High-Isolation Compact Wideband MIMO Antennas for 5G Smartphones with Unbroken Metal Frames

This article presents a novel design method for realizing wideband operation and excellent isolation in fifth-generation (5G) multiple-input multiple-output (MIMO) systems. The proposed MIMO antenna employs a ground-plane slot in the shape of the Chinese character “工” and maintains an unbroken metal frame, thereby avoiding slot openings on the rim. The theory of characteristic modes (TCM) is applied to determine appropriate feeding structures and locations for two functional antenna modules. This design achieves wide bandwidth and high isolation without requiring additional decoupling structures, simplifying the overall system. Two prototype arrays, consisting of four and eight antenna elements, were implemented for 5G operation in the 3.4–5.0 GHz band. The measured results confirm isolation levels above 21.6 dB and 16.2 dB for the four- and eight-element arrays, respectively, with envelope correlation coefficients (ECCs) below 0.16. These results indicate that the proposed design is a promising solution for integration into 5G smartphones.