# Miniaturized Antenna Array-Based Novel Metamaterial Technology for Reconfigurable MIMO Systems
## 1 Introduction  
Recently, microstrip antennas were introduced in many wireless communication networks including 5G systems for their desired properties, including a low profile and inexpensive fabrication costs [1]. Nevertheless, MTM and specific composite right/left-handed (CRLH) structures are introduced to the microstrip antenna designs from different research groups to enhance their limitations including low gain bandwidth products [2]. Therefore, many researchers applied their scientific research and theoretical studies to develop different microstrip antennas for different applications. One of the most popular applications of CRLH-based microstrip antennas is the MIMO antenna array [3]. This technology received high attention due to its unique specifications, among many others, its high channel capacity with minimum latency [3]. For this, the authors in [4] developed an array of two printed transparent circuit antennas with low gain and limited bandwidth for MIMO applications at sub-6 GHz. In [5], a design of four elements was sequentially fed and mounted on a substrate backed with an electromagnetic bandgap structure of sub-6 GHz bands. The antenna's orientation was introduced to avoid a high coupling effects due to skew waves. However, the proposed MTM was presented to minimize the surface wave's effects from the antenna back panel. Other researchers developed, for example, a low-profile antenna in [2] based on antenna elements of Hilbert-shaped fractalmetamaterials to form a 2D-rectangular array fed with a coplanar waveguide feed line. Another design was proposed for sub-6 GHz applications based on a wideband elliptically slotted semi-circular patch element with a defected ground plane structure [4]. In [5], efforts were applied to design a directive pointer antenna for sub-6 GHz systems including 5G applications. Another work was published in [6] to realize a design of MIMO antenna based 2 elements at sub-6 GHz frequency bands for portable devices. A dual frequency band reconfigurable antenna-based portable device was developed for 5G systems in [7]. A dual band antenna based on compact flexible structure for GSM and Wi-Fi applications [8]. A design of an antenna is based MTM structure for reconfigurable systems at sub-6 GHz bands [9]. A MIMO antenna array system was proposed based on a pair of transparent antenna elements at sub-6 GHz frequency bands [10]. In [11], a metasurface reflector with a four-port wideband MIMO antenna array systems was developed to support 5G sub-6 GHz applications. Later, several studies were applied to design an antenna that supports more than one application; for instance, in [12] an antenna was developed for 5G MIMO systems and the Internet of Things. In [13], a dual-band antenna based on composite right-and left-handed transmission lines was developed for MIMO systems at sub-6 GHz bands. Additionally, in [14], a design was introduced for the applications of 5G MIMO antenna arrays to be applied in smartphones at the frequencies in the ranges of 3.41-3.63 and 4.82-5.23 GHz. In [15], two wideband PITA elements were offered as a possible solution for constructing a miniaturized MIMO array to suit 5G mobile handsets [15]. A design of a compact broadband bandstop filter based on composite right/left handed transmission line was proposed to reduce the mutual coupling between the antenna elements in MIMO systems at 3.5 GHz bands [16]. In the same manner, a design was suggested in [17] using a split ring resonators to reduce the mutual coupling between antenna at sub-6 GHz for 5G MIMO systems. A reconfigurable antenna element was designed to provide a wide and narrow frequency band for smart MIMO antenna arrays [18]. A design of an MTM-based ultra-wideband antenna was developed to form a MIMO array [19]. A design of a solar panel integrated 3D MIMO antenna array for 5G self-powered communication systems was proposed in [20]. For frequencies between 3.4 and 3.8 GHz, a MIMO antenna array was developed with a partial ground plane [21].