Eight-port multiband MIMO antenna design with high isolation for 5G smartphones

Abstract

This paper presents the design and analysis of a compact eight-port multiband multiple-input–multiple-output (MIMO) antenna for 5G smartphones. The proposed antenna structure is designed using meandering elements, as radiator, on the FR4 substrate of 150 × 80 × 0.8 mm with loss tangent (tan δ) of 0.02 and relative permittivity (ε_r) of 4.4. The proposed antenna resonates at 2.4, 3.5, and 5.5 GHz, and it covers the bandwidth of 2%, 6.28%, and 2.53%, respectively. The measured results provide an omnidirectional radiation pattern with 58%–78% of efficiency in all operating bands. The eight-port multiband MIMO design provides a high isolation of 17.5 dB, envelope correlation coefficient < 0.04, diversity gain of 9.98 dB, total active reflection coefficient < −10 dB, and channel capacity loss of <0.25 bits/s/Hz. Also, the hand phantom is designed to analyze the reflection coefficients and efficiency of the proposed antenna.

Introduction

Multiple-input–multiple-output (MIMO) systems with frequency diversity are the optimal choice for improved coverage and ultra-fast transmission rates [1–3]. The authors of reference [4] presented a MIMO antenna with a square patch and a modified complementary electric inductive capacitive resonator designed on the FR4 substrate, resonating at 2.45, 3.5, and 4.4 GHz. This design features a peak isolation of 47 dB due to hexagonal-shaped split ring resonators (SRRs) in the ground plane. In reference [5], the authors proposed a 20 × 20 mm MIMO antenna on FR4 substrate that produces triple bands at 1.8, 2.4, and 3.5 GHz with the help of meta-resonator to achieve gain of 1.9, 1.75, and 1.52 dBi, respectively. The references [4, 5] present the different techniques used for multiband operation, reduced antenna size, and frequency diversity. The MIMO antenna has a challenge of coupling among the resonating elements, which can be encountered with the help of defected ground surface, parasitic reflectors and neutralization lines. In reference [6], a single band MIMO antenna of 26.43 × 38.25 mm, with four ports designed on FR4 substrate, is used for 4.4–5 GHz band, wherein the radiators are orthogonally placed to produce an isolation of 25 dB, reducing mutual coupling and obtain a peak gain of 2.8 dBi used for 5G applications. MIMO antenna system that covers 3.4–3.6 GHz with H-shaped resonators, of size of 20 × 20 mm, in orthogonal arrangement used for 5G applications is reported in reference [7]. This offers a bandwidth of 200 MHz and isolation of >12 dB with FR4 substrate of 0.8 mm thickness. A quad-port MIMO antenna of 45 × 45 mm with FR4 as a substrate is proposed in reference [8], which consists of a circular stub in the ground to achieve ultra-wideband range of 3.1–11 GHz for Wi-MAX, WLAN, and C-band applications. A four-port antenna of 60 × 60 mm with FR4 substrate is presented in reference [9], in which two arms with an open slot ground plane is used for bandwidth enhancement that operates at 3.4–3.8 GHz. The single band and low-level isolation limit its practical application.

In reference [10], the authors proposed a MIMO antenna of 46 × 21 mm, designed on the FR4 substrate of 1.6 mm thickness, with a circular-shaped SRR on top and a defected ground at the bottom to produce dual bands that provide linear polarization and circular polarization in each band. The gain of the antenna is 3.25 and 3.4 dBi in 3.4–3.6 GHz and 4–8 GHz bands, respectively, with more than 15 dB isolation used for 5G and C band applications. In reference [11], the authors proposed a four-port SRR induced inverted L-monopole antenna on the FR4 substrate of 40 × 40 mm. The lower frequency mode merges with SRR to obtain a wider bandwidth of 35.21% and gain of 4 dBi at 2.9 GHz for 5G applications. It also presents MIMO parameters like envelope correlation coefficient (ECC), total active reflection coefficient (TARC), channel capacity loss (CCL) with acceptable value limits. A MIMO antenna of 35 × 28 mm designed on FR4 substrate is presented in reference [12] to produce dual-band operation, with the help of closely spaced U-slots, at 2.6 and 3.6 GHz. It has parasitic structures of C-shape to reduce mutual coupling in the operating bands, where the isolation obtained is of 13 and 10 dB at resonant frequencies. Antenna configurations proposed in references [10–12] have larger substrate area, complicated design, and poor performance.

In reference [13], a dual-band MIMO antenna with FR4 substrate is reported with dimensions of 7 × 6.2 mm for 5G mobile applications, and use a parasitic stub method to cover 3.4–3.93 GHz and 4.5–5.3 GHz with isolation of 10 dB, efficiency > 50%, and ECC < 0.23. In reference [14], a T-shaped monopole MIMO antenna with FR4 substrate is presented for 5G mobile applications. The antenna has dimensions of 21.8 × 7 mm, produces an isolation of 12.8 dB, ECC lower than 0.18 and efficiencies of 71% and 68% in 3.3–3.6 GHz and 4.4–5 GHz bands, respectively. In reference [15], a four-port low profile MIMO antenna is presented for 5G mobile applications mounted on FR4 substrate with wideband shared-radiator. The antenna with dimensions of 19 × 19 mm has four corner-cut patches with grounded square analyzed through characteristic mode analysis, and it produces isolation of 10 dB and efficiencies of 40.3%–48.5% in the frequency range of 4.4–5 GHz. Another eight-element MIMO antenna for 5G mobile applications is presented in reference [16], which resonates at 3.1–3.7 GHz, 4.47–4.91 GHz, and 5.5–6.0 GHz. The antenna dimensions are 19 × 7 mm and produces spatial diversity characteristics with efficiencies of 62% and 78%, isolation greater than 16 dB and 5.8 dBi of peak gain. In reference [17], a mobile phone antenna is designed of dimensions of 46 × 1 mm with dual feed shared radiator on FR4 substrate to cover Global Positioning System (GPS)and Long Term Evolution (LTE) bands at 1172–1205 MHz and 1410–2790 MHz with improved isolation of 14.8 dB and efficiencies from 52.5%–63.9%. In reference [18], an 8 × 8 MIMO antenna with dimensions of 3 × 21.5 mm is presented, which has isolation greater than 17.5 dB with the help of balanced open-slot design between the ports. The antenna is designed for 5G mobile applications, and it has an efficiency of >62% and ECC < 0.05. The designs in the above references are used in miniaturizing MIMO antennas in smartphones. However, these designs cannot reduce the size of antenna further with a greater number of antenna elements. Therefore, designing a miniaturized antenna with larger number of antenna elements, which can be used to develop a MIMO antenna for smartphones, is still a challenge.

In this paper, the designed antenna is a compact multiband MIMO configuration for 5G smartphones with improved isolation. The proposed antenna resonates at 2.4, 3.5, and 5.5 GHz with omnidirectional radiation patterns. The objectives of the proposed eight-port multiband MIMO antenna are:

• • To achieve the frequency bands that resonates at 2.4, 3.5, and 5.5 GHz without any parasitic elements.

• • To obtain isolation of >17.5 dB between the antennas.

• • To study the antenna characteristics on a hand phantom model to check its performance such as S₁₁, total efficiency, and ECC.

This work is organized as follows: The “Design of an eight-port multiband MIMO antenna” section elaborates the design of an eight-port multiband MIMO antenna, the “Analysis of the eight-port multiband MIMO antenna” section describes the analysis of the proposed MIMO antenna, the “Performance analysis using hand-phantom mode” section analyzes the efficiency, ECC, and S-parameters using hand phantom model, and the “Conclusion” section describes the conclusion of the work.

Design of an eight-port multiband MIMO antenna

The proposed antenna has meandering elements that are etched on 150 × 80 mm FR4 substrate (${\varepsilon _r}$ = 4.4 and tan δ = 0.02) with 0.8 mm of thickness, and the single antenna has a size of 16.5 × 8.5 mm (0.132λ ₀ × 0.068λ ₀, where λ ₀ is the free space wavelength calculated at the lowest frequency). The antenna has eight elements (ANT 1–8), in which, antenna 1 and antenna 5 are placed in horizontal direction along the short edges, and antennas 2–4 and 6–8 are placed in vertical direction along the long edges.

The expanded layout of the proposed antenna is indicated in Fig. 1. The proposed eight-port multiband MIMO antenna is simulated using CST Microwave Studio Suite software, and Table 1 shows the dimensions of the proposed eight-port multiband MIMO antenna. It has an inverted E-shaped slot radiator on the ground plane, which has two open ends. In the radiator, there is a branch slot on the either of the open-ended vertical slot, and a microstrip line is used in the slot radiator to feed it.

Figure 1. (a), (b). Top and bottom view of the proposed multiband MIMO antenna.

Table 1. Dimensions of the proposed eight-port multiband MIMO antenna

Every antenna element is designed to resonate at 2.4, 3.5, and 5.5 GHz using equations (1–3), respectively.

(1)\begin{eqnarray} {{f}_{3.5\text{GHz}}}=\frac{0.61*c}{\begin{align} & \left( {{L}_{1}}+{{W}_{1}}+{{L}_{3}}+{{W}_{3}}+{{L}_{4}}+{{W}_{4}}+{{L}_{5}} \right. \\ & \ \ \ \ +{{W}_{5}}+\left. {{L}_{6}}+{{W}_{6}}+{{L}_{7}}+{{W}_{7}} \right)*\sqrt{\frac{{{\varepsilon }_{r}}+1}{2}} \\ \end{align}}\ \notag\end{eqnarray}

(2)\begin{equation}{f_{2.4\,{\text{GHz}}}} = \frac{{0.56\,*\,c}}{{\left( {{L_1}\, + \,{W_1}\, + \,d} \right)\,*\,\sqrt {\frac{{{\varepsilon _r}\, + \,1}}{2}} }}\end{equation}

(3)\begin{equation}{f_{5.5\,{\text{GHz}}}} = \frac{{0.623\,*\,c}}{{\left( {{L_1}\, + \,{W_1}\, + \,{L_3}\, + \,{W_2}} \right)\,*\,\sqrt {\frac{{{\varepsilon _r}\, + \,1}}{2}} }}\end{equation}

Design novelty

The proposed MIMO antenna composed of eight resonators with an ordinary ground plane and no decoupling element among the radiators is used to improve isolation.

• • The antenna elements 1 and 5, 2 and 8, 3 and 7, and 4 and 6 are mirror images of each other so the coupling current vectors are in the opposite direction. Due to this mirror-image arrangement of the resonating elements, the proposed MIMO antenna achieves polarization diversity thereby providing high isolation without the need for an additional decoupling mechanism.

• • The proposed antenna design achieves multiband by using an E-shaped slot in the ground plane, and the ground plane disrupts unwanted current paths, reducing coupling between antenna elements.

• • The meandering of the conductive patch reduces the physical dimensions of the antenna while maintaining the desired electrical length. This allows for a more compact design, which is beneficial in space-constrained applications like mobile devices.

• • Despite the compact size, the proposed antenna elements can achieve good radiation efficiency ensuring strong signal transmission and reception, particularly in low-power applications.

• • The antenna element can be tailored to meet specific design requirements, such as desired resonant frequency, bandwidth, and radiation pattern. This flexibility makes it adaptable to integrate into printed circuit boards (PCBs) and other electronic devices, facilitating the design of compact and integrated wireless gadgets/devices.

• • Mirrored antenna elements in the proposed antenna can help to achieve better isolation between individual antenna elements, improving the performance of MIMO algorithms that rely on separating the multiple transmitted and received signals.

• • With reduced correlation, the proposed MIMO antenna can achieve higher diversity and multiplexing gains, resulting in better performance metrics such as signal-to-noise ratio.

• • The planar structure of the proposed meandered line antenna contributes to a low-profile design, which is aesthetically pleasing and practical for embedding in thin devices like smartphones and tablets.

Evolution stages of the proposed multiband single antenna

The evolution stages of the proposed antenna are shown in Fig. 2. The antenna has a meandering element over the feed lines to resonate at 3.5 GHz, as shown in Fig. 2(a). To cover the 5.5 GHz frequency for Wi-MAX application, an additional meandering element is introduced internally, as shown in Fig. 2(b). Finally, to obtain the WLAN operating frequency at 2.4 GHz, external meandering element is incorporated in the antenna, as shown in Fig. 2(c). The S₁₁ of the three stages are given in Fig. 3.

Figure 2. (a–c). Evolution stages of the proposed multiband single antenna.

Figure 3. S₁₁ of the evolution stages of the proposed multiband single antenna.

Eight-port multiband MIMO antenna placement

The placement of the radiating elements for an eight-port antenna is decided by placing them in different positions and analyzing their S-parameters.

Case 1

In this case, the position of the radiating elements 1 and 5 is reversed, as shown in Fig. 4(a), so that the open ends of the two antennas (ANT 1 and ANT 5) are very close to the PCB corners. Due to this, the eight-port arrangement for the multiband MIMO antenna does not resonate at 3.5 GHz frequency band. Also, the isolation between antennas 1 and 2 and antennas 4 and 5 (S₂₁ and S₅₄) is 13.5–17.5 dB, as shown in Fig. 4(b).

Figure 4. (a), (b). Design and its S-parameters of Case 1.

Case 2

In this case, the position of the radiating elements 2, 4, 6, and 8 is reversed, as shown in Fig. 5(a). The isolation between the antennas is reduced to 12 dB, as shown in Fig. 5(b). As the open ends of the radiating elements are facing each other, due to the lesser distance between both the antennas, an undesirable isolation is obtained.

Figure 5. (a), (b). Design and its S-parameters of Case 2.

It is observed that the two antennas that are adjacently placed causes poor isolation than the antennas placed away from each other. Therefore, the two face to face adjacent radiating elements should be placed away from each other (as in antennas 1 and 6 and antennas 5 and 8) to obtain enhanced isolation.

Case 3

This case shows the orthogonal arrangements of the radiating elements in which they are placed perpendicularly, as shown in Fig. 6(a). The orthogonal antenna polarization does not cover the resonating frequencies (3.5 and 5.5 GHz) and provides deteriorated isolation, as shown in Fig. 6(b). This case cannot yield good antenna performance.

Figure 6. (a), (b). Design and its S-parameters of Case 3.

Case 4

To improve the isolation between the antenna elements, the antennas 1 and 5 are placed away from the nearby antennas 2 and 4, as shown in Fig. 1. This produces the bandwidth of 2%, 6.28%, and 2.53% at 2.4, 3.5, and 5.5 GHz, respectively. From Fig. 7, it is observed that the proposed eight-port multiband MIMO antenna operates at 2.4, 3.5, and 5.5 GHz with an isolation of 17.5 dB.

Figure 7. (a), (b). Simulated S-parameters of the eight-port multiband MIMO antenna.

Current distribution

The two antennas connecting at the common null are in reverse direction. As there is a less current distribution in the ground, a high isolation level is achieved. Also, as stipulated by the current distribution in Fig. 8, if the antennas are in perpendicular direction, the x-axis and y-axis generate orthogonal polarizations, resulting in low isolation.

Figure 8. (a–c) Surface current distribution at 2.4, 3.5, and 5.5 GHz.

The antenna proposed in this work resonates at 2.4, 3.5, and 5.5 GHz, making it suitable for 5G smartphones while having smaller dimensions. The fabrication process is simple because the substrate used in this work, FR4 with a thickness of 0.8 mm, is widely available. FR4 is inexpensive while comparing with other high-performance substrates, making it a cost-effective choice for production of PCBs. It is a mechanically robust material, providing good support and stability to the antenna design that helps the PCB to withstand physical stress during manufacturing. Also, FR4 is easy to process and handle during PCB manufacturing, including drilling, cutting, and etching.

Figure 9 shows the prototype of the proposed eight-port multiband MIMO antenna whose S-parameter and far-field characteristics are tested using vector network analyzer and anechoic chamber. The measured S-parameters and transmission coefficients are shown in Fig. 10(a) and (b).

Figure 9. (a–c). Prototype of the proposed MIMO antenna.

Figure 10. (a), (b). Measured S-parameters of the proposed MIMO antenna.

Radiation characteristics

The radiation patterns for antennas 1–4 operating in the three bands are given in Fig. 11, which that shows the patterns of the antennas are omnidirectional. The radiation patterns of antennas 5–8 are not presented, as they are nearly the mirror images of antennas 1–4. Figure 12 shows the gain and efficiency of the proposed eight-port multiband MIMO antenna. The obtained gain values are 2.78, 3.014, and 3.438 dBi and efficiencies are 78%, 71%, and 58% at 2.4, 3.5, and 5.5 GHz, respectively.

Figure 11. Radiation patterns at 2.4, 3.5, and 5.5 GHz.

Figure 12. Gain and efficiency of the proposed antenna.

Analysis of the eight-port multiband MIMO antenna

To verify the proposed multiband MIMO characteristics, several metrics such as CCL, ECC, diversity gain (DG), total efficiency, and TARC are studied in this section.

The ECC explains the correlation between the radiating elements. The actual value of ECC is zero but in practical, 0.5 is the acceptable limit. The ECC calculated using equation (4) [18] is less than 0.04, as shown in Fig. 13.

Figure 13. Measured ECC of the antenna.

(4)\begin{equation}{\rho _e} = \frac{{|\mathop \iint \nolimits \left[\,\overrightarrow {{F_1}} \left( {\theta ,\varphi } \right).\overrightarrow {{F_2}} \left( {\theta ,\varphi } \right.\right]d\Omega |}^2}{\mathop \iint \nolimits {|\overrightarrow {{F_1}} \left( {\theta ,\varphi } \right)|}^{2} d\Omega \,\mathop \iint \nolimits{ |\overrightarrow {{F_2}} \left( {\theta ,\varphi } \right)|}^{2} d\Omega }\end{equation}

where F_i denotes far-field radiated by the antenna. $\left( {\theta ,\varphi } \right)$ are the angles (elevation, azimuth) and $\Omega $ is the solid angle. The measured DG shown in Fig. 14 is mathematically related to ECC as given in equation (5) [14] whose value is >9.98 for the proposed MIMO antenna.

Figure 14. Measured DG of the antenna.

(5)\begin{equation}DG = 10\sqrt {1 - EC{C^2}} \end{equation}

TARC describes the effective operating bandwidth of the antenna system, whose obtained value is less than –10 dB, is shown in Fig. 15, and can be given by equation (6) [17].

(6)\begin{equation}TARC = \frac{{\sqrt {\mathop \sum \nolimits_{i = 1}^N {{\left| {{b_i}} \right|}^2}} }}{{\sqrt {\mathop \sum \nolimits_{i = 1}^N {{\left| {{a_i}} \right|}^2}} }}\end{equation}

Figure 15. Measured TARC for the proposed MIMO antenna.

CCL, shown in Fig. 16, compares the performance of the MIMO system with a single antenna system which produces CCL less than 0.25 bits/Hz/s that can be calculated using equation (7) [16, 19]. In Fig. 17, it is observed that the total efficiency of the antennas 1–4 is 55%–78%, and the antennas 5–8 are nearly the mirror image of antennas 1–4.

(7)\begin{equation}CCL = - lo{g_2}det\left( {{\theta ^\mu }} \right)\end{equation}

(8)\begin{equation}{\theta ^\mu } = \left[ {\begin{array}{*{20}{c}} {{\xi _{11}}}&{{\xi _{12}}} \\ {{\xi _{21}}}&{{\xi _{22}}} \end{array}} \right]\end{equation}

Figure 16. Measured CCL of the antenna.

Figure 17. Total efficiency of the proposed antenna.

where ${\xi _{11}} = 1 - \left[ {{{\left| {{S_{11}}} \right|}^2} + {{\left| {{S_{12}}} \right|}^2}} \right]$; ${\xi _{11}} = - \left[ {S_{11}^{\text{*}}{S_{12}} + S_{21}^{\text{*}}{S_{12}}} \right]$; ${\xi _{11}} = - \left[ {S_{22}^{\text{*}}{S_{21}} + S_{12}^{\text{*}}{S_{21}}} \right]$; ${\xi _{11}} = 1 - \left[ {{{\left| {{S_{22}}} \right|}^2} + {{\left| {{S_{21}}} \right|}^2}} \right]$.

Performance analysis using hand-phantom model

In 5G smartphones, where data communication plays a dominant role compared to voice calls, understanding the impact of user hands on antenna performance becomes crucial [20, 21]. This section explains the influence of user’s single and double hands on the proposed eight-port multiband MIMO antenna. Through simulations employing a standard hand phantom model, the antenna’s behavior is analyzed under single hand mode (SHM) and dual hand mode (DHM) scenarios, as shown in Fig. 18.

Figure 18. (a), (b). SHM and DHM model of the proposed antenna.

Figure 19 shows the S-parameters for SHM operation. From the figure, it is observed that, in SHM, shifting of frequencies affect the antennas 2, 7, and 8, which have direct contact with the fingers. The efficiency values of antennas 2, 7, and 8 are below 30% due to the absorption of energy in the user’s hand, as shown in Fig. 20.

Figure 19. (a), (b). S-parameters of the proposed antenna in SHM.

Figure 20. Total efficiency of the proposed antenna in SHM.

Similarly, the scattering parameters for DHM operations are shown in Fig. 21, which shows that there is a shifting of resonant frequencies in antennas 1 and 5, that have direct contact with the thumb. The scattering parameters of antennas 3 and 6–8 are almost the same, as they are distant from the hand phantom. From Fig. 22, in antennas 1 and 5, the total efficiencies are reduced to 25%. Figure 23 shows that the ECC values of the affected antennas 2, 7, and 8 in SHM and antennas 1 and 5 in DHM gets shifted.

Figure 21. (a), (b). S-parameters of the proposed antenna in DHM.

Figure 22. Total efficiency of the proposed antenna in DHM.

Figure 23. (a), (b). ECC of SHM and DHM of the proposed antenna.

From the analysis of S-parameters, total efficiencies, and ECCs, the proposed MIMO antenna has good performance under SHM and DHM hand-phantom conditions. Table 2 provides the comparison between the hand-held and non-hand-held states of the proposed antenna design.

Table 2. Comparison of antenna performance under hand-held and non-hand-held states

Table 3 shows the comparison of the proposed antenna with the related published works, which can be explained as,

• • The proposed eight-port multiband MIMO antenna has a compact size of 0.132λ ₀ × 0.068λ ₀ compared to the designs reported in references [6–8, 12, 14, 17, 18].

• • The proposed eight-port multiband MIMO antenna covers three operating frequencies at 2.4, 3.5, and 5.5 GHz, unlike reported in references [6–8, 12–15, 17, 18].

• • The proposed eight-port multiband MIMO antenna produces improved ECC value of 0.04 than reported in references [6, 8, 12–18].

• • The proposed eight-port multiband MIMO antenna produces improved DG value of 9.98 dB than reported in references [6, 7, 15, 16].

• • The proposed eight-port multiband MIMO antenna produces improved CCL of 0.25 bits/Hz/s than reported in references [7, 8, 15, 16].

• • The proposed eight-port multiband MIMO antenna produces isolation of 17.5 dB unlike in references [6–8, 12–18].

Table 3. Comparison of the proposed antenna with other works

Conclusion

In this work, an eight-port antenna is proposed with small size, which is easy to fabricate and resonates at 2.4, 3.5, and 5.5 GHz. It provides an isolation of >17.5 dB, antenna efficiency (58%–78%) and its parameters such as low ECC < 0.04, high DG > 9.98 dB, improved TARC < –10 dB and CCL < 0.25 bits/Hz/s are observed. The proposed antenna also exhibits good characteristics with hand-phantom. Based on the simulated and measured results, the proposed multiband MIMO antenna is a suitable design for 5G applications.