# Novel MIMO Antenna System for Ultra Wideband Applications
## 1 Introduction
The MIMO antenna elements were composed of a combination of rectangle and semi-circular arc fed using a stepped microstrip fee line. For improved isolation between MIMO elements, a Jerusalem cross-shaped FSS structure and a meandered-shaped decoupling structure were designed. All the above-reported designs have EBG/FSS structures, which are complex to fabricate as they have a \"via\" that connects the top and ground plane and are large in size. The use of neutralization lines or decoupling structures between the radiators can also provide high isolation. In [13], a decoupling strip was diagonally placed between the antenna elements for isolation enhancement. In addition, a dumbbell-shaped stub was designed with the ground plane, and the same was placed on the top side of the substrate. From the presented configuration, an isolation of \(>\)15 dB was achieved in the frequency range of 3-11 GHz. In [14], a four-port dual-band notch UWB MIMO antenna was presented. Four rhombic-shaped patch radiators were arranged in a polarization diversity configuration. Two CSRRs were etched from the radiators to reject WLAN and WiMAX frequency bands. A plus-shaped metal strip was designed with the ground plane for enhanced isolation. In [15], a design for a compact super wideband (SWB) antenna was presented. The presented SWB radiator geometry consists of a combination of circular, rectangular, and trapezoidal shapes. The bandwidth of the antenna was enhanced by placing slots in the radiator and by a modified ground plane. Different MIMO configurations were analyzed, such as spatial, pattern, and polarization diversity with a common ground plane. From the presented configurations, an isolation of \(\geq\)30 dB was achieved. In [16], an octagonal band notch UWB MIMO antenna design was demonstrated. Two hexagonal-shaped CSRRs (HCSRRs) were etched from the octagonal radiator to achieve band stop characteristics around the WLAN and WiMAX frequency bands. A decoupling structure consisting of three thin metal strips was designed in the middle of the radiation elements to achieve an isolation of \(>\)18 dB in the UWB frequency range. The same kind of isolation enhancement technique was described in [17]. In [18], a simple polarization diversity-based four-element MIMO configuration was presented for UWB applications. The single antenna element consists of a combination of rectangular and circular patch radiators. In [19], an eight-element semi-circular arc-based MIMO antenna design was presented for UWB applications. The UWB response was achieved by etching a semi-elliptical slot in the ground plane and designing an inverted L-shaped strip in conjunction with the ground plane. Initially, the authors realized a four-element MIMO antenna design placed in a plus-shaped configuration. After that, an eight-element MIMO antenna configuration was demonstrated for UWB applications. Thin metallic strips were designed between the radiators to achieve an isolation of \(>\)15 dB in the frequency range of 2.84-11 GHz. In the literature, some researchers utilized defected ground structures (DGSs) to improve the isolation between MIMO antennas. A two-element aperture-coupled complementary Sierpinski gasket equilateral triangular fractal MIMO antenna was designed for UWB performance in [20]. For enhanced isolation, a complementary Archimedean spiral-based DGS was utilized. The use of DGS led to an isolation level of \(\geq\)15.8 dB. In [21], for UWB applications, a MIMO antenna with good isolation was demonstrated. For MIMO antenna design, slot elements having quasi F-shaped radiators and L-shaped open-slots were adopted. In the ground plane, a decoupling structure consisting of a thin slot and a fork-shaped slot was implemented to increase isolation between the radiators and in the lower band. The techniques presented above provide better isolation between antenna elements, but they are quite complex and their fabrication is difficult. In this paper, a novel MIMO antenna configuration is presented for UWB applications.