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Article

Design and Integration of Millimeter-Wave 5G and WLAN Antennas in Perfect Full-Screen Display Smartphones

by
Fei-Peng Lai
,
Shih-Yuan Mi
and
Yen-Sheng Chen
*
Department of Electronic Engineering, National Taipei University of Technology, Taipei 10608, Taiwan
*
Author to whom correspondence should be addressed.
Electronics 2022, 11(6), 957; https://doi.org/10.3390/electronics11060957
Submission received: 18 February 2022 / Revised: 14 March 2022 / Accepted: 17 March 2022 / Published: 19 March 2022
(This article belongs to the Special Issue Metastructures and Antennas with Enhanced Properties for Modern Microwave and Millimeter-Wave Applications)
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Abstract

:
Smartphone industries are seeking to maximize screen size, but a perfect full-screen phone is not yet available due to the unoccupied area required for the mobile antennas. Since a touchscreen is coated with transparent conductive layers, antennas below the touchscreen cannot create outward radiation. Accordingly, a non-metallic area is left for the antennas, which reduces the screen size. In this paper, we propose a design and integration of antennas for future millimeter-wave 5G smartphones to suit a perfect full-screen display. The proposed design consists of three modules, including a 1 × 12 patch antenna array at 28 GHz, a 1 × 8 patch array at 38 GHz, and a loop antenna at 2.45 GHz. The design environment consists of fully metallic top and bottom enclosures, mimicking the touchscreen and the printed circuit board, respectively. The three antennas were implemented on the flanks. Although the design area was limited and contained in parallel conductive plates, the three antennas still provided broad impedance bandwidths (26.5–32.0 GHz, 35.2–42.0 GHz, and 2.41–2.48 GHz) and high gains (16.7 dBi, 16.4 dBi, and 4.3 dBi). The isolation was larger than 20 dB, and the scanning ranges were ±45°. The proposed scheme is the first antenna system designed for perfect full-screen display phones.

1. Introduction

The demand for maximized screen sizes has gathered great importance for smartphone industries. This requirement is particularly urgent for millimeter-wave fifth generation (5G) communications, as soaring data throughput is associated with high-resolution applications. A perfect full-screen display device, which employs the entire front panel as a screen, is a game changer for screen maximization purposes; however, this technology has not yet been developed due to the unoccupied area necessary for the mobile antennas. As a touchscreen consists of transparent conductive coating layers, the touchscreen and a printed circuit board (PCB) are constructed as parallel conductive plates. Antennas cannot create radiation unless the touchscreen is demetalized. Thus, smartphones currently notch the screen display for the antenna arrangement.
In fact, the influence of the conductive surface of a touchscreen has not been given the attention it needs, even though there has been a dramatic proliferation in research on millimeter-wave 5G handset antennas. The antenna performance from recent studies concerning millimeter-wave 5G handsets is provided in Table 1 [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. To create millimeter-wave 5G antenna arrays, these earlier studies employed substrate-integrated-waveguide (SIW) structures [1,8], patches [2,4,6,20,22], monopoles along with slots [3], cavity-backed slots [5,7,11,12,14,15,18], dielectric resonators [9], embedded structures in a fourth generation (4G) structure [10,13,19], snowflake monopoles [16], quasi-Yagi structures [17], and ring resonators [21]. This academic research has attempted to enhance antenna performance in terms of miniaturization, bandwidths, gain, and isolation, but few studies have developed antennas to suit a large area for the conductive touchscreen [14,21]. Developing an antenna for a conductive full-screen display must address two challenges. First, the environment of parallel conductive plates makes it difficult to enhance the bandwidth and gain of the antennas. Second, the smartphone is required to deploy an antenna for a wireless local area network (WLAN) at 2.45 GHz, but designing a WLAN antenna to suit parallel conductive plates is more difficult than a millimeter-wave antenna due to its larger dimensions.
The purpose of this study was to move mobile antenna development from an ideal PCB environment to a conductive touchscreen device with a maximized screen size. Three antenna modules were integrated in a perfect full-screen display smartphone, the front and back of which were fully coated with copper. The three antenna modules included a 1 × 12 patch array at 28 GHz, a 1 × 8 patch array at 38 GHz, and a loop antenna at 2.45 GHz. These frequencies follow the standards for 5G and WLAN. In particular, the 3rd Generation Partnership Project (3GPP) regulates 5G New Radio (NR) Frequency Ranges 2 (FR2) for data throughput up to 12 Gbps, where n257, n260, and n261 utilize 26.5–29.5 GHz, 37.0–40.0 GHz, and 27.5–28.4 GHz, respectively. All three are fully covered by the proposed antennas with impedance bandwidths of 26.5–32.0 GHz and 35.2–42.0 GHz. Additionally, the proposed WLAN antenna meets the IEEE 802.11g standard. The performance of the proposed antennas is summarized in Table 1. Although the design space included parallel conductive plates, superior performance including sufficient bandwidths, high isolation, and high gains were achieved. The antenna design, analysis, and the results will be presented in the following sections.

2. Antenna Design

Figure 1 shows the environment of the mobile handset used in this study. The dimensions of the touchscreen display are the same as the PCB, namely, 74 × 150 mm2. The touchscreen is emulated by using a capacitive resistive approach [23], which uses optically transparent microwires made of copper [24]. Since the conductivity of the transparent copper microwires varies depending on the manufacturing process, this study emulates the conductivity (σ) in a worst-case scenario, namely, σ = 5.8 × 107 S/m. The spacing between the conductive touchscreen display and the PCB is 5.8 mm. The 28 GHz, 38 GHz, and 2.45 GHz antennas are located on the upper, right, and left flanks, respectively, whereas the lower half is reserved for a hand-held position.
Figure 2a shows the geometry of the 28 GHz antenna array, which comprises 12 identical elements printed on a 0.787 mm-thick Rogers RT5880 substrate (dielectric constant εr = 2.2 and loss tangent tanδ = 0.0009). The distance between the phase centers of two adjacent elements is 6.2 mm. Figure 2b depicts the proposed element, a revised capacitive coupled patch. It consists of three patches, namely, a left coupled patch with a rectangular notch, a feed strip, and a right coupled patch. This configuration is different from an earlier design [4] owing to the rectangular notch. In comparison with a design without the notch [4], the proposed unit element reduces the vertical dimension by 33% and the horizontal dimension by 11%; therefore, the proposed antenna is capable of fitting in a slim and low-profile handset. Besides this modification, this element inherits the characteristic broad bandwidths of the earlier design [4], which is significantly larger than conventional patch antennas. Figure 2c depicts the simulated surface current distributions using HFSS. When the feed strip is excited, the horizontal edges of the left coupled patch produces strong currents in the same direction. Such current distributions are comparable with a conventional patch antenna excited along the z-axis, thereby creating broadside radiation with y-axis polarization.
Figure 3a depicts the geometry of the 38 GHz 8-element antenna array printed on a 0.787-mm-thick Rogers RT5880 substrate where the adjacent elements are separated by 5.1 mm. Detailed dimensions for the unit element are provided in Figure 3b. This element is also a capacitive coupled patch antenna [25], where only two patches—a feed strip and a right coupled patch—are employed. When the strip is fed by a 38 GHz signal, it couples electromagnetic energy to the adjacent patch antenna. The simulated current distributions for the patches are shown in Figure 3c. In contrast to the previous design for the 28 GHz antenna, strong and in-phase currents flow along the vertical edges of the two patches, creating z-axis polarization radiation. This configuration provides a broader impedance bandwidth as compared to microstrip-line-fed patch antennas, albeit at the cost of a slight asymmetry in its radiation characteristics. Although designing millimeter-wave elements with dual polarization can achieve polarization diversity, this study utilizes single linear polarization to reduce the footprint of the antenna. By arranging the proposed antenna with dual-polarized base stations [26,27], the communication system can still be operated with enhanced spectral efficiency.
Figure 4a details the topology of the 2.45 GHz antenna. Along with the current pathway on the PCB ground, the proposed antenna has a half-wavelength loop printed on a 0.6-mm-thick FR4 substrate (εr = 4.4 and tanδ = 0.02). The WLAN requires a significantly lower frequency than millimeter-wave applications, so it is challenging to integrate this antenna with the two aforementioned antenna arrays. Thus, a loop antenna was designed to suit the design space on the flank. The current distributions are shown in Figure 4b. The currents along the horizontal segment change direction at the midpoint, while the currents along the vertical segments show relatively strong magnitudes, which follows the operating principle of a half-wavelength loop. Additional studies were performed to create a meandered inverted L antenna (ILA) that resonated at 2.45 GHz. However, narrow impedance bandwidths were observed for the meandered ILA (2.43–2.46 GHz in terms of a −10 dB reflection coefficient). Although meandered ILAs are a common choice for WLAN modules in mobile handsets, the proposed loop antenna is more suitable for an environment that has parallel conductive plates. In particular, thanks to the design on the edge and currents on the ground, this loop antenna is more compact compared to earlier designs for WLAN antennas, as shown in Table 2 [28,29,30,31,32,33,34,35,36,37,38,39]. The area occupied by this antenna is only 96.3 mm2, compared to at least 210 mm2 for the earlier designs.
For modern mobile phones, consumers desire a light weight, reduced volume, and a low cost. In addition, the transceiver modules and the battery occupy large areas in the handset. These characteristics demand that the design space for the antennas be miniaturized. When compared to the state-of-the-art, the proposed three antennas are very compact, and thus, they fulfill this requirement for modern mobile devices.

3. Results and Discussion

The dimensions of the three antennas were optimized using parametric studies. Figure 5 illustrates the full-screen handset model flanked by the three antenna prototypes. The antennas are soldered to semi-rigid 0.047 coaxial cables from Woken Technology Inc., Taiwan, for measurement purposes.
Figure 6 shows the reflection coefficients for the unit elements. Referring to Figure 2, Figure 3 and Figure 4, the input ports were evaluated at F1, F13, and F21, respectively. With respect to a −10 dB threshold, the simulated (and measured) impedance bandwidths were 26.5–32.0 GHz (25.7–28.0 GHz), 35.2–42.0 GHz (34.0–38.1 GHz), and 2.41–2.48 GHz (2.40–2.48 GHz) for each antenna. Slight frequency shifts were observed for the millimeter-wave elements. These discrepancies were mainly attributed to the soldering process, since soldering a connector within parallel conductive plates adds to the manufacturing complexity. According to the 5G NR FR2 standard, the maximum input power for the antenna is 26 dBm. When this input power was transmitted by the antenna, less than 10% was reflected to the transceiver. This indicates that the proposed antenna is capable of transmitting and receiving signals in practical use. It should be noted that satisfying the bandwidth requirement for the WLAN antenna was challenging due to the presence of the parallel conductive plates, yet the proposed loop antenna achieved the desired bandwidth.
Figure 7 provides mutual coupling between adjacent elements for the millimeter-wave arrays. The results for the 28 GHz array were measured between ports F1 and F2, whereas ports F13 and F14 were used for the 38 GHz array. The isolation was generally larger than 20 dB, which indicates that less than 1% of power was coupled to adjacent ports when one port was excited. This isolation is sufficient to guarantee the integrity of the signal. Although the measured mutual coupling was obviously larger than the simulated results on a logarithmic scale, the difference was insignificant when plotted on a linear scale. Note that there were other mutual coupling coefficients for the proposed arrays. Among all these mutual coupling coefficients, the worst-case scenario applies to adjacent elements. These other coefficients were lower than the results shown in Figure 7, which are intended to demonstrate the worst-case scenario for each array.
Next, the isolation of the 28 GHz and 38 GHz bands was investigated. While there are three operating bands, the isolation of the two millimeter-wave bands is more important, since the frequency of the WLAN band is more than 10-times lower than the millimeter-wave bands. The first element of the 28 GHz band (F1) and that of the 38 GHz band (F13) were excited, and the results for their simulated and measured mutual coupling are shown in Figure 8. Due to the physical separation of the elements and the frequency separation, mutual coupling was generally below −40 dB, indicating high isolation of the two bands. As a result, when the two modules are operated simultaneously, the interference will be insignificant.
Figure 9 illustrates the antenna efficiency and realized peak gain for each module. For the two arrays, only single ports (F1 and F13) were excited individually. At 28 GHz, the simulated (measured) efficiency and peak gain were 96.4% (91.4%) and 6.3 dBi (6.6 dBi), respectively, whereas at 38 GHz the simulated (measured) efficiency and peak gain were 96.2% (69.5%) and 7.7 dBi (6.0 dBi), respectively. The simulated (measured) antenna efficiency and realized peak gain at 2.45 GHz were 86.3% (62.6%) and 4.3 dBi (3.3 dBi), respectively. The sufficient antenna efficiency indicates that the power collected from the transceivers can be radiated to free space. Since these performance indices also jointly evaluated the reflection coefficients, a shift of resonant frequencies was responsible for the slight discrepancies between the simulations and the measurements for the two arrays.
Figure 10 demonstrates the simulated and measured radiation patterns at 28.0 GHz, 38.0 GHz, and 2.4 GHz. The main beams of the two capacitive coupled patches were directed broadside. For the antenna array operated at 28.0 GHz, the half-power beam widths (HPBWs) were about 90°. In addition, the 2.45 GHz antenna emitted conventional loop-wise radiation patterns, and thus, it is suitable for WLAN applications. The maximum difference between the simulated and measured main beams was less than 3 dB for each of the patterns. Considering the errors related to fabrication, measurement, and calculation, this discrepancy is not significant. To demonstrate the capability of 5G beamforming, Figure 11 presents the simulated realized peak gains and the scanning ranges for the 28 GHz and 38 GHz arrays. The maximum gains for the proposed arrays were 16.7 dBi and 16.4 dBi, respectively. In terms of a 3 dB scan loss, the scanning ranges at 28 GHz and 38 GHz were about ±45°.

4. Conclusions

The design, integration, and analysis of two millimeter-wave 5G antenna arrays at 28/38 GHz and a WLAN antenna at 2.45 GHz have been demonstrated for a perfect full-screen display smartphone. The novelty of the proposed antennas is threefold. First, while few studies have designed millimeter-wave antenna arrays with a consideration for a conductive touchscreen, we have developed three antenna modules for a handheld device where the front and back panels are fully coated with copper. Second, although the design space for parallel conductive plates adds some complexity, the 28 GHz and 38 GHz antenna arrays still deployed 12 and 8 elements, respectively, with broad impedance bandwidths (26.5–32.0 GHz and 35.2–42.0 GHz) and sufficient realized peak gains (16.7 dBi and 16.4 dBi). Third, the relatively large antenna design for the WLAN module was integrated into the proposed design, and this antenna provided the desired performance. It is important to emphasize that the proposed design neglects fourth generation (4G) transceiver modules. Our design is only suited for future mobile handsets that are deployed for millimeter-wave 5G mobile communications. It is expected that the proposed antennas will contribute to these scenarios with a maximized screen size.

Author Contributions

Conceptualization, Y.-S.C.; methodology, S.-Y.M.; software, S.-Y.M. and F.-P.L.; validation, S.-Y.M. and F.-P.L.; formal analysis, S.-Y.M.; investigation, S.-Y.M. and F.-P.L.; resources, Y.-S.C.; data curation, S.-Y.M. and F.-P.L.; writing—original draft preparation, F.-P.L.; writing—review and editing, F.-P.L. and Y.-S.C.; visualization, F.-P.L.; supervision, Y.-S.C.; project administration, Y.-S.C.; funding acquisition, Y.-S.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Ministry of Science and Technology, Taiwan, under Contract MOST 110-2636-E-027-003.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The simulated and measured data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Design environment for the three antenna modules.
Figure 1. Design environment for the three antenna modules.
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Figure 2. Geometry of (a) the 28 GHz 1 × 12 coupled patch antenna array and (b) the unit element (all measurements are in mm). (c) The surface current distributions at 28 GHz.
Figure 2. Geometry of (a) the 28 GHz 1 × 12 coupled patch antenna array and (b) the unit element (all measurements are in mm). (c) The surface current distributions at 28 GHz.
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Figure 3. Geometry of (a) the 38 GHz 1 × 8 coupled patch antenna array and (b) the unit element (all measurements are in mm). (c) The surface current distributions at 38 GHz.
Figure 3. Geometry of (a) the 38 GHz 1 × 8 coupled patch antenna array and (b) the unit element (all measurements are in mm). (c) The surface current distributions at 38 GHz.
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Figure 4. (a) The geometry and (b) surface current distributions for the 2.45 GHz loop antenna (all measurements are in mm).
Figure 4. (a) The geometry and (b) surface current distributions for the 2.45 GHz loop antenna (all measurements are in mm).
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Figure 5. The prototype of the proposed antenna design and integration.
Figure 5. The prototype of the proposed antenna design and integration.
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Figure 6. Reflection coefficients for the 28 GHz unit element, 38 GHz unit element, and 2.45 GHz loop antenna.
Figure 6. Reflection coefficients for the 28 GHz unit element, 38 GHz unit element, and 2.45 GHz loop antenna.
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Figure 7. Mutual coupling between adjacent elements of the two antenna arrays.
Figure 7. Mutual coupling between adjacent elements of the two antenna arrays.
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Figure 8. Mutual coupling of the millimeter-wave elements. (a) 28 GHz and (b) 38 GHz bands.
Figure 8. Mutual coupling of the millimeter-wave elements. (a) 28 GHz and (b) 38 GHz bands.
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Figure 9. (a) Antenna efficiency and (b) realized peak gain for the 28 GHz unit element, 38 GHz unit element, and 2.45 GHz loop antenna.
Figure 9. (a) Antenna efficiency and (b) realized peak gain for the 28 GHz unit element, 38 GHz unit element, and 2.45 GHz loop antenna.
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Figure 10. Radiation patterns for (a) the 28 GHz unit element, (b) 38 GHz unit element, and (c) 2.45 GHz loop antenna.
Figure 10. Radiation patterns for (a) the 28 GHz unit element, (b) 38 GHz unit element, and (c) 2.45 GHz loop antenna.
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Figure 11. Beam scanning performance of (a) the 28.0 GHz and (b) 38.0 GHz antenna arrays.
Figure 11. Beam scanning performance of (a) the 28.0 GHz and (b) 38.0 GHz antenna arrays.
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Table
Table 1. Comparison of millimeter-wave 5G antennas for mobile handsets.
Table 1. Comparison of millimeter-wave 5G antennas for mobile handsets.
No.Impedance Bandwidth (GHz)Number of ElementsIsolation (dB)Peak Gain (dBi)Proportion of Metallic Area on Screen (%)
[1]26.5–29.4820.014.00
[2]34.2–38.81N.A.5.70
[3]26.0–28.48N.A.8.20
[4]23.2–29.81216.016.50
[5]27.5–30.0817.015.60
[6]25.6–29.61619.019.90
[7]27.3–29.81N.A.12.40
[7]26.9–30.61N.A.7.60
[8]27.0–29.44N.A.12.60
[9]27.2–28.5224.09.00
[10]25.0–30.04N.A.5.00
[11]27.0–30.81N.A.4.90
[12]26.9–28.44N.A.11.50
[13]22.0–31.0413.09.50
[14]26.7–33.31N.A.5.175
[14]37.0–40.01N.A.7.075
[15]25.0–30.0421.514.00
[16]25.4–29.04N.A10.10
[17]26.5–30.5315.06.00
[18]27.6–28.34N.A6.40
[19]22.0–28.0225.08.00
[20]26.1–31.01226.0N.A.0
[21]23.5–32.0818.011.086
[22]25.0–31.010N.A.11.70
This study26.5–32.01222.316.7100
This study35.2–42.0820.016.4100
This study2.41–2.471N.A.4.3100
Table
Table 2. Comparison of antenna dimensions for WLAN applications.
Table 2. Comparison of antenna dimensions for WLAN applications.
No.Dimensions (mm2)Total Area Occupied by the Antenna (mm2)
[28]40 × 281120
[29]28.2 × 36.71034.97
[30]30 × 30900
[31]27 × 25675
[32]10 × 40400
[33]15 × 20300
[34]15 × 14210
[35]26.5 × 25662.5
[36]180 × 15027,000
[37]46 × 351610
[38]54 × 10540
[39]39 × 25975
This study5.8 × 16.696.3
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Lai, F.-P.; Mi, S.-Y.; Chen, Y.-S. Design and Integration of Millimeter-Wave 5G and WLAN Antennas in Perfect Full-Screen Display Smartphones. Electronics 2022, 11, 957. https://doi.org/10.3390/electronics11060957

AMA Style

Lai F-P, Mi S-Y, Chen Y-S. Design and Integration of Millimeter-Wave 5G and WLAN Antennas in Perfect Full-Screen Display Smartphones. Electronics. 2022; 11(6):957. https://doi.org/10.3390/electronics11060957

Chicago/Turabian Style

Lai, Fei-Peng, Shih-Yuan Mi, and Yen-Sheng Chen. 2022. "Design and Integration of Millimeter-Wave 5G and WLAN Antennas in Perfect Full-Screen Display Smartphones" Electronics 11, no. 6: 957. https://doi.org/10.3390/electronics11060957

APA Style

Lai, F.-P., Mi, S.-Y., & Chen, Y.-S. (2022). Design and Integration of Millimeter-Wave 5G and WLAN Antennas in Perfect Full-Screen Display Smartphones. Electronics, 11(6), 957. https://doi.org/10.3390/electronics11060957

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