## 3 Ultra-Wideband (UWB) MIMO Antenna Designs
A compact wideband MIMO antenna has two identical monopole radiating components engraved on top of the substrate along with a GP including a flag-shaped stub and strips at the bottom [25]. UWB frequency is achieved by adding the slits and slots situated next to the flag-shaped stub. This antenna supports wireless systems such as LTE, WiMAX, WLAN, and HIPERLAN and runs on several standards. In [26], the neutralization line is used between the radiating elements to cancel the field between  
Figure 8: Simulated and measured S–parameters of the antenna [23]. Figure 7: Fabricated prototype of the structure [23]. the neighboring radiating components. The antenna impedance changes as a result of the field cancellation and the coupling between the antennas is decreased. To enhance impedance bandwidth, a UWB MIMO antenna implements two symmetric slot antenna components with quasi F-shaped radiators and L-shaped open-slots [27]. The currents flowing between the closely spaced patches are ejected through a tapered slot carved on the array GP, resulting in a novel antenna with a decoupling structure as suggested in [28]. The idea adopted in this design is to separate coupled currents by passing them through a tapered slot. In [29], the wideband operation is achieved by a central conjoined circular slot. In [30], a UWB MIMO design achieves better isolation without the need of any decoupling techniques taking full use of the monopole asymmetrical structure. In this design, wide bandwidth is achieved by using a radiating patch with a split form. To increase the isolation, two elements on the front side of the MIMO antenna are placed orthogonal to the back-side elements and a circular radiating patch provides good wideband operation [31]. In addition to that, decoupling structures are placed on the top and bottom layers of the substrate to further reduce the mutual coupling between the ports. To decrease mutual coupling, spatial diversity and polarization diversity techniques are applied and a staircase-shaped patch provides UWB frequency range [32]. In [33], four drop-shaped slot antennas are placed perpendicular to one another but in separate layers to limit the transmission of the surface current to the other radiators for improving the isolation. Seven vertical metallic barriers are placed on the front side of the antenna and two L-shaped stubs along with a vertical metallic barrier printed on the back side to strengthen the isolation between the two radiating elements [34]. In addition to that, a circular radiating patch with a small hemisphere on the top of the circle is utilized to achieve wider bandwidth. The introduction of a fence-type decoupling structure at the GP improves the isolation in the working band and the rectangular slot on the radiation patch enhances the resonance characteristics [35]. Moreover, two L-shaped parasitic branches are added on the surface of the dielectric substrate with the goal of improving the mutual coupling at lower frequency (3-3.4 GHz) and the impedance matching performance. In [36], a MIMO antenna is made up of two identical antenna elements that are symmetrically arranged in opposite directions on the same dielectric substrate. High isolation and good impedance matching are accomplished by loading three X-shaped stubs between two disconnected ground planes. In [37], the antenna is made up of four similar patches arranged in an orthogonal pattern. The total size of the antenna is reduced by using a rectangular GP below the radiating patch to produce a monopole design. The use of T- and C-shaped stubs/slots on the radiating patch reduces interfering bands for WiMAX, LTE43, and WLAN and achieve super wideband [38]. Dielectric resonator antennas (DRA) [39; 40; 41] can also be used to develop a dual-element wide band MIMO antenna system. The wide bandwidth is achieved by merging the two neighboring modes of a wideband A-shaped DRA stimulated by a simple rectangular conformal strip [39].