## 3 Multiple-Input-Multiple-Output CP DRA  
In recent years, multiple-input-multiple-output (MIMO) technologies have been in continuous development for modern telecommunication systems such as for wireless local area networks (WLANs), long-term evolution (LTE), and fifth-generation (5G) communications. In a practical communication system, the space for the MIMO antenna is generally limited. Moreover, the mutual coupling between antennas will degrade the efficiency and performance of the MIMO system. It also can affect the antenna's radiation pattern. MIMO antennas designed through patch antennas tend to have lower efficiencies and insufficient isolation between unit antennas, rendering them unsuitable in most applications. Compact DRAs with high radiation efficiency will be much more favorable to combat the issue, observing no unwanted effects even in array configurations with better isolation. Numerous works on reducing mutual coupling between DRA elements have been reported, such as by using defected ground structures (DGS), electromagnetic band-gap (EBG) structure, metamaterials, and frequency selective surfaces (FSSs). DGS technique is applied in [63; 64] to control the impedance transmission capacity and reduce mutual coupling. However, the proposed antennas are LP and would suffer in antenna misalignment, thus degrading the antenna performance. In [65], an EBG structure was added between two CP DRAs to reduce the mutual coupling. The proposed antenna consists of a rectangular DR placed on top of the cross-ring slot, as shown in Figure 14. The two opposite corners of the DR are truncated at 45\({}^{\circ}\) and combined with a cross-ring coupling slot with an optimum ratio to obtain CP radiation with a wideband AR performance. The antenna is fed with a 50 \(\Omega\) microstrip feedline for good impedance matching. A total of 25.9% and 18.7% measured impedance and AR bandwidths, respectively, are recorded from a 3.3 to 3.8 GHz frequency range for 5G applications. As this design has decoupling structures placed between the antenna elements, the system complexity will be increased. In [66], an integrated four-element DR based on a MIMO polarization diversity antenna is demonstrated for vehicular communication. The proposed antenna comprises two conformal open-loop fed DRA elements (Ant. 1 and Ant. 2), and two microstrip-slot fed DRA elements (Ant. 3 and Ant. 4). As shown in Figure 15, Ant. 1 and Ant. 2 are located at the top of the substrate and placed diagonally. RHCP and LHCP waves are excited in Ant. 1 and Ant. 2, respectively, to establish polarization diversity. Similarly, Ant. 3 and Ant. 4 are positioned at the substrate's bottom side and generate LHCP and RHCP waves, respectively. Thus, it will create polarization diversity and reduce the interference between the field components. The proposed antenna obtains a 500 MHz measured impedance bandwidth from 3.22 to 3.72 GHz, and a 200 MHz measured AR bandwidth from 3.34 to 3.54 GHz. However, to meet the needs of a small size MIMO antenna, the antenna will suffer from a large mutual coupling effect and a reduction in the antenna performances. Figure 14: Proposed MIMO DRA: (**a**) front view; (**b**) fabricated antenna [65]. Metal was can also be used to decrease the mutual coupling between the DRA elements. In [67], four metal strips on the lateral sides of each DRA are introduced, as shown in Figure 16. With that, the rotation direction of the coupled E-field in the passive DRA can be changed and made orthogonal to that of the active DRA. Owing to the polarization orthogonality, the isolation between the two DRAs is enhanced to 31 dB at 2.43 GHz without affecting the reflection coefficient, radiation pattern, and AR. The measured impedance bandwidth of the proposed antenna is approximately 9.3% from 2.36 to 2.59 GHz, and the measured AR bandwidth is 4.9%, ranging from 2.39 to 2.51 GHz. The length of the orthogonal rectangular slots should be unequal to generate the CP field. Otherwise, the antenna will radiate in linear polarization.