Wideband 5G N77/N79 4 $ \times $ 4 MIMO Antenna Featuring Open and Closed Stubs for Metal-Rimmed Smartphones With Four Slits

This letter presents a compact wideband 4 $ \times $ 4 multiple-input–multiple-output (MIMO) antenna integrated within the four slits of a 5G metal-rimmed smartphone. Open and closed stubs are devised and integrated with each slit to enable pair of antenna elements at N77 (3.3–4.2 GHz) and N79 (4.4–5.0 GHz) spectrum. Furthermore, an additional bent stub is introduced within each pair to suppress mutual coupling within the aforementioned spectrum. Four antenna pairs are placed at each four edges of the smartphone mockup to realize a 4 $ \times $ 4 MIMO antenna array and the entire system is simulated, fabricated, and measured. The results indicate the isolations are greater than 11.5 dB and the envelope correlation coefficients are lower than 0.2 across the operating frequency bands. The measured total efficiency ranges from 38% to 52%. Compatibility with metal rim is considered in all processes to incorporate the realistic implementation of the devised antenna. We forecast that the proposed eight-antenna MIMO array exhibits high potential for practical applications targeted for future 5G smartphones.