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An 8-port multiple-input multiple-output (MIMO) array operating in the long-term evolution (LTE) band 42 (3400-3600 MHz) based on the characteristic mode theory (CMT) is described. As designing a multi-antenna system with multiple significant modes is difficult for the CMT, a new method is, therefore, proposed to effectively provide useful physical insights and improve the envelope correlation coefficient (ECC). Although many of the same modes are excited by two antennas, the proposed method is still effective to improve the ECC of the two antennas. A detailed and reasonable framework adopting the CMT to analyze geometries with multi-significant modes is presented. The consideration of the mode number can be avoided by using this method. For an arbitrary structure, it is verified that the modal weighting coefficient (MWC) phases of the same modes excited by two ports are in phase or out of phase. Therefore, for the design of a multi-port MIMO array, the same modes excited by different ports can be divided into two types: equiphase modes and antiphase modes in the proposed method, and the tradeoff of the equiphase modes and the antiphase modes is first proposed and analyzed as an efficient way to improve the ECC. Furthermore, the proposed method can also be applied to asymmetric structures. In order to prove the effectiveness of the proposed method in the design of MIMO antennas with desirable ECC, an 8-port MIMO array for 5G mobile applications is implemented and measured.INDEX TERMS Multiple-input multiple-output (MIMO), characteristic modes, 5G mobile application.

In this paper, a new design method to enhance the isolation of a two-antenna building block is introduced. Here, a two-antenna building block (Ant 1 and Ant 2) that is composed of a gap-coupled loop antenna (Ant 1) and a loop antenna (Ant 2) is meticulously designed to allow Ant 1 to excite a standingwave region that can be formed on the structure of Ant 2. To exhibit high isolation (26 dB) between the two antennas, the stable current null point existed in the standing wave region is exploited and designed to fall at the feed point of Ant 2 during the excitation of Ant 1. By employing this proposed building block, a fourport multiple-input multiple-output (MIMO) system and an eight-port MIMO system operating in the long term evolution (LTE) band 42 (3.4-3.6 GHz) are implemented. The proposed four-antenna and eight-antenna MIMO arrays can yield desirable measured isolation of better than 23 dB and 17.9 dB, respectively, over the band of interest, and their respective measured ECCs (envelope correlation coefficients) were lower than 0.032 and 0.075. To further evaluate the MIMO performances of the presented MIMO arrays, their ergodic channel capacities are also investigated.INDEX TERMS Fifth-generation (5G) smartphone, multiple-input multiple-output (MIMO), building block, high isolation.

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