## 4 Dual-Band MIMO Antenna Designs
In [47], back-to-back C-type slot resonators are connected in a radiating patch of a CPW-fed dual-band MIMO antenna using an artificial magnetic conductor as shown in Figure 10. The antenna elements are separated by a gap of 8.3 mm, and the antenna resonates at 3.5 and 5.2 GHz (Figures 11 and 12). Although, there is already a significant amount of isolation at the lower band, a U-shaped slot is introduced in the GP to further boost the isolation at the upper frequency band. A 2 \(\times\) 2 dual-band MIMO antenna with size of 46 \(\times\) 30 \(\times\) 1.6 mm\({}^{3}\) is reported in [48]. A swastika-shaped slot in the rectangular patch makes up the intended dual-band CPW-fed MIMO antenna as shown in Figure 13. Moreover, to increase the isolation between the two radiating components, a T-shaped narrow conducting stri  
Figure 10: Prototype of CPW-fed MIMO design [47]. Figure 9: Dual-band MIMO antenna design approaches. It is found that lower and higher frequency bandwidths of the design are 64.96% (1.85-3.63 GHz) and 44.36% (5.07-7.96 GHz), with isolation values \(\leq\)\(-\)17.21 dB and \(\leq\)\(-\)22.42 dB, respectively (Figure 14). The observed realized gain varies between 1.14 and 4.12 dBi (lower band) and 1.42 and 4.78 dBi (upper band), with a radiation efficiency \(>\) 72% for both frequency bands. This MIMO antenna covers the applications in DCS, LTE2300/2500, Bluetooth, ISM, and WLAN. The radiation patterns at 3.12, 5.29 and 7.18 GHz are evaluated and it is found that the E-plane pattern is dumb-bell shaped while the H-plane is omnidirectional in nature. The T-shaped strip embedded in the ground plane minimizes the electromagnetic coupling between the patches and hence the radiation patterns are least affected. Figure 11: Simulated S–parameters for MIMO antenna [47]. Figure 12: Measured S–parameters for dual-band MIMO antenna [47]. Figure 13: Fabricated two-element dual-band MIMO antenna [48]. In [49], a trident-shaped dual-port MIMO antenna with dimensions 62 \(\times\) 25.6 \(\times\) 1.524 mm\({}^{3}\) is shown in Figure 15.  
To obtain the dual-band characteristics, an arrow-shaped strip is placed between the U-shaped patch with two L-shaped slots on the GP. The dual frequency bands cover the frequency range from 2.99 to 3.61 GHz (lower band) and 4.53 to 4.92 GHz (higher band) with isolation \(\leq-25\) dB and \(\leq-16\) dB, respectively (Figure 16). This MIMO antenna effectively covers the 5G and sub 6G n77/n78/n79 spectrum. Figure 14: S–parameter of the dual-band MIMO antenna [48]. Figure 10.0 Top and bottom view of fabricated antenna [49]. The directional dual-band planar inverted-F antenna (PIFA) element with a small dimension of 31 \(\times\) 17 mm\({}^{2}\) is presented in [50], in which two antenna elements share a common ground with a 3.0 mm edge-to-edge inter-element spacing. Further, it is found that the inverted L-shaped metal arm dimensions are responsible for dual band operation. An inverted T-Shape slot is created on the GP to produce lower and higher frequency bands [51]. A T-shaped strip and a rectangular strip make up the radiating element in [52] and the lower and higher frequency bands are primarily matched by the top and bottom half of the T-shaped strip and the rectangular strip. In [53], six rectangular slots on the trapezoidal structure patch produce two frequency bands and they are achieved by adjusting the length of the rectangular slots. In addition to that, a T-shaped branch contributes to the improvement in isolation of the designed MIMO antenna. In [54], a MIMO antenna with decoupling structure is reported consisting of four L-shaped branches arranged in a counterclockwise direction. Dual band response of the antenna is achieved by varying the length and width of the L-shaped branches. In [55], inverted L-shaped monopole antennas loaded with split-ring resonators (SRRs) are placed in a rotationally symmetric pattern.