# Novel MIMO Antenna System for Ultra Wideband Applications
## 1 Introduction  
Recent progress in wireless communication necessitates a system capable of sharing massive amounts of data quickly with increased capacity, dependability, and security with less complexity. Ultra wideband (UWB) technology can meet these expectations since it allows high data rates while employing low-cost infrastructure [1, 2]. The traditional UWB technology, on the other hand, has problems with multipath propagation. To mitigate this effect, multiple-input multiple-output (MIMO) technology has been introduced. Furthermore, one of the most promising solutions for meeting the needs of UWB systems is to design MIMO antennas. It is a difficult task to design such an antenna because it adds extra performance characteristics to consider. Antenna element isolation is one of the most essential performance characteristics, which should be as low as possible to avoid channel capacity loss. Several configurations for improving isolation have been published in the literature. Designing stubs with the ground plane or the radiating element helps improve isolationbetween MIMO antenna elements. In [3], a spatial diversity-based MIMO antenna was designed for UWB applications. To obtain a wide impedance bandwidth, a triangular-shaped patch radiator with a modified ground plane was used. The radiators do not radiate directly towards each other, which can lead to better isolation. In the ground plane, inverted L-shaped stubs and a complementary split ring resonance (CSRR) were designed to improve isolation even further. The authors in [4] utilized the same kind of configuration. In this design, a tapered fed polygon-shaped patch radiator was designed to achieve UWB response. The authors in [5] used the same type of isolation enhancement technique as presented in [3; 4]. They designed a co-planar waveguide (CPW)-fed bevealed-shaped patch radiator to achieve a UWB response. In addition, the radiating structure was composed of two half-wavelength rectangular single CSRRs (RSCSRRs) of different dimensions to achieve band notch characteristics for Worldwide Interoperability for Microwave Access (WiMAX) and Wireless Local Area Network (WLAN) frequency bands. In [6], an eye-shaped slot radiator-based MIMO antenna design was presented. The ground plane was developed with an extruded T-shaped stub to improve isolation. This simple configuration tends to achieve an isolation of \(>\)20 dB and ECC \(<\) 0.02. In [7], a 2-element dual-ring-based MIMO antenna was designed. The main patch radiator consists of a ring element, while the second ring behaves as a parasitic element. To improve the isolation between the antennas, two inverted U-stubs were designed into the ground plane. The employment of electromagnetic bandgap (EBG) structures, metamaterials, and frequency-selective surfaces (FSSs) to increase MIMO antenna element isolation has been proposed in the literature as a viable technology. In [8], a modified radiator with a FSS was reported. The design was non-planar, with four elements placed around a cube of polystyrene. A quad-port cognitive radio MIMO antenna was presented in [9]. The single antenna consists of a circular monopole geometry and a \(\lambda/4\) monopole for the notch band. For high isolation, multiple EBG cells having different dimensions and an isolating stub between the UWB and notch band radiators were employed. A two-port MIMO antenna with improved E-plane isolation was presented in [10]. A meandered EBG structure was designed on both sides of the substrate to improve isolation. In [11], a MIMO antenna design was presented having arrow shaped radiator. The presented design operates within the UWB spectrum with three notch bands obtained by cutting the slots within the radiator. To improve the isolation performance, a T-shaped stub was used with the ground plane, and two EBG cells were placed on each side of the feeding line. Additionally, the size of the structure is quite large for a two-port configuration. In [12], a miniaturized UWB MIMO antenna design was presented.