# Meta Surface-Based Multiband MIMO Antenna for UAV Communications at mm-Wave and Sub-THz Bands
## 1 Introduction  
The fast progression of the mobile Internet and the Internet of Things (IoT) has caused a significant increase in mobile data traffic. As a result, improving the capacity and data transmission rates of communication systems is crucial. This high-data-rate transmission is increasingly essential for unmanned aerial vehicles (UAVs) to enable real-time sensor and camera data transmission, remote control, and situations where UAVs function as aerial base stations. Within fifth-generation (5G) wireless technology, the adoption of the millimeter-wave (mm-wave) frequency range holds promise for enabling rapid data transfer and extensive throughput for large-scale IoT devices and other user equipment [1; 2; 3]. Furthermore, mm-wave signals (up to sub-THz) are widely anticipated for their exceptional capabilities, such as high-angle resolution, sensing, and robust resistance to interference,leading to widespread application in various scenarios [4]. However, the propagation of mm-wave signals encounters significant hurdles, such as substantial path loss and limited transmission range [5]. These challenges could potentially hinder the progress of mm-wave wireless communications.  
The adaptable nature of communication systems supported by UAVs holds promise in addressing many limitations of mm-wave signals, offering wireless connectivity for radiofrequency (RF) devices in areas with signal shadows [6]. Aside from providing ample bandwidth, mm-wave and sub-THz frequency bands possess the potential to enable high data rates with minimal latency [7]. Moreover, connections to UAVs often operate in a line-of-sight (LOS) manner, bypassing common obstruction issues encountered in mm-wave deployments for cellular and local area networks (LANs). However, mm-wave communication with UAVs presents several technical challenges. These concerns include range limitations, power usage, and the capability to maintain directional alignment, particularly in high-mobility situations.  
The design and placement of antennas play a pivotal role in determining communication range, with multi-array configurations offering significant advantages in long-range applications. As UAVs prioritize a flat and compact design to reduce wind resistance, they favor planar antenna structures for their lightweight and small-scale wireless systems. Apart from the miniaturizations, other factors such as resonant frequency, impedance bandwidth, polarization, and radiation pattern affect antenna performance in UAV-assisted communications. Studies have shown that lowering antenna weight by 25% can save around 10% of a UAV's energy consumption [8].  
Research on mm-wave UAV communication is relatively nascent, and the existing articles consider either the analysis and investigation of UAV communication networks or the required antennas for UAV communications at mm-wave band up to sub-THz. For example, the authors of [9] conducted laboratory tests to gauge the efficacy of beam tracking, while those of [10] employed ray tracing and channel sounding methods to evaluate directional antennas' capabilities for drone-to-ground UAV communications operating at 28 GHz and 60 GHz. However, since this article focuses on antenna design and its analysis for UAV communications (D2D and D2S) at mm-wave and sub-THz, the state of the art will move further towards antennas designed for these UAV communications.