Introduction
Fifth-generation (5G) mobile communication has three features: Ultra-Reliable and Low Latency Communications, Enhanced Mobile Broadband, and Massive Machine Type Communications. In release technical specification, 38 of the 3 GPP, 5G mobile communications operating bands are in the Sub-6-GHz band (410–7125 MHz) [Reference Dubazane, Kumar and Afullo1–Reference Ullah, Ullah, Faisal, Ullah, Mabrouk, Hasan and Kamal28]. The European Commission has designated a frequency range of 3.4–3.8 GHz specifically for the deployment of 5G technology [Reference Sheriff, Kamal, Tariq Chattha, Kim Geok and Khawaja5]. The 3.5-GHz (3.4–3.6 GHz) frequency band is now the most commonly used frequency band [Reference Parchin, Al-Yasir, Abdulkhaleq, Basherlou, Ullah and Abd-Alhameed3–Reference Muhsin, Salim and Ali18]. Multi-input multi-output (MIMO) technology is used to increase data transmission speed. MIMO technology uses spatial diversity to increase data transmission speed [Reference Alwarafy, Sulyman, Alsanie, Alshebeili and Behairy19]. MIMO technology does not require extra bandwidth or transmission power and significantly increases channel capacity (CC) and spectral efficiency. Therefore, MIMO technology is critical for 5 G communication systems [Reference Dubazane, Kumar and Afullo1–Reference Ullah, Ullah, Faisal, Ullah, Mabrouk, Hasan and Kamal28].
Antenna types include inverted-F, open slot, patch, fractal geometry, and loop [Reference Liao6, Reference Muhsin, Salim and Ali18, Reference İncesulu, Ulutaş, Bilge, İmeci and Durak20–Reference Mok, Wong, Luk and Lee22]. There are many types of MIMO antennas because fractal geometry and loop antennas have a small footprint, are inexpensive, have a simple structure, and allow easy impedance matching [Reference Wong, Tsai and Lu8–Reference Zhao and Ren11]. A previous study [Reference Muhsin, Salim and Ali18] arranged eight antennas on the two long side edges and printed them on the inner and outer surfaces of the frame. An I-shaped feeding part and a modified Hilbert fractal monopole antenna comprise the structure of the antenna. The Hilbert space-filling property is used to achieve significant miniaturization. Loop antennas also have a variety of shapes, such as circles, triangles, and rectangles, all of which have similar far-field patterns. A loop antenna can be coupled through the feeding structure on the inner surface to reduce the size of the antenna so a fractal geometry and a loop antenna can be used for a mobile phone with a high screen-to-body ratio [Reference Alja’afreh9, Reference Piao, Jin and Qu10, Reference Guo, Cui, Li and Sun13, Reference Muhsin, Salim and Ali18].
Antennas in the MIMO antenna system must be isolated, but mobile phones are becoming more compact so mutual coupling is increased. Mutual coupling results in poor isolation and a low data transmission rate. Physical separation, a metasurface [Reference Dubazane, Kumar and Afullo1, Reference Dubazane, Kumar and Afullo2], a one-wavelength mode [Reference Liao6], self-decoupling [Reference Piao, Jin and Qu10], a single common grounding branch [Reference Ren, Zhao and Wu15], decoupling structures [Reference Liu, Zhao, Jing, He, Xi and Zhao17], a neutralized line [Reference Wang and Du23], diverse polarization [Reference Parchin, Al-Yasir, Abdulkhaleq, Basherlou, Ullah and Abd-Alhameed3, Reference Sheriff, Kamal, Tariq Chattha, Kim Geok and Khawaja5, Reference Serghiou, Khalily, Singh, Araghi and Tafazolli7, Reference Vaughan24], and a decoupling network [Reference Zhuang, Feng and Wu25] are used to increase isolation. A metasurface, a common grounding branch, decoupling structures, a neutralized line, and a decoupling network require additional elements. Self-decoupling, a single common grounding branch, and diverse polarization require more ground plane space.
This study proposes a 12 × 12 MIMO antenna array that requires no external passive elements. The proposed single-loop antenna element has a compact uniplanar structure with a footprint of 5.85 × 4.9 mm2 (0.068λ 0 × 0.057λ 0). Table 1 shows a comparison between the proposed antenna and those of other studies. The proposed antenna has the smallest dimensions, so it is suitable for a mobile phone with a high screen-to-body ratio. The transmission coefficient for the proposed antenna is less than −12 dB without any decoupling structure between adjacent antennas. The measured 6-dB impedance bandwidth is from 3.4 to 3.73 GHz and covers the long-term evolution 42 (LTE42) (3.4–3.6 GHz) band.
Table 1. Comparison between the proposed antenna and those of other studies
* The proposed antenna
Design for the proposed antenna
Antenna structure and principle
Figure 1(a) shows the positions of antennas for a MIMO antenna array in a mobile phone. The size of the mobile phone is 150 mm (length) × 75 mm (width) × 7 mm (height). The blue part on the back of the FR4 substrate that is 0.8 mm thick is the system ground plane (ε r = 4.4, tanδ = 0.02). The antennas for the proposed 12 × 12 MIMO antenna array are placed on two side edges of the mobile phone. Figure 1(b) and (c) shows the geometric and the detail parameters for the proposed single antenna element. Figure 1(b) and (c), respectively, shows the front view and the lateral view of the proposed single antenna element. The single antenna element is composed of a serpentine-structure loop antenna and an inverted-L feeding structure. The serpentine-structure loop antenna has a resonance frequency of 3.5 GHz. The length of the loop antenna is 43.8 mm, which is around half the wavelength of 3.5 GHz.
Figure 1. (a) The position of the MIMO antenna array in a mobile phone; the detail parameters for the proposed single antenna element in (b) the front view and (c) the lateral view.
The inverted-L feeding structure, which is a capacitive coupling structure, is connected to a 50 Ω transmission line. In Fig. 1(b) and (c), the red rectangle is the feeding point for the proposed antenna and the blue area is the ground plane in the mobile phone. The size of a single antenna element is 5.85 × 4.9 mm2 (0.068λ 0 × 0.057λ 0). The width of all lines for the proposed antenna is 0.2 mm, except lines that are marked with a yellow T, which are 0.3 mm, and all gaps between line and line of the antenna are 0.25 mm except for gaps that are marked with a red G, which are 0.3 mm, as shown in Fig. 1(c). The simulated reflection coefficients for the proposed MIMO antenna array are shown in Fig. 2, and the 6-dB impedance bandwidth covers 3.4–3.6 GHz.
Figure 2. Reflection coefficients for the proposed MIMO antenna array.
Antenna design process and current distribution
In Fig. 3(a), the bilateral-symmetry serpentine loop antenna, which is called Ref.1, initially had a T-shape feeding structure. Ref.1 excites three resonating modes at around 3.3, 5.4, and 7.5 GHz and the reflection coefficient is shown in Fig. 3(b). Ref.1 is a bilateral-symmetry structure, so the current distribution is 5.4 GHz on the right-hand side and the left-hand side of Ref.1, as shown in Fig. 3(c). Therefore, Ref.1 can be divided into two identical half-antennas. The half-antenna that is fed by an L-shape feeding structure and is connected by a straight strip to the ground is called Ref.2, as shown in Fig. 3(d). The L-shape feeding structure in Ref.2 is split from the T-shape feeding structure in Ref.1. The L-shape feeding structure is compressed, so line ab and line cd are shorter, as shown in Fig. 3(d).
Figure 3. The antenna design process shows (a) Ref. 1, (d) Ref. 2, and (g) the proposed Ant. The reflection coefficients are for (b) Ref. 1, (e) Ref. 2, and (h) the proposed Ant. The current distributions are for (c) Ref. 1, (f) Ref. 2, and (i) the proposed Ant.
Ref.2 uses a straight strip that is connected to the ground, so Ref.2 is longer than half of the length of Ref.1. Therefore, the first resonant frequency of Ref.2 is lower than the second resonant frequency of Ref.1. The first resonant frequency for Ref.2 is 4.6 GHz and the length of the current path, which is about 0.5λ 0 at 4.6 GHz, is 32 mm, as shown in Fig. 3(e). However, the operating band is still not close to the desired band of 3.4–3.6 GHz. Extending the length of the current path for Ref.2 forms the proposed antenna, as shown in Fig. 3(g). The size of the proposed antenna is also increased to 5.85 × 4.9 mm2. The reflection coefficient for the proposed antenna is shown in Fig. 3(h). The resonant frequency is around 3.5 GHz, and the operating bandwidth covers the required band of 3.4–3.6 GHz. The current distribution for the proposed antenna at 3.5 GHz is shown in Fig. 3(i). The length of the proposed antenna is 43.8 mm, which is half of the wavelength for the resonant frequency of 3.5 GHz.
Parametric analysis
The parameter H, which is the distance between the upper and lower arms of the S-shaped strip line for the proposed antenna, is shown in Fig. 4(a). The reflection coefficients are specified relative to the variations in the parameter H, as shown in Fig. 4(b). As the value for the parameter H increases, the current path becomes longer and the resonant frequency decreases. As the value of the parameter H decreases, the resonance frequency shifts increase. Therefore, a value for H of 2.1 mm equates to the lowest resonance frequency of 3.25 GHz and a value for H of 1.1 mm equates to a resonating frequency of 3.8 GHz. The optimal value for H is 1.6 mm because the 6-dB impedance frequency covers the desired band.
Figure 4. (a) The parameter H for the proposed antenna and (b) the variation in the reflection coefficient as H varies.
The width (parameter W) of the straight shorting line that is connected to the ground is also studied, as shown in Fig. 5(a). The value of the parameter W is used to fine-tune the resonance frequency. As W becomes wider, the resonance frequency increases slightly. The optimal value for parameter W is 0.2 mm because the 6-dB impedance bandwidth is from 3400 to 3730 MHz and covers 3400–3600 MHz, as shown in Fig. 5(b). Adjusting the length (parameter F) of the L-shaped feeding structure in Fig. 6(a) affects the impedance matching. From Fig. 6(b), it can be observed that the longer parameter F has better impedance matching. However, a greater value for parameter F creates a small value for gap1, which is the gap between the feeding structure and the serpentine antenna, as shown in Fig. 6(a). gap1 is too small to be fabricated so the optimal value for the parameter F is 4.95 mm and impedance matching is sufficient (reflection coefficient is around −30 dB). The value for the parameter S (Fig. 7(a)) is used to fine-tune the operating frequency such as the parameter W, as shown in Fig. 7(b). As the value for the parameter S is increased, the resonance frequency decreases slightly. The optimal value for the parameter S is 4.3 mm, and the 6-dB impedance bandwidth covers the LTE42 band.
Figure 5. (a) The parameter W for the proposed antenna and (b) variation in the reflection coefficient as W varies.
Figure 6. (a) The parameter F for the proposed antenna and (b) variation in the reflection coefficient as F varies.
Figure 7. (a) The parameter S for the proposed antenna and (b) variation in the reflection coefficient as S varies.
Measurements and discussion
Figure 8(a–c) shows front, rear, and lateral views of the proposed MIMO antenna. The simulated and measured reflection coefficients for Ant1 to Ant6 are shown in Fig. 9. The solid lines in Fig. 9 are measurements and the dotted lines are simulations. The MIMO antenna is symmetrical, so only Ant1 to Ant6 are measured. All of the measured reflection coefficients are similar to the simulated results, but the frequency shifts slightly. The 6-dB impedance bandwidth for the entire antenna covers the LTE42 band.
Figure 8. Photographs of the fabricated 12-element MIMO antenna: (a) front, (b) back, and (c) lateral view of the single antenna element.
Figure 9. Reflection coefficients for simulation and measurement.
Figure 10 shows simulated and measured transmission coefficients for Ant1, Ant2, and Ant3 to other antennae. Only the transmission coefficients between Ant1 to Ant3 and other antennae are shown because the arrangement of those antennas is symmetrical in the mobile phone. Figure 10(a) and (b) shows the simulated and measured transmission coefficients between Ant1 and other antennae. Figure 10(c) and (d) shows the simulated and measured transmission coefficients between Ant2 and other antennae. Figure 10(e) and (f) shows the simulated and measured transmission coefficients between Ant3 and other antennae. All of the measured transmission coefficients are less than −12 dB. Figure 10 shows that there is good agreement between the measured and simulated results.
Figure 10. Transmission coefficients for simulation and measurement (a) between Ant1 and Ant2 to 6, (b) between Ant1 and Ant7 to 12, (c) between Ant2 and Ant3 to 6, (d) between Ant2 and Ant7 to 12, (e) between Ant3 and Ant4 to 6, and (f) between Ant3 and Ant7 to 12.
The performance of the proposed MIMO antenna is determined using the envelope correlation coefficient (ECC) and the diversity gain (DG). The ECC represents the degree of correlation between the signal that is received by different antenna elements and DG measures the benefits of having multiple uncorrelated signals in the system [Reference Malviya, Panigrahi and Kartikeyan26]. ECC and DG are, respectively, defined as (1) and (2):
\begin{equation}\rho_\text{e}=\frac{{\vert\int{}\int{{\overrightarrow E}_i(\theta,\phi)}{\overrightarrow E}_j{(\theta,\phi)}{}\text{d}\Omega\vert}^2}{{\vert{\overrightarrow E}_i{(\theta,\phi)}\vert}^2{\vert{\overrightarrow E}_j{(\theta,\phi)}\vert}^2{}\text{d}\Omega},\end{equation}
\begin{equation}{\textrm{DG}} = 10\sqrt {1 - \rho _{\textrm{e}}^2} \end{equation}[Reference Mashagba27]. Figure 11(a–c), respectively, shows the calculated values for ECC for Ant1, Ant2, and Ant3 to other antennae. These ECC values are less than 0.42 at 3.4–3.6 GHz. Figure 12(a–c), respectively, show the calculated DG values for Ant1, Ant2, and Ant3 to other antennas. The DG values are more than 9 dB at 3.4–3.6 GHz. The ergodic CC for the proposed MIMO antenna is determined using Equation (3) as [Reference Ahmad12, Reference Ullah, Ullah, Faisal, Ullah, Mabrouk, Hasan and Kamal28]:
\begin{equation}{\textrm{CC}} = E\left\{ {{{\log }_2}\left[ {\det \left( {I + {{{\textrm{SNR}}} \over {{m_{{\textrm{tr}}}}}}} \right){\textrm{H}}{{\textrm{H}}^{\textrm{T}}}} \right]} \right\}.\end{equation}
Figure 11. Calculated ECC values for (a) Ant1, (b) An2, and (c) Ant3 to other antennae.
Figure 12. Calculated DG values for (a) Ant1, (b) Ant2, and (c) Ant3 to other antennae.
In Equation (3), an equal amount of power is allocated to each transmit antenna, but there is no prior knowledge of the channel state information included. The calculated CC value for the proposed MIMO antenna is between 45 and 50 bps/Hz at 3.4–3.6 GHz, as shown in Fig. 13. Figure 14 shows the simulated and measured total efficiency and peak gain for Ant1 to Ant3. The MIMO antenna is symmetrical so only Ant1 to Ant3 are shown. The simulated total efficiency for the proposed MIMO antenna is between 40% and 57%, and the measured total efficiency for the proposed MIMO antenna is between 39% and 56%, as shown in Fig. 14(a). Figure 14(b) shows that the simulated and measured peak gain for the proposed MIMO antenna is between 1 and 2.6 dBi and between 0 and 3.2 dBi, respectively.
Figure 13. Calculated channel capacity for the proposed MIMO antenna array.
Figure 14. Simulated and measured (a) total efficiency and (b) peak gain.
The 2D normalized radiation pattern for Ant1 to Ant3 in the xz-, yz-, and xy-planes at 3.5 GHz is shown in Fig. 15. Figure 15(a–c) shows the radiation patterns for Ant1 in the xz-, yz-, and xy-planes at 3.5 GHz. Figure 15(d–f) shows the radiation patterns for Ant2, and Fig. 15(g–i) shows the radiation patterns for Ant3. The loop antenna is a magnetic dipole so the Eϕ value for Ant1 to Ant3 in the xz-plane features an omnidirectional radiation pattern, as shown in Fig. 15(a), (d), and (g).
Figure 15. Simulated and measured radiation patterns for the proposed MIMO antenna at 3.5 GHz: (a) xz-plane, (b) xy-plane, and (c) yz-plane for Ant1, (d) xz-plane, (e) xy-plane, and (f) yz-plane for Ant2, and (g) xz-plane, (h) xy-plane, and (i) yz-plane for Ant3.
Conclusion
A compact 12 × 12 MIMO loop antenna array is proposed for 5 G mobile phone applications. The single antenna element uses a fold and serpentine design to reduce the size, which is 5.85 × 4.9 mm2 (0.068λ 0 × 0.057λ 0). The MIMO antenna has a small transmission coefficient of less than −12 dB, and no decoupling element is used. The measured 6-dB impedance bandwidth for the antenna is 3.4–3.73 GHz and covers the LTE 42 (3.4–3.6 GHz) band. The measured total efficiency for the proposed antenna is 39–56%. The ECC value is less than 0.42, the CC is 45–50 bps/Hz, and the peak gain is 0–3.2 dBi.
Funding Statement
The authors would like to thank the Ministry of Science and Technology (Taiwan) and National Pingtung University for partially sponsoring this study. The project number for the MOST is MOST 106-2633-E-153-001.
Competing interests
The authors report no conflict of interest.
Ji-Peng Jhuang received his B.S. degree in the Department of Computer and Communication from National Pingtung University, Taiwan, in 2021. In 2022, he joined the Department of Computer Science and Information Engineering, Taiwan. His research interests include MIMO antenna design, fabrication, and measurements, as well as Sub-6-GHz band and mobile phone antennas. His current research interests involve the design of access point antennas, dual-polarization antennas, and MIMO loop slot antennas for Sub-6-GHz applications.
Hsin-Lung Su was born in Kaohsiung, Taiwan. He received the B.S. degree in electronic engineering from National Taiwan Institute of Technology, Taipei, and the M.S. and Ph.D. degrees in electrical engineering from National Sun Yat-Sen University (NSYSU), Kaohsiung, Taiwan. From 2007 to 2013, he was with the Department of Computer and Communications, National Pingtung Institute of Commerce, Pingtung, Taiwan. He has been serving in the Department of Computer and Communications, National Pingtung University (NPTU) since 2013. He is currently an Associate Professor with NPTU. His current research interests include antennas, metamaterials, metamaterial-inspired structures, EBG structures, conducted emission of electromagnetic compatibility, and the application of EM waves of the anisotropic media. Dr. Su serves as the Vice Chair in APSTNC (IEEE Antennas and Propagation Society Tainan Chapter) from 2016 to 2022. Now, he serves as the Chair in APSTNC.
