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A Compact Planar Omnidirectional MIMO Array Antenna with Pattern Phase Diversity Using Folded Dipole Element

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A Compact Planar Omnidirectional MIMO Array Antenna with Pattern Phase Diversity Using Folded Dipole Element 1688 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 67, NO. 3, MARCH 2019 A Compact Planar Omnidirectional MIMO Array Antenna With Pattern Phase Diversity Using Folded Dipole Element Libin Sun ,YueLi , Senior Member, IEEE, Zhijun Zhang , Fellow, IEEE, and Magdy F. Iskander, Life Fellow, IEEE Abstract— This paper presents a planar omnidirectional omnidirectional patterns [2]–[14]. In [2]–[5], some wide- multiple-input multiple-output (MIMO) antenna with pattern band or multiband dual-polarized omnidirectional antennas phase diversity. The profiles of conventional dual-polarized are proposed with the circular dipole array for horizontal omnidirectional MIMO antennas are too high to be planar integrated due to the limited bandwidth of vertical-polarized polarization (HP) and the discone antenna for vertical polar- element. Therefore, a dual-channel horizontal-polarized MIMO ization (VP). An artificial magnetic conductor reflector is antenna is proposed for the planar integration of omnidirectional appended in [9] to reduce the profile of the structure in [3]. MIMO antenna. The antenna is formed with four-element folded In [6] and [7], the cross bow-tie dipole is utilized for HP, while dipole array, and a compact feed network with two sets of 90° the inverted-cone antenna is utilized for VP. In [10] and [11], progressive phases is employed to feed the two channels with shared elements, thus the antenna can be integrated into a the vertical and horizontal slots are combined for the HP sheet of substrate with an ultralow profile of 0.8 mm. Although and VP, respectively. A novel sabre-like structure is proposed the polarization and radiation pattern (amplitude) of the two in [12] for the compactness of the dual-polarized antenna with colocated channels are the same with each other, a high isolation the resonant cavity for HP and declining monopole for VP. The is still achieved as a result of the orthogonal pattern phase. aforementioned antennas in [2]–[12] are all 3-D structures with A prototype was fabricated to validate the performance. The measured highest isolation between two channels is 31.7 dB, high profiles, and it is hard for them to be integrated into a pla- and −10 dB isolated bandwidth is 2.29–2.57 GHz (11.7%) with nar platform. To satisfy the requirement of planar integration, S11 < −14 dB and envelope correlation coefficient less than some planar dual-polarized omnidirectional MIMO antennas 0.05, which shows a good diversity performance. Therefore, are presented in [13] and [14]. However, the bandwidths of the proposed dual-channel antenna gives a feasible solution for them are narrow owing to the intrinsic narrowband property the planar integration of the omnidirectional MIMO system. of the low-profile omnidirectional VP element. To solve this Index Terms— Antenna diversity, antenna radiation patterns, problem, a planar dual-channel HP omnidirectional MIMO multiple-input multiple-output (MIMO), pattern phase diversity, antenna is proposed in this paper. Because the two channels planar integration. are both HP, the bandwidth of the proposed antenna will not be limited. The high isolation between two copolarization I. INTRODUCTION channels is achieved by a pattern phase diversity technique. N RECENT years, dual-polarized antennas are widely In traditional MIMO systems, polarization diversity Iemployed in multiple-input multiple-output (MIMO) sys- [2]–[14], spatial diversity [15]–[19], and radiation pattern tems to enhance the channel capacity [1]. In addition, diversity [20]–[33] techniques are exploited to enhance the omnidirectional pattern antennas have received great atten- port isolations between multiple channels. In polarization tion in some wireless communication systems such as the diversity, two orthogonal polarizations are utilized to decou- wireless local area networks and base stations. Therefore, pling between two channels. In spatial diversity, parasitic many researchers focused on the dual-polarized antennas with structures [15], slot etching [16], electromagnetic bandgap structures [17], and neutralization lines [18], [19] are applied Manuscript received September 27, 2018; revised November 14, 2018; accepted December 4, 2018. Date of publication December 24, 2018; date to reduce the mutual coupling with closely spaced ele- of current version March 5, 2019. This work was supported in part by the ments. In radiation pattern diversity, there are three dif- National Natural Science Foundation of China under Contract 61525104 and ferent ways to decoupling between multiple channels. The Contract 61771280 and in part by the Beijing Natural Science Foundation under Contract 4182029. (Corresponding author: Zhijun Zhang.) first way is employing different modes of patch antenna to L. Sun, Y. Li, and Z. Zhang are with the Beijing National Research generate broadside and conical radiation patterns [20]–[26]. Center for Information Science and Technology, Tsinghua University, Beijing, This method can be realized in a single radiator and thus 100084, China (e-mail: [email protected]). M. F. Iskander is with the Hawaii Center for Advanced Communica- has a simple antenna structure. The second way is using tions, University of Hawai’i at M¯anoa, Honolulu, HI 96822 USA (e-mail: different radiators to generate different beam-pointing pat- [email protected]). terns [27]–[30]. The third way is utilizing different feed Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. phases for the antenna array to achieve different radiation Digital Object Identifier 10.1109/TAP.2018.2889608 patterns [31], [32]. The above-mentioned pattern diversity 0018-926X © 2018 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. SUN et al.: COMPACT PLANAR OMNIDIRECTIONAL MIMO ARRAY ANTENNA 1689 techniques proposed in [20]–[32] are all based on the ampli- tude diversity of the radiation patterns, but cannot generate two isolated omnidirectional radiation channels. To decou- pling in the same polarization and beam pointing, a pat- tern phase diversity technique is exploited in this paper to achieve a high isolation between two shared-aperture channels. Except the dual-polarized omnidirectional antenna, some co-VP omnidirectional antennas are proposed in the full- duplex systems [33]–[36]. A VP progressively phased circular Fig. 1. Two different phase distributions of the omnidirectional radia- array used in antenna isolation is first proposed in 1974 [33]. tion pattern in the azimuth plane (normalized by the phase at ϕ = 0°). There are two isolated channels in [33], one channel is a (a) Increasing phase distribution (+1 mode). (b) Decreasing phase distribution − progressively phased four-element circular dipole array, and ( 1 mode). another channel is a single dipole placed at the center of the circular array, which is the electric field null of the first thus the goal of the MIMO antenna is to reduce the cor- channel, thus a high isolation can be acquired between them. relation between every channel. The envelope correlation To avoid affecting the omnidirectional radiation pattern of coefficient (ECC) is a significant parameter to evaluate the the inner element, the inner element is placed in a higher correlation between two channels, and it can be calculated by step in [34]. The inner element can also be replaced by the the complex radiated far field, the formulation is shown as progressively phased circular array, thus two progressively follows [37]: phased circular arrays with different feed phases are proposed ρ ≈ |ρ |2 e c in [35]. An improved broadband version is presented in [36]. 2 A12 (θ,ϕ) sin θdθdϕ The aforementioned research studies [33]–[36] are all dual- = (1) (θ,ϕ) θ θ ϕ · (θ,ϕ) θ θ ϕ channel omnidirectional antennas, but the transmit and receive A11 sin d d A22 sin d d antennas are not shared-aperture and the profiles are too high where to be planar integrated. = (θ, ϕ) · ∗ (θ, ϕ) + (θ, ϕ) · ∗ (θ, ϕ). To achieve a planar omnidirectional MIMO antenna with Aij Eθ,i Eθ,j Eϕ,i Eϕ,j (2) adequate bandwidth, a co-HP omnidirectional MIMO antenna Here, Eθ,i and Eϕ,i are the complex electric field of channel is proposed for the first time in this paper. Four-element planar i in the elevation and azimuth planes, respectively. ()∗ is the folded dipole array is fed in two different progressive phases conjugate operator. to achieve two isolated channels, and identical HP omnidi- Two different phase distributions of the far field with omni- rectional patterns for both channels are obtained. The high directional radiation pattern in the azimuth plane are illustrated isolation between two channels is achieved by the orthogonal in Fig. 1. As shown in Fig. 1(a), the phase increases from 0 to pattern phase distribution in the azimuth plane. In addition, 2π with ϕ increased, let us define it as +1 mode. As for a compact feed network is placed in the center of the planar Fig. 1(b), the phase decreases from 2π to 0 with ϕ increased, folded dipole array to integrate the whole antenna system in a and let us define it as −1 mode. Note that the polarizations and single sheet of substrate. Therefore, the profile of the proposed radiation beams of these two modes are identical to each other. omnidirectional MIMO antenna can be reduced to 0.8 mm To quantitatively evaluate the diversity performance of the λ ) (0.006 0 and easy to be planar integrated. Compared with pattern phase diversity technique, the ECC between these two the traditional dual-polarized planar omnidirectional MIMO modes is calculated. If only considering the complex electric antenna [13], [14], the narrowband problem can be solved for field in the azimuth plane, (1) can be simplified as the absence of VP channel. Consequently, a planar omnidirec- π 2 tional MIMO antenna with adequate bandwidth is realized by 2 A (ϕ)dϕ ρ ≈|ρ |2 = 0 12 a pattern phase diversity technique.
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