Kun Li

• PhD
• Professor (Associate) at University of Electro-Communications

Characterizing specific absorption rate (SAR) of multi-antenna system is a facing challenge for implementing large-scale multiple-input multiple-output (MIMO) communications in mobile devices. For body-worn antennas, active circuit components capable of overcoming dynamic body area network (BAN) channels make MIMO-SAR complex in compliance testing....

In the last few decades, extensive efforts have been dedicated to developing computational methods for modeling the interaction of the human body with electromagnetic fields (EMFs). These studies are crucial for the establishment of exposure limits in international standards and guidelines for human protection from EMF, as well as for advancing per...

This paper presents a highly compact multiple-input multiple-output (MIMO) antenna system that maintains a high and stable channel capacity above 20 bits/s/Hz in dynamic channels for human-centric Internet of Things (IoT) devices. The MIMO array consists of four compact antenna elements and four compact control circuits operating at 2.45 GHz. This...

In the context of forthcoming sixth-generation (6G) wireless communication, the sub-terahertz and terahertz frequency spectrum are anticipated. At such high frequencies, electromagnetic field (EMF) exposure assessment becomes significantly challenging, requiring substantial computational resources. This paper is the first to utilize machine learnin...

The recently revised ICNIRP guidelines specify basic restrictions for whole-body exposure in terms of whole-body-average specific absorption rate (WBASAR) up to 300 GHz. However, numerical assessments of WBASAR above 10 GHz are difficult to be carried out due to the significant computational burden involved. To overcome this difficulty, this study...

International guidelines and standards for human protection from electromagnetic fields have been revised recently. The Epithelial/absorbed power density (APD) has been used as a new physical quantity for local exposure to frequencies >6 GHz, related to the temperature rise in the superficial layer. The assessment methods of APD for practical expos...

This paper presents a novel method for assessing spatially averaged absorbed power density (sAPD) in irregular human body parts that are exposed to electromagnetic fields (EMFs) at millimeter wave (MMW) bands. To ensure high precision in sAPD assessment, different surface area weights are assigned to surface voxel blocks based on a defined step siz...

A novel wideband dual‐band bandstop filter (DBBSF) with multi‐transmission zeros (TZs) and multi‐reflection zeros (RZs) is presented. It consists of two pairs of symmetric open‐ended coupled lines at two sides and (2N–1) pairs of coupled‐line sections (CLSs) with an open‐circuited stub (OCS) in the middle. Through even‐ and odd‐mode analyses, gener...

This study presents a statistical assessment of clothed human skin model exposure from 20 to 100 GHz. Dielectric property data for two typical textile materials, i.e., cotton and wool, were provided for the first time over the entire frequency range. A statistical analysis of the ratio of absorbed power density (APD) to skin temperature elevation w...

This study presents a parametric analysis of the steady-state temperature elevation in rat skin models due to millimeter wave exposure at frequencies from 6–100 GHz. The statistical data of the thickness of skin layers, namely epidermis, dermis, dermal white adipose tissue, and panniculus carnosus, were measured for the first time using the excised...

The next-generation mobile communication technology may exploit the higher frequency spectrum beyond 100 GHz, while most studies on electromagnetic field exposure from mobile communication devices focus on frequencies below 100 GHz. In this article, the electromagnetic field exposure from subterahertz antennas designed for future mobile communicati...

Background/aim: The widespread use of fifth-generation 5G millimeter-waves (MMW) generates concern about potential adverse health effects. The latest international guidelines for MMW exposure adopt an absorbed power density (APD) of 200 W/m2 to avoid a local temperature rise of 5°C in human tissues as an operational adverse health effect threshold...

The lack of freely available standardized datasets represents an aggravating factor during the development and testing the performance of novel computational techniques in exposure assessment and dosimetry research. This hinders progress as researchers are required to generate numerical data (field, power and temperature distribution) anew using si...

International organizations have collaborated to revise standards and guidelines for human protection from exposure to electromagnetic fields. In the frequency range of 6–300 GHz, the permissible spatially averaged epithelial/absorbed power density, which is primarily derived from thermal modeling, is considered as the basic restriction. However, f...

This study presents a fast and simple estimation method for dosimetry assessment of a clothed human body exposed to electromagnetic fields in millimeter-wave bands. A multivariate regression algorithm was given instead of conventional numerical methods for the first time, which utilized the exact value of skin surface temperature rise that was comp...

This study presented an implementation of weighted-polarization multiple-input–multiple-output (MIMO) antenna for wireless body area networks at 2.45 GHz. The weight function determined by considering the variation in the cross-polarization power ratio (XPR) and the antenna tilt angle was realized by a two-way power divider (PD), where tunable po...

With the advancement of wireless technologies and electronic/electrical devices, humans are exposed to more complicated electric, magnetic, and electromagnetic fields (EMF), which has raised public concerns on potential health effects. Researchers have recently conducted a series of studies on diverse exposure scenarios. In addition, international...

This study presents an investigation of clothed human skin exposure to obliquely incident electromagnetic waves at 60 GHz. We clarified the combined impacts of the cloth material, incidence angle, and polarization on the assessment of transmittance and absorbed power density (APD) at the skin surface. A Monte Carlo simulation was conducted consider...

In this letter, a novel microstrip switchable bandpass filter (BPF) is presented. By switching RF p-i-n diodes on/off, the proposed topology could provide three different filtering states: broadband bandpass filter (BBPF), dual-band bandpass filter (DBPF) and tri-band bandpass filter (TBPF). For each state, the relationship between bandwidths and r...

Recently, international exposure guidelines/standards for human protection from electromagnetic fields were revised. For frequencies between 6-300 GHz, the permissible incident power density is defined as the reference level, which is derived from a new metric “absorbed/epithelial power density” based on thermal modeling. However, only a few groups...

This paper presents a quantitative analysis of the differences between the various definitions of spatially averaged power densities (sIPDs) for localized exposure to electromagnetic near-fields at frequencies from 6 to 100 GHz. The spatially averaged modulus of the complex Poynting vector (sIPDmod) and spatially averaged norm of the real part of t...

This article reports on an intercomparison study on the effect of the incidence angle on the spatial average of incident power density (PD) and resultant temperature rise using computational and thermographic measurement approaches. Two definitions of the spatial average of incident PD—the peak spatial-average normal component of the Poynting vecto...

In this paper, we present a new estimation technique with K-order expressions for evaluating the maximum electric fields radiated from multiple-antenna transmitters such as those in mobile phones, WiFi access points, and cellphone base stations. We start by explaining the need for K-order estimation to provide precisely estimated electric fields, t...

This study presents an investigation of human skin exposure to obliquely incident electromagnetic waves at frequencies from 6 GHz to 1 THz. We aim to clarify the relationship between the power density and the skin surface temperature elevation under various exposure conditions. A Monte Carlo simulation was conducted to assess the transmittance and...

This study aims to clarify the accuracy of a method for assessing the power density in close proximity to a wireless communication device operating above 6 GHz for the assessment of compliance with radio-frequency exposure guidelines. We focused on a near-field reconstruction technique that estimates the power density in close proximity to a wirele...

A new Waffle-iron Ridge Guide (WRG) structure that has the ability to control both wavelength and impedance is proposed. With the proposed structure, not only can the wavelength be controlled over a wide range for both fast- and slow-waves in free space but the impedance can also be controlled. These features can improve the performance of array an...

A mechanical linear sliding phase shifter using waffle-iron ridge guides (WRG) and its application to a beam steering antenna are shown. The structure of the new phase shifter is simpler than we have presented previously in that the number of metal layers has been reduced. Another advantage of the new phase shifter is that with it simple beam steer...

In this paperwe use equivalent circuits to analyze the wave-lengths in a Fast and Slow wave Waffle-iron Ridge Guide (FS-WRG). An equivalent circuit for the transverse direction is employed and the transverse resonance method is used to determine the fast wave wavelength. Another equivalent circuit, for the inserted series reactance in the waveguide...

This paper presents a novel design for a two-element stub-loaded dipole array antenna. The purpose is to obtain the decoupling and orthogonal polarization characteristics using a small antenna configuration. A quarter-wavelength stub loaded on one side of the two elements is operated as a phase shifter to create the opposite phase of current distri...

This paper presents an experimental methodology for bit-error-rate over-the-air (BER-OTA) testing to evaluate the communication quality of body area network (BAN) antennas. We proposed a dual-discrete control method to detect an instantaneous BER using a fading emulator with a dynamic human phantom. Particular emphasis is placed on how to implement...

A simple but efficient method for evaluating the channel capacity of 2 × 2 multiple-input multiple-output (MIMO) antenna systems is proposed. First, the channel capacity of a half-wavelength dipole array antenna is calculated using the Monte Carlo method by changing the incident-wave signal-to-noise power ratio, the power difference between two ele...

This paper presents a calibration method for measuring the channel capacity using a three-dimensional multiple-input multiple-output (MIMO) Over-The-Air (OTA) apparatus. The reference level is calibrated to establish the Gaussian angular power spectra in elevation in consideration of various power losses that exist in the fading emulator. The measu...

This paper presents a methodology of controlling the spatial correlation of BS (base station) antenna realized by a bilateral fading emulator for MIMO-OTA (Over-The-Air) testing. On the basis of Jakes theory, the base station correlation is controlled by setting the initial phases of scatterers in a two-dimensional fading emulator. The experimental...

This paper presents a method to achieve the maximum MIMO channel capacity using a weighted-polarization MIMO antenna. Particular emphasis is placed on the way to adjust the phase shift value between the antenna elements to an optimum value in accordance with the variation of cross polarization power ratio (XPR) and the antenna inclination angle. Bo...

This paper presents experimental studies on the improvement of RF-DC conversion efficiency of the rectenna using a high Q λ/4-coaxial dielectric resonator and distributed capacitors at 850 MHz. In order to achieve an ultimate efficiency, we have constructed a rectenna using both a coaxial dielectric resonator and a film capacitor. The measured resu...

This paper presents a weighted-polarization antenna applied to BAN (Body Area Network) on-body communication systems. A primal objective is to achieve the radio link enhancement in both downlink and uplink on-body channels. The proposed antenna obtains an optimum signal level using a weight function considering the BP-XPR (Body Proximity Cross-Pola...

This paper presents a measurement method on the Bit-Error-Rate (BER) Over-The-Air (OTA) Testing of FSK signals for BAN antennas. The human body shadowing effects when a human walks in a multiple radio wave propagation environment are performed by a developed fading emulator with a dynamic phantom. The results show that the shadowing effect caused b...

This paper presents a new methodology of the over-theair (OTA) assessment for vertically arranged multiple-input multiple-output (MIMO) array antennas. Particular emphasis is placed on how well handset MIMO antennas with a vertically arranged structure are characterized using the limited number of scatterers implemented in a fading emulator. First...

In this paper, we present a weighted-polarization wearable multiple-input multiple-output (MIMO) antenna that is based on radiofrequency (RF) signal processing to realize ultra-high-speed and highcapacity mobile communications. The proposed antenna is comprised of three orthogonal dipoles, two of which can be selected according to a weight function...

This paper presents a preliminary investigation on the body proximity cross-polarization properties considering human walking motion for the applications to BAN (Body Area Network) On-Body wireless communication systems. Firstly, a novel concept of the BP-XPR (Body Proximity Cross-Polarization Power Ratio) in on-body channel is defined, which has b...

This paper presents an automatic impedance-matching algorithm that can sequentially realize a simultaneous conjugate-matching state for all the elements in an N-element array antenna. The analytical results show that the converged solution obtained from the proposed algorithm agrees well with the solution calculated by the analytical deterministic...

This paper presents a three-axis decoupling stubloaded parallel dipole array antenna with tri-orthogonal polarization directivity. The decoupling characteristics are realized using the phase shifter composed of a quarter-wavelength stub placed at two sides of three parallel dipole array antenna for creating the opposite phase of the current distrib...

This paper presents a preliminary investigation on the body proximity cross-polarization properties considering the human walking motions for BAN on-body wireless communication systems. A novel concept of the BP-XPR (Body Proximity Cross-Polarization Power Ratio) in on-body channel is defined, which has been quantified based on relative antenna loc...

This paper presents a dual-polarized turn-style patch antenna for BAN on-body communication systems. Our target is aiming at achieving an orthogonally-polarized omnidirectional radiation pattern in the mounting plane of the antenna, which is similar to the turn-style operating principle. The analytical results of a design example show that the desi...

This paper presents a bilateral fading emulator for MIMO-OTA testing with a function of controlling the correlation of base station. On the basis of the Jakes theory, a methodology for achieving the spatial correlation of a base station antenna by setting the initial phases of scatterers in the two-dimensional channel model is introduced. The exper...

This paper presents a disk-loaded monopole stacked with patch antenna (DLMP) with a low-profile structure. The DLMP has three driven ports, each of which can create independent radiation patterns with orthogonally polarized characteristics. A design example is presented to achieve an overall height of 10 mm at 2 GHz. The calculated results show tha...

This paper presents a new methodology for realizing a Rice channel in BAN Over-The-Air (OTA) testing using a fading emulator with a dynamic phantom. For the proposed apparatus to be effective, the fading emulator must be provided with an appropriate K-factor that represents the actual propagation environment indoors. Further, an implementation of t...

This paper presents an automatic impedance-matching algorithm that can sequentially realize a simultaneous conjugate-matching state for all the elements in an N-element array antenna. The analytical results show that the converged solution obtained from the proposed algorithm agrees well with the solution calculated by the analytical deterministic...

This paper presents a 3-axis weighted-polarization active antenna in order to realize ultra high-speed and high-capacity mobile communications. The proposed antenna is comprised of three orthogonal antennas, two of which can be selected according to a weight function in different situations. The weight function is created based on the variation of...

This paper presents a new methodology of the OTA assessment for 2-dimensional arrangement MIMO array antennas. We have studied the minimum number of scatterers required for conducting a 3-dimensional OTA in a practical apparatus. Particular emphasis is placed on how well handset MIMO antennas with a 2D-structure are characterized using the limited...

This paper investigates the mutual interactions between MIMO antenna radiation and multipath characteristics to obtain useful knowledge that can be employed in the design of a three-dimensional OTA system. In investigating factors that may affect the MIMO channel capacity, we focus on two key parameters, the incident wave power distribution in the...

This paper presents a preliminary experiment for evaluating the channel capacity of a 2 × 2 polarization-controlled MIMO antenna that takes the variation of the radio wave propagation environment and the antenna inclination angle into consideration simultaneously. Firstly, the radiation pattern of a 2 × 2 polarization-controlled MIMO antenna was me...

This paper presents a new methodology of an Over-the-Air (OTA) testing using an arm-swinging dynamic phantom applied to wireless Body Area Network (BAN) systems. The phantom has unique features, in that swinging of the right and left arms and the direction and speed of swing can be controlled independently, leading to excellent capability for repli...

This paper presents the shadowing analysis of a body area network (BAN) diversity antenna based on the statistical measurements of the human walking motion. First, the dynamic characteristics of the arm-swing motion were measured using human subjects, and a statistical analysis was then carried out using the measured data to extract useful informat...

This paper presents a new methodology for BAN-OTA testing using a fading emulator with a dynamic phantom. The key to the success of the proposed apparatus is to provide the fading emulator with a proper K-factor that can represent an actual propagation environment under consideration. Hence, experiments for getting the knowledge of K-factor in indo...

This paper presents analytical results on the BER degradation of QPSK signals for BAN antennas for the case where the combined outcome of shadowing and multipath fading emerge simultaneously and different types of human walking models are employed in the analysis. The results show that a probability of a low signal level is higher than that for the...

This paper presents analytical results on the BER degradation of QPSK signals for BAN diversity antennas for the case where the combined outcome of shadowing and multipath fading emerge simultaneously and statistical model of human walking motion is employed in the analysis. The results show that a probability of a low signal level is higher than t...

This paper presents a new methodology for BAN-OTA Testing using a developed fading emulator with a dynamic phantom. The key to the success of the proposed apparatus is to provide the fading emulator with a proper K-factor that can represent an actual propagation environment under consideration. Firstly, a configuration of the developed fading emula...