# Design of Wideband Decoupling Antenna Array for 5G Smartphones at N77/N78/N79/WLAN 5 GHz Bands
## 2 Design and Evolution of Antenna Systems
### Design Process and Analysis
The tight arrangement of the antenna elements reduces the size of the antenna pair but causes the isolation of the antenna pair to become unsatisfactory. Figure 3b shows that Case 3 has the weakest isolation at 4.07 GHz with only \(-\)9.02 dB. Figure 7a shows that the surface currents of the antenna pair in the range of 3.3-5 GHz are mainly distributed at the edge of the common ground plane between Ant 1 and Ant 2, which is the main cause of mutual coupling. Therefore, we try to add a 10.1 mm long defective ground structure between Ant 1 and Ant 2 (Case 4). Figure 7b shows that the surface currents that originally flowed from Ant 1 to Ant 2 were effectively curtailed and instead distributed near the defective ground, and the surface currents near the L-shaped coupling feed patch of Ant 2 were also effectively attenuated. Figure 3b shows that the isolation of the antenna pair is significantly improved after the unfavorable surface currents are controlled, and the S\({}_{12}\) decreases from \(-\)9.02 dB to \(-\)16.7 dB at 4.07 GHz, and the maximum value of the S  
Figure 4: CMA of Case 1. (**a**) MS. (**b**) MWC. (**c**) Characteristic current distributions. decreases from \(-9.02\) dB to \(-13.77\) dB within 3.3-5.8 GHz. It suggests that the defective ground structure incorporated by Case 4 has good band elimination characteristics. On the other hand, it can be clearly observed in Figure 3a that the \(-10\) dB impedance bandwidth of Case 4 is substantially improved compared to Case 3, which completely covers 3.3-5.83 GHz. It indicates that the defected ground structure is able to improve the impedance matching of the antenna pair while weakening the coupling. Figure 5: CMA of Case 2. (**a**) MS. (**b**) MWC. (**c**) Characteristic current distributions. Figure 6: CMA of Case 3. (**a**) MS. (**b**) MWC. The increase in the number of antennas can effectively enhance the capacity of the communication system, so we utilize the proposed antenna pairs to construct a MIMO antenna array. As shown in Figure 1, four antenna pairs are arranged on both sides of the bezel to form an eight-element MIMO system. Due to the increase in the size of the metal ground plane, new coupling is created between Ant 2 and Ant 3, which leads to the deterioration of the impedance matching and isolation of the antenna system. Therefore, we add a new composite decoupling structure between Ant 2 and Ant 3. The composite decoupling structure consists of an \(11\times 1\) mm\({}^{2}\) strip defective ground structure with two 6 \(\times\) 0.5 mm\({}^{2}\) decoupling strips extending from the common ground plane to the bezel (Figure 1), and this composite decoupling structure can effectively isolate the common ground plane current between Ant 2 and Ant 3. Figure 8 shows that the composite decoupling structure improves the isolation between Ant 2 and Ant 3 from \(-\)7.5 dB to \(-\)12.42 dB compared to the no decoupling structure or single decoupling structure. This situation suggests that the proposed composite decoupling structure is well suited to be applied to eliminate coupling between two antenna pairs. Figure 8: Simulated S-parameters with different decoupled structure. Figure 7: Electric current distribution. (**a**) Case 3. (**b**) Case 4.