0018-926X (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TAP.2019.2955166, IEEE Transactions on Antennas and Propagation IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION 1 Phase Compensation for Decoupling of Large-scale Staggered Dual-polarized Dipole Array Antennas Changning Wei, Zhen-Yuan Zhang, and Ke-Li Wu, Fellow, IEEE Abstract—Staggered array antenna is a common array configuration for large-scale array antennas due to its favorable radiation characteristics and relatively large element spacing. In developing a compact staggered dipole array, the most challenging issue is how to simultaneously reduce the four mutual couplings taking place between adjacent co-polarized antenna elements with diversified phase laggings. A large difference in the phase of different mutual couplings makes simultaneous reduction of all the mutual couplings by applying the recently developed array-antenna decoupling surface (ADS) technique difficult. In this paper, a phase compensation method by using a staple-shaped probe for alleviating the largest phase offset is proposed conceptually and verified experimentally. With the proposed phase compensation method, the ADS technique can be effectively applied to a compact staggered dipole array with a wide band simultaneous decoupling. The design guideline for the phase compensation probe is presented by EM simulation and a parametric study. Two practical design examples of dual polarized staggered dipole arrays are given to demonstrate the effectiveness of the proposed phase compensation method in conjunction with ADS, showing a promising potential for wide band simultaneous decoupling of a large-scale dual polarized staggered dipole array-antenna. Index Terms—M-MIMO, large-scale array antenna, mutual coupling, decoupling, array-antenna decoupling surface, staggered array. I. INTRODUCTION ITH the spectrum being at a premium, especially in the sub-6 GHz bands, and the demands for high data throughput, the massive multiple input and multiple output (M-MIMO) technology has been recognized as the most compelling spatial multiplexing technology and will be adopted in the 5G and future wireless communication systems [1]. It has been shown that the M-MIMO technology can provide unprecedented spatial multiplexing gain, excellent spectral efficiency and superior energy efficiency [1 - 4]. Theoretically, the larger the number of antenna elements on base-station array antennas, the higher the spectral efficiency of an M-MIMO system. A great deal of effort has been paid to the research of various challenging problems facing to M- MIMO systems. One of the critical issues is how to reduce the mutual couplings among all the adjacent antenna elements in a 2D dual polarized array antenna [4], [5]. This issue appears to be intolerable for a compact M-MIMO array antenna, in which the center-to-center spacing is equal to or less than half a wavelength. Mutual coupling in a large-scale array antenna has received tremendous attention in the domains of phased array radars, wireless communications, and array signal processing since array antennas were put into use after World War II [6]. In practice, mutual coupling arouses the following major concerns: 1) narrowed scan angle due to substantial active impedance changes in a phased array [7, 8]; 2) reduced mean channel capacity due to the degraded signal-to-interference-noise ratio (SINR) [9]; 3) deteriorated radiation efficiency [10]; 4) degraded efficiency and linearity of power amplifiers in an M-MIMO system due to an unpredictable loading condition determined by the channel dependent precoding in the presence of the mutual coupling [11]; and 5) distorted radiation patterns of antenna elements. It will be shown in this study that the element radiation patterns in an M-MIMO array antenna will be severely suffered from distortion and gain drop in the principal directions due to mutual coupling, which leads to a degraded beamforming performance. Although a lot of researches have been done to reduce the mutual coupling in an array antenna, the majority of the works fall in the realm of antenna arrays with few antenna elements. In dealing with decoupling of two antenna elements, the mainstream method is to cancel the mutual admittance by an additional shunt connected circuit, for instance, [12 - 15]. A conceptual breakthrough called self- curing decoupling technique for two inverted-F antennas is reported very recently [16], by which no inter-connected circuit nor destructive structure on the ground between two antennas is needed. Reduction of mutual coupling among multiple antennas is more challenging and should take different decoupling philosophy. Among very few available approaches for decoupling of multiple antennas, the dummy element method with and without reactive loads sounds to be an effective approach for low gain antennas [17, 18]. An attempt of using metamaterial to enhance the isolation of a 2 by 2 inter-orthogonal linearly polarized slot array antenna is reported recently [19]. All of the aforementioned decoupling techniques are not suitable for a dual polarized broad side array antenna, which is the mainstream array configuration for M-MIMO systems. The main difficulties for the techniques to be adopted for practical M-MIMO array antennas lie in the fact that none of W Manuscript received October 23, 2018; revised xxxx. xx, 2018; accepted xxx. xx, 2019. This work was supported by Postgraduate Scholarship of The Chinese University of Hong Kong. (Corresponding author: Ke-Li Wu.) Authors are with the Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong, (emails: cnwei@ee.cuhk.edu.hk, zyzhang@ee.cuhk.edu.hk, klwu@cuhk.edu.hk ). Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on February 16,2020 at 03:13:25 UTC from IEEE Xplore. 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