###### Abstract  
In this article, a miniature eight-port multiple-input multiple-output (MIMO) antenna array is proposed for fifth-generation (5G) sub-6 GHz handset applications. The individual antenna element comprises a radiator shaped like the Chinese character \"\(\mathbb{E}\)\" (phonetically represented as \"Wang\") and three split-ring resonators (SRR) on the metal frame. The size of the individual antenna element is only 6.8 \(\times\) 7 \(\times\) 1 mm\({}^{3}\) (47.6 mm\({}^{3}\)). The proposed antenna element has a \(-\)10 dB impedance bandwidth of 1.7 GHz (from 3.3 GHz to 5 GHz) that can cover 5G New Radio (NR) sub-6 GHz bands N77 (3.3-4.2 GHz), N78 (3.3-3.8 GHz), and N79 (4.4-5 GHz). The evolution design, the current distribution, the effects of single-handed holding, and the analysis of the parameters are deduced to study the approach used to design the featured antenna. The measured total efficiencies are from 40% to 80%, the isolation is better than 12 dB, the calculated envelope correlation coefficient (ECC) is less than 0.12, and the calculated channel capacity (CC) ranges from 35 to 38 bps/Hz. The presented antenna array is a good alternative to 5G mobile handsets with wideband operation, a metal frame, and minimized spacing. 5G; MIMO antenna; C-band; sub-6 antenna; split-ring resonator; Wang-shaped radiator +
Footnote †: journal: _Acticle_  
0000-0002-4000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-0000-00000-00000-0000-00000-00000-000000-00000-00000-00000-00000-000000-000  
feed strip on the top of the substrate and a shared circular patch on the bottom of the substrate are incorporated to achieve a 121% fractional bandwidth of 2.6-10.2 GHz. Two coupled-loop structures are printed on both sides of the frame, which makes the bandwidth wide enough to cover all of the N77, N78, and N79 frequency bands [14]. In [15], a tuning stub and a meandered line on the feed strip form a -6 dB bandwidth of 3.3-7.1 GHz. In [16], a C-shaped slot on the ground plane, an I-shaped slot on the frame, and a tuning stub are combined to realize an ultra-wideband operation. Although the structure of the antenna branches and parasitic components is complex, it achieved broadband coverage and high efficiency [18]. In [19], the feeding strip connected to the tuning branch effectively excites the double coupled-fed loop modes and a slot mode to operate in the 3.3-6 GHz range. MIMO antenna technology has been widely used in the design of 5G terminal antennas since it can increase spectral efficiency without excess power consumption and spectrum. However, the restricted space of general-purpose smartphones complicates the design of MIMO antennas. Various decoupling techniques have been presented to achieve high isolation, such as neutralization line structures [20], a T-shaped decoupling stub [21], a rectangular stub with a defected ground plane [21, 22], and two H-shaped slots on the ground plane with a fork-shaped microstrip line [23]. In [24, 25], four antenna elements are placed at the four corners of the metal frame to realize spatial orthogonal decoupling. In [26], the high isolation of the presented MIMO antenna array is achieved by the combination of quasi-orthogonal polarization, balanced mode excitation, and pattern diversity. A parasitic decoupling structure, aligned with the mesh grid, is specifically designed to improve the isolation between the elements of the MIMO antenna without compromising transparency [27]. This innovative design ensures that the decoupling structure effectively mitigates interference and enhances isolation between the antenna elements while maintaining the desired level of transparency for the antenna configuration. In this paper, a miniature MIMO antenna array for 5G broadband metal-frame mobile phone applications is proposed. An SRR structure on the frame and a Chinese character Wang-shaped radiator are utilized to achieve a \(-10\) dB bandwidth of 1.7 GHz (3.3-5 GHz), which can cover 5G N77 (3.3-4.2 GHz), N78 (3.3-3.8 GHz), and N79 (4.4-5 GHz).