A Quad-Port Dual-Band MIMO Antenna Array for 5G Smartphone Applications

electronics

Article A Quad-Port Dual-Band MIMO Antenna Array for 5G Smartphone Applications

Jianlin Huang, Guiting Dong, Jing Cai, Han Li and Gui Liu *

College of Electrical and Electronics Engineering, Wenzhou University, Wenzhou 325035, China; [email protected] (J.H.); [email protected] (G.D.); [email protected] (J.C.); [email protected] (H.L.) * Correspondence: [email protected]

Abstract: A quad-port antenna array operating in 3.5 GHz band (3.4–3.6 GHz) and 5 GHz band (4.8–5 GHz) for fifth-generation (5G) smartphone applications is presented in this paper. The single antenna element consists of an L-shaped strip, a parasitic rectangle strip, and a modified Z-shaped strip. To reserve space for 2G/3G/4G antennas, the quad-port antenna array is printed along the two long frames of the smartphone. The evolution design and the analysis of the optimal parameters of a single antenna element are derived to investigate the principle of the antenna. The prototype of the presented antenna is tested and the measured results agree well with the simulation. The measured total efficiency is better than 70% and the isolation is larger than 16.5 dB.

Keywords: 5G; sub-6 GHz; MIMO; dual-band antenna; smartphone

1. Introduction With the increasing demand for the quality of wireless communication, the fifth- Citation: Huang, J.; Dong, G.; Cai, J.; generation (5G) mobile communication technology provides a promising solution to higher Li, H.; Liu, G. A Quad-Port transmission rates, shorter latency, more connection density, and larger communication Dual-Band MIMO Antenna Array for capacity. Recently, many 5G sub-6 GHz Multiple-Input Multiple-Output (MIMO) antennas 5G Smartphone Applications. have been developed for mobile terminals and base stations [1–5]. However, with the Electronics 2021, 10, 542. https:// limited internal space of mobile phones, it is a challenging task to integrate many antennas doi.org/10.3390/electronics10050542 with high isolation and low envelope correlation coefficient. Dual-band four-port MIMO antenna can expand system capacity and realize multi-mode communication. To minimize Academic Editor: Faisel Tubbal the unwanted mutual coupling, various isolation techniques have been reported in recent years [3–14]. The rectangle slots of the defected ground plane and two rectangular mi- Received: 7 February 2021 crostrip lines are employed to decrease the mutual coupling in [3]. In [4], the vertical patch Accepted: 23 February 2021 Published: 25 February 2021 is used to offset the coupling between the antenna and nearby components. A ground- connected T-shaped decoupling stub and an additional modified T-shaped decoupling

Publisher’s Note: MDPI stays neutral stub is inserted between the two mirrored dual-antenna arrays to reduce the coupling π with regard to jurisdictional claims in in [5]. By introducing two short T-shaped strips and a -shaped strip, the isolation be- published maps and institutional afﬁl- tween two antenna elements is enhanced [6]. An inverted-F antenna is used to reduce the iations. mutual coupling of antenna elements [7]. Other decoupling techniques have also been presented, such as using a short neutral line [8,9], high-pass ﬁlter [10], pattern diversity [11], self-decoupling [12], defected ground structure [13], and metamaterial structure [14]. In the World Radiocommunication Conference 2015, the 3.5 GHz (3.4–3.6 GHz) frequency band became one of the 5G mobile phone spectrums. The Ministry of Industry and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Information Technology of China approved 3.3–3.6 GHz and 4.8–5 GHz as the sub-6 GHz This article is an open access article 5G frequency bands. Moreover, except for the sub-6-GHz spectrum, 5G smartphones are distributed under the terms and also expected to be phased array millimeter-wave antenna at 28 GHz frequency band [15]. conditions of the Creative Commons In this paper, a novel dual-band antenna operating at 3.5 GHz band (3.4–3.6 GHz) Attribution (CC BY) license (https:// and 5 GHz band (4.8–5 GHz) is proposed for the applications of a 5G MIMO antenna in creativecommons.org/licenses/by/ smartphones. The proposed antenna shows high isolation, high gain and low ECC, which 4.0/). can be used to achieve high transmission rates and large channel capacity. The proposed

Electronics 2021, 10, 542. https://doi.org/10.3390/electronics10050542 https://www.mdpi.com/journal/electronics Electronics 2021, 10, x FOR PEER REVIEW 2 of 9

In this paper, a novel dual-band antenna operating at 3.5 GHz band (3.4–3.6 GHz) Electronics 2021, 10, 542 2 of 9 and 5 GHz band (4.8–5 GHz) is proposed for the applications of a 5G MIMO antenna in smartphones. The proposed antenna shows high isolation, high gain and low ECC, which can be used to achieve high transmission rates and large channel capacity. The proposed antennaantenna array array consists consists of an of L-shaped an L-shaped feeding feeding strip, a strip, parasitic a parasitic rectangle rectangle strip, and strip, a mod- and a ifiedmodiﬁed Z-shaped Z-shaped radiating radiating strip. Different strip. Different evaluative evaluative structures structures and andoptimal optimal parameter parameter analysisanalysis are arecarried carried out outfor forthe theproposed proposed compact compact quad-port quad-port dual-band dual-band MIMO MIMO antenna. antenna.

2. Antenna Geometry 2. Antenna Geometry TheThe geometry geometry of the of theproposed proposed quad-port quad-port dual-band dual-band MIMO MIMO antenna antenna array array is shown is shown in Figure1. There are two types of printed circuit boards (PCBs) including a main in Figure 1. There are two types of printed circuit boards (PCBs) including a main board board and two side boards, as depicted in Figure1a. The size of the main board is and two side boards, as depicted in Figure 1a. The size of the main board is 150 mm × 75 150 mm × 75 mm × 0.8 mm, and the size of the side board is 150 mm × 6.2 mm × 0.8 mm. mm × 0.8 mm, and the size of the side board is 150 mm × 6.2 mm × 0.8 mm. Four antenna Four antenna elements are printed on two side boards which are positioned vertically elements are printed on two side boards which are positioned vertically to the main board. to the main board. Both the side boards and the main board are printed on an FR4 sub- Both the side boards and the main board are printed on an FR4 substrate with εr = 4.4 and strate with ε = 4.4 and tan δ = 0.02. The side boards are bonded with the main boards tan δ = 0.02. Ther side boards are bonded with the main boards by metal adhesive. Other by metal adhesive. Other spaces of both frames (side boards) are reserved for 2G/3G/4G spaces of both frames (side boards) are reserved for 2G/3G/4G or other wireless commu- or other wireless communication antennas in the mobile handsets. Figure1b shows the nication antennas in the mobile handsets. Figure 1b shows the detailed dimensions of sin- detailed dimensions of single antenna element. The single antenna element includes a gle antenna element. The single antenna element includes a L-shaped feed strip, a para- L-shaped feed strip, a parasitic rectangular strip and a modiﬁed Z-shaped radiation strip siticwhich rectangular is connected strip and to a the modified ground Z-shaped plane. The radiation total size strip of which a single is connected antenna element to the is ground14.9 mmplane.× 7The mm total× 0.8 size mm. of a single antenna element is 14.9 mm × 7 mm × 0.8 mm.

(a)

(b)

FigureFigure 1. Geometry 1. Geometry of the of theproposed proposed quad-port quad-port MIMO MIMO antenna antenna (in (inmillimeters). millimeters). (a) ( aProspective) Prospective view; view;(b )(b Single) Single antenna antenna element. element.

3. Antenna3. Antenna Analysis Analysis In orderIn order to understand to understand the themechanism mechanism of the of theproposed proposed MIMO MIMO antenna, antenna, the thedesign design evolutionevolution of the of theantenna antenna element element and and the theoptimal optimal parameter parameter analysis analysis are areboth both carried carried out. out. SinceSince the thestructure structure of the of thefour four antenna antenna elements elements is identical, is identical, the theparameters parameters of a of single a single element are used for analysis. The 3D full-wave electromagnetic simulator HFSS is used for design and optimization.

Electronics 2021, 10, x FOR PEER REVIEW 3 of 9

element are used for analysis. The 3D full-wave electromagnetic simulator HFSS is used Electronics 2021, 10, 542 3 of 9 elementfor design are and used optimization. for analysis. The 3D full-wave electromagnetic simulator HFSS is used for design and optimization. 3.1. Analysis of the Design Evolution of the Proposed Antenna Element 3.1.3.1. AnalysisAnalysisThe proposed ofof thethe DesignDesign antenna EvolutionEvolution is generated ofof thethe Proposed Proposedby the evolution AntennaAntenna Element Elementdemonstrated in Figure 2. Ant. 1 is aTheThe structure proposedproposed composed antennaantenna of isis an generatedgenerated L-shaped byby feed thetheing evolutionevolution strip and demonstrateddemonstrated a Z-shaped radiation inin FigureFigure strip.2 2.. Ant. Ant. By 11adding isis aa structure a parasitic composed rectangle of of stripan an L-shaped L-shaped to Ant.1, feed feedingAningt. 2 isstrip stripobtained. and and a Z-shaped aCompared Z-shaped radiation with radiation Ant.2, strip. strip. Ant. By Byadding3 has adding an a additionalparasitic a parasitic rectangle L-shaped rectangle strip strip. strip to Ant.1, toThe Ant. ga Anps 1, t. Ant.between 2 is 2obtained. is obtained.the L-shaped Compared Compared and with Z-shaped with Ant.2, Ant. stripAnt. 2, Ant.3are has 0.5 3 an hasmm additional an and additional 0.2 mm, L-shaped respectively. L-shaped strip. strip. The Compared ga Theps gapsbetween wi betweenth Ant. the 3,L-shaped the Ant. L-shaped 4 cuts and a andrectangularZ-shaped Z-shaped strip slit stripareon the0.5 are additionalmm 0.5 mmand and0.2 L-shape mm, 0.2 mm, respectively. strip. respectively. Compared Compared with with Ant. Ant. 3, Ant. 3, Ant. 4 cuts 4 cuts a rectangular a rectangular slit sliton onthe theadditional additional L-shape L-shape strip. strip.

Ant. 1 Ant. 2 Ant. 1 Ant. 2

Ant. 3 Ant. 4 (proposed)

Ant. 3 Ant. 4 (proposed) Figure 2. Design evolution of a single MIMO antenna element. FigureFigure 2.2.Design Design evolutionevolution ofof aa singlesingle MIMOMIMOantenna antennaelement. element. Figure 3 depicts the simulated S11 parameters of the evolution antennas. It can be seen Figure3 depicts the simulated S 11 parameters of the evolution antennas. It can be seen that parasiticFigure 3 depictsrectangular the simulated strip can increaseS11 parameters the bandwidth, of the evolution and L-shaped antennas. strip It can can be create seen that parasitic rectangular strip can increase the bandwidth, and L-shaped strip can create a thata 3.4 parasitic GHz resonant rectangular point. strip Furthermore, can increase th thee modified bandwidth, Z-shaped and L-shaped strip can strip shift can the create fre- 3.4 GHz resonant point. Furthermore, the modiﬁed Z-shaped strip can shift the frequency aquency 3.4 GHz band resonant to the higherpoint. Furthermore,frequencies whic theh modified can cover Z-shaped 4.9 GHz (4.8–5strip can GHz) shift frequency the fre- band to the higher frequencies which can cover 4.9 GHz (4.8–5 GHz) frequency band. quencyband. band to the higher frequencies which can cover 4.9 GHz (4.8–5 GHz) frequency band.

Figure 3. Simulated S11 of various antennas. Figure 3. Simulated S11 of various antennas. Figure 3. Simulated S11 of various antennas. 3.2. Analysis of the Variables of the Proposed Antenna Element

Figures 4 and5 show simulated S 11 of a single antenna element as a function of L and H, respectively. The value of L can be effectively used to change the resonant frequency of the lower frequency band. The higher resonance of a single element can be optimized

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3.2. Analysis of the Variables of the Proposed Antenna Element Electronics 2021, 10, 542 4 of 9 Figures 4 and 5 show simulated S11 of a single antenna element as a function of L and H, respectively. The value of L can be effectively used to change the resonant frequency of the lower frequency band. The higher resonance of a single element can be optimized byby tuningtuning thethe valuevalue ofof H.H. Eventually,Eventually, thethe antennaantenna cancan operateoperate atat frequenciesfrequencies rangingranging fromfrom 3.43.4 GHzGHz toto 3.63.6 GHzGHz andand 4.84.8 GHzGHz toto 55 GHz.GHz.

11 FigureFigure 4.4. SimulatedSimulated SS1111 of the proposed antenna withwith different values ofof L.L.

Figure 5. Simulated S11 of the proposed antenna with different values of H. FigureFigure 5.5. SimulatedSimulated SS1111 of the proposed antenna withwith different values of H. 4.4. ExperimentalExperimental ResultsResults andand DiscussionDiscussion Electronics 2021, 10, x FOR PEER REVIEW4. Experimental Results and Discussion 5 of 9

ToTo verifyverify thethe proposedproposed design,design, anan antennaantenna prototypeprototype waswas fabricatedfabricated usingusing thethe opti-opti- mizedmized dimensionsdimensions listedlisted inin FigureFigure1 1.. Figure Figure6 6is is the the photograph photograph of of the the prototype prototype antenna. antenna.

FigureFigure 6. 6. PhotographPhotograph of of the the fabricated fabricated antenna antenna prototype. prototype.

FigureFigure 77 showsshows the the S-parameters S-parameters which which are are measured measured by by Keysight Keysight Vector Vector Network Network An- alyzer N5224A. It can be seen that the measured S can cover both 3.5 GHz (3.4–3.6 GHz) Analyzer N5224A. It can be seen that the measured11 S11 can cover both 3.5 GHz (3.4–3.6 GHz)and 4.9 and GHz 4.9 (4.8–5 GHz GHz)(4.8–5 frequencyGHz) frequency bands, andbands, the and isolation the isolation is higher is than higher 16.5 than dB. The16.5 mea- dB. Thesured measured isolation isolation between between various various ports agrees ports wellagrees with well the with simulated the simulated results. results. The slight The frequency offset is mainly due to SMA connective errors. slight frequency offset is mainly due to SMA connective errors.

Figure 7. Simulated and measured S-parameters of the proposed antenna.

In Figure 8, the simulated surface current at 3.5 GHz is mainly distributed at the gap between the rectangular strip and the Z-shaped strip. At 4.9 GHz, strong current intensity is observed at the gap between the L-shaped strip and the modified Z-shaped strip. The distribution of the electric field is mostly the same as the distribution of electric current.

120

70 J-surf (A/m) (A/m) J-surf 10

(a)

200

100

E-Field (V/cm) E-Field 30

(b) Figure 8. The surface current and electric field distributions of the proposed antenna element at (a) 3.5 GHz. (b) 4.9 GHz.

Electronics 2021, 10, x FOR PEER REVIEW 5 of 9 Electronics 2021, 10, x FOR PEER REVIEW 5 of 9

Figure 6. Photograph of the fabricated antenna prototype. Figure 6. Photograph of the fabricated antenna prototype. Figure 7 shows the S-parameters which are measured by Keysight Vector Network Figure 7 shows the S-parameters which are measured by Keysight Vector Network Analyzer N5224A. It can be seen that the measured S11 can cover both 3.5 GHz (3.4–3.6 Analyzer N5224A. It can be seen that the measured S11 can cover both 3.5 GHz (3.4–3.6 GHz) and 4.9 GHz (4.8–5 GHz) frequency bands, and the isolation is higher than 16.5 dB. GHz) and 4.9 GHz (4.8–5 GHz) frequency bands, and the isolation is higher than 16.5 dB. Electronics 2021, 10, 542 The measured isolation between various ports agrees well with the simulated results. The5 of 9 The measured isolation between various ports agrees well with the simulated results. The slight frequency offset is mainly due to SMA connective errors. slight frequency offset is mainly due to SMA connective errors.

Figure 7. Simulated and measured S-parameters of the proposed antenna. FigureFigure 7.7. SimulatedSimulated andand measuredmeasured S-parametersS-parameters ofof thethe proposedproposed antenna.antenna. In Figure 8, the simulated surface current at 3.5 GHz is mainly distributed at the gap InIn FigureFigure8 8,, thethe simulated surface surface current current at at 3.5 3.5 GHz GHz is ismainly mainly distributed distributed at the at the gap between the rectangular strip and the Z-shaped strip. At 4.9 GHz, strong current intensity gapbetween between the rectangular the rectangular strip and strip the and Z-shaped the Z-shaped strip. At strip. 4.9 GHz, At 4.9 strong GHz, current strong intensity current is observed at the gap between the L-shaped strip and the modified Z-shaped strip. The intensityis observed is observed at the gap at thebetween gap between the L-shaped the L-shaped strip and strip the and modified the modiﬁed Z-shaped Z-shaped strip. strip. The distribution of the electric field is mostly the same as the distribution of electric current. Thedistribution distribution of the of theelectric electric field ﬁeld is mostly is mostly the the same same as asthe the distribution distribution of ofelectric electric current. current.

120 120

70 70 J-surf (A/m) (A/m) J-surf

10 (A/m) J-surf 10 (a) (a) 200 200

100 100

E-Field (V/cm) E-Field 30 E-Field (V/cm) E-Field 30 (b) (b) FigureFigure 8. The 8. Thesurface surface current current and electric and electric field ﬁelddistribu distributionstions of theof proposed the proposed antenna antenna element element at (a) at Figure 8. The surface current and electric field distributions of the proposed antenna element at (a) 3.5 GHz. (b) 4.9 GHz. (3.5a) 3.5GHz. GHz. (b) (4.9b) 4.9GHz. GHz. The radiation patterns of the proposed antenna element at 3.5 GHz and 4.9 GHz are shown in Figure9, respectively. The measured co-pol and cross-pol are represented by different symbol lines. E-plane represents the direction in which the feed current ﬂows, and H-plane is perpendicular to that direction. As shown in the E-plane radiation patterns

of Figure9b, the radiation between the feeding strip and the ground is taken into account, resulting in a similar gain between the co-pol and cross-pol. Electronics 2021, 10, x FOR PEER REVIEW 6 of 9 Electronics 2021, 10, x FOR PEER REVIEW 6 of 9

The radiation patterns of the proposed antenna element at 3.5 GHz and 4.9 GHz are The radiation patterns of the proposed antenna element at 3.5 GHz and 4.9 GHz are shown in Figure 9, respectively. The measured co-pol and cross-pol are represented by shown in Figure 9, respectively. The measured co-pol and cross-pol are represented by different symbol lines. E-plane represents the direction in which the feed current flows, different symbol lines. E-plane represents the direction in which the feed current flows, and H-plane is perpendicular to that direction. As shown in the E-plane radiation patterns and H-plane is perpendicular to that direction. As shown in the E-plane radiation patterns Electronics 2021, 10, 542 of Figure 9b, the radiation between the feeding strip and the ground is taken into account, 6 of 9 of Figure 9b, the radiation between the feeding strip and the ground is taken into account, resulting in a similar gain between the co-pol and cross-pol. resulting in a similar gain between the co-pol and cross-pol.

E-plane H-plane E-plane H-plane (a) (a)

E-plane H-plane E-plane H-plane (b) (b) Figure 9. Radiation patterns of the proposed antenna. (a) 3.5 GHz. (b) 4.9 GHz. FigureFigure 9.9.Radiation Radiation patternspatterns ofof thethe proposedproposed antenna.antenna. ((aa)) 3.53.5 GHz.GHz. ((bb)) 4.94.9 GHz.GHz. In Figure 10, the 3D radiation patterns of a single antenna element as seen from the In FigureIn Figure 10, the 10, 3D the radiation 3D radiation patterns patterns of a single of a single antenna antenna element element as seen as from seen the from the front side and back side of the substrate are presented, respectively. The maximum gain front sidefront and side back and side back of side the substrate of the substrate are presented, are presented, respectively. respectively. The maximum The maximum gain at gain at 3.5 GHz and 4.9 GHz are 4.9 dBi and 5.1 dBi, respectively. The gains are high enough 3.5 GHzat and3.5 GHz 4.9 GHz and are4.9 GHz 4.9 dBi are and 4.9 5.1dBi dBi, and respectively. 5.1 dBi, respectively. The gains The are gains high enoughare high to enough to meet the requirements of most mobile phone antennas. meet theto meet requirements the requirements of most mobile of most phone mobile antennas. phone antennas.

4.9 4.9

−2.5 −2.5 Electronics 2021, 10, x FOR PEER REVIEW 7 of 9 −8.4

GainTotal (dBi) (dBi) GainTotal −8.4 Front Back GainTotal (dBi) (dBi) GainTotal Front Back (a) (a) 5.1

2.2

GainTotal (dBi) (dBi) GainTotal −1.2 Front Back

(b)

Figure Figure10. 3D radiation 10. 3D radiation patterns patterns of a single of a element. single element. (a) 3.5 GHz (a) 3.5 (b GHz) 4.9 GHz. (b) 4.9 GHz.

The measured total efficiency and peak gain, as shown in Figure 11, are better than 70% and 4 dBi, respectively. The 3.5 GHz and 4.9 GHz frequency points obtain the maximum total efficiency about 85% and 82%, respectively, while the 3.6 GHz and 5 GHz frequency points have the maximum peak gain about 4.7 dBi and 5 dBi, respectively. The measured efficiency is generally lower than that of simulation, which is mainly due to the current skin effect caused by excess solder and the loss of the chamber. The measured total efficiency and peak gain are in good agreement with the simulation results.

Figure 11. Simulated and measured peak gain and total efficiencies of the proposed antenna.

The envelope correlation coefficient (ECC) between radiating elements is a critical parameter of the MIMO antenna. The ECC can be calculated by the measured S-parameters according to the below equation. ∗ ∗ | + | = (1) (1 − || −|| )(1 − || −|| ) The calculated ECC of the proposed MIMO antenna is shown in Figure 12. The values of ECC in the working frequency bands are all smaller than 0.01, which means that the proposed quad-port MIMO antenna achieves good MIMO diversity performance.

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5.1

2.2

GainTotal (dBi) (dBi) GainTotal −1.2 Front Back Electronics 2021, 10, 542 7 of 9 (b) Figure 10. 3D radiation patterns of a single element. (a) 3.5 GHz (b) 4.9 GHz.

TheThe measuredmeasured total total efficiency efficiency and and peak peak gain, gain, as shownas shown in Figure in Figure 11, are11, better are better than 70%than and70% 4 and dBi, 4 respectively.dBi, respectively. The 3.5 The GHz 3.5 andGHz 4.9 and GHz 4.9 frequencyGHz frequency points points obtain obtain the maximum the maxi- totalmum efficiency total efficiency about 85%about and 85% 82%, and respectively, 82%, respectively, while thewhile 3.6 the GHz 3.6 and GHz 5 GHz and frequency5 GHz fre- pointsquency have points the have maximum the maximum peak gain peak about gain 4.7 dBiabout and 4.7 5 dBi,dBi and respectively. 5 dBi, respectively. The measured The efficiencymeasured is efficiency generally is lower generally than thatlower of simulation,than that of whichsimulation, is mainly which due is to mainly the current due to skin the effectcurrent caused skin effect by excess caused solder by excess and the solder loss an ofd the the chamber. loss of the The chamber. measured The totalmeasured efficiency total andefficiency peak gain and arepeak in gain good are agreement in good agreement with the simulation with the simulation results. results.

FigureFigure 11.11. SimulatedSimulated andand measuredmeasured peakpeak gaingain andand totaltotalefﬁciencies efficiencies of of the the proposed proposed antenna. antenna.

TheThe envelopeenvelope correlationcorrelation coefﬁcientcoefficient (ECC)(ECC) betweenbetween radiatingradiating elementselements isis aa criticalcritical parameterparameter of of the the MIMO MIMO antenna. antenna. The The ECC ECC can can be be calculated calculated by by the the measured measured S-parameters S-parame- accordingters according to the to below the below equation. equation. ∗ ∗ ∗| +∗ 2 | = |S mmSmn + S mmSmn| ECC = (1)(1) (1 − || −|2| )(1 − || −|2| ) 1−|Smm|2−|Snm| 1−|Snn|2−|Smn| The calculated ECC of the proposed MIMO antenna is shown in Figure 12. The values of ECCThe in calculated the working ECC frequency of the proposed bands MIMO are all antenna smaller is than shown 0.01, in which Figure means12. The that values the Electronics 2021, 10, x FOR PEER REVIEWproposed quad-port MIMO antenna achieves good MIMO diversity performance. 8 of 9 of ECC in the working frequency bands are all smaller than 0.01, which means that the proposed quad-port MIMO antenna achieves good MIMO diversity performance.

FigureFigure 12.12.Calculated Calculated ECCECC ofof thethe proposedproposed antenna. antenna.

Table 1 shows the performance comparison between the proposed antenna and the previously reported smartphone MIMO antennas. It could be concluded from Table 1 that the proposed MIMO antenna had high isolation, high efficiency and low ECC performance.

Table 1. Performance comparison with smartphone MIMO antennas.

Bandwidth Isolation Efficiency Size Reference ECC (GHz) (dB) (%) (mm3) [1] 3.3–3.6 (−6 dB) 20 33–47 0.4 150 × 75 × 5.3 [2] 3.4–3.6 (−6 dB) 13 50–60 0.15 136 × 68 × 1 3.3–4.2 (−6 dB) 11.5 53.8–76.5 0.1 [5] 150 × 75 × 5.5 4.8–5.0 (−6 dB) 15 62.6–79.1 0.12 3.1–3.85 (−10 dB) 17 65–75 0.06 [8] 150 × 70 × 6 4.8–6 (−10 dB) 18 60–71 0.06 3.4–3.6 (−6 dB) 41–72 0.08 [9] 11.5 150 × 75 × 7 4.8–5.1 (−6 dB) 40–85 0.05 [12] 3.3–4.2 (−6 dB) 11.5 63.1–85.1 0.2 150 × 75 × 7.5 2.45–2.65 (−10 dB) 40–65 [15] 3.4–3.75 (−10 dB) 11 50–70 0.01 150 × 75 × 1.6 5.6–6.0 (−10 dB) 60–80 3.4–3.6 (−10 dB) 85 This work 16.5 0.01 150 × 75 × 6.2 4.8–5.0 (−10 dB) 82

5. Conclusions In this paper, a quad-port MIMO antenna covering 3.5 GHz (3.4–3.6 GHz) and 4.9 GHz (4.8–5 GHz) frequency bands is proposed. The proposed antenna is printed on two long frames of the smartphone, which reserves some space for other wireless communication antennas. The antenna is verified by both simulation and measurement. The size of a single antenna element is only 14.9 mm × 7 mm × 0.8 mm. The measured maximum peak gain at 3.6 GHz and 5 GHz are 4.7 dBi and 5 dBi, respectively. The measured total efficiency is greater than 70% and the isolation is better than 16.5 dB. The proposed MIMO antenna is a good candidate for 5G mobile handsets.

Author Contributions: Conceptualization, J.H.; methodology, G.L.; investigation, G.D., J.C.; writing—original draft preparation, J.H.; writing—review and editing, H.L., G.L.; supervision and funding acquisition, G.L. All authors have read and agreed to the published version of the manuscript.

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Table1 shows the performance comparison between the proposed antenna and the previously reported smartphone MIMO antennas. It could be concluded from Table1 that the proposed MIMO antenna had high isolation, high efﬁciency and low ECC performance.

Table 1. Performance comparison with smartphone MIMO antennas.

Bandwidth Isolation Efﬁciency Size Reference ECC (GHz) (dB) (%) (mm3) [1] 3.3–3.6 (−6 dB) 20 33–47 0.4 150 × 75 × 5.3 [2] 3.4–3.6 (−6 dB) 13 50–60 0.15 136 × 68 × 1 3.3–4.2 (−6 dB) 11.5 53.8–76.5 0.1 [5] 150 × 75 × 5.5 4.8–5.0 (−6 dB) 15 62.6–79.1 0.12 3.1–3.85 (−10 dB) 17 65–75 0.06 [8] 150 × 70 × 6 4.8–6 (−10 dB) 18 60–71 0.06 3.4–3.6 (−6 dB) 41–72 0.08 [9] 11.5 150 × 75 × 7 4.8–5.1 (−6 dB) 40–85 0.05 [12] 3.3–4.2 (−6 dB) 11.5 63.1–85.1 0.2 150 × 75 × 7.5 2.45–2.65 (−10 dB) 40–65 [15] 3.4–3.75 (−10 dB) 11 50–70 0.01 150 × 75 × 1.6 5.6–6.0 (−10 dB) 60–80 3.4–3.6 (−10 dB) 85 This work 16.5 0.01 150 × 75 × 6.2 4.8–5.0 (−10 dB) 82

5. Conclusions In this paper, a quad-port MIMO antenna covering 3.5 GHz (3.4–3.6 GHz) and 4.9 GHz (4.8–5 GHz) frequency bands is proposed. The proposed antenna is printed on two long frames of the smartphone, which reserves some space for other wireless communication antennas. The antenna is veriﬁed by both simulation and measurement. The size of a single antenna element is only 14.9 mm × 7 mm × 0.8 mm. The measured maximum peak gain at 3.6 GHz and 5 GHz are 4.7 dBi and 5 dBi, respectively. The measured total efﬁciency is greater than 70% and the isolation is better than 16.5 dB. The proposed MIMO antenna is a good candidate for 5G mobile handsets.

Author Contributions: Conceptualization, J.H.; methodology, G.L.; investigation, G.D., J.C.; writing— original draft preparation, J.H.; writing—review and editing, H.L., G.L.; supervision and funding acquisition, G.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported in part by National Natural Science Foundation of China under Grant No. 61671330and No. 61340049, the Science and Technology Department of Zhejiang Province under Grant No. LGG19F010009, and Wenzhou Municipal Science and Technology Program under Grant No. C20170005 and No.2018ZG019. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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