This study investigates a compact and straightforward self-decoupled $2\times 2$ multiple-input multiple-output (MIMO) antenna set and its applications for current and future 5G terminal devices. The proposed self-decoupled MIMO antennas include a popularly used loop antenna and a compact loop-type ground-radiation antenna without utilizing any supplementary decoupling structures or undergoing complex tuning process. It is revealed that the loop antenna and the ground-radiation antenna can be modeled as an electric-current element and a magnetic-current element, respectively. This orthogonality allows the self-decoupled characteristic of the proposed MIMO antennas even though the antenna elements are tightly arranged and collocated together. An $8\times 8$ MIMO antenna system is further demonstrated for 5G MIMO applications, where both simulation and measurement are conducted to validate the feasibility of the proposed technique. It is concluded that the proposed MIMO antenna system is a smart way to generate high isolation and low correlation characteristics while having advantages of low profile and easy fabrication so that it can be recognized as a promising candidate for 5G applications.

This paper investigates the design of a compact multiple-input multiple-output (MIMO) ground-radiation antenna system for 5G terminal devices. Ground-radiation antennas are small loop-type resonators that can excite the ground plane as radiators while having advantages of small size and convenient tunability, but their compact MIMO systems remain as unsolved issues. In this study, a novel and straightforward decoupling technique based on a closed-decoupling-loop is presented for MIMO applications in the first of its kind. The proposed closed-decoupling loop can be viewed as a coupling transformer between two antenna elements by acting as a half-wavelength loop-type resonator and by introducing a current null at its center; in this way, a tremendous isolation improvement is accomplished even though the distance between two antennas is only 1 mm. Therefore, a compact 2 × 2 MIMO ground-radiation antenna system and its 8 × 8 MIMO system are demonstrated for 5G applications. Both simulation and measurement are taken to validate the proposed decoupling technique and the compact MIMO ground-radiation antenna system.

This paper studies an isolated ground-radiation antenna (iGradiANT) that has inherent isolation with another closely-located antenna element and demonstrates its applications in 5G multiple-input multiple-output (MIMO) antenna array. The proposed iGradiANT is accomplished by merely employing a small out-of-ground loop into a traditional ground-radiation antenna (GradiANT). Hence, in contrast to the traditional GradiANT, the proposed iGradiANT can simultaneously support an in-ground loop-type current mode and an out-of-ground loop-type current mode, responsible for far-field radiation and near-field energy cancellation, respectively. In this way, the proposed iGradiANT can exhibit an intrinsic decoupling effect with the adjacent antenna element. Hereby, a typically used inverted-F antenna (IFA) and a normal loop antenna are adopted to separately validate the functionality and versatility of the proposed iGradiANT in establishing 2 × 2 MIMO antenna sets without any separation. Furthermore, an 8 × 8 MIMO antenna array is demonstrated for the usage of 5G terminal devices; both simulation and measurement are conducted to verify its radiation performance and diversity performance.

In this paper, we propose a compact four-port coplanar antenna for cognitive radio applications. The proposed antenna consists of a coplanar waveguide (CPW)-fed ultra-wideband (UWB) antenna and three inner rectangular loop antennas. The dimensions of the proposed antenna are 42 mm × 50 mm × 0.8 mm. The UWB antenna is used for spectrum sensing and fully covers the UWB spectrum of 3.1–10.6 GHz. The three loop antennas cover the UWB frequency band partially for communication purposes. The first loop antenna for the low frequency range operates from 2.96 GHz to 5.38 GHz. The second loop antenna is in charge of the mid band from 5.31 GHz to 8.62 GHz. The third antenna operates from 8.48 GHz to 11.02 GHz, which is the high-frequency range. A high isolation level (greater than 17.3 dB) is realized among the UWB antenna and three loop antennas without applying any additional decoupling structures. The realized gains of the UWB antenna and three loop antennas are greater than 2.7 dBi and 1.38 dBi, respectively.

This communication proposes a compact, wideband, two-port substrate-integrated waveguide (SIW) antenna with a central, double-slotted metallic plate flanked by two paired corrugations for radar applications. The proposed antenna uses a two-layer structure, with a slotted metallic plate with corrugations on the top layer and an FR-4 substrate SIW on the bottom layer. The antenna has compact dimensions of 63 mm $\times$ 54 mm ( $2.32\lambda _{0} {\times}\,\, 1.99\lambda _{0}$ ) with a low profile of 7 mm ( $0.26\lambda _{0}$ ). To understand the operating principle of the two-port SIW antenna clearly, modal analysis is performed for single-port SIW antenna. The antenna satisfies a −10 dB reflection coefficient from 10.15 to 11.58 GHz. Although the separation between the two central slots is minimized to 4 mm ( $0.14\lambda _{0}$ ), the isolation performance remains excellent (less than −30 dB). The gain enhancement, with a narrow beam pattern for radar applications, is realized using constructive superposition of in-phase electromagnetic energy from the two slots. In addition, because the antenna has two ports, it also has a beam scanning function from −12° to +12° in its boresight direction. The experimental and simulated results from the proposed antenna are in good agreement.