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Top1. Introduction
Nowadays and future wireless applications, there is an insatiability demand for having high data rate, high link quality, large bandwidth antenna at both transmitter and receiver. MIMO technologies have gain much attention recently. The potential for MIMO antenna systems to improve reliability and enhance channel capacity in wireless mobile communications has generated great interest (Foschini & Gans,1998). A major consideration in MIMO antenna design is to reduce correlation between the multiple elements, and in particular the mutual-coupling electromagnetic interactions that exist between multiple elements are significant, because at the receiver end this effect could largely determine the performance of the system. Lower mutual coupling can result in higher antenna efficiencies and lower correlation coefficients (Thaysen & Jakobsen, 2007) . Significant research efforts to reduce mutual coupling have been reported. For example inserting stubs on ground plane (Zhao et al., 2014; Zhou F et al.,2010), two long protruding ground stubs (Liu et al., 2013) and T-shaped stub (Xiong & Gao, 2012) added on ground plane. Moreover, other methods to enhance isolation have been made using defected ground structure (DGS) (Prasanna & Behera, 2013; Sharmaa & Khanb,2013; Zhu et al., 2009). The structure consists of hexagonal, rectangular and dumb bell structures etched on ground plane to reduce mutual coupling between element. In Singh et al. (2013) the authors proposed by connecting an additional non radiating folded shorting strip between antenna element and ground plane, port to port isolation lower than –28 dB achieved for lower WLAN band.These studies include that by applying T-shape slot impedance transformer to both single and dual band PIFA’s, the isolation over 20 dB is obtained (Zhang et al., 2013). In Zuo et al. (2010), the authors proposed the method to obtained low mutual coupling by cutting two quarter wavelength slots into the ground plane. The use of planar soft surfaces proposed in Queredo Teruel et al. (2010) to reduce mutual coupling. Other methods to reduce mutual coupling and enhanced isolation of the MIMO antenna, such as inserting slits on ground plane (Li et al.,2012; Wu et al., 2013), I-shaped conductor in modified ground plane(C.See et al., 2013), using electromagnetic bandgap (EBG) and (Abidin et al., 2011) and using split ring resonator (Bait Suwailam et al., 2010).It is also intresting that neutralization line is one of the promising method to reduce coupling. By inserting neutralization line between antenna element(Peng et al., 2013; See et al., 2012; Su Lee & Chang,2012; Wang & Du, 2013) mutual coupling reduced as good as 22 dB can be realized.