# Wideband Eight-Antenna Array Designs for 5G Smartphone Applications
## 1 Introduction  
In today's modern society, the construction of Fifth Generation (5G) Mobile Communication is gradually being completed. Due to the increasing demands for data throughput and bandwidth utilization from technologies such as the Internet of Things (IoT), electric vehicles, and smart-home appliances, the bandwidth provided by Fourth Generation Mobile (4G) Communication is inadequate to fulfill the demands of the market. To enable higher data transmission within the 5G bands, MIMO technology has become crucial in smartphone and base station technology. Additionally, MIMO technology can effectively enhance spectrum utilization and channel capacity (CC) [1; 2]. In 2018, the 3 rd Generation Partnership Project (3 GPP) officially defined the specifications for 5G NR Release 15 [3]. According to these specifications, 5G NR in the sub-6 GHz range includes three operating bands: Band n77 (3.3-4.2 GHz), Band n78 (3.3-3.8 GHz), and Band n79 (4.4-5.0 GHz). Consequently, countries around the world have selected their respective 5G bands from these frequencies. With the increasing market and technological demands, not only the aforementioned licensed bands, but also the application of an unlicensed spectrum has gained significant attention. The main relevant frequency band for this is the Unlicensed National Information Infrastructure (U-NII) band, and corresponding to this band, LTE Band 46 has been proposed. In 2020, 3 GPP officially introduced Release 16 [4], which highlights the 5G New Radio Unlicensed (5G NR-U) band. This unlicensed band is due to the development of the unlicensed spectrum LTE-U (LTE in Unlicensed Spectrum) and Licensed-Assisted Access (LAA) from 3 GPP Release 13/14. As a result, NR-U equipment will operate within 6 GHz band for the first time. In the United States, the NR-U is divided into four sub-bands, namely, U-NII-5 to U-NII-8 (5925-7125 MHz), and is classified as the new version of Wi-Fi, widely known as Wi-Fi 6E. Finally, 3 GPP's Release 16 officially added the LTE Band 46(5.15-5.925 GHz) to the 5G NR FR 1, designated as 5G NR-U n46; the 5.925-7.125 GHz band is designated as 5G NR-U n96. In the early designs of 5G mobile phone antennas, to induce a resonance mode at 3.5 GHz, a common approach was to etch an open-slot antenna with a length of \(\lambda/4\) in the ground plane [5; 6; 7] or to generate a quarter-wavelength mode using a monopole antenna [8]. However, present smartphone antennas are often collocated along the phone's frame, leading some researchers to design inverted-F antennas (IFAs) [9] and loop antennas [10] on the frame to excite a 3.5 GHz mode. However, narrowband antennas for smartphone applications are no longer sufficient to meet the market demands. Consequently, many papers have reported combined IFAs and loop antennas to form hybrid antenna designs [11; 12; 13], which can produce dual-band or wideband modes. In the limited space of a smartphone, accommodating multiple antennas presents a significant challenge for maintaining isolation between antennas. Various methods to improve isolation have been proposed in the open literature; for instance, decoupling structures [14; 15; 16; 17; 18; 19; 20; 21] can effectively reduce the coupling energy between antennas, and orthogonal polarization [22; 23; 24; 25; 26; 27] uses polarization diversity to enhance antenna isolation. Besides the above techniques, the self-isolated designs [28; 29; 30; 31; 32; 33] can also improve isolation efficiency by reducing the impact of ground plane currents. For example, [31] capture currents on the ground plane of the antenna (wave trap) to minimize the influence on adjacent antennas, while [32; 33] found that symmetric structures effectively reduce the ground plane current, thereby lowering interference between antennas. Notably, the work in [34] points out that the antenna feed position affects isolation performance, suggesting that placing antennas at current null points can improve isolation.