## 1 Introduction  
Antenna design has changed extensively over the past couple of decades. The antenna size has reduced to a great extent in this time. The antenna designs have shifted from bulkier horn antennas to small microstrip antennas. However, there is still scope for improvement, and researchers are working on achieving smaller antennas to accommodate small handheld portable devices. The antenna's reduction in size and weight can be handy in mobile devices as it can create a space for new applications to be accommodated in portable devices. The size reduction and high gain are the two critical parameters that can apply to many applications if achieved together. The metamaterial can be used to accomplish these essential parameters in multiple-input multiple-output (MIMO) antennas. Comparing a MIMO system to a single-input single-output (SISO) system, the former is superior in terms of gain, bandwidth, channel capacity, and diversity performance. As a MIMO system does not have to retransmit information repeatedly, the data throughput is significantly increased. An additional benefit of a MIMO system in a dense scattering environment is a higher data rate with less channel capacity loss. This is achieved without increasing the transmitted power or the spectrum used [1]. A combination of different methods is used to improve MIMO performance. Short-range wireless local area networks (WLANs) are one of several wireless communications applications that use antennas. Researchers are keen on WLAN MIMO systems because of their potential to improve the quality of wireless connections between various electronic gadgets. WLAN networks rely on base stations, sometimes called access points, to link wireless clients to the network [2]. Metamaterials are artificial materials that can be incorporated into antennas to improve their parameters such as gain, size, bandwidth, etc. Metamaterial antennas can enhance the bandwidth and gain by incorporating metamaterial elements in antennas [3]. Metamaterial components can be added to the dipole antenna to reduce its size and achieve reconfiguration [4]. Gain and bandwidth enhancement are achieved using Radar cross-section (RCS) reduction with a resonating antenna with a metamaterial superstrate [5]. Pattern reconfiguration and frequency tunability is achieved using metamaterial antennas [6]. Liquid metamaterial is a new concept, and the metamaterial antenna based on this new concept is implemented in [7]. Multiband antenna design is presented with metamaterial superstrates. Metamaterial superstrates are formed with liquid split ring resonator (SRR) and copper SRR [8]. Metamaterials can also be added to the MIMO antenna to improve its parameters. Metamaterial MIMO antennas can be used to enhance parameters such as isolation, gain, bandwidth, and size. The size and gain of the MIMO antenna can be improved with a metamaterial superstrate [9]. Antenna diversity may be accomplished using various approaches, including polarization, space, and pattern diversities, or by combining these and other methods. One or more of these many diversity schemes may be used to achieve diversity gain, depending on various considerations, including space, environment, and interferences [10]. Ultrawideband (UWB) MIMO technology combines UWB and MIMO technologies to maximize the benefits of each, allowing for a faster data transmission rate and increased channel capacity while mitigating multipath fading. Scholars have also found UWB MIMO antennas to be an exciting area of study in recent years. The main issue with UWB MIMO antennas is their poor coupling. Due to the high speed of today's mobile terminals, mobile communications and wireless devices are shrinking and converging. As UWB MIMO antennas continue to shrink in size, electromagnetic interference will become more severe due to the decreased spacing between antenna components and increased strength of mutual interaction [11].