# Meta Surface-Based Multiband MIMO Antenna for UAV Communications at mm-Wave and Sub-THz Bands
## 1 Introduction
### The State of the Art and Originality of Study
Several techniques, like metamaterials and metasurfaces, were utilized to enhance the performance of antennas at microwave and mm-wave bands. Researchers have introduced MTSs and metamaterials into the realm of study to develop broadband antennas that offer advantages like low profile, affordability, and seamless integration. Metamaterials' three-dimensional (3D) patterns can be transformed into two-dimensional (2D) patterns on a surface or interface by arranging multiple electrically small elements or apertures. This 2D version of a metamaterial is known as a metasurface (MTS). Unlike bulky 3D metamaterials, MTSs take up less physical space, resulting in lower losses. They hold great potential for revolutionizing various photonic and electronic device technologies. MTSs are engineered structures that exhibit unusual electromagnetic characteristics, such as negative permittivity and permeability. These MTSs, along with metamaterials, have garnered significant attention due to their unique properties, making them a promising technology for enhancing the performance of microwave components and overcoming limitations, particularly in antennas [23; 24]. Initial investigations employed composite periodic structures such as EBG (electromagnetic bandgap) and AMC (artificial magnetic conductor) to create artificial-impedance-controlled surfaces. These structures aimed to reduce size, achieve wide bandwidth, and minimize back radiation. A cut circular ring MTS structure with a diameter of 34.7 mm was utilized to enhance an antenna's gain up to 3 dB for the Sub-6 GHz band and reduce the radar cross-section of the antenna. The antenna obtained a maximum gain of 11.4 dBi. However, the complex structure operated at only a single broadband [25]. The unique shape, a slotted cutting edge, rectangular shape, that was used for the MTS structure, obtained a 5.66 dBi gain increment for the Sub-6 GHz band with the dimensions of 80 \(\times\) 80 \(\times\) 1.5 mm\({}^{3}\). Cutting the slot from the MTS gave a CP feature but only at a single band [26]. The authors of [27; 28] presented rectangular and slotted mushroom MTS structures for antenna performance enhancement for mm-wave and higher 5G bands, respectively. Despite obtaining high gains of 11 and 19 dBi, the structure was complex and multilayered, with higher dimensions related to the designing frequency. Furthermore, their MTS structures' unique multimode resonance characteristics and rectangular ring and split ring resonators have proven advantageous in broadband, multi-band, and omnidirectional radiation applications. Nevertheless, they could offer only a single wide band [29; 30]. MTS antennas maintain characteristics like low profile and ease of integration and usually exhibit a structure comprising multiple patch resonators. Having multiple arrays of resonators enables them to achieve broadside radiation gain and broadband bandwidth. It only had a single wide band of 3.8-8.5 GHz with a gain of almost eight dBi, but the antenna had a complex structure and a thickness of more than 4 mm [31]. Using characteristic mode analysis (CMA) may alter the structures of MTS, which significantly tightens the coupling between MTS structures and inevitably affects the desired modes. Moreover, segmented small-size patches uniformly impact the desired radiation pattern, making it challenging to adjust a specific mode. Complex feeding structures can also influence the MTS's characteristic mode, complicating the MTS antenna design. For example, dipole feeding and slot coupling feeding introduce new modes that expand antenna bandwidth. However, this demands a tighter correlation between the design of the MTS and the feeding structure, potentially increasing the antenna's profile [32]. Table 1 indicates the performance comparison of some recent, related, state-of-the-art UAV communications at microwave and mm-wave bands and several works utilizing MTS structures for both bands.