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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In this paper, a single-layer wideband differential-fed microstrip patch antenna (DMPA) with complementary split-ring resonators (CSRRs) loaded is proposed. First, a quarter-wave microstrip-line resonator is introduced as a capacitive coupling feeding structure which can generate a non-radiative resonant mode. Second, a set of rectangular parasitic patches is loaded co-planarly along the non-radiative edges of the DMPA. Thus, a parasitic radiative mode at the high side of the operation band is added. Meanwhile, with this arrangement, the field distribution of the DMPA has also changed, and a radiation null at high band edge is formed. Then, a pair of CSRRs is etched on the ground beneath the feeding lines, which has compensated the mismatched impedance of the DMAP at the low side of the operation band. As a result, the low-frequency bandwidth is significantly increased. Finally, the proposed antenna is fabricated and measured, and the measured results are in good agreement with the simulation ones. The measured bandwidth of the proposed antenna is ranged from 5.35 GHz to 7 GHz (about 26.7%) while keeping a low profile of 0.051λ (λ is the free-space wavelength at the center frequency) without any air gaps. Besides, the broadside gain is from 7.7 dBi to 10.7 dBi. INDEX TERMSDifferential-fed patch antenna, wideband antenna, single-layer, parasitic patches, complementary split ring resonator. YI-XUAN ZHANG (S'18) received the B.Eng. degree in electronic information engineering from Xidian University (XDU), Xi'an, China, in 2017, where he is currently pursuing the Ph.D. degree with the Department of National Key Laboratory of Antennas and Microwave Technology. His current research interests include high performance electromagnetic optimization, phased array, and 5G communication technology.

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