Dual-Band Eight-Element MIMO Array Using Multi-Slot Decoupling Technique for 5G Terminals

This paper presents a dual-band eight-element multiple-input multiple-output (MIMO) array using a multi-slot decoupling technique for the fifth generation (5G) mobile communication. By employing a compact dual-loop antenna element, the proposed array obtains two broad bandwidths of 12.2% and 15.4% for sub-6GHz operation. To reduce the mutual coupling between antenna elements, a novel dual-band decoupling method is proposed by employing a multi-slot structure. The proposed MIMO array achieves 15.5-dB and 19.0-dB isolations across the two operating bands. Furthermore, three decoupling modes generated by different bent slots can be independently tuned. Zero ground clearance is also realized by the coplanar arrangement of the antenna elements and decoupling structures. The proposed MIMO array was simulated, fabricated, and measured. Experimental results agree well with the simulations, showing that the dual-band MIMO array has good impedance matching, high isolation, and high efficiency. In addition, the envelope correlation coefficient and channel capacity are calculated and analyzed to validate the MIMO performance of the 5G terminal array. Such a dual-band high-isolation eight-element MIMO array with zero ground clearance is a promising candidate for 5G or future mobile applications.

In [11], a platform-free planar inverted-F antenna (PIFA) array with 7% and 8% bandwidths for 5G MIMO applications was reported. This array can be compatible with different platforms by adding vertical patches. Besides, an ultra-wideband four-element MIMO array is studied in [12], which can cover 3300-6000MHz with 10-dB isolation. For multi-element MIMO array, it is challenging to achieve low mutual coupling in the limited area. To solve this problem, various decoupling techniques have been used in 5G terminals, such as the orthogonal mode [13]- [15], balanced slot mode [16], grounded strips [17], pattern diversity [18], [19], neutralization line [20], [21], and inherent decoupling structure [22], [23]. A tightly-arranged array based on orthogonal mode method was developed in [15]. The isolation of the 8×8 MIMO system is better than 17 dB across the operating band. In [16], by combining two decoupling techniques of the balanced slot mode and polarization diversity, an eightelement array realized 17.5-dB isolation in the 3.5-GHz band.
In [17], by inserting a metal ground between the decoupling shorting strips, the resonant frequencies of the decoupling strips can be tuned independently. A pattern-diversity-based decoupling method without any decoupling structure was presented in [18], and more than 15-dB isolation is obtained in the eight-element array. In [20], by introducing the neutralization lines into the middle elements of a dual-band MIMO array, the isolation was improved by 3 dB at the 3.5-GHz band. In [23], a dual-band dual-element MIMO array is studied. By using the proposed inherent decoupling structures, good isolation can be obtained in both two frequency bands. These reported decoupling techniques can realize a reduction of the mutual coupling for terminal antennas. However, it is difficult to improve the isolation for 5G multi (eight or more) elements dual-band MIMO array.
In this paper, a novel multi-slot dual-band decoupling technique is proposed to enhance the isolation of a dual-loop antenna array for 5G MIMO terminals. In this design, a compact dual-loop antenna element is developed for dual-band sub-6 GHz operation. Dual resonances are generated by two different split loops, which produce two wide operating bands of 12.2% and 15.4%, respectively. To obtain low mutual coupling within the limited space of 5G terminals, a new dual-band decoupling technique is proposed by employing a multi-slot structure. Three bent slots produce different decoupling modes with independent tuning characteristic at desirable frequencies to improve the isolation in the lower and higher bands simultaneously. Moreover, thanks to the coplanar design of antenna elements and decoupling structures on the side edges, zero ground clearance is realized in the proposed MIMO array, which is promising to place this design along a narrow region and reserve more space for other electronic components.
To validate the design concept, the proposed dual-band eight-element array is designed, fabricated, and measured. The measured results show that the proposed MIMO array achieves the 15.5-dB and 19-dB isolations across the two operating bands. The antenna efficiencies of 42%-83% in the lower band and 40%-85% in the higher band are obtained. Low envelope correlation coefficients (ECCs) and high channel capacities are achieved within the two bands, which indicate the proposed eight-element array can have a good MIMO performance for the dual-band 5G terminal applications.

A. MIMO ARRAY CONFIGURATION
The configuration of the proposed dual-band eight-element array for 5G MIMO terminal applications is shown in Fig. 1. There are three printed circuit boards (PCBs) in the design, including Sub 1, Sub 2, and Sub 3. All of them are 0.8 mm-thick FR-4 substrates (relative permittivity of 4.65, loss tangent of 0.02). Sub 1 is employed as the system circuit board for a 5.5-inch mobile terminal with a size of 150 mm × 75 mm. The 50-Ohm microstrip feedlines and metal ground plane are printed on the top and bottom layers of Sub 1, respectively. Sub 2 and Sub 3 have the same size of 150 mm × 7 mm, which are perpendicularly placed along the two side edges of Sub 1. On the inner sides of Sub 2 and Sub 3, the antenna elements and decoupling structures are symmetrically arranged along the two long edges of the system circuit board.
The dual-loop antenna elements have a compact size of 14.8 mm × 4.3 mm, which are placed at the same distance of 22.2 mm (0.25λ at 3.4 GHz) between the edges of adjacent antenna elements. As shown in Fig. 1(c), the proposed antenna element is composed of two folded strips and a T-shaped feeding line, which form two split loops. Multi-slot decoupling structures are designed on the same layer as the VOLUME 7, 2019 antenna elements. As shown in Fig. 1(d), three bent slots with an identical width of 0.4 mm are embedded on a grounded patch with a size of 13 mm × 7 mm. Note that there is no need of any ground clearance for this design as it is realized by using a coplanar arrangement of the antenna elements and decoupling structures on the side edges.

B. DUAL-BAND DUAL-LOOP ANTENNA ELEMENT
The proposed antenna consists of two split loops with different lengths, which are formed by two folded strips and a T-shaped coupled feedline. Fig. 2 shows the simulated |S 11 | of the antenna element (Ant 1). The simulation was performed by using ANSYS High-Frequency Structure Simulator (HFSS). The −6-dB impedance bandwidths are 500 MHz (3.34-3.84 GHz) and 850 MHz (4.66-5.51GHz), which covers the sub-6 GHz 5G frequency bands, as well as WLAN and WiMAX. The simulated input impedance for the Ant 1 is shown in Fig. 3. It can be seen that two resonances are generated at 3.6 GHz and 4.9 GHz, and good impedance matching is obtained in both desired bands. This result is in good agreement with the simulated result of |S 11 | shown in Fig. 2.  To investigate the dual-band operation mechanism of the proposed antenna element, the simulated surface current distributions at 3.6 GHz and 4.9 GHz are shown in Fig. 4. The electromagnetic energy is coupled from the T-shaped feeding structure to the open ends of two folded strips. At 3.6 GHz, the surface current is concentrated along the loop path of ABCD of Ant 1. While at 4.9 GHz, strong surface current density is observed on the loop path of AEFG. Total lengths of coupling loop paths ABCD and AEFG are 20.1 mm and 17.6 mm, which are corresponding to the quarter free-space wavelength at 3.6 GHz and 4.9 GHz, respectively. Therefore, the dualloop antenna can generate two independent resonances for the dual-band operation. By employing capacitively coupling, the proposed dual-loop antenna design achieves a compact size and two wide bandwidths, as compared with the traditional loop antennas using half-wavelength modes [24]- [26].

C. DUAL-BAND MULTI-SLOT DECOUPLING STRUCTURE
For MIMO antenna array in sub-6 GHz bands, it is challenging to obtain low mutual coupling within a limited space. In this work, a multi-slot decoupling structure is developed to obtain dual-band high isolation between the antenna elements. Three different bent slots are embedded on a grounded patch, and its occupied area is only 13 mm × 7 mm on the side edges. Thanks to the coplanar design with the antenna elements, the decoupling structures do not occupy any extra ground.
To illustrate the effect of the multi-slot decoupling technique, the simulated S-parameters of the proposed antenna array (Ant 1-Ant 4) with and without the decoupling structures are shown in Fig. 5. The results of Ant 5-Ant 8 are not given for brevity due to the symmetric layout of the eight-element array. As shown in Fig. 5(a), when the antenna array is without the decoupling structures, the port isolation in the lower and higher bands are only 11.5 dB and 12.5 dB, respectively. Owing to the strong mutual coupling between adjacent elements, the antenna impedance bandwidths are deteriorated in the lower band, which can only cover 3.43-3.67GHz.
In order to improve the dual-band isolation of the antenna array, multiple decoupling slots are introduced on a rectangular metal patch, which is vertically connected to the ground plane. By employing three bent slots with different lengths, the presented decoupling structure generates three decoupling modes to reduce the mutual coupling both in the lower and higher bands. As shown in Fig. 5(b), three decoupling modes, Slot-1, Slot-2, and Slot-3 modes are obtained at 3.2 GHz, 3.4 GHz, and 4.8 GHz, respectively. By using the multi-slot decoupling structures, the dual-band isolations are enhanced from 11.5 dB to 16 dB in the lower band and from 12.5 dB to 19 dB in the higher band. Moreover, all the antenna elements can operate over the 3.40-3.84 GHz and 4.72-5.33 GHz frequency ranges for dual-band 5G terminal applications.
To investigate the multi-slot decoupling technique, different decoupling structures involved in the design evolution process are shown in Fig. 6. As shown in Fig. 6(a), when there is only the rectangular metal patch connected to the ground without the decoupling slots, the transmission coefficients of the antenna elements are almost not affected by the grounded patch. Then, the slot 1 is introduced into the lower half of the patch, as shown in Fig. 6(b). This can excite a decoupling mode at 3.2 GHz to improve the isolation between antenna elements from 11.5 dB to 13.5 dB in the lower band. To further enhance isolation in the lower frequency band, Slot 2 is embedded into the upper half of the grounded patch. As shown in Fig. 6(c), an additional decoupling mode appears at 3.4 GHz. After combining the Slot-1 and Slot-2 modes, a good isolation level of 16 dB is achieved in the lower band. To reduce the mutual coupling in the higher band, Slot 3 is inserted between Slot 1 and Slot 2, which produces a decoupling mode at 4.8 GHz. The isolation of 19 dB is obtained in the higher band as plotted in Fig. 6(d). In this way, three decoupling slots can be appropriately arranged to reduce the mutual coupling effect between the antenna elements for dual-band sub-6 GHz operation. The design of the decoupling structure involves the following two steps: (1) introducing Slot 1 and Slot 2 to reduce the effect of mutual coupling in the lower band. (2) introducing Slot 3 to reduce the effect of mutual coupling in the higher band.
To further illustrate the operating mechanism of the dual-band multi-slot decoupling technique, Fig. 7 shows the simulated surface current distributions of Ant 1 and Ant 2 with and without the decoupling structure when Ant 1 is excited. As shown, without the decoupling structures, strong currents are coupled from Ant 1 to Ant 2 at the two resonant frequencies of the dual-loop antenna. This leads to poor isolation between the antenna elements. By introducing the multi-slot decoupling structure between the Ant 1 and Ant 2, the strong current density is observed on the Slot 1 and Slot 2 at 3.6 GHz in the lower band, as shown in Fig. 7(a). Similarly, it also can be seen the surface current distributions are concentrated on the Slot 3 at 4.9 GHz in the higher band, as shown in Fig. 7(b). There is very weak energy coupled from Ant 1 to Ant 2 at the lower and higher center frequencies, which means that the multi-slot decoupling structure can effectively reduce the mutual coupling between the antenna elements. The observed current distributions are consistent with the design concept, which validates the proposed dual-band multi-slot decoupling approach. Fig. 8 shows the simulated |S 12 | of the antenna array for various lengths of the three decoupling slots. In Fig. 8(a), it can be seen that the first decoupling mode can be manipulated by changing the length L 1 of Slot 1, when L 2 and L 3 are fixed. Meanwhile, there is little effect on the Slot-2 and Slot-3 modes. Similar operating characteristics of Slot 2 and Slot 3 are also observed in Fig. 8(b) and (c), respectively. Thereby, it can be concluded that three decoupling modes can be controlled independently by adjusting the lengths of the corresponding slots.

D. ANTENNA EFFICIENCY
As a key figure of merit for 5G MIMO array design, the antenna total efficiency should be considered for a multi-element array. Therefore, the simulated antenna efficiencies of the proposed antenna array with and without the decoupling structures are shown in Fig. 9. As can be seen in Fig. 9(a), in the lower band, the simulated antenna efficiencies are worse than that in Fig. 9(b). This is mainly because the MIMO array has a narrower impedance bandwidth compared with the array using decoupling structures, as shown in Fig. 5. By introducing the proposed multi-slot decoupling structures, the antenna efficiencies are improved from 28% to 40% in the lower band. Meanwhile, 50% antenna efficiencies in the higher band are also obtained. In this array design, the resonant frequency of the decoupling modes is slightly deviated from the resonant frequency of the antenna to avoid the unstable efficiency in the lower band. Fig. 10 shows  the simulated transmission coefficients and antenna efficiencies when the decoupling modes are designed at 3.5 GHz, 3.7 GHz, and 4.8 GHz. It can be seen that in this case, isolation better than 15 dB is obtained in the lower band. However, the antenna elements suffer unstable efficiencies in the lower band, and Ant 3, Ant 4 only have 20% total efficiencies at 3.4 GHz. Therefore, the MIMO array using the presented decoupling structures can realize a good dual-band operating performance with total efficiency of more than 40% over both frequency bands.

A. S-PARAMETERS
To validate the design concept, a dual-band eight-element array using the multi-slot decoupling technique was fabricated and measured. Fig. 11 shows the photographs of the fabricated eight-element array. The eight antenna elements with six decoupling structures are symmetrically arranged along the two side edges. Each antenna element is fed by a 50-Ohm SMA connector, which is mounted below the system circuit board.
The measured S-parameters are plotted in Fig. 12. Because of the symmetric layout of the eight-element array, only the results of Ant 1-Ant 4 are shown. As can be seen, the experimental results agree well with the simulated ones in Fig. 4(b). By using the proposed dual-loop antenna elements, two resonances are excited with a good impedance matching of 440 MHz (3.38-3.82 GHz) and 800 MHz (4.80-5.60 GHz) under the criterion of less than −6-dB reflection coefficient. Furthermore, high isolations between antenna elements are obtained, which are better than 15.5 dB and 19 dB in the lower and higher bands, respectively.

B. RADIATION PERFORMANCES
Figs. 13 and 14 show the simulated and measured normalized radiation patterns in xy-plane at 3.6 GHz and 4.9 GHz, respectively. Due to the symmetrical array layout, only radiation patterns of Ant 1-Ant 4 are given in the figure for brevity. As can be observed, the measured radiation patterns agree reasonably well with the simulated results at the two operating frequencies. The radiation performance measurements were carried out in a near-field antenna measurement system at Xidian University. The measured antenna total efficiencies  of the four representative antenna elements (Ant 1-Ant 4) are presented in Fig. 15. It can be seen that the measured antenna efficiencies are about 42%-83% in the lower band, and 40%-85% in the higher band. The experimental results indicate the proposed eight-element array achieves good radiation performances in the two operating bands, which are suitable for the practical MIMO terminal operation.

C. MIMO PERFORMANCES
To evaluate the potential MIMO performance of the proposed antenna array, the ECCs and channel capacities are calculated and analyzed. Fig. 16 shows the results of ECCs of Ant 1-Ant 2, Ant 2-Ant 3, and Ant 3-Ant 4, which are calculated from the measured radiation patterns [27] and S-parameters. It can be seen in Fig. 16(a) that the ECCs are less than 0.07 and 0.06 in the lower and higher bands respectively, which is in good agreement with the results calculated by the measured S-parameters. Both the results guarantee a good diversity performance of the MIMO antenna array. Besides, the calculated ergodic channel capacities of the fabricated eight-element array are shown in Fig. 17. The channel capacities with a 20-dB SNR vary from 37.3 to 38.3 b/s/Hz in the two bands, which are 3.2 times larger  than the upper limit of a 2×2 MIMO system. Compared with the upper limit of 46 b/s/Hz for an 8×8 MIMO antennas, the proposed array exhibits desirable channel capacities that are only 8 b/s/Hz less than the ideal case. Therefore, based on the calculated ECC and channel capacities, the proposed dual-band eight-element array is capable of providing good MIMO performances for 5G terminal applications.

D. USER'S HAND EFFECTS
To investigate the operating performance of the antenna array in practical scenarios, the effects of the user's hand are studied in the subsection. As shown in Fig. 18, there are two typical 5G usage scenarios, the single-hand operation (SHO) and dual-hand operation (DHO). Figs. 19 and 20 show the simulated reflection coefficients, transmission coefficients, and antenna total efficiencies of the proposed array in the SHO mode and DHO mode, respectively. For the SHO mode, it can be seen in Fig. 19(a) that the reflection coefficients of the proposed array are not affected drastically by the hand, except for some small frequency fluctuations at the edges of the working band. In Fig. 19(b), the isolation between antenna elements becomes better with the hold of the hand. This is because some EM energy has been absorbed by the hand. However, the antenna efficiencies of the proposed array are affected significantly owing to the absorption effect of the user's hand. As shown in Fig. 19(c), the antenna efficiencies of Ant 2, Ant 7, and Ant 8 are declined to 18% because they are very close to the hand. For DHO mode, a similar phenomenon can be observed  in Fig. 20. The reflection coefficients and the isolations have little deterioration. The eight-element array can still cover the two working bands with the two hands holding. Meanwhile, the antenna efficiency decreases significantly due to the absorption effect of the hands. The antenna efficiencies of only 17% are obtained in DHO mode. Table 1 compares the presented design with the recently reported 5G terminal MIMO antennas. To the best knowledge of the authors, most of the reported antennas for sub-6 GHz 5G terminals are single-band operating, and there are few reported works on the use of the dual-band decoupling technique for multi-element terminal systems. The designs reported in [4], [5], [15] and [16] are the single-band antennas operating at the 3.5-GHz band. Compared with these designs, the proposed design shows two wider operating bandwidths with comparable isolation level and total efficiency. In [6], [8] and [20], the reported MIMO antennas are of dual-band operation. Compared with these three designs, the presented antenna array exhibits much higher isolations with comparable total efficiency. The reported MIMO antenna in [23] consists of only two antenna elements for dual-band operation. Compared with this design, the proposed MIMO antenna array has a larger MIMO order and higher channel capacity. It should be noted that most of the reported mobile MIMO antennas require some ground clearance. The presented MIMO array does not need any ground clearance, and this allows it to be placed along a narrow region and reserved more space for other electronic components.

IV. CONCLUSION
A novel method to design a dual-band MIMO array using the multi-slot decoupling technique is presented in this paper. The proposed terminal array is composed of eight compact dual-loop antenna elements, which can generate two quarter-wavelength resonances with 12.2% and 15.4% bandwidths, respectively. To obtain low mutual coupling within a limited space, a new multi-slot structure is introduced between the antenna elements. High isolations of 15.5 dB and 19.0 dB are achieved in the two sub-6 GHz operating bands. To validate the design concept, the MIMO array prototype was fabricated and measured. Good agreement is obtained between the simulated and the measured results. Owing to the dual wideband, high isolation, zero ground clearance, high efficiency, large channel capacity, and low ECC level, the proposed eight-element array can be a desirable candidate for 5G and future MIMO terminal applications. He has authored and coauthored more than 50 journal articles and 60 conference papers. He holds 30 Chinese patents. His current research interests include waveguide slot antennas, microstrip antennas for radar, compact ultrawideband for wireless communications, microwave passive devices and circuits, and microwave/millimeter systems. Dr. Wang is a Senior Member of the Chinese Institute of Electronics, Beijing, China. He was a recipient of many awards, including one Second Class Award for Scientific and Technology of National Progress, two Second Class Awards for Scientific and Technology Progress of National Defense Industry, one First Class and one Third Class Awards for scientific and technology achievements in China Electronic Technology Group Corporation, one First Class Award for Electronic Information Science and Technology of China Electronic Institute, and one First Class and one Second Class Awards for Excellent Academic Papers in Natural Science of Anhui Province.