# 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  
Since the characteristic mode theory can reveal the resonant modes of the object itself and is independent of excitation, it has received widespread attention in antenna design. In this study, the bandwidth expansion process adopts CMA. By capturing different modes and observing the corresponding characteristic current distributions, the geometry of the antenna pair is optimized at the more sensitive parts of the characteristic currents. A new mode is introduced to make the modes better excited. The evolution of the antenna pair is shown in Figure 2. Modal significance (MS) is used to indicate the potential of each mode to  
Figure 1: Antenna system structure diagram. be excited under each geometry during the evolution of the antenna pair. The modal weighting coefficient (MWC) evaluates the extent to which the modes or combinations of modes are effectively excited. Due to the symmetry of the structure, the evolutionary process of the antenna pair is built on a quarter of the substrate with the bezel (75 mm \(\times\) 35 mm), and only port 1 is analyzed for the excitation simulation. The simulation results of the reflection and transmission coefficients of the evolution process are shown in Figure 3.  
The basis for bandwidth expansion is an S-shaped dual-frequency antenna pair (Figure 2, Case 1). The MS of Case 1 is shown in Figure 4a. Mode 1 and mode 2 do not form a broadband mode but have the potential to form a broadband mode. MWC shows (Figure 4b) that mode 1 and mode 2 are effectively excited at 3.3 GHz and 5.4 GHz, respectively. Figure 4c shows that the characteristic current of mode 1 is mainly concentrated in the part of the patch near the common ground plane, and mode 2 forms two loops between the L-shaped coupled feed patch and the S-shaped external radiator. What the two characteristic currents have in common is that they are both very sensitive at the long side of the S-shaped patch near the common ground plane. Next, to increase the broadband potential of Case 1, we extended an L-shaped patch at the end of the S-shaped radiator toward the center of the antenna pair and widened the characteristic current-sensitive part of the external radiator connected to the common ground plane (Figure 2 Case 2). Figure 5a shows the MS for both modes of Case 2. The variation of MS shows that Case 2 possesses better broadband potential compared to Case 1. Figure 5b shows that Case 1 and Case 2 obtained better excitation at 3.5-5.7 GHz and ideal resonance near 4.0 GHz and 5.5 GHz. Figure 5c shows that due to the loading of the L-shaped patch, the original characteristic currents are redistributed. The characteristic currents in the low-frequency portion of Case 1 move away from the common ground plane and are concentrated on the new loaded L-shaped patch. This change effectively reduces the surface current between Ant 1 and Ant 2 at the edge of the common ground  
Figure 3: Evolution of S-parameters. (**a**) S\({}_{11}\). (**b**) S\({}_{12}\). Figure 2: Evolution of antenna pairs. plane and improves the isolation between the ports of the low-frequency part. Nonetheless, Case 2 is still unable to meet the demands of 5G terminals, and the bandwidth needs to continue to be expanded. In order to excite more modes in the same plane, we optimized the structure of the external radiator by adding a new L-shaped patch and rectangular patches, whose spatial position exactly corresponds to the L-shaped coupled feed patch (Figure 2 Case 3). This helps to improve the impedance matching of the antenna in the broadband range. Figure 6a shows the MS of Case 3, which introduces two new resonant modes near the center frequency compared to Case 2. The MS of the four modes is mostly greater than 0.7 in the frequency band of interest, and the antenna pair already has good broadband potential. Figure 6b shows that the four modes are able to be excited simultaneously by the same feed source and that mode 1 resonates with mode 4 near 5.7 GHz, producing the most significant resonance point in the 3.3-5.8 GHz range.