Designing Eight-port Antenna Array for Multi-Band MIMO Applications in 5G Smartphones

 This article introduces a dual-functional low-profile compact multiple input multiple output (MIMO) antenna array for multi-band communication in 5G smartphones. The proposed eight elements of the antenna array form two different 4×4 MIMO systems. The first four elements are placed at the four mobile corners and operate in a single band of 3.445–3.885 GHz for 5G n77 and n78 applications. The other system, in which four antennas are positioned in the middle of the terminal board, supports two wide bands of 1.684–2.300 GHz and 4.432–5.638 for n2, n3, n39, n65, n66, n79, and WLAN applications. The second iteration of a modified Peano-type fractal geometry served as the design foundation for the proposed antenna element. The system’s ground plane design is based on self-isolated and spatial diversity methods. The single-band MIMO system’s isolation is better than 23 dB. In the dual-band MIMO system that is based on self-isolation, isolation equals approximately 16 dB in the higher band and 10 dB in the lower band. To evaluate performance, radiation-related and total antenna efficiencies, scattering parameters and gains were investigated. Additionally, ECCs have been considered to evaluate MIMO performance. According to the results, such design constitutes a valuable option for MIMO applications in 5G smartphones.


Introduction
The constantly increasing data transmission rates and the advent of smart services motivate researchers to develop wireless communication techniques.This leads to the need of an antenna system that can simultaneously operate with different frequencies, providing high data rates, good quality of service, and very low latency [1], [2].The 5G technology and the Internet of Things (IoT) are recognized as the key trends capable of transforming education, industry, healthcare, and other public areas in the future [3].The MIMO antenna technology is utilized for 5G mobile terminals to enhance spectrum efficiency and channel capacity [4], [5].However, an increase in the number of antennas creates integration issues due to the constrained space available in mobile devices [6], [7].Many techniques have been used to deal with this problem, including the multimode decoupling method [8] and the hybrid decoupling method (neutralization line and ground slot) [9].Nevertheless, antenna efficiency was significantly reduced because of additional isolation-related components.The self-isolated approach has been revealed to provide high isolation and antenna efficiency [10], [11].However, such antenna systems operate in a single band only.As a result, developing a small, multi-band MIMO antenna with good performance and a high isolation rate has become a significant challenge.A compact dual-functional antenna array is presented in this study for use in multi-band MIMO applications in mobile devices supporting 5G.Adequate isolation is accomplished without inserting any decoupling structures.Besides, the proposed MIMO antenna array is capable of achieving a good antenna efficiency and MIMO performance.CST Microwave Studio software has been relied upon to analyze the proposed design.

Proposed Antenna Array
The design of an eight-element dual MIMO antenna array for 5G handset applications is shown in Fig. 1.The antenna array comprises two different 4×4 MIMO systems.The four antennas (ant.[1][2][3][4] in the corners of the main PCB are relied upon to build a MIMO system which can operate in a single 3.445-3.885GHz band, as part of the 5G n77 and n78 NR (new radio) bands.The other four MIMO elements (ant.[5][6][7][8] are placed in the middle of the PCB and support wide bands of 1.684-2.300GHz for n2, n3, n39, n65, n66 4.432-5.638GHz for n79, and 5.15-5.63GHz WLAN.All eight antenna elements are located on the top layer made of a 0.8 mm thick double-sided FR4 substrate with a relative permittivity of 4.3 and a loss tangent of 0.02, as shown in Fig. 1a.The thickness of the conductive layer is 35 µm.The system's circuit board has the size of 150×75 mm -a standard dimension for mobile phones.The ground bottom layer enhances the antenna's impedance matching and bandwidth, as shown in Fig. 1b.However, in the proposed design, the ground layer is slightly modified to achieve two different 4×4 MIMO systems.Figure 2 presents the design of a single compact monopole antenna element with its dimensions being   based on the second iteration of a modified Peano-type fractal geometry [12].The process of generating a modified Peano pre-fractal curve is shown in Fig. 3.
The proposed fractal geometry is combined with a 50 Ω transmission line.RF signal is fed from a coax connector to the hole in the ground layer.

Performance Analysis
Figure 4 presents the results of S-parameter simulations for the proposed single-band, four-element MIMO antenna.One may notice that the desired single operating band of 3.445-3.885GHz for 5G n77 and n78 bands applications is achieved with an acceptable impedance matching of less than -6 dB, meaning that a 3:1 voltage standing wave ratio (VSWR) has been achieved, as shown in Fig. 4a.As illustrated in Fig. 4b, decent isolation better than 23 dB is achieved by applying the self-isolated and spatial diversity methodologies, where the four MIMO antennas are placed at the corners of the PCB.The transmission coefficients are better than: S 12 -23 dB, S 13 -30 dB, S 14 -37 dB (diagonal distance), S 15 -26 dB, and S 16 -35 dB.
Figure 5 shows the S-parameters of the dual band MIMO antenna design proposed.As shown in Fig. 5a, the operating range of 1.684-2.300GHz for n2, n3, n39, n65, and n66   bands, and 4.432-5.638GHz for n79 and 5.15-5.63GHz for WLAN bands, works with an impedance matching score of less than -6 dB (3:1 VSWR).From Fig. 5b, suitable isolation of better than 16 dB in the higher band and acceptable isolation better than 10 dB in the lower band is attained using the self-isolated method, i.e., without inserting any decoupling parameters or/and deploying any isolation techniques, where the four MIMO antennas are located closely to each other, in the middle of the mobile phone's PCB.Only the essential transmission coefficients of the lower band are introduced, which are all better than the accepted level of -10 dB.These are as follows: the S 51 ≈ -14 dB, S 53 ≈ -19 dB, S 56 ≈ -10 dB, S 57 ≈ -10 dB, and S 58 ≈ -13 dB.In contrast, the transmission coefficients of the higher band are as follows: S 51 ≈ -20 dB, S 53 ≈ -29 dB, S 56 ≈ -26 dB, S 57 ≈ -16 dB, and S 58 ≈ -28 dB. Figure 6 illustrates the efficiencies of the single-band four MIMO antennas setup.As illustrated, radiation antenna efficiency and total antenna efficiency levels that are acceptable for mobile phone applications are obtained.The radiation and total antenna efficiencies are between 67-82% and 50-76%, respectively.As presented in Fig. 7, good antenna gains are obtained, where the maximum gain in the working band is almost 2.6 dB.The efficiencies of the four antennas of the dual-band MIMO antenna system are shown in Fig. 8. Decent radiation and total antenna efficiencies are attained, remaining between 81-90% and 47-60%, for the lower band.For the higher band, the scores of between 79-87% and 56-84% are obtained.High antenna gains can be also observed in Fig. 9, with the maximum gain that can be reached in the lower operating band equaling approximately 3.5 dB, and approximately 4.3 dB in the higher operating band.For clarity and due to the similarity of results, one-sided antennas (ant. 1, 3, 5, and ant.6) are considered.Figure 10 presents two-dimensional radiation patterns at the working bands, while Fig. 11 depicts, for comparison, three-dimensional radiation patterns.As shown in the pictures, each antenna element has a maximum gain direction that differs from the maximum gain orientations of the other antennas, suggesting decent pattern diversity characteristics.Furthermore, each antenna's radiation pattern covers both sides of the mobile device PCB board.As a result, the proposed MIMO antenna systems achieve the expected level of radiation coverage.

Performance of MIMO Array
The envelope correlation coefficient (ECC) is a recognized metric used to assess functionality of a MIMO antenna sys-  tem [13], [14], showing the correlation or isolation of the multipath communication channels [15].The ECCs can be   calculated based on far-field radiations and/or S-parameters data of the antenna system.Here, Eq. ( 1) is used to calculate the ECCs based on far-field radiations [16].
where − → F 1 (θ, φ) denotes the three-dimensional far-field radiation pattern and Ω indicates the solid angle.An ECCs calculation based on the far field radiations of the single-band 4-element MIMO antenna system is sketched in Fig. 12. Val- ues lower than 0.007 in the desired band are obtained, which is considerably below the accepted criterion according to which ECCs should be less than 0.5 in the MIMO systems.
As far as a dual-band MIMO antenna system is concerned, as shown in Fig. 13, all the ECCs are lower than 0.317 within the lower working band, while the ECCs of the higher band are below 0.009.Thus, the proposed four-element MIMO antenna offers outstanding diversity performance due to its low ECC values.One of the effective performance criteria for MIMO systems is the mean effective gain (MEG), defined as the ratio between the mean received power of the antenna and the total mean incident power [17].MEG can be calculated using Taga's [18] formula: The cross-polarization power ratio (XPR) is the mean incident power ratio of vertically and horizontally polarized waves.P θ and P ϕ represent the theta and phi components of the normalized angular power density functions of the incoming plane waves, respectively.In contrast, G θ and G ϕ represent the antenna gain components.It is worth noting that the MEGs of the MIMO antennas should be close to equal to ensure good MIMO system diversity performance and system power balancing [19].Figure 14 illustrates the MEG of the proposed single-band and dual-band MIMO antenna systems.The MEGs for all antennas of the proposed single-band and dual-band antenna systems are stable within the operational bands.Furthermore, in the MIMO system, the MEGs approximately satisfy the equality criteria applicable to antenna elements.The diversity gain of the proposed single-band and dual-band MIMO antenna systems can be estimated with the use of a formula relative to the correlation coefficient G DG [20], [21], such as: High-diversity gain performance of the single-band and dualband MIMO antenna system is shown in Fig. 15.It is better than 9.96 dB in the single-band antenna system and better than 9.82 dB in the dual-band antenna system.

Comparison with Other Works
Table 1 compares the performance of the proposed singleband and dual-band MIMO antenna systems with that of other recent MIMO antenna systems developed for 5G mobile terminals.As shown, the proposed solution offers a compact size antenna element, expected isolation, and adequate total antenna efficiencies thanks to employing spatial diversity and self-isolated methodology for the single-band antenna system and self-isolated technique for the dual-band antenna system.Therefore, no antenna efficiency losses are caused by other isolation elements and/or decoupling techniques.

Conclusions
This article presents and discusses an eight-port dualfunctional MIMO antenna array for 5G mid-band MIMO applications for mobile devices.The antenna miniaturization with multi-band capability is achieved using the modified Peano-type fractal geometry space-filling feature and its selfsimilarity property.The design contains two four-element MIMO antennas working in different bands to serve various 5G mobile phone applications.By adopting the spatial and self-isolated techniques, suitable isolation is attained.Therefore, it is not necessary to use matching circuits, decoupling components, or re-optimizing the antenna's construction, which would increase system complexity and degrade its efficiency.An antenna element may function simultaneously as a radiated and isolated component, as it offers suitable self-isolated features.The modified Peano-type fractal geometry exhibits the ability to realize the self-isolated attribute of the antenna's components.It is evident from simulation results that the two proposed MIMO antenna systems are an attractive option when it comes to using them in mobile communication equipment.

Fig. 1 .
Fig. 1.Proposed MIMO antenna array: a) top view and b) back view.

Fig. 3 .
Fig. 3.The process of generating a modified Peano pre-fractal curve.
Tab. 1.Comparison of proposed antenna features with other recent works.