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.

A dual‐band metamaterial microwave absorber for X band (8–12 GHz) applications is proposed. A unit cell of the absorber consists of an electric LC (ELC) resonator, a backing ground plate, and an FR4‐epoxy substrate with 0.8 mm thickness. To realize efficient space utilization for the dual resonance, an ELC resonator consisting of asymmetric triangle split‐ring resonators is adopted. Using effective medium parameters extracted from the S‐parameters, the EM behavior is clarified to describe the operating mechanism. The miniaturized dimensions of the unit cell are 8 mm × 8 mm × 0.8 mm. The measured results exhibit two absorptivity peaks stronger than 98.5% and FWHMs of 8.4% and 7.25% at 8.2 GHz and 12 GHz, respectively. Additionally, high absorptivity performance is achieved for EM waves from 0° to 60° regardless of the incident angle. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2052–2057, 2016

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.

A compact ultra-wide band antenna with a quadruple band-notched characteristic is proposed. The proposed antenna consists of a slotted trapezoid patch radiator, an inverted U-shaped band stop filter, a pair of C-shaped band stop filters, and a rectangular ground plane. To realize the quadruple notch-band characteristic, a U-shaped slot, a complementary split ring resonator, an inverted U-shaped band stop filter, and two C-shaped band stop filters are utilized in this antenna. The antenna satisfies the -10 dB reflection coefficient bandwidth requirement in the frequency band of 2.88-12.67 GHz, with a band-rejection characteristic in the WiMAX (3.43-3.85 GHz), WLAN (5.26-6.01 GHz), X-band satellite communication (7.05-7.68 GHz), and ITU 8 GHz (8.08-8.87 GHz) signal bands. In addition, the proposed antenna has a compact volume of 30 mm× 33.5 mm× 0.8 mm while maintaining omnidirectional patterns in the H-plane. The experimental and simulated results of the proposed antenna are shown to be in good agreement. 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. ⓒ