# A Design for a Wide-Band Antenna Pair Applied for Mobile Terminals at the Sub-6 GHz Band
## 1 Introduction  
Wireless communication has entered the fifth-generation mobile communication technology (5G) period. According to the third generation partnership project (3 GPP) protocol, the main working operation frequency is divided into the sub-6 GHz (450 MHz-6 GHz) and millimeter-wave (24.25 GHz-52.6 GHz) bands, of which sub-6 GHz has the most development potential in the field of mobile terminal applications. To meet the increasing demand for better communication rates, multiple antenna array technology is being developed, in which a multiple input-multiple output (MIMO) antenna system is one of the research hotspots. For the MIMO systems used in mobile terminals, an antenna with the characteristics of a low profile, compact structure, and excellent radiation performance is essential. In mobile terminals, there is a trend toward decreasing the size and increasing the number of components. Due to the limitations of physical size, a system with multiple antenna elements not only has a small element size but also has close spacing. Therefore, severe electromagnetic interference is generated in adjacent antenna elements, which is known as the mutual coupling effect of antennas. The mutual coupling effect will damagethe independence of the different antennas, that is, isolation and spatial correlation. In addition, it also affects the radiation patterns, gain, and efficiency of the antenna, which makes the channel capacity of the MIMO system reduced. Many decoupling types of research have been proposed, such as neutralization lines (NL) [1; 2], decoupling networks [3; 4; 5], parasitic elements [6; 7], defected ground structures (DGS) [8; 9], and an electromagnetic band-gap (EBG) [10; 11]. There are also decoupling methods that do not require additional decoupling structures, such as orthogonal polarization [12; 13] and multiple polarization [14] techniques. Among them, DGS and EBG are more commonly applied in planar antenna array decoupling, due to their trapping properties for electromagnetic waves, which allow surface electromagnetic waves to resonate on the etched grooved surface. In a previous study [15], the resonant currents were concentrated on the edges of radiation patches and the ground plane. By introducing slots on the ground plane, an isolation performance was achieved below \(-20\) dB and showed less impact on impedance matching. In another study [16], multiple DGS were etched on the ground plane for isolation enhancement. Such decoupling structures were noted to be equivalent to band-stop filters, based on a parallel resonator. A modified EBG was used by another group of authors for antenna elements, with an edge-to-edge spacing of 50 mm [17]. Isolation of lower than \(-23\) dB was obtained over the UWB frequency spectrum. Recently, in other research, four- or eight-port MIMO antenna system designs for mobile terminals have been proposed. Those antenna schemes demonstrate compact antennas with decoupling methods, placed on the frames or sides of mobile terminals, that are practical and effective. Resolving the conflict between compact structure and good radiation uncorrelation is a challenge in MIMO antenna system design in the case of mobile terminals. Previous research [18; 19; 20; 21; 22; 23; 24; 25; 26; 27] presented MIMO antenna system designs applied to mobile terminals at sub-6G bands as shown in Table 1. In [18], an eight-port MIMO system was printed along the inner side of two sideboards. A minimum antenna spacing of 21 mm was used, and a \(12.6\times 3.5\) mm\({}^{2}\) parasitic patch was placed between adjacent antenna elements, which achieved isolation below \(-14.5\) dB. A MIMO system composed of eight slot antennas on the outside of the metal bezel was designed [19]. With the addition of I-shaped slots on the metal bezel and C-shaped DGS on the ground plane, isolation was achieved below \(-22\) dB. Four loop antenna elements were placed on the quad corners of the mobile phone board [20]. It easily achieved good radiation uncorrelation and isolation of less than \(-10\) dB.