## 1 Introduction  
Due to the enormous capabilities of 5G technologies, there has been a rapid increase in the deployment of IoT (Internet of Things) devices for a diverse range of applications, such as in smart cities, healthcare, smart manufacturing, logistics, intelligent transport systems, agriculture, etc. [1, 2, 3, 4]. These smart IoT devices must be compact and energy-carrying, they must have low end-to-end latency, and they must be capable of transmitting large volumes of data with excellent signal quality. The surging requirement for super high data rates and bandwidth in wireless communication has driven the adoption of the mmWave (millimeter-wave) frequency band [5]. Compared to its sub-6 GHz counterparts, the mmWave spectrum provides superior data rates and bandwidth. IoT-based applications, such as in industrial environments, smart health systems, autonomous vehicles, etc., require high rates. But, with the use of single antennas in the mmWave band, there is very high signal degradation due to path attenuation [6]. Therefore, mmWave antennas are more suitable for indoor short-range applications [7], such as in industrial automation, smart homes, smart healthcare, etc. To achieve optimal performance in the mmWave region and to mitigate the challenge of high attenuation, a MIMO configuration is implemented. This enables superior data rates and maintains adequate signal quality, even in adverse communication scenarios, through the use of spatial multiplexing [8; 9]. Additionally, because of the shorter wavelength in mmWave, a smaller antenna size can be achieved that can support massive MIMO and beamforming [10; 11]. MIMO, along with beamforming, can be implemented in IoT scenarios in Industry 4.0 or smart cities for efficient signal delivery. This enables the IoT devices to transmit large chunks of data in a shorter span of time, thus reducing the power consumption of IoT devices. The use of massive MIMO can support a large number of IoT devices, which is a primary requirement in smart cities and industrial IoT applications. In addition to supporting massive IoT deployment, the combined use of MIMO with Intelligent Reflecting Surfaces (IRSs) provides promising advantages, such as enhancement of the coverage footprint, signal quality, and reliability in densely cluttered areas [12]. MIMO antennas can also be implemented in dynamic, movable scenarios, such as in autonomous vehicles, drones, factory robots, etc. In order to attain better spatial Diversity Gain in such environments, the transmitter and/or the receiver may dynamically take positions with better channel conditions [13]. This can potentially help overcome multipath fading effects, reduce interference, enhance coverage, and improve the user experience [14]. However, because there is a common dielectric in MIMO configurations, mutual coupling due to closely placed antenna elements should be handled with diligence. There are several techniques given in the literature to minimize mutual coupling/interference, such as placing antennas elements orthogonally to each other and use of metamaterials, parasitic elements, slotted elements, neutralization lines [15], a Defected Ground Structure (DGS) [16; 17], an electromagnetic band-gap (EBG), etc. [18]. However, use of these additional structures for ensuring isolation also introduces power losses and consumes more space. Among the mentioned isolation techniques, placing antenna elements orthogonally not only reduces coupling but also the space requirements. The combined use of Ultra-Wideband (UWB) with MIMO technology finds its potential implementation in short-range communications, which can simultaneously enable enhanced data speed, wider bandwidths, decreased effects of multipath fading, and enhanced capacity [19; 20]. The main parameters that need to be addressed in designing UWB-MIMO antennas are maintaining the compact size, enabling a wide bandwidth, reducing mutual coupling, etc. [21].