Safieddin Safavi-Naeini

A hybrid technique based on combining ray tracing and FDTD methods for site-specific modeling of indoor radio wave propagation

The paper presents a hybrid technique based on combining ray tracing and finite-difference time-domain (FDTD) methods for site-specific modeling of indoor radio wave propagation. Ray tracing is used to analyze the wide area and FDTD is used to study areas close to complex discontinuities where ray-based solutions are not sufficiently accurate. The hybrid technique ensures improved accuracy and practicality in terms of computational resources at the same time since FDTD is only applied to a small portion of the entire modeling environment. Examples of applying the method for studying indoor structures and penetration of wave from outdoor to indoor are given at 2.4 GHz. Numerical results are compared with known exact solutions or results of the full wave analysis or traditional ray model to demonstrate the accuracy, efficiency, and robustness of the novel method. Numerical results are also compared with reported measurement results for waves at 1.29 GHz penetrating an external wall with metal-framed windows. Cumulative distributions of field envelope obtained from the hybrid method show close resemblance to the Rayleigh distribution, which conforms to the reported measurement results.

Computer diagnosis and tuning of RF and microwave filters using model-based parameter estimation

This paper describes an efficient and robust approach for the computer diagnosis and tuning of RF and microwave filters relying upon model-based parameter estimation (MBPE) and multilevel optimization. The frequency sampled S-parameters are obtained from the measurement, and then an MBPE procedure based on adaptive sampling is employed to approximate the frequency-domain behavior of S-parameters in terms of rational functions. This approach uses a reduced-order system model. The effect of measurement noise is also considered. The approach is applied to coupled resonator filters that are modeled by a general equivalent circuit. The loss of each resonator is included in the model by a series resistor. A simple and efficient error function is used to reduce the computational effort of the optimization while improving the speed and robustness of diagnosis process for lossy filters. This approach can be applied to many classes of filters. The proposed approach is demonstrated through numerical examples and application to the manufactured filter.

An FDTD/ray-tracing analysis method for wave penetration through inhomogeneous walls

A novel method of studying wave penetration through inhomogeneous walls using the hybrid technique based on combining finite-difference time-domain (FDTD) and ray tracing methods is presented . The FDTD method is used to analyze the transmission characteristics of inhomogeneous walls. Using the knowledge of the tangential electric and magnetic field distributions along the borders of the FDTD computation domain, rays are sent out to cover the rest of the environment so that prediction of signal coverage can be made more efficiently without compromising the accuracy. Numerical results of the method have been compared and shown to agree very well with those of measurement and those of full wave analysis. Examples have shown the inadequacy of the traditional ray tracing method in the presence of walls made of concrete blocks. However, the proposed method can accurately predict signal coverage by taking into account the scattered fields by the inhomogeneity inside the walls. The method does not add much to computational complexity. Reduction in computation time is even more significant when the incident waves can be approximated to be plane waves and the wall structure is periodic.

On the determination of resonant modes of dielectric objects using surface Integral equations

Although surface integral equations have been extensively used for solving the scattering problem of arbitrarily shaped dielectric objects, when applied to the resonance problem, there are still some issues not fully addressed by the literature. In this paper, the method of moments with Rao-Wilton-Glisson basis functions is applied to the electric field integral equation (EFIE) for solving the resonance problem of dielectric objects. The resonant frequency is obtained by searching for the minimum of the reciprocal of the condition number of the impedance matrix in the complex frequency plane, and the modal field distribution is obtained through singular value decomposition (SVD). The determinant of the impedance matrix is not used since it is difficult to find its roots. For the exterior EFIE, the original basis functions are used as testing functions; for the interior EFIE, the basis functions rotated by 90/spl deg/ are used as testing functions. To obtain an accurate modal field solution, the impedance matrix needs to be reduced by half before SVD is applied to it. Numerical results are given and compared with those obtained by using the volume integral equation.

Rapid calculation of the Green's function in a rectangular enclosure with application to conductor loaded cavity resonators

A new technique for rapid calculation of the Greens functions in a rectangular cavity is presented. The method is based on a best polynomial approximation in three dimensions, which is implemented through a fast cosine transform. Generating the required samples for polynomial modeling is greatly accelerated through Ewald summation technique. To validate the efficiency of the resulting Chebyshev series for the potential Greens functions, a surface integral-equation (SIE) formulation is used to compute the resonant frequency of conductor loaded cavity resonators. The new scheme is proved to be considerably faster than Ewald transform in filling the method of moments (MoM) matrix. A SIE with the MoM can now be efficiently used for electromagnetic analysis and optimization of conductor or dielectric loaded resonators and filters with rectangular enclosures.

Design and analysis of a combination antenna with rectangular dielectric resonator and inverted L-plate

A wide-band compact antenna combining a rectangular dielectric resonator (DR) with a grounded and inverted L-plate is proposed. Two different impedance matching circuits, a vertical T-shaped strip attached to one side of the DR and a microstrip stub terminated into a T-branch, are introduced to enhance the impedance matching and to economize the space utilization by the matching circuits. A two-step procedure is applied to design the radiation element and the feed structure of the antenna. Approximate formulas are used to obtain initial estimation of the antenna dimensions. Then finite-difference time-domain method (FDTD) with the help of fast Fourier transform/Pade/spl acute/ method is used for fine-tuning these dimensions to obtain satisfactory resonant frequency and quality factor. Next, the feed structure is designed and adjusted to obtain the optimal impedance matching. Since the radiation element and the feed structure are designed separately, the design process is simplified and both their performances are optimized within the frame of design restrictions. Two antennas, for 2.4 and 5.8 GHz Bluetooth and wireless local-area network applications, are designed as examples. The -10 dB bandwidths between 210 and 240 MHz are obtained. The antenna gains are 1.0 and 1.5 dBi, respectively.

A new representation for the Green's function of multilayer media based on plane wave expansion

In this paper, a new representation for the space domain Greens function of general multilayer media is presented. This approach is based on an efficient plane wave expansion of the source incident field. Using the transmission line model for the multilayer medium, the effect of the layered medium on the incident plane waves is determined by a transmission coefficient, and the amplitude and phase of each plane wave at the field point are obtained. The total field is evaluated by summing the resultant plane waves at the field point. The main advantage of the proposed method is that the spatial domain Greens function of multilayer media can be obtained easily as a summation of simple exponential functions, without the need for Sommerfeld integration or complex image approximation. The plane wave approximation is independent of the parameters of the medium and is valid over a wide frequency range. Furthermore, the exponential form of the plane wave solution makes it possible to compute the method of moments matrix elements analytically for most important types of basis functions in multilayer problems.

Physical modeling and frequency-response analysis of a high-temperature Superconducting terahertz photomixer

A physical modeling and a rigorous theoretical analysis consisting of the two-temperature model and the generalized transmission-line model is used to evaluate a photo-induced terahertz continuous-wave voltage from a dc current-biased high-temperature superconducting (HTS) transmission line by optical heterodyne photomixing. The electrical and optical frequency-response analyses show the amplitude of this voltage increases with increasing beat frequency and decreasing optical frequency of laser beams. Its maximum frequency is found to be limited by the gap frequency of the HTS material, which is consistent with the available experiments reported in the literature. The developed model along with our numerical simulation reveal the ways to produce a coherent traveling-wave and high-power terahertz signal with proper choice of an HTS material, bias condition, geometrical configuration, and parameters of the transmission line and characteristics of two laser beams. This HTS photomixer can play an important role in terahertz transceivers as broadly tunable local oscillators with low-noise/low-power consumption characteristics.

Generalized reaction and unrestricted variational formulation of cavity resonators. I. basic theory

Based on the reciprocity theorem, the reaction concept in electromagnetic theory is generalized to the cases where both surface electric and magnetic currents overlap across boundaries, i.e., neither the E-field, nor H-field meets the continuity conditions. An improved systematic method is then developed to obtain unrestricted variational expressions in a cavity resonator for which the tangential components of the trial fields can be discontinuous across its interior boundaries.

Generalized reaction and unrestricted variational formulation of cavity resonators. II. Nonorthogonal and free-boundary mode-matching method

For pt. I see ibid., vol. 50, no. 11, p. 2480-90 (2002). This paper addresses a systematic method whereby the conventional mode-matching method is generalized to the cases where the set of modes used for the field expansion within a cavity resonator are relaxed to be orthogonal or satisfy any specific boundary conditions. It is shown that this approach is based on the unrestricted variational formulation of a cavity resonator. Reciprocity theorem and generalized reaction are the mathematical foundations of this new formulation. We have shown that the conventional mode-matching method is a special case of this generalized formulation and indeed is variational in nature. More precisely, we have proven that, if the field distribution obtained based on the conventional mode-matching method is used as a trial one in some variational formulas, the resonant frequency will be the same as the one obtained by the mode-matching method.