- Email: [email protected]
Wi-Fi application
Int. J. Electron. Commun. (AEÜ) 111 (2019) 152893
Contents lists available at ScienceDirect
International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
Regular paper
Design of a compact quad-radiating element MIMO antenna for LTE/Wi-Fi application Abubeker A. Yussuf ⇑, Selcuk Paker Department of Electronic and Communication Engineering, Istanbul Technical University, Ayazaa-Maslak, Saryer, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 1 April 2019 Accepted 30 August 2019
Keywords: MIMO antenna Cross-shaped stub Ring-shaped stripes Decoupling Channel capacity loss
a b s t r a c t In this paper, a compact quad-element multiple-input-multiple-output (MIMO) antenna for LTE/Wi-Fi application is proposed. The four dual-elliptically tapered antenna elements are an orthogonal orientation to one another. The MIMO antenna comprises a cross-shaped stub between the quad-radiating elements and ring-shaped stripes between the partially ground planes that act as a decoupling element to suppress mutual coupling. The dimension of the designed MIMO antenna is ð66 66 1:6Þ mm3 . A prototype MIMO antenna was fabricated and S-parameters were measured in order to characterize the performance parameters of the MIMO antenna. The designed MIMO antenna operates at 2.3 to 2.7 GHz frequency band with high isolation exceeding 17 dB. Thus, the measured results give an envelope correlation coefficient below 0.04 and a channel capacity loss is lower than 0.6 b/s/Hz, which reasonably agrees with the simulated results. Ó 2019 Elsevier GmbH. All rights reserved.
1. Introduction Rapid development of cutting-edge wireless technology demands significant improvement in the data rate and channel capacity of wireless communication systems in abundant scattering environments. MIMO antennas are verified to be effective for enhanced channel capacity, improved data rate and diminishing multipath fading in abundant scattering environment [1]. In order to accomplish a higher data rate without adding transmission power, the MIMO system deploys various transmitter and receiver antennas at each end for wireless communication devices. Various methods have been developed to maximize isolation and reduce coupling among the antenna radiating elements. For instance, in a study by Katie et al. on isolation enhancement, a ground plane with flag-shaped stub was reported to decrease mutual coupling between two antenna elements sharing a single ground plane [2]. The slits and slots were also placed beside the flag-shaped stub, which created notch-band out of the preliminary antennas band. In their work, they eventually created an additional coupling route that eliminated the original coupling current between antenna elements. Kiem et al. also showed another approach to diminish the coupling between radiating antennas by using a stub, based on the principle of microstrip multimode resonator (MMR) structure. Fur⇑ Corresponding author. E-mail address: [email protected] (A.A. Yussuf). https://doi.org/10.1016/j.aeue.2019.152893 1434-8411/Ó 2019 Elsevier GmbH. All rights reserved.
ther, they generated a notched band using a mushroom-like electromagnetic band-gap (EBG) structures [3]. Their technique primarily acted as a band-stop filter which minimized the effect of surface current on the antenna elements. Ding et al. also presented another method to enhance the isolation between antenna elements using a parasitic radiator. The antenna element composed of the parasitic strip located between parabolic reflector and mender dipole, which was used not only to enhance impedance matching, but also to improve the gain and eventually widen the operating bandwidth [4]. The parasitic radiator generated an induced coupling current out of phase that mitigated the primary coupling current between antenna elements. Using a magnetic waveguide metamaterial (MTM) based on the Hilbert-shaped between two closely placed patches, significant improvement in isolation was accomplished by Xu et al. in [5]. Their method initially applied as a band-stop filter which reduced the effect of surface current on the antenna elements. A neutralization line technique, which basically provided decoupling current path with different dimensions mainly to eliminate the coupling current on the ground plane and also used to minimize mutual coupling between antenna elements [6,7]. The neutralization line eliminated the induced current with the primary current, since two current flows was out of phase. But, despite the enormous research that have been undertaken on design MIMO antennas, the design of compact, low-profile and multi-standard antennas for wireless communication devices remains a challenging issue.
2
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In this study, a compact four tapered radiator antenna element is proposed for MIMO applications. The dual-elliptically tapered antenna has advantages of enhanced impedance matching across the desired frequency, symmetrical radiation pattern and easily fabricate. The designed MIMO antenna can be adopted in portable devices, which supports Wi-Fi and LTE applications. The decoupling technique is accomplished by adding both cross-shaped stub between the orthogonal symmetric of four-radiating elements and ring-shaped stripes between the partially ground planes. The designed MIMO antenna system has a compact size with low envelope correlation coefficient and high inter-element isolation. The simulated and measured results are used to validate the proposed design methodology. 2. Antenna configuration Fig. 1 exhibits the geometry of the designed dual-elliptically tapered slot antenna. It comprises of the tapered radiating slot, the microstrip feed line and the partial ground plane. The tapered radiating slot is fed by a 50 X microstrip line. The antenna is designed on FR4 substrate with a thickness of 1.6 mm, a relative permittivity of 4.3, and a loss tangent of 0.035. The thickness of the copper layer on the substrate is 0.035 mm. The overall size of the designed MIMO antenna was 30:75 35:25 mm2 . The optimum dimensions of the designed dual-elliptically tapered slot antenna are given in Table 1. The designs and simulation were performed in commercial software CST.
Fig. 2. Return loss for varying length, Rmx1, of the tapered radiating slot.
3. Parametric study on the impedance characteristics Parametric study is undertaken to characterize the impedance matching and to miniaturize the size of dual-elliptically tapered antenna. Fig. 2 exhibits the return loss of the designed antenna with varying length of tapered slot, Rmx1. The smaller length, Rmx1, of tapered slot have little effect on the impedance matching of the designed antenna. Thus, a compact dual-elliptically tapered Fig. 3. Return loss for varying width, w4, of tapered radiating slot.
Fig. 1. Geometry of the designed dual-elliptically tapered antenna.
Table 1 Optimum dimensions of the designed dual-elliptically tapered antenna. Parameters
W
L
Rmx
Rmx1
w
lg
Rmi
Rmi1
Value
35.25
30.75
29.25
19.75
2.85
10
18
15.15
3
A.A. Yussuf, S. Paker / Int. J. Electron. Commun. (AEÜ) 111 (2019) 152893
Fig. 4. Geometry of the designed MIMO antenna for LTE/Wi-Fi application.
Fig. 5. Fabricated of the designed MIMO antenna for LTE/Wi-Fi application.
can be realizable by reducing the length of microstrip line. Fig. 3 shows the impact of varying width, w4, of tapered radiating slot on return loss. The width, w4, of the tapered radiating slots have slight effects on the impedance characteristic. The larger w4 is preferable for better impedance matching at lower frequencies, while the smaller w4 makes the radiating taper too narrow to maintain the impedance characteristics at lower frequency.
4. MIMO Antenna configuration Fig. 4 exhibits the geometry of the designed quad-element MIMO antenna using dual-elliptically tapered slot radiator. The
designed dual-elliptically tapered antenna has advantages of enhanced impedance matching across the desired frequency and symmetrical radiation pattern. The designed MIMO antenna was simulated using FR4 substrate with relative permittivity of 4.4, a thickness of 1.6 mm and loss tangent of 0.035. The overall size of the designed MIMO antenna was 66 66 mm2 . The dimensions of the various geometrical parameters are given in Table 2. Initially, a single radiating antenna was designed with micro-strip fed, which contributed a 50X of characteristic impedance. The dimensions of the radiating antenna element were optimized using a parametric optimization technique to operate LTE (2.3–2.4 GHz, 2.496–2.69 GHz) and Wi-Fi (2.4 GHz) frequency band. The parametric optimization was conducted using computer simulation
Table 2 Dimensions of the designed MIMO Antenna. Parameters
L
W
Rmx1
Rmi1
w
S
l1
w1
R
Value
66
66
19.75
15.15
2.85
5.3
4.75
3
1.5
Parameters
l2
w2
Rmx
Rmi
l3
w3
w4
w5
arc1
Value
14
3
29.25
18
30.75
10
9.5
8
5
4
A.A. Yussuf, S. Paker / Int. J. Electron. Commun. (AEÜ) 111 (2019) 152893
Fig. 6. Designed quad-element MIMO antenna without stub and with the stub.
tary spiral ring resonators (CSRs) and ring-shaped stripes techniques were basically used to improve the frequency response of the reflection coefficient and reduce the mutual coupling. Fig. 6 shows the four dual-elliptically tapered slot MIMO antenna with and without cross-shaped stub between elements. The cross-shaped stub reduces the mutual coupling and enhances impedance matching over frequency band. Fig. 7 depicts simulated S-parameters of the quad dual-elliptically tapered slot MIMO antenna with and without cross-shaped stub between elements.
Fig. 7. Simulated S-parameters of quad-element MIMO antenna without stub and with stub between elements.
tools (CST). Then, four such radiating elements were replicated into orthogonal orientation to each other on the same substrate. The cross-shaped stub and ring-shaped stripes were added to enhance the isolation between the adjacent and the diagonal antennae elements. The manufactured MIMO antenna is depicted in Fig. 5. 5. Analysis of quad-element MIMO antenna In this section, a parametric study is conducted to observe the effect of an isolation techniques in MIMO antennae performance using CST Microwave studio. The cross-shaped stub, complemen-
Fig. 9. Simulated S-parameters of quad-element MIMO antenna with ring-shaped and with CSRs between elements.
Fig. 8. Designed quad-element MIMO antenna with ring-shaped and with CSRs.
A.A. Yussuf, S. Paker / Int. J. Electron. Commun. (AEÜ) 111 (2019) 152893
5
Fig. 10. Designed quad-element MIMO antenna with stub and with stub and ring-shaped stripes.
Fig. 11. Simulated S-parameters of quad-element MIMO antenna with stub and with stub and ring-shaped stripes.
The MIMO antenna with cross-shaped stub can cover a 10 dB reflection loss over the frequency band between 2.3 and 2.8 GHz. The figure also shows that the mutual coupling between diagonally placed elements (S31) and adjacent placed elements (S21) are low (less than 20 dB) throughout the frequency band 2.3 to 2.8 GHz. Additionally, in comparison to the results without cross-shaped stub, the mutual coupling between diagonally placed elements (S31) is considerably improved by around 10 dB at the center frequency of 2.45 GHz. Fig. 8 shows the four dual-elliptically tapered slot MIMO antenna with ring-shaped and complementary spiral ring resonators between elements.The ring-shaped strips can be defined as a matching circuit is designed from capacitances (space between the ring-shaped stripes) and inductances(Width ring-shaped stripes). The ring-shaped strips were provided in order to shift the resonant frequencies downwards. Fig. 9 shows the resulted S-parameters with ring-shaped and complementary spiral ring resonators (CSRs). As a result, the mutual coupling (S21) between the antenna elements with the CSRs is slightly reduced throughout the desired frequency band. Fig. 10 shows the four dual-elliptically tapered slot MIMO antenna with and without ring-shaped stripes between the partially ground planes. Fig. 11 depicts a comparison between Sparameters of the quad dual-elliptically tapered slot MIMO antenna with and without ring-shaped stripes between the partially ground planes. The MIMO antenna with the ring-shaped stripes covers the frequency band from 2.2 to 2.7 GHz having a bandwidth of 500 MHz for S11 < 10 dB and the mutual coupling
between diagonally placed elements (S31) and adjacent placed element pair (S21) are less than 20 dB throughout the frequency band 2.2 to 2.7 GHz. As a result, the isolation between the antenna elements is slightly enhanced throughout the desired frequency band. The methods used to analyze mutual coupling effects of the proposed MIMO antenna are also demonstrated by surface current distribution. The current distribution of a compact quad-element MIMO antennas without stub, with cross-shaped stub and ringshaped stripes at frequency 2.45 GHz are presented in Fig. 12. The cross-shaped stub and ring-shaped stripes generated an induced surface current to suppress the initial coupling between the radiating antenna elements. Thus, the cross-shaped stub and the ring-shaped stripes played an important role in reducing mutual coupling. The current distribution of a compact quadelement MIMO antennas with ring-shaped stripes and complementary spiral ring resonators (CSRs) at frequency 2.45 GHz are presented in Fig. 13. In Fig. 13b, the CSRs in MIMO antenna are used to obstruct radiated energy and lessen transmitted energy between adjacent elements due to the induced current that flow within the metallic stub and CSRs, generating a balanced inductive-capacitive effect. Therefore, the mutual coupling (S21) between the adjacent elements is slightly enhanced.
6. Results and discussion The designed antenna was fabricated (Fig. 5) and then Sparameter was measured using a network analyzer (Rohde and Schwarz, FSH). During the measurement process, the two ports of antenna are excited, the others are terminated with the 50X load. Fig. 14 is given to display the simulated and measured reaction coefficients of the MIMO antenna designed in this study. It can be observed from the shape of the plot in Fig. 14 that the simulated reflection coefficient results are in considerable agreement to the experimental results. One can see from the Fig. 14 that the designed quad-element MIMO antenna operates in the frequency band range from 2.3 GHz to 2.7 GHz for reflection coefficient of less than 10 dB. Both the simulation and measurement based results of the radiating antenna elements achieved a good resonance at a centre frequency of 2.46 GHz. Preliminary simulation results were performed to select the terminal prototype of the designed antenna based on their performance in providing low mutual coupling and good impedance matching. The simulated and measured results, as presented in Fig. 15, illustrate that the mutual coupling between the adjacent (S21, S14, S43, S32) and diagonal (S31, S24) paired elements were found to be greater than 17 dB and 21 dB, respectively, throughout the desired frequency band. The isolation measurement of the
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Fig. 12. Simulated current distributions at 2.45 GHz when port 1 is excited of the designed MIMO antenna.
experimental and the simulation-based results at the operating frequency 2.3 to 2.7 GHz were all found to be below 17 dB. This indicates that the experimental and simulation-based results are in reasonable agreement. The minor variation between the simulated and measured results are mainly due to fabrication imprecision, soldering, and the use of lossy material. The mutual coupling (S21, S14, S43, S32) is partially degraded in the measured result. Our work has successfully reduced the mutual coupling (S21, S14, S43, S32) but as suggested by Xu et al.’s work on electromagnetic coupling reduction, metamaterials can be utilized to further reduce the mutual coupling [5,8], an area which we could explore in a our future studies.
The simulated radiation patterns of individual antenna elements at 2.45 GHz for all four ports excited are shown in Fig. 16. Simulated radiation pattern for all four modes in E-plane (Co-pol. and Cross-pol.) and H-plane (Co-pol. and Cross-pol.) of the antenna shows that microsrip feed Port1, Port2, Port3 and Port4 have polarization planes perpendicular to each other, which provides diversity patterns. The antennae elements are linearly polarized, which displays good cross-polarization performance at E-pane about 15 dB at 2.45 GHz. Figs. 17 and 18 show the simulated radiation patterns of quad-radiating antenna elements for E-plane (Co-pole. and Cross-pole.) and H-plane (Co-pole. and Cross-pole.) at 2.3 GHz and 2.6 GHz, respectively. In the designed
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Fig. 13. Simulated current distributions at 2.45GHz when port 1 is excited of the designed MIMO antenna with ring-shaped and with CSRs.
7. MIMO antenna performance To validate the ability of designed quad-radiating element MIMO antenna, it is needed to determine several MIMO diversity performance parameters. These MIMO performance parameters are evaluated by the commercial software tools CST and these results are corroborated with the experimental measured result. The envelope correlation coefficient computes how much the adjacent and diagonal MIMO antenna elements are correlated to each other, where a low envelope correlation coefficient indicates better performance of antenna diversity. The envelop correlation coefficient (e) between antenna elements are calculated using scattering parameters, as defined below; (see Blanch et al., 2003 [9]; Malviya et al., 2017 [10]; Ali, 2017 [11]).
Fig. 14. Simulated and measured reaction coefficient of the designed MIMO antenna.
qe ði; j; NÞ ¼
2 X N Si;n Sn;j n¼1 Y
1
N X Sk;n Sn;k
!
ð1Þ
n¼1
k¼i;j
In Fig. 20, the measured results of the envelope correlation coefficient has been shown to be very close to the simulated results. The values between the adjacent and diagonal elements are lower than 0.04 and 0.03 respectively. It shows good diversity performance of the dual-radiating element MIMO antenna. Channel capacity is the maximum attainable limit at which the information can be transmitted through a MIMO channel. Channel capacity loss is the upper threshold of data rate at which information can be continually transmitted without obtaining significant error [12]. Channel capacity loss, a vital diversity performance parameter used for MIMO antenna characterization was also computed [13,14]. The simplified channel capacity loss of an N x N MIMO system can be evaluated by using the following equation:
C loss ¼ log2 detðwÞR
ð2Þ
R
where w is the receiving antenna correlation matrix.
2 Fig. 15. Simulated and measured isolation of the designed MIMO antenna.
MIMO antenna, the dual-tapered radiating slot are placed symmetrically, therefore, it provides the diversity patterns. In Fig. 19, the designed MIMO antenna maintains gain between 3.2 dB and 3.7 dB and efficiency ranging from 86% to 91% across the working frequency band.
q11 q12 . . . q1j 3 6q 7 6 21 q22 . . . q13 7
wR ¼ 6 6 .. 4 .
qi1
.. .
..
.
7 .. 7 . 5
ð3Þ
qi2 . . . qNN
P P i = j qij ¼ 1 Nn¼1 Si;n Sn;j and i – j qij ¼ Nn¼1 Si;n Sn;j The channel capacity loss of the measurement and simulation of the MIMO antenna are evaluated based on scattering parameters in if
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Fig. 16. Simulated radiation patterns of the proposed MIMO antenna at 2.45 GHz in E-plane and H-plane.
Fig. 17. Simulated radiation patterns of the proposed MIMO antenna at 2.30 GHz in two principle plane.
Eq. (2). Fig. 21 shows that the average value in the entire working frequency band is lower than 0.6 b/s/Hz, which is convenient for practical MIMO systems. Little variety between the results can be accounted to fabrication imprecision, soldering, and feed cable effects. The isolation, gain, and bandwidth of the designed MIMO antenna were compared with four-element MIMO antennas
from the literature and the results are presented in Table 3. The proposed MIMO antenna is the smallest in dimension and its isolation is more than 17 dB over the operating frequency band (2.32.7 GHz). Although antennas in [15,16] offer high gain as compared to the designed antenna, the size of the antenna is larger as compared to the proposed antenna in this study.
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Fig. 18. Simulated radiation patterns of the proposed MIMO antenna at 2.60 GHz in two principle plane.
Fig. 19. Simulated Rad. efficiency and gain of the designed MIMO antenna.
Fig. 21. Simulated and measured capacity loss of the designed MIMO antenna.
Fig. 20. Simulated and measured envelop correlation of the designed MIMO antenna.
antenna consists of four identical radiating elements with a cross-shaped stub on top of the substrate and a partial ground plane with ring-shaped stripes placed at the bottom. The crossshaped stub and the ring-shaped stripes induce coupling current to suppress the initial coupling current among the radiating antenna elements. Thus, the decoupling technique used for accomplishing enhanced isolation is in close proximity to radiating antenna elements. The mutual coupling is reduced by more than 17 dB between adjacent antenna elements and 21 dB between diagonal antenna elements. The characteristic performance was found to be lower than 0.6 b/s/Hz and 0.04 for capacity loss and ECC, respectively. These features of the designed MIMO antenna are indications of its applicability to LTE (2.3–2.4 GHz, 2.496– 2.69 GHz) and Wi-Fi (2.412–2.484 GHz) frequency band. The tapered slot antennas are good candidates for MIMO communication applications because it has stable directional patterns and a consistent impedance matching over a wide operating frequency band.
8. Conclusion Declaration of Competing Interest In this study, a compact, four-port MIMO antenna using dualelliptically tapering shaped element was designed. The designed
The authors declared that there is no conflict of interest.
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Table 3 Comparison between the designed antenna and the previous reported four-element MIMO antennas. Ref. No.
Size, mm2
No. elm.
Operating bands, GHz
Isolation, dB
Peak Gain, dBi
Substrate
[15] [16] [4] [17] Proposed Antenna
70 70 100 50 85 85 45 90 66 66
4 4 4 4 4
2.37–2.69 2.40–2.80 2.32–2.95 2.23–2.64 2.30–2.70
> 17 > 12 > 17 > 13 > 17
3.98 Not reported 3–5.5 2.7 3.67
FR4 FR4 Ro4003C FR4 FR4
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.aeue.2019.152893.
References [1] Malik J, Patnaik A, Kartikeyan MV. Compact antennas for high data rate communication: ultra-wideband (UWB) and multiple-input-multiple-output (MIMO) technology. Springer International Publishing; 2018. https://doi.org/ 10.1007/978-3-319-63175-2. [2] Katie MO, Jamlos MF, Alqadami AS, Jamlos MA. Isolation enhancement of compact dual-wideband mimo antenna using flag-shaped stub. Microwave Opt Technol Lett 2017;59:1028–32. https://doi.org/10.1002/mop.30450. [3] Kiem NK, Phuong HN, Chien DN. Design of compact 4 4 uwb-mimo antenna with wlan band rejection. Int J Antenn Propag 2014;2014:1–11. https://doi. org/10.1155/2014/539094. [4] Ding K, Gao C, Qu D, Yin Q. Compact broadband mimo antenna with parasitic strip. IEEE Antenn Wirel Propag Lett 2017;16:2349–53. https://doi.org/ 10.1109/lawp.2017.2718035. [5] Xu H, Wang G, Qi M. Hilbert-shaped magnetic waveguided metamaterials for electromagnetic coupling reduction of microstrip antenna array. IEEE Trans Magn 2013;49:1526–9. https://doi.org/ 10.1109/TMAG.2012.2230272. [6] Zhang S, Pedersen GF. Mutual coupling reduction for uwb mimo antennas with a wideband neutralization line. IEEE Antenn Wirel Propag Lett 2016;15:166–9. https://doi.org/10.1109/lawp.2015.2435992. [7] Kayabasi A, Toktas A, Yigit E, Sabanci K. Triangular quad-port multi-polarized uwb mimo antenna with enhanced isolation using neutralization ring. AEU Int J Electron Commun 2018;85:47–53. https://doi.org/10.1016/j. aeue.2017.12.027.
[8] Xu H, Wang G, Qi M, Zeng H. Ultra-small single-negative electric metamaterials for electromagnetic coupling reduction of microstrip antenna array. Opt Express 2012;20(20):21968. https://doi.org/10.1364/oe.20.021968. [9] Blanch S, Romeu J, Corbella I. Exact representation of antenna system diversity performance from input parameter description. Electron Lett 2003;39:705. https://doi.org/10.1049/el:20030495. [10] Malviya L, Panigrahi R, Kartikeyan M. Mimo antennas with diversity and mutual coupling reduction techniques: a review. Int J Microwave Wireless Technol 2017;9:1763–80. https://doi.org/10.1017/s1759078717000538. [11] Ali WA, Ibrahim AA. A compact double-sided mimo antenna with an improved isolation for uwb applications. AEU - Int J Electron Commun 2017;82:7–13. https://doi.org/10.1016/j.aeue.2017.07.031. [12] Choukiker YK, Sharma SK, Behera SK. Hybrid fractal shape planar monopole antenna covering multiband wireless communications with mimo implementation for handheld mobile devices. IEEE Trans Antenn Propag 2014;62:1483–8. https://doi.org/10.1109/tap.2013.2295213. [13] Sharma Y, Sarkar D, Saurav K, Srivastava KV. Three-element mimo antenna system with pattern and polarization diversity for wlan applications. IEEE Antenn Wirel Propag Lett 2017;16:1163–6. https://doi.org/10.1109/ lawp.2016.2626394. [14] Kumar A, Ansari AQ, Kanaujia BK, Kishor J. A novel iti-shaped isolation structure placed between two-port cpw-fed dual-band mimo antenna for high isolation. AEU - Int J Electron Commun 2019;104:35–43. https://doi.org/ 10.1016/j.aeue.2019.03.009. [15] Malviya L, Panigrahi RK, Kartikeyan MV. A 2 2 dual-band mimo antenna with polarization diversity for wireless applications. Progress Electromagnet Res C 2016;61:91–103. https://doi.org/10.2528/pierc15110401. [16] Sharawi MS, Khan MU, Numan AB, Alo i DN. A csrr loaded mimo antenna system for ism band operation. IEEE Trans Antenn Propag 2013;61:4265–74. https://doi.org/10.1109/tap.2013.2263214. [17] Malviya L, Kartikeyan M, Panigrahi R. Multi-standard, multi-band planar multiple input multiple output antenna with diversity effects for wireless applications. Int J RF Microwave Comput-Aid Eng 2019;29. https://doi.org/ 10.1002/mmce.21551.
Contents lists available at ScienceDirect
International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
Regular paper
Design of a compact quad-radiating element MIMO antenna for LTE/Wi-Fi application Abubeker A. Yussuf ⇑, Selcuk Paker Department of Electronic and Communication Engineering, Istanbul Technical University, Ayazaa-Maslak, Saryer, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 1 April 2019 Accepted 30 August 2019
Keywords: MIMO antenna Cross-shaped stub Ring-shaped stripes Decoupling Channel capacity loss
a b s t r a c t In this paper, a compact quad-element multiple-input-multiple-output (MIMO) antenna for LTE/Wi-Fi application is proposed. The four dual-elliptically tapered antenna elements are an orthogonal orientation to one another. The MIMO antenna comprises a cross-shaped stub between the quad-radiating elements and ring-shaped stripes between the partially ground planes that act as a decoupling element to suppress mutual coupling. The dimension of the designed MIMO antenna is ð66 66 1:6Þ mm3 . A prototype MIMO antenna was fabricated and S-parameters were measured in order to characterize the performance parameters of the MIMO antenna. The designed MIMO antenna operates at 2.3 to 2.7 GHz frequency band with high isolation exceeding 17 dB. Thus, the measured results give an envelope correlation coefficient below 0.04 and a channel capacity loss is lower than 0.6 b/s/Hz, which reasonably agrees with the simulated results. Ó 2019 Elsevier GmbH. All rights reserved.
1. Introduction Rapid development of cutting-edge wireless technology demands significant improvement in the data rate and channel capacity of wireless communication systems in abundant scattering environments. MIMO antennas are verified to be effective for enhanced channel capacity, improved data rate and diminishing multipath fading in abundant scattering environment [1]. In order to accomplish a higher data rate without adding transmission power, the MIMO system deploys various transmitter and receiver antennas at each end for wireless communication devices. Various methods have been developed to maximize isolation and reduce coupling among the antenna radiating elements. For instance, in a study by Katie et al. on isolation enhancement, a ground plane with flag-shaped stub was reported to decrease mutual coupling between two antenna elements sharing a single ground plane [2]. The slits and slots were also placed beside the flag-shaped stub, which created notch-band out of the preliminary antennas band. In their work, they eventually created an additional coupling route that eliminated the original coupling current between antenna elements. Kiem et al. also showed another approach to diminish the coupling between radiating antennas by using a stub, based on the principle of microstrip multimode resonator (MMR) structure. Fur⇑ Corresponding author. E-mail address: [email protected] (A.A. Yussuf). https://doi.org/10.1016/j.aeue.2019.152893 1434-8411/Ó 2019 Elsevier GmbH. All rights reserved.
ther, they generated a notched band using a mushroom-like electromagnetic band-gap (EBG) structures [3]. Their technique primarily acted as a band-stop filter which minimized the effect of surface current on the antenna elements. Ding et al. also presented another method to enhance the isolation between antenna elements using a parasitic radiator. The antenna element composed of the parasitic strip located between parabolic reflector and mender dipole, which was used not only to enhance impedance matching, but also to improve the gain and eventually widen the operating bandwidth [4]. The parasitic radiator generated an induced coupling current out of phase that mitigated the primary coupling current between antenna elements. Using a magnetic waveguide metamaterial (MTM) based on the Hilbert-shaped between two closely placed patches, significant improvement in isolation was accomplished by Xu et al. in [5]. Their method initially applied as a band-stop filter which reduced the effect of surface current on the antenna elements. A neutralization line technique, which basically provided decoupling current path with different dimensions mainly to eliminate the coupling current on the ground plane and also used to minimize mutual coupling between antenna elements [6,7]. The neutralization line eliminated the induced current with the primary current, since two current flows was out of phase. But, despite the enormous research that have been undertaken on design MIMO antennas, the design of compact, low-profile and multi-standard antennas for wireless communication devices remains a challenging issue.
2
A.A. Yussuf, S. Paker / Int. J. Electron. Commun. (AEÜ) 111 (2019) 152893
In this study, a compact four tapered radiator antenna element is proposed for MIMO applications. The dual-elliptically tapered antenna has advantages of enhanced impedance matching across the desired frequency, symmetrical radiation pattern and easily fabricate. The designed MIMO antenna can be adopted in portable devices, which supports Wi-Fi and LTE applications. The decoupling technique is accomplished by adding both cross-shaped stub between the orthogonal symmetric of four-radiating elements and ring-shaped stripes between the partially ground planes. The designed MIMO antenna system has a compact size with low envelope correlation coefficient and high inter-element isolation. The simulated and measured results are used to validate the proposed design methodology. 2. Antenna configuration Fig. 1 exhibits the geometry of the designed dual-elliptically tapered slot antenna. It comprises of the tapered radiating slot, the microstrip feed line and the partial ground plane. The tapered radiating slot is fed by a 50 X microstrip line. The antenna is designed on FR4 substrate with a thickness of 1.6 mm, a relative permittivity of 4.3, and a loss tangent of 0.035. The thickness of the copper layer on the substrate is 0.035 mm. The overall size of the designed MIMO antenna was 30:75 35:25 mm2 . The optimum dimensions of the designed dual-elliptically tapered slot antenna are given in Table 1. The designs and simulation were performed in commercial software CST.
Fig. 2. Return loss for varying length, Rmx1, of the tapered radiating slot.
3. Parametric study on the impedance characteristics Parametric study is undertaken to characterize the impedance matching and to miniaturize the size of dual-elliptically tapered antenna. Fig. 2 exhibits the return loss of the designed antenna with varying length of tapered slot, Rmx1. The smaller length, Rmx1, of tapered slot have little effect on the impedance matching of the designed antenna. Thus, a compact dual-elliptically tapered Fig. 3. Return loss for varying width, w4, of tapered radiating slot.
Fig. 1. Geometry of the designed dual-elliptically tapered antenna.
Table 1 Optimum dimensions of the designed dual-elliptically tapered antenna. Parameters
W
L
Rmx
Rmx1
w
lg
Rmi
Rmi1
Value
35.25
30.75
29.25
19.75
2.85
10
18
15.15
3
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Fig. 4. Geometry of the designed MIMO antenna for LTE/Wi-Fi application.
Fig. 5. Fabricated of the designed MIMO antenna for LTE/Wi-Fi application.
can be realizable by reducing the length of microstrip line. Fig. 3 shows the impact of varying width, w4, of tapered radiating slot on return loss. The width, w4, of the tapered radiating slots have slight effects on the impedance characteristic. The larger w4 is preferable for better impedance matching at lower frequencies, while the smaller w4 makes the radiating taper too narrow to maintain the impedance characteristics at lower frequency.
4. MIMO Antenna configuration Fig. 4 exhibits the geometry of the designed quad-element MIMO antenna using dual-elliptically tapered slot radiator. The
designed dual-elliptically tapered antenna has advantages of enhanced impedance matching across the desired frequency and symmetrical radiation pattern. The designed MIMO antenna was simulated using FR4 substrate with relative permittivity of 4.4, a thickness of 1.6 mm and loss tangent of 0.035. The overall size of the designed MIMO antenna was 66 66 mm2 . The dimensions of the various geometrical parameters are given in Table 2. Initially, a single radiating antenna was designed with micro-strip fed, which contributed a 50X of characteristic impedance. The dimensions of the radiating antenna element were optimized using a parametric optimization technique to operate LTE (2.3–2.4 GHz, 2.496–2.69 GHz) and Wi-Fi (2.4 GHz) frequency band. The parametric optimization was conducted using computer simulation
Table 2 Dimensions of the designed MIMO Antenna. Parameters
L
W
Rmx1
Rmi1
w
S
l1
w1
R
Value
66
66
19.75
15.15
2.85
5.3
4.75
3
1.5
Parameters
l2
w2
Rmx
Rmi
l3
w3
w4
w5
arc1
Value
14
3
29.25
18
30.75
10
9.5
8
5
4
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Fig. 6. Designed quad-element MIMO antenna without stub and with the stub.
tary spiral ring resonators (CSRs) and ring-shaped stripes techniques were basically used to improve the frequency response of the reflection coefficient and reduce the mutual coupling. Fig. 6 shows the four dual-elliptically tapered slot MIMO antenna with and without cross-shaped stub between elements. The cross-shaped stub reduces the mutual coupling and enhances impedance matching over frequency band. Fig. 7 depicts simulated S-parameters of the quad dual-elliptically tapered slot MIMO antenna with and without cross-shaped stub between elements.
Fig. 7. Simulated S-parameters of quad-element MIMO antenna without stub and with stub between elements.
tools (CST). Then, four such radiating elements were replicated into orthogonal orientation to each other on the same substrate. The cross-shaped stub and ring-shaped stripes were added to enhance the isolation between the adjacent and the diagonal antennae elements. The manufactured MIMO antenna is depicted in Fig. 5. 5. Analysis of quad-element MIMO antenna In this section, a parametric study is conducted to observe the effect of an isolation techniques in MIMO antennae performance using CST Microwave studio. The cross-shaped stub, complemen-
Fig. 9. Simulated S-parameters of quad-element MIMO antenna with ring-shaped and with CSRs between elements.
Fig. 8. Designed quad-element MIMO antenna with ring-shaped and with CSRs.
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Fig. 10. Designed quad-element MIMO antenna with stub and with stub and ring-shaped stripes.
Fig. 11. Simulated S-parameters of quad-element MIMO antenna with stub and with stub and ring-shaped stripes.
The MIMO antenna with cross-shaped stub can cover a 10 dB reflection loss over the frequency band between 2.3 and 2.8 GHz. The figure also shows that the mutual coupling between diagonally placed elements (S31) and adjacent placed elements (S21) are low (less than 20 dB) throughout the frequency band 2.3 to 2.8 GHz. Additionally, in comparison to the results without cross-shaped stub, the mutual coupling between diagonally placed elements (S31) is considerably improved by around 10 dB at the center frequency of 2.45 GHz. Fig. 8 shows the four dual-elliptically tapered slot MIMO antenna with ring-shaped and complementary spiral ring resonators between elements.The ring-shaped strips can be defined as a matching circuit is designed from capacitances (space between the ring-shaped stripes) and inductances(Width ring-shaped stripes). The ring-shaped strips were provided in order to shift the resonant frequencies downwards. Fig. 9 shows the resulted S-parameters with ring-shaped and complementary spiral ring resonators (CSRs). As a result, the mutual coupling (S21) between the antenna elements with the CSRs is slightly reduced throughout the desired frequency band. Fig. 10 shows the four dual-elliptically tapered slot MIMO antenna with and without ring-shaped stripes between the partially ground planes. Fig. 11 depicts a comparison between Sparameters of the quad dual-elliptically tapered slot MIMO antenna with and without ring-shaped stripes between the partially ground planes. The MIMO antenna with the ring-shaped stripes covers the frequency band from 2.2 to 2.7 GHz having a bandwidth of 500 MHz for S11 < 10 dB and the mutual coupling
between diagonally placed elements (S31) and adjacent placed element pair (S21) are less than 20 dB throughout the frequency band 2.2 to 2.7 GHz. As a result, the isolation between the antenna elements is slightly enhanced throughout the desired frequency band. The methods used to analyze mutual coupling effects of the proposed MIMO antenna are also demonstrated by surface current distribution. The current distribution of a compact quad-element MIMO antennas without stub, with cross-shaped stub and ringshaped stripes at frequency 2.45 GHz are presented in Fig. 12. The cross-shaped stub and ring-shaped stripes generated an induced surface current to suppress the initial coupling between the radiating antenna elements. Thus, the cross-shaped stub and the ring-shaped stripes played an important role in reducing mutual coupling. The current distribution of a compact quadelement MIMO antennas with ring-shaped stripes and complementary spiral ring resonators (CSRs) at frequency 2.45 GHz are presented in Fig. 13. In Fig. 13b, the CSRs in MIMO antenna are used to obstruct radiated energy and lessen transmitted energy between adjacent elements due to the induced current that flow within the metallic stub and CSRs, generating a balanced inductive-capacitive effect. Therefore, the mutual coupling (S21) between the adjacent elements is slightly enhanced.
6. Results and discussion The designed antenna was fabricated (Fig. 5) and then Sparameter was measured using a network analyzer (Rohde and Schwarz, FSH). During the measurement process, the two ports of antenna are excited, the others are terminated with the 50X load. Fig. 14 is given to display the simulated and measured reaction coefficients of the MIMO antenna designed in this study. It can be observed from the shape of the plot in Fig. 14 that the simulated reflection coefficient results are in considerable agreement to the experimental results. One can see from the Fig. 14 that the designed quad-element MIMO antenna operates in the frequency band range from 2.3 GHz to 2.7 GHz for reflection coefficient of less than 10 dB. Both the simulation and measurement based results of the radiating antenna elements achieved a good resonance at a centre frequency of 2.46 GHz. Preliminary simulation results were performed to select the terminal prototype of the designed antenna based on their performance in providing low mutual coupling and good impedance matching. The simulated and measured results, as presented in Fig. 15, illustrate that the mutual coupling between the adjacent (S21, S14, S43, S32) and diagonal (S31, S24) paired elements were found to be greater than 17 dB and 21 dB, respectively, throughout the desired frequency band. The isolation measurement of the
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Fig. 12. Simulated current distributions at 2.45 GHz when port 1 is excited of the designed MIMO antenna.
experimental and the simulation-based results at the operating frequency 2.3 to 2.7 GHz were all found to be below 17 dB. This indicates that the experimental and simulation-based results are in reasonable agreement. The minor variation between the simulated and measured results are mainly due to fabrication imprecision, soldering, and the use of lossy material. The mutual coupling (S21, S14, S43, S32) is partially degraded in the measured result. Our work has successfully reduced the mutual coupling (S21, S14, S43, S32) but as suggested by Xu et al.’s work on electromagnetic coupling reduction, metamaterials can be utilized to further reduce the mutual coupling [5,8], an area which we could explore in a our future studies.
The simulated radiation patterns of individual antenna elements at 2.45 GHz for all four ports excited are shown in Fig. 16. Simulated radiation pattern for all four modes in E-plane (Co-pol. and Cross-pol.) and H-plane (Co-pol. and Cross-pol.) of the antenna shows that microsrip feed Port1, Port2, Port3 and Port4 have polarization planes perpendicular to each other, which provides diversity patterns. The antennae elements are linearly polarized, which displays good cross-polarization performance at E-pane about 15 dB at 2.45 GHz. Figs. 17 and 18 show the simulated radiation patterns of quad-radiating antenna elements for E-plane (Co-pole. and Cross-pole.) and H-plane (Co-pole. and Cross-pole.) at 2.3 GHz and 2.6 GHz, respectively. In the designed
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Fig. 13. Simulated current distributions at 2.45GHz when port 1 is excited of the designed MIMO antenna with ring-shaped and with CSRs.
7. MIMO antenna performance To validate the ability of designed quad-radiating element MIMO antenna, it is needed to determine several MIMO diversity performance parameters. These MIMO performance parameters are evaluated by the commercial software tools CST and these results are corroborated with the experimental measured result. The envelope correlation coefficient computes how much the adjacent and diagonal MIMO antenna elements are correlated to each other, where a low envelope correlation coefficient indicates better performance of antenna diversity. The envelop correlation coefficient (e) between antenna elements are calculated using scattering parameters, as defined below; (see Blanch et al., 2003 [9]; Malviya et al., 2017 [10]; Ali, 2017 [11]).
Fig. 14. Simulated and measured reaction coefficient of the designed MIMO antenna.
qe ði; j; NÞ ¼
2 X N Si;n Sn;j n¼1 Y
1
N X Sk;n Sn;k
!
ð1Þ
n¼1
k¼i;j
In Fig. 20, the measured results of the envelope correlation coefficient has been shown to be very close to the simulated results. The values between the adjacent and diagonal elements are lower than 0.04 and 0.03 respectively. It shows good diversity performance of the dual-radiating element MIMO antenna. Channel capacity is the maximum attainable limit at which the information can be transmitted through a MIMO channel. Channel capacity loss is the upper threshold of data rate at which information can be continually transmitted without obtaining significant error [12]. Channel capacity loss, a vital diversity performance parameter used for MIMO antenna characterization was also computed [13,14]. The simplified channel capacity loss of an N x N MIMO system can be evaluated by using the following equation:
C loss ¼ log2 detðwÞR
ð2Þ
R
where w is the receiving antenna correlation matrix.
2 Fig. 15. Simulated and measured isolation of the designed MIMO antenna.
MIMO antenna, the dual-tapered radiating slot are placed symmetrically, therefore, it provides the diversity patterns. In Fig. 19, the designed MIMO antenna maintains gain between 3.2 dB and 3.7 dB and efficiency ranging from 86% to 91% across the working frequency band.
q11 q12 . . . q1j 3 6q 7 6 21 q22 . . . q13 7
wR ¼ 6 6 .. 4 .
qi1
.. .
..
.
7 .. 7 . 5
ð3Þ
qi2 . . . qNN
P P i = j qij ¼ 1 Nn¼1 Si;n Sn;j and i – j qij ¼ Nn¼1 Si;n Sn;j The channel capacity loss of the measurement and simulation of the MIMO antenna are evaluated based on scattering parameters in if
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Fig. 16. Simulated radiation patterns of the proposed MIMO antenna at 2.45 GHz in E-plane and H-plane.
Fig. 17. Simulated radiation patterns of the proposed MIMO antenna at 2.30 GHz in two principle plane.
Eq. (2). Fig. 21 shows that the average value in the entire working frequency band is lower than 0.6 b/s/Hz, which is convenient for practical MIMO systems. Little variety between the results can be accounted to fabrication imprecision, soldering, and feed cable effects. The isolation, gain, and bandwidth of the designed MIMO antenna were compared with four-element MIMO antennas
from the literature and the results are presented in Table 3. The proposed MIMO antenna is the smallest in dimension and its isolation is more than 17 dB over the operating frequency band (2.32.7 GHz). Although antennas in [15,16] offer high gain as compared to the designed antenna, the size of the antenna is larger as compared to the proposed antenna in this study.
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Fig. 18. Simulated radiation patterns of the proposed MIMO antenna at 2.60 GHz in two principle plane.
Fig. 19. Simulated Rad. efficiency and gain of the designed MIMO antenna.
Fig. 21. Simulated and measured capacity loss of the designed MIMO antenna.
Fig. 20. Simulated and measured envelop correlation of the designed MIMO antenna.
antenna consists of four identical radiating elements with a cross-shaped stub on top of the substrate and a partial ground plane with ring-shaped stripes placed at the bottom. The crossshaped stub and the ring-shaped stripes induce coupling current to suppress the initial coupling current among the radiating antenna elements. Thus, the decoupling technique used for accomplishing enhanced isolation is in close proximity to radiating antenna elements. The mutual coupling is reduced by more than 17 dB between adjacent antenna elements and 21 dB between diagonal antenna elements. The characteristic performance was found to be lower than 0.6 b/s/Hz and 0.04 for capacity loss and ECC, respectively. These features of the designed MIMO antenna are indications of its applicability to LTE (2.3–2.4 GHz, 2.496– 2.69 GHz) and Wi-Fi (2.412–2.484 GHz) frequency band. The tapered slot antennas are good candidates for MIMO communication applications because it has stable directional patterns and a consistent impedance matching over a wide operating frequency band.
8. Conclusion Declaration of Competing Interest In this study, a compact, four-port MIMO antenna using dualelliptically tapering shaped element was designed. The designed
The authors declared that there is no conflict of interest.
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Table 3 Comparison between the designed antenna and the previous reported four-element MIMO antennas. Ref. No.
Size, mm2
No. elm.
Operating bands, GHz
Isolation, dB
Peak Gain, dBi
Substrate
[15] [16] [4] [17] Proposed Antenna
70 70 100 50 85 85 45 90 66 66
4 4 4 4 4
2.37–2.69 2.40–2.80 2.32–2.95 2.23–2.64 2.30–2.70
> 17 > 12 > 17 > 13 > 17
3.98 Not reported 3–5.5 2.7 3.67
FR4 FR4 Ro4003C FR4 FR4
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.aeue.2019.152893.
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