for characterizing a RC, based on the enhanced backscatter coefficient (a quantity analogous to the enhanced backscatter that has been derived for scattering by a random medium) measured in the chamber.

Spherical Wave Scattering Matrix Description of Antenna Coupling in Arbitrary Environments

A framework is presented for investigating antenna coupling in arbitrary environments by way of antenna and environment scattering parameter matrices. The environment scattering parameter matrices are bona fide scattering parameters that describe the environment when all radiation ports are terminated with nonreflecting loads. Simplification of the antenna coupling equations leads to a second formulation that is compatible with classically-defined antenna scattering parameter matrices. A third formulation, based on an auxiliary set of matrices describing the environments scattering when all unexcited radiation ports are open-circuited, extends the coupling formulation used in spherical near-field antenna measurements to arbitrary environments.

MIMO Channel Capacity in 2D and 3D Isotropic Environments

We analyze theoretical distributions of MIMO channel capacity for different antennas in 2D and 3D statistically isotropic environments, which may be generated by multiprobe anechoic and reverberation chambers, respectively. We observe that the two environments yield comparable capacity distributions provided that (1) the 2D statistically isotropic environment’s capacity data are taken at many different antenna orientations and (2) the radiation elements have a low directivity. When these conditions are met, we find that the relative error between the 2D statistically isotropic environment’s orientation-combined capacity distribution and the 3D statistically isotropic environment’s capacity distribution is typically less than 10% for signal-to-noise ratios greater than 5 dB.