Low-profile 2$$\times $$2 MIMO antenna array with dense 16-port configuration for 6G upper mid-band mobile systems

This work proposes a novel 2 \\(\\times \\) 2 MIMO antenna array with a dense 16-port configuration, engineered for seamless integration into the rear cover of mobile devices while meeting compliance requirements for next-generation 6G applications. Designed on a low-profile footprint of 55 mm \\(\\times \\) 55 mm \\(\\times \\) 1.8 mm and optimized through full-wave simulations in Ansys HFSS, the antenna operates within the emerging international mobile telecommunications (IMT) spectrum of 7–24 GHz. Resonance is observed in the 10.7–13.25 GHz sub-band, one of the widest contiguous spectrum allocations within the FR3 range and an operational region that remains largely unexplored in recent compact MIMO antenna designs. An air-gap structure is incorporated between the substrate and ground plane to shift the resonance from the lower-frequency region and enhance the operational bandwidth. The proposed antenna is fabricated and experimentally characterized, with measured results showing good agreement with simulations. The fabricated prototype achieves reflection coefficients better than \\(-6\\) dB and inter-port isolation better than \\(-10\\) dB across the operating band, while maintaining an average realized gain exceeding 6 dB. MIMO performance evaluation yields ECC below 0.35, DG above 9.4 dB, \\(\\Delta \\) MEG below 0.6 dB, and radiation efficiency between 73% and 87% across the investigated frequency range. Surface power density analysis confirms compliance with ICNIRP guidelines, and the proposed antenna provides a useful contribution toward the development of future 6G mobile-device antennas operating within the emerging upper mid-band spectrum.