A Miniaturized MIMO Antenna With Dual-Band for 5G Smartphone Application

In this paper, a dual-band multiple-input and multiple-output (MIMO) antenna with ceramic substrate is designed for the miniaturized application in 5G smartphone. The proposed method of miniaturization is different from the traditional ceramic loading technique. It is a new method which not only associated with the path extending technique but also combined with the parasitic sub method so that a 68% size miniaturization is achieved. The antenna prototype is fabricated to demonstrate the proposed method. The simulated and measured results indicate that the MIMO antenna has an excellent performance in the frequency band of 3.4 – 3.93 GHz & 4.5 – 5.3 GHz (reflection coefficients < −6 dB) where its isolations are over 10 dB, envelope correlation coefficients (ECCs) are lower than 0.23 and efficiencies are higher than 50%. At last, the on-body effects of the proposed design are analyzed to guarantee the robustness in practical application.


I. INTRODUCTION
I N ORDER to improve the channel capacity and spectrum efficiency, MIMO (multiple-input and multiple-output) technology has become more and more important [1]. Considering the limited space in the mobile terminal, how to integrate more antennas is a challenge of the antenna design for 5G smartphone, and therefore, it is particularly important to reduce the size of MIMO antenna element.
In this paper, a miniaturized antenna element with dualband performance is proposed for the construction of an 8-MIMO antenna for 5G smartphone application. The proposed antenna element is evolved from an inverted-F antenna (IFA) whose fundamental IFA mode is miniaturized based on the methods of ceramic loading and path folding. Then, the second resonant mode is introduced by further extending the path of IFA to construct a loop path and miniaturized by adding a parasitic stub at another extremity of the IFA. The proposed method of miniaturization is different from the traditional ceramic loading technique. It is a new method which not only associated with the path extending technique but also combined with the parasitic sub method, and therefore, results in a high-level miniaturized antenna with dual-band performance. At last, eight elements which are designed with compact size and dual-band performance have been arranged on the bezels of the smartphone to constitute an 8-MIMO antenna. The proposed MIMO antenna covers the 5G NR frequency bands: 3.4 -3.93 GHz and 4.5 -5.3 GHz with isolations over 10 dB, ECCs lower than 0.23 and total efficiencies higher than 50%. Finally, the MIMO antenna prototype has been fabricated and its performance has been measured. The measured results show that the proposed MIMO antenna is a promising candidate for 5G smartphone applications.

II. ANTENNA DESIGN AND ANALYSIS
The geometry of the proposed MIMO antenna and the detailed dimensions of the miniaturized element are shown in Fig. 1. The model of the 5G smartphone is constructed with the copper-covered FR4 substrate whose thickness is 0.8 mm, relative permittivity is 4.4, and loss tangent is 0.02. The dimension of the model is 150 × 75 × 7 mm 3 . The element is fabricated with the silver-covered substrate of ceramics (JJD12) whose thickness is 2.5 mm, relative permittivity is 12, and loss tangent is 0.02. The dimension of the element is 7 × 6.2 mm 2 (0.08·λ 0 × 0.07·λ 0 , where λ 0 is corresponding to 3.4 GHz). There are 8 such elements for the construction of the MIMO antenna and each 4 elements are symmetrically arranged on the two bezels of the model, respectively.

A. DESIGN STRATEGY OF ANTENNA ELEMENT
The design strategy of miniaturization and dual-band is analyzed in this paragraph. The proposed antenna element is evolved from an IFA as in Fig. 2. Firstly, the Ant. I which is an IFA shown as in Fig. 2 (a) resonates in its fundamental IFA mode at 5.6 GHz as shown with the black line in Fig. 3. Then, the ceramics is loaded as a substrate of the IFA and the length of the IFA is remained unchanged as shown with Ant. II in Fig. 2 (b). At the same time, the resonant frequency moves to 4.4 GHz as shown with the red line in Fig. 3. After that, the path of the IFA is folded and its length is increased from L1 to L2 as shown with Ant. III in Fig. 2 (c). As a result, the resonant frequency shifts to 4.2 GHz as shown with the green line in Fig. 3. To further improve the miniaturized level, the path of the IFA is extended from L2 to L3 as shown with Ant. IV in Fig. 2 (d), so that the resonant frequency of the IFA mode drops to 3.8 GHz and, meanwhile, there is a new resonant mode introduced at the frequency of 5.7 GHz as shown with the blue line in Fig. 3. The freshly introduced resonant mode is considered as a loop mode which comes from the circular path of the IFA. To prove this inference, a parasitic stub with the length of L4 is extended from another extremity of the IFA and the path length of IFA remains unchanged as shown with Ant. V in Fig. 2 (e). It is clearly that, the resonant frequency of IFA mode almost stays constant at 3.8 GHz while the frequency of introduced loop mode moves from 5.7 to 4.7 GHz due to the increased path of loop as shown with the orange line in Fig. 3. Finally, the miniaturized antenna element with dual band performance is achieved and the electric current distribution of these two resonant modes is consistent with the analysis as shown in Fig. 4 (a).

B. MIMO ARRANGEMENT
The dimensions of the proposed antenna element are optimized and the final parameters are shown in Fig. 1. The proposed antenna-element can cover the frequency bands of 3.4 -3.93 GHz and 4.5 -5.3 GHz with reflection coefficient lower than 6 dB (3:1 voltage standing-wave ratio) as shown in Fig. 4 (b). In order to support the high transmission rate in 5G mobile communication, an 8-MIMO antenna composed of eight such antenna-elements is arranged symmetrically as shown in Fig. 1.

III. RESULTS AND DISCUSSION
The prototype of the proposed MIMO antenna is fabricated to demonstrate the dual-band miniaturization as shown in Fig. 5. There are eight SMA connectors soldered underground for the convenience of measurement. Owing to the centrosymmetric structure of the MIMO antenna, the simulated and measured results of partial antenna elements are provided for brevity.

A. REFLECTION AND TRANSMISSION COEFFICIENTS
The simulated and measured reflection coefficients of element 1 & 2 show a good agreement in Fig. 6 (a).   because they are adjacent to each other and the coupling between them is stronger than other pairs. The simulation and measurement agree well and the isolations are higher than 10 dB across the two frequency bands.

B. FAR-FIELD PERFORMANCES
Firstly, the simulated and measured total efficiencies of element 1 & 2 are more than 50% across the two frequency bands as shown in Fig. 7 (a). Then the envelope correlation coefficients (ECCs) of adjacent elements such as elements 1 & 2, 2 & 3, 1 & 8, and 2 & 7 have been calculated with the radiated far-field data from simulation and measurement as shown in Fig. 7 (b). The magnitude of ECCs remains lower than 0.23 in both two frequency bands. The 2-D radiation patterns of element 1 & 2 in both x-o-z and y-o-z planes which have been measured and simulated at 3.6 GHz and 5 GHz are shown in Figs. 8 and 9, respectively.

C. USER'S HANDS EFFECTS
Finally, the smartphone model integrated with the proposed 8-MIMO antenna is analyzed under the scenarios of a single-handhold and a double-handhold as shown in Figs. 10 and 11.
In Fig. 10, the MIMO antenna system is simulated with a single-hand model, it is found that the performance of element 8 has a significant deterioration due to the direct touching with the thumb, while the performance of other elements still remains stable as in Figs. 10 (b), (c), and (d).
In Fig. 11, the MIMO antenna system is simulated with a double-hand model. The total efficiencies of antennas 5 and 8 drop to 40% due to the influence of double-handhold. Additionally, the frequencies of antennas 3, 4, 5 and 6 move slightly to the high frequency.
The result of this analysis prove that the proposed MIMO antenna has a promising stability in the practical applications.

D. COMPARISON
The proposed MIMO antenna is compared with the stateof-the-art MIMO antennas which has been reported for 5G smartphone applications in Table 1. It is obviously that the proposed MIMO antenna with the smallest electrical size can cover two wide frequency band with decent isolation and radiation performance. It can be found that, the ECCs of the proposed antenna is higher than that of other reported designs. This is because that the ECCs in this paper is calculated with the radiation patterns and the radiation patterns of the IFA and loop modes are nearly omnidirectional as shown in Figs. 8 and 9. Additionally, the ECCs is required to below 0.3 in the international standard in mobile communication, and therefore, the ECCs of the proposed MIMO antenna can satisfy the requirement in practical application.

IV. CONCLUSION
An 8-MIMO antenna composed of eight miniaturized antenna elements with dual band performance has been investigated in this paper. The MIMO antenna covers two frequency band of 3.4 -3.93 GHz and 4.5 -5.3 GHz where its element-isolations are higher than 10 dB, total efficiencies are better than 50% and ECCs are lower than 0.23. What's more, the performance remains decent under the scenarios of a single-handhold in the practical applications. Consequently, the proposed method of dual-band miniaturization for MIMO antenna design in this paper is a promising technology for 5G smartphone application.