Glass Penetrating Transparent Surface with Transmission Improvement for 5G Indoor Communications
Article information
Abstract
A glass penetrating transparent surface (GPTS) is typically implemented using metasurface technology. A GPTS, such as a glass window, must be visually unaffected in environments where propagation control is required. Therefore, we aimed to improve the performance of indoor wireless communication systems for 5G and 6G. In this study, we designed and investigated two different frequency selective surface (FSS) structures, FSS-1 and FSS-2, based on miniaturized elements applied on two separate glasses. FSS-1 was applied on borosilicate glass (GPTS-1), and the results revealed a stable filtering passband around 28 GHz in the Ka band with an impressively low insertion loss of less than 1.7 dB. FSS-2 was applied on conventional glass (GPTS-2), and the measurement results revealed a high insertion loss of 3.7 dB at 30 GHz. However, by adding dielectric slabs to one side and both sides of the conventional glass, the insertion loss improved, reaching 1.7 dB at 30 GHz and 1.08 dB at 27.16 GHz, respectively. To explain this behavior, we introduced a simple equivalent circuit model. Overall, our measured results indicate that the proposed concepts hold potential as viable solutions for signal filtering at millimeter waves on glass windows.
I. Introduction
The fifth-generation (5G) mobile communication system has recently been implemented in several countries, and the expectations for its use in a variety of applications have been increasing [1]. In the process of enhancing coverage for 5G and 6G development, the idea of an intelligent radio environment (IRE) or a reconfigurable intelligent surface (RIS) has attracted much attention [2, 3]. One of the advancements in IRE/RIS is the transparency of the frequency selective surface (FSS), which is used on windows and domes for observation and communication in space [4]. For example, by selectively focusing on frequency, a design can be tailored to applications that demand ultra-high data transfer speeds, which is a key aspect of 5G networks.
This idea, which is referred to as a glass penetrating transparent surface (GPTS), provides an opportunity to incorporate transparent metal surfaces into glass [5, 6]. This integration is essential for enhancing communication quality while maintaining aesthetics and creating multimode communication windows, as illustrated in Fig. 1. Selectively focusing on a specific frequency range helps optimize the transmission process by reducing interference, improving signal quality, and making it more efficient [1].
Concept of signal selectivity through GPTS for 5G indoor communications.
There are numerous studies on GPTS, and they have been categorized into two primary groups: optimizing wave performance—including signal communication [7–13], electromagnetic (EM) shielding [14], and EM absorbers [15]—and controlling EM waves [16–18]. In the context of optimizing wave performance, most studies have focused on using metal mesh as a transparent electrode [7, 8, 11] or using glass windows as substrates with minimal penetration loss [7, 8, 11, 12].
There has been limited research on GPTSs for frequency filtering in 5G millimeter wave communication [7–9]. In [7] and [8], studies based on multilayer FSSs were conducted, revealing moderate transparency, complex structures, and high costs. In [9], a proposed design with a single-layer FSS on 0.8 mm-thick quartz glass faced challenges in building implementation due to the thinness of the glass.
Meanwhile, glass windows play a very important role in the design of transparent FSSs on glass. Millimeter and terahertz waves are clearly affected not only by conventional glass windows—because of penetration losses—but also by environmental factors [19]. One of the easiest ways to improve transmission is to use glasses with low penetration losses (low dielectric losses, low permittivity, smooth surfaces, etc.), such as glass ceramics [20]. However, this approach is very expensive in bulk deployment because of its high cost. An alternative approach involves attaching a dielectric slab of half- or quarter-wavelength thickness onto the glass to enhance transmission [21–23]. This method is often called an anti-reflection coating or dielectric coating. The idea behind this approach is to reduce the reflection of light at the interface between two different media (e.g., glass, air) by introducing a thin layer of material with a refractive index that is carefully chosen to minimize reflections.
In this study, we developed a GPTS design that uses a single-layer FSS on a glass window for 5G indoor communication (n257; 26.5–29.5 GHz). To reduce fabrication costs, the metal surface on the FSS was formed from miniaturized elements, as reported in studies focused on FSS [24–26]. FSS-1 was applied on borosilicate glass (GPTS-1) with a low penetration loss, while FSS-2 was applied on conventional glass (GPTS-2) with a high penetration loss.
A simple equivalent circuit model (ECM) was introduced to clarify the behavior of the GPTSs. GPTS-1 achieved high transparency (>57%), and the bandwidth percentage was 5% at 28 GHz. For GPTS-2, due to the high penetration loss effect on transmission, dielectric slabs with a quarter-wavelength thickness were used on the glass to enhance transmission. By adding dielectric slabs to one side and both sides of the glass windows, the insertion loss improved, equaling 1.7 dB at 28 GHz and 1.08 dB at 27.16 GHz, respectively. The close alignment between the measurement outcomes and the theoretical predictions, along with the full-wave simulations, highlights the robustness of our findings.
II. Design of the Transparent Unit Cell and the Operating Principle
1. Effect of Glass Windows
Millimeter waves, which are the frequencies used in 5G, have shorter wavelengths than lower frequency signals. These waves tend to be more sensitive to obstacles, including glass windows [19]. The type of glass used in windows, such as conventional glass, coated glass, or specialized materials (low-emissivity glass), can affect how millimeter wave signals interact with the surface. The effect of window glass on 5G millimeter wave signals must be carefully considered during the design process, as shown in Fig. 2.
Simulation results for transmission at 28 GHz for glass with varying thicknesses and dielectric constants (ɛr).
In the simulation, two Floquet ports were used based on boundary conditions to excite the structure with incident wave angles of TE 0°. The considered glass had a variable dielectric constant (3, 5, 7.38, and 9) and variable thickness (2–6 mm). In most cases, increasing the dielectric constant or thickness of the glass material resulted in a higher transmission loss. The loss was primarily due to dielectric absorption, which is a process in which the dielectric material absorbs and then re-emits energy or in which signals must traverse a longer distance through the dielectric material, leading to signal attenuation. Additionally, low penetration loss glasses have low dielectric losses, resulting in less effect on transmission than high penetration loss glasses [27].
2. Principles of Operation
The primary distinction between the two FSSs lies in their substrates. As shown in Fig. 3(a), GPTS-1 utilized borosilicate glass, a low-loss substrate with a thickness of 3.3 mm and a dielectric constant (ɛr) and a loss tangent (tanδ) of 4.6 and 0.0037, respectively. In contrast, GPTS-2 employed conventional glass, a high-loss substrate (ɛr = 7.38, tanδ=0.06) with a thickness of 3 mm, as shown in Fig. 3(b). This difference in substrate properties had a significant impact on the characteristics and performance of the respective FSSs. The metal lines on both FSSs were made of copper with a thickness of 35 μm. These metal lines were printed on an ultra-thin polyethylene terephthalate (PET) film (ɛr = 3.5, tanδ=0.022), which had a thickness of 100 μm.
Unit cells of the GPTSs: (a) FSS-1 on borosilicate glass and its dimensions (h1=0.035, h2=0.1, h3=3.3, d=3.5, d1=2.07, d2=3.54, w=s=0.2) and (b) FSS-2 on conventional glass and its dimensions (h1=0.035, h2=0.1, h3=3, d=3.2, d1=2.2, d2=3, w=s=0.2) (unit: mm).
The dimension of the aperture dipole (L) was initially determined by [28] as follows:
where L represents the slot lengths (d2), s represents the slot widths, ɛa is the effective dielectric permittivity (ɛa = ɛ0 +ɛr)/2, ɛ0 is the dielectric constant of the outermost layer (e.g., air), and fc is the resonant frequency at 28 GHz. Compared to glass, PET is very thin. Therefore, the value of ɛr of PET was equal to the value ɛr of glass.
The dimensions of the two FSSs based on a periodic unit structure were then optimized in a simulation conducted using CST software. As shown in Fig. 4(a), if the incident wave was polarized perpendicular to the narrow strips, leading to the accumulation of positive and negative charges on the lower and upper strips, respectively, the shunt impedance became capacitance (C) [22, 23]. In addition, when the incident wave was polarized parallel to the strips, the shunt impedance became impedance (L). Here, L is equal to the sum of the values of L1 and L2. Evidently, the current length of L1 was significantly shorter than that of L2, resulting in an inductance value L1 ≪ L2. For rough estimates or a first-order approximation of capacitance (C), we can ignore inductance L1. The effective capacitance and inductance values can be calculated in accordance with [24, 25]. By combining capacitive and inductive elements within a compact structure, as shown in Fig. 4(b), the resonant frequencies (fc) were expressed as follows:
(a) Capacitance and inductance formed by small metallic strips, (b) equivalent circuit of two GPTSs describing the LC formation on FSS-1. For GPTS-1, L1=0.082 nH, L2=0.263 nH, C1=0.35pF, Zsub=175.77, hsub=3.3, and for GPTS-2, L1=0.087 nH, L2=0.68 nH, C1=0.36pF, Zsub=138.77, hsub=3.
Fig. 5 compares the simulated transmission coefficients between the full-wave EM and the ECM, showing comparable shapes in the transmission responses of both FSSs. FSS-1 and FSS-2 without glass showed almost no reflection at 36 GHz and 39 GHz, respectively. By applying FSSs onto glass, it became evident that glass affects transmission and reduces transmission ability.
Simulated transmission coefficients through the full-wave EM and the ECM: (a) FSS-1 without/with borosilicate glass and (b) FSS-2 without/with conventional glass.
3. Analysis of the Equivalent Circuit Model
Since both GPTSs had the same ECM, to enhance the Q-factor and transmission, the key parameters associated with GPTS-1 were investigated, as illustrated in Fig. 6. Fig. 6(a) shows that the Q-factor increased as the capacitance value (C1) increased and that this occurred concurrently with a decrease in the distance between the two microstrip lines (s) [25]. As shown in Fig. 6(b), an increase in the Q-factor was observed when the inductance values (L2) decreased in correspondence with an increase in the width of the line (w) [25]. L2 induced a transmission zero at around 33 GHz, and the L2 value decreased, the frequency of this transmission zero correspondingly increased.
Transmission coefficient by ECM analysis of GPTS-1 with changing values: (a) capacitance (C1), (b) inductance (L2), (c) the relative dielectric constant of glass (ɛr), and (d) the thickness of the glass (hsub).
In addition, as the dielectric constant of glass decreased, not only did the Q-factor increase, but the transmission was also enhanced, as shown in Fig. 6(c). This demonstrates that the penetration loss of the substrate had a significant effect on transmission. Fig. 6(d) shows that a transmission peak of less than 12 dB occurred at approximately 10.6 GHz. This phenomenon occurred because the thickness of the glass altered the propagation characteristics of EM waves through the material
Based on the above analysis and the results in Fig. 2, the easiest way to improve transmission is to choose a substrate with a low penetration loss (e.g., borosilicate glass for GPTS-1). However, for conventionally available glass windows, such as GPTS-2, which have a high penetration loss, there is an approach that can enhance transmission by incorporating an additional dielectric substrate.
4. Transparency of the FSS Unit Cell
Fig. 7(a) shows that, in the case of GPTS-1, when only the metal line width (w) increased, a constant total area for the unit cell was maintained, and the transmission response in the out-of-band signal typically decreased. This was similar to the results for GPTS-2. In addition, we investigated how the width of the metal line affected both the metal area and the transparency of the two FSSs, as shown in Fig. 7(b). The transparency of the FSS can be calculated as the percentage of light passing (without metal) per the total area of the unit cell [14, 29, 30]. As the metal line width (w) increases, there is an accompanying rise in the metal area, leading to a decrease in the optical transparency (OT). In the present study, opting for a narrow metal line width (100 μm) led to excellent transparency, with FSS-1 of 76% and FSS-2 of 79%. However, it is also crucial to address the challenge of minimizing fabrication costs while attaining the desired transparency. We aimed to improve transmission for frequency filtering at 28 GHz over glass windows using inexpensive dielectric slabs instead of multilayer FSSs, which are costly and complex [7–9]. Therefore, a metal line width of 200 μm was chosen, and the moderate OT values were 57% for FSS-1 and 62% for FSS-2. The flexible application of FSS on glass with multimode windows (Fig. 1) did not significantly affect the aesthetics or transparency of the glass window.
Transmission coefficient by changing the width of the line: (a) GPTS-1 and (b) optical transparency (two FSSs) and the metal area with an adjusted line width.
III. Methods to Enhance Transmission
1. Theory for Single and Double Dielectric Slabs
The utilization of matching and propagation matrices offers a straightforward approach for addressing various interface problems. As shown in Fig. 8, we focused on scenarios involving uniform plane waves that are normally incident on material interfaces. The two-interface problem in Fig. 8(a) depicts a dielectric slab η1 dividing the semi-infinite media ηa and ηb from one another. The slab had a width of l1, a refractive index of n1, a propagation wave number k1 = ω/c1, and a wavelength within the slab λ1 = 2π/k1 = λ1/n1. Additionally, λa was the wavelength inside the medium ηa. We assumed that the incoming wave went from the left medium ηa through slab η1 and exited medium ηb. Based on [21–23], the reflection coefficient (Γ1) of medium ηa was written as follows:
Reflection coefficients for (a) a dielectric slab, (b) glass with a dielectric slab, and (c) glass with two dielectric slabs.
where the two-way travel time delay in the z-domain is denoted as z−1 = e−jωT = e−2jk1l1, and ρ1, ρ2 are the elementary reflection coefficients from two interfaces corresponding to the following:
To enhance transmission from medium ηa to ηbor η1, a reflectionless interface corresponding to Eq. (3) must be equal to zero, which occurs when ρ1 + ρ2z−1 = 0. This led to two cases. Case 1 was expressed as follows:
which translated into the half-wavelength condition of l1 = mλ1/2, which was based on Eq. (4), and corresponded to ηa = ηb.
Case 2 was expressed as follows:
This provided the quarter-wavelength condition l1= (2m + 1)λ1/4, used Eq. (4), and correspond to
From Eq. (5), the following reflection coefficient (Γ1) for the half-wavelength case was obtained:
Similarly, for Eq. (6), the reflection coefficient (Γ1) for the quarter-wavelength case was given as follows [21–23]:
As shown in Fig. 8(b), if the left side of the slab was air, the right side was glass (ɛr = 7.38, n = 1.5), and there was no backward-moving wave in the rightmost medium (Γ3 = 0), the scenario depicted in Fig. 8(a) occurred. We used Eq. (8) so that there was no reflection and chose a dielectric slab made of polyvinyl chloride (PVC) (ɛr = 2.8, tanδ = 0.022, and n = 1.53). The quarter-wavelength thickness of PVC at 28 GHz was calculated to be 1.5 mm. Therefore, the reflection coefficient was approximately 0.03.
However, as shown in Fig. 8(b), assuming that Γ3 ≠ 0, the addition of a dielectric slab on the right side of the glass was necessary to prevent reflections at the glass layer (Fig. 8(c)). In this case, the reflection coefficient (Γ3) was written as follows:
For the anti-reflection, the chosen thickness and permittivity of the dielectric slab were similar to the case shown in Fig. 8(b).
2. Theory Verification by Simulation and Measurement
By varying the dielectric constant of the glass according to Eq. (8), the values of the quarter-wavelength-thick dielectric slabs were calculated. In Fig. 9(a), using a 3 mm-thick glass, the values of the dielectric slab based on Eq. (8) were 2.32 (slab-1), 2.52 (slab-2), and 2.72 (slab-3) when the dielectric constant changed to 5.38 (glass-1), 6.38 (glass-2), and 7.38 (glass-3), respectively. The simulation results for transmission showed improvements of 1.7 dB, 2.03 dB, and 2.2 dB without/with a dielectric slab corresponding to glass-1, glass-2, and glass-3, respectively.
Simulated transmission loss of glass without/with a dielectric slab: (a) change dielectric constant of glass and (b) simulations and measurements of conventional glass (ɛr =7.38) without/with dielectric slabs.
To confirm the theory and simulation, measurement results were studied for the conventional glass (3 mm thick and ɛr = 7.38) without/with a dielectric slab of PVC (1.5 mm thick and ɛr = 2.8), as illustrated in Fig. 9(b). The measurement results for the glass were better than the simulation results. The reason for this discrepancy could be related to the EM properties of the glass at the specified frequency, including its dielectric constant or refractive index. Here are some potential reasons why this might have occurred:
1) Inaccurate material models: Simulations based on mathematical models to describe the EM properties of materials, including their dielectric constants and thicknesses. Material models are often approximations and might not capture all the nuances of glass behavior, especially at specific frequencies.
2) Frequency-dependent behavior: Glass exhibits frequency-dependent behavior that was not adequately accounted for in the simulation. In the simulation, the dielectric constant measured up to 1 GHz and had an average value of 7.38, and a loss tangent of 0.06 was used for the simulation at the full frequency range.
3) Differences arose because of unavoidable experimental errors, in which power was excited and received by real horns instead of the waveguide port in CST. Additionally, the measurement setup, calibration, and environmental conditions could have introduced uncertainties.
At 28 GHz, the transmission results (glass + slab) increased by 2.2 dB in the simulation and by 1.52 dB in the measurement when compared to the case without a slab. When two slabs adhere to the glass (slab + glass + slab), this configuration resembles a sandwich with glass in the middle. Compared to the case with glass only, at 28 GHz, the transmission showed a 3.2 dB improvement in the simulation and a 2.38 dB increase in the measurement.
IV. Fabrication and Measurement Method
To experimentally validate the performance of the proposed FSS, transparent samples of both FSSs, each measuring 20 cm × 20 cm, were fabricated. The copper clad laminate (CCL) fabrication technique developed by DIC Co. Ltd. in the Republic of Korea was used. The CEF0501 transparent adhesive (25-μm-thick, ɛr = 3.72 at 500 kHz) from 3M Co. was also used. Fig. 10(a) depicts the measurement setup with the transmission response for GPTS-1. The horn antenna had dimensions of 37 mm × 47 mm. In our experiment, we positioned GPTS at the center, flanked by two horn antennas with a 1 m separation, to utilize the far-field region and generate a uniform plane wave on the FSS surface [30]. Fig. 10(b) shows a comparison of the OT of conventional glass without/with FSS-1. Fig. 10(c) and 10(d) depict the fabrication of FSS-1 and FSS-2 in a close view.
(a) Measurement setup for the transmission of the proposed GPTS-1, (b) photograph of the OT without/with FSS-1, (c) prototype of the OT FSS-1 on paper, and (d) prototype of the OT FSS-2 on paper.
V. Simulation and Measurement Results
1. GPTS-1
Fig. 11(a) and 11(b) show the simulated and measured transmission coefficients of GPTS-1 with incident wave angles of 0° and 30°, respectively. The measurement results revealed a slight right shift compared to the simulation, possibly due to imperfections in adhering the FSS to the glass. The transparent adhesive was omitted from the simulation. Borosilicate glass has a low-loss substrate, resulting in its 28 GHz insertion loss value outperforming that of conventional glass in the measurements and simulations by approximately 1.7 dB. For the normal incidence waves, the measurement results for GPTS-1 showed a 3 dB transmission bandwidth extending from 26 to 31.5 GHz and a maximum insertion loss of 1.4 dB at 28.4 GHz. While the transmission of GPTS-1 did not reveal a significant improvement compared to glass, it was evident that it effectively blocked the out-of-band frequency. This contributes to an overall enhancement in transmission speed and a significant reduction in potential issues that can affect data transfer [1]. Fig. 11(b) illustrates the TE mode at a 30° incident angle, revealing a maximum frequency deviation of 3%, while the TM mode exhibited a deviation of 1.7%. The 3 dB transmission bandwidths of the TE/TM modes were 26.2–29.44 GHz/25.8–28.56 GHz.
Simulated and measured transmission coefficients of GPTS-1 (a) at normal incidence and (b) with a change in the angle of incidence at 30°.
2. GPTS-2 without/with Dielectric Slabs
Fig. 12 shows the simulated and measured transmission coefficients for GPTS-2 without/with dielectric slabs. For GPTS- 2/Slab (Fig. 12(a)), the slab was placed on the opposite side of the FSS-2 through the glass. In the case of the two slabs adhering to GPTS-2, this configuration resembled a sandwich with GPTS-2 in the middle, as shown in Fig. 12(b).
Simulated configuration with a periodic condition for (a) GPTS-2/Slab and (b) Slab/GPTS-2/Slab.
As shown in Fig. 13(a) and 13(b), when there was no dielectric slab (GPTS-2 only), the simulation and measurement results both indicated that the transmission bandwidths were lower than 3 dB. Fig. 5 clearly depicts the impact of glass on GPTS transmission. Borosilicate glass (low penetration losses) and GPTS-1 both had better transmission results (>–3 dB) than conventional glass (high penetration losses) and GPTS-2.
Simulated and measured transmission coefficients (a) without/ with dielectric slabs at a normal incidence wave, (b) the millimeter-wave band from 20 to 34 GHz, (c) GPTS-2/Slab with a 30° incidence angle, and (d) Slab/GPTS-2/Slab with a 30° incidence angle.
To improve transmission, we proposed using a dielectric slab, as discussed in Section III. When a dielectric slab was present (GPTS-2/Slab), the simulated transmission results improved by 1.84 dB at 28 GHz, while the measured transmission results exhibited a 1.7 dB enhancement at 30 GHz.
The measurement results showed a 3 dB transmission bandwidth extending from 28 to 31 GHz. Additionally, when using two dielectric slabs (Slab/GPTS-2/Slab), the measured transmission results exhibited a 1.08 dB enhancement at 27.16 GHz and a 3 dB transmission bandwidth from 23.36 to 29.24 GHz. In this case, the transmission bandwidth shifted toward lower frequencies, which can be explained based on Eq. (1).
In Fig. 13(c), the GPTS-2/Slab configuration showed 3 dB transmission bandwidths of 28.84–30.6 GHz and 27.44–29.6 GHz for the TE/TM modes at a 30° incident angle, with maximum transmission coefficients of 2.15 dB/2.2 dB. In Fig. 13(d), the Slab/GPTS-2/Slab configuration revealed 3 dB transmission bandwidths of 23.24-29.2 GHz and 23.12–28.48 GHz for the TE/TM modes at a 30° incident angle, with maximum transmission coefficients of 1.27 dB/1.18 dB.
Notably, the measurement results for the transmission of glass were better than the simulation results, as explained in Section III-2. This led to differences in the measured transmission results for GPTS-2 with and without a dielectric slab. A slight rightward frequency shift occurred because of imperfections in the adhesion of the FSS onto the glass, resulting in the formation of small air gaps.
Table 1 presents a comparison of our proposed FSS and recent designs with similar applications. The analysis highlights that our designs in having the narrowest 3 dB fractional bandwidth (FBW), minimal transmission loss, satisfactory OT, and a lower fabrication cost compared to metal mesh or metal coating. Notably, our work, with its crucial window glass thickness for stability in this application, has proven to be more suitable for building use than the methods outlined in prior studies [9, 10, 13]. For the incident angle, when TE polarization was considered, the incident angle increased, and wave impedance (ZTE = Z0/cos(θ)) increased, whereas glass impedance (ZS) remained constant [31]. This caused an impedance mismatch between the incident wave and the glass, resulting in a higher transmission loss.
Comparison of the proposed FSS performance with that of existing references
VI. Conclusion
In this study, we considered the design of a single-layer FSS based on small-width metal lines. These metal lines not only improved the transparency of the FSS but also significantly reduced fabrication costs compared to the metal mesh film. The transparency results for FSS-1 and FSS-2 were 57% and 62%, respectively. The validity of this approach was demonstrated through numerical simulations and experiments. Additionally, a method for improving transmission through glass windows at millimeter wave frequencies was introduced by adding dielectric slabs, and it was subsequently validated through a combination of theoretical analysis, simulation, and measurement. The main advantages of the proposed FSS include its simple design, transparency, and low-cost fabrication, and it could also be optimized for single glass windows.
Notes
This work was supported by the Institute for Information and Communications Technology Planning and Evaluation (IITP), funded by the Korean government (MSIT) (Grant No. RS-2023-00216221).
References
Biography
Tien Dat Nguyen, https://orcid.org/0000-0002-7499-2386 received his B.S. degree in electrical and electronics engineering from the Hanoi University of Science and Technology (HUST), Ha Noi, Vietnam, in 2020 and his M.S. degree from the Graduate School of Nano IT Design Fusion at Seoul National University of Science and Technology, Seoul, South Korea. He is currently pursuing a Ph.D. degree in the Department of Semiconductor Engineering at Seoul National University of Science and Technology, Seoul, South Korea. His research interests include EMI/EMC, RF filters, frequency selective surfaces (FSSs), and transparent antennas.
Chang Won Jung, https://orcid.org/0000-0002-8030-8093 received his B.S. degree in radio science and engineering from Kwangwoon University, Seoul, South Korea, in 1997, his M.S. degree in electrical engineering from the University of Southern California, Los Angeles, CA, USA, in 2001, and his Ph.D. degree in electrical engineering and computer science from the University of California at Irvine, Irvine, CA, USA, in 2005. He was a research engineer in the Wireless Communication Department at LG Information and Telecommunication, Seoul, South Korea, from 1997 to 1999. From 2005 to 2008, he was a senior research engineer in the Communication Laboratory at the Samsung Advanced Institute of Technology, Suwon, South Korea. Since 2008, he has been a professor at the Graduate School of Nano IT Design Technology, Seoul National University of Science and Technology, Seoul, Korea. He has authored over 120 papers in refereed journals and conference proceedings, a book, and more than 50 international patents. His current research interests include antennas for multimode multi-band communication systems, multifunctional reconfigurable antennas, electromagnetic interference/electromagnetic compatibility, millimeter-wave applications, and wireless power transfers for energy harvesting.