# A Design for a Wide-Band Antenna Pair Applied for Mobile Terminals at the Sub-6 GHz Band
## 4 Performance of the Eight-port MIMO System  
As shown in Figure 9, an eight-port MIMO system composed of four antenna pairs, as proposed above, is constructed. The fabricated MIMO system consists of three parts, including two \(150\times 6\times 0.8\) mm\({}^{3}\) sides in the vertical direction and one \(150\times 75\times 0.8\) mm\({}^{3}\) floor in the horizontal direction. The sides and floor are connected by six solder points and eight SMA connectors are added to the ports for antenna performance testing. To demonstrate the effectiveness and practicality of the antenna design, the proposed MIMO system is simulated and measured. Due to the symmetry of the MIMO antenna layout, the simulation and test results are shown for some of the ports. Figure 8: Comparisons of the antenna pair surface current distributions with and without a decoupling structure. (**a**) Surface currents at 4.0 GHz. (**b**) Surface currents at 5.5 GHz. Figure 6: Antenna pair with different DGS. (**a**) Model 3 without DGS. (**b**) Model 3 with strip DGS. (**c**) Model 3 with a T-shaped DGS. (**d**) Model 3 with the proposed DGS. Figure 7: S-parameters of the antenna pair with different DGS. (**a**) Reflection coefficients. (**b**) Transmission coefficients. Figure 10 shows the simulated and measured S-parameters results of the proposed eight-port MIMO antenna system. Compared with the S-parameter results of a single antenna pair, the results have some changes. This is due to the influence of the longer surface current path caused by the larger size of the dielectric substrate and the coupling effects from the near antenna elements and the far antenna elements. One can see that there are minor differences in the simulation results and measurement results, which is unavoidable due to measurement errors and the addition of SMA connectors, as well as the addition of solder points. The \(-6\) dB impedance bandwidth depicted in Figure 10a is 3.4-5.2 GHz, which indicates that the proposed MIMO system can work at the N77/N78/N79 bands of 5G communication bands. The isolation depicted in Figure 10b within a common antenna pair is lower than \(-12.5\) dB. Due to the influence of the intersecting polarization directions, the worst isolation exists in Antenna 2 and Antenna 3, and in Antenna 6 and Antenna 7, which still remain below \(-10\) dB. Figure 11 shows the two-dimensional measured radiation patterns of the antenna pair on the MIMO system, observed from 3.6 and 4.8 GHz resonant points. It can be seen that in the xoy plane, the gain of the phi direction is dominant, and in the yoz plane, the gain of the theta direction is dominant. Antenna 1 and Antenna 2 have complementary radiation patterns, which will ensure full coverage of the radiation direc  
Figure 10: Simulated and measured S-parameters of a proposed MIMO system. (**a**) Reflection coefficients. (**b**) Transmission coefficients. Figure 9: Photograph of the proposed eight-port MIMO system. diversity. The envelope correlation coefficients (ECC) of the antenna elements are used to evaluate the spatial irrelevance of the antenna radiation. To further verify the coupling suppression ability of the proposed antennas, the ECCs of radiation elements are measured. It can be observed from Figure 12 that the value of both ECCs is lower than 0.1 in the whole of the working frequency bands, which proves that the different antenna radiation patterns of such a MIMO system have good spatial uncorrelation. Figure 11: Measured two-dimensional radiation patterns of antenna elements. (**a**) Results of Antenna 1 and Antenna 2 in the xyo plane. (**b**) Results of Antenna 1 and Antenna 2 in the yoz plane. It can be observed from Figure 13a that the total efficiency in the whole working band is better than 46%. The maximum efficiency can reach 84% at 3.6 GHz resonant points. Figure 13b shows that the peak gain of all antennas is higher than 4.6 dB on the working frequency bands. The maximum gain can reach 7.8 dB at 4.3 GHz resonant points.