Davy P. Gaillot

Magnetic control of negative permeability metamaterials based on liquid crystals

We report on the tunability, by a magnetic field, of a negative permeability metamaterial consisting of stacked arrays of broadside-coupled split ring resonators infiltrated with liquid crystals (LCs). The resonant frequency shift was numerically assessed by a rigorous anisotropic analysis of the reorientation of LC molecules. Experiments were carried out with a prototype designed and fabricated for X-band operation and infiltrated with a nematic compound with optical birefringence Δn=0.18. Scattering parameters vectorial analysis shows a good agreement between the resonant frequency shifts predicted under anisotropic conditions and those measured under static magnetic control.

Joint Carrier Frequency Offset and Fast Time-Varying Channel Estimation for MIMO-OFDM Systems

In this paper, a novel pilot-aided iterative algorithm is developed for MIMO-OFDM systems operating in fast time-varying environment. An L-path channel model with known path delays is considered to jointly estimate the multi-path Rayleigh channel complex gains and Carrier Frequency Offset (CFO). Each complex gain time-variation within one OFDM symbol is approximated by a Basis Expansion Model (BEM) representation. An auto-regressive (AR) model is built for the parameters to be estimated. The algorithm performs recursive estimation using Extended Kalman Filtering. Hence, the channel matrix is easily computed and the data symbol is estimated with free inter-sub-carrier-interference (ICI) when the channel matrix is QR-decomposed. It is shown that only one iteration is sufficient to approach the performance of the ideal case for which the knowledge of the channel response and CFO is available.

Experimental Analysis of Dense Multipath Components in an Industrial Environment

This work presents an analysis of dense multipath components (DMC) in an industrial workshop. Radio channel sounding was performed with a vector network analyzer and virtual antenna arrays. The specular and dense multipath components were estimated with the RiMAX algorithm. The DMC covariance structure of the RiMAX data model was validated. Two DMC parameters were studied: the distribution of radio channel power between specular and dense multipath, and the DMC reverberation time. The DMC power accounted for 23% to 70% of the total channel power. A significant difference between DMC powers in line-of-sight and nonline-of-sight was observed, which can be largely attributed to the power of the line-of-sight multipath component. In agreement with room electromagnetics theory, the DMC reverberation time was found to be nearly constant. Overall, DMC in the industrial workshop is more important than in office environments: it occupies a fraction of the total channel power that is 4% to 13% larger. The industrial environment absorbs on average 29% of the electromagnetic energy compared to 45%-51% for office environments in literature: this results in a larger reverberation time in the former environment. These findings are explained by the highly cluttered and metallic nature of the workshop.

Millimeter-Wave Propagation: Characterization and modeling toward fifth-generation systems. [Wireless Corner]

The World Radiocommunication Conference 2015 (WRC-15) identified a number of frequency bands between 24 and 86 GHz as candidate frequencies for future cellular networks. In this article, an extensive review of propagation characteristics and challenges related to the use of millimeter wave (mm-wave) in future wireless systems is presented. Reference to existing path-loss models including atmospheric and material attenuation in recommendations of the International Telecommunication Union (ITU) in Geneva, Switzerland, is given, and the need for new multidimensional models and measurements is identified. A description of state-of-the-art mm-wave channel sounders for single and multiple antenna measurements is followed by a discussion of the most recent deterministic, semideterministic, and stochastic propagation and channel models. Finally, standardization issues are outlined with recommendations for future research.

An all-dielectric route for terahertz cloaking

An original all-dielectric design that performs cloaking at terahertz frequencies is demonstrated. The cloak consists of radially positioned discretized micrometer-sized cylindrical elements. Based on Mie theory and under adequate excitation conditions (H along the rod axis), high-κ cylinders exhibit a strong magnetic resonance dependent on the cylinder radii and material properties. Full-wave simulations coupled with a field-summation retrieval technique were employed to adjust the electromagnetic response of individual ferroelectrics rods (Ba0.5Sr0.5TiO3; ε = 200 - tan δ = 2.10-2). The rods magnetic plasma frequency was engineered such that the full cloak displays a progressive variation in its permeability radial component; hence satisfying, for this polarization, the reduced equations derived from the conformal transformation theory. The cloaking performance was assessed by modelling the complete micro-structured device. Results unambiguously show that cloaking of any wavelength scaled objects located inside the cloak is achieved above the Mie resonance frequency at 0.58 THz for the present device. In particular, the phase fronts of the electric field behind the device are well reconstructed with a high value in transmission of the incident plane wave. This also means that the absorption losses are small within the cloak in comparison with the metallic systems originally proposed. Although cloaking is observed in a narrow band, this all-dielectric configuration provides an attractive route for designing cloaking devices at microwave and terahertz frequencies.

Validation of experimental whole‐body SAR assessment method in a complex indoor environment

Experimentally assessing the whole-body specific absorption rate (SAR(wb) ) in a complex indoor environment is very challenging. An experimental method based on room electromagnetics theory (accounting only the line-of-sight as specular path) is validated using numerical simulations with the finite-difference time-domain method. Furthermore, the method accounts for diffuse multipath components (DMC) in the total absorption rate by considering the reverberation time of the investigated room, which describes all the losses in a complex indoor environment. The advantage of the proposed method is that it allows discarding the computational burden because it does not use any discretizations. Results show good agreement between measurement and computation at 2.8 GHz, as long as the plane wave assumption is valid, that is, at large distances from the transmitter. Relative deviations of 0.71% and 4% have been obtained for far-field scenarios, and 77.5% for the near field-scenario. The contribution of the DMC in the total absorption rate is also quantified here, which has never been investigated before. It is found that the DMC may represent an important part of the total absorption rate; its contribution may reach up to 90% for certain scenarios in an indoor environment.

Deterministic and Experimental Indoor mmW Channel Modeling

This letter presents an extensive multidimensional analysis of line-of-sight (LOS) experimental data and simulations at 60 GHz over a 9-GHz bandwidth. Numerical versions of the measured multiple-input-multiple-output (MIMO) channel transfer functions were obtained with a ray-tracing engine that includes single-order diffuse scattering. The received power, RMS delay spread (DS), and maximum excess delay (MED) computed from both measured and simulated data indicate that diffuse scattering improves ray-tracing-based modeling. Moreover, the multipath components (MPCs) extracted from both sets of data using the high-resolution estimator RiMAX were statistically compared. The analysis of the results shows that even a raw description of the environment can be used to predict millimeter-wave (mmW) propagation with ray tracing.

Double Directional Channel Measurements in an Arched Tunnel and Interpretation Using Ray Tracing in a Rectangular Tunnel

The objective of this paper is to study the wideband characteristics of the radio channel in a tunnel environment, not only the delay spread, but also the angle of departure/arrival of the rays, their relative weights and their delays, which are important values for Multiple-Input Multiple-Output applications. In order to achieve this goal, a measurement campaign has been carried out in a straight arched tunnel over a frequency band extending from 2.8 to 5.0GHz and distance varying from 50m up to 500m. First, the variations of the channel impulse response and of the delay spread versus the distance between the transmitter and the receiver are analyzed. Then, the bidirectional channel characteristics have been extracted from the measured channel matrices using a high resolution estimation algorithm. The main contribution of this paper is to clearly show the quantitative variation of the delay spread and the angle of departure/arrival of the rays along a real tunnel and to investigate the possibility of using the ray theory in a rectangular tunnel to interpret experimental results obtained in an arched tunnel.

Assessing Whole-Body Absorption Cross Section For Diffuse Exposure From Reverberation Chamber Measurements

An original experimental protocol is developed to assess the whole-body absorption cross section of objects with arbitrary shapes and materials in diffuse fields at any operating frequency. This approach is important for dosimetry specifically in realistic environments wherein diffuse fields can be prominent. For this application, the knowledge of the whole-body specific absorption rate is critical and can be determined from the human wholebody absorption cross section. The whole-body absorption cross section is obtained from measurements performed in a stirred-mode reverberating chamber processed with the high-resolution parameter estimator RiMAX. To validate the proposed approach and highlight its robustness, the whole-body absorption cross section of a cylindrical phantom is experimentally and numerically determined at 1800 MHz. For both methods, the whole-body absorption cross section is shown to be independent on the orientation of the transceivers, indicating that it is indeed caused by diffuse fields. A good agreement is obtained between experimental and numerical finite-difference time-domain results with a relative deviation of about 17%. From the validation of this approach, the measurement protocol is applied to a real human at 1800 MHz resulting in a whole-body absorption cross section of 0.95 m2, 1.01 m2, and 1.11 m2 for a sitting, standing, and standing with stretched arms posture, respectively.

On-body calibration and measurements using personal radiofrequency exposimeters in indoor diffuse and specular environments.

For the first time, response of personal exposimeters (PEMs) is studied under diffuse field exposure in indoor environments. To this aim, both numerical simulations, using finite-difference time-domain method, and calibration measurements were performed in the range of 880-5875 MHz covering 10 frequency bands in Belgium. Two PEMs were mounted on the body of a human male subject and calibrated on-body in an anechoic chamber (non-diffuse) and a reverberation chamber (RC) (diffuse fields). This was motivated by the fact that electromagnetic waves in indoor environments have both specular and diffuse components. Both calibrations show that PEMs underestimate actual incident electromagnetic fields. This can be compensated by using an on-body response. Moreover, it is shown that these responses are different in anechoic chamber and RC. Therefore, it is advised to use an on-body calibration in an RC in future indoor PEM measurements where diffuse fields are present. Using the response averaged over two PEMs reduced measurement uncertainty compared to single PEMs. Following the calibration, measurements in a realistic indoor environment were done for wireless fidelity (WiFi-5G) band. Measured power density values are maximally 8.9 mW/m(2) and 165.8 μW/m(2) on average. These satisfy reference levels issued by the International Commission on Non-Ionizing Radiation Protection in 1998. Power density values obtained by applying on-body calibration in RC are higher than values obtained from no body calibration (only PEMs) and on-body calibration in anechoic room, by factors of 7.55 and 2.21, respectively. Bioelectromagnetics. 37:298-309, 2016.