I. Introduction
In 2002, Federal Communications Commission (FCC) declared a large bandwidth from 3.1 to 10.6 GHz as unlicensed for commercial use applications and was known as UWB technology [1]. Due to the large bandwidth, data rate, channel capacity, and quality of transmission are possible for portable wireless short distance communications. Nevertheless, it is suffering from multipath fading in the free space environment. Later, new technology came into wireless communication known as Multiple Input Multiple Output (MIMO) technology. MIMO technology uses more than one antenna element used at transmitting and the same thing in receiving end, and it combats the problem of multipath fading [2]–[3]. The data rate and channel capacity are depended on the bandwidth and number of antenna elements. Beyond the limit, we cannot increase the bandwidth, so the alternative is increasing the number of antenna elements. However, in MIMO technology, one main limiting factor is mutual coupling. Hence, compromising bandwidth, the number of elements, and mutual coupling factors are more important when designing a MIMO antenna structure [4]. The literature below from [5]–[12] is a four-element MIMO structure with good isolation for UWB applications are presented, and different coupling reductions methods are used to improve the isolation. Coplanar waveguide-fed 4-port MIMO antenna is designed for the UWB application, and polarisation diversity is used for isolation improvement [5]. A novel structure of 4-element MIMO antenna is designed for UWB region applications to enhance the isolation polarisation diversity & pattern diversity techniques are used [6]. Compact quad-port MIMO antenna is designed for UWB applications, and optimized dimensions of strips are placed between antenna elements for isolation enhancement [7]. Electromagnetic Band Gap (EBG) structures are used for isolation improvement in 4-port MIMO antenna structures [8]. A 4-port slot antenna is designed for LTE, WiFi, WiMAX, and WLAN applications [9]. A circular patch with ring slotted MIMO antenna is designed for UWB applications is presented [10]. A novel neutralization line technique is used for isolation improvement in a 4-port MIMO antenna for UWB applications is presented [11]. A wideband MIMO antenna is designed for 5G sub: 6GHz bands and WLAN applications are presented [12].