# Miniaturized MIMO Antenna Array with High Isolation for 5G Metal-Frame Smartphone Application
###### Abstract  
In this paper, a highly isolated multiple-input multiple-output (MIMO) antenna array is proposed for fifth-generation (5G) metal frame smartphones. The eight identical small-sized inverted F-shaped folded slots are etched on the metal frame as a MIMO antenna. The bandwidth of the antenna can be adjusted by changing one of the short branches of the antenna. The bandwidth of the antenna can reach the N79 band (4.4-5.0 GHz). By carefully arranging the positions of the eight antenna elements, ideal spatial diversity can be successfully achieved to mitigate the coupling between the antenna elements effectively. What is more, a small combination slot of C-shape (0.0078 \(\times\) 0.047\(\uplambda^{2}\)) and vertical I-shape (0.12 \(\times\) 0.004\(\uplambda^{2}\)) between each antenna element is introduced to improve the element isolation of the MIMO antenna system. The proposed MIMO array has been simulated, fabricated, and measured. The results show good impedance matching (return loss > 6 dB) and high isolation (>22 dB). Due to the decent element isolation, the envelope correlation coefficient (ECC) between each antenna element is below 0.049. It can provide a reliable anti-interference performance for the MIMO antenna system. In addition, the measured radiation efficiencies of the MIMO antenna system are higher than 50%. The interaction of the hand model with the MIMO antenna system is also investigated, including the specific absorption rate (SAR). multiple-input-multiple-output (MIMO) antenna; N79 band; high isolation; metal frame; SAR +
Footnote †: journal: _micromachines_  
00222022022022022022  
wideband antenna with slots on the metal frame and back cover, but the antenna uses lumped elements to adjust impedance matching and uses the differential balun chips for differential feeding, which increases the complexity of the antenna design. This is not the target that we pursue. In [34; 36], the designed broadband antennas are very similar. The main contribution to the broadband is the slotted part of the back cover. It is at the expense of a larger size, which is also far higher than the goal we pursue. In the designs of [31; 32; 33], the MIMO antenna system is integrated into the metal back cover of the mobile phone. However, the distance between the radiation patch and the ground is too large to integrate into practical application. If the metal back covers with a large area is used as the antenna ground, it will affect the radiation performance making the impedance broadband not reach the -6 dB requirement [31]. Various types of decoupling techniques reported are effective methods to raise the isolation between each MIMO antenna element, such as parasitic decoupling techniques [3], neutral lines [4; 6], and loading decoupling elements [16]. It is reported in [5; 37; 38] that the diversity technique can increase the space between the antennas to improve isolation. Both frequency selective surfaces [39] and metasurfaces [40] have been used to mitigate mutual coupling in MIMO antenna systems. Additionally, I-shaped and H-shaped ground slots are introduced between the antenna elements to improve the element isolation in [4; 35], respectively. In [41], the small T-shaped open slot is used to reduce the coupling between two closely spaced antenna elements, too. However, a chip capacitor is loaded to reduce the size of the slot in this method (parasitic scatterers decoupling). The isolation can be adjusted by changing the capacitor value, but this method is limited in reducing the size of the isolation element. Plus, the isolation level is restricted by two aspects (T-shaped slot size and the chip capacitor value), which increases the complexity of the antenna. Inspired by [35; 41], we presented C-shaped and I-shaped combination slots between the antenna elements. The footprint can be reduced to 3 mm \(\times\) 0.5 mm (0.0078 \(\times\) 0.047\(\lambda^{2}\) (\(\lambda\) corresponds to 4.7 GHz)).