H-Shaped Eight-Element Dual-Band MIMO Antenna for Sub-6 GHz 5G Smartphone Applications

The design of an eight-element H-shaped dual-band multiple-input multiple-output (MIMO) antenna system for sub-6 GHz fifth-generation (5G) smartphone applications is presented in this work. The radiating elements are designed on the side edge frame of the smartphone, placed on both sides of the main printed circuit board (PCB). Each side edge consists of four radiating elements, which ensures low mutual coupling between antenna elements. The total size of the main PCB is <inline-formula> <tex-math notation="LaTeX">$150\times 75$ </tex-math></inline-formula> mm<sup>2</sup>, while the size of the side edge frame is <inline-formula> <tex-math notation="LaTeX">$150\times 7$ </tex-math></inline-formula> mm<sup>2</sup>. A single antenna consists of an H-shaped radiating element fed using a <inline-formula> <tex-math notation="LaTeX">$50\Omega $ </tex-math></inline-formula> microstrip feeding line designed on the main board of the smartphone. The results show that, according to −6 dB impedance bandwidth criteria, the designed MIMO antenna radiates at two different frequency ranges within the allocated 5G spectrums, i.e., 3.1–3.78 GHz and 5.43–6.21 GHz with 680 MHz and 780 MHz bandwidths, respectively. It is also observed that the antenna elements are able to provide pattern diversity for both the frequency bands. Furthermore, an isolation of >12 dB is observed between any two given radiating elements. Numerous MIMO critical performance characteristics are assessed, including diversity gain (DG), envelope correlation coefficient (ECC), and channel capacity (CC). A prototype is built, measured, and it is observed that the measured and simulated data correspond well. On the basis of performance characteristics, it can be claimed that the suggested MIMO system may be used in 5G communication networks.


I. INTRODUCTION
Fifth-generation (5G) cellular networks are considered to be a viable alternative for overcoming current communication technology limitations [1]- [4]. The millimeter-wave (mmwave) spectrum (24 GHz,28 GHz, 37-39 GHz, and 60 GHz) The associate editor coordinating the review of this manuscript and approving it for publication was Shah Nawaz Burokur . has been recommended by the Federal Communications Commission (FCC) as the operational spectrum for 5G communication. However, the proposed spectrum has issues and flaws in terms of propagation, which could have an impact on network implementation. As a result, the 5G mid-band has been designated by the International Telecommunication Union (ITU) for broadband cellular communication systems. The sub-6 GHz band, commonly known as the 5G mid-band, can provide more coverage over a larger area while minimizing propagation losses. The 5G mm-wave frequency bands are ideal for dense 5G smallcell networks in metropolitan regions with increased capacity demands, as well as macro-cells that give more coverage. To accommodate a considerable volume of traffic at a high data rate, 5G services require roughly 100 MHz of bandwidth in the sub-6 GHz spectrum and 1 GHz of bandwidth in the mm-wave band. In addition, a multiple-input multiple-output (MIMO) antenna system for 5G communication systems is required to fulfill the requirements of high data rate and increased channel capacity (CC).
Li et al. [5] designed a 10-port dual-band MIMO antenna configuration for 3.5 GHz and 5.5 GHz mobile phone applications. They utilized a T-shaped resonator to realize a single radiating element. An L-shaped microstrip feeding line was utilized for the excitation of the radiating element. The presented results show that the isolation between closely spaced radiating elements is >10 dB. They also presented an 8-port MIMO antenna in [6] for the 3.5 GHz frequency spectrum. In this design, a fork-shaped radiating element was used instead of the T-shaped resonator. Although the isolation between antenna elements is high, it suffers from poor antenna efficiency. In [7], orthogonal mode MIMO antennas were designed for smartphone applications. The MIMO's single antenna element consisted of a pair of curved monopoles and an edge-fed dipole. The results show that the utilization of orthogonal mode configuration tends to achieve high isolation without using an additional decoupling network. Moreover, the efficiencies of the designed MIMO antennas varied in a range of 51.7-84.5% and 49-72.9%, respectively. The same type of configuration was represented in [8]. In these designs, a common grounding branch was designed between the antenna elements to act as a decoupler.
Zhang et al. [9] designed an 8-port MIMO antenna for wideband sub-6 GHz services. The single radiation element was composed of a 50 feed line, an open-circuit stub, a U-slot on the ground plane, and a slot on the side-edge. The authors observed that the designed antenna offered a wideband response from 3.3 GHz to 6 GHz. In [10], an eightelement dual-band monopole slot MIMO antenna design was presented for 5G services. They utilized the same feeding technique as presented in [5] and [6]. For 3.5 GHz 5G smartphone applications, Jiang et al. [11] developed a MIMO antenna system using an L-shaped coupled antenna element and a U-shaped loop antenna. To improve the isolation between the antenna elements, a neutralization line and an inverted I-slot were designed between the antennas. In [12], an L-shaped microstrip line-fed dual-polarized diamond slot MIMO antenna was designed for 5G MIMO applications. The authors demonstrated that the orthogonal arrangement of feed lines led to achieving both polarization and pattern diversity with an isolation of more than 20 dB between radiation elements. They also designed a multiband MIMO antenna configuration in [13] for sub-6 GHz services. In this design, a ring slot was etched onto the ground plane and excited with orthogonally arranged fork-shaped feed lines.
In [14], an 8-port self-isolating MIMO antenna for the 3.5 GHz frequency band was designed. A T-shaped and inverted U-shaped radiating structures connected to the ground plane were used to achieve resonance for the 3.5 GHz frequency band. To minimize antenna dimensions, two vertical stubs were inserted in the main radiating element. This type of configuration was shown to provide ≥20 dB of isolation. The same type of configuration was presented in [15]- [17]. In [18], a uni-planar cellular MIMO loop antenna was designed and presented for sub-6 GHz applications. The antenna elements were arranged in such a way that they provide polarization and pattern diversity in the band of interest. For improved isolation, an arrow-shaped strip was designed between the antenna elements to improve isolation. A similar MIMO antenna design was shown in [19]. A multimode planar inverted-F antenna (PIFA) was used instead of a loop-shaped element. The designed MIMO antenna provides resonance in three different frequency bands, i.e., 2.5-2.7 GHz, 3.43-3.75 GHz, and 5.6-6 GHz. In [20] and [21], a co-planar waveguide (CPW)-fed MIMO antennas were designed for sub-6 GHz 5G communication systems. The authors demonstrated that the use of the CPW technique led to achieving enhanced isolation in the operating bandwidth.
This paper presents the design of an eight-element dualband MIMO antenna for cell phone applications. To make room for other components of the cell phone, the radiating elements are designed on the side edge frame of the cell phone board. It is found that the MIMO antennas perform well in the 3.5 GHz and 5.8 GHz frequency bands and also provide pattern diversity. The isolation between the antenna elements is >12 dB, resulting in a low envelope correlation coefficient (ECC) and high CC.

II. ANTENNA DESIGN
A. SINGLE ELEMENT DESIGN Figure 1 shows the design of a single antenna. From the figure, it can be seen that an H-shaped radiating element was chosen for a single antenna design. It is printed on a 0.8 mm thick FR-4 substrate with a relative permittivity of 4.4. Please note that the antenna element is etched on the side edge frame of the smartphone. A coaxially fed 50 microstrip line is used to excite the radiating element, which is located on the main printed circuit board (PCB). The overall dimensions of the single antenna element are: The construction of the proposed radiating element is divided into two parts. First, the bottom part of the radiating element is designed, whose dimensions are noted to be L 1 and L 3 , as shown in the inset of Figure 2. It is observed from the result of Figure 2 that the bottom narrow strip provides resonance at a 5.8 GHz frequency band (black curve). In the second and last step, a stub and another narrow strip are added   with the step-1 design (see inset of Figure 2). The addition of the stub and second strip provides a dual-band response at 3.5 GHz and 5.8 GHz frequency bands (see Figure 2, red curve). This effect may also be seen in Figure 3, which shows the surface current distribution for a single antenna element. One can observe from the result that for 3.5 GHz, a dense current is distributed on the top strip, stub, and in the center of the bottom strip, while for 5.8 GHz, the current is distributed on the surface of the bottom strip. Figure 4(a) shows the simulated radiation and total efficiency of the proposed single antenna element. It can be seen that the radiation efficiency for 3.5 GHz and 5.8 GHz is ≥80% and >70%, respectively. On the other hand, the total efficiency for both bands is >60%. Moreover, the realized gain of a single antenna element is shown in Figure 4(b). The gain varies in the range of 2.5 dBi to 5 dBi for the 3.5 GHz frequency, while it varies in the range of 3-5.8 dBi for the 5.8 GHz frequency band. The simulated far-field radiation characteristics of the proposed single antenna element are shown in Figure 5. It is observed that the antenna offers a quasi-omnidirectional radiation pattern for both the frequency bands in yz-and xz-plane.
A parametric study was conducted to understand the behavior of a single antenna element. Two main variables, L 1 and L 2 are being investigated. For L 1 = 10 mm, the antenna has a single frequency response around 3.5 GHz (see Figure 6a). This value shifts around 4.75 GHz for L 1 = 12 mm and 14 mm, as shown in Figure 6(a). The antenna provides a dual-band response for values greater than 14 mm, and the desired results are obtained for L 1 = 18 mm, as shown in Figure 6(a). On the other hand, when the value of L 2 changed from 10 mm to 20 mm, the resonant frequencies shift from higher bands to lower bands (see Figure 6b). In case of L 2 , the desired response is noted for a value of 18 mm, as shown VOLUME 10, 2022  in Figure 6(b). Therefore, from the presented results, one can tune the dual-band response at desired frequencies by changing the values of L 1 and L 2 .

B. MIMO DESIGN
After analyzing the performance of a single antenna element, an eight-element MIMO array is constructed and analyzed. Figure 7(a) shows the perspective view of the proposed MIMO antenna system. One can note from the figure that the MIMO antenna consists of two sub-arrays of four radiating elements (see Figure 7b) printed on the side edge frame of the smartphone PCB. This technique provides space to accommodate batteries, liquid crystal devices (LCDs), radio frequency (RF) systems and subsystems, cameras, and sensor modules. The gap between each antenna element is optimized to achieve low mutual coupling. Furthermore, as shown in Figure 7(a), the sub-arrays are placed in two different planes, ensuring pattern diversity configuration. For the excitation of antenna elements, a 50 microstrip feeding line is used, and the signal is provided through a coaxial connector placed on the bottom side of the smartphone PCB (see Figure 7a). The remaining parameters are as follows: L S1 = 150, W S1 = 7, S = 22, and g = 6 (all dimensions in mm).
The simulated reflection coefficients of the proposed MIMO antenna are shown in Figure 8   elements. It can be seen that the isolation between adjacent antenna elements is >12 dB for 3.5 GHz and >10 dB for the 5.8 GHz frequency band. The maximum isolation is observed between antenna-1 and antenna-4, which is >25 dB and >20 dB for the 3.5 GHz and 5.8 GHz frequency bands, respectively. The MIMO antenna's simulated radiation and total efficiency for a single sub-array is shown in Figure 9. Both frequency bands have radiation efficiencies greater than 70% (see Figure 9a), while total efficiency varies between 50% and 78% for the 3.5 GHz frequency band and 42-70% for the 5.8 GHz frequency band, as shown in Figure 9(b).

III. FABRICATION AND MEASUREMENTS
The prototype of the proposed MIMO antenna is shown in Figure 10. A Precision Network Analyzer (PNA) E8363C from Agilent Technologies was used to measure the S-parameters. The measured reflection coefficients and port isolations are shown in Figure 11. As can be observed from Figure 11(a), the antenna elements are able to resonate in the frequency range of 3.1-3.9 GHz and 5.5-6.3 GHz. The measured −6 dB impedance bandwidth is observed to be 800 MHz for both the 3.5 GHz and 5.8 GHz frequency bands (see Figure 11a). Figure 11(b) shows the isolation between the antenna elements. The isolation between adjacent antenna elements is noted to be >12 dB  for both the frequency bands. The maximum isolation is observed between antenna-1 and antenna-4, which is >25 dB and >20 dB for the 3.5 GHz and 5.8 GHz frequency bands, respectively. From Figure 11, some difference between the simulated (see Figure 8) and measured results is observed, which is due to manufacturing tolerances and losses in the measurement setup.
The far-field radiation characteristics of the presented MIMO antenna are assessed using a traditional method. As a reference antenna, a dual-ridge horn antenna with a frequency range of 1-18 GHz is employed, and the designed MIMO antenna is mounted on a turntable that is positioned on the opposite side. RF absorbers are used to cover the anechoic chamber's walls in order to eliminate reflections. Figures 12 and 13 depict the computed and measured radiation properties for antenna-1, antenna-2, and antenna-5 at resonant frequencies of 3.5 GHz and 5.8 GHz. Figures 12(a) and 12(c) show that, for the 3.5 GHz frequency band, antenna-1 and antenna-5 exhibit quasi-omnidirectional radiation characteristics in both the yz-and xz-planes. In case of antenna-2, a butterfly like radiation pattern is observed for both the planes (see Figure 12b). It can also be observed from Figure 12(b) that the main beam for both the planes is directed towards 315 • . Furthermore, from Figures 12(a) and 12(c), it is noted that antenna-1 and antenna-5 provide pattern diversity in both planes, which is a useful characteristic for modern smartphone applications.
In the 5.8 GHz frequency band, antenna-1, antenna-2, and antenna-5 provide quasi-omnidirectional radiation properties in both the yz-and xz-planes (see Figures 13a-c). In addition, just like in the 3.5 GHz frequency band, antenna-1 and antenna-5 provide pattern diversity in both planes, as shown in Figures 13(a) and 13(c). This effect can also be observed from the plot of Figure 14 where three-dimensional (3-D) radiation patterns of antenna-1 and antenna-5 are provided. From the result, one can clearly see that both the antenna elements are able to provide pattern diversity.

IV. DIVERSITY PERFORMANCE
In this section, a detailed description of the diversity performance of the designed MIMO antenna system is presented. The performance is assessed in terms of ECC, diversity gain (DG), and CC.

A. ENVELOPE CORRELATION COEFFICIENT (ECC)
ECC shows how effectively radiating elements are isolated within an array [22], [23]. It can be expressed as [24], [25] (1), as shown at the bottom of the next page, where M a and M b represent far-field characteristics of port a and port b.
It has been described that the value of ECC should be <0.5 for practical applications [26]. From Figure 15(a), it is observed that for the proposed MIMO antenna, the simulated value of ECC for both the frequency bands is less than 0.035, while the measured values are noted to be less than 0.1 and 0.12 for the 3.5 GHz and 5.8 GHz frequency bands, respectively, as shown in Figure 15(b). This result indicates that the interference between the elements is minimal.

B. DIVERSITY GAIN (DG)
Another important feature that needs to be assessed is DG. The DG of the MIMO antennas should be high (≈10 dB) in the operating bandwidth. It is expressed in terms of ECC as [24]: (2) Figure 16 depicts the DG of the proposed MIMO antenna. The simulated value of DG for both frequency bands is >9.8 dB (see Figure 16a), whereas the measured value is >9.4 dB (see Figure 16b).

C. CHANNEL CAPACITY (CC)
The ergodic CC of the proposed MIMO antenna is shown in Figure 17. It is evaluated by averaging 10,000 realizations of Rayleigh fading with a reference signal-to-noise ratio (SNR) of 20 dB [27]. Mathematically, the CC of any MIMO system can be calculated using the expression mentioned in [23]. For our proposed MIMO antenna, the value of CC lies in the range of 36-41 bps/Hz for both the frequency bands (see Figure 17), which is very close to the ideal value [28]. A comparison among proposed and previously presented MIMO antennas is presented in Table 1. From the data in the table, it is observed that the proposed MIMO antenna offers high efficiency compared to the designs presented in [5]- [11], [14], [17], and [19]. In addition, the presented MIMO antenna provides low ECC compared to the designs of [5], [8], [9], [11], and [17], which tend to achieve high CC in the bands of interest.

V. CONCLUSION
This work presents the design of an eight-element dualband MIMO antenna system for sub-6 GHz 5G technology. The MIMO's single antenna element is composed of an H-shaped radiator designed on the side edge frame of the smartphone, while each side edge has four elements. It is observed that the antenna elements resonate around the 3.5 GHz and 5.8 GHz frequency bands. The bandwidth of the radiating elements is noted to be 680 MHz ( He is also a Lecturer of the electromagnetic fields and electromagnetic laboratory with the Department of Signal Theory and Communications. His research interests include electromagnetic systems, antennas and wave-propagations, metamaterials and metasurfaces, synthetic aperture radars (SAR), multipleinput multiple output (MIMO) systems, RFID tag antennas, substrate integrated waveguides (SIWs), impedance matching circuits, microwave components, millimeter-waves and terahertz integrated circuits, gap waveguide technology, beamforming matrix, and reconfigurable intelligent surfaces (RIS). The above research lines have produced more than 200 publications on international journals, presentations within international conferences, and book chapters with a total number of the citations more than