Minhyuk Kim

A New Weighted Correlation Coefficient Method to Evaluate Reconstructed Brain Electrical Sources

Various inverse algorithms have been proposed to estimate brain electrical activities with magnetoencephalography (MEG) and electroencephalography (EEG). To validate and compare the performances of inverse algorithms, many researchers have used artificially constructed EEG and MEG datasets. When the artificial sources are reconstructed on the cortical surface, accuracy of the source estimates has been difficult to evaluate. In this paper, we suggest a new measure to evaluate the reconstructed EEG/MEG cortical sources more accurately. To validate the usefulness of the proposed method, comparison between conventional and proposed evaluation metrics was conducted using artificial cortical sources simulated under different noise conditions. The simulation results demonstrated that only the proposed method could reflect the source space geometry regardless of the number of source peaks.

Precise Estimation of Correlated Bio-Electromagnetic Activities in Deep Source Space

In this paper, an improved Wiener inverse technique for precisely estimating the correlated bio-electromagnetic activities in the deep source space is proposed. A novel weighting matrix building method obtained from the sensitivity similarity degree of the sensor array between the sources is presented, so as to enhance the property representation for the correlated deep sources. The results confirm that the proposed technique provides more detailed information for the source estimation, improves the result accuracy, and is physically more reasonable than the conventional Wiener and beamformer techniques.

Numerical Exposure Assessment Method for Low Frequency Range and Application to Wireless Power Transfer

In this paper, a numerical exposure assessment method is presented for a quasi-static analysis by the use of finite-difference time-domain (FDTD) algorithm. The proposed method is composed of scattered field FDTD method and quasi-static approximation for analyzing of the low frequency band electromagnetic problems. The proposed method provides an effective tool to compute induced electric fields in an anatomically realistic human voxel model exposed to an arbitrary non-uniform field source in the low frequency ranges. The method is verified, and excellent agreement with theoretical solutions is found for a dielectric sphere model exposed to a magnetic dipole source. The assessment method serves a practical example of the electric fields, current densities, and specific absorption rates induced in a human head and body in close proximity to a 150-kHz wireless power transfer system for cell phone charging. The results are compared to the limits recommended by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the IEEE standard guidelines.

Numerical Method for Exposure Assessment of Wireless Power Transmission under Low-Frequency Band

In this paper, an effective numerical analysis method is proposed for calculating dosimetry of the wireless power transfer system operating low-frequency ranges. The finite-difference time-domain (FDTD) method is widely used to analyze bio-electromagnetic field problems, which require high resolution, such as a heterogeneous whole-body voxel human model. However, applying the standard method in the low-frequency band incurs an inordinate number of time steps. We overcome this problem by proposing a modified finite-difference time-domain method which utilizes a quasi-static approximation with the surface equivalence theorem. The analysis results of the simple model by using proposed method are in good agreement with those from a commercial electromagnetic simulator. A simulation of the induced electric fields in a human head voxel model exposed to a wireless power transmission system provides a realistic example of an application of the proposed method. The simulation results of the realistic human model with the proposed method are verified by comparing it with the conventional FDTD method.

A consumer transceiver for long-range IoT communications in emergency environments

As an emerging technology, IoT enables a machine-to-machine connectivity without a network infrastructure. Therefore, IoT is strongly considered a technology for communications in emergency environments. Especially, a WiFi protocol is under standardization as IEEE802.11ah for IoT-based long-range communications. However, the service range is not enough to cover a communication area in emergency situations. This paper addresses a consumer transceiver for long range communications of IoT in emergency environments. The transceiver is based on IEEE802.11ah WiFi protocol which is under standardization for low-cost and low-power services. In this paper, a novel architecture for the transceiver is proposed in order to increase the service range of IEEE802.11ah, which is necessary for the long-range IoT communication of emergency messages in emergency situations. Experimental results show that the presented architecture is suitable for the long-range IoT communication of emergency alert messages.

Comparison and Improvement of Inverse Techniques for MEG Source Connectivity Network Reconstruction

Recent studies on bio-electromagnetic inverse problems have shown that a satisfactory understanding of source mechanisms requires to perform source connectivity analyses. This paper focuses on the comparison of inverse techniques for reconstructing the source connectivity network. The results confirm that the noise effect for linear estimation technique is direct, while, for spatial filtering technique the effect is indirect. Linear estimation is advantageous for the connectivity reconstruction of high quality magnetoencephalography (MEG) data, while, the benefit for the case of spatial filter is low SNR environments. This paper also proposes a modified spatial filtering method to improve the source connectivity reconstruction by using the correlation gram matrix. The results show that the proposed method can increase the reconstruction accuracy, decrease the error fluctuation and enhance the representation for profiles of the original source connectivity network.

An advanced numerical technique for a quasi-static electromagnetic field simulation based on the finite-difference time-domain method

Abstract In this paper, an advanced numerical technique is proposed for a low-frequency electromagnetic field simulation. The finite difference time domain (FDTD) method is one of the best methods for analyzing electromagnetic field problems, which require high levels of precision, such as a heterogeneous whole-body voxel human model. However, directly using the standard FDTD method in the quasi-static frequency band requires an extra-large number of iterations. We overcome this drawback using the hybrid of surface equivalence theorem and quasi-static FDTD (QS-FDTD) method. The surface equivalence theorem is used to consider arbitrary shape of source and the QS-FDTD is applied to obtain electromagnetic response quickly in the low-frequency region. The results of the proposed method are in good agreement with those from a commercial electromagnetic simulator. The proposed method is valid where the quasi-static approximation is satisfied.

A real-time locating system for localization of high-speed mobile objects:

This article addresses a novel real-time locating system for localization of high-speed mobile objects in fading environments. The proposed locating system exploits time difference of arrival measurements based on ultra-wideband signals. However, the ultra-wideband signals cause a frequency-selective fading due to their short time duration, which induces severe inter-symbol interference. Moreover, high-speed objects cause fast fading due to large Doppler spread. Therefore, the fading cases considerably reduce the localization performance. The presented locating system relies on a new localization approach in order to overcome the fading issues, which utilizes a modification of extended Kalman filtering. Especially, the suggested locating method works well even in the zero time difference of arrival case, which occurs due to a very deep fading. Experiment results verify that the proposed real-time locating system gives excellent localization performance in severe fading environments. The results also exhibit that the presented locating system is superior to the conventional locating systems in the localization of high-speed mobile objects under fading environments.

Numerical method for exposure assessment in the low-frequency ranges

This paper proposes an effective numerical method for electromagnetic field exposure assessment using anatomical human-body voxel model in the low-frequency ranges. The proposed method is based on the quasi-static finite-difference time-domain method which can rapidly solve the electromagnetic interaction with heterogeneous dielectric and conductive body excited by plane wave even in low-frequency ranges. The equivalent principle is employed to solve the arbitrary source problems. To verify the proposed approach, the internal electric field induced in the simplified human phantom of sphere model exposed to magnetic dipole source is obtained by our approach and compared to those from theory.

The investigation of measurement method for HPM radiation to the analog switch chip

In this paper we suggest a special measurement system using an optical transformation when an analog switch chip is exposed by an external High Power Microwave(HPM). The proposed measurement system can compare a normal operation with a malfunction of the chip, analyzed signals at each pin of the chip when the HPM radiates to the chip. To analyze accurately the operation of the chip proposed measurement system uses an optical communication method. An electrical signal of the chip is modulated to an optical signal by a laser diode(LD) within the range of the HPM. After that, the modulated optical signal is demodulated to the electrical signal by a photo diode(PD) beyond the range of the HPM and then signal is measured. Because this system has no interference between frequency of microwaves and optical waves, an accurate analysis is possible.