Jonas Fridén

Transient electromagnetic wave propagation in anisotropic dispersive media

In this paper transient electromagnetic wave propagation in a stratified, anisotropic, dispersive medium is considered. Specifically, the direct scattering problem is addressed. The dispersive, anisotropic medium is modeled by constitutive relations (a 3 × 3 matrix-valued susceptibility operator)con taining time convolution integrals. In the general case, nine different susceptibility kernels characterize the medium. An incident plane wave impinges obliquely on a finite slab consisting of a stratified anisotropic medium. The scattered fields are obtained as time convolutions of the incident field with the scattering kernels. The scattering (reflection and transmission)k ernels are uniquely determined by the slab and are independent of the incident field. The scattering problem is solved by a wave splitting technique. Two different methods to determine the scattering kernels are presented; an imbedding and a Green functions approach. Explicit analytic expressions of the wave front are given for a special class of media. Some numerical examples illustrate the analysis. (Less)

Inverse scattering for anisotropic mirror image symmetric media

An inverse medium scattering problem for a homogeneous slab of anisotropic dispersive medium is presented. The inverse problem is to recover the dyadic susceptibility kernel from the dyadic reflection kernels for obliquely incident transient electromagnetic plane waves. A time domain technique, based on wave splitting and invariant imbedding, is used. Mirror images enter into the formalism in a natural way and are therefore discussed. This leads to a simplified problem for a class of mirror image symmetric media. This is solved numerically and the stability of the algorithm is tested using noisy data.

Antenna Current Optimization for Lossy Media With Near-Field Constraints

Optimization of the current distribution is used to analyze how small antennas are affected by amplitude constraints on the near field and by lossy background media. The optimal antenna current that minimizes the stored energy, for a prescribed radiated field in a given direction, and with limited near field in a set of control points, is formulated as a convex optimization problem. The analysis is also extended to antennas in lossy media by using a frequency-derivative approximation of the stored energy. The results suggest that many fundamental antenna problems involving near-field constraints and lossy background media can be analyzed using convex optimization.

Radio frequency electromagnetic field compliance assessment of multi‐band and MIMO equipped radio base stations

In this paper, different methods for practical numerical radio frequency exposure compliance assessments of radio base station products were investigated. Both multi-band base station antennas and antennas designed for multiple input multiple output (MIMO) transmission schemes were considered. For the multi-band case, various standardized assessment methods were evaluated in terms of resulting compliance distance with respect to the reference levels and basic restrictions of the International Commission on Non-Ionizing Radiation Protection. Both single frequency and multiple frequency (cumulative) compliance distances were determined using numerical simulations for a mobile communication base station antenna transmitting in four frequency bands between 800 and 2600 MHz. The assessments were conducted in terms of root-mean-squared electromagnetic fields, whole-body averaged specific absorption rate (SAR) and peak 10 g averaged SAR. In general, assessments based on peak field strengths were found to be less computationally intensive, but lead to larger compliance distances than spatial averaging of electromagnetic fields used in combination with localized SAR assessments. For adult exposure, the results indicated that even shorter compliance distances were obtained by using assessments based on localized and whole-body SAR. Numerical simulations, using base station products employing MIMO transmission schemes, were performed as well and were in agreement with reference measurements. The applicability of various field combination methods for correlated exposure was investigated, and best estimate methods were proposed. Our results showed that field combining methods generally considered as conservative could be used to efficiently assess compliance boundary dimensions of single- and dual-polarized multicolumn base station antennas with only minor increases in compliance distances.

MIMO Performance of Realistic UE Antennas in LTE Scenarios at 750 MHz

Multiple-input-multiple-output (MIMO) is a technique to achieve high data rates in mobile communication networks. Simulations are performed at both the antenna level and Long-Term Evolution (LTE) system level to assess the performance of realistic handheld devices with dual antennas at 750 MHz. It is shown that MIMO works very well and gives substantial performance gain in user devices with a quarter-wavelength antenna separation.

Quick SAR assessment using dual-plane amplitude-only measurement

Current standardized procedures for measurements of the Specific absorption rate (SAR) of mobile phones and radio base station antennas include a volumetric scan of the electric field strength induced in a head or body phantom. Assessment of multi-band and whole-body SAR requires repeated volumetric scanning over a large part of the phantom and is time-consuming. In order to reduce the total evaluation time, different methods have been proposed to estimate the SAR from measurement data based on sparse volumetric scanning and surface scanning. These methods rely on data fitting with underlying assumptions about the spatial distribution of the fields. In order not to be biased by previous or current antenna design, and to be able to use currently available assessment systems, a model-independent dual-plane-scan method is investigated based on amplitude measurements of the electric field components. The amplitude of the electric field components are measured in two planes close to the phantom surface, and the phase is recovered using an iterative process. The plane wave spectrum of the resulting complex electric field components is then used to propagate the field into the phantom. The measurement time is typically reduced by a factor 5 and in some cases even more. Furthermore, the plane wave spectrum is utilized for fast calculation of the mass-averaged local SAR values. A numerical tolerance study, using single and multi- peak fields with relevant errors superposed, is performed to demonstrate the robustness of the method. The resulting errors in the estimated SAR values are below 1% for realistic positioning errors and signal to noise ratio. Comparisons with measurements in a flat phantom are also made. Moreover, the underlying algorithm can be applied to curved surfaces.

Transient external 3D excitation of a dispersive and anisotropic slab

Propagation of a transient electromagnetic field in a stratified, dispersive and anisotropic slab and related direct and inverse problems are investigated. The field is generated by a transient external 3D source. The analysis relies on the wave splitting concept and a two-dimensional Fourier transformation in the transverse spatial coordinates. An investigation of the physical properties of the split fields is made. To solve the direct and inverse scattering problems, wave propagators are used. This method is a generalization and a unification of the previously used imbedding and Green functions methods. The wave propagator approach provides an exact solution of the transmission operator. From this solution it is possible to extract the first precursor (the Sommerfeld forerunner). These results also hold for a bi-anisotropic slab. An inverse problem is outlined using reflection and transmission data corresponding to four, two-dimensional Fourier parameters. Due to the stratification of the medium, the inverse Fourier transformation is not needed in the inverse problem. (Less)

INVERSE SCATTERING FOR THE HOMOGENEOUS DISPERSIVE ANISOTROPIC SLAB USING TRANSIENT ELECTROMAGNETIC FIELDS

Abstract An inverse scattering problem for a slab containing a homogeneous dispersive anisotropic medium is investigated. The inverse problem is to recover two three-dimensional dyadic susceptibility kernels from knowledge of the scattering kernels. Time domain techniques involving transient electromagnetic plane waves, wave splitting, invariant imbedding and a Green function technique are used. The inverse problem is separated into two parts: The Dynamics Inverse Problem (DIP) and Retrieval of Interior Parameters (RIP). Furthermore, mirror images and the Mirror Image Pair (MIP) are discussed. The DIP is solved numerically by using an inverse algorithm and scattering data from one MIP. The RIP turns out to be well posed (system of Volterra equations of the second kind) and needs in general two MIPs. In the DIP, the equations for initial values using transmission data have in general not a unique solution. Constraints and simplifications for certain classes of media are pointed out. Numerical examples, including noisy data, illustrate the analysis.

Calculation of antenna radiation center using angular momentum

An algorithm to compute the radiation center of an antenna based on the Spherical wave expansion (SWE) is presented. The method is based on the angular momentum vector quantity that is uniquely defined for any antenna far field pattern. The radiation center is defined as the unique point where the magnitude of the angular momentum is minimized with respect to active translations of the far field. This corresponds to minimizing the phase variations in the antenna far field pattern. In addition, the current distribution axis can be determined, corresponding to minimization of the vertical component of angular momentum with respect to rotations.

Practical multi-antenna terminals in LTE system performance simulations

In cellular radio network simulations the modeling of terminal antennas is often extremely simplified. In this paper the results from a study on the impact of using realistic terminal antennas in LTE system simulations are presented. Results from downlink simulations using measured radiation patterns of a number of typical multi-antenna terminals have been compared with results using an ideal antenna model. The results showed that the impact was weak in a scenario with high intercell interference while a substantial performance degradation could be observed in a scenario with low interference. Cell-edge throughput was found to be the most sensitive performance metric which in the worst case suffered a 53% throughput reduction compared to the ideal model. An analysis of the relation between antenna properties and system performance is also presented. It was observed that the geometric mean of the eigenvalues of the pattern covariance matrix has a distinct connection to system performance. It was also found that efficiency is important for cell-edge throughput and that the pattern correlation has impact on peak throughput.