## 3 Ultra-Wideband (UWB) MIMO Antenna Designs  
Ultra-wideband (UWB or digital pulse wireless) is a radio-based wireless communication technology that allows the transmission of a huge amount of digital data over a broad range of frequency bands with extremely little power over short distances. In this section, the different UWB MIMO antennas are reported which includes modification in the radiating patches, ground plane (GP) and isolation techniques (Figure 2). UWB technology offers a frequency range of 3.1 to 10.6 GHz with channel bandwidths of more than 500 MHz [16]. For the decoupling of a small UWB MIMO antenna, a broadband neutralized line is employed [17]. With a mutual coupling of less than \(-22\) dB, this UWB MIMO antenna has covered the lower UWB range of 3.1-5 GHz. MIMO designs must display wideband features to meet the demands of increased spectral efficiency,  
Figure 1: Categorization of MIMO antenna designs. provided that the reciprocal coupling between the radiating components is kept to a minimum [17; 18; 19]. The decoupling network using two inverters and two connected split-ring resonators (SRRs) in [20] provides excellent isolation between the ports. A four-element compact MIMO antenna with a total volume of 26 \(\times\) 26 \(\times\) 0.8 mm\({}^{3}\) and a 4 mm edge-to-edge distance between the radiating patches is reported in [21]. A groove is carved in the center of the GP (Figure 3) to prevent mutual coupling. The MIMO antenna functions with isolation of more than 15.4 dB in the frequency region from 5.6 to 5.8 GHz (Figure 4). The maximum achieved gain for this antenna is >1.41 dBi across the applicable band and can be implemented in portable devices for WLAN applications at 5.7 GHz. Figure 3: The fabricated prototype of 4 \(\times\) 4 MIMO antenna [21]. **(Left side: ground plane and **right side**: microstrip line). Figure 2: Typical UWB MIMO antenna design approaches. Figure 5 shows the structure of the 4 \(\times\) 4 UWB MIMO antenna as reported in [22]. The dimensions of the reported antenna is 48 \(\times\) 34 \(\times\) 1.6 mm\({}^{3}\). The MIMO design consists of the rectangular patch attached with a half-circular disc of radius 4.8 mm. The lower corners of the rectangular patch are carved with a small amount of copper to achieve UWB characteristics. The neutralization line connects the two symmetrical antennas to increase the isolation between the two radiating patches. The corresponding S-parameters are plotted and presented in Figure 6. The observed bandwidth and isolation of 2 \(\times\) 2 and 4 \(\times\) 4 MIMO antennas are 95.22% (3.51-9.89 GHz) and \(-\)24 dB, and 96.47% (3.52-10.08 GHz) and \(-\)23 dB, respectively. Figure 4: S–parameters of the MIMO antenna. — simulated and - - measured [21]. Figure 5: Fabricated 4 \(\times\) 4 MIMO antenna design [22]. Figure 6: S–parameters of the designed antenna [22]. In [23], a UWB MIMO antenna with defective ground structure (DGS), U-shaped branch step impedance stub, and slotted patch with dimensions of 80 \(\times\) 35 \(\times\) 0.508 mm\({}^{3}\) is reported and shown in Figure 7. The DGS improves the antenna bandwidth significantly. By adding U-shaped branches, the antenna achieves different resonance characteristics. Step impedance converters produce better impedance matching as compared to standard linear transmission lines. The diagonal arrangement of U-strips is employed to increase the isolation of the antenna. The antenna gain ranges from 3.58 to 5.79 dBi, and from Figure 8 it is found that the relative impedance bandwidth of the design is 131% (2.57-12.2 GHz). The maximum measured value of isolation is \(-\)15 dB with an ECC value \(<\) 0.005. A corner-truncated rhombus-shaped radiating patch is responsible for UWB range [24]. Moreover, to enhance the decoupling effect, a small rectangular stub is connected in the GP and placed between two ports. The MEG is measured using a far-field radiation pattern, which shows a value of \(-\)6 dB.