Each generation of mobile communications introduces new frequency spectrums and new communication systems to increase user’s experienced data rates. For instance, the fifth-generation (5G) communication introduces the 4×4 multi-input-multi-output (MIMO) system (4 receive antennas for 4 spatial streams) for mobile devices such as the smartphone, especially in 3.3-5.0 GHz in the mid-band, to achieve increased data rates for the user. Also, the MIMO antennas are generally deployed along the frame of the smartphone. For future six-generation (6G) communications, the new spectrum of 6.425-8.4 GHz in the upper mid-band is envisioned as a potential 6G prime mobile spectrum. Additionally, higher-order MIMO operations such as the 8×4 MIMO (eight receive antennas for four spatial streams) are promising for 6G smartphones to achieve increased data rates for the user. In this study, to conveniently employ 5G mid-band/6G upper mid-band MIMO antennas in future smartphones, we introduce for the first time hybrid MIMO antennas comprising 4 frame antennas provided by two ultra-wideband (UWB) frame conjoined two-antenna (FC2A) elements in 3.3-8.4 GHz and four backcover antennas provided by a low-profile backcover four-antenna (B4A) module or modular MIMO antennas in 6.425-8.4 GHz. The four frame antennas not only operate for 5G mid-band 4×4 MIMO in 3.3-5.0 GHz but also can combine with four backcover antennas to operate for 6G upper mid-band 8×4 MIMO in 6.425-8.4 GHz. We address details of the hybrid MIMO antennas formed by the FC2A element and B4A module in this study. We also apply the fabricated hybrid MIMO antennas in the mid-band 4×4 and upper mid-band 8×4 MIMO systems in the outdoor field test to evaluate their MIMO performance for 5G/6G smartphones. The results obtained demonstrate that the hybrid MIMO antennas are promising for the deployment of 5G/6G MIMO antennas in future smartphones.
