# Wideband Eight-Antenna Array Designs for 5G Smartphone Applications
## 1 Introduction
Additionally, [35] mentions that adding capacitors or inductors between adjacent antennas can effectively eliminate the coupling. As stated in [36], the construction of 5G mobile communication infrastructure for the sub-6 GHz band has been gradually completed in many developed countries. To achieve higher data throughput in the 5G operating frequency bands below the sub-6 GHz band, one of the leading 5G technologies is MIMO technology, which plays a pivotal role in both smartphone and base station technology. As aforementioned, many single-band or dual-band operation MIMO antenna arrays have been investigated for 5G smartphones; however, these antenna designs can only cover part of the 5G bands (3300-5000 MHz). Therefore, it is vital to implement a wideband operation 5G MIMO antenna array design that can cover the 5G bands of interest [37; 38; 39; 40; 41; 42; 43; 44; 45; 46]. In [38; 39; 40], MIMO antenna array designs for metallic frames are adopted. In [38; 39], an open slot is cut into a metallic frame with an extended slot loaded into the ground plane, and an additional tuning stub is implemented to achieve a good 6 dB overlapped impedance bandwidth across the bands-of-interest. In comparison, [40] requires two slots (inverted T-shaped and inverted C-shaped) to be loaded into the metallic frame, and these slots are fed by a meticulously designed L-shaped feed-line. Besides applying antenna array structures with metallic frames, other designs have been explored. In [41], the individual array element consists of an inverted-F antenna with shorting branches and parasitic elements. Impedance matching for the mid-band at 5.2 GHz can be fine-tuned using an L-shaped parasitic element and a slot. In [42], an inverted-F antenna (IFA) unit structure is implemented for 4 \(\times\) 4 MIMO operation, and it is coupled-fed by a meandering line coupled feeding structure. Notably, a 4 \(\times\) 4 MIMO operation IFA pair is also implemented in [43], featuring a two-antenna conjoined type with two identical IFA structures (back-to-back) linked to the same grounding structure. However, [43] only applied the IFA pair for 4 \(\times\) 4 MIMO operation, and such a design requires the excitation of three different modes (loop, IFA, and slot) to achieve wideband operation, resulting in diverse radiation patterns across the bands-of-interest. Compared to the IFA pair design in [43; 44], a different integrated slot antenna pair for 8 \(\times\) 8 MIMO operation is implemented. This design employs a dual-mode decoupling technique specifically for closely spaced open-slot antennas, but a horizontal connecting line must be used to link the two open-slot antennas to achieve desirable isolation of \(>\)10.8 dB. In this paper, a modified loop antenna element (MLAE) is proposed as the antenna unit for an 8 \(\times\) 8 MIMO antenna array. This MLAE design is derived from a modified loop antenna type fed by a T-shaped coupled feeding structure [46]. However, the loop antenna in [45] could only excite a fundamental loop mode, resulting in a narrow operating bandwidth of 3400-3600 MHz. To achieve a better impedance bandwidth, the design in [46] was further modified into a wideband loop antenna. This wideband design was achieved by disassembling the loop antenna into four sections and applying a coupling method, where two sections located on one side of the substrate are coupled to the two collocated on the other side. This reconfigures the loop antenna into a segmented coupled loop antenna type. By applying this method, the coupling structure can excite an additional 0.75-wavelength loop mode. Combining this mode with the 0.5-wavelength fundamental loop mode and the 1.0-wavelength higher resonant loop mode yields a wideband operation covering the 5G bands-of-interest (3300-5925 MHz). Nonetheless, to avoid the complexity of the segmented coupled loop antenna, which requires printing antenna structures on both sides of the substrate, we propose a novel MLAE, and its corresponding contributions are as follows:  
1.