## 1 Introduction  
With the progress of the Internet of Things (IoT) and 5G communication, modern healthcare is advancing day by day. Therefore, modern communication devices and wearable biomedical telemetry systems need such frequency bands that transmit large data, ensuring the quality of audio and video signal [1]. At present, the multiple-input-multiple-output (MIMO) antennas in the millimeter frequency range are the suitable choice of researchers to transmit/receive the signals with high-speed data rate [2]. Since there are many advantages of the MIMO antenna over a single-element antenna, the MIMO designs are highly recommended for the high-speed 5G communication technologies [3]. However, there are certain constraints while designing the MIMO antenna such as the size of the antenna and the isolation between the radiating elements [4]. To address these issues, attempts were made in [5; 6] to minimize the coupling effect between the MIMO elements in the proximity. In [7], a dual-band mm-wave MIMO antenna was designed for 28/38 GHz with good isolation, but the antenna has very narrow operating bands. The orthogonal elements in the MIMO design can largely improve the isolation as reported in [8], in which the composite radiating structures were used to operate at 28/38 GHz. In addition to that, a square slot was etched at the center of the ground plane to further improve the isolation > 33 dB at both the resonance frequencies. Along with the orthogonal arrangement of MIMO elements, the frequency selective surface (FSS) can be used to reduce the mutual coupling [9]. Due to the use of a 7 \(\times\) 7 array of unit cells in FSS and printed at a suitable position in the MIMO design, the reported MIMO antenna exhibits the increased gain of 8.6 dBi and isolation > 26 at the operating frequency of 28 GHz. The implementation of a metamaterial structure in a coplanar MIMO structure is also one of the methods to improve the isolation. In [10], a split-ring resonator (SRR) with some modification was used between the two radiating elements. The dual-port MIMO antenna was operating at 28/38 GHz with isolation > 28 dB. A dual-band two-port mm-wave MIMO antenna was designed in [11] operating at 24/36 GHz. The isolation was improved by etching the complementary split-ring resonator (CSRR) structure in the ground plane. To improve the overall gain, an array of four radiating elements with corporate feed technique [12] was used to design the dual-port MIMO antenna covering the operating band of 37 GHz. This design demonstrates the enhanced gain of 12.8 dB with an overall antenna dimension of 20 \(\times\) 40 \(\times\) 0.254 mm\({}^{3}\). In [13], multiple intersecting rings were used as MIMO elements to enhance the antenna gain up to 11 and 10.9 dBi at two resonant frequencies of 28 and 38 GHz, respectively. This 4-port MIMO design has linear arrangement of elements, and the total size of the antenna is 48 \(\times\) 12 \(\times\) 0.254 mm\({}^{3}\). Four mm-wave array along with two sub-6 GHz radiating elements were utilized as a co-designed system [14]. Further, a PIN diode was used to address the issue of beam steering in mm-wave frequency range. This antenna design was working in the millimeter frequency range (27.5-28.35 GHz) and sub-6 GHz (0.79-0.96 GHz and 1.7-5 GHz) applicable for 5G smartphones. Moreover, the PIN diode and shorting pins were used along with an H-shaped slot in the ground plane of the MIMO antenna to achieve the dual-frequency band [15]. In the upcoming advancement in wireless technology, the wearable MIMO antennas are widely used in the field of robotics, IoT, biotechnologies, medical sectors, and many more [16; 17; 18]. A semi-flexible 2-port MIMO antenna working in the 27 GHz and 29 GHz frequency bands was proposed in [19]. This antenna had a size of 36 mm \(\times\) 22.5 mm and was proposed for biomedical applications. Recently, a 4-port mm-wave antenna of size 18 mm \(\times\) 8.5 mm for wearable application was proposed in [20]; this antenna was working in the dual-frequency bands 28 GHz and 38 GHz.