Luca Bastianelli

Reliable Finite-Difference Time-Domain Simulations of Reverberation Chambers by Using Equivalent Volumetric Losses

A numerical code based on the finite-difference time-domain technique is adopted to simulate a large reverberation chamber including the stirrers and the actual antenna presence in the frequency range 200-1000 MHz. The power of the code lies in the parallelization that allows the execution on high-performance computers. Chamber losses are accounted for by introducing virtual volumetric losses in the hypothesis of ideal conductor walls. The correction that arises from this choice which allows us to speed up the numerical computation is verified by a comparison with the case of wall losses implemented by assuming a finite conductivity. The analysis of the statistics of the chamber field, stirrer uncorrelated positions, Rician K-factor, backscatter, and field uniformity returns overlapping results for the two techniques. The analysis of the spatial field correlation within a spatial grid shows a correlation greater than 0.5 in the whole frequency range and for all stirrer angles, so confirming the equivalence of the two methods to manage losses.

Stirrer Efficiency as a Function of its Axis Orientation

The performance of a mechanical stirrer is numerically investigated, when its rotation axis is moved parallel to the three Cartesian axes in the reverberation chamber. We present results for the same stirrer, and for a stirrer with the same paddles but a different length to match the chamber sides. The number of uncorrelated stirrer positions is calculated using our finite-difference time-domain code, optimized for reverberation chamber simulations in high-performance computers. No significant differences were found in the simulation results when the stirrer axis and length are changed. However, a significant stirrer performance enhancement occurs when the blade width is increased.

Uncorrelated frequency steps in a reverberation chamber: A multivariate approach

The use of correlation matrices to estimate the number of uncorrelated frequency steps of the reverberation chamber field is presented and discussed. Finite-difference time-domain simulations of an actual reverberation facility are performed through an in-house parallel code. The efficiency of this code allows for capturing the field behavior through large spatial volumes. Spatial field samples are used to construct the multivariate correlation function and to evaluate the number of uncorrelated frequencies over a broad band. Results are obtained for single rectangular component and total field. It is found that traditional (autocorrelation based) methods tend to overestimate the number of uncorrelated frequency steps, with respect to the proposed (multivariate) approach.

Effect of absorber number and positioning on the power delay profile of a reverberation chamber

The paper shows how a reliable numerical code allows to optimize the number and the position of absorbers inside a reverberation chamber (RC) to properly reduce the chamber decay time. Simulation carried out on different situations demonstrate that the chessboard strategy is the best choice. Results are compared to measurements showing a good agreement. The RC power delay profile can be tuned close to that of real life environments.

A Comparison Between Different Reception Diversity Schemes of a 4G-LTE Base Station in Reverberation Chamber: A Deployment in a Live Cellular Network

The uplink performance of a real fourth-generation long-term-evolution (LTE) frequency-division multiplexing base station was observed by adopting a reverberation chamber as propagating environment. In the downlink direction, the reception of the LTE signal is limited to mobile station receivers. On the other hand, different reception schemes could be implemented by the base station in the uplink direction. Interference rejection and coordinated multipoint reception criteria are based on spatial diversity and are analyzed in a rich multipath environment. These options could be susceptible to impairments due to the presence of Gaussian noise, also including the more realistic case of a discontinuous noise source. The testing session was carried out under a collaboration program between TIM S.p.A., Nokia, and Università Politecnica delle Marche, and it ended up by introducing envisioned reception solutions of the base station in a live customers cellular network.

Helical Stirring for Enhanced Low-Frequency Performance of Reverberation Chambers

A novel class of volumetric mechanical mode stirring methods is introduced. One particular type, denoted as helical stirring, combines rotation and translation in a single motion. This technique is implemented, and its measured performance is compared to conventional circular stirring by the same paddle. It is observed that helical stirring can significantly increase the number of uncorrelated stir states at sufficiently low frequencies. Higher maximum- and lower minimum-to-average ratios of the magnitude and intensity of the received field are obtained that are traceable to a heavier tail of their probability density functions. While the helix pitch can be optimized per frequency, suboptimal values also yield considerable improvement. The use of a stir matrix facilitates the data analysis and estimation of ensemble statistics for arbitrary values of the helix pitch, beyond its experimentally realized discrete values.

Analysis of field probe perturbation in a mode stirred reverberation chamber

The residual stirring action of two commercially available electric field probes is experimentally and numerically analyzed. The capability to stir the reverberation chamber field is not negligible, and it alters the independent positions of mechanical rotating paddles when the spatial correlation matrix method is applied to evaluate them. The numerical code adopted for the simulations reveals its reliability and it can be employed to check probes during the design stage. Finally, we numerically evaluated the stirring effect of a conductor tripod that increases the independent positions.

Evaluation of stirrer efficiency varying the volume of the reverberation chamber

A full-wave finite-difference time-domain code is adopted to simulate a large reverberation chamber including stirrers, actual antennas, and losses. Maintaining the stirrer geometry unaltered, the chamber performance is analyzed by varying its volume. In particular, stirrer uncorrelated positions and field uniformity are analyzed for one and two stirrers. In the case of a chamber with only one stirrer, the number of uncorrelated positions depends on the chamber volume. It increases by reducing the volume. On the contrary, the field uniformity continues to gain benefits from a chamber enlargement. A chamber with two stirrers operating in a interleaved mode has already a very good performance which shows a weakly dependence on the volume variation.

Base-Case Model for Measurement Uncertainty in a Reverberation Chamber Including Frequency Stirring

We address the uncertainty of reverberation chamber (RC) measurements in presence of both mechanical and frequency stirring (FS). A base-case model is derived for reverberation fields affected by the measurement uncertainty due to the lack of a perfect statistical uniformity of fields in a RC. It is found that the measurement uncertainty associated with the FS depends on both the total uncorrelated samples and the local insertion loss (IL). The local IL depends on the frequency stirring bandwidth (FSB). The model allows us for obtaining separate measurement uncertainty contributions. Measurements support the achieved uncertainty model. In particular, results show that the dependence on the IL is normally rather weak also when very wide FSBs are used.

Full wave analysis of chaotic reverberation chambers

In this paper, we study possible ways to improve the Reverberation Chamber (RC) behavior. There could be benefits in deforming the RC rectangular geometry of the baseline cavity by inserting spherical diffractors into the RC or by titling the walls of the RC. We demonstrate that increased field disorder and randomization can be achieved with both the asymmetric structures in presence of the same mechanical stirrer. It is found that the improvement is prominent at low frequencies, with respect to the lowest usable frequency (LUF), which is consistent with results obtained by other investigators on similar RC structures. Numerical Finite-Difference Time-Domain (FDTD) simulations are performed to demonstrate the field chaoticity improvement in terms of both uncorrelated excitation frequencies and stirrer configurations.