# 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  
This section presents recent studies on antennas designed for UAV communications, including D2G, D2D, and D2S. It also considers the long- (low-frequency) and short-range (mm-wave and sub-THz) UAV communications. The researchers who wrote [11] employed reconfigurable radiation beams to guarantee extensive signal coverage for air-to-ground communications. Despite obtaining a high gain of 10.2 dBic, adjusting these beams required supplementary feed networks, which added complexity to the UAV design. Furthermore, it also offered size increments and thicknesses of up to 13 mm, escalating energy consumption. The authors of [12] introduced wide-band mm-wave antennas tailored for high-speed UAV communications. However, their designs encountered challenges such as low gain, increased thickness and larger size compared to the center frequency of 28.3 GHz, and a single-band dipole designed for D2G communication. It offered a deficient gain of 1.76 dBi, having a length of 110 mm. It could be considered for long-range communication, but obtaining a meager gain affects signal transmission, brings a high chance of signal interference, and results in poor link quality [13]. The dual-band MIMO antenna was designed for long-range and D2G communications. Using this, they obtained a peak gain of 8.7 dBi and an impedance BW of 21% at 2.4 GHz. The antenna also showed high isolation between the two ports and had a low profile. However, the BW was limited [14]. Another dual-band antenna was designed for D2G communication with a maximum gain of 9.55 dBi and a diameter of 76 mm. However, narrow BW and polarization assessment were not considered [15]. Last but not least, a single-band horn antenna was designed for the C-band at 5.4 GHz, offering a high gain of up to 16.25 dBi. However, the antenna was -bulky (0.5 kg), nonplanar, and had linear polarization [16]. All the long-range antennas presented for D2G communication showed limitations such as narrow BW, oversized and heavy structure, lower gain compared to their size, single/dual bands, and linear polarization [11; 12; 13; 14; 15; 16]. Researchers used slightly higher bands, mm-wave and sub-THz, to address these limitations. Alternatively, a separate strategy involves using dual-band leaky-wave antennas, offering frequency-scanning beams for prospective micro- and mm-wave mobile communications, covering both low and higher bands. They offer high gain but with high complexity and an extended length of more than 25 cm [17]. Additionally, the antenna design presented in [18] operates at 2.4/5.2 GHz and 60 GHz bands, catering to existing WiFi connections and anticipated high-data-rate Wi-Gig applications. The antenna offered high gain, having large dimensions and linear polarizations. Nonetheless, many of these designs encounter issues like elevated profiles and limited bandwidths in the mm-wave spectrum, rendering them capable of only a restricted performance for UAV applications. Moreover, their radiation directions could not be flexibly adjusted to the upper and lower half-spaces based on UAV mission requirements. In addition, some studies have delved beyond the typical frequency range, exploring communication frequencies above 100 GHz, where the available bandwidths surpass the lower mm-wave frequencies [19]. For instance, some worked on communication protocols and cooperative communication for UAVs at sub-THz and THz bands [20; 21]. Not much work was performed considering the antenna design for both mm-wave and sub-THz bands. A single study examined two distinct systems: (a) a 28 GHz system resembling existing 5G new radio (NR) mm-wave deployments; (b) a theoretical 140 GHz system employing a connection akin to NR. The 140 GHz band represents a prospective sub-terahertz frequency spectrum for forthcoming 6G systems. They simulated and proved that the antenna works at these two bands when the drone is far from the base station, with a vertical distance of 25 m and a horizontal distance of 700 m [22].