## 2 Decoupling Techniques (DTs) in Ultra-Wideband MIMO Antennas (UMAs)
### Decoupling Structure (DS)  
In this DT, a conducting decoupling stub-like structure is attached to the CG between AEs to enhance isolation. In reference [50], the antenna involving an open annular slot (monopole-like) and two indistinguishable CPW strips ending in a U-shaped stub was proposed, as illustrated in Figure 4a. High isolation was accomplished using a diagonally integrated strip in the ground, which increases the current path between AEs. The ground plane was designed as an open annular-like slot, which decreased the size of the antenna. Likewise, an impedance transformer (one-stage) was fused into the CPW transmission line to improve matching. The feeding structure (U-shaped) gave a compelling coupling between the feeding and the slot. An arc-slot resonator was etched on the U-shaped stubs to obtain notch bands. In reference [51], the antenna contained two CPW-feed modified staircase radiating patches with orthogonal directed radiation patterns. A quarter circular slot was added to the ground surface, and a rectangular stub was located at 45\({}^{\circ}\) between two AEs, which not only extended the current path but also provided high isolation, as shown in Figure 4b. By adding two slots in the patch radiators, a band-notched performance was achieved at 5.5 GHz. In [52], each AE contained a half fan-shaped radiator with a complimentary cut in the ground, as presented in Figure 4c. The AEs were positioned orthogonally to obtain polarization diversity. In this design, high isolation was attained by adding an arrow-like  
Figure 3: AEs placed in 180\({}^{\circ}\) orientation: (**a**) [47], (**b**) [48], and (**c**) [49]. stub to the ground. A notched band for WLAN was designed by adding a bent slit in each radiator. In reference [53], the antenna was composed of dual identical CPW-fed rectangular radiators, as displayed in Figure 4d. To reduce mutual coupling as well as to expand bandwidth, a rectangular strip was added to the ground surface. To reject the WLAN band, an inverted U-slot was introduced in the feed line. In reference [54], the AE consisted of a circular radiator in which the eye slot was etched to achieve a better impedance bandwidth, as displayed in Figure 4e. The microstrip line feeding was connected at the lower ends of each radiator. The two AEs have a common rectangular-shaped ground located beneath the radiating AEs to maintain the compactness of the designed antenna. Furthermore, the ground was modified via the addition of a T-shaped stub to achieve better isolation between the two radiators. The antenna in [55] had a modified substrate geometry and was composed of a circular arc-shaped conductive element on top, as shown in Figure 4f. Polarization diversity and isolation were achieved by replicating the elements orthogonally, forming a plus-shaped structure. The modified ground plane consisted of an inverted L-shaped strip and a semi-ellipse slot over the partial ground that helped the antenna achieve wide bandwidth. In paper [56], the antenna consisted of four symmetric circular elements with a slotted ground plane, as shown in Figure 4g. The two sides of the substrate were symmetrical, and each side consisted of two radiators with partial ground planes associated with the two other elements mounted on the other side. The two AEs of the front side were orthogonal to the two other elements of the back side in order to increase the isolation between elements. For further reduction in the coupling between AEs, decoupling structures were presented in the top and bottom layers of the substrate. In [57], the MIMO antenna contained four monopole AEs with rectangular radiators, each fed via an L-shaped microstrip line, as displayed in Figure 4h. The high isolation was accomplished via the orthogonal direction of AEs and a cross-shaped DS. The designed antenna can realize WLAN and X-band NBs by etching two C-shaped slits from each radiator and Wi-MAX NB by introducing an L-shaped slit in the ground.