Two ground radiation antennas with polarisation diversity performance were proposed for MIMO WLAN applications in wearable devices. The antennas can simultaneously excite the vertical mode and horizontal mode of the ground plane and the phase difference between the two modes can be controlled by utilising an inductor‐loaded metal strip in the ground plane, generating circular polarisation. A 3:1 VSWR bandwidth of 140 MHz with high isolation (above 17 dB) and a 3 dB axial ratio bandwidth of 100 MHz was obtained. The opposite rotations generated by the two antennas resulted in good diversity performance, which was verified by the measured envelope correlation coefficient.

This study investigates a compact and straightforward self-decoupled 2 × 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 selfdecoupled characteristic of the proposed MIMO antennas even though the antenna elements are tightly arranged and collocated together. An 8 × 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. INDEX TERMS Multiple-input multiple-output (MIMO), 5G, terminal devices, loop antenna, groundradiation antenna, orthogonality.

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.INDEX TERMS Multiple-input multiple-output (MIMO), ground-radiation antenna, 5G, terminal devices, decoupling technique, closed-decoupling-loop.

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 multipleinput 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. INDEX TERMS Isolated ground-radiation antenna (iGradiANT), inherent isolation, 5G, multiple-input multiple-output (MIMO), terminal devices. I. INTRODUCTION With the explosion of the user number and a burst of powerful cellular devices, there is a tremendous demand for fast data rates. The next-generation communication (5G) is proposed to address this demand by employing the unprecedented spectrum of sub-6 GHz band and millimeter wave (mmWave) band, such that characteristics of ultrafast speeds, low latency, and excellent reliability, can be supported [1, 2]. In the sub-6 GHz layer, the allocation of the 3.5 GHz band (3.4-3.6 GHz) for 5G wireless communication [3] has brought an explosion of groundbreaking research works in multiple-input multiple-output (MIMO) antenna systems for current and future terminal devices [4-32]. It has been proved that the integration of more antenna elements into terminal devices can significantly increase energy efficiency, spectral efficiency, robustness, and reliability, thus satisfying the growing demands of 5G wireless communication.

In this paper, an asymmetric compact multiband slot antenna is proposed for global positioning system (GPS), worldwide interoperability for microwave access (WiMAX), and wireless area network (WLAN) applications. The top plane, a ground is composed of a rectangular slot with a trapezoidal-like stub, an inverted U-shaped slot at the right side of the rectangular slot, an inverted L-shaped slot at the left side of the rectangular slot, and three stubs. The proposed antenna is fed by an asymmetric cross-parasitic strip on the bottom plane. By properly designing the slots and stubs, four resonant frequency bands are achieved with -10 dB reflection coefficient bandwidths of 50 MHz, 400 MHz, 390 MHz, and 830 MHz in the 1.57 GHz GPS band, 2.4 GHz WLAN band, 3.5 GHz WiMAX band, and 5.5 GHz WLAN bands, respectively. The antenna has a total compact size of 13 mm × 32 mm × 0.8 mm. Simulated and measured results indicate that the proposed antenna has sufficient bandwidth and good radiation performance in each band.Key Words: GPS, Quad-Band, Slot Antenna, WiMAX, WLAN. This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. ⓒ