Eight-Port Orthogonally Dual-Polarized Antenna Array ... - IEEE Xplore

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 64, NO. 9, SEPTEMBER 2016

Eight-Port Orthogonally Dual-Polarized Antenna Array for 5G Smartphone Applications Ming-Yang Li, Yong-Ling Ban, Zi-Qiang Xu, Gang Wu, Member, IEEE, Chow-Yen-Desmond Sim, Senior Member, IEEE, Kai Kang, Member, IEEE, and Zhe-Feng Yu Abstract— A dual-polarized hybrid eight-antenna array operating in the 2.6-GHz band (2550–2650 MHz) for 5G communication multi-input multi-output (MIMO) operation in the smartphone is presented. The proposed hybrid antenna array elements are symmetrically placed along the long edges of the smartphone, and they are composed of two different four-antenna array types (C-shaped coupled-fed and L-shaped monopole slot) that exhibit orthogonal polarization. Therefore, coupling between the two antenna array types can be reduced, and the MIMO system performances are enhanced. A prototype of the proposed eight-antenna array is manufactured and measured. A good impedance matching (10 dB return loss or better), desirable cross-polarization discrimination (better than 15 dB), and an acceptable isolation (better than 12.5 dB) are obtained. Envelope correlation coefficient and channel capacity are also calculated to evaluate the MIMO performances of the proposed antenna array. Index Terms— 5G communication, dual polarization, multi-input multi-output (MIMO), smartphone antenna.

I. I NTRODUCTION

A

LTHOUGH the 4G communication system is presently undergoing intensively studies, the next-generation communication (5G) is now drawing even more interest and attention [1]. Because the universal standard for 5G communication has not been finalized, massive multi-input multi-output (MIMO) system based on multiantennas will become one

Manuscript received April 1, 2016; revised May 26, 2016; accepted June 15, 2016. Date of publication June 22, 2016; date of current version September 1, 2016. This work was supported in part by the National Natural Science Foundation of China under Grant 61471098 and in part by the Mainland-Hong Kong-Macau-Taiwan Science and Technology Cooperation Project under Grant 2015DFT10170. (Corresponding author: Yong-Ling Ban.) M.-Y. Li is with the School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China, and also with the with the School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China (e-mail: [email protected]). Y.-L. Ban and K. Kang are with the School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China (e-mail: [email protected]; [email protected]). Z.-Q. Xu is with the School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China (e-mail: [email protected]). G. Wu is with the National Key Laboratory of Science and Technology on Communication, University of Electronic Science and Technology of China, Chengdu 611731, China (e-mail: [email protected]). C.-Y.-D. Sim is with the Department of Electrical Engineering, Feng Chia University, Taichung 40724, Taiwan (e-mail: [email protected]). Z.-F. Yu is with the China Aerodynamics Research and Development Center, Mianyang 621000, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2016.2583501

of the core technologies for 5G operation [2]. Even though applying the massive MIMO technology into a smartphone can enhance its channel capacity, arranging a large number of antennas into a limited space (provided by a smartphone) will also lead to deteriorated isolations and efficiencies. Therefore, loading multiple antennas into a smartphone for 5G communication is presently a challenging topic for antenna engineers. In recent years, a number of antenna arrays designed for 3G/Long Term Evolution (LTE) MIMO smartphones have been reported [3]–[12]. An eight-antenna array is designed for the LTE 3.5-GHz band (3400–3600 MHz) [8], and at a 20-dB signal-to-noise ratio (SNR), the ergodic channel capacity of approximately 15.5 b/s/Hz can be achieved in a 2 × 8 MIMO system. Even though good results are obtained, the eightantenna elements are distributed along all four edges of the ground, allowing no further spaces available for integrating 2G/3G/4G antennas into the mobile terminal. In [9], a tenantenna array for the 3.6-GHz band (3400–3800 MHz) with good performance is demonstrated. A maximum channel capacity of approximately 47 b/s/Hz with a 20-dB SNR can be obtained for the ten-antenna array applied in a 10 × 10 MIMO system. Diversity techniques (spatial, polarization, and pattern diversity) can provide good solutions to improve the isolation between antenna elements, if more antennas are to be embedded into a smartphone [13]. Furthermore, it has also been verified that using diversity technique can increase channel capacity in some scattering environment. However, due to limited space for smartphone devices, it is difficult to apply spatial or radiation pattern diversity [14]–[17]. In [18], a hybrid dual antenna formed by an Inverted F Antenna (IFA) and an open-slot antenna have shown good performances. However, the system ground in the structure works as the director to the IFA. Thus, if placed on both edges of the smartphone to form a four- or eight-antenna array, isolation between the two IFAs will be undesirable. Although applying polarization diversity technique into smartphones is rarely reported, its superiority in enhancing the isolation and channel capacity makes it a good candidate for MIMO smartphones design. Thus, it is a good practice to utilize dual-polarized antenna arrays into smartphones for 5G MIMO operation, and as far as the authors are concerned, such kinds of antenna designs are not reported anywhere else in the open literature. In this paper, a dual-polarized hybrid eight-antenna array with simple structure operating in the 2.6-GHz band (2550–2650 MHz) for 5G MIMO smartphone applications

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LI et al.: EIGHT-PORT ORTHOGONALLY DUAL-POLARIZED ANTENNA ARRAY

Fig. 1. Geometry of the proposed eight-antenna array for 5G MIMO operation in smartphone, and detailed dimensions of the L-shaped monopole slot and C-shaped coupled-fed antenna.

is proposed. Here, four of the antenna elements are C-shaped coupled-fed type, whereas the other four are L-shaped monopole slots, and the two different antenna array types are symmetrically placed along the two long edges of the system ground plane. Because of this arrangement, the proposed antenna array can generate dual-polarized radiation according to the different radiating mechanism between the coupled-fed antenna and monopole slot antenna. In this case, the four C-shaped coupled-fed antennas will generate linearly polarized (LP) radiation along the long edges of the ground, while the four monopole slots also radiate LP wave but along the narrow edges. Thus, each pair of antenna unit will excite orthogonal polarization that will aid in enhancing the MIMO performances of the proposed eight-antenna array. Besides showing desirable impedance matching and isolation results across the 2.6-GHz band, the envelope correlation coefficient (ECC) between antennas and the ergodic channel capacity of the 8 × 8 MIMO system are also calculated and discussed. II. P ROPOSED E IGHT-A NTENNA A RRAY S TRUCTURE The proposed dual-polarized hybrid eight-antenna array for MIMO operation in the 5G smartphone application is shown in Fig. 1. In Fig. 1, the antenna elements are loaded symmetrically along the two long edges of a 68 mm × 136 mm system ground plane, typically used in a 5.3-in smartphone. The system ground plane in this case is printed on a 1-mm-thick FR4 substrate of relative permittivity 4.4 and loss tangent 0.026. The four L-shaped monopole slot antennas (ant1–ant4) are separately loaded near the four corners, and the four C-shaped coupled-fed antennas (ant5–ant8) are perpendicularly loaded in the middle section of the system ground. The two

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rectangular clearance regions colored in yellow at the top and bottom sides of the system ground plane are reserved for accommodating two LTE/Wireless Wide Area Network antennas for 2G/3G/4G operations. Detail dimensions of the L-shaped monopole slot antenna and C-shaped coupled-fed antenna are also shown in Fig. 1. The monopole slot antenna has a dimension of 4 mm × 17.8 mm, and the total length of the slot is about 1/4 λg at 2.6 GHz. Here, the open end of the monopole slot is bent back to the opposite edge of the ground plane, which will enhance the isolation between the two adjacent units. The monopole slot is coupled fed by an L-shaped feedline that is connected directly to a 50- Sub-Miniature-A (SMA) connector via the system ground. This L-shaped feed-line has a length of 5.8 mm (section A A shown in Fig. 1), and by tuning this length, good impedance matching below 10-dB return loss can be obtained, and it can fully cover the 2.6-GHz band (2550–2650 MHz). The C-shaped radiating element of coupled-fed antenna has a width of 2 mm, and is fabricated using thin FR4 substrate of 1 mm. Its horizontal section (parallel with the ground) is 31.2 mm long, and the two vertical sections extended at both ends (bending ends) are 2 mm long. By further observing Fig. 1, each C-shaped element is coupled fed by a 1 mm × 4 mm feed-line connected directly to a 50- SMA connector. III. D ESIGN P ROCESS The design process of proposed eight-antenna array is discussed in this section. The initial condition is to reserve the top and bottom sections of the ground plane for accommodating two identical 2G/3G/4G antennas. In order to obtain higher channel capacity for the MIMO system, the polarization diversity method is applied in this work, so that all eight-antennas can be allowed into the limited space of a smartphone, and good isolations between antenna elements are also achieved. Notably, conventional design methods to achieve dual polarization are not suitable for smartphones [19]–[22]. A. C-Shaped Coupled-Fed Antenna Array To obtain relatively pure LP radiation, a single straight antenna unit (2 mm × 33 mm) coupled fed by a 50- feedline is initially studied, as shown in Fig. 2(a). A long rectangular notch is removed from the ground plane edge to allow this single antenna to be loaded into. In Fig. 2, a resonance of approximately 2.6 GHz is excited, because this antenna has a length of 33 mm, which is about 1/2 λg at 2.6 GHz. Furthermore, simulated S11 (reflection coefficient) of less than −12 dB was also observed across the desired 2.6-GHz band. Based on the single straight coupled-fed antenna unit design, four identical ones (forming a four-antenna array) are loaded along the two long edges of the main system ground, as shown in Fig. 2(b). Here, the surface current that flows along each straight radiating array element can still exhibit very good LP radiation along the y-axis (here defined as vertical polarization). In Fig. 2, it is apparent that the introduction of the

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Fig. 3. Simulated radiation patterns in the xy-plane at 2600 MHz of the proposed C-shaped coupled-fed antenna array excited separately by different ports. (a) port1. (b) port2. (c) port3. (d) port4.

Fig. 2. Design process of the C-shaped coupled-fed antenna array. (a) Single straight coupled-fed antenna unit. (b) Four straight coupled-fed antenna units. (c) Proposed C-shaped coupled-fed antenna array mounted perpendicularly on the ground plane.

other three antennas does not affect the impedance matching across the 2.6-GHz bands of interest. In addition to that, isolations between any two antennas are better than 18 dB. Even though good performances are obtained in this case, the four notches loaded along two long edges (in the middle section of ground plane) cannot meet the demands for narrow frame characteristic, which is now a trend for smartphone design. Therefore, as shown in Fig. 2(c), the four coupled-fed antenna array elements are mounted perpendicularly on the ground plane, and they still remain along the middle section of the two long edges. Because of the perpendicular arrangement, it has two advantages: no requirement for clearance on the system ground and easy to integrate with the smartphone’s frame. By further observing Fig. 2(c), the shape of coupledfed antenna is now designed as an inverted C-shaped type. As the strength of surface current disperses quickly from the

center of the C-shaped element toward both ends, bending both ends (two vertical sections) toward the system ground will not influence the polarization of the C-shape coupled-fed antenna. Due to its desirable impedance bandwidth and port isolation, the proposed coupled-fed array structure is a good candidate for 5G MIMO antenna type. Simulated radiation patterns of the proposed C-shaped coupled-fed antenna array (four-antenna array) excited separately by different ports are shown in Fig. 3. In Fig. 3, the antennas excited via port1 and port4 [in the array arrangement shown in Fig. 2(c)] mainly radiate toward the +x orientation. In comparison, the antennas excited via port2 and port3 separately show maximum radiation orientated in the −x direction. The simulated cross-polarization discrimination (XPD) of all antenna elements is better than 20 dB. B. L-Shaped Monopole Slot Array As the four-antenna array presented in Section III-A is vertically polarized, introducing another four-antenna array with horizontally polarized radiation can reduce the coupling effects between the two antenna array types. Therefore, a fourantenna L-shaped monopole slot array is further proposed in this work. The concept of monopole slot antenna design is stemmed from a patent reported in [23], and its merits as reported in [24]–[26] have shown that this design is very suitable for mobile handset designs. The design process of this four-antenna L-shaped monopole slot array is shown in Fig. 4. A straight monopole slot (SMS) etched in the top left-hand corner of the ground (along the long edge) is initially investigated, as depicted in Fig. 4(a). It has a length of 17.7 mm, which is near 1/4 λg at 2.6 GHz. Because the electric field on the monopole slot is perpendicular


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Fig. 6. Simulated surface current distributions on the L-shaped slot and electric field distributions inside the slot region for the proposed design.

Fig. 4. Structures of the monopole slot. (a) Single SMS along the long edge. (b) Four single SMS array arrangement. (c) Proposed four L-shaped monopole slot array.

Fig. 7. Simulated radiation patterns in the xy plane at 2600 MHz of the proposed L-shaped monopole slot antenna array excited separately by different ports. (a) port1. (b) port2. (c) port3. (d) port4.

Fig. 5. Simulated S-parameters. (a) Single SMS and four SMS arrays. (b) Four L-shaped monopole slot array.

to the slot, therefore, very good LP radiation in the x-axis direction (here defined as horizontal polarization) is exhibited. Next, a four-antenna array that composed of these four identical SMSs etched into the four corners of the ground, as shown in Fig. 4(b) is also studied, and their simulated S-parameters are also depicted in Fig. 5(a). In Fig. 5, compared with the single SMS type, the resonance of four SMS antenna type is shifted slightly from approximately 2.6 to 2.58 GHz, and better impedance bandwidth is also exhibited. However, the isolation (S21 ) between two adjacent slots is poor at approximately 8 dB. Therefore, the technique of bending the open end of each SMS that results in forming an L-shaped monopole slot is employed, as shown in Fig. 4(c). Because the two adjacent slots (for example, at port1 and port2) have

open ends that are in opposite direction, enhanced isolations of better than 12 dB are exhibited, as shown in Fig. 5(b). The simulated electric field and surface current distribution of a single L-shaped monopole slot at 2.6 GHz are shown in Fig. 6. Strong electric fields are observed at the open end of the monopole slot and gradually decrease to the closed end of the monopole slot. In comparison, its corresponding surface current flows along the edges of monopole slot clearly indicate that the slot can provide a 1/4 λg resonant path at 2.6 GHz. It is noteworthy that bending the open end of the slot will inevitably introduce some XP components; however, these XP effects are insufficient to change the main polarization characteristics. Simulated radiation patterns of the proposed L-shaped monopole slot antenna array excited separately by different ports are shown in Fig. 7. In Fig. 7, the antennas excited via port1 and port2 [in the array arrangement shown in Fig. 4(c)] mainly radiates toward the −y orientation, and the remaining ones via port3 and port4 shows maximum radiation in the +y direction. Here, a good XPD better than 20 dB is realized as well. C. Combination of the Two Orthogonally Polarized Arrays Based on the aforementioned two orthogonally polarized four-antenna array designs, a dual-polarized hybrid

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Fig. 8.


Three referential arrangements of the two arrays.

eight-antenna array antenna is therefore proposed, in which the C-shaped coupled-fed antenna array exhibits vertically polarization, whereas the L-shaped monopole slot antenna array radiates horizontally polarized wave. Furthermore, from the simulated results shown in Figs. 3 and 7, the two array types radiate in different directions. As shown in Fig. 1, the proposed hybrid eight-antenna array is formed by four L-shaped monopole slot antenna array and four C-shaped coupled-fed antenna array mounted along the two long edges of the system ground. Even though the isolation between these two arrays can be enhanced due to orthogonal polarization characteristic, it can also be affected by the surface current distributed on the system ground. Thus, the coupled-fed type should not be very close to the monopole slot type. To further comprehend the effects of altering the locations of these two array types, Fig. 8 shows three referential arrangements of the two arrays (cases I–III), and their corresponding simulated S-parameters are shown in Fig. 9(a)–(c), respectively. In case I, the closed ends of monopole slots are reversed and point away from the coupledfed type. In case II, the positions of the two arrays are exchanged. In case III, the four monopole slots of case II are reversed and point away from the coupled-fed type. As shown in Fig. 9(a), the 10-dB impedance bandwidth of coupled-fed antenna (e.g., S55 ) is unable to fully cover the 2.6-GHz operating band. Furthermore, because the open end of the monopole slot is in close proximity to the coupled-fed type (shown in case I), a large portion of the energy radiated by the monopole slot will be coupled to the coupled-fed array. Thus, isolations between some antenna array elements are poor (e.g., ant1 and ant5, S51 = −11 dB at 2.6 GHz). As for case II, because the open ends of two monopole slots (on the same edge) are too close to each other, and their feeding points are also in close proximity to the feeding point of coupled-fed type, therefore, as shown in Fig. 9(b), poor S41 and S51 of −9 dB and −10 dB, respectively, are demonstrated at 2.6 GHz. Due to the very close proximity of the open end of the monopole slot and the feeding point to the coupled-fed type, as shown in Fig. 9(c) for case III, the values of S51 across

Fig. 9.

Simulated S-parameters. (a) Case I. (b) Case II. (c) Case III.

the entire 2.6-GHz operating bands have exhibited very poor isolation of larger than −8 dB. According to the above analysis of the three referential antennas (cases I–III), the optimized arrangement for the two array types is proposed and shown in Fig. 8 (right bottom corner). The simulated S-parameters of this proposed antenna array is depicted in Fig. 10. In Fig. 10, the proposed hybrid eight-antenna array can cover the entire 2.6-GHz operating band (return loss better than 10 dB), and all isolations between any two antenna elements are better than 12 dB as well. IV. R ESULTS AND D ISCUSSION In this section, the merits of proposed eight-antenna array are analyzed by measured and computed results. First, the measured S-parameters are presented, followed by the discussion of the polarization and radiation efficiency.


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Fig. 10. Simulated S-parameters of the proposed hybrid eight-antenna array.

Fig. 12. Measured S-parameters of the proposed hybrid eight-antenna array. (a) Measured and simulated reflection coefficient. (b) Measured port isolation.

Fig. 11. Photos of the fabricated eight-antenna array. (a) Overall view of the proposed antenna. (b) Enlarged photo of the monopole slot. (c) Enlarged photo of the coupled-fed type.

Finally, the ECC and channel capacity are calculated via the measured results to evaluate the potential MIMO performance of the proposed array [27], [28].

for 5G operation. Fig. 12(b) shows the isolations between antenna elements of the proposed eight-antenna array. Because the array is symmetrically placed and some isolation curves are better than 20 dB within the entire operating band, only six curves are plotted. In Fig. 12, within the bands of interest, the isolations between ant1 and ant4 are better than 12.5 dB, while ant1 and ant5 are better than 13.5 dB. Here, the coupling between ant1 and ant4 mainly derives from nearfield radiation, while the coupling between ant1 and ant5 comes from near-field radiation and current flow on the system ground. As for the other four S-parameter curves (S21 , S31 , S81 , and S85 ), due to orthogonal polarization and optimized arrangement of the two arrays, acceptable isolations of better than 15 dB are obtained.

A. S-Parameters The proposed eight-antenna array was fabricated and its photo pictures (front and back views) are shown in Fig. 11. The simulated results are obtained by High Frequency Structure Simulation version 15.0, and the measured results are performed by an Agilent N5247A vector network analyzer. The simulated and measured reflection coefficients are shown in Fig. 12(a). Even though slight discrepancies are observed due to minor fabrication error, the two results still validate well with each other, because their resonances do not deviate away from 2.6 GHz. From the measured results, it indicates that both the horizontally and vertically polarized antennas have a good 10-dB impedance matching (across 2:1 Voltage Standing Wave Ratio (VSWR)) over the 2.6-GHz band (2550–2650 MHz), which is a potential band

B. Radiation Performances The simulated and measured radiation efficiencies that include the effect of mismatching loss are plotted in Fig. 13. Due to slight fabrication errors and machining accuracy, approximately 10% discrepancies between the two results are observed. As shown in Fig. 13(a), the measured antenna efficiencies of the L-shaped monopole slot antenna array (ant1–ant4) were approximately 48%–55% over the desired band, while the simulated ones were slightly higher at 53%–60%. Fig. 13(b) shows the simulated and measured efficiencies of the C-shaped coupled-fed antenna array (ant5–ant8). Even though the measured efficiencies are 10% lower than the simulated ones, acceptable radiation efficiencies at 55%–63% in the desired band are still obtained.

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Fig. 15. Measured radiation patterns in the xy-plane at 2600 MHz of proposed L-shaped monopole slot antenna array excited separately by different antennas. (a) ant1. (b) ant2. (c) ant3. (d) ant4.

Fig. 13. Measured radiation efficiencies of the proposed hybrid eight-antenna array. (a) L-shaped monopole slot array (ant1–ant4). (b) C-shaped coupled-fed array (ant5–ant8).

orientations in Fig. 15 are either in the +y or −y direction. For the C-shaped coupled-fed antenna array (ant5– ant8) at maximum radiating directions, the copolarization (Ephi) is 13 dB larger than its XP (Etheta) counterpart. Therefore, ant5–ant8 can exhibit a relatively good vertical polarization. In comparison, for the L-shaped monopole slot antenna array (ant1–ant4) at maximum radiating directions, the copolarization (Ephi) is approximately 8–9 dB larger than its XP (Etheta) counterpart. Hence, ant1–ant4 do not possess good horizontal polarization purity. Nevertheless, even though ant1–ant4 with horizontal polarization cannot achieve high XP level, good isolation between the horizontally and vertically polarized antenna array units can still prove the utility of this arrangement. C. MIMO Performance

Fig. 14. Measured radiation patterns in the xy plane at 2600 MHz of the proposed C-shaped coupled-fed antenna array excited separately by different antennas. (a) ant5. (b) ant6. (c) ant7. (d) ant8.

The measured radiation patterns in the xy-plane at 2.6 GHz of the proposed C-shaped coupled-fed antenna array (ant5–ant8) and the L-shaped monopole slot antenna array (ant1–ant4) excited separately by different antennas are shown in Figs. 14 and 15, respectively. The results are similar to the simulated ones shown in Figs. 3 and 7. As shown in Fig. 14, maximum radiating orientations are either in the +x or −x direction, whereas the maximum radiating

The ECC, mean effective gain (MEG), and channel capacity are calculated to evaluate the potential MIMO performance of the proposed antenna array. The ECC and MEG are calculated from the measured 3-D electric field patterns of ant1–ant8 of the fabricated antenna array, with the assumption of uniform incident wave environment. Typical ECC results between (ant1 and ant2), (ant1 and ant3), (ant1 and ant4), (ant1 and ant5), (ant1 and ant6), (ant5 and ant6), (ant5 and ant7), and (ant5 and ant8) are presented in Fig. 16(a). The results shown in Fig. 16 indicate that the calculated ECC is less than 0.15 over the entire operating band, which satisfies the criterion of acceptable ECC (