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In this communication, the novel compact tightly arranged pairs are employed to form a 4 × 4 multiple-input-multiple-output (MIMO) system and an 8 × 8 MIMO system operating at 3.4-3.6 GHz for fifth-generation (5G) mobile phones. Each tightly arranged pair is composed of a bent monopole and an edge-fed dipole with a compact size of 7 × 12 mm 2 . The orthogonal-mode method is presented to mitigate the mutual coupling of the tightly arranged pairs without any external decoupling structure. With the help of the orthogonal mode, isolation performances across the desired band of the 4 × 4 MIMO system and the 8 × 8 MIMO system are better than 20 and 17 dB, respectively, with the elements closely spaced. The measured efficiencies are 51.7%-84.5%/49%-72.9%, and the measured envelope correlation coefficients are less than 0.06/0.07 for the 4 × 4/ 8 × 8 MIMO system. The proposed MIMO systems provide a promising solution to compact 5G MIMO mobile phone antennas with good isolation and diversity performance.
Sun et al. (2018) studied this question.