Low-profile closely coupled dual-band MIMO antenna using double layer metasurface

Abstract

A two-element low-profile closely coupled dual-band MIMO antenna is demonstrated for WiMAX applications. Based on the principle of metasurface (MTS) decoupling, a double-layer MTS consisting of pairs of elliptic patches with two different sizes is proposed. The MTS is loaded above a coupled dual-band MIMO antenna, and the mutual coupling in the lower and upper band is reduced by the larger and smaller elliptic patches, respectively. The edge-to-edge distance of antenna elements is only 0.01λ₀ (λ₀ is the free-space wavelength at 2.6 GHz). The measured results show that the working bandwidths of the MIMO antenna are 2.5–2.69 and 3.4–3.69 GHz. The −10 dB impedance bandwidths in two bands are 8.83% (2.49–2.72 GHz) and 8.50% (3.38–3.68 GHz), and the isolation between antenna elements is enhanced by 13.5 and 18.4 dB in two bands, respectively. Moreover, broadside radiation performances in two bands are obtained.

Introduction

With the rapid development of mobile communication technology, multi-band antenna system becomes the main development trend due to its high data rate and channel capacity. Multiple-input-multiple-output (MIMO), as a key technique to improve channel capacity, has been widely used in wireless communication systems. The mutual coupling between the MIMO antenna elements degrades the system performance. Traditionally, there has been many methods for reducing the mutual coupling between multi-band MIMO antenna elements, such as decoupling network [1, 2], defective ground structure (DGS) [3] and electromagnetic band-gap (EBG) [4, 5]. In [2], a decoupling network composed of a pair of open-loop square ring resonators is introduced for the decoupling of dual-band antennas. In [3], a DGS composed of a slot with two meander lines inside is proposed to improve isolation of a MIMO antenna. In [5], a double-ring mushroom-like EBG is employed to achieve dual-band decoupling. However, the antennas in [1–5] still suffer the disadvantage of complex structure, low radiation efficiency and large edge-to-edge distance.

With the continuing pursuit of the miniaturization of a system platform, the antenna elements are closer to each other. However, the closely coupling between the elements leads to the reduction of the channel capacity and efficiency. In recent years, MTS has been widely used in closely coupled antenna decoupling, due to its strong electromagnetic wave regulation characteristics. Several closely coupled single-band MIMO antennas based on MTS decoupling have been reported. Wang et al. propose a MTS consisting of split ring resonators to enhance the isolation of the antenna more than 20 dB [6, 7]. In [8], a MTS formed by periodic ring slots is employed to achieve single-band decoupling more than 40 dB. In [9, 10], a double-layer MTS composed of rectangular patches is loaded to improve the isolation of antenna elements by 15 dB. Recently, some closely coupled dual-band MIMO antennas based on MTS decoupling have been reported. In [11], an array-antenna decoupling surface (ADS) consisting of compact square patches is adopted to reduce mutual coupling between the dual-band array antenna by 10 and 20.4 dB. In [12], a modified ADS is proposed to enhance the isolation by 28.5 dB in the lower band and a H-shaped DGS is loaded to enhance the isolation by 19.4 dB in the upper band. In [13], a double layer MTS formed by rectangular strips with different lengths is employed to improve the isolation of the dual-band MIMO antenna by 20 and 11 dB in two bands. For the aforementioned antennas, the lowest profile of the closely coupled dual-band antennas is 0.27λ ₀, which still suffers the disadvantage of high profile. Therefore, it is a great challenge to achieve low profile characteristic for closely coupled dual-band MIMO antennas based on MTS decoupling.

In this paper, a low-profile closely coupled dual-band MIMO antenna is proposed using MTS decoupling. Based on the principle of MTS decoupling, a double-layer MTS composed of pairs of elliptic patches with two different sizes is proposed. The MTS is loaded above a coupled dual-band MIMO antenna, and the mutual coupling in the lower and upper band is reduced by the larger and smaller elliptic patches, respectively. The edge-to-edge distance of antenna elements is only 0.01λ ₀ and the profile height is 0.09λ ₀. The measurement results show that the MIMO antenna can work in 2.5–2.69 and 3.4–3.69 GHz, and the isolation is enhanced by 13.5 and 18.4 dB in two bands, respectively. Moreover, broadside radiation performances in two bands are obtained.

Dual-band MIMO antenna based on MTS

Principle of MTS decoupling

A two-element MIMO antenna can be equivalent to a two-port network, and its relationship of input and output voltages is expressed by S-parameters [14]:

(1)$$\left[{\matrix{ {V_1^{\rm o} } \cr {V_2^{\rm o} } \cr } } \right] = \left[{\matrix{ {S_{11}} & {S_{12}} \cr {S_{21}} & {S_{22}} \cr } } \right]\left[{\matrix{ {V_1^{\rm i} } \cr {V_2^{\rm i} } \cr } } \right]$$

where S ₁₁ and S ₂₂ are the reflection coefficients of two ports, S ₁₂ and S ₂₁ are the coupling coefficients, V ₁ⁱ, V ₂ⁱ, V ₁^o and V ₂^o are the input and output voltages of two ports, respectively. When the network is lossless, reciprocal and matched, formula (1) can be expressed as:

(2)$$\left[{\matrix{ {V_1^{\rm o} } \cr {V_2^{\rm o} } \cr } } \right] = \left[{\matrix{ 0 & {S_{12}} \cr {S_{21}} & 0 \cr } } \right]\left[{\matrix{ {V_1^{\rm i} } \cr {V_2^{\rm i} } \cr } } \right]$$

When S ₁₂ = S ₂₁, the mutual coupling between two antenna elements will disappear, and the mutual impedances Z ₁₂ and Z ₂₁ should be reactive, thus the single negative metamaterial can be employed to reduce mutual coupling.

The principle of MTS decoupling is shown Fig. 1. A two-element MIMO antenna is arranged along the x-axis, and a single negative MTS is placed above the antenna, when the Ant.1 is excited and the Ant.2 is terminated with a 50 Ω matched load. The coupled electromagnetic waves propagate along the positive x-direction. When loading a single negative metamaterial (ɛ _r < 0, μ _r > 0 or ɛ _r > 0, μ _r < 0), the wavenumber k can be expressed as [6]:

(3)$$k = w\sqrt {\mu \cdot \varepsilon } = \omega \cdot \sqrt {\varepsilon _{\rm r}\varepsilon _0\cdot \mu _{\rm r}\mu _0} = jk_0\cdot \sqrt {\vert {\mu_{\rm r}} \vert \cdot \vert {\varepsilon_{\rm r}} \vert \;} $$

Fig. 1. Principle of MTS decoupling.

The electric field along the positive x-direction can be expressed as:

(4)$$E( {x, \;t} ) = E_0e^{\,jkx}e^{\,jwt} = E_0e^{{-}k_0\sqrt {\vert {\mu_{\rm r}} \vert \vert {\varepsilon_{\rm r}} \vert } \cdot x}\cdot e^{\,jwt}$$

Therefore, the coupled electric field along the x-axis is evanescent, and the electromagnetic wave mainly propagates along the z-direction. In a word, the mutual coupling between antenna elements can effectively reduce by loading the single negative MTS.

MTS unit cell

Figure 2 plots the configuration of an unsymmetrical double-layer MTS unit cell. It consists of pairs of elliptic patches with two different sizes and is printed on the F4B substrate with a dielectric contains of 2.2, a loss tangent of 0.002 and a thickness of 1 mm. The master-slave boundaries are assigned on the sidewalls, and a pair of Floquet ports are applied on the x–y plane, as plotted in Fig. 2(a). The reflection and transmission coefficients of the unsymmetrical double-layer MTS unit cell are simulated with ANSYS HFSS. As shown in Fig. 2(b), the reflective type MTS unit cell operates at 2.6 and 3.5 GHz, and the electromagnetic waves can radiate effectively due to the anisotropic nature of the MTS [10]. The permittivity ɛ and permeability μ of the MTS unit cell are extracted using the standard parameter retrieval method [15] and the inversion algorithm [16] programmed by MATLAB in Appendix. Figure 2(c) plots the extracted ɛ and μ of the unit cell illuminated by y-polarized plane wave, it is observed that the results of two algorithms agree well. The ɛ and μ are positive and negative around 2.6 GHz while negative and positive around 3.5 GHz. Besides, the extracted permittivity has a spike at 3.2 GHz due to the coupling between the elliptic patches with different sizes in different layers.

Fig. 2. MTS unit cell, (a) simulation model; (b) S-parameters; (c) permittivity and permeability.

Closely coupled dual-band MIMO antenna based on MTS

The configuration of the closely coupled dual-band MIMO antenna is shown in Fig. 3. A double-layer MTS formed by the proposed unit cells is loaded above a two-port coupled dual-band MIMO antenna to reduce the mutual coupling between the antenna elements. A pair of open slots is etched on the antenna element to achieve dual-band and two C-shape slots with different lengths are etched to increase the matching bandwidths in the lower and upper band, respectively. Two rows of the MTS unit cells are chosen to obtain better decoupling performance, and the distance between the MTS and the coupled antenna is determined by the theory of Fabry–Perot (F–P) cavity [17]. The coupled MIMO antenna is fabricated on the F4B substrate with a dielectric contains of 2.2 and a loss tangent of 0.002. The edge-to-edge distance of two antenna elements is 1 mm (0.01λ ₀ at 2.6 GHz). All analysis and simulations are carried out by HFSS 13.0, and the dimensions are listed in Table 1. Figure 4 shows the simulated S-parameters of the antenna with/without MTS. It can be seen that, the bandwidth in the lower band is increased from 4.26% (2.53–2.64 GHz) to 8.43% (2.5–2.72 GHz) and that in the upper band is increased from 4.62% (3.38–3.54 GHz) to 8.18% (3.4–3.69 GHz). Also, after loading the MTS, the isolation in two bands is improved by 12.5 and 15.4 dB, respectively.

Fig. 3. Configuration of antenna structure, (a) side view; (b) top view of MTS; (c) top view of the coupled MIMO antenna.

Fig. 4. S-parameters of antenna with/without MTS.

Table 1. Dimensions of antenna structure (Unit: mm)

A thinner substrate minimizes the surface waves and the mutual coupling between the closely coupled antennas is mainly due to the near-field space waves. The surface current distribution for antenna with/without MTS is investigated to illustrate the working mechanism of the MTS for mutual coupling reduction. Figure 5 shows the surface current distribution of the antenna when the Ant.1 is excited and the Ant.2 is terminated by a 50 Ω load. It is observed that without the MTS, a strong coupling current is distributed on the Ant. 2. However, after loading the MTS, the coupling current on the Ant. 2 at two bands is dramatically reduced as shown in Figs 5(c) and 5(d). In other words, the MTS has the favorable decoupling capability for a dual-band MIMO antenna.

Fig. 5. Surface current distribution of antenna, (a) at 2.6 GHz without MTS; (b) at 3.5 GHz without MTS; (c) at 2.6 GHz with MTS; (d) at 3.5 GHz with MTS.

Parametric analysis

The effect of the structural parameters on the antenna performance is conducted. It is found that the length of C-shape slots (ls₁ = w ₅ + 2 × l ₅ + 2 × w ₆, ls₂ = w ₃ + 2 × l ₃ + 2 × w ₄) and the distance between the MTS and coupled antenna (h _a) play a significant role.

Figure 6 shows the S-parameters for different ls₁ and ls₂. It is observed that the length of C-shape slots has significant effects on the impedance bandwidths and slight effects on the isolation. With the decrease of ls₁, the impedance bandwidth in the lower band decreases, while that in the upper band is insensitive to ls₁. When ls₂ decreases, the bandwidth in the lower band increases and that in the upper band increases significantly. As shown in Fig. 7, the resonance frequency and isolation are sensitive to h _a. With the decrease of h _a, the resonance in the lower band increases and the second resonance of the upper band increases significantly. In a word, the resonance frequency could be controlled by h _a. Meanwhile, the decoupling effect in two bands becomes poorer with the decrease of h _a. When ls₁ = 59.4 mm, ls₂ = 48 mm, h _a = 6 mm, the S ₁₁ of antenna covers 2.5–2.69 and 3.4–3.69 GHz, and the S ₂₁ is below −18 and −17 dB in two bands.

Fig. 6. S-parameters for different length of C-shaped slots, (a) l _s1; (b) l _s2.

Fig. 7. S-parameters for different h _a.

Experimental results

A prototype of the proposed closely couple dual-band MMO antenna is fabricated and measured. Figure 8 shows the photographs of the MIMO antenna. The S-parameter is measured by an Agilent N5221A vector network analyzer, and the radiation pattern is measured by a Lab-Volt 8092 antenna training and measuring system.

Fig. 8. Photograph of antenna, (a) MTS; (b) coupled MIMO antenna.

Figure 9 shows the measured and simulated S-parameters of the antenna. It is seen that the measured results agree basically with the simulated ones and the measured isolation is deeper than the simulated one. The simulated −10 dB impedance bandwidths of two bands are 8.43% (2.5–2.72 GHz) and 8.18% (3.4–3.69 GHz), and the corresponding isolation between antenna elements is improved to 18 and 17 dB in two bands. The measured results show that the −10 dB impedance bandwidths of two bands are 8.83% (2.49–2.72 GHz) and 8.50% (3.38–3.68 GHz), and the isolation can be improved to 19 and 20 dB, respectively. The difference between the measurement and simulation results is mainly due to the fabrication errors and the deviation of dielectric constant.

Fig. 9. S-parameters of antenna.

Figures 10 and 11 show the simulated and measured patterns when the Ant.1 and Ant.2 are independently excited. It can be seen that a good agreement between the simulated and measured results is obtained. After loading the MTS, the dual-band MIMO antenna remains stable broadside radiation patterns except the cross-polarization are slightly changed. Meanwhile, the significant back radiation and the wider beamwidth with MTS is mainly because the MTS substrate is larger than the coupled antenna substrate. Figure 12 shows the simulated and measured gains. It is known that the MTS could be regarded as a superstrate to improve the gain of antenna, and the flat gain is obtained by forming a Fabry–Perot cavity [18]. The peak gains of the MIMO antenna with MTS are improved to 8.26 and 7 dBi in two bands, and the measured ones are 6.9 and 6.38 dBi, respectively.

Fig. 10. Radiation patterns when the Ant.1 is excited, (a) at 2.6 GHz; (b) at 3.5 GHz.

Fig. 11. Radiation patterns when the Ant.2 is excited, (a) at 2.6 GHz; (b) at 3.5 GHz.

Fig. 12. Gain of antenna.

Finally, Table 2 lists the comparison of the reported and proposed closely couple dual-band antennas based on MTS. It is observed that the proposed antenna has the lowest profile. Compared to the antennas with smaller bandwidths [11, 12], the antenna in this work has the smallest edge-to-edge distance. Moreover, the proposed antenna has bigger bandwidth in the upper band compared to the antenna in [13].

Table 2. Performance comparison of dual-band antennas based on MTS decoupling

Conclusion

This paper presents a low-profile closely coupled dual-band MIMO antenna for WiMAX applications. A double-layer MTS composed of pairs of elliptic patches is loaded above the dual-band coupled antenna to reduce the mutual coupling. The edge-to-edge distance between antenna elements is only 1 mm (0.01λ ₀) and the profile height is 10 mm (0.09λ ₀). The measured results show that the −10 dB impedance bandwidths of the dual-band MIMO antenna are 8.83 and 8.50%, and the isolation in two bands is improved by 13.5 and 18.4 dB, respectively. Also, the proposed antenna has potential to decouple dual-band antenna array with more elements.