## 1 Introduction  
The use of ultra-wideband (UWB) technology for wireless applications has seen a surge after the Federal Communications Commission (FCC) specified the 3.1-10.6 GHz band as an unlicensed band [1]. Recently, UWB antennas have attracted the focus of RF engineers and researchers due to their extensive usage in sensing networks, cognitive radios, microwave imaging, wearable devices, military applications, wireless personal area networks, and high data rate communication systems. The monopole-based UWB antennas of various shapes and sizes have been explored by the researchers for various applications [2, 3]. The features, such as low-profile, miniature size, light-weight, good radiation efficiency, and simple integration into communication devices, make planar monopole antennas a preferred choice for UWB transceiver systems [4]. However, UWB antennas suffer from the disadvantage that radiation can be transmitted to smaller distances only, which is due to the use of low power for transmission, as mentioned by the Federal Communications Commission (FCC) [5]. The limitations of low power and short-range transmission can be encountered using multiple-input-multiple-output (MIMO) technology along with UWB. It uses multiple radiating elements for transmitting and receiving wireless signals, but the placement of multiple elements in a limited space within a communication device/system is a challenge. The close placement of resonating elements will result in poor isolation and high envelope correlation coefficient (ECC), which deteriorates the functioning of the designed system. An inverse relationship exists between the mutual coupling and the space present among the resonating elements. Various methods have been explored and proposed by the researchers for designing MIMO antennas. In literature, different configurations of dual-element antennas have been reported, but these antennas provide limited diversity. The designing of quad-port MIMO systems having four identical radiating elements is more complex as compared to the designing of dual-port systems having two identical elements, due to a higher level of mutual coupling in quad-port systems [6, 7]. Moreover, various commercially available devices use Wi-MAX and WLAN bands for performing operations like data transfer, wireless connectivity, etc., which may interfere with the UWB and thus affects the performance of the UWB system. Thus, it is preferable to eliminate these bands for better functioning of the UWB system [8, 9]. In the literature, many band-notched UWB MIMO/diversity antennas have been explored [10, 11, 12, 13, 14, 15, 16, 17, 18]. A MIMO antenna with two G-shaped elements was reported, where isolation between the resonating elements was enhanced by a T-shaped metal strip designed on the ground surface [10]. In [11], a compact MIMO design comprised of a coplanar waveguide-fed (CPW) monopole antenna and a half-slot antenna was presented, where a T-shaped parasitic stub, a protracted Z-shaped stub, a rectangular slot, and a combination of C-shaped slots were implanted to notch 5-6 GHz band. In [12], a small-sized two-port MIMO antenna comprised of square-shaped monopole resonators and a T-shaped metal stub embedded in the ground surface to reduce mutual coupling was proposed. In [13], a MIMO antenna composed of two staircase-shaped resonating elements was reported, where band rejection property was realized by engraving split-ring resonator (SRR) slits on the patch elements. In [14], a planar filtenna with shunt short-circuited stubs and three interdigital edge-coupled patterns was suggested for UWB MIMO applications, where dual notched-band characteristics were realized by introducing a complementary split-ring resonator (CSRR). A small-sized differential stepped-slot MIMO antenna was proposed with notched band rejection capability [15], where four U-shaped stubs were implanted in the radiators to notch the WLAN band.