Low Mutual Coupling Design for 5G MIMO Antennas Using Multi-Feed Technology and Its Application on Metal-Rimmed Mobile Phones

In this paper, an effective method for developing a multi-feed multiple-input multiple-output (MIMO) antenna system using the metal rims of a mobile phone without any structural interruption on the long edge frames is proposed. Instead of traditionally designing MIMO antennas with multiple similar antenna structures on the mobile phone frame or its ground plane, the MIMO antennas in this work are realized by reasonably constructing feeding points and grounding points on the entire edge frame. The MIMO antenna system established by such multi-feed technique is capable to achieve much simpler structural configuration, excellent isolation and matching performance while multiple antennas are connected. A planar eight-port MIMO antenna system operating in the 5G frequency band of 3.4~3.6 GHz is investigated initially, and subsequently applied to the curved metal-rims which comply with the industry design of a modern flagship mobile phone. The prototypes are fabricated and measured, evaluation results show that isolations among all ports are better than 15 dB with reflection coefficients below -10 dB within the band of 3.4~3.7 GHz and -6 dB within the band of 3.3~3.8 GHz. In addition, more than 57% total efficiency and envelope correlation coefficient (ECC) of less than 0.3 are obtained. The satisfactory performance reveals the proposed design of multi-feed MIMO antennas can be a promising candidate for 5G metal-rimmed mobile phone applications.

Furthermore, it is well-known that the clearance area for mobile phone antennas is getting very stringent, which in turn squeezes the space for MIMO antenna system deployment. When multiple antennas of the MIMO system are placed closely to each other in smartphones, the co-existence or the mutual coupling problem of the MIMO antennas would become very serious [2], [3]. There are many decoupling approaches reported in literatures and the majority of them introduce additional structures like artificial structures and neutralization-lines to achieve isolation improvement [4]- [8].
Using decoupling networks made of lumped elements or coupled resonators are also reported in [9]- [14]. A recent progress on MIMO antenna decoupling by using characteristic mode without adding any extra structures or circuits have been reported [15], [16]. Good isolation is achieved by fixing an antenna while placing other antennas at the stable null-points of the electric-fields excited by the fixed antenna [16]. From another perspective, applying diversity techniques on antenna polarization and pattern can also achieve high isolation and low envelope correlation coefficient (ECC) among multielement antennas, but it has to occupy a sufficient space in the mobile terminals to realize such techniques [17]- [20], which is infeasible for future 5G phones integrated with more and more functional electronic components to realize more fantabulous performances. Recently, spatial reuse method has been widely used in designing at least 8 MIMO antennas in a mobile terminal. Orthogonal radiation modes is obtained by properly designing the antennas within an antenna unit made of two antennas, and has achieved very good results [27]- [32]. However, there are no further measures taken to improve the isolation between the antenna units, in most cases, they are naturally placed and separated. Therefore, regarding practical applications, the above-mentioned decoupling approaches and the MIMO antennas placement strategies are difficult to implement to full-screen mobile terminals surrounded by metal frames/rims as signals would be blocked by the rims. A traditional solution to alleviate such problem is to destroy the structural integrity and cut slots on the metal rims of the phones so as to achieve good electromagnetic (EM) wave radiation and proper antenna operation frequency. Thus, it forms the motivation of this paper to explore an efficient design methodology to realize the MIMO antenna system by directly exciting the metal rims of the mobile phone without adding additional structures, components or breaking the integrity of the metal rims on both long edge of a mobile terminal.
This work proposes an 8-element MIMO antenna system with good isolation and radiation for 5G smartphone applications. Compared with existing decoupling methods, it possesses the following unique features: 1) The structural configuration of the proposed MIMO antenna is simple and easy for practical mobile phone implementation. The 8-element MIMO antenna system is formed by directly exciting the side metal rims based on the multi-feed technology [21]. No additional antenna structures or circuits are required.
2) The radiation patterns of the proposed MIMO antenna element have a wide coverage range. Since there are two groups of orthogonal radiation patterns within the 8-element antennas which are complementary to each other, the proposed antenna system is robust with respect to different usage scenarios and holding positions of the mobile phone.
3) The proposed antenna system is only designed on the two sides of the mobile metal rims without destroying their integrity. It not only well maintains the robustness of the mobile phone, but also is space-saving.
The remaining part of this paper is organized as follows. Section II introduces the antenna evolution process and theoretical analysis of the proposed multi-feed MIMO antenna system realized in a planar style. An 8-port MIMO antenna system designed in practical curved metal-rims of mobile phone associated with experimental results is illustrated in detail in Section III. Section IV draws the conclusions of the whole paper. VOLUME XX, 2017 1

II. DESIGN EVOLUTION OF MIMO ANTENNAS IN PLANAR STRUCTURE
In this section, the design evolution process of the proposed planar MIMO antennas from a dual-feed element (Antenna A) to an eight-feed antenna system (Antenna D) is illustrated in Fig. 1. Antenna A is the basic dual-feed antenna module which acts as the building block for the final multi-feed antenna system. Antenna B is a quad-feed antenna subsystem without any breaking points, formed by two dual-feed antenna modules. An eight-feed MIMO antenna system (Antenna C) can be considered as formed by two Antenna Bs arranged at two sides of the mobile phone. Additionally, four grooves are etched on the ground plane of the final planar MIMO antenna system in order to further reduce the mutual coupling between the metallic radiators on both sides, as shown in the configuration of Antenna D in Fig.1. The design details and antenna performances will be discussed as follows.

A. Design of the Dual-Feed Antenna Module
A detail view of the dual-feed antenna module (Antenna A) is shown in Fig. 2, which can be considered as an evolution from the combination of two types of radiators: a planar inverted-F antenna (PIFA) and a loop antenna [22], [23]. The antenna module is designed on a 90 mm × 50 mm printed circuit board (PCB) with low-cost FR-4 substrate, the substrate thickness, copper layer thickness, dielectric constant and loss tangent of which are 0.8 mm, 0.035 mm, 4.4 and 0.02, respectively. The printed copper line marked with orange color is the main radiator with four branches from the top view, two of which are shorted branches with length and width of 7 mm and 1.5 mm respectively, and the other two are feeding branches with 8.5 mm long and 1.5 mm width. The two antennas share the shorted branch between Port 1 and Port 2. In addition, the common ground plane of the PIFA radiator and the loop radiator is printed on the bottom-side of the substrate, as the grey color portion marked in Fig. 2. The detail dimensions of the dual-feed antenna module are already given in Table I.    In order to achieve good decoupling performance, the spacing between the two feeding ports and the shorted branch in the middle should satisfy equation (1) as it is known that the current distribution on the radiator would reach its minimum after travelling a quarter wavelength In equation (1), the lengths of Lf, d1 and Ls are around 0.1 λ, 0.08 λ and 0.08 λ, respectively (λ is the wavelength of the center operating frequency 3.5 GHz in free-space). In order to further understand the working and decoupling mechanism, the dual-feed antenna module is separated into two independent antennas along the shorted branch between the two feeding ports for better illustration, as shown in Fig.  3. The separated model can be considered as counterpart antennas to the proposed antenna module shown in Fig. 2, the corresponding dimensions of which are exactly the same as the proposed dual-feed antenna. From the numerical results in Fig. 4, the simulated reflection coefficient of the proposed antenna and the counterpart antennas are close to each other, which proves that the proposed dual-feed antenna is basically made up of the two independently-working antennas but merged into one antenna by sharing the common shorted branch and almost no effect is introduced into their resonance thanks to the spacing relationship shown in equation (1). Fig. 5 shows the simulated S-parameter of the proposed dual-feed MIMO antenna module.  The operating frequency band of the PIFA antenna (Port 1 excited) is 3.25~3.8 GHz (15.6% relative bandwidth) and the loop antenna (Port 2 excited) is 3.38~3.64 GHz (7.4% relative bandwidth) with reflection coefficient less than -10 dB. Both of the bandwidths with reflection coefficient less than -6 dB are able to cover the frequency band of interest (3.4~3.6 GHz, 5G NR band N78) for 5G mobile applications. In addition, the isolation between the two antennas is better than 25 dB in the desired frequency band.
To further intuitively demonstrate the electrical performance of the proposed dual-feed antenna module, its current distribution and illustrative far-field radiation patterns are shown in Fig. 6 and Fig. 7 respectively. It can be seen from Fig. 6 that when Port 1 is excited, the antenna is working in PIFA mode as the majority of currents are concentrating in its radiating arm, while Port 2 is activated, it works as a full-wavelength loop antenna mode as there are two stable current nulls existing at the center of the main loop radiator. One also can observe in Fig. 6 that no matter which feed port is under operation, there is almost no current coupled to the other port, which shows the proposed design has great advantage in guaranteeing an excellent isolation between two connected antennas without any additional decoupling structures. Moreover, as depicted in Fig. 7, the far-field performance under the two ports fed separately show the radiation pattern of the loop antenna is perpendicular to the substrate while that of the PIFA antenna mainly radiates outward paralleled to the PCB. Therefore, the two types of radiation patterns are approximately orthogonal with each other, which can effectively reduce the coupling between the antennas and further verify the great isolation characteristic of the dual-feed antenna module.

B. Construction of Unilateral Quad-Feed MIMO Antenna Subsystem
A unilateral quad-feed MIMO antenna subsystem comprised of two dual-feed antenna modules with mirrorsymmetrical placement along the y-axis is presented in Fig.  8. From the side-view of the quad-feed antenna, its main radiator can be considered as a metal rim with the length of L0, the dimension of which is close to that of a commercial 6-inch flagship mobile phone. In order to maintain the integrity of the phone rim without any breaks, a section of metal line with its both end shorted is added between the two dual-feed antenna modules and such short-circuit connection would not affect the overall MIMO antenna's performance. Note that some parameters of the quad-feed MIMO antenna subsystem are slightly different from those of the dual-feed MIMO antenna module for the purpose of obtaining a better impedance matching performance. The detail dimensions of the quad-feed antenna subsystem are listed in Table II. Fig. 9 shows the simulated S-parameters of the quad-feed subsystem, from which it can be seen that the isolation level among the four antennas achieves more than 17 dB within the band of 3.4~3.6 GHz with reflection coefficient less than -10 dB. The good isolation phenomenon can also be viewed in the current distribution when one of the antenna ports is excited, as plotted in Fig. 10, where limited current distribution is found at its adjacent ports. Though compared with the dual-feed antenna module, the decoupling performance would be affected after integrating more antennas on a single metal rim, the isolation of the quad-feed antenna subsystem is still acceptable for MIMO communication of mobile terminals with such multiple antenna system.

C. Decoupling of Planar Eight-Feed MIMO Antenna System
Based on the multi-feed antenna architecture proposed in the previous sections, the final geometry of the planar eightfeed MIMO antenna system is obtained and shown in Fig. 11 [24], the dimension of which is 130 mm × 70 mm. The detail dimensions are given in Table III. As the eight-feed MIMO antenna system is comprised of two quad-feed MIMO antenna subsystems located on both long edge of the mobile terminal, the isolation between unilateral ports can be enhanced by fine tuning several key parameters such as d2, Ls and Lf. However, strong coupling still exists among the feed ports opposite to each other along the two metal rims on both long edge of the terminal, i.e., Port 1 and Port 5, Port 2 and Port 7. In order to reduce such mutual couplings, four choking grooves are etched along the center line of the ground plane with different lengths and widths, which can be separated into two groups, i.e., groove 1 (L1, W1) and groove 2 (L2, W2). Fig. 12 shows the parametric study of the two groups, from which it can be observed that the grooves' lengths of L1 and L2 have varying degrees of impact on the isolation of the opposite feed-ports. Considering isolation level of more than 15 dB as a benchmark, as well as taking their influence on the resonance frequency into account,    In order to futher demonstrate the decoupling capability of the four chocking grooves, the vector current diagram of the eight-feed MIMO antenna system when Port 1 and Port 2 are excited respectively with and without the grooves is depicted in Fig. 13. It can be seen from the current distributions that after adding the choking grooves, the coupling currents concentrate on the grooves instead of flowing to the opposite antennas so that the mutual coupling is greatly reduced. Additionally, as comparison, two counterpart eight element MIMO antenna arrays constructed in a traditional way without any additional decoupling structures are also investigated in this work. Each of the counterpart MIMO antenna array consists of eight identical PIFA antennas with different intervals of array arrangement on the PCB, as shown in  Fig. 14(a) and Fig. 14(b) respectively. (c) and (d) are the simulated and measured isolation of the array shown in Fig. 14(a) respectively. (e) and (f) are the simulated and measured isolation of the array shown in Fig. 14(b) respectively. FIGURE 16. Prototype photograph of the planar eight-feed MIMO antenna system. Fig. 14 (a) and Fig. 14 (b). The simulated and measured Sparameters of the two counterpart MIMO antenna arrays are shown in Fig. 15, from which one could observe that, though the operating bandwidth of the two eight-feed antenna arrays can cover the band of interest in this work, their mutual couplig among the typical adjacent or opposite ports is quite serious, which is only better than 10 dB for the antenna array shown in Fig. 14     performance of the proposed design is much better than that of the traditional ones as shown in Fig. 14. Furthermore, the radiation pattern and efficiency of the proposed antenna are conducted in a near-field anechoic chamber, as shown in Fig.  18. Both of the simulated and measured two-dimensional (2-D) radiation patterns are plotted in Fig. 19 and a threedimensional (3-D) radiation pattern exported from simulation is given in Fig. 20. It can be concluded from these results that: 1) The radiation patterns of the whole eight-feed antenna system can be separated into four identical groups. 2) Within each group, there are two almost orthogonal radiation patterns generated while one of the two ports are excited, respectively. 3) Within each group, the radiation patterns are also complementary to each other, enabling quasiomnidirectional beam coverage capability in a real complex multipath environment. Besides, the experimental results of the total efficiencies against frequencies are presented in Fig. 21. The total efficiencies of the PIFA antenna excited at Port 1 and the loop antenna excited at Port 2 achieve 65% and 55% at 3.5 GHz, respectively.

D. Impact of User's Body
In this section, to analyze the impacts of the human body on the antenna array, the simulated 8 × 8 MIMO antenna array with one hand and two hands modes are analyzed in HFSS.
For the one hand mode, as shown in Fig. 22(a) and (c), due to the appearance of the thumb, it covered a huge area of a Loop antenna's radiation space, which caused the offset of the impedance matching of Ant2 which is acceptable in general. However, the rest of the impedance matching almost unchanged and all of the isolation of ports had different degrees of improved for 1-2dB as illustrated in Fig. 22(e) and (g). Yet, gain and total efficiency had a reasonable loss due to the high absorption loss of the user's hand as seen in Fig.  22(i).
For the two hands held mode, the antenna array is clipped into the palm of the two hands which kept the ten fingers away from radiation area, as shown in Fig. 22(b) and (d), so that it causes an extremely subtle impacts on the impedance matching and isolation of all ports as illustrated in Fig. 22(f) and (h). Yet, gain and total efficiency had a subtle loss because of the two hands' slight absorption in the near field as seen in Fig. 22(j). To analyze the user's hand and head impacts on the proposed MIMO antenna arrays, we simulated this mode that combine with the one hand mode and a human head as seen in Fig. 23(a). As shown in Fig. 23(b)-(d), apparently, the hand had a major influence on the radiation of antenna array in this mode because of its coverage and therefore have similar performance with a hand mode above. Yet, gain and total efficiency had a great loss as illustrated in Fig. 23(e) because the head and hand compare with one hand and two hands modes.  To calculate the SAR of the antenna, we simulated 8 × 8 MIMO antenna array with one hand held near the head which selected from component libraries in CST microwave studio as shown in Fig. 24(a)-(b). We calculated SAR of the antenna in two different standards which is 10gm and 1gm of tissue.

III. EIGHT-FEED MIMO ANTENNA SYSTEM DESIGNED WITH PRACTICAL CURVED METAL-RIMS
In order to reduce the space occupation of the planar eightfeed MIMO antenna system to the mobile phone and make the proposed antenna further adapted to practical phone industry design (ID), the 2-D eight-feed antenna scheme is transformed to 3-D MIMO antenna structure and an eightfeed MIMO antenna system designed on practical curved metal-rims is proposed in this section. The top-view and front-view of the antenna configuration are sketched in Fig.  25 and Fig. 26, the size of which is similar to the commercial phone model iPhone Xs Max. The curved metal-rims are manufactured with the advantage of 3-D printing technology [25]. Fillets with radius, R, are employed at the four corners of the FR-4 substrate according to the actual phone' appearance. After applying the multi-feed technology and the proposed design methodology to realize the eight-feed MIMO antenna system on the practical phone rims, it can leave enough space to the top and bottom portion of the mobile phone to accommodate other functional antennas like 3G/4G antennas and WBG (Wi-Fi Bluetooth and GPS) antennas, which makes the proposed 5G MIMO antenna system very compatible with the existing antennas.    Similar to the planar eight-feed MIMO antenna system in Fig. 11, four choking grooves are also etched on the ground plane to improve the port isolation. It is acceptable since even commercial flagship phones have almost periodic grooves on the main board for heat dissipation and weight reduction. There are four assembly holes designed on the metal-rims to ease the fixation between the rims and the substrate. Furthermore, different from the planar model, the short metal line is replaced by an open-circuit branch with size of (lp, p) for the integrity of the whole metal rim. Meanwhile, rectangular metal-rings with size of (lc, t, w) are added in the metal-rims in order to reduce the actual electrical length of the loop antennas. The detail dimensions of the eight-feed MIMO antenna system with curved metalrims are given in Table IV.
In order to further improve the impedance matching of the complete MIMO antenna system, a π-shaped LC matching network can be inserted to the feed-ports. LC matching network is an efficient impedance tuning technique commonly used by and widely known to professional antenna engineers, which is demonstrated in Fig. 27. Therefore, according to the on-site measured results of antenna reflection coefficient, different values of inductor or capacitor can be easily introduced to improve the ports' impedance matching with the help of VNA and smith chart.

A. Experimental Results
The final prototype of the proposed eight-feed MIMO antenna system realized in the curved metal-rims is shown in Fig. 28. The comparison between the simulated and measured S-parameters is given in Fig. 29, from which it can be observed that a good agreement is achieved and the operating bandwidth can well cover the desired frequency band of 3.4~3.6 GHz with reflection coefficient less than -10 dB or 3.3~3.8 GHz with reflection coefficient less than -6 dB when all port isolations are better than 15 dB.  In order to analyze the influence of the added rectangular metal-rings shown in Fig. 25 on the antenna impedance matching, parametric study on the key parameter, t, is conducted, which is directly relevant to the perimeter of the ring. As seen from the results shown in Fig. 30(a), the resonant frequencies of the loop antenna (Port 2 excited) shift to lower band when t increases, which verifies that the additional metal-rings can effectively adjust the electrical length of the loop antenna. Meanwhile, Fig. 30 Radiation patterns of the eight-feed MIMO antenna system are also tested in the anechoic chamber for complete performance verification, as shown in Fig. 34. Similar to the planar eight-feed MIMO antenna mentioned in the last section, the proposed metal-rimmed MIMO antenna system also possesses approximately orthogonal radiation patterns between the PIFA modes and the loop modes, which can be viewed in the 2-D radiation patterns plotted in Fig. 33 and the 3-D simulated radiation patterns plotted in Fig. 34. Such radiation patterns diversity is beneficial to the isolation enhancement and good for mobile wireless communication with reduced cross-talk.
The measured efficiencies of the eight-feed MIMO antenna system with curved metal-rims are given in Fig. 35. High total efficiencies of 66% and 57% at 3.5 GHz can be achieved when Port 1 and Port 2 are functioned respectively. VOLUME XX, 2017 9    Moreover, ECC is another key indicator to evaluate the data transmission rate for 5G application. After obtaining the field data from the measured radiation patterns, the ECC can be calculated through the following equation: where XPD is the cross-polarization discrimination; ( ) and ( ) are angular distribution of the θ and ϕ component.   In the context of mobile terminals, uniform angular distribution and equal polarization (XPD=1) is always assumed, therefore, equation (2) can be further simplified to be: Where ⃑ ( , ) • ⃑ ( , ) = ( , ) ⋅ * ( , ) + ( , ) ⋅ * ( , ) (4) Among the eight antennas shown in Fig. 36, the results of the calculated ECC from measure radiated fields shown in Fig. 37 reveal that the overall ECCs are lower than 0.3 in the band of interest, which satisfies the required ECCs<0.5 for the MIMO system and validates the correlation level of the proposed MIMO antenna system is very low [26].

B. Performance Comparison with Reported Designs
In order to highlight the advantages of the proposed multifeed MIMO antenna with curved metal-rims, Table V summarizes the performance comparison between this work and the reported designs in literatures [28]- [31]. One can observe from the table that the proposed antenna system has simpler structure of realizing an eight-feed MIMO antenna system by directly exciting the metal frame, which does not occupy extra space of the smartphone and possesses an excellent compatibility with existing antennas. Therefore, the proposed design is a promising antenna candidate for the application to various terminal devices with enhancing the metal rims' integrity and robustness.

IV. CONCLUSION
In this paper, an eight-port metal-rimmed MIMO antenna system using multi-feed technology is proposed for 5G mobile phone applications. The MIMO antenna system is simply constructed on the side metal-rims without breaks, but meanwhile realizes satisfactory isolation levels among the antennas without introducing additional decoupling structures.
The proposed work starts with the investigation of a dual-feed antenna module, the operating principle of which has been analyzed in detail based on the multi-feed technology. Subsequently, a quad-feed and eight-feed MIMO antennas with planar structure are designed and their performance has been confirmed with experimental results. In order to adapt the proposed design to a more practical mobile phone, an eightfeed MIMO antenna system realized in curved metal-rims is finally constructed and investigated. Experimental results show that the isolation levels among the eight feed-ports are more than 15 dB from 3.4GHz to 3.6 GHz with reflection coefficient less than -10 dB and the isolation levels among the eight feed-ports are more than 15 dB from 3.3GHz to 3.8 GHz with reflection coefficient less than -6 dB. The radiation efficiency and ECC reach to at least 57% and less than 0.3 at the center frequency respectively. Such satisfactory performance validates the proposed multi-feed MIMO antenna design is particularly suitable for mobile terminal devices which require the metal-rims to function as MIMO antennas but maintain the structural integrity and robustness.