Broadband Compact MIMO Antenna Employing CRLH Transmission Lines with High Isolation

A broadband and planar multiple-input multiple-output (MIMO) antenna with pattern diversity is presented in this letter. e four-port MIMO antenna is designed by using a novel metamaterial structure with small dimensions of 60× 60×1.6mm. Each radiating element of the MIMO antenna is comprised of composite right/left-hand (CRLH) transmission lines (TLs). Between its four elements, there are four open L-shaped slot loading on the ground plane of the patch antenna system.e antenna obtains the wide frequency range of 2.31–4.99GHz. In addition, e proposed antenna has an isolation greater than 20 dB between its four elements. e MIMO antenna has excellent bandwidth performance and good diversity properties between elements.


Introduction
Design of MIMO antennas encounters many challenges, such as miniaturization and high isolation. Unlike the traditional MIMO antennas, the metamaterial MIMO antennas have many advantages of electrically small size, high isolation, low pro le, and broadband width. e development of high speed, high capacity, energy saving, and emission reduction in the global mobile communication network has led to the characteristics of miniaturization, broadband, and integration on the development trend of mobile communication antenna. By multiple antennas, the multiple-input multipleoutput (MIMO) technology is used to suppress multipath channel fading, improve system capacity, and reduce channel error rate. To set up multiple antennas on platforms with limited mobile terminals, it requires miniaturization of antennas and small cell spacing. e research of comb-line inductor, complementary open resonant ring and slot capacitor provides new ideas for miniaturization, high isolation, wide bandwidth, and low mutual coupling of MIMO antenna elements. e high isolation of 5G smart phone antenna unit is realized by a comb-line structure.
Metamaterials with negative permeability and permittivity can be used to design subwavelength cavity resonators, which miniaturizes the antenna [1]. Itoh reports the basic principles of compound left/right-handed (CRLH) transmission lines. e broadband and miniaturization of antenna unit is realized by CRLH transmission line structure with tunable radiation angle [2]. A MIMO antenna using slotted-complementary split-ring resonators can minimize mutual coupling between two coplanar microstrip antennas [3]. A planar compact metamaterial-substrate antenna array based on the resonator structures is presented [4]. e monopole antenna loading on metamaterial can e ectively control electromagnetic wave propagation [5]. A compact wideband MIMO antenna loaded a split-ring resonator (SRR) with the orthogonal arrangement, having a good isolation and low correlation diversity performance between elements, is presented [6]. Based on the theory of characteristic modes (TCM), a planar UWB mobile MIMO antenna is able to excite di erent modes, which realize the desired diverse radiation patterns and high isolation [7]. e mushroom body metamaterial dielectric structure is used to reduce the space size of base station antenna and isolation can be improved by introducing planar-modi ed mushroom structures [8]. e electromagnetic bandgap structures and metasurfaces can enhance the isolation [9,10]. A four-element MIMO antenna system loading complementary split-ring resonator (CSRR) on its ground plane has highly compact patch elements [11]. e parasitic elements are introduced to reduce mutual coupling [12,13]. e defected ground structures (DGS) with the opposite original characteristic modes currents blocks the coupling modes, thus reducing mutual coupling and improving port isolation [14]. However, this antenna requires a large overall antenna size and the antenna gain is approximate 0 dB. By self-cancellation of the near-field currents and induced ground, the mutual coupling of two close metamaterial-inspired antennas is reduced [15]. A pattern-diversity-based method is proposed to reduce mutual coupling [16,17]. DGS can be introduced to 5G MIMO antenna [18,19].

International Journal of Antennas and Propagation
Based on the CRLH-TL concept, the metamaterial MIMO antenna of this project has a smaller space size, wider bandwidth, smaller coupling between units, which can support high data transmission rate, high frequency spectrum utilization, and greater flexibility. Unlike the existing mobile terminal antenna mentioned above, the ground and inverted-F patch form a CRLH-TLs resonance. It should be pointed out that the pattern diversity method can excite different characteristic modes to obtain better isolation. A metal patch embedded with two interdigital capacitors acts as the main radiator. A small comb strip is placed between the microstrip line and ground. Four same antenna elements are placed on the orthogonal edges of a rectangular ground plane to meet the MIMO

Antenna Design
e structure of the mobile MIMO antenna is shown in Figure 1. e MIMO antenna is composed of three parts: four inverted-F radiation elements, four feed ports, and a coplanar ground plane. e antenna is printed on the FR4 substrate with a dielectric constant of 4.4, a thickness of 1.6 mm, and a loss tangent of 0.025. e thickness of covered copper layers is 35 um. A set of interdigital gap is etched on between the host inverted-F patch and the coplanar ground, which forms left-handed series capacitance. e left-handed shunt inductance is resulted from the small comb strip. e    left-handed series capacitance and the left-handed shunt inductance can provide a via-free left-handed transmission line, which can excite the backward radiating wave. At the same time, the series inductance L R is formed from the current flow on the inverted-F patch. e shunt capacitance C R results from the electric field between the inverted-F patch and coplanar ground. e series inductance and the shunt capacitance compose a right-handed transmission line which can induce a forward radiating wave. e physical dimensions of optimized parameters are summarized in Table 1.
An equivalent circuit model for the via-free CRLH is shown in Figure 2. A small comb strip is placed between the feed point and the coplanar ground plane. e inverted-F patch and the coplanar ground form a CRLH-TLs resonant cavity. Because the phase of backward wave is complementary, the zeroth-order resonance (ZOR) is inspired. e size of four element antenna is effectively decreased, which can realize a subwavelength resonant cavity.
According to the Bloch-Floquet theorem and the equivalent circuit diagram of the CRLH-TLs resonant cavity [1], the dispersion relation can be calculated as follows: e CRLH-TLs subwavelength resonance has many resonant frequencies with the phase constant of electromagnetic waves β � 0. An subwavelength resonance occurs when β n � n π l n � 0, ± 1, . . . , ± (N − 1), where n is the mode numbers and l is the electrical length of the CRLH-TLs resonator. Because of the novel CRLH-TL subwavelength resonance, the antenna effective length has been greatly reduced. e antenna bandwidth has been broadened simultaneously. e S-parameter simulation and measurement are convenient, and the transfer matrix is easy to be converted into parameters. It is easy to judge the characteristics of the transfer matrix by using the S-parameter simulation and measured values. e dispersion diagram of the CRLH TL is obtained using (3).
e dispersion diagram of the CRLH-TL antenna is plotted in Figure 3. e ZOR occurs at 3.46 GHz, where the value of βΔx � 0. When the frequency is low, it shows lefthanded characteristics. e phase velocity in the transmission line is negative and there is backward wave transmission. When the frequency is high, it shows the right-hand characteristics. e phase velocity in the transmission line is positive and there is forward wave transmission. Owing to infinite wavelength of the ZOR frequency, the design and fabrication dimensions are particularly compact. e fabricated MIMO antenna is shown in Figure 4. Four SMA connectors are used to feed to the antenna from the bottom.

Simulation and Experimental Results
To verify the simulations and optimizations, the planar MIMO antenna has been performed using HFSS16 Microwave Studio simulation tool. e effects of the -10 dB impedance bandwidth are presented in Figures 5 and 6. e effects of different width L8 parameters on the impedance bandwidth of the MIMO antenna are illustrated in Figure 5. As shown in figure 5, the resonance frequencies shift ward with the decrease of L8. With the decrease of L8, the bandwidth becomes wider and the resonance of the intermediate frequency part becomes deeper.
It is observed in Figure 6 that the resonance frequencies shift downward with the increase of W7. With the increase of W7, the bandwidth becomes wider. e impedance matching becomes good and the bandwidth is gradually widen. e S parameters of the four-port MIMO antenna is measured by using a network analyzer Agilent E8361 A as shown in Figure 7. e simulated −10 dB bandwidth is 2.69 GHz (2.28-4.97 GHz). e measured −10 dB bandwidth is 2.68 GHz (2.31-4.99 GHz). e fractional bandwidth for the four ports is as much as 73.8%. e simulated and measured results agree well.
S parameters S 21 , S 31 , and S 41 of the MIMO antennas are presented in Figure 8, respectively. Because of the symmetrical structure, S 21 and S 41 are almost identical. e isolation without decoupled structure is relatively low.
To improve the isolation, the defected ground structures (DGS) are introduced to block the coupling modes at certain locations. DGS can significantly affect the surface current of the chassis and the noncoupling modes. Figure 9 shows the simulated and measured isolation performance of the MIMO antenna with DGS. As can be seen from figure  9, the measured values of S 21 , S 31 , and S 41 are basically consistent with the simulated values. Compared with S 21 and S 41 , S 31 is smaller due to the long distance. As can be seen from figure 9, the MIMO antenna with decoupling structure has a small value of S 21 , S 31 , and S 41 . It is already obvious that the all S 21 , S 31 , and S 41 values are less than −20 dB at entire operating impedance bandwidth. In the entire operating band, the MIMO antenna has a low envelope co-relation coefficient (ECC<0.003), a high diversity gain (DG > 9.999). e simulated electric field intensity in 3.63 GHz is analyzed in Figure 10. e electric field around the ground gap is stronger. e defected ground structures improve antenna isolation. e normalized measured gain patterns for the 4-element MIMO antenna at 3500 MHz are shown in Figure 11. e measured highest gain is 4.25 dBi. Figure 12 shows the measured and simulated radiation gain of the CRLH-TLs MIMO antenna. e total radiation efficiencies are above 75%. e peak efficiency is 90% at 3.8 GHz.  International Journal of Antennas and Propagation e proposed MIMO technique is compared with some previously presented antennas as shown in Table 2.
where λ 0 is the free space wavelength at the center resonant frequency.

Conclusions
In this letter, a novel broadband metamaterial MIMO antenna has been presented. Miniaturization and broadband are finally accomplished by introducing an organic integration of metamaterials, orthogonal structure, and separated four-directional L-shaped slot decoupling. e simulation and measurement results confirm that the MIMO antenna has excellent bandwidth performance. Both the simulated and measured results of S parameters demonstrate that the four-port MIMO antenna could offer an higher isolation of better than 20.0 dB in the operating range. With an overall dimension of 60 × 60 × 1.6 mm 3 , the bandwidth of the antenna is 2.68 GHz (2.31-4.99 GHz). e proposed MIMO antenna is a potential candidate in portable wireless routers for a 5G communication system. A metal inverted-F patch embedded with a set of interdigital gap acts as the main radiation element.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding this article.