An Ultra-Wideband Microstrip MIMO Antenna with EBG Loading for WLAN and Sub-6G Applications

TL;DR

This paper introduces an ultra-wideband MIMO antenna with EBG loading designed for WLAN and Sub-6G applications, achieving high isolation and low correlation over 2.4-6.5 GHz.

Contribution

It presents a novel microstrip MIMO antenna with EBG structures that significantly improves isolation and performance across the targeted frequency bands.

Findings

Return loss covers 2.4-6.5 GHz with S11 under -10 dB.

Isolation of about 20 dB across the bands.

Envelope correlation coefficient under 0.02.

Abstract

This manuscript presents an ultra-wideband Microstrip multiple-input-multiple-output (MIMO) antenna covering the 2.4GHz to 6.5 GHz wireless local networks (WLAN) and Sub-6G bands. The applied MIMO antenna is composed of two symmetrical microstrip antenna elements. Each microstrip is designed based on the stepped impedance resonator (SIR) technique and closely positioned with about 0.25lambda of the lower band (2.5 GHz). The proposed MIMO antenna is incorporated with the EBG structure to enhance the radiation characteristic and isolation between two radiating components. The return loss (S11 under -10) covers 2.4 GHz to 6.5 GHz, and the proposed configuration obtains about 20 dB isolation within the 2.4 GHz to 6.5 GHz Sub-6G and WLAN bands, which shows an improvement compared to the initial design of the MIMO antenna. The envelope correlation coefficient (ECC) is under 0.02 over the frequency band. The simulation results offer that the proposed microstrip MIMO antenna system with the different levels of the EBG structure is suitable for WLAN and Sub-6G applications.

Tables

Table 1. TABLE I: Dimensions of the original antenna units: m ​ m 𝑚 𝑚 mm

Variable	Value	Variable	Value	Variable	Value
$l_1$	17.5	$l_7$	4	$w_5$	2.5
$l_2$	2	$l_p$	16.3	$w_6$	0.5
$l_3$	2.5	$w_1$	1.07	$w_7$	0.5
$l_4$	2.5	$w_2$	0.4	$w_8$	0.5
$l_5$	4	$w_3$	2.8
$l_6$	4	$w_4$	2.265

Table 2. TABLE II: Dimensions of the EBG’s unit cell: m ​ m 𝑚 𝑚 mm

Variable	$a$	$b$	$t_1$	$t_2$
Value	$4$	$2$	$0.5$	$0.4$

Equations

$$\rho_{e}=\frac{\Big{|}S^{*}_{11}S_{21}+S^{*}_{22}S_{12}\Big{|}^{2}}{\Big{|}(1-|S_{11}|^{2}-|S_{21}|^{2})(1-|S_{22}|^{2}-|S_{12}|^{2})\Big{|}}$$

Full text

I Introduction

Multiple-input-multiple-output (MIMO) radio frequency (RF) architectures that incorporate IEEE standards 802.11a and 802.11b, can offer high data rates and high reliability by delivering higher receive gains and network capacities. It can address higher-speed internet services for handheld computers and smartphone users, high level of throughput, and good directivity for the local wireless networks (WLAN). Hence, many communication systems, such as 4G/5G technologies and WiFi systems, have included the MIMO technology in the system specifications.

A crucial challenge in designing the MIMO WLAN system is fabricating compact, low-cost, relatively simple multiband, and broadband antennas whilst maintaining: $i)$ high isolation or interference between radiation elements, and $ii)$ low envelope correlation. In recent years, various approaches, such as space/polarization/pattern diversity [1, 2, 3], electromagnetic bandgap (EBG) structure [4], T-slots plus a meander-line resonance [5], passive resonator, T-shape or L-shape ground branches [6], and parasitic elements, to name a few, have been reported for reducing the mutual couplings in the design of MIMO antennas.

One solution to reduce the mutual coupling is using dual-band decoupling networks like neutralization lines [7], T-stub circuits [8], and lumped components [9] by placing them between the radiating elements, which increases the complexity of the antenna. Ground plane structure (DGS) is another newly introduced method for coupling reduction between the antenna’s radiating elements [10, 11]. In this method, the bottom layer of the antenna contains the DGS etched out from the ground plane. High isolation, however narrowband in the desired frequency band, is reported by employing this method. In [5], a meandering resonant branch and an inverted T-shaped slot as a parasitic element are used for the higher and lower bands to achieve high isolation properties.

In this letter, a single microstrip antenna is designed using the SIR technique. Next, two single antennas with bend transmission lines are united to construct the MIMO antenna. Later, a new design that makes use of a metamaterial-inspired EBG is proposed to improve the mutual coupling between the radiating elements, and the radiation characteristics. This MIMO antenna is operating in the $2.4\sim 6.5$ GHz WLAN frequency range. The detail of the proposed antenna is discussed in the following.

II Antenna Design

The single antenna’s configuration used to construct the two ports MIMO antenna is shown in Fig. 1. The design is imprinted on a Rogers 4350 substrate with a dielectric constant of 3.66, a loss tangent of 0.004, and a thickness of $0.508\text{\,}\mathrm{m}\mathrm{m}$, with a width and length of $36\text{\,}\mathrm{m}\mathrm{m}$ ($W$)\times$$2\text{\,}\mathrm{m}\mathrm{m} ($L$). The antenna is coaxially fed, and the optimized antenna dimensions are listed in Table I. A stepped impedance resonator (SIR) structure is employed here to design the radiating element. It implies cascading high-impedance and low-impedance transmission lines from the feed line to the radiate microstrip [12]. Therefore, a wider bandwidth can be achieved.

The preliminary design of the shown microstrip antenna has been evaluated and optimized by using the commercial software CST Studio Suite 2022. The simulated return loss in dB of the initial design of the single antenna is shown in Fig. 2. It can be seen that the return loss is lower than $-10\text{\,}\mathrm{d}\mathrm{B}$ from $2.5\sim 6.5$ GHz, as expected by employing the SIR structure that delivers an ultra-wideband performance. The radiation patterns of the simulated antenna are given in Fig. 3 in vertical and paralleled planes corresponding to the frequencies of $2.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$, $5.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$, respectively.

III Design of the Dual-Band MIMO Antenna

III-A Without the EBG Loading

The general schematic of the ultra-wideband MIMO antenna (Ant I) is shown in Fig. 4, in addition to the side view and back view. The distance between the two radiating elements is $0.25\lambda$ at the $2.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$ and $0.58\lambda$ at $5.8\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$. A microwave bend is used to place the coaxial feed at the side of the MIMO antenna. It is observed the small capacitance that is usually a result of bending the transmission line just slightly changes the antenna performance. The S-parameters of the MIMO antenna is represented in Fig. 5. The return loss is under $-10\text{\,}\mathrm{d}\mathrm{B}$ in $2.0\sim 3.07$ GHz, and $4.18\sim 6.5$ GHz. The isolation between two radiating elements is about $10\text{\,}\mathrm{d}\mathrm{B}$ in all frequency bands.

III-B With the EBG Loading

By adding EBG to the MIMO antenna, the structure shown in Fig. 6 is obtained. Fig. 6 (a) represents one row of the EBG loading (Ant II), and Fig. 6 represents three rows of the EBG loading (Ant II). The unit cell of the planar EBG employed here is semi-periodic metallic patches printed on the same side of the ground, however, it doesn’t cover all the width of the ground. The optimized EBG unit cell’s dimensions are listed in Table II.

The $S_{11}$ and $S_{21}$ of the first and second radiating elements of the Ant II with one row of EBG loading are illustrated in Fig. 7. The S-parameters of the MIMO antennas represent satisfactory behavior in the entire frequency band of $2.0\sim 6.5$ GHz, compared to the Ant I without the EBG loading, which can cover WiMax ($2.5/3.5/5.5$ GHz), WLAN ($5.2/5.8$-GHz), Sub-6G, as shown in Fig. 7. The isolation between two ports of the MIMO antenna is about $13\text{\,}\mathrm{d}\mathrm{B}$ in almost all frequency bands which shows $3\text{\,}\mathrm{d}\mathrm{B}$ improvement compared to the Ant I.

In order to find the influences on the resonant frequencies and bandwidths of corresponding structural parameters of the EBG, a parametric study has been fulfilled. By increasing the width of the single cell of the EBG, and keeping constant the height of that, the resonance frequency of the lower band ($2.4\sim 3.0$ GHz) almost remains constant. However, the resonance frequency of the higher band ($5.0\sim 6.5$ GHz) decreases and gets wider. The isolation ($S_{21}$) between two ports also increases by increasing the width of the unit cell. By increasing the length of the unit cell in a fixed width, the resonance frequency in the lower band increases, and the resonance frequency of the higher band decreases and gets wider.

The isolation between two ports of the previously introduced MIMO antenna can be further improved. In this respect, two more rows of semi-periodic EBG loading are added to the ground of the antenna (Ant III), as shown in Fig. 6 (b). Integrating more rows of the EBG loading (in both $x$ and $y$ direction) into the MIMO antenna will curb the current distribution between two transmission lines and result in improving the isolation. The S-parameters of the first and second radiating elements are shown in Fig. 8. Compared to the S-parameters of the Ant II with one row of EBG, $6\text{\,}\mathrm{d}\mathrm{B}$ improvement at $2.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$, and $8\text{\,}\mathrm{d}\mathrm{B}$ improvement at $5.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$ can be obtained. However, the return loss in structure (b) is under $-10\text{\,}\mathrm{d}\mathrm{B}$ from $2.4\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$ rather than $2\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$ in structure (a), which still is covering the mentioned applications’ operating frequencies.

The surface current of the proposed MIMO antenna with/without EBG at the frequencies of $2.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$ and $5.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$ are shown in Fig. 9, and 10, 11, to better compare the effect of the EBG. The figures depict that the coupling at the $2.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$ and $5.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$ bands have been interrupted by using the EBG. Moreover, an improvement can be observed in the radiating. Increasing the EBG loading in $y$-direction has led to improve the isolation as shown in Fig. 11.

To better represent the working mechanism of the proposed MIMO antenna, the radiation patterns of the simulated Ant II are given in Fig. 12 in vertical and paralleled planes corresponding to the operating frequencies of $2.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$, and $5.5\text{\,}\mathrm{G}\mathrm{H}\mathrm{z}$, respectively. It can be seen that the patterns are quasi-omnidirectional, which is suitable for wireless communication terminals to receive signals from any direction.

In the MIMO communication system, the ECC parameter represents how much the channel is uncorrelated to obtain if the designed antenna is appropriate for the MIMO application. Here, we use the S-parameter method to calculate the ECC of the designed MIMO antenna. Less correlation (more channel capacity) between the MIMO antenna’s elements can be found by lowering the value of the ECC. The ECC is calculated as follows

[TABLE]

The ECC of the proposed MIMO antennas with and without the EBG is illustrated in Fig. 13. As can be seen from 13, the ECC of Ant II and Ant III is lower than 0.02 at $2.0\sim 6.5$ GHz, which means a good MIMO performance. However, the ECC of Ant III with more EBG loading is clearly better than Ant II without one row of EBG.

IV Conclusion

In this manuscript, an ultra-wideband microstrip antenna with integrated EBG loading operating at $2.0\sim 6.5$ GHz WLAN and Sub-6G bands is designed and analyzed. Two microstrip antennas, designed using the SIR technique, were unified to construct two ports MIMO antenna. The isolation of the proposed MIMO antenna has been improved to $20\text{\,}\mathrm{d}\mathrm{B}$ by employing EBG loading at the ground plane. The MIMO antenna represents a quasi-omnidirectional pattern, which is suitable for wireless communication. The proposed MIMO antenna has a compact size, lower envelope correlation coefficient, and low cost and can be used in different wireless devices.

Acknowledgment