Abstract
A coplanar waveguide- (CPW-) fed dual-band multiple-input multiple-output (MIMO) antenna for 2.45/5.5 GHz wireless local area network (WLAN) applications is presented in this paper. The presented MIMO antenna consists of two identical trapezoidal radiating elements which are perpendicular to each other. The size of the entire MIMO antenna is 50 × 50 × 1.59 mm3, which is printed on a FR4 substrate. The measured impedance bandwidth of the proposed antenna is 2.25–3.15 GHz and 4.89–5.95 GHz, which can cover IEEE 802.11 a/b/g frequency bands. A rectangular microstrip stub is introduced to achieve a good isolation which is less than −15 dB in both operation frequency bands. The measured peak gain is 5.59 dBi at 2.45 GHz and 5.63 dBi at 5.5 GHz. The measured antenna efficiency is 77.8% and 80.4% in the lower and higher frequency bands, respectively. The ECC values at the lower and higher frequencies are lower than 0.003 and 0.01, respectively.
1. Introduction
The development of modern wireless communication systems requires high-speed and high-reliability data transmission. Due to the limitation of spectrum resources, there are higher requirements for wireless communication technology. Since there are several wireless communication systems in a mobile terminal, designing and placing many internal antennas in a limited space remains a big challenge. Dual-band antenna is one of the solutions of reducing the antenna numbers in a terminal. Many dual-band mobile terminal antenna structures have been reported, including hook-shaped strip structure [1], an open-loop ring resonator feeding element and a T-shaped radiating element [2], and substrate-integrated suspended line structure [3].
Multiple-input multiple-output antennas have been widely studied in recent years, which can increase the channel capacity and data transmission rate [4–14]. Due to the use of multi-port structures, placing many antenna elements close to each other would increase the mutual coupling between the elements and affect the performance of the MIMO antenna. A decoupling structure is needed to enhance the isolation between radiator elements of MIMO antenna. In recent years, many decoupling techniques have been proposed, including defected ground structure [4, 5], electromagnetic bandgap structure [6, 7], open-slot [8], π-shaped decoupling structure [9], and neutral line [10]. In [11], the stub-loaded meander-line antenna is loaded with split-ring resonators (SRRs) arranged in the form of a ring to enhance isolation by 17 dB at 2.4 GHz. In [12], by sharing one common grounding branch for the two adjacent antenna elements, a compact self-decoupled antenna pair can be obtained.
There are many technologies to implement dual-band MIMO antennas. For example, a microstrip line feed antenna with two quarter-wavelength slots is used as an antenna element that achieves high radiation efficiency under dual band [15]. In [5], a novel dual-band frequency reconfigurable MIMO patch-slot antenna based on a microstrip feedline is designed, which has a compact size and strong isolation. Inverted F structure is a good way to achieve dual-band MIMO antenna [16–18]. In [16], an inverted F-shaped radiator is used to realize dual-band MIMO antenna, which achieves better impedance matching by etching two U-shaped slits on a 50 Ω feeder. The coplanar waveguide (CPW) feed structure is a single-plane structure that can achieve a wider bandwidth. At present, many CPW-fed dual-band MIMO antennas have been proposed, such as circularly polarized planar slot antenna [19] and dual-polarized loop-slot planar antenna [20].
In this paper, a CPW-fed dual-band MIMO antenna suitable for WLAN applications is presented. The presented antenna consists of two trapezoidal radiating elements, two rectangular slits, an L-shaped slit, and a defected ground plane. The rectangular slits and L-shaped slit are used to obtain good impedance matching in the lower and higher frequency bands, respectively. Parametric study has been carried out to further investigate the mechanism of the presented antenna.
2. Antenna Geometry and Design
The geometry of the proposed CPW-fed dual-band MIMO antenna is shown in Figure 1. The antenna is fabricated on a FR4 substrate with relative permittivity εr of 4.4, loss tangent δ of 0.02, and thickness of 1.59 mm. The radiating elements, the defected ground, and the isolation structure are all printed on one side of the substrate. The photograph of the manufactured antenna is shown in Figure 2.
The two identical antenna elements are perpendicular to each other. The antenna elements are fed by coplanar waveguides. The width of the CPW signal line is 3 mm. The radiating element is composed of a trapezoidal patch with two rectangular slits, an L-shaped slit, and a semicircular slit. The L-shaped slit and rectangular slits can realize impedance matching at the lower and higher frequency bands, respectively. The length of the ground plane is L1, and a rectangular stub with a length of W8 and a width of L5 is introduced at the middle of the ground plane to improve the isolation between two radiating elements.
To investigate the influence of the critical parameters on the frequency bands of the proposed antenna, the parametric study is carried out by changing the values of antenna dimensions. The current distribution of the presented antenna in different frequency bands is shown in Figure 3. Most of the surface currents are concentrated on the L-shaped slit and vertical rectangular slit at 2.45 GHz. At 5.5 GHz, the currents mainly concentrate along two rectangular slits. Therefore, we can adjust the dimension of the rectangular slit and the L-shaped slit to optimize the key performance of the antenna. Figure 4 illustrates the reflection coefficients of the presented antenna with different values of W6 and W14, respectively. In Figure 4(a), it can be observed that the value of W6 mainly affects the lower frequency band. Figure 4(b) shows the effect of different W14 values on the higher frequencies. The optimum values of W6 and W14 are 2.1 mm and 7.2 mm, respectively.
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The strong isolation between the two antenna elements is mainly achieved by introducing rectangular stub and defected ground structure. The isolation performance of the MIMO antenna with and without rectangular stub is shown in Figure 5. It can be seen that the isolation is enhanced obviously with rectangle stub at the operating band. The final dimensions of the proposed antenna are listed in Table 1.
3. Results and Discussion
Figure 6 shows the simulated and measured S-parameter. There is a little difference between the measurement results and the simulation results, which may be caused by measurement errors. From the measurement results, we can see that the bandwidth with reflection coefficient S11 less than −10 dB is 900 MHz (2.25–3.15 GHz) and 1060 MHz (4.89–5.95 GHz). The realized two frequency bands can cover 802.11 a/b/g (WLAN, 2.4–2.4835 GHz, 5.15–5.875 GHz) frequency bands. The measured mutual coupling S21 between the two antenna elements is less than −15 dB in the desired frequency bands (2.4–2.4835 GHz, 5.15–5.875 GHz). The designed antenna shows good isolation performance in the desired frequency band.
Figure 7 shows the simulated and measured normalized E-plane and H-plane radiation patterns at 2.45 GHz and 5.5 GHz. Since the proposed antenna structure is symmetrical, one port is fixed and the other port is connected to a 50 Ω load during measurement.
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The measured peak gain and total efficiency are shown in Figure 8. The measured gain is 5.59 dBi at 2.45 GHz and 5.63 dBi at 5.5 GHz. The measured antenna efficiency is 77.8% and 80.4% in the lower and higher frequency bands, respectively.
4. MIMO Antenna Diversity Performance
Diversity performance of the designed MIMO antenna is mainly judged in terms of envelope correlation coefficient (ECC), diversity gain (DG), total active reflection coefficient (TARC), and channel capacity loss (CCL).
For MIMO antenna, the envelope correlation coefficient (ECC) can be used for diversity performance evaluation. ECC (ρe) can be calculated using radiation patterns of two antennas as follows [5]:where is the radiation patterns related to i antenna element, and another antenna is terminated by a matched load. Since the lower envelope correlation leads to a larger channel capacity, the ECC value is required to be less than 0.5. As we can observe from Figure 9, the ECC values at the lower and higher frequencies are lower than 0.003 and 0.01, respectively.
The diversity gain of the two-element MIMO antenna can be calculated as
Generally, the acceptable DG value should be close to 10 dB. As shown in Figure 9, the diversity gain of the proposed MIMO antenna in the desired frequency band is 9.99 and 9.95, respectively.
In a 2-port antenna system, adjacent antenna elements will interfere with each other when they work at the same time. The performance of the actual MIMO antenna system will not be predicted only by S parameters, so another important parameter, TARC (), is explored to verify the diversity performance of the MIMO antenna system. TARC provides information about the return loss of a MIMO antenna system, which can be calculated aswhere θ represents the phase difference between the excitation signals. For MIMO wireless communication system, the TARC should be less than 0 dB. As shown in Figure 10, the TARC value of the designed MIMO antenna is less than −20 dB in the desired frequency band. Therefore, the designed antenna has good diversity performance.
For MIMO antenna systems, the upper limit of the information transmission rate in the wireless communication channel can be judged by the channel capacity loss (CCL), and usually the CCL should be less than 0.5 bits/sec/Hz. CCL can be expressed aswhere ψP is the element matrix, which provides the performance of the MIMO system. For a two-port antenna, ψP can be expressed as
Matrix elements can be obtained through S parameters:where i = 1, j = 2, and n = 1, 2. The measured result of CCL is depicted in Figure 10. It can be seen that CCL has lower values of 0.2 and 0.1 at 2.45 GHz and 5.5 GHz.
Finally, Table 2 summarizes the performance comparison between the proposed MIMO antenna and the other reported antennas. From the comprehensive comparison, it is observed that the proposed antenna has shown higher gain and efficiency. The proposed MIMO antenna supports dual-frequency operation and has low ECC, which can guarantee good MIMO performance.
5. Conclusions
A novel coplanar waveguide- (CPW-) fed dual-band antenna is proposed. By embedding rectangular slits and L-shaped slit in two identical trapezoidal patch elements, the dual-frequency operation of the designed antenna can be realized. The proposed antenna is manufactured on a FR4 substrate with the size of 50 × 50 × 1.59 mm3. Moreover, the proposed antenna exhibits good isolation performance and stable radiation patterns in the desired frequency bands. The proposed antenna is a suitable candidate for IEEE 802.11 a/b/g applications.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgments
This study was supported in part by the National Natural Science Foundation of China under grant no. 61671330, the Science and Technology Department of Zhejiang Province under grant no. LGG19F010009, and the Wenzhou Municipal Science and Technology Program under grant nos. C20170005 and 2018ZG019.