Two-Port CPW-Fed Dual-Band MIMO Antenna for IEEE 802.11 a/b/g Applications

A coplanar waveguide(CPW-) fed dual-band multiple-input multiple-output (MIMO) antenna for 2.45/5.5GHz wireless local area network (WLAN) applications is presented in this paper. *e presented MIMO antenna consists of two identical trapezoidal radiating elements which are perpendicular to each other. *e size of the entire MIMO antenna is 50× 50×1.59mm, which is printed on a FR4 substrate. *e measured impedance bandwidth of the proposed antenna is 2.25–3.15GHz and 4.89–5.95GHz, 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.*emeasured peak gain is 5.59 dBi at 2.45GHz and 5.63 dBi at 5.5GHz.*e measured antenna efficiency is 77.8% and 80.4% in the lower and higher frequency bands, respectively. *e ECC values at the lower and higher frequencies are lower than 0.003 and 0.01, respectively.


Introduction
e 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][5][6][7][8][9][10][11][12][13][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.
ere 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][17][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. e 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. e presented antenna consists of two trapezoidal radiating elements, two rectangular slits, an L-shaped slit, and a defected ground plane. e 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.

Antenna Geometry and Design
e geometry of the proposed CPW-fed dual-band MIMO antenna is shown in Figure 1. e antenna is fabricated on a FR4 substrate with relative permittivity ε r of 4.4, loss tangent δ of 0.02, and thickness of 1.59 mm. e radiating elements, the defected ground, and the isolation structure are all printed on one side of the substrate. e photograph of the manufactured antenna is shown in Figure 2.
e two identical antenna elements are perpendicular to each other. e antenna elements are fed by coplanar waveguides. e width of the CPW signal line is 3 mm. e radiating element is composed of a trapezoidal patch with two rectangular slits, an L-shaped slit, and a semicircular slit. e L-shaped slit and rectangular slits can realize impedance matching at the lower and higher frequency bands, respectively.
e length of the ground plane is L 1 , and a rectangular stub with a length of W 8 and a width of L 5 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. e 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. erefore, 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 W 6 and W 14 , respectively. In Figure 4(a), it can be observed that the value of W 6 mainly affects the lower frequency band. Figure 4(b) shows the effect of different W 14 values on the higher frequencies. e optimum values of W 6 and W 14 are 2.1 mm and 7.2 mm, respectively. e strong isolation between the two antenna elements is mainly achieved by introducing rectangular stub and defected ground structure. e 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. e final dimensions of the proposed antenna are listed in Table 1. Figure 6 shows the simulated and measured S-parameter.

Results and Discussion
ere 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 S 11 less than −10 dB is 900 MHz (2.25-3.15 GHz) and 1060 MHz e 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. e measured peak gain and total efficiency are shown in Figure 8. e measured gain is 5.59 dBi at 2.45 GHz and 5.63 dBi at 5.5 GHz.
e measured antenna efficiency is 77.8% and 80.4% in the lower and higher frequency bands, respectively.

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 (ρ e ) can be calculated using radiation patterns of two antennas as follows [5]: where F i → (θ, ϕ) 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   International Journal of Antennas and Propagation 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. e 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. e performance of the actual MIMO antenna system will not be predicted only by S parameters, so another important parameter, TARC (Γ t a ), 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 as where 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. erefore, 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 as where ψ P is the element matrix, which provides the performance of the MIMO system. For a two-port antenna, ψ P can be expressed as Matrix elements can be obtained through S parameters: where i � 1, j � 2, and n � 1, 2. e 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  is observed that the proposed antenna has shown higher gain and efficiency. e proposed MIMO antenna supports dualfrequency operation and has low ECC, which can guarantee good MIMO performance.

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. e proposed antenna is manufactured on a FR4 substrate with the size of 50 × 50 × 1.59 mm 3 . Moreover, the proposed antenna exhibits good isolation performance and stable radiation patterns in the desired frequency bands. e proposed antenna is a suitable candidate for IEEE 802.11 a/b/g applications.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper. International Journal of Antennas and Propagation 7