- Email: [email protected]
3.5 GHz MIMO applications
Optik 126 (2015) 1175–1180
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
A novel eight ports dual band antenna array for 2.4/3.5 GHz MIMO applications Yun-qiang Xia, Xiao-rong Chen, Tao Tang ∗ Electronic Engineering College, Chengdu University of Information Technology, Chengdu 610225, China
a r t i c l e
i n f o
Article history: Received 19 February 2014 Accepted 4 March 2015 Keywords: Multiple-input multiple-output (MIMO) Eight ports Dual band Antenna array
a b s t r a c t An eight ports multiple-input multiple-output (MIMO) patch antenna array operating at 2.4 GHz and 3.5 GHz band is presented in this paper, which can be used for the Long Term Evolution (LTE), Microwave Access (WiMAX) and the low frequency band of Wireless Fidelity (WiFi). The presented antenna is fabricated on a F4B-2 substrate (εr = 2.65, tan ı = 0.002), it consists of four coplanar E-shaped patches which are orthogonal to each other. Two feed ports are respectively located at two adjacent sides of each patch. Double symmetrical slots are etched out in each patch to improve the antenna bandwidth, a 46 mm × 33 mm metal parasitic patch is placed above each patch to improve the gain. The maximum gain of the proposed antenna array is more than 8 dB at 2.4 GHz band and 9 dB at 3.5 GHz band respectively. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction Due to an expanding number of applications in modern wireless communication technologies, such as LTE, WiMAX and WiFi, the MIMO technology [1–4] has attracted much attention in recent years [5–7]. The MIMO can substantially increase the wireless channel capacity in scattering environments without the additional power [8,9]. In most of the prior researches, some MIMO systems with two or more antennas operating at the same frequency bands have been proposed [10–12]. An antenna array consisting of three equilateral triangular-patch antenna elements has been proposed in [13], which operates at 2.65 GHz with good polarization. Another two ports MIMO for 2.4 GHz WiFi is designed in [14], which consists of a square ring patch antenna and a PIFA co-located inside. Ref. [15] proposed a 5 GHz 4 or 8-element MIMO antenna system for IEEE 802.11 ac devices, which consists of 2 × 2 or 2 × 4 MIMO antenna systems. Recently, the miniaturized indoor base station has become an industry-wide potential interest for the wireless access. In order to achieve the seamless link in wireless communication among the LTE, WiMAX and WiFi, it is necessary to combine the band coverage among them. Based on these multi-purpose considerations, some wideband or UWB, dual band as well as triple band MIMO has been developed [16–20]. Ref. [21] proposes a two-element MIMO arrays for WLAN/WiMAX application, which covers 2.4-/5.8GHz WLAN and 2.5-/3.3-/3.5-/3.7-/5.5-GHz WiMAX. A dual-loop
∗ Corresponding author. Tel.: +86 28 85966898. E-mail addresses: [email protected] (Y.-q. Xia), [email protected] (X.-r. Chen), [email protected] (T. Tang). http://dx.doi.org/10.1016/j.ijleo.2015.03.024 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
antenna array with high gain (with peak gain of about 7 dB for the three operating bands) for MIMO has been demonstrated in [22], which operates at the 2.4/5.2/5.8 GHz WiFi band. A dualbroadband MIMO for GSM1800/1900, UMTS2000, LTE2300/2600, and WLAN2.4/5-GHz has been developed in [23], which is composed of two dual-broadband antenna elements, each of which consists of an outer loop coupled by an inner loop. In general, a MIMO system consists of multiple antennas at the receiver and transmitter, which increases channel capacity but suffers from mutual interference. The mutual coupling from closed antennas is mainly through two ways [24]: radiation emission (RE, through electromagnetic coupling) and conduction emission (CE, through a common conductor such as the ground plane). It is a critical problem for a MIMO system because it deteriorates the performance of MIMO system including the capacity [25–27]. Various methods have been used to reduce the coupling between the antennas [28], such as decoupling network, inductor coil, ground branches, electromagnetic band-gap (EBG) structure and an inverted-Y shaped stub inserted on the ground plane for two-element MIMO antennas. For four-element MIMO antennas, a series of slits etched into the ground plane, antennas placed uniquely and orthogonal arrangement of antennas were introduced. In this paper, a new low profile, eight channels antenna with good isolation (<−20 dB) for MIMO applications is proposed, which can be used for LTE, WiFi and WiMAX 2.3–2.5 GHz band and WiMAX 3.5 GHz band due to its dual band performance. It consists of four E-shaped patch antennas, where each one has two microstrip feed ports that are respectively located at two adjacent sides of the Eshaped patch. The configuration and the design concept used in this
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Fig. 1. Geometries of the proposed antenna. (a) Top view. (b) Lateral view. (c) Single E-shaped patch.
paper will be explained in Section 2. Parametric studies have been carried out in Section 3. Finally, conclusions are drawn in Section 4. 2. Antenna design and results The proposed eight channels MIMO antenna, which consists of four patch antennas placed orthogonal to each other, is shown in Fig. 1. As Fig. 1(a) shows, two symmetrical rectangular slots are located on each patch to constitute the E-shaped patch antenna. A ground plane is located on the substrate bottom, the substrate between patches and ground is F4B-2 (εr = 2.65, tan ı = 0.002) with the thickness 1 mm. In order to obtain dual band and orthogonally polarization, two microstrip feed ports are located at two adjacent sides of each E-shaped patch. A 46 mm × 33 mm metal parasitic patch is located above each E-shaped patch, the gap between the parasitic patch and the E-shaped patch is 8 mm. The overall size of the MIMO is 120.75 mm × 120.75 mm × 9 mm. The port impedance matching is a key factor in the antenna design, especially for this design, that two ports corresponding different frequency band placed on the same E-shaped patch. Fig. 2 shows the ports input impedance after optimization. When each port of the single E-shaped patch antenna in Fig. 1(a) works alone, the simulated and measured input reflection coefficients S11 and S22 corresponding to port 1 and port 2 respectively are shown in Fig. 3. One can note that the operate frequency of port 1 covers the LTE, WiFi and WiMAX 2.3–2.5 GHz, and port 2 covers the WiMAX 3.5 GHz. Fig. 4 shows the simulated and measured radiation patterns of the single E-shaped antenna at 2.4 GHz and 3.5 GHz, respectively. The maximum antenna patch gain, resulting from port 1 and port 2 is respectively larger than 8 dB and 9 dB (Fig. 5).
Fig. 2. Input impedance. (a) Port 1. (b) Port 2.
3. Parametric studies 3.1. Antenna element Since the slots created on metallic patches can help increase inductance value of the antenna, hence changing of the slots dimension will cause the changes of antenna inductance. As we know that the antenna resonant frequency is inversely proportional to inductance and capacitance value, which defined as: 1 ω= √ , LC
(1)
So, there is a simple method to reduce the overall size of antenna by adjusting the slots. Besides, for the E-shaped patch antenna, two parallel slots are incorporated to introduce a second resonant
mode, resulting in a dual-band antenna [29]. If two parallel slots are appropriately placed in the patch, a wideband performance can be achieved. To further illustrate the function of the two slots, an unslotted antenna element but with the parasitic element and the same overall size was analyzed. The reflection coefficient of S11 in dB is shown in Fig. 6. From Fig. 6(a) one can note that the reflection coefficient of the unslotted antenna is always larger than −10 dB in LTE, WiFi and WiMAX band, which indicates that the antenna impedance match poorly in this band. From Fig. 6(b), we can see that the bandwidth of the unslotted antenna is less than the proposed E-shaped antenna. It is more important that the resonant frequency of the unslotted antenna is not located at the desired frequency band. According
Y.-q. Xia et al. / Optik 126 (2015) 1175–1180
Fig. 3. The simulated and measured input reflection coefficient (S11 and S22 ) of the single E-shaped antenna. (a) Port 1 (S11 ). (b) Port 2 (S22 ).
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Fig. 6. The comparison of input reflection coefficient. (a) Port 1 (S11 ). (b) Port 2 (S22 ).
Fig. 4. The simulated and measured radiation patterns of the single E-shaped antenna. (a) Port 1 (2.4 GHz). (b) Port 2 (3.5 GHz).
Fig. 5. E-shaped patch and its Equivalent parallel resonant circuit.
to antenna theory, the antenna size should be extended to get a lower resonant frequency, but this does not meet with the design concept. Fortunately, as previously mentioned, we can achieve the miniaturization purpose by creating slots in the patches of the antenna. A metal parasitic element is introduced in the antenna structure as Fig. 1 shows, so that a corresponding air substrate is inserted between the parasitic element and the E-shaped patch. The air substrate can improve the antenna bandwidth. Besides,
Fig. 7. The comparison of input reflection coefficient and the radiation patterns of the E-shaped antenna with and without the parasitic element.
the air can reduce the effective synthesized dielectric constant of the antenna and as a result to increase the peak gain. Fig. 7 shows the reflection coefficient and the radiation patterns of the single antenna E-shaped antenna with and without the parasitic element. Both the bandwidth and gain of the antenna with parasitic element are better than the antenna without the parasitic element.
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Table 1 The minimum distance to meet the isolation (−20 dB) for various arrangement. No.
Arrangement model
Distance of the element D (mm)
a
45
b
26
c
15
3.2. Antenna array 3.2.1. The effect of antenna arrangement model Each element of the proposed MIMO has two feed ports corresponding to two channels and two work frequency bands. Orthogonal polarization can be formed by two channels. As we know that each element in an antenna array will be influenced by another nearby antenna, so, in order to improve the isolation between antenna’s ports, the antenna elements must be appropriately arranged. We use a two-element array to illustrate this point. The arrangement model of the two elements array has several ways, but if the feed line is located between the array elements, antenna size will increase accordingly. So, considering the miniaturization requirement, the arrangement of the two elements array mainly has three models, which are shown in Table 1. The isolation between ports corresponding the same frequency band must be taken into account, such as port 1, port 3 and port 2, port 4 of the array as Table 1 shows. Generally, the isolation between ports should be less than −20 dB [10]. In order to meet the isolation requirement, we investigate the minimum distance of D between two elements by reducing for the three arrangement models. The results show that the distance of D between two elements needs to equal 45 mm for model (a), about 25 mm for model (b), but only 15 mm for model (c). This is because a strong electric field coupling between the parallel ports in model (a) and (b), but in mode (c), the ports corresponding the same frequency band are orthogonal to each other, so the coupling between these ports will be minimized in the mode (c). 3.2.2. The effect of antenna distance for four elements According to the analysis of the previous section, the MIMO antenna of the proposed four elements array are placed orthogonal to each other in a row as Fig. 1(a) shows. The distance between each antenna is 15 mm for two elements array as Table 1 model (c) shows, but for the four elements array, port 1 and port 5 are parallel to each other, port 3 and port 7, port 2 and port 6, port 4 and port 8 are paralleled too. Strong coupling will occur between them, which results in poor isolation between these ports corresponding same frequency band. For example, If we set the distance D = 15 mm, the isolation between port 1 and port 5 is about −16 dB.
Fig. 8. The isolation between ports correspond the same frequency.
Considering the isolation and overall performances of the four elements array, the antenna elements are separated by 25 mm (about /5 at 2.4 GHz) finally. The isolation between the ports corresponding the same frequency band is shown in Fig. 8. Within the desired frequency band, the isolation is less than −20 dB, which meets the design requirements. When all ports of the MIMO work together, the scattering matrix does not accurately characterize the radiation efficiency and bandwidth of a MIMO antenna [30,31]; instead of input reflection coefficients, the array’s Total Active Refection Coefficient (TARC) can be used so that it accounts for both coupling and random signal combination, for a lossless N port antenna, the TARC can be described as
N |bi |2
at =
i=1
, N |ai |2
(2)
i=1
where ai is the incident signal vector with randomly phased elements and bi is the reflected signal vector. Set ai = 1, bi can be calculated by [b] = [S][a],
(3)
Using the measured scattering matrix, the measured TARC can be got by Eq. (3). Fig. 9 shows the TARC of the proposed MIMO. A high radiation efficiency (high TARC) is expected around the 2.5 GHz and 3.5 GHz band as Fig. 9 shows. The radiation patterns of the proposed MIMO at some key frequency (LTE/WiFi/WiMAX band) are shown in Fig. 10. The maximum gain which can be seen in Fig. 10 is more than 8 dB at 2.35 GHz and 2.4 GHz band, as well as 9 dB at 3.5 GHz band respectively.
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Fig. 9. The prototype of the MIMO and its total active refection coefficient (TARC in dB).
Fig. 11. The correlation coefficient.
4. Conclusion A novel eight channels dual band MIMO antenna which can be used for LTE, WiMAX and the low frequency band of WiFi indoor base station has been designed. It consists of four E-shaped patch antennas, wherein each has two ports corresponding to two different operating frequency bands. The results show that the proposed MIMO antenna’s gain is more than 8 dB at 2.4 GHz and 9 dB at 3.5 GHz respectively. Acknowledgment We would like to thank anonymous reviewers. This work was supported by the National Natural Science Foundation of China (No. 61201095) and the China Postdoctoral Science Foundation (2014M562299) as well as the Sichuan Science and Technology Support Program (2015GZ0282) and the Sichuan Education Department Research Projects (13ZB0076). References Fig. 10. Radiation patterns of the eight ports MIMO.
The correlation coefficient can be calculated by S-parameters [32] e =
∗ S ∗ 2 |S11 12 + S21 S22 |
(1 − |S11
|2
− |S21 |2 )(1 − |S22 |2 − |S12 |2 )
,
(4)
The simulated correlation coefficients of the proposed fourelement E-shaped array are given in Fig. 11.
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[7] Mina Ayatollahi, Qinjiang Rao, Dong Wang, A compact, high isolation and wide bandwidth antenna array for long term evolution wireless devices, IEEE Trans. Antennas Propag. 60 (10) (2012) 4960–4963. [8] Y. Ding, Z. Du, K. Gong, Z. Feng, A novel dual-band printed diversity antenna for mobile terminals, IEEE Trans. Antennas Propag. 55 (7) (2007) 2088–2096. [9] M.K. Meshram, R.K. Animeh, A.T. Pimpale, N.K. Nikolova, A novel quad-band diversity antenna for LTE and Wi-Fi applications with high isolation, IEEE Trans. Antennas Propag. 60 (9) (2012) 4360–4371. [10] S.L. Zuo, Y.Z. Yin, Z.Y. Zhang, W.J. Wu, J.J. Xie, Eigenmode decoupling for MIMO loop-antenna based on 180◦ coupler, Prog. Electromagn. Res. 26 (2011) 11–20. [11] A. Jain, P.K. Verma, V.K. Singh, Performance analysis of PIFA based 4 × 4 MIMO antenna, Electron. Lett. 48 (9) (2012) 474–475. [12] Saou Wen Su, Cheng Tse Lee, Fa Shian Chang, Printed MIMO-antenna system using neutralization-line technique for wireless USB-dongle applications, IEEE Trans. Antennas Propag. 60 (2) (2012) 456–463. [13] Haili Zhang, Zhihong Wang, Jiawei Yu, Jia Huang, A compact MIMO antenna for wireless communication, IEEE Antennas Propag. Mag. 50 (6) (2008) 104–107. [14] H. Li, J. Xiong, Z. Ying, S.L. He, Compact and low profile co-located MIMO antenna structure with polarisation diversity and high port isolation, Electron. Lett. 46 (2) (2010) 108–110. [15] M.S. Sharawi, A 5-GHz 4/8-element MIMO antenna system for IEEE 802.11ac devices, Microw. Opt. Technol. Lett. 55 (7) (2013) 1589–1594. [16] R. Sadeghi Fakhr, Compact size and dual band semicircle shaped antenna for MIMO applications, Prog. Electromagn. Res. C 11 (2009) 147–154. [17] Chan Hwang See, R.A. Abd Alhameed, Z.Z. Abidin, N.J. McEwan, P.S. Excell, Wideband printed MIMO/diversity monopole antenna for WiFi/WiMAX applications, IEEE Trans. Antennas Propag. 60 (4) (2012) 2028–2035. [18] A.N. Kulkarni, S.K. Sharma, A multiband antenna with MIMO implementation for USB dongle size wireless devices, Microw. Opt. Technol. Lett. 54 (8) (2012) 1990–1994. [19] S.A. Rezaeieh, N. Pouyanfar, Double channel triple band MIMO antenna with high isolation performance and pattern diversity for wireless applications, Microw. Opt. Technol. Lett. 54 (12) (2012) 2689–2691.
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
A novel eight ports dual band antenna array for 2.4/3.5 GHz MIMO applications Yun-qiang Xia, Xiao-rong Chen, Tao Tang ∗ Electronic Engineering College, Chengdu University of Information Technology, Chengdu 610225, China
a r t i c l e
i n f o
Article history: Received 19 February 2014 Accepted 4 March 2015 Keywords: Multiple-input multiple-output (MIMO) Eight ports Dual band Antenna array
a b s t r a c t An eight ports multiple-input multiple-output (MIMO) patch antenna array operating at 2.4 GHz and 3.5 GHz band is presented in this paper, which can be used for the Long Term Evolution (LTE), Microwave Access (WiMAX) and the low frequency band of Wireless Fidelity (WiFi). The presented antenna is fabricated on a F4B-2 substrate (εr = 2.65, tan ı = 0.002), it consists of four coplanar E-shaped patches which are orthogonal to each other. Two feed ports are respectively located at two adjacent sides of each patch. Double symmetrical slots are etched out in each patch to improve the antenna bandwidth, a 46 mm × 33 mm metal parasitic patch is placed above each patch to improve the gain. The maximum gain of the proposed antenna array is more than 8 dB at 2.4 GHz band and 9 dB at 3.5 GHz band respectively. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction Due to an expanding number of applications in modern wireless communication technologies, such as LTE, WiMAX and WiFi, the MIMO technology [1–4] has attracted much attention in recent years [5–7]. The MIMO can substantially increase the wireless channel capacity in scattering environments without the additional power [8,9]. In most of the prior researches, some MIMO systems with two or more antennas operating at the same frequency bands have been proposed [10–12]. An antenna array consisting of three equilateral triangular-patch antenna elements has been proposed in [13], which operates at 2.65 GHz with good polarization. Another two ports MIMO for 2.4 GHz WiFi is designed in [14], which consists of a square ring patch antenna and a PIFA co-located inside. Ref. [15] proposed a 5 GHz 4 or 8-element MIMO antenna system for IEEE 802.11 ac devices, which consists of 2 × 2 or 2 × 4 MIMO antenna systems. Recently, the miniaturized indoor base station has become an industry-wide potential interest for the wireless access. In order to achieve the seamless link in wireless communication among the LTE, WiMAX and WiFi, it is necessary to combine the band coverage among them. Based on these multi-purpose considerations, some wideband or UWB, dual band as well as triple band MIMO has been developed [16–20]. Ref. [21] proposes a two-element MIMO arrays for WLAN/WiMAX application, which covers 2.4-/5.8GHz WLAN and 2.5-/3.3-/3.5-/3.7-/5.5-GHz WiMAX. A dual-loop
∗ Corresponding author. Tel.: +86 28 85966898. E-mail addresses: [email protected] (Y.-q. Xia), [email protected] (X.-r. Chen), [email protected] (T. Tang). http://dx.doi.org/10.1016/j.ijleo.2015.03.024 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
antenna array with high gain (with peak gain of about 7 dB for the three operating bands) for MIMO has been demonstrated in [22], which operates at the 2.4/5.2/5.8 GHz WiFi band. A dualbroadband MIMO for GSM1800/1900, UMTS2000, LTE2300/2600, and WLAN2.4/5-GHz has been developed in [23], which is composed of two dual-broadband antenna elements, each of which consists of an outer loop coupled by an inner loop. In general, a MIMO system consists of multiple antennas at the receiver and transmitter, which increases channel capacity but suffers from mutual interference. The mutual coupling from closed antennas is mainly through two ways [24]: radiation emission (RE, through electromagnetic coupling) and conduction emission (CE, through a common conductor such as the ground plane). It is a critical problem for a MIMO system because it deteriorates the performance of MIMO system including the capacity [25–27]. Various methods have been used to reduce the coupling between the antennas [28], such as decoupling network, inductor coil, ground branches, electromagnetic band-gap (EBG) structure and an inverted-Y shaped stub inserted on the ground plane for two-element MIMO antennas. For four-element MIMO antennas, a series of slits etched into the ground plane, antennas placed uniquely and orthogonal arrangement of antennas were introduced. In this paper, a new low profile, eight channels antenna with good isolation (<−20 dB) for MIMO applications is proposed, which can be used for LTE, WiFi and WiMAX 2.3–2.5 GHz band and WiMAX 3.5 GHz band due to its dual band performance. It consists of four E-shaped patch antennas, where each one has two microstrip feed ports that are respectively located at two adjacent sides of the Eshaped patch. The configuration and the design concept used in this
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Fig. 1. Geometries of the proposed antenna. (a) Top view. (b) Lateral view. (c) Single E-shaped patch.
paper will be explained in Section 2. Parametric studies have been carried out in Section 3. Finally, conclusions are drawn in Section 4. 2. Antenna design and results The proposed eight channels MIMO antenna, which consists of four patch antennas placed orthogonal to each other, is shown in Fig. 1. As Fig. 1(a) shows, two symmetrical rectangular slots are located on each patch to constitute the E-shaped patch antenna. A ground plane is located on the substrate bottom, the substrate between patches and ground is F4B-2 (εr = 2.65, tan ı = 0.002) with the thickness 1 mm. In order to obtain dual band and orthogonally polarization, two microstrip feed ports are located at two adjacent sides of each E-shaped patch. A 46 mm × 33 mm metal parasitic patch is located above each E-shaped patch, the gap between the parasitic patch and the E-shaped patch is 8 mm. The overall size of the MIMO is 120.75 mm × 120.75 mm × 9 mm. The port impedance matching is a key factor in the antenna design, especially for this design, that two ports corresponding different frequency band placed on the same E-shaped patch. Fig. 2 shows the ports input impedance after optimization. When each port of the single E-shaped patch antenna in Fig. 1(a) works alone, the simulated and measured input reflection coefficients S11 and S22 corresponding to port 1 and port 2 respectively are shown in Fig. 3. One can note that the operate frequency of port 1 covers the LTE, WiFi and WiMAX 2.3–2.5 GHz, and port 2 covers the WiMAX 3.5 GHz. Fig. 4 shows the simulated and measured radiation patterns of the single E-shaped antenna at 2.4 GHz and 3.5 GHz, respectively. The maximum antenna patch gain, resulting from port 1 and port 2 is respectively larger than 8 dB and 9 dB (Fig. 5).
Fig. 2. Input impedance. (a) Port 1. (b) Port 2.
3. Parametric studies 3.1. Antenna element Since the slots created on metallic patches can help increase inductance value of the antenna, hence changing of the slots dimension will cause the changes of antenna inductance. As we know that the antenna resonant frequency is inversely proportional to inductance and capacitance value, which defined as: 1 ω= √ , LC
(1)
So, there is a simple method to reduce the overall size of antenna by adjusting the slots. Besides, for the E-shaped patch antenna, two parallel slots are incorporated to introduce a second resonant
mode, resulting in a dual-band antenna [29]. If two parallel slots are appropriately placed in the patch, a wideband performance can be achieved. To further illustrate the function of the two slots, an unslotted antenna element but with the parasitic element and the same overall size was analyzed. The reflection coefficient of S11 in dB is shown in Fig. 6. From Fig. 6(a) one can note that the reflection coefficient of the unslotted antenna is always larger than −10 dB in LTE, WiFi and WiMAX band, which indicates that the antenna impedance match poorly in this band. From Fig. 6(b), we can see that the bandwidth of the unslotted antenna is less than the proposed E-shaped antenna. It is more important that the resonant frequency of the unslotted antenna is not located at the desired frequency band. According
Y.-q. Xia et al. / Optik 126 (2015) 1175–1180
Fig. 3. The simulated and measured input reflection coefficient (S11 and S22 ) of the single E-shaped antenna. (a) Port 1 (S11 ). (b) Port 2 (S22 ).
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Fig. 6. The comparison of input reflection coefficient. (a) Port 1 (S11 ). (b) Port 2 (S22 ).
Fig. 4. The simulated and measured radiation patterns of the single E-shaped antenna. (a) Port 1 (2.4 GHz). (b) Port 2 (3.5 GHz).
Fig. 5. E-shaped patch and its Equivalent parallel resonant circuit.
to antenna theory, the antenna size should be extended to get a lower resonant frequency, but this does not meet with the design concept. Fortunately, as previously mentioned, we can achieve the miniaturization purpose by creating slots in the patches of the antenna. A metal parasitic element is introduced in the antenna structure as Fig. 1 shows, so that a corresponding air substrate is inserted between the parasitic element and the E-shaped patch. The air substrate can improve the antenna bandwidth. Besides,
Fig. 7. The comparison of input reflection coefficient and the radiation patterns of the E-shaped antenna with and without the parasitic element.
the air can reduce the effective synthesized dielectric constant of the antenna and as a result to increase the peak gain. Fig. 7 shows the reflection coefficient and the radiation patterns of the single antenna E-shaped antenna with and without the parasitic element. Both the bandwidth and gain of the antenna with parasitic element are better than the antenna without the parasitic element.
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Table 1 The minimum distance to meet the isolation (−20 dB) for various arrangement. No.
Arrangement model
Distance of the element D (mm)
a
45
b
26
c
15
3.2. Antenna array 3.2.1. The effect of antenna arrangement model Each element of the proposed MIMO has two feed ports corresponding to two channels and two work frequency bands. Orthogonal polarization can be formed by two channels. As we know that each element in an antenna array will be influenced by another nearby antenna, so, in order to improve the isolation between antenna’s ports, the antenna elements must be appropriately arranged. We use a two-element array to illustrate this point. The arrangement model of the two elements array has several ways, but if the feed line is located between the array elements, antenna size will increase accordingly. So, considering the miniaturization requirement, the arrangement of the two elements array mainly has three models, which are shown in Table 1. The isolation between ports corresponding the same frequency band must be taken into account, such as port 1, port 3 and port 2, port 4 of the array as Table 1 shows. Generally, the isolation between ports should be less than −20 dB [10]. In order to meet the isolation requirement, we investigate the minimum distance of D between two elements by reducing for the three arrangement models. The results show that the distance of D between two elements needs to equal 45 mm for model (a), about 25 mm for model (b), but only 15 mm for model (c). This is because a strong electric field coupling between the parallel ports in model (a) and (b), but in mode (c), the ports corresponding the same frequency band are orthogonal to each other, so the coupling between these ports will be minimized in the mode (c). 3.2.2. The effect of antenna distance for four elements According to the analysis of the previous section, the MIMO antenna of the proposed four elements array are placed orthogonal to each other in a row as Fig. 1(a) shows. The distance between each antenna is 15 mm for two elements array as Table 1 model (c) shows, but for the four elements array, port 1 and port 5 are parallel to each other, port 3 and port 7, port 2 and port 6, port 4 and port 8 are paralleled too. Strong coupling will occur between them, which results in poor isolation between these ports corresponding same frequency band. For example, If we set the distance D = 15 mm, the isolation between port 1 and port 5 is about −16 dB.
Fig. 8. The isolation between ports correspond the same frequency.
Considering the isolation and overall performances of the four elements array, the antenna elements are separated by 25 mm (about /5 at 2.4 GHz) finally. The isolation between the ports corresponding the same frequency band is shown in Fig. 8. Within the desired frequency band, the isolation is less than −20 dB, which meets the design requirements. When all ports of the MIMO work together, the scattering matrix does not accurately characterize the radiation efficiency and bandwidth of a MIMO antenna [30,31]; instead of input reflection coefficients, the array’s Total Active Refection Coefficient (TARC) can be used so that it accounts for both coupling and random signal combination, for a lossless N port antenna, the TARC can be described as
N |bi |2
at =
i=1
, N |ai |2
(2)
i=1
where ai is the incident signal vector with randomly phased elements and bi is the reflected signal vector. Set ai = 1, bi can be calculated by [b] = [S][a],
(3)
Using the measured scattering matrix, the measured TARC can be got by Eq. (3). Fig. 9 shows the TARC of the proposed MIMO. A high radiation efficiency (high TARC) is expected around the 2.5 GHz and 3.5 GHz band as Fig. 9 shows. The radiation patterns of the proposed MIMO at some key frequency (LTE/WiFi/WiMAX band) are shown in Fig. 10. The maximum gain which can be seen in Fig. 10 is more than 8 dB at 2.35 GHz and 2.4 GHz band, as well as 9 dB at 3.5 GHz band respectively.
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Fig. 9. The prototype of the MIMO and its total active refection coefficient (TARC in dB).
Fig. 11. The correlation coefficient.
4. Conclusion A novel eight channels dual band MIMO antenna which can be used for LTE, WiMAX and the low frequency band of WiFi indoor base station has been designed. It consists of four E-shaped patch antennas, wherein each has two ports corresponding to two different operating frequency bands. The results show that the proposed MIMO antenna’s gain is more than 8 dB at 2.4 GHz and 9 dB at 3.5 GHz respectively. Acknowledgment We would like to thank anonymous reviewers. This work was supported by the National Natural Science Foundation of China (No. 61201095) and the China Postdoctoral Science Foundation (2014M562299) as well as the Sichuan Science and Technology Support Program (2015GZ0282) and the Sichuan Education Department Research Projects (13ZB0076). References Fig. 10. Radiation patterns of the eight ports MIMO.
The correlation coefficient can be calculated by S-parameters [32] e =
∗ S ∗ 2 |S11 12 + S21 S22 |
(1 − |S11
|2
− |S21 |2 )(1 − |S22 |2 − |S12 |2 )
,
(4)
The simulated correlation coefficients of the proposed fourelement E-shaped array are given in Fig. 11.
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